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Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse Emily C. Neeve,† Stephen J. Geier,§ Ibraheem A. I. Mkhalid,‡ Stephen A. Westcott,*,§ and Todd B. Marder*,†

† Institut für Anorganische Chemie, Julius-Maximilians-UniversitatWü ̈rzburg, Würzburg 97074, Germany § Mount Allison University, Department of Biochemistry and Chemistry, Sackville, New Brunswick E4L 1G8, Canada ‡ Department of Chemistry, King Abdulaziz University, Jeddah 21413, Saudi Arabia

ABSTRACT: Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B−C bonds and other processes.

CONTENTS 4. Boryl Addition (Hydroboration) Reactions 9115 4.1. α,β-Unsaturated Carbonyls, Imines, and 1. Introduction 9092 Related Compounds 9115 2. Synthesis, Structure, and Bonding of Diboron(4) 4.2. Alkynes 9119 Compounds 9093 4.3. Alkenes 9121 2.1. B2X4 Compounds 9093 4.4. Dienes, Enynes, and Allenes 9122 2.2. B2H4 Chemistry 9094 4.5. Aldehydes and Imines 9123 2.3. B2(NR2)4 Compounds 9094 4.6. Ring-Opening Reactions 9123 2.4. B2(alkyl)n Compounds 9095 4.7. Borylative Cyclizations and Related Intermo- 2.5. B2(OR)4 Compounds 9095 lecular Reactions 9124 2.5.1. Synthesis of B2(OR)4 9095 4.8. Boron-Element Additions Across Mutiple 2.5.2. Adducts of B2(OR)4 Compounds 9096 Bonds 9125 2.6. Three-Membered Aromatic Heterocycles − 4.9. Diagram Summarizing Section 4 9126 Containing B B Bonds 9098 5. Boryl Substitutions 9126 2.7. Diboryl Groups as Ligands 9098 5.1. Coupling Reactions 9126 2.8. [2]-Borametalloarenophanes 9098 − 5.2. Allylic and Propargylic Substitutions 9127 2.9. B B Bonding 9099 5.3. Alkyl Substitutions 9130 3. 1,2- and 1,4-Diborations 9101 5.4. Alkenyl Substitutions 9131 3.1. Mechanistic Studies of 1,2-Diboration Pro- 5.5. Aromatic Substitutions 9131 cesses 9101 5.5.1. Aryl Halides 9131 3.2. Alkynes 9104 5.5.2. Aryl C−O Electrophiles 9135 3.3. Alkenes (Aliphatic) 9105 5.5.3. Aryl C−N Bonds 9136 3.4. Alkenes (Vinyl Arenes) 9107 5.5.4. Aryl Nitriles 9136 3.5. Alkenyl and Alkynyl Boronate Esters 9108 5.6. Diagram Summarizing Section 5 9137 3.6. Dienes 9109 6. Conclusion 9137 3.7. Allenes 9110 Author Information 9137 3.8. Carbonyls and Thiocarbonyls 9112 Corresponding Authors 9137 3.9. Imines 9113 3.10. Other Substrates 9113 3.11. Unsaturated Carbonyls 9114 3.12. Diagram Summarizing Section 3 9115 Received: March 22, 2016 Published: July 19, 2016

© 2016 American Chemical Society 9091 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Figure 1. Diboron compounds.

Notes 9137 1b).11 This compound was arduously prepared by the slow Biographies 9137 passage of B5H9, in an atmosphere of dihydrogen, through an Acknowledgments 9138 electric discharge between copper electrodes. Not surprisingly, References 9138 while several theoretical and physical studies have focused on − this complex,12 22 its complicated synthesis has limited the synthetic potential of this unique diboron compound, and its ff 1. INTRODUCTION chemistry remains largely unexplored. A di erent structural − isomer of B10H16, also bearing a central B B bond where The advent of diboron(4) compounds in 1925 introduced a neither boron atom bears a terminal hydride, was prepared by new group of reagents that would eventually prove to be very Sneddon and co-workers through the dehydrocoupling of versatile and extremely useful in many synthetic pathways to B H .23 form a range of valuable natural products, pharmaceutical 5 9 1 Brown further extended research on diboron compounds by intermediates, and biologically active compounds. Further- developing a pantheon of alkyl- and dialkylboranes by the more, organic compounds containing boron are ideal addition of BH ·LB (where LB = Lewis base, such as THF or candidates for green chemistry as they are generally considered 3 SMe2) to alkenes. For example, the addition of borane to nontoxic to plants, mammals, and other complex life forms and ff 2 cyclooctadiene a ords 9-borabicyclo[3.3.1]nonane (9-BBN, are therefore environmentally benign. The past few decades 24 Figure 1c), which has found extensive use in hydroboration have seen remarkable progress in both inorganic and organic reactions, wherein the B−H bond of the borane adds to an boron chemistry, with several people awarded the Nobel Prize − unsaturated organic fragment to generate the corresponding for their ground-breaking research in these emerging areas.3 5 organoborane. In the absence of a Lewis base, dialkylboranes William Lipscomb and Herbert Brown were awarded the usually exist as dimers, in which the two boron fragments are Nobel Prize in 1976 and 1979, respectively, for their once again connected by two three-center, two-electron B−H− contributions to the field of boron chemistry. Lipscomb’s main contribution in this field came from deducing the nature B bridges. In 2010, the Nobel Prize was awarded to Professor Akira of chemical bonding in boranes, such as B2H6, diborane(6), and clusters (Figure 1a), although he also made significant Suzuki for his outstanding work on cross-coupling reactions contributions to both nuclear magnetic resonance spectroscopy and the corresponding organoborane products and boronate reagents that have all but usurped the role of their more toxic and in the chemistry of large biomolecules. Using X-ray 5,25 crystallography as a method of structural determination, tin counterparts. − − Lipscomb and others6 9 were able to assess the nature of The Suzuki Miyaura reaction uses organoboranes (primarily “ ” aryl boronic acids, ArB(OH)2, and their boronate ester B2H6, where the two BH3 fragments were connected to one another through two B−H−B interactions with a pair of derivatives, ArB(OR)2) and organic halides (ArX or RX), in electrons shared among the three atoms. No significant boron− the presence of a catalyst, to generate organic products − boron interaction is believed to occur in these three-center two- containing a new C C bond (Scheme 1a and 1b). Although electron bonds, and the chemistry of diborane(6) proceeds aryl boronic acid derivatives have traditionally been prepared primarily via cleavage, either homolytically or heterolytically, of using Grignard and organolithium reagents, alternative − − borylation reactions using dialkoxyboranes or diboron(4) the two B H B bonds. 26 27,28 Furthering the understanding of boron chemistry, Nobel compounds, such as B2cat2 (cat = 1,2-O2C6H4), B2pin2 29 Prize laureate Roald Hoffmann was a doctoral student in (pin =1,2-O2C2Me4, Figure 1d), and B2neop2 (neop = Lipscomb’s laboratory and later responsible, in part, for OCH2CMe2CH2O, Figure 1e), have been developed (Scheme developing the extended Hückel method for calculating 1c). These diboron(4) compounds are relatively stable and easy molecular orbitals, the fragment molecular orbital (FMO) to handle and are increasingly utilized in all aspects of approach, and for developing the isolobal analogy well beyond chemistry. Indeed, it is now difficult to keep pace with the latest the boron cluster field.10 Other notable boron chemists that developments using these tetraalkoxydiboron(4) compounds, including tetrahydroxydiboron, as their full potential is still worked with Lipscomb included M. Frederick Hawthorne, − pioneer in the synthesis of boron and carborane clusters and coming to light (Figure 2).30 56 The volume of papers their metal-containing compounds, and Russell Grimes, who published in this area has grown exponentially, with bis- found that a remarkable boron hydride, namely, B10H16, (pinacolato)diboron appearing as a reactant or reagent in as contains a direct and unsupported B−B bond, where neither many as 8193 publications (SciFinder, 01.02.2016). The utility of the central boron atoms has a terminal hydride (Figure of these remarkable compounds continues to grow rapidly,

9092 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

63−70 Scheme 1. (a) Generic Suzuki−Miyaura Cross-Coupling preparation of B2Cl4 have been examined, including an Reaction; (b) Cross-Coupling of a Diborated Alkene with early reaction using microwave excitation of gaseous boron Iodobenzene; (c) Miyaura Borylation of an Aryl Halide with trichloride,71 these attempts suffered from low yields and harsh B2pin2 reaction conditions. Interestingly, diboron tetrachloride has been prepared by reaction of boron trichloride with boron ° 72 monoxide, x(BO)x, at 200 C. The starting boron monoxide was generated by dehydration of tetrahydroxydiboron(4), B2(OH)4, previously prepared by the hydrolysis of tetrakis- (dimethylamino)diboron(4), B2(NMe2)4, in aqueous hydro- chloric acid solution. Furthermore, Schlesinger et al. inves- fi tigated the reaction of B2Cl4 with BBr3 to obtain the rst proof 63 for the formation of B2Br4. However, later, the diboron tetrabromide species was prepared using a similar methodology to the B2Cl4 system with tetramethoxydiboron(4) and boron tribromide.68,73 The importance of these tetrahydroxy-, tetramethoxy-, and tetraaminodiboron(4) compounds will be addressed later (sections 2.3 and 2.5). Investigations into the synthesis of related diboron tetrahalides, B2I4 and B2F4, were also conducted. The preparation of diboron tetraiodide was first reported by Schumb in 1949 using electrodeless radiofrequency discharge 74,75 to reduce BI3, while B2F4 was initially prepared, almost a 76 decade later, in 1958 from the reaction of B2Cl4 and SbF3. In 1967, Timms reported the formation of B2F4, as well as triboron pentafluoride, during the co-condensation of BF with − ° 77 BF3 at 196 C. Furthermore, several years later, he also showed that a similar system could be used to synthesize 66,69,78 B2Cl4. While the existence of homoatomic bonds was well known for carbon, nitrogen, and oxygen, the possibility that an electropositive element such as boron could form an unsupported bond with itself gave birth to a whole new area Figure 2. Number of publications featuring bis(pinacolato)diboron as of chemistry. Early studies on the structure79 and reactiv- a reactant or reagent per year. − ity75,80 86 of boron subhalides found that these species behaved as bifunctional Lewis acids. For example, addition of PCl3 to warranting this new review which details many of the recent · B2Cl4 gave the adduct (PCl3)2 B2Cl4 as the major boron- developments in this evolving area. However, due to the scale containing species.81 However, more interestingly, the first of papers published in this field, this paper will not include diboration of ethylene using B2Cl4 was reported by Schlesinger catalytic C−H borylation, for which there are already several 80 fi 51,57−60 et al. in 1954. The stereochemistry of this reaction was found excellent reviews focusing speci cally on this area. to be specific for cis addition and consistent with a four- 82 2. SYNTHESIS, STRUCTURE, AND BONDING OF centered transition state (Scheme 2). While this remarkable DIBORON(4) COMPOUNDS Scheme 2. Diboration Reaction of Ethylene Using B2Cl4 Compounds such as B2pin2 and B2neop2 (Figure 1) are referred to by some as diboron(4) and by others as diborane(4) species. The IUPAC nomenclature for such compounds also derives from their ring sizes, such that the correct name for ′ ′ ′ ′ bis(pinacolato)diboron, B2pin2, is 4,4,4 ,4 ,5,5,5 ,5 -octameth- yl-2,2′-bi-(1,3,2-dioxaborolane), whereas for bis(neopentyl ′ ′ glycolato)diboron, B2neop2, the correct name is 5,5,5 ,5 - tetramethyl-2,2′-bi-(1,3,2-dioxaborinane). We employ the reaction was later expanded to include B F along with a family simple and widely used abbreviations and the general term 2 4 of unsaturated hydrocarbons, including vinyl halides, butadiene, diboron(4) for such compounds throughout the review. and acetylene, the synthetic utility of this methodology was 2.1. B2X4 Compounds once again limited by the difficult and harsh reaction conditions The genesis of diboron(4) compounds occurred in 1925 when necessary to produce the starting diboron(4) compounds.83 Stock, Brandt, and Fischer prepared the first so-called boron Recently, Fernandez,́ and Brown conducted a DFT study, subhalide, B2Cl4, by striking an arc across zinc electrodes revisiting the reactions of B2X4 (X = Cl, Br, and F) with various immersed in liquid boron trichloride.1 Diboron tetrachloride unsaturated hydrocarbons, such as ethene or benzene, in the was initially prepared in about 1% yield and in low purity (less absence of a transition metal.87 The calculations showed that than 90%) and found to react violently with moisture or the addition of B2Cl4 to alkenes is extremely plausible and oxygen. Conducting the reaction in the vapor phase using a lower in energy than the alternative B2X4 additions, apart from 61,62 mercury arc improved the yield considerably. Although B2Br4. Furthermore, it was reported that reactions of B2Cl4 with ff fi ffi numerous e orts to nd more e cient methods for the naphthalene and C60 were easily accessible, and B2X4

9093 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review compounds could undergo 1,4-addition to dienes, especially Scheme 3. Formation of a Dinuclear Cobalt Complex 87 108 cyclopentadiene in the case of B2F4. Containing a Nonsubstituted Borylene Fragment 2.2. B2H4 Chemistry

After the discovery of the stable D2d and D2h forms of subhalides, there was considerable interest in the parent B2H4 compound. Mohr and Lipscomb used computational studies to suggest that the D2d form is more stable than the other forms (Figure 3), including the nonplanar doubly bridged structure by group 9 metals are thought to be regarded as early stage oxidative addition of the boron−boron bond, leading to diboryl compounds.121 Metal complexes derived from addition of base- stabilized diboron(4) compound have been studied from a structural standpoint, but their reactivity with organic molecules has yet to be investigated. Interestingly, in a recent computa- tional study, Jemmis and co-workers analyzed the interaction Figure 3. Potential geometries for B2H4. 122 between transition-metal complexes and B2H4 as a ligand. A 88 unique case study for the Dewar−Chatt−Duncanson model only 1.5 kcal/mol. Additional computational studies have ff was proposed, in which B2H4 could be stabilized by formation been performed in an e ort to assist in the experimental π 89,90 of transition-metal -complexes such as [Cr(CO)4B2H4], identification of this parent diboron(4) compound, as well 122 [Mn(CO)CpB2H4], [Fe(CO)3B2H4], and [CoCpB2H4]. as in the study of the interaction between B2H4 with proton donors, such as HF, HNC, HCl, HCN, and HCCH.91 The 2.3. B2(NR2)4 Compounds calculations showed that the diboron(4) compound interacts A significant breakthrough in diboron(4) chemistry arose with with the proton donors to form hydrogen-bond complexes with the discovery of tetrakis(dimethylamino)diboron(4), 123 C2v symmetry. B2(NMe2)4. In 1954, Urry and co-workers observed the The elusive B2H4 species has been subsequently detected and formation of B2(NMe2)4 in the reaction of diboron analyzed using photoionization mass spectrometry and found tetrachloride with dimethylamine but only gave elemental 64 to be consistent with the doubly bridged C2v structure for both analysis as evidence. It was not until 6 years later that a more neutral and ionic forms.12 Interestingly, early computational convenient route to prepare this important compound was work using the MNDO method has shown that in the reaction developed by Brotherton and co-workers.123 In this report, the π of B2H4 with ethylene, a three-centered -complex arising from diboron species was prepared by the addition of halo- electron donation of the alkene to only one boron atom is bis(dimethylamino)boranes to highly dispersed molten sodium. preferred over a four-centered intermediate.92 Furthermore, a Three equivalents of the starting monohaloboranes B- recent publication by Cheng et al. reported the observation of a (NMe2)2X were generated via a comproportionation reaction novel B2H4 prototype compound, with a molecular structure with 2 equiv of B(NMe2)3 and 1 equiv of the corresponding containing two terminal and two bridging hydrogen atoms, boron trihalide (BX3). Furthermore, Brotherton and co- from irradiation of diborane(6) dispersed in solid neon at 3 workers also observed that transamination with a number of K.93 The photochemically formed diboron(4) species showed secondary amines gave the corresponding aminodiboron(4) many new absorptions in both the infrared spectrum and the species with loss of dimethylamine (Scheme 4).123 Two years ultraviolet absorption and emission spectra. One of the sets of later, Silver and co-workers reported that by using a similar absorptions, determined by analysis of the isotopic shifts of 10B transamination reaction with several different difunctional and 11B, was identified as the diboron(4) species.93 aliphatic secondary diamines and trimethylenediamine, a While the parent compound has continued to elude synthetic monomeric diboron compound with the general formula chemists, the B2H4 molecule has been captured with the use of B2(NR(CH2)nNR)2 was formed upon the release of dimethyl- Lewis bases, such as phosphines and amines. Numerous amine from tetrakis(dimethylamino)diboron(4).124 The struc- examples exist in the literature in which adducts of B2H4 tures of these diboron compounds, B2(NR(CH2)nNR)2, were have been synthesized by cleavage of larger polyhedral borane analyzed further by Shore using mass spectrometry125 and the − complexes.94 100 For example, the bis-phosphine adduct crystal structure was initially reported by Nöth and co-workers · 126 (Ph3P)2 B2H4 was originally prepared by the addition of excess in 1976. · ° 99 ̈ triphenylphosphine to (Me3N) B3H7 in benzene at 50 C. In 1984, Noth and co-workers reported adducts of these Kodama and co-workers elegantly used these adducts as compounds from the reaction of B2(NR(CH2)nNR)2 with building blocks for a variety of borane framework expansion various trihalides, EX (E = B, Al, Ga; X = Cl, Br, I).127 The − 3 reactions to generate higher order polyhedral boranes.101 107 Lewis acid binds to the N atom on the backbone of the diboron The only other application of these base-stabilized species has compound, with both mono- and diadducts of the diamino been in their use as ligands for transitions metals, where the compound observed, depending on the equivalents of EX3 borane usually coordinates to one metal center using one or used. Furthermore, it is noteworthy that B2(NMe2)4 also − − 108−120 two M H B interactions. An interesting exception was reacted with H2S to form heterocycles containing B2(NMe2)2 the formation of a rare cobalt complex bridged by a fragments bridged by two sulfur atoms.128,129 An interesting use nonsubstituted borylene (BH) group, which was isolated by for these compounds was investigated by Patton and co- · treatment of Co2(CO)8 with a 2-fold excess of (Me3P)2 B2H4 workers, who discovered a novel synthetic pathway to various (Scheme 3).108 diboron-bridged bis(amide)−base complexes, starting from A report by Himmel and co-workers confirms that bonding tetrakis(dimethylamino)diboron(4) (Scheme 5).130 This was with group 6 metals occurs primarily through the hydrogen initially reacted with HCl to form the mixed diamino−dichloro atoms; however, base-stabilized diboron(4) interactions with diboron compound, before reaction with the respective lithium

9094 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 4. Formation and Reactivity of Tetrakis(dimethylamino)diboron(4), B2(NMe2)4

Scheme 5. Synthesis of Metal Complexes with Mixed structural features and photophysical properties of these Diaminodiboron Species compounds.177

Scheme 6. Synthesis and Reduction of Dithieno-1,2-dihydro- 1,2-diborin

Power and co-workers178 and Nöth and co-workers168 prepared diboron(4) compounds containing mesityl groups, which show dramatically enhanced stability over their tert-butyl analogues. Actual applications of these compounds have so far been surprisingly limited; however, the landscape of synthetic chemistry changed drastically with the advent of dialkoxybor- ane(4) compounds.

2.5. B2(OR)4 Compounds

2.5.1. Synthesis of B2(OR)4. The parent B2(OR)4 compound, B2(OH)4, is normally prepared through the 179−181 154 123,156 hydrolysis of B2X4, B2(OR)4, or B2(NR2)4 species. In the solid state, B2(OH)4 is found to exist in hydrogen-bonded sheets. The B−B bond lengths for two crystallographically inequivalent molecules are 1.715(5) and 1.710(4) Å.181 In a serendipitous reaction, trace amounts of anilinide to form the predicted diboron(4). The lithium salt was water in the presence of B (NMe ) allowed for the formation then reacted with several different transition metals to 2 2 4 of a six-membered ring containing two B (OH) fragments synthesize the desired complex.130 2 2 bridged by two oxygen atoms, a presumed intermediate in the Unlike the subhalide derivatives, B (NMe ) is stable in air 182 2 2 4 formation of [BO] polymers (Figure 4). up to 200 °C, although it is still moisture sensitive. This x stability was believed to be due to extensive N−B π-bonding and steric crowding of the dimethylamino group (Scheme 4). The molecular structure of this species was determined using electron diffraction in 1981, and the amino groups were reported to adopt a fully staggered conformation (D ), 2d Figure 4. Formation of novel compounds containing B4O2 rings. presumably for steric reasons.131 While a number of − tetraaminodiboron(4) compounds are now known,28,132 159 In their seminal work, Brotherton and co-workers also the angle of rotation about the B−B bonds in their solid-state established a general route to dialkoxyborane(4) compounds by structure usually lies in the range between 55° and 90°, 154 addition of alcohols to B (NMe ) (Scheme 7, route 1). depending upon the steric requirements of the amido 2 2 4 159 Several modifications to this synthetic procedure have occurred groups. 26,28,29,155,183−187 over the following decades including a 2.4. B2(alkyl)n Compounds selective reduction of specifically substituted halocatecholbor- In an attempt to enhance the applicability of these compounds, anes. Additionally, Hartwig established a novel synthetic route a number of second-generation diboron(4) compounds, based to prepare a series of previously known substituted bis- on tetrakis(dimethylamino)diboron(4), have been prepared (catecholato)diboron(4) compounds in moderate to high andstudied.Althoughafewmixedaminoandhalide yields by the reaction of 1% sodium/mercury amalgam with compounds have been synthesized,128,160,161 asignificant the corresponding halocatecholboranes (Scheme 7, route 2).188 amount of research has focused on generating organoborane(4) A recent study by Braunschweig and Guethlein has shown that − 68,162−175 derivatives containing B C bonds. Of synthetic both B2pin2 and B2cat2 can be prepared by the metal-catalyzed interest is a report by Wakamiya, Yamaguchi et al., who dehydrogenative coupling of the starting boranes HBpin and − prepared and studied the boracycle dithieno-1,2-dihydro-1,2- HBcat, respectively (Scheme 7, route 3).189 191 These studies diborin and its dianion as potential building blocks for extended were based on previous observations by Marder et al., who π− electron systems for making optoelectronic materials observed the formation of B2pin2 from the rhodium-catalyzed (Scheme 6).176 The same group went on to synthesize two dehydrocoupling of HBpin; however, there were only ca. 7 other diarene-fused diborin dianions and investigate the turnovers, with the major product being the C−H borylation of

9095 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 7. Synthetic Routes to Tetraalkoxydiboron(4) Compounds

Scheme 8. Reactions of Platinum−Phosphine Complexes with Catecholborane

the benzene solvent.192 This methodology is extremely (dab = 1,2-diaminobenzene) and derivatives205,206 are becom- important as it allows for a new one-step procedure, from ing more significant (vide infra). easily prepared boranes to two of the most widely used and synthetically pertinent diboron(4) compounds. Marder, Braunschweig and co-workers also reported the reverse reaction, namely, the synthesis of boranes from diboron(4) compounds via hydrogenolysis of the B−B bond, using heterogeneous Group 10 catalysts, with palladium on charcoal found to be the most effective.193 Hartwig et al. observed a similar reaction taking place to generate DBpin via the 194 deuterolysis of B2pin2, catalyzed by an iridium complex. A recent experimental and computational study has gathered evidence for several postulated intermediates in the platinum- catalyzed dehydrocoupling of HBcat, including a platinum boryl hydride species, resulting from the oxidative addition of HBcat, and a platinum bis-boryl species.191 Bulky cyclohexyl groups from the phosphine ligands block reaction of the metal center with a second equivalent of HBcat, preventing dehydrocoupling Figure 5. Alternative dialkoxydiboron(4) compounds. and allowing for the isolation of the trans-hydridoboryl fl platinum species (Scheme 8, left). More exible P(CH2Cy)3 ligands allow for reaction with a second equivalent of borane, A recent study by Bryce and Perras showed an interesting prompting dehydrocoupling and the formation of B2cat2 development in the use of NMR spectroscopy to analyze (Scheme 8, right). A similar process was also reported by B2(OH)4, several dialkoxyborane(4) species, and the literature Marder et al. in their investigations into the mechanism of the known adducts of these compounds (vide infra).207 By using rhodium phosphine-catalyzed borylation reaction.192,195 The 11B DQF J-resolved NMR spectroscopy, the J(11B,11B) values observed B2pin2 was computed to be generated via a reaction of were shown to correlate with the energy of the B−B σ-bonding i 195 a [(P Pr3)2Rh(Bpin)] species with HBpin. natural bond orbital (NBO). Therefore, this allows direct The development of new tetraalkoxydiboron(4) and related information to be obtained about the strength of the B−B bond derivatives is an area that had received little attention, but of and the hybridization of the boron orbitals involved in this notable exception are the bis(dithiocatecholato)-, bis- bond.207 − (catecholato)-, and bis(pinacolate)diboron(4) ana- 2.5.2. Adducts of B2(OR)4 Compounds. Lewis acid base 29,33,187,196−200 logues and bis(neopentylglycolato)diboron adduct formation of B2cat2 was examined with 4-picoline, and 29 (B2neop2, Figure 1), the latter two are now being widely the addition of 1 equiv of base led to rapid exchange between used in catalytic borylation reactions. Interestingly, unsym- the two boron atoms (Scheme 9, route 1).208 Addition of a metrical diboron compounds are beginning to find a niche in second equivalent of 4-picoline proceeded to give the organic synthesis as different reactivities and regioselectivities bis(adduct) (Scheme 9, route 2).208 Further studies by Ingleson can be achieved in reactions using these unusual species. and co-workers showed that, under specific solvent conditions, · Therefore, compounds such as pinacolato diisopropanolamina- the 1,2-B2cat2 (pic)2 isomer could be isolated and crystal- 201,202 203 209 to diboron (PDIPA, Figure 5), BpinBMes2, BpinBdan lized. Additionally, alternative Lewis base adducts of B2cat2 (dan = naphthalene-1,8-diaminato, Figure 5),204 and BpinBdab were structurally characterized, confirming the formation of

9096 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 9. Lewis Base Adducts of B2cat2 with 4-Picoline

both 1,1 and 1,2 isomers of B2cat2 with DBN (1,5- the exchange involved dissociation and reassociation of the diazabicyclo[4.3.0]non-5-ene).209 Phosphine adducts of bis- NHC rather than an intramolecular pathway.213 The thiocatecholato196 are known, though only (mono and bis) monoadduct was isolated and structurally characterized by X- 210 ff 213 PMe3 adducts have been suggested with B2cat2. Other bis- ray di raction. Further investigations of the NHC adducts of dithiolatodiboron species and their adducts have been prepared diboron(4) compounds, such as B2cat2, were investigated in a by Norman and co-workers.161,211 Despite the lack of evidence collaboration among the Marder, Radius, and Ingleson 216 · for adduct formation at room temperature by solution NMR groups. Initially, the adduct B2cat2 Me4Im was isolated spectroscopy, cocrystals of 1:1 complexes between TCNQ (Figure 6), but unlike the previous NHC adduct of B2pin2,no (7,7,8,8-tetracyano-p-quinodimethane) or TCNE (tetracyanoe- dissociation of the NHC was observed on the NMR time scale. · thene) and B2cat2 or bis(dithiocatecholato)diboron have been The bis-adduct B2cat2 (Me4Im)2 was also synthesized and isolated and structurally characterized.212 structurally characterized by solid-state NMR spectroscopy. A detailed NMR spectroscopic study with B2pin2 and the More interestingly, it was observed that, unlike the mono- NHC, 1,3-bis(cyclohexyl)imidazol-2-ylidene, revealed that adduct, the bis-adduct was unstable at elevated temperatures rapid exchange of the NHC between the two boron atoms and underwent ring expansion of the NHC to form a six- was occurring in solution (Figure 6).213 The adduct was initially membered hetereocycle after several hours (Figure 6).216,217 proposed by Hoveyda to be involved in metal-free borylation of The interest in Lewis base adducts of B2pin2 has grown over α,β-unsaturated compounds214 and its NMR assignment the past few years, due to the proposed requirement of such corrected subsequently.215 DFT calculations suggested that compounds in the catalytic cycle of borylation reactions, in the presence or absence of a transition metal (vide − infra).43,45,218 222 The formation of such compounds had been proposed by Miyaura and co-workers as early as 2000223 as a means of enhancing the nucleophilicity to generate a boryl anion equivalent. Miyaura et al. thus proposed that the in-situ − − formation of [B2pin2OAc] aided in the formation of a Cu boryl complex, required to facilitate the copper-catalyzed β- borylation of α,β-enones.223,224 Furthermore, anionic adducts, such as [B pin OR]−, have been discussed and observed in 2 2 − computational and spectroscopic studies;219,221,222,225 231 however, the full characterization of a series of anionic sp2− 3 sp Lewis base adducts of B2pin2, such as [K(18-Crown- 6)][B2pin2OMe] and [Me4N][B2pin2F], was only published recently by Marder et al. (Figure 7).218,232 The reactivity of these isolated adducts with aryl electrophiles will be discussed later (section 5.5). In comparison, Santos et al. reported a similar neutral intramolecular sp2−sp3 diboron(4) compound, in the absence of base.201,202 The interaction of the amino group with one of the boron centers alters the hybridization of 2 3 Figure 6. NHC adducts of diboron(4) and the ring-expansion product the B moiety from sp to sp , providing a more nucleophilic · formed from B2cat2 (Me4Im)2 (RER = ring-expansion reaction). Bpin fragment, which accelerates the transmetalation step in

9097 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

2− 3 Figure 7. Anionic sp sp adducts of B2pin2. the copper-catalyzed borylation of electron-poor olefins 2.7. Diboryl Groups as Ligands (section 4.1).202 − In the late 1990s, Braunschweig and co-workers began Several anionic adducts of [B2pin2R] have also been reporting reactions of diamino−dichloro diboron(4) com- observed in NMR studies but without crystallographic 233,234 pounds with transition metals. An early reaction with 2 equiv of evidence. In 2014, Wang and co-workers discovered an anionic manganese hydride complex resulted in the − · that the boryl borate, Li[B2pin2Ph] 1/8B2pin2,formeda formation of a borylene-bridged bimetallic species (Scheme 241−247 frustrated Lewis pair with B(C6F5)3 that could activate 11, see also Scheme 3 for a similar reaction). Reactions dihydrogen, pinacolborane, and ethylene (Figure 7),234 while Bedford et al. observed the formation of the adduct t 11 Scheme 11. Synthesis of Borylene-Bridged Bimetallic Li[B2pin2 Bu] by B NMR spectroscopy from the reaction of t 233 Species from Anionic Manganese Hydride Complex B2pin2 with BuLi. Anionic adducts of unsymmetrical diboron(4) compounds have also been synthesized. Kleeberg and co-workers showed that an unsymmetrical diboron(4) compound, BpinBdmab (dmab =1,2-di(methylamino)benzene), reacted with [K(18- Crown-6)OtBu] to form an anionic sp2−sp3 adduct, [K(18- Crown-6)][BpinBdmabOtBu], with the tert-butoxide group attached to the more Lewis acidic Bpin moiety.205 Yamashita et al. also observed the formation of a diboran(4)- of related diboron(4) compounds with sodium salts of anionic yl radical anion, [K([2,2,2]-cryptand)]+[BpinBmes]•−, from the iron and tungsten complexes resulted in an anion exchange and 235 reduction of BpinBMes2. the formation of transition-metal-substituted diboron(4) 49,245,246,248−250 2.6. Three-Membered Aromatic Heterocycles Containing compounds. Analogous reactions were carried B−B Bonds out with potassium salts of anionic molybdenum and ruthenium complexes.251,252 A unique class of compounds possessing a boron−boron bond Platinum(II) diboryl complexes could be formed from the contains a three-membered aromatic ring, where two π- oxidative addition of B−X bonds of B X (NMe ) at zerovalent electrons are shared between the three atoms. Thus, the 2 2 2 2 platinum centers.253 These diboryl groups were found to have a boron−boron bonds tend to be somewhat shorter than single strong trans influence, resulting in a lengthening and labilizing bonds, owing to the aromatic character of these species. Of ff − − 2− − e ect on trans-Pt Br and Pt I bonds compared to the those systems containing boron−boron bonds, B ,BC , and 253 3 2 − dihaloboryl analogues. In a unique reaction, the addition of a a variety of B E (E = NR, O) heterocycles are known.133,236 240 2 chelating diphosphine ligand to trans-[Pt(PiPr ) Br(B(NMe )- In the case of azadiboriridines (E = NR), steric bulk is vital to 3 2 2 B(NMe )Br)] not only displaced phosphine ligands from the prevent dimerization (Scheme 10, left). Although some of these 2 platinum complex but also resulted in rearrangement to a cis- bis-boryl species (Scheme 12,left).254 This reaction is Scheme 10. Reactions of Azadiboriridines presumed to proceed through rearrangement and reductive elimination of the dibromodiboron, followed by a subsequent oxidative addition of the boron−boron bond. Reaction of other zerovalent platinum tetraphosphine compounds with B2X2Ar2 also resulted in oxidative addition of the boron−halogen bond.255,256 The products of these reactions show a significant interaction between the electron-rich metal center and the β- boron atom, due to its electron deficiency. The magnitude of this interaction was found to be strongly dependent on the halide substituent. A platinum(II) diboryl complex could also be reduced to a platinum zero diborene complex by a magnesium(I) dimer (Scheme 12, right).257,258 It is noteworthy that the diboron B2I2(NMe2)2 undergoes oxidative addition when reacted with 2 equiv of Pt(PEt3)3, forming a bimetallic species bridged by a diboron(4)-1,2-diyl ligand.259 2.8. [2]-Borametalloarenophanes [2]-Boraferrocenophanes are a unique family of organometallic complexes containing a boron−boron bridge between the two heterocycles have proven to be quite reactive in B−B addition cyclopentadienyl rings of ferrocene. The parent compound fi ̈fl chemistry (Scheme 10), generation of these species involves (Scheme 13, center), was rst prepared by Herberhold, Dor er, and Wrackmeyer through the reaction of dilithioferrocene with lengthy and highly air- and moisture-sensitive procedures, 260 B2Cl2(NMe2)2. A closely related species containing sulfur limiting their synthetic utility. atoms between each boron atom and the cyclopentadienyl rings

9098 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

a Scheme 12. Synthesis and Reactions of Platinum(II) Diboryl Complexes

a dur = 2,3,5,6-Me4C6H.

Scheme 13. Reactions of [2]-Boraferrocenophanes with active catalysts in ethylene polymerization and ethylene/1- 262,275 B2(diol)2 and Pt(PEt3)3 hexene copolymerization. 2.9. B−B Bonding Much like its neighbor in the periodic table, carbon, boron can form a wide variety of homoatomic bonds. In addition to single, double, and triple bonds, singly reduced diboron(4) com- pounds having a formal bond order of 1.5 are also − known.162,173,247,258,276 285 The discussion here will be confined to diboron(4) species and reactions of these species. was prepared by Norman and co-workers.261 Braunschweig and Most diboron(4) compounds have a relatively simple co-workers subsequently developed a synthesis for the bis- structure with a single B−B bond and no bridging atoms; cyclopentadienyl-diboron ligand, allowing for the preparation of however, there are some examples of species with bridging 262 other transition-metal complexes. Related sandwich com- hydrides. The simplest diboron(4) compound, B2H4, was found plexes containing B−B bonds as bridging atoms were prepared to have a doubly bridged C2v structure (section 2.2)by 263−266 12 either through anionic diboryl species or via deprotona- photoionization mass spectrometry. The staggered D2d tion of the transition-metal precursor and subsequent addition isomer was found to have a very similar energy. Though rare, 267,268 μ of dichlorodiborons. For example, the reaction of 2 equiv the doubly bridged B2( -H)2 core has been observed under of lithioferrocene with B2Cl2(NMe2)2 generated the diboron- ambient conditions using very bulky aromatic terminal 170 bridged bis-ferrocene complex Fc(NMe2)BB(NMe2)- substituents (Scheme 14a). Fc.68,264,269 The B−B bond length of 1.4879(7) Å observed in the The B−B bond in [2]-borametalloarenophanes has been molecular structure of the hydrogen-bridged species was found cleaved by addition to alkynes using palladium and platinum to be significantly shorter than experimental values for neutral catalysts, inserting the carbon−carbon triple bond into the B−B B−B double bonds (1.560(18)281 and 1.546(6) Å282) but − bridge.270 273 Braunschweig and co-workers were also able to comparable to experimental (1.449(3) Å)282 and calculated isolate heterobimetallic compounds resulting from oxidative (1.455 Å)286 values for B−B triple bonds. The short distance addition of the B−B bond to a platinum(0) compound along with nearly linear B−B−C bond angles implied sp 267,268,271,274 fi (Scheme 13, right). Reaction with B2pin2 or B2cat2 hybridization, an assertion that was con rmed through a resulted in boryl exchange and loss of the B−B bridge (Scheme computational study.170 Thus, the species can be considered to 13, left).270 Additionally, these complexes were found to be contain a doubly protonated basic boron−boron triple bond.

μ Scheme 14. (a) Synthesis of the Stable Doubly Hydrogen-Bridged Diboron B2Eind2( -H)2, and (b) Synthetic Route to the μ Isolable Diboron(4) Compound B2(MPind)2H2 and the Diboron B2Eind2( -H)2

9099 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 15. Reactions of B2Br2Mes2 with PEt3 and PMeCy2

Further studies by Tamao et al. observed the first isolable adopts a bridging position (Scheme 16, right).291 A recent diboron(4) compound with terminal B−H bonds in both report by the same group expanded upon the scope of Lewis solution and the solid state (Scheme 14b).287 The terminal B− base addition to unsymmetrical diboron(4) compounds, H bonds were protected by 1,1,7,7-tetramethyl-3,3,5,5- demonstrating the formation of adducts of 1,1-dimesityl-2,2- tetrapropyl-s-hydrindacen4-yl (MPind) groups, which are difluorodiboron with Lewis bases attached, somewhat surpris- bulkier than the Eind group previously used, and therefore ingly, to the more sterically congested diaryl boron atom.292 aid in stabilizing the compound with terminal hydrogens. The Though covalently bound bidentate substituents generally characteristics of the compounds were shown to be very bind to one boron atom, there are several cases in which these different, with the MPind-based diboron compound having a substituents are covalently bound to each boron atom. In 1994, single B−B bond, compared to the multiple bonding character Siebert and co-workers produced one such compound through 287 293 shown previously in the Eind-based compound. reaction of Li2(1,2-(CH2)2C6H4) with B2Cl2(NMe2)2. This Bridging halides have also been observed by Braunschweig compound was found to insert an alkyne, resulting in ring and co-workers upon adding a phosphine to Mes(Br)BB(Br)- expansion. A related species was produced by Lesley, Norman, Mes (Mes = 2,4,6-trimethylphenyl).288 The bridging bromide and co-workers through the reaction of disodium catecholate 294 was found to have a minimal impact on the B−B bond length; with B2Cl2(NMe2)2 (Scheme 17a, right). They noted that however, the bond angles at boron differed significantly from the B−B bond in this cyclic species is slightly longer than that 2 3 the idealized sp and sp geometries. The product distribution of B2cat2. Surprisingly, the authors found that the analogous in the reaction was found to depend heavily on the steric bulk reaction of B2Cl2(NMe2)2 with dilithium thiocatecholate of the phosphine (Scheme 15). An analogous rearrangement of produced the 1,1-isomer (Scheme 17a, left). A molecular mechanics study has suggested that 1,1-diboron compounds are B2X2Mes2 involving 1,2-migration of the mesityl group has generally more stable than their cyclic 1,2-counterparts.295 been previously observed in the reaction of B2Cl2Mes2 with an N-heterocyclic carbene.289 Norman and co-workers prepared a variety of 1,1- and 1,2- diaminodiboron species and found that the binding mode Remarkably, Braunschweig and co-workers reported the 132 synthesis of 5 different product types based on the reaction of a depended on the diamine used. In the case of 1,2- diaminobenzene, a mixture of 1,1- and 1,2-isomers was Lewis base with B2X2R2 species through variation of the base prepared by reaction of the diamine with B2(NMe2)4 (Scheme used and the groups present on the diboron compound 160 (Scheme 16).290 Increasing bulk of the base and halide was 17b). The molecules were easily separated due to the poor found to increase the likelihood of halide migration. Yet solubility of the 1,2-derivative. Molecular structures reveal that the B−B bond length is significantly contracted in the 1,2- another type of product was observed when 2 equiv of NEt3 − derivative (1.649(3) and 1.651(3) Å vs 1.678(5) Å for the 1,1- was added to B2I2Mes2 in pentane. Precipitation of Et3N HI − was observed along with the product of an intramolecular C−H isomer). Likewise, the B N bond lengths also contract. activation of the NHC-NtBu group; the remaining iodine atom Lithiation of the 1,1-derivative, followed by reaction with 2 equiv of Cl(NMe2)BB(NMe2)Cl, formed a new polycyclic 296 Scheme 16. Reactions of B R X with Lewis Bases borazine species. A tetra-N-methylated derivative of the bis- 2 2 2 1,2-phenylenediaminato derivative could be quantitatively 297 oxidized to a radical cation using Ag[Al(OC(CF3)3)4]. An X-ray crystallographic study revealed that this oxidation caused a contraction of C−N bond lengths in one ring but had no effect on the B−B bond length. One-electron reductions have been known for diborons for quite some time, and the products were initially characterized by EPR spectroscopy.298 Structural characterization of these singly reduced monoanionic species show B−B distances shortened by approximately 5%, with a formal π-bond order of 0.5.280 Similarly, these species with one-electron π-bonds are accessible through the oxidation of electron-rich diborene compounds.299 Two-electron reductions of diboron(4) com- pounds increase the formal π-bond order to 1; however, the boron−boron contacts do not shorten substantially, an effect explained by the repulsive effect of adjacent negative charges on − each boron atom in the dianion.162,173,278 280 In a related reaction reported by Wagner and co-workers, the one-electron reduction of a biaryl bis-borane gave a one-electron σ-bond, the first time this type of bonding has been crystallographically

9100 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 17. (a) Reactions of B2Cl2(NMe2)2 with Dilithium Thiocatatecholate (left) and Disodium Catecholate (right); (b) Reaction of 1,2-Diaminobenzene with B2(NMe2)4

characterized (Scheme 18).300 A boron−boron bond length of sluggish, presumably as a result of the reduced Lewis acidity at 2.265(4) Å was observed. boron arising from increased π-dative bonding with the smaller fluorine atoms. These uncatalyzed diboration reactions Scheme 18. Formation of a B·B One-Electron σ-Bond generally yield cis-1,2-addition products. This type of reaction Confirmed by X-ray Crystallography was initially thought to proceed via a 4-centered transition state with the unsaturated substrate coordinated to the vacant p-type orbitals on both boron atoms; however, subsequent computa- 92 tional studies of reactions of B2H4 with ethylene and acetylene302 using the MNDO method revealed that this substrate−diboron interaction was taking place through just one of the boron atoms. This initial interaction results in a significant lengthening of the B−B bond, and the rearrange- ment of this adduct intermediate to the diboration product is Power and co-workers found that some dianions can undergo − − 173 nearly thermoneutral. intramolecular C HorC C bond activation (Scheme 19). While these early diboration reactions were generally They proposed that these rearrangements proceed through successful, compounds with B−Cl bonds are quite air and initial reduction, followed by loss of a methoxide group. A − moisture sensitive; therefore, recent developments have focused similar C H bond activation of a mesityl group was observed on diborations with more stable, electron-rich heteroatom- by Braunschweig and co-workers upon reduction of an NHC 70 289 substituted diboron sources. However, these more electron- adduct of Cl2BBMes2 (Scheme 16). A comprehensive rich diborons are generally not as reactive, due to the reduced summary of reductions of B−B bonds was published by 247 − Lewis acidity at the boron center; thus, transition-metal Braunschweig and Dewhurst. The boron boron single bond catalysts are often required. Their ease of synthesis, stability, found in typical diboron compounds is approximately 1.7 Å (2 × and moderate reactivity have made B2(OR)4 species preferred 0.85 Å, the approximate atomic radius for boron) and is quite diboron reagents. In particular, bis(pinacolato)diboron electron rich due to the relatively electropositive nature of (B2pin2), bis(catecholato)diboron (B2cat2), and BpinBdan boron. have recently been extensively used in transition-metal- 3. 1,2- AND 1,4-DIBORATIONS catalyzed reactions. This section will focus on reactions of diboron(4) compounds 3.1. Mechanistic Studies of 1,2-Diboration Processes where both boryl (BR2) groups are added to the substrate. Platinum(0) complexes have generally proven to be the most Uncatalyzed reactions of this type using diboron tetrahalides effective and general catalysts for diboration reactions.53,68,70,301 have been known for quite some time but were often While the substrate scope for this reaction is quite uncontrollable and gave unstable products (see section extensive31,32,52,53 and includes alkynes, alkenes, dienes, allenes, 301 2.1). The addition of B2Cl4 to a variety of unsaturated vinyl boronate esters, carbonyls, unsaturated carbonyls, and substrates proceeds in the absence of catalyst, often at low imines, the mechanisms are often quite similar. Mechanistic temperatures. Analogous reactions of B2F4 are much more studies suggested that the catalytic cycle proceeded via initial

Scheme 19. Intramolecular C−H (left) and C−C (right) Bond Activations by 2,6-Dimesitylphenyl-Substituted Diborons

9101 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review oxidative addition of the diboron(4) species to the metal oxidative addition/reductive elimination of B2(OMe)4 at center, followed by coordination of the unsaturated substrate platinum(0) is reversible and temperature dependent.318 A and insertion into one of the Pt−B bonds. A subsequent similar phenomenon was reported by Smith and co-workers in ff reductive elimination a orded the corresponding product and their investigations into the catalytic capability of [Pt(PR3)2(B- 245,303 312 regenerated the active catalyst (Scheme 20). Early (OR)2)2] in the diboration of alkynes (vide supra). It was observed that [Pt(PPh3)2(Bcat)2] could be generated along Scheme 20. Proposed Catalytic Cycle for the Diboration of with B2pin2 from the reaction of [Pt(PPh3)2(Bpin)2] with 312 Alkynes Using Zerovalent Platinum Complexes B2cat2. Marder, Baker, and co-workers demonstrated alkene insertions into rhodium−boron bonds319 and oxidative addition of diborons at rhodium29,320 and iridium320,321 centers, suggesting that diborations using these metals as catalysts could proceed through a similar mechanism. However, σ-bond metathesis reactions between rhodium(III) bis-boryl species and other diboron compounds suggest the mechanism may be 322 somewhat more complex. Oxidative addition of B2cat2 with a rhodium(I) boryl complex has also been observed, resulting in the generation of a rhodium(III) tris-boryl complex.323 Perutz, Marder, and co-workers later showed oxidative addition of 324 325,326 B2pin2 to a rhodium(I) center. Cobalt(0), osmi- um(0),327,328 and ruthenium(0)328 complexes were also found to oxidatively add B2cat2 (Scheme 21). Similar oxidative

Scheme 21. Oxidative Addition Reactions of B2cat2 at Zerovalent Cobalt, Osmium, and Ruthenium Centers

computational studies have shown that the initial oxidative addition step is unlikely to occur in the case of palladium − complexes.56,112,304 306 However, a more recent study has contradicted these early works, demonstrating that B−B oxidative addition can occur at zerovalent palladium centers.307 Experimental evidence has been found for each step of the catalytic cycle.49,56 Marder and Norman reported the crystal addition reactions of substituted bis(catecholato)diborons(4) 329 330 structures of the organic products from the diboration of have been observed with Fe(CO)4 and Cp2W, generated 308 309 alkynes with either B2pin2 or B2cat2. The groups of in situ from Fe(CO)5 and Cp2WH2 (or [Cp2WH(Bcat)]), Marder310 and Smith311,312 reported the isolation and respectively, using photolysis. characterization of [Pt(PR3)2(B(OR)2)2] complexes, which Products of B2F4 oxidative addition to platinum and iridium proved to be catalytically competent in the diboration of centers are also known.331,332 A report in 2007 described alkynes. These species were, in fact, found to be more active oxidative addition using a variety of diborons with Pt(PMe3)4, 313,314 than the initially used catalyst precursor Pt(PPh3)4. In the resulting in the loss of two phosphine ligands and the formation same studies, further evidence was gathered that suggested that of square planar platinum(II) complexes.333 Reaction of Vaska’s phosphine dissociation is a key step in the reaction. If a compound, trans-[Ir(CO)Cl(PPh3)2], with B2F4 generated fac- 334 chelating bis-phosphine such as dppe (1,2-bis- [Ir(BF2)3(PPh3)2(CO)] (Scheme 22). Presumably, the (diphenylphosphino)ethane) is used instead of two mono- ’ dentate phosphine ligands, the platinum complex is found to be Scheme 22. Reaction of Vaska s Compound with B2F4 catalytically inactive. The addition of excess phosphine to the reaction mixture was also found to slow the reaction significantly. Other studies have shown base-free metal complexes to be effective catalysts for 1,2-diboration reactions.315,316 Later, Norman and co-workers reported the oxidative addition of a variety of diborons, including B2(X4- 200 210 ’ cat)2 (X = Cl, Br) and Cl(NMe2)BB(NMe2)Cl to chloride ligand from Vaska s compound is lost through zerovalent platinum centers. Braunschweig and co-workers transmetalation or reductive elimination as BF2Cl. Attempts examined the reductive elimination of (alkyl)Bcat from to isolate products from the reaction with just 1 equiv of B2F4 platinum(II) complexes upon addition of excess B2pin2 or were unsuccessful. Thus, the oxidative addition/reductive B2cat2 or alkynes; however, they were unable to distinguish elimination mechanism depicted in Scheme 20 has also been between metathesis and reductive elimination pathways.317 The implicated in diboration reactions catalyzed by other metals, − same group also recently discovered a system where the including some rhodium(I)335 341 and palladium cata-

9102 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 23. Generation of a Nickel(II) Boryl Complex via Addition of B2cat2 to a Nickel(II) Alkoxide Species (left) and a Nickel(I) Dimer (right) by Mindiola and Co-workers

lysts.307,342 While rhodium(I) catalyst systems are usually through the use of two different diborons, suggesting that each believed to proceed through an oxidative addition/reductive boryl group of the product came from a different diboron elimination mechanism (Scheme 20), mixtures of products are molecule.346 often observed as β-hydride elimination can compete with An intriguing recent discovery by Fernandeź and co-workers reductive elimination, generating borylation or dehydrogenative is that diborations of various unsaturated substrates with B pin − 2 2 borylation products. can take place without a transition-metal catalyst.225 227,354 An Marder and co-workers found that B2cat2 is, not surprisingly, excess amount of alcohol and a catalytic amount of a base are more active in transition-metal-catalyzed alkyne diborations present and serve to generate the catalytically active anionic t 310 − than B2pin2, while the B2(4- Bucat)2 is less active than B2cat2. species, [B2pin2OR] , which is an alkoxide adduct of B2pin2 It is significant to note at this time that organoboron products (for further information see section 2.5.2). Upon formation of containing Bpin groups are relatively stable to air, moisture, and this adduct, the boron−boron bond becomes strongly polarized column chromatography and thus offer a synthetic advantage and interacts with the substrate.219 It was proposed that the over their catecholato counterparts, which decompose in air. intermediate rearranges to the diboration product, while the While many 1,2-diborations operate through the oxidative alkoxide ion is abstracted from the product by protonation addition/reductive elimination mechanism outlined above, (Scheme 25). The use of a chiral alcohol produced diboration other reports have highlighted similar additions, which are not believed to operate via this mechanism. In particular, the Scheme 25. Proposed Catalytic Cycle for the Metal-Free ability of copper(I) complexes to catalyze the diboration of Diboration of Alkenes unsaturated organic molecules is emerging as a remarkably − useful synthetic tool.70,343 345 This reaction operates through a series of σ-bond metathesis steps, resulting in the overall transfer of two boryl groups to the substrate. Other catalysts believed to proceed through a similar mechanism include gold nanoparticles,346 palladium(II)−NHC complexes,48,347 and some iridium catalysts.204,348 Mindiola demonstrated stoichio- metric σ-bond metathesis reactions of nickel(II) and cobalt(II) tert-butoxide complexes with B2cat2 and B2pin2, respectively, to generate metal−boryl species (Scheme 23,left).349 The nickel(II) boryl species is also accessible through addition of 350 B2cat2 to a nickel(I) dimer (Scheme 23, right). In the case of copper(I) catalysts, the reaction has been studied extensively through density functional theory (DFT) − calculations.43,351 353 Further support for the stepwise transfer of boryl groups (Scheme 24) was shown by Fernandeź and co- 226 workers, who were able to generate the mixed bis-borylalkane products with modest enantiomeric excesses (ee’s). These promising reports highlight an efficient, affordable, and quite environmentally benign method to generate 1,2-diboron Scheme 24. Proposed Catalytic Cycle for the Diboration of 351,355,356 Alkenes Using Copper(I) Complexes species and their corresponding 1,2-diols. One example has emerged in the literature showing B−B addition across nitrogen−palladium bonds (Scheme 26),357 suggesting another plausible mechanism to consider for this

− Scheme 26. Addition of B2cat2 across a Palladium Nitrogen Bond

9103 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 27. Functionalization of Alkyne Diboration Products

Scheme 28. Internal-Selective Cross-Coupling Reactions of 1,2-Diborylalkenes

Scheme 29. Catalyzed Diborations of Alkynes with [B(NMe2)Cl]2

363−365 326 important reaction. Though the reaction results in cleavage of supported platinum nanoparticles, and Co(PMe3)4. the B−B bond, it does not result in a net oxidation of the metal Nanoporous gold and platinum metals have also been found to center. There are extensive examples of additions across metal− be effective catalysts for the diboration of internal alkynes with 366 ligand bonds in the literature, most notably the addition of H2 B2pin2. While the platinum metal was found to leach into the across a metal−ligand bond in Noyori-type hydrogenation reaction mixture, gold did not; thus, the heterogeneous gold catalysis. This reaction suggests the possibility of delivering catalyst was easily reused after washing with acetone. A cross- both boryl groups from the same diboron molecule to the addition experiment using the nanoporous gold catalyst with substrate, without formal oxidative addition. B2pin2 and B2hex2 (hex = hexylene glycolato, 2-methyl-2,4- 3.2. Alkynes pentanediolato) showed a mixture of products, including the  cross-addition products, suggesting the reaction does not In general, terminal alkynes (RC CH) are considerably more proceed through oxidative addition.366 reactive than the corresponding internal alkynes (RCCR′). The alkene products resulting from alkyne diboration contain Thus, an efficient yet regioselective method for adding boron two boron groups and can be further transformed to other reagents to internal alkynes remains a challenging problem. For useful organic molecules. For example, Morken and co-workers example, hydroboration reactions are plagued not only by a studied the rhodium-catalyzed asymmetric hydrogenation of mixture of isomers but also from products arising from a these substrates as an effective method of ultimately generating second addition reaction to the activated boron-containing 367 fl alkenes. Remarkably, the first diboration reaction of alkynes was 1,2-diols (Scheme 27, right). Electrophilic uorination of 83 these diboron products affords α,α-difluorinated carbonyl reported in 1959, where acetylene was observed to react with 368 − ° compounds (Scheme 27, left), which can be transformed diboron tetrachloride at 45 C. While garnering some initial 369 attention,358 over 20 years would pass before a computational subsequently into the corresponding imine derivatives. Armstrong and co-workers used a diborated internal alkyne study was carried out on the diboration of acetylene with B2H4 302 for two subsequent Suzuki reactions, the second of which using the MNDO method. The reaction was shown to be 370 − exothermic and proceed through an intermediate with the triple attaches the molecule to a solid support. Suzuki Miyaura bond interacting once again with one boron atom. cross-coupling of alkyne diboration products with alkyl and aryl halides has been used in the synthesis of semiconductor The reaction of B2Cl4 with other alkynes was subsequently − 359 materials,371 tetrasubstituted alkenes,372 374 benzo[b]- studied and found to give the cis-1,2-diboration products. 375 376 t t oxepine, and 1-boryl-alkenes and the introduction of a Interestingly, reactions of BuClBBCl Bu with TMS-substituted 377 alkynes were found to yield 1,1-diboration products,360 thought methyl group to a cycloalkynone. to occur through rearrangement of the initially generated 1,2- While the majority of these diboration reactions utilize diboration product. bis(pinacolato)diboron (B2pin2) or bis(catecholato)diboron The first synthetically relevant diboration of both internal (B2cat2), the use of BpinBdan as the diborating agent in Ir(I)- and terminal alkynes was reported in 1993 by Miyaura, Suzuki, and Pt(0)-catalyzed diborations provides access to unsym- and co-workers using B2pin2 and a catalytic amount of a metrically substituted bis-boronated alkenes, allowing for platinum(0) complex, typically Pt(PPh ) .313 Alkenylboron regioselectivity in subsequent cross-coupling reactions comple- 3 4 204,378 products were generated in high yields (>90%) when reactions mentary to their symmetrical counterparts (Scheme 28). were carried out in DMF at 80 °C for 24 h in the presence of 3 High regioselectivities were achieved for products bearing the mol % of a zerovalent platinum complex. The formation of the naphthalene-1,8-diaminatoboryl group on the terminal carbon. cis isomer was established with high selectivity (>99%). The more inert electron-rich Bdan group protects the normally Subsequent studies resulted in the discovery of other effective more reactive terminal position;376 thus, subsequent cross- catalyst precursors, including cis-[Pt(B(OR)2)2(PPh3)2] com- coupling reactions occur selectively at the internal site. 310,312 η2 310 315 pounds, [Pt(PPh3)2( -C2H4)2], [Pt(COD)Cl2], Lesley, Norman and co-workers demonstrated platinum- − 361 η2 362 Pt isocyanide complexes, [Pt(PR3)( -C2H4)2], solid- catalyzed alkyne diboration with Cl(NMe2)BB(NMe2)Cl

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(Scheme 29).379 In this instance, the ability of the dimethyl- amine group to bridge the two boron atoms, along with the differing electronic effects of the chloride and dimethylamine groups, induced rearrangement of the diboron substituents in the product. B2pin2 wasfoundtoaddtoalkynylboronatesand alkynylphosphonates producing tris-boronated alkenes and cis-1,2-diboronated vinylphosphonates, respectively, provided Figure 8. F050 and F1070, two biologically active molecules prepared that catalyst and diboron were heated together prior to addition through the diboration (and subsequent Suzuki−Miyaura cross- of the alkyne.380 A noteworthy observation from this study is coupling) of a transient aryne. that hydrodeboronation of the alkynylboronate, presumably caused by the presence of trace amounts of water, was observed The authors believed this replacement to be caused by when substrate, catalyst, and diboron were mixed together elimination of copper(I) methoxide from the organocopper simultaneously. The initial addition of diboron and the intermediate, resulting in the formation of an allene, which is − platinum catalyst likely removes trace water from solution, then inserted into a boron copper intermediate. The believed to occur through reaction of a platinum bis-boryl insertion/CuOMe-elimination process is repeated with the species with water, forming a platinum bis-hydride and an oxo- intermediate allene, resulting in the formation of a 1,3-diene, bridged diboron. Arynes, generated in situ, can also be which subsequently undergoes copper(I)-catalyzed 1,4-dibora- tion.356,386,387 The selective diboration of bis(2-bromophenyl)- diborated by B2pin2 with the Pt(dba)2/isocyanide catalyst system.361 Alternative substituents on the alkyne were recently acetylene with B2neop2 (neop = neopentylglycolato) was used in the preparation of a unique 10-borylated dibenzoborepin examined by Nishihara and co-workers in the selective synthesis 388 of vic-diborylated vinylsilanes from the relevant alkynylsilane system (Scheme 32). 374 As the search for cheaper, more environmentally friendly and B2pin2 and by Ohmiya and Sawamura in the diboration of alkynoates using a phosphine organocatalyst.381 The same replacements for precious metals such as Pt, Rh, and Pd continues, Nakamura and co-workers examined the utility of authors went on to discover that similar reactions could be 389 conducted with catalytic amounts of Brønsted bases, with the iron catalysts in the diboration of alkynes. It was shown that expansion of the substrate scope to incorporate propiolamides a variety of internal alkynes were diborylated by B2pin2 in high and 2-ethynylazoles.382 Furthermore, Uchiyama et al. utilized yields in the presence of an iron salt, FeBr2, and a base LiOMe, as well as MeOBpin, which was observed to be required to the presence of a hydroxy substituent in the trans-diboration of 387 propargylic alcohols in the absence of a transition-metal catalyst provide the second boron moiety in the borylation reaction. (Scheme 30).383 The MeOBpin could be replaced by an alkyl halide to facilitate a carboboration reaction to obtain the monoborylated alkene instead of the previously isolated diborated product.387,389 The Scheme 30. trans-Diboration of Propargylic Alcohols by ability to conduct these reactions in the absence of a transition Uchiyama et al. metal using specific reaction conditions has already been discussed (vide supra), but one more report that is noteworthy is the diboration of terminal alkynes with B2pin2 reported by Ogawa et al.390 This reaction could be conducted in the presence of catalytic amounts of organosulfides under photo- irradiation conditions.390 3.3. Alkenes (Aliphatic) fi Simple aliphatic alkenes were rst diborated using B2Cl4 in Fernandeź and Perez demonstrated that a copper(I)−NHC 1954.80 The initial study examined the diboration of ethylene, catalyst was capable of the catalytic diboration of alkynes with while subsequent studies expanded the scope of these additions B cat in refluxing THF.345,384 Subsequent computational using tetrahalogenato diborons and provided more detail on 2 2 − studies helped confirm the reaction mechanism (following the nature of the observed products.82,83,153,391 396 These initial the general pathway outlined in Scheme 24)351 and explained reactions have recently been used as the basis of a 352 the enhanced reactivity observed for B2cat2 over B2pin2. A computational study by Brown and co-workers on the simple 87 later report showed that copper(I) phosphine complexes are addition of B2X4 to alkenes (see section 2.1). effective catalysts for the diboration of alkynes and arynes with The influential work by Miyaura, Suzuki, and co-workers on 301,356,385−387 B2pin2. This system was also found to operate catalytic diboration of alkynes found alkenes to be unreactive through the pathway shown in Scheme 24. Further support for under identical conditions,313 due to the weaker binding ability this mechanism came in the form of the stoichiometric reaction of these substrates, preventing them from displacing the − of an aryl copper species with B2pin2, which resulted in the phosphine ligands of the transition-metal catalyst precursor. In formation of an aryl−Bpin species.356 The authors, Yoshida and an early report by Baker, Marder, and co-workers, vinyl arenes co-workers, also made use of the diboration of alkynes in the and the activated alkene allylbenzene were diborated with preparation of two biologically active molecules (Figure 8). B2cat2 using a gold(I)/phosphine catalyst system (see section From this study it is noteworthy that, in contrast to the cis-1,2- 3.3).335 Other early reports described the successful catalytic diboration product observed with Pt(0) catalysts, when methyl diboration of aliphatic alkenes utilizing zerovalent platinum propargyl ether substrates were employed, the authors observed catalyst precursors bearing only labile ligands: norbornene,316 replacement of the methoxy group(s) of the substrate with cyclooctadiene,316 and dibenzylideneacetone.314 These initial Bpin groups, in addition to diboration of the triple bond reports included diborations of both internal and terminal 314 316 (Scheme 31). alkenes with B2pin2 and B2cat2, showing tolerance of ester

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Scheme 31. (a) Tetraborylation of 1,4-Dimethoxy-2-butyne; (b) Proposed Mechanism for the Tetraborylation Process

Scheme 32. Synthesis of an Unusual 10-Borylated Dibenzoborepin

groups and chlorine atoms in the substrate. The platinum- group identified TADDOL-derived L1 (Figure 9) as the catalyzed diboration of aliphatic alkenes follows the same optimal ligand and found the reaction to be quite insensitive to general oxidative addition/reductive elimination mechanism shown in Scheme 20. These initial reports found the substrate scope to be limited to terminal alkenes and strained cyclic alkenes. Marder and co-workers also demonstrated the catalytic diboration of norbornene and styrenes (vide infra) with the η6 336 zwitterionic rhodium(I) catalyst, [Rh(dppm)( -Bcat2)]. Several years later, chiral rhodium(I)337,338,397,398 and 399 platinum(0) catalysts were utilized with B2cat2 and B2pin2, respectively, by Morken and co-workers to effect the enantioselective diboration of internal and terminal al- 378,400 Figure 9. TADDOL-derived chiral ligands for enantioselective kenes. A later report by Toribatake and Nishiyama diborations. demonstrated the high activity and enantioselectivity of a rhodium(III)−pincer bis-acetate catalyst precursor in the 401 the platinum(0) starting material, with the optimal catalytic diboration of terminal alkenes. The diborylalkanes generated conditions being 1 mol % platinum and 1.2 mol % ligand.400,407 from this reaction were subsequently oxidized to generate the Kinetic experiments showed the reactions to be nearly zero corresponding chiral diols.400,401 Further investigations by the order in B2pin2 and alkene, suggesting that the rate-determining same group used a similar reaction to synthesize the optically step does not involve association or dissociation of either active 3-amino-1,2-diols via Rh-catalyzed diboration of the substrate at the platinum center. The regioselectivity of alkene 402 respective N-acyl-protected allylamines. Other reports have insertion shown in Scheme 33 is consistent with the observed also utilized this facile method to generate a range of 1,2- enantioselectivity, the selectivity favoring the same enantiomer 403−406 diols. The resulting diborylalkanes have likewise been as reactions of vinyl arenes (whereby a benzylic intermediate is subjected to terminal-selective cross-coupling reactions and assumed for these substrates), and with DFT calculations. subsequent oxidation to generate carbohydroxylation prod- Armed with a detailed knowledge of the diboration reaction, ucts.403 Morken and co-workers formulated a one-pot route using the A thorough experimental and computational study of diboration reaction in tandem with a Suzuki−Miyaura cross- enantioselective zerovalent platinum-catalyzed alkene dibora- coupling reaction to synthesize a variety of chiral allylic alcohol tion has been published by Morken and co-workers.407 This and amine compounds.397,400,408 Notably, this synthetic route

9106 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 33. Proposed Mechanism for the Enantioselective Another major development came in 2011 from the Diboration of Alkenes Fernandeź group, who discovered that metal-free systems could catalyze the diboration of unactivated olefins.219,225 In this case, a combination of base and alcohol served to activate the diboron reagent to facilitate addition across the alkene C C bond (Scheme 25). An asymmetric variation of this method was subsequently published by the same group, in which chiral alcohols served to induce low to moderate levels of enantioselectivity.226 Recently, Fernandeź and Fujita published a report on the use of the crystalline sponge method to confirm that the metal-free diboration reaction goes through a syn- addition mechanism, which supports the previously predicted mechanism, using DFT calculations.415 A number of sym- metrical diboron reagents were tested, with the highest yields ́ starts with nonfunctionalized, readily available terminal alkenes shown when B2pin2 was used. Fernandez and co-workers also and is able to generate a variety of biologically active reported the ability of the mixed diboron system, BpinBdan, to compounds (Scheme 34). The authors demonstrated that bis- diborate a range of aliphatic alkenes;219,416 however, unlike boryl alkanes are actually more reactive in the cross-coupling B2pin2, the same system could not diborate vinyl arenes, with reaction than analogous boryl alkanes. The same group only the monoborylated product being obtained. Recently, continued their investigations into the tandem reactions, Morken and co-workers used a similar principle to facilitate the which led to the observation that after the initial diboration diboration of alkenes using carbohydrate-derived catalysts. It the subsequent cross-coupling reaction could be directed by the was suggested that 1,2-bonded diboronates were possible 409 presence of an hydroxy group β to the pinacol borane. This intermediates in the catalytic cycle, with these species route was developed to generate γ-oxygenated boronates from previously discussed in section 2.5.2.417 the sequential directed diboration and cross-coupling reactions, Allylic alkenes are also susceptible to diboration by various 409 followed by either acylation or silylation. different catalysts. In 2006, a zerovalent platinum−NHC The use of NHCs in exchange for phosphorus-based ligands catalyst was used in the diboration of these compounds, has also been investigated. Fernandeź and co-workers initially along with allylsulfones, using B cat or B pin for the − 2 2 2 2 observed that a silver(I) NHC complex was competent in preparation of 1,2-dihydroxy compounds under mild con- 410 418 catalyzing the diboration of vinylcyclohexane with B2cat2, ditions. The same group also found that an iridium(III)- while several years later the same group reported that based catalyst could add B2cat2 to allylic alkenes in the presence palladium(II)−NHC complexes could facilitate the diboration 348 347 of excess NaOAc. Interestingly, Alonso et al. observed of terminal aliphatic alkenes and vinyl arenes. Contrary to recently that a titania-supported platinum nanoparticle system the catalytic cycle proposed for platinum(0) complexes, these was able to promote diborations of allylic alkenes in the palladium catalysts are believed to operate through the absence of solvent or added ligand.364 transmetalation pathway similar to that shown in Scheme 24. 3.4. Alkenes (Vinyl Arenes) Additionally, palladium and platinum complexes were later shown to be effective catalyst precursors for the diboration of The first report of catalytic diboration of vinyl arenes came in cyclic alkenes,411 as well as phenyl vinyl sulfide, which was 1995 from Baker, Marder, and co-workers for 4-vinylanisole 412 335 subjected to diboration with B2cat2,usingPt(dba)2. utilizing B2cat2 and a rhodium(I) catalyst. Due to Bimetallic palladium(II) complexes have also been used for competition resulting from β-hydride elimination of the tandem diboration (with B2cat2) and Suzuki cross-coupling benzylic intermediate (Scheme 35, bottom left), yields of the reactions of alkenes, although the mechanism for the diboration diboration product were quite low. However, a zwitterionic step has not been studied in detail.413,414 rhodium(I) catalyst precursor containing the dppm ligand,

a Scheme 34. Synthesis of Enantiomerically Enriched Lyrica, Featuring Asymmetric Alkene Diboration

aRuPhos = 2-dicyclohexylphosphino-2′,6′-diisopropoxybiphenyl.

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Scheme 35. Proposed Mechanism for the Diboration of diborations of vinyl arenes, as well as allylic alkenes. In the case Vinyl Arenes of the iridium(III) catalyst, an excess of NaOAc was required for good conversion, and modest enantioselectivity was achieved (10% ee). Dinuclear palladium(II) complexes have also been shown to catalyze not only the diboration of vinyl arenes but also the subsequent cross-coupling step.413 In addition, gold(I),419 copper(I),345 and silver(I)−NHC410,419 complexes have been used as catalysts for the addition of B2cat2 to styrene. In 2011, metal-free diboration of vinyl arenes using base in MeOH was established by Fernandeź et al. when they investigated the diboration reaction of a range of alkenes with 225 B2pin2. They subsequently reported the metal-free dibora- tion of indene under similar conditions in the presence of 2 η6 226 [Rh(dppm)( -Bcat2)], prepared from [Rh(dppm)(acac)] equiv of chiral alcohol, B2pin2, and Cs2CO3; however, the showed good selectivity and yield while operating under milder conversion, chemoselectivity (diboration vs hydroboration), conditions (vide supra).336 Chiral rhodium(I) catalysts were and enantioselectivity were poor. also utilized by Fernandeź and co-workers and shown to be 3.5. Alkenyl and Alkynyl Boronate Esters competent catalysts for the diboration of vinyl arenes with a Vinyl boronate esters are often products of diboration variety of diborons; however, selectivity for the diboration reactions; however, these species can also readily undergo product and asymmetric induction values were modest.339 reactions with diborons. For example, tetraborylethene is Subsequent work by Toribatake and Nishiyama demonstrated presumably an intermediate in the synthesis of a hexaboryl- that changing the catalyst to a rhodium(III)−pincer bis-acetate ethane from a diborylacetylene and could be isolated through catalyst precursor facilitated the very rapid diboration of vinyl 420,421 modification of the reaction conditions (Scheme 36a). arenes in the presence of B pin and catalytic NaOtBu.401 2 2 The diborations of these species often resulted in mixtures of The same initial report by Marder and co-workers also products. Nishihara et al. also used alkynylboronate esters in examined gold(I) phosphine complexes as catalyst precur- the synthesis of gem-diboryl alkenes. The boryl acetylene sors.335 The gold-catalyzed reactions, while slow, proved to undergoes diboration followed by a Suzuki−Miyaura coupling produce much higher yields of the desired diboration product 422 to generate the desired product (Scheme 36b). than the initially examined rhodium(I) complex. Subsequent Marder and co-workers reported the rhodium(I)-catalyzed work by the Fernandeź group, examining diborations catalyzed 423 diboration of E-styryl boronate esters with B cat . Perhaps by alternative gold(I) complexes, showed that reactions were 2 2 surprisingly, the dominant product of the reaction with several accelerated by the introduction of excess B cat .346 This 2 2 different catalysts was a 1,1,1-triborylalkane (Scheme 37), observation led to a more thorough examination of the reaction, which revealed that the catalytically active species in Scheme 37. Rhodium-Catalyzed Diboration of E-Styryl this case was in fact gold nanoparticles. As B2cat2 reduces gold(I) to zerovalent gold, excess diboron reagent accelerated Boronate Esters the reaction. A head-to-head comparison of diboration of vinyl arenes and bis-hydroboration of aryl alkynes with rhodium(I) catalysts found that bis-hydroboration produced higher yields and increased selectivities.339 Marder and co-workers later reported the diboration of vinyl 199 arenes using Pt2(dba)3 and chiral diborons. However, only low to modest diastereomeric excesses were obtained. which was produced through a sequence of alkene insertions Fernandeź and co-workers were able to facilitate the diboration into the Rh−B bond and β-hydride eliminations from benzylic − of vinyl arenes with B2cat2 using a zerovalent platinum NHC intermediates (see Scheme 35) following the initial insertion of catalyst under ambient conditions.418 Furthermore, the same the CC bond into a Rh−B bond, whereupon a final C−H platinum(0) nanoparticles bound to a titania surface,364 the reductive elimination provided the 1,1,1-triborylalkane. Re- chiral iridium(III) catalyst348 and palladium(II)-NHC com- cently, Zhang and Huang used the combination of cobalt- plex347 discussed previously (section 3.3), were able to promote catalyzed dehydrogenative borylation and hydroboration to

Scheme 36. (a) Diboration and Bis-Diboration of an Alkynyl Bis-Boronate Ester; (b) Diboration of Borylacetylene and a Subsequent Cross-Coupling

amida = N-methyliminoidiacetic acid.

9108 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

314 synthesize a range of similar products from vinyl arenes with Pt(dba)2 resulted in 1,2-diboration. Shortly thereafter, 424 B2pin2. Marder, Norman, and co-workers showed that other diborons, 3.6. Dienes including chiral derivatives, could be added to 1,3-dienes in a 1,4-fashion as well; however, little asymmetric induction was An early example of diboration of dienes was reported by 198 Haubold and Stanzl in 1979. It was observed that the reaction achieved in this manner. Morken and co-workers later used bulky chiral phosphine of B2Cl4 or B2F4 with 1,3-butadiene generated the diborated product, 1,4-bis(dihalogenoboryl)-2-butene;425 however, the ligands, which, when added to a Pt2(dba)3-catalyzed diboration reaction mixture, produced 1,4-diols with high enantioselectiv- product was only characterized via elemental analysis. The next 427−430 study of the diboration of conjugated 1,3-dienes using ity, following oxidative workup. The bulky, electron-rich zerovalent platinum catalysts was not conducted until almost cyclic phosphine L2 (Figure 9), when added to a zerovalent 20 years later in 1996 by Miyaura and co-workers.426 The platinum precursor proved to generate the most active catalyst for this transformation. The diastereoselective 1,4-diboration of reaction of 1,3-dienes with B2pin2, catalyzed by Pt(PPh3)4 resulted in 1,4-diboration (Scheme 38a).341,426 During their 1,3-dienes has been applied as a convenient route to 1,4-diols in the synthesis of several natural products. Initially, (+)-trans- Scheme 38. (a) Proposed Mechanism for the Platinum- dihydrolycoricidine was synthesized from a readily available 431 Catalyzed Diboration of 1,3-Dienes; (b) Borylative Coupling cyclohexadiene derivative (Scheme 39), while another of Isoprene attempted synthesis used a singlet oxygen cycloaddition reaction with the cyclohexadiene derivative; however, this route provided no diastereoselectivity.431 Morken and co- workers also utilized the enantioselective 1,4-diboration of trans-pentadiene in the synthesis of (+)-discodermolide.432 Morgan and Morken applied the product of a 1,4-diboration using a chiral diboron to induce good enantioselectivity in the next step of the reaction, wherein the boron-containing product was added to an aldehyde.433 Coordination of the aldehyde at a boron center activated the substrate toward nucleophilic attack by the π-bond, creating a new carbon−carbon bond and cleaving a carbon−boron bond, where the net result was a chiral allylboration reaction (Scheme 40). A similar reaction sequence was used to diborate dienes catalytically with a zerovalent nickel catalyst wherein the diboration product added in an intramolecular fashion to an aldehyde then to a second aldehyde, resulting in the formation of four contiguous stereogenic centers.434 Morken and co- workers studied the zerovalent nickel-catalyzed allylboration of study, the authors noted that using a phosphine-free catalyst aldehydes, finding that upon mixing the 3 substrates (diboron, Pt(dba)2 in the reaction of B2pin2 with isoprene gave a new aldehyde, and diene) together they obtained the complemen- product, resulting from a tandem dimerization/diboration tary product to the platinum-catalyzed reaction (Scheme 41, sequence. The use of 3 equiv of isoprene resulted in left). However, when diene and diboron were mixed initially, quantitative formation of this coupling product (Scheme products with the opposite regioselectivity were observed 38b). The same authors later reported that, at room (Scheme 41, right).435 This finding suggests that stepwise − temperature, diboration of neoprene with B2pin2 catalyzed by diboration allylboration was not occurring when all of the

Scheme 39. Use of a Diastereoselective Diboration in the Total Synthesis of (+)-trans-Dihydrolycoricidine

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Scheme 40. Diboration/Allylboration of a 1,3-Diene

Scheme 41. Regioselectivity in the Allylboration of 1,3-Dienes

components were added simultaneously, but the three Scheme 42. Proposed Mechanism for the Zerovalent Nickel- components come together at the metal center during the Catalyzed Diboration of 1,3-Dienes reaction. Remarkably, switchingtheligandtoP(SiMe3)3 resulted in yet another change in regioselectivity in the nickel-catalyzed allylboration reaction (Scheme 41, bottom).436 The authors speculated that the change in regioselectivity was an electronic effect, causing the reductive elimination to occur prior to allyl isomerization at the metal center. Following the discovery of their excellent activity as aldehyde allylboration catalysts,434,435 Morken and Ely were subsequently motivated to study the activity of zerovalent nickel complexes in the diene diboration reaction.437 Catalyst systems comprising Ni(COD)2 (COD = cis-1,4-cyclooctadiene) and PCy3 were found to be highly active and stereoselective catalysts for the 1,4-diboration of 1,3-dienes. Importantly, reactions using these tions.198,426,428,433 The 1,2-diboration reaction was paired with catalysts showed no evidence of 1,2-diboration and were a subsequent allylboration of an aldehyde in a one-pot reaction, effective in the diboration of internal 1,3-dienes, a class of allowing for access to a wide range of chiral diols, following substrates inert under platinum-catalyzed reaction conditions. oxidation. Alternatively, the terminal boronate ester could be However, like the zerovalent platinum systems, only dienes subjected to hydrodeboronation (Scheme 43, bottom) or capable of adopting the s-cis configuration were reactive. The homologation (Scheme 43, right). The ability to generate chiral observation that styrene and 2-vinylnaphthalene did not centers, stereoselectively, and even enantioselectively makes the undergo 1,2-diboration prompted the authors to suggest that catalytic diboration of 1,3-dienes a useful reaction in the the standard oxidative addition/insertion mechanism did not synthesis of natural products and other biologically active apply in this case. Instead, they suggested an alternate molecules. mechanism involving initial coordination of the diene to the 3.7. Allenes nickel complex, followed by reaction with B2pin2, resulting in a Allenes were first shown to be competent substrates in the nickel(II)−allyl intermediate, which generated the 1,4-dibora- diboration reaction by Miyaura and co-workers in 1998.439 As tion product upon reductive elimination (Scheme 42). with many other substrate families, diboration reactions were Interestingly, the Morken group later reported the initially studied using B2pin2 with zerovalent platinum unexpected enantioselective 1,2-diboration of 1,3-dienes.438 complexes as the catalyst precursors. Regioselectivity for The reversal in selectivity is achieved using cis-1,3-dienes or 1,1- monosubstituted allenes favored the internal double bond, disubstituted-1,3-dienes as opposed to the trans-1,3-diene, while 1,1-disubstituted and alkoxy-substituted allenes were previously employed in platinum-catalyzed dibora- selectively diborated at the terminal double bond. The

9110 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 43. Diboration and Subsequent Functionalization of a 1,3-Diene

diboration of phenylallene showed differing regioselectivity Scheme 45. (a) Three-Component Coupling Reaction of an depending on the catalyst system used: for the Pt(PPh3)4 Allene, a Diboron, and an Alkyl Iodide; (b) Proposed catalyst system, diboration occurred preferentially at the Catalytic Cycle for the Alkyl Iodide-Promoted Catalytic internal double bond, while diboration at the terminal double Diboration of Allenes bond was favored for the Pt(dba)2/PCy3 catalyst system. A subsequent computational study found that the presence of an electron-withdrawing substituent on the allene generally favors diboration of the terminal double bond, while electron- donating substituents favor diboration of the internal double bond (Scheme 44a).440 A recent publication by Santos and co-

Scheme 44. (a) Calculated Regioselectivities for the Diboration of Monosubstituted Allenes Using Pt(PH3)2 as the Model Catalyst; (b) Diboration of Allenes with BpinBdan Using Pt(dba)3/Sphos as Catalyst

Transmetalation with diboron results in the elimination of workers examined the use of the unsymmetrical diboron(4), the diboration product and regeneration of the palladium boryl 441 BpinBdan, instead of B2pin2, in a similar system. It was iodide complex (Scheme 45b). Further support for the shown that 1,1-disubstituted alkenes underwent diboration in mechanism comes with the observation that the addition of the presence of a Pt catalyst and SPhos to give the diborated equimolar amounts of two different diborons to methylallene product. It was demonstrated that there was a difference afforded a nearly statistical mixture of diboration products, between the regioselective addition of the two boron moieties including different boryl substituents, which is not expected if to the terminal position. In the major isomer, the Bdan moiety the reaction proceeds through the B−B oxidative addition was transferred to the terminal position, while the Bpin group mechanism normally observed for zerovalent platinum- was transferred to the position at the internal sp-hybridized catalyzed diborations. carbon (Scheme 44b). Morken and co-workers developed enantioselective palla- Alternative metal complexes were also reported for the dium-based catalysts for the diboration of allenes.307,342,430 In diboration of allenes, for example, palladium(II) or zerovalent these cases, an excess of a phosphorus-based ligand was added palladium complexes, along with an alkenyl- or aryliodide as to the reaction mixture, along with Pd2(dba)3, resulting in the cocatalysts.442 These reactions occur with complete regiose- generation of the active catalytic species. Simple nonchiral lectivity, with diboration taking place exclusively at the terminal phosphines, PCy3, PPh2Cy, and P(NMe2)3, performed best. double bond. The proposed mechanism begins with the three- Chiral phosphoramidite ligands also led to excellent con- component coupling of the organic iodide, diboron, and allene. versions, while the use of the bulky TADDOL-derived ligand The allene−carboboration coupling product is observed in 3% L3 (Figure 9) provided good yields along with high yield by a GC/MS examination of the reaction mixture enantioselectivities. Subsequent in situ addition of benzalde- (Scheme 45a). Oxidative addition of the boron−iodine bond hyde to the diboration product followed by oxidative workup initiates the catalytic cycle, and the terminal double bond of the produced good yields and excellent chirality transfer to the β- allene is then inserted into the palladium−boron bond. hydroxyketone product. In the same report, Morken and co-

9111 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 46. Allylboration and Subsequent Functionalization of an Allene

Scheme 47. One-Pot Diboration and Aminoallylation (left) or Hydroboration/Cross-Coupling (right)

workers stated that initial attempts at rhodium-catalyzed 3.8. Carbonyls and Thiocarbonyls diboration of allenes suffered from poor yields, regioselectiv- 342 One early example of the reduction of a carbonyl group by a ities, and enantioselectivities. The synthetic utility of these diboron reagent is the reaction reported by Sadighi and co- reactions was illustrated further when a step in the synthesis of workers, who showed that an (NHC)copper(I)−tert-butoxide the complex terpenoid natural product, Solanoeclepin A, was complex could catalytically reduce CO2 to CO with B2pin2 (the 343 the diboration of allenes using Pd2(dba)2, reported previously oxygen atom is lost as pinB-O-Bpin). A DFT study by Zhao, by Morken et al.443 Lin, and Marder demonstrated that this reaction is promoted This chiral diboration/allyl addition reaction was improved by the high nucleophilicity of the boryl−copper species, which and the substrate scope expanded to include a wide variety of results in insertion of a CO bond into the Cu−B bond, with allenes and aldehydes.444 Aldehyde addition products could copper and not boron bound to the oxygen atom in the 444 444 − 448 also be subjected to acetic acid, NaOH/I2, or Suzuki resulting intermediate. Boryl migration from carbon to Miyaura coupling reactions without the need for additional oxygen results in the generation of a copper(I) (OBpin) species 444 palladium catalyst (Scheme 46). and the release of CO. Lastly, transmetalation with B2pin2 − Furthermore, Morken and co-workers used a chiral regenerates the copper boryl species, along with O(Bpin)2 phosphine ligand along with [(allyl)PdCl] to execute a (Scheme 48). 2 fi diboration/asymmetric allyl−allyl coupling reaction with an In a related reaction, sulfoxides were deoxygenated to sul des 445 fl 449 by B2pin2 in the presence of zinc(II) tri ate. Amine- and allyl halide, producing chiral borylated 1,5-hexadienes. In 450 α pyridine-N-oxides can also be reduced by B2pin2 or B2cat2, addition, allene diboration products were found to undergo - 451 aminoallylation (Scheme 47, left)446 or hydroboration/cross- and pyridine-N-oxides can be reduced with B2(OH)4, coupling (Scheme 47, right).447 without the need for a catalyst. Additionally, Huang and co- Indeed, the diboration of allenes has helped provide access to workers established that B2pin2 could be utilized as the reducing agent in the nickel-catalyzed reductive tetramerization a huge family of chiral alcohols. Despite previous computational 452 of alkynes, while Ogoshi et al. reported that B2pin2, in the studies suggesting the unfavorable nature of a palladium diboryl t 112,304,305 presence of NaO Bu, was used as the reductant in the copper- oxidative addition product, a recent experimental and 453 catalyzed reaction of trifluoromethylketone with aldehyde. computational study of the present palladium-catalyzed allene fi The diboration of aldehydes with B2pin2 using a modi ed diborations showed that the reaction may in fact proceed via − 307 copper(I) NHC catalyst at room temperature was reported by this oxidative addition step. The use of a mixture of D24- Sadighi and co-workers in 2006.344 A DFT study of the B2pin2 and B2pin2 as the diborating agent resulted in the aldehyde diboration reaction revealed that it proceeds through generation of only D24 and H24 products by electrospray mass a transmetalation pathway similar to that shown in Scheme spectrometry, indicating both boryl groups in the product came 24.355 The boryl group attacks the carbonyl carbon acting as a from the same B2 molecule. In contrast to zerovalent platinum nucleophile, rather than an oxophile. Notably, in the absence of systems, which showed a first-order dependence on alkyne diboron, the copper alkoxide Cu−OCRHBpin intermediate concentration, the palladium systems seemed to show a first- rearranges to the more thermodynamically favorable organo- order dependence on B2pin2, suggesting that oxidative addition cuprate Cu−CRHOBpin product. Subsequently, the same is the rate-limiting step for these palladium-catalyzed reactions. copper(I)−NHC complex was reported to catalyze the

9112 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 48. Deoxygenation of Carbon Dioxide Catalyzed by initially reduced to Pt(0) prior to the reaction taking place. The a Copper(I) Complex addition of any electron-donating reagents, including coordi- nating solvents, hinders the reaction significantly in an analogous fashion to that observed with alkyne diboration. It should be noted that bis(pinacolato)diboron has also been used to affect the reduction of nickel(II) to zerovalent nickel.460 Morken and Hong recently generated N-silyl aldimines from aldehydes in situ and added B2pin2 using a chiral zerovalent platinum catalyst.461 These silylimines subsequently undergo acylation, resulting in one-pot asymmetric aminoborylation of the starting aldehyde (Scheme 50). 3.10. Other Substrates In addition to the 1,2-diboration of CE multiple bonds (E = C, N, O, S), a few related examples have been reported. For example, the diboration of NN bonds was first reported t using dichlorodiborons RClBBClR (R = Bu, NMe2)in 1995.462 Braunschweig and co-workers reported the platinum- promoted stoichiometric and catalytic insertion of azobenzene into the B−B bond of [2]-bora-metalloarenophanes (Scheme 51, for further information see section 2.8).463 These results prompted a computational study examining the diboration of diboration of a wide range of ketones, displaying a tolerance for −  − alkenes and esters, along with other functional groups.454 Many Ph E E Ph (E = Group 15 element) species, which found α that the NN bond is more reactive than the heavier group 15 of the isolated products were -hydroxyboronate esters, 464 resulting from hydrolysis of the boron−oxygen bond during element homoatomic multiple bonds. workup. Clark and co-workers expanded on these results to In 2001, Srebnik and co-workers reported the addition of β B2pin2 to diazomethane, resulting in 1,1-diboration and loss of facilitate the formation of -hydroxyboronate esters from the 465 respective aldehyde by an initial Cu-catalyzed diboration nitrogen. As this reaction occurred only with the assistance followed by a Matteson homologation reaction.455 This route of a platinum(0) catalyst, it is believed to proceed through was found to favor the β-product over the more sensitive α- oxidative addition of the B−B bond, followed by insertion of − product. The diboration of ketones to form 1,1-disubstituted CH2 into a Pt B bond and reductive elimination to give the and trisubstituted vinyl boronate esters was also investigated by 1,1-diborated product. Several other diazo compounds were the same group. By using an initial copper-catalyzed diboration subsequently found to undergo the same transformation reaction and subsequent acid-catalyzed elimination, the desired (Scheme 52, right).466 The substrate scope was expanded products were obtained in good yields.456 Recently, the 1,1- upon by Kingsbury and Wommack in 2014 to obtain chiral diboration of aldehydes and ketones with the unsymmetrical tertiary boronate esters and alcohols.467 A silylene can similarly 468 diboron(4) compound, BpinBdan, has been conducted under react with B2pin2 to form a diboryl silane. Likewise, diboryl transition-metal-free conditions by formation of a diazo silanes were formed from compounds bearing Si−Si double intermediate.457,458 bonds, though yields are modest (Scheme 52, bottom).469 Interestingly, only a single thiocarbonyl diboration has been Tosylhydrazones were found to undergo 1,1-diboration with 412 reported in the literature, catalyzed by [RhCl(PPh3)3]. While B2pin2, in the presence of a base, to isolate the monoborated other thiocarbonyl diborations were attempted, only (1R)- product (Scheme 52, left).470,471 During investigations into the (−)-thiocamphor underwent diboration. mechanism, it was discovered that 1,1-diboronates could be 3.9. Imines formed by thermal reaction of B2pin2 with the in situ-formed N- An early attempt to diborate imines was reported in 1998.459 tosylhydrazone sodium salt. Due to the presence of NaH, the protonation of the 1,1-diboronate product is no longer possible Here, rhodium catalysts were used in the reaction of B2cat2 with 472 ketimines. Instead of the diborated product, a 1:1 mixture of N- because of the release of H2 upon formation of the salt. The boryleneamine and N-borylamine products was observed reaction also can be conducted with BpinSiPhMe2 in place of (Scheme 49). B2pin2. In related chemistry, isocyanides are also found to undergo 1,1-diboration by [2]-borametalloarenophanes.473 Scheme 49. Early Attempt To Diborate an Imine Miyaura and co-workers reported the ring-opening plati- num(0)-catalyzed diboration reactions of methylenecyclopro- panes (Scheme 53, left).474 More recently, Matsuda and Kirikae reported the diboration of biphenylene, catalyzed by palladium(0) complexes (Scheme 53, right).475 The proposed mechanism involves stepwise oxidative addition of the B−B The first successful catalytic diboration of imines emerged in bond and the strained C−C bond, resulting in a Pd(IV) 2000 from Baker and co-workers.315 Imines were diborated intermediate. This intermediate undergoes two reductive using B2cat2 with [PtCl2(COD)] as the catalyst precursor. elimination steps to give the ring-opened diborated product However, this method only produced good yields in the case of and regenerate the palladium(0) catalyst. Competitive hydro- bulky diaryl aldimines. Experimental evidence suggested that boration was also observed and was in fact quantitative when the commercially available and air-stable [PtCl2(COD)] is Pd(PPh3)3 was employed as the catalyst precursor.

9113 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

a Scheme 50. One-Pot Aminoborylation of Aldehydes

aL=L4, see Figure 9.

Scheme 51. Insertion of Azobenzene Into the B−B Bond of a 3.11. Unsaturated Carbonyls [2]-Boraferrocenophane While the diboration of α,β-unsaturated carbonyls is undoubtedly part of some β-borylation reactions reported later in section 4.1, where only one boryl group is retained in the product, we limit the discussion here to the rare cases where characterization of the diboration product is reported. a fi α β Scheme 52. 1,1-Diboration of Carbene-Like Substrates The rst report of diborations of , -unsaturated carbonyls came from Marder, Norman, and co-workers in 1997.478 A

[Pt(PPh3)2(C2H4)] catalyst was used initially, before a bulky platinum diimine catalyst was subsequently employed in the α β reaction of B2cat2 or B2pin2 with , -unsaturated ketones, resulting in 1,4-diboration. Regioselectivity was found to be reversed for α,β-unsaturated ester substrates, which underwent 3,4-diboration (Scheme 55).479 Following hydrolysis of the O- or C-bound boron enolate intermediate, β-boryl carbonyls were a Tbt =2,4,6-(CHSiMe3)3-C6H2. isolated in each case. A computational study by Marder and Lin helped elucidate Scheme 53. Borylative Ring-Opening Reactions the mechanism of this diboration reaction.480 Rate-determining − oxidative addition of B2pin2 occurs with a platinum diimine substrate adduct. The axial boryl group of the distorted trigonal bipyramidal complex electrophilically attacks the sp2 oxygen of the α,β-unsaturated substrate, generating a boron enolate intermediate, with platinum bound at the 4-position. This Pyrazines can be diborated at the 1- and 4-positions by species coordinates another substrate molecule and then B2pin2 and other tetra-alkoxydiborons without the need for a catalyst (Scheme 54).476 This was subsequently followed by the undergoes reductive elimination, releasing the 1,4-diboration product. The electrophilic attack of the boryl ligand on the Scheme 54. 1,4-Diboration of Pyrazine bound substrate is particularly interesting given the boryl ligand’s normal behavior as a nucleophile when bound to Group 11 or 12 metals such as Cu or Zn or to main group metals (Scheme 56).43,220,221 In cases where 3,4-diboration products were observed, these compounds presumably arise from a thermodynamically favored migration of the boryl group from oxygen to carbon. The relative stabilities and isomerization between O- and C- bound B−,Si−,andCu−enolates were also examined theoretically.353 Hoveyda and co-workers have shown that copper-catalyzed examination of sterically hindered pyrazine derivatives in the presence of substituted 4,4′-bipyridine as an organocatalyst.477 and metal-free NHC-catalyzed borylations can proceed through 214 Further mechanistic studies were conducted which showed that diborated intermediates (Scheme 57). These species can the catalytic cycle involved two key steps. The initial step was only be observed in the absence of MeOH, which is normally − the reductive addition of the B B bond to the bipyridine present in the borylation reaction mixture. Aqueous workup ligand, and the second step was the oxidative boryl transfer to − β the pyrazine from this intermediate to regenerate the bipyridine hydrolyzed the boron oxygen bond and produced the -boryl giving the pyrazine diboration product. ketone.

9114 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 55. Diboration of α,β-Unsaturated Carbonyls and Subsequent Hydrolysis

Scheme 56. Calculated Mechanism for the Pt-Catalyzed 1,4- development of the field warrants an up-to-date summary of Diboration of α,β-Unsaturated Carbonyls progress in this area. 4.1. α,β-Unsaturated Carbonyls, Imines, and Related Compounds The so-called β-borylation of α,β-unsaturated ketones was first reported by Marder, Norman, and co-workers in 1997.478 This reaction proceeds through initial diboration at the 1- and 4- ° positions with B2cat2 or B2pin2 at 80 C, catalyzed by 5% η2 [Pt(PPh3)2( -C2H4)], followed by quenching with H2Oto hydrolyze the B−O bond, leaving a β-boryl enol which rearranges to the ketone (see Section 3.10). The 1,4-diboration products were also observed. Later work expanded the substrate scope for the platinum-catalyzed borylation reac- tion.465 Subsequent reports by the groups of Miyaura223 and Hosomi488 demonstrated copper(I)-promoted boryl additions α β to α,β-unsaturated ketones, while the Miyaura group also Scheme 57. Diboration of , -Unsaturated Ketones, α β Catalyzed by a Copper(I) Complex, or an N-Heterocyclic included other Michael acceptors including , -unsaturated Carbene esters and nitriles. The reactions are proposed to proceed through a boryl−copper intermediate, which then attacks the electrophilic substrate.224 In the Miyaura reports, stoichiometric amounts of a base additive (potassium acetate) and CuCl were required for optimal reactivity, suggesting that coordination of the acetate anion to either the diboron or the copper complex may also be an important step. Hosomi used only catalytic amounts of CuOTf or CuCl and a tri-n-butylphosphine additive (Scheme 58). Interestingly, reactions did not proceed without the phosphine additive or when using a chelating diphosphine ligand. 3.12. Diagram Summarizing Section 3 Scheme 58. Copper(I)-Catalyzed β-Borylation of an α,β- Unsaturated Ketone

Marder and Lin studied the mechanism of these reactions and confirmed that they proceed via CC insertion into a copper−boron bond, followed by keto−enol isomerization to a copper−alkoxide species which then undergoes protonation or metathesis with B2pin2, generating the borylation product or the 1,4-diboration product, respectively.43,353 Interestingly, they 4. BORYL ADDITION (HYDROBORATION) REACTIONS note that a protic additive is vital to the borylation of methyl This section will focus on reactions wherein one boryl group of acrylate due to its inability to undergo keto−enol isomerization. the diboron is added to an unsaturated substrate while the Thus, the organocuprate intermediate, which is inert to fi 355 other is sacri ced. These addition reactions are often quenched metathesis with an incoming B2pin2 molecule, must be by a proton from the solvent or an additive; thus, these protonated by the alcohol (Scheme 59). Generally, the boron− additions can result in net hydroboration (addition of H and oxygen bond of the 1,4-diboration products is hydrolyzed upon β B(OR)2). The hydroboration of imines and enones has already workup, and the products are typically isolated as -borylated been discussed above in sections 3.9−3.11. While recent species. It is important to note that the overall “hydroboration” reviews and perspective articles exist for the β-borylation of α,β- of these species using diboron reagents offers complementary − − unsaturated compounds,45,48,68,70,218 221,228,481 487 the rapid regioselectivity to hydroborations with boron hydride reagents,

9115 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 59. Proposed Mechanism for the Copper(I)- Scheme 60. (a) Borylation of α,β-Unsaturated Imines (1- Catalyzed Borylation of α,β-Unsaturated Carbonyls azadienes); (b) Asymmetric Borylation and Subsequent Oxidation of α,β-Unsaturated Sulfones; (c) Generation of Chiral Phosphine Oxide Boronates from α,β-Unsaturated Phosphine Oxides

which generate CO reduction products with boron bound to oxygen.43,486 The mechanisms for these reactions are reminiscent of the − − copper-catalyzed diboration reaction, shown in Scheme 24, the Fernández,398, 508 512 Mazet,485 Ma,513 515 and − only difference being that, in the present case, the organo- others.496,516 521 Enantioselective borylation of α,β-unsaturated cuprate intermediate is quenched by a proton from solution aldehydes has been employed in tandem with a Wittig reaction instead of a second boryl group. Yun and co-workers reported to provide access to homoallylboronates (Scheme 61).522 The the copper-catalyzed β-borylation of α,β-unsaturated carbon- amine cocatalyst is proposed to activate the substrate to − yls,384,489 491 nitriles,489,492 phosphates,489 and even β,β- nucleophilic attack by transiently generating a chiral iminium disubstituted α,β-unsaturated esters493 using CuCl and a chiral cation from the aldehyde group. Asymmetric copper(I)- or nonchiral phosphine as their catalyst, along with a base catalyzed borylation of α-dehydroamino acid derivatives was additive. They found that the addition of a catalytic amount of used to afford chiral β-hydroxy-α-amino acids.523 Although base and an excess of alcohol is key to the reaction. An enantioselectivities were excellent for this reaction, an impregnated copper on magnetite system has also been approximately 1:1 diastereomeric mixture of products was effective for the β-borylation reaction, offering the advantage obtained. of an easily separable and reusable catalyst.494 Kinetic resolution of a racemic mixture of β-borylated A ligandless copper catalyst system CuCl and NaOtBu was products through lipase-catalyzed hydrolysis or transesterifica- used to perform the β-borylation of α,β-unsaturated carbonyls tion reactions has also been reported.524 Santos exploited the β- in water.495 The same CuCl/NaOtBu combination was borylation reaction in the synthesis of β-boryl carboxylic acids employed along with chiral phosphine ligands for asymmetric (Scheme 62a), which can be reacted with amines and − copper-catalyzed borylations of unsaturated carbonyls,496 498 hydrolyzed to generate N-terminal boronic acids, compounds amides,499 and nitriles.498 Similar conditions were also used by which have proven to be useful protease inhibitors.525 Hall et al. to generate 1,1-diboron compounds from β-boronyl Fernandeź and co-workers noted that borylation and electro- acrylates. The Bpin group was reacted subsequently to form the philic fluorination reactions of α,β-unsaturated ketones can be trifluoroborate salt and then chemoselectively cross-coupled to done in a one-pot fashion (Scheme 62b).526 form benzylic or allylic boronate esters.500 Additionally, the use Furthermore, Fernandeź and co-workers prepared γ-amino of a copper(0) bipyridine-based catalyst for the borylation of alcohols in a highly enantioselective fashion through a one-pot α,β-unsaturated carbonyls in water was reported.501 copper(I)-catalyzed condensation/borylation/reduction/oxida- Fernandeź and Soléreported the copper-catalyzed β- tion reaction from imine starting materials.527,528 Ketones could borylation of α,β-unsaturated imines (1-azadienes) using the also be subjected to the same protocol (Scheme 63). same CuCl/base (NaOtBu, NaOAc, NaOMe, NaOH) combi- The reduction step of the borylation/reduction/oxidation 502 nation along with PCy3 (Scheme 60a). Another report by sequence depicted in Scheme 63 had previously been studied ́ 529 the Fernandez and Whiting groups detailed the use of Cu2Oas by Whiting and co-workers. The same borylation/reduction/ a catalyst for the base-free β-borylation of α,β-unsaturated oxidation sequence could be performed with α,β-unsaturated imines.503 A chiral copper(I) complex was also established as a ketones.527 Fernandeź and Whiting also introduced four- and good catalyst for the β-borylation of α,β-unsaturated sulfones, five-step one-pot sequences, including a borylation step, in the producing chiral β-hydroxy sulfones upon oxidation (Scheme preparation of γ-aminoalcohols and 1,3-oxazines, respec- 60b),504 while a similar system was also used to develop the tively.530 A double asymmetric borylation method could be first asymmetric synthesis of ambiphilic phosphine oxide used to desymmetrize prochiral 1,4-Michael acceptors.531 The boronates (Scheme 60c).505 same groups published a report documenting the conjugate Related asymmetric copper(I) catalyst systems for the addition to unsaturated bulky aldimines, generated in situ from borylation of α,β-unsaturated conjugated systems were enals (Scheme 64). The use of chiral diphosphine auxiliary reported by the groups of Hoveyda,506 McQuade,507 ligands led to high enantioselectivity in the borylation step. The

9116 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 61. One-Pot Borylation/Wittig Reaction of an α,β-Unsaturated Aldehyde

Scheme 62. (a) Preparation of β-Boryl Carboxylic Acids; (b) Borylation and Electrophilic Fluorination of α,β-Unsaturated Carbonyls

Scheme 63. One-Pot Borylation/Reduction/Oxidation Scheme 65. Copper(I) Bifluoride-Catalyzed Borylation Reaction of an α,β-Unsaturated Species

products of these reactions could be subjected to an oxidative workup, forming chiral γ-amino alcohols, or hydrolysis, forming the chiral β-boryl aldehydes.532 Similar reactions were also conducted in an attempt to make the β-boryl aldehyde by serves as the nucleophile in the reaction and is eventually initially converting the α,β-unsaturated aldehyde in situ into the transferred to the substrate. respective imine, followed by catalytic borylation and then In a surprising observation, Santos and co-workers workup; however, the product was extremely unstable, and discovered that a copper(II) chloride/amine system could therefore, subsequent Wittig olefination was used to facilitate catalyze the borylation of α,β-unsaturated ketones and esters in the generation of a stable and versatile homoallylic boronate air using water as the solvent.538 Although a variety of nitrogen- ester.533 Furthermore, starting from the α,β-unsaturated containing bases were effective, catalyst systems containing 4- aldehyde, such a process has been utilized in the total syntheses picoline were found to provide the highest conversion in the β- of (R)-fluoxetine and (S)-duloxetine.534 borylation of 2-cyclohexen-1-one. Kobayashi and co-workers A novel copper(I) bifluoride NHC complex showed simultaneously published a report with an asymmetric variation relatively high activity for the β-borylation of an α,β- of the copper(II)/water reaction conditions (Scheme 66a).539 unsaturated ester (Scheme 65); however, a copper complex A similar copper system was developed for industrial with a chiral NHC ligand used in the study showed poor yield applications, combining basic CuCO3 and PPh3 to catalyze and enantioselectivity.535 Borylations of α,β-unsaturated the β-borylation of a range of Michael acceptors in water.540 carbonyls using bulky copper(I) phosphine catalysts and The borylation of a selection of model alkenes and alkynes was 536 B2(OH)4 have also been reported. Santos and co-workers also reported. reported the copper(I)−NHC-catalyzed β-borylation of α,β- Kobayashi et al. recently adapted their previously reported unsaturated carbonyls and nitriles by PDIPA (Figure Cu(II)/water system to facilitate the boron conjugate addition 201,202,537 α β 541 5). As expected, the three-coordinate boron center using B2pin2 to , -unsaturated imines, cyclic and acyclic

Scheme 64. In Situ Generation and Borylation of Bulky Aldimines, and Subsequent Reactions of the Borylated Products

9117 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 66. (a) Chiral Copper(II)-Catalyzed β-Borylation in Scheme 67. (a) Monoborylation of Enones in the Absence of Water; (b) Enantioselective 1,6-Boration to Synthesize a Transition Metal; (b) Metal-Free Catalytic Hydroboration Allylboronates of Aldimines and Ketimines

used a similar NHC catalyst system to establish a novel route for the enantioselective boryl conjugate addition to enones, in the presence of a catalytic amount of base, DBU (DBU = 1,8- enones,542 α,β-unsaturated esters, amides, and nitriles,542 as diazabicyclo[5.4.0]undec-7-ene).552 Similar to the metal-free well as the β-borylation of α,β,γ,δ-dienones and dienoesters.543 diboration mechanism shown in Scheme 25, coordination of However, for the latter report, the route was only successful if the carbene to one of the boron centers dramatically increases substrates disubstituted at the β-position were used. Therefore, the nucleophilicity of the other boron atom (see section − Lam and co-workers went on to develop a system which could 2.5.2).213 215 Additional mechanistic studies on these reactions successfully facilitate the 1,6-boration of α,β,γ,δ-unsaturated have recently been conducted by Hoveyda and co-workers to esters and ketones without a required substituent blocking the try to gain more insight into why excess base and methanol are β-position (Scheme 66b).544 necessary in many of these reactions and why the reactions are Investigations into the catalytic potential of zerovalent selective.553 This methodology was subsequently extended to palladium or nickel complexes in asymmetric β-borylation of include chiral boronation of α,β-unsaturated ketones, esters, α,β-unsaturated esters were conducted by Fernandez,́ Westcott, aldehydes, amides, and nitriles.554 An NHC bearing a pendant and co-workers,545 while Oshima et al. reported a zerovalent amine group was also competent as a catalyst for the borylation nickel phosphine system that catalyzed the borylation of α,β- of α,β-unsaturated ketones.555 Likewise, an amine/NHC- unsaturated esters and amides.546 Similar reactivity has been catalyzed β-borylation reaction has also been applied in tandem α β observed with [RhCl(PPh3)3] for the borylation of , - with a Wittig reaction, allowing access to homoallylboro- 556 unsaturated carbonyls and nitriles with B2pin2 and nates. Sun and co-workers developed a protocol involving an 547 − B2neop2, while recently a CCC NHC pincer Rh complex NHC-precursor (imidazolium salt) and a base (DBU) in was shown to quantitatively borylate α,β-unsaturated carbonyl methanol for the borylation of α,β-unsaturated ketones, esters, fi compounds with B2pin2, with the latter used to generate and sul nyl aldimines under mild conditions and open to the products with boron-substituted quaternary carbon centers.548 atmosphere (Scheme 67b).557 A similar protocol has been Additionally, Nishiyama and co-workers reported the rhodium- reported by Song, Ma, and co-workers for the enantioselective (I)-catalyzed enantioselective β-borylation of α,β-unsaturated β-boration of acyclic enones.558 549,550 ́ ketones, esters, and amides with B2pin2. In contrast to Fernandez and co-workers applied a similar protocol in the Fernandeź ’s palladium and nickel systems,545 the rhodium(I) phosphine-promoted asymmetric boronation of α,β-unsatu- catalyst showed decreased enantioselectivity with bulkier ester rated ketones,559 esters,559 and tosylaldimines.230 They also functionalities.549,550 The use of palladium as a catalyst for the examined the use of iron(II) salts as Lewis acid activators for borylation reaction was further exploited by Fernandeź and co- the Michael acceptors.354 The protocol evolved further as the workers, who performed a tandem borylation/arylation of α,β- stoichiometric amount of base initially used is not required228 unsaturated carbonyls with the use of a single palladium and a simple Bronsted base/alcohol system proved capable of catalyst.551 the β-borylation reaction (Scheme 68).228 The mechanism Hoveyda and co-workers noted that, in contrast to many originally proposed (Scheme 68a) has been superseded by an other copper catalysts, the active catalyst derived from the alternative one in which the phosphine attacks the substrate reaction of an NHC with CuCl and 2 equiv of NaOtBu showed (Scheme 68b). In cooperation with Whiting, the same group 214 − 232 high activity in the absence of MeOH. The authors propose established that in situ formation of [B2pin2OMe] could the strong σ-donor ability of the ligand to be responsible for the facilitate the asymmetric β-boration of α,β-unsaturated imines facile release of the product from the organocuprate in the presence of a chiral phosphine.560 intermediate. The same group has shown that these conjugate The use of alternative diboron(4) reagents has also been additions of boryl groups can be done in the absence of a investigated in transition-metal-free borylations. One example transition-metal catalyst (Scheme 67a).214,215 Instead, they established that BpinBdan could react in situ to form a similar utilized N-heterocyclic carbenes, generated in situ from sp2−sp3 Lewis base adduct to transfer the Bdan group to the β- imidazolium salts and alkoxide, as catalysts for the addition of carbon in the α,β-unsaturated compound,561 while another a boryl group to α,β-unsaturated ketones. Additionally, they example showed that the enantioselective borylation of enones

9118 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 68. Proposed Mechanisms for the Phosphine-Catalyzed Borylation of α,β-Unsaturated Species: (a) Original Proposal Involving R3P Coordination to B2pin2; (b) Subsequent Proposed Mechanism

Scheme 69. Enantioselective Bis-Borylation of an Alkyne

with B2neop2 using O-monoacyltartaric acid as the catalyst then subjected to a subsequent hydroboration through a similar could be achieved.562 pathway. This reaction has been expanded to include 384,565 4.2. Alkynes silylalkynes and shows generally superior functional group tolerance when compared to the diboration of alkenes. Alkynes have already been mentioned previously in section 3.2, Monoboration of alkynes was also of great interest with with the diboration of these substrates shown to provide a wide many different groups exploring the reactivity of both terminal variety of compounds that can be used as building blocks in many synthetic pathways. However, an excellent alternative and internal alkynes with diboron(4) compounds to form the monoborylated product.301,384,385,387,566 In 2001, Miyaura et al. route to 1,2-diborylalkanes is through the double hydroboration − of alkynes.53,301 While this reaction is known for more common investigated the addition of the proposed copper boryl 563 complex, discussed in section 4.1, to terminal alkynes, hydroborating agents such as pinacolborane (HBpin) and 224 catecholborane (HBcat),339 the same reaction can be generating the desired alkenylboronate. This work has subsequently been investigated by many research groups, accomplished using B2pin2 as the borylating agent. Hoveyda 224 and co-workers reported the enantioselective double borylation expanding greatly upon the initial results. Son and Yun of alkynes affording chiral diborylalkanes with high enantiose- introduced bulky copper(I) complexes, which showed excellent 567,568 lectivity (Scheme 69).564 These reactions were catalyzed by regioselectivity in the borylation of activated alkynes. chiral copper(I) complexes in the presence of an alcohol. They subsequently elaborated on this approach by introducing Following alkyne insertion into the copper−boron bond, the the one-pot copper-catalyzed borylation and asymmetric alkyl group is protonated by the alcohol. The alkoxy group is reduction of activated alkynes, providing an elegant route then abstracted from copper by the diboron, regenerating the from alkynes to chiral boryl alkanes (Scheme 70).569 copper−boryl species. The initially produced alkenylborane is Furthermore, investigations into the origins of the regiose-

9119 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 70. Borylation and Reduction of an Activated catalyzed synthesis of branched alkenylboronate esters, instead a Alkyne of the more commonly observed linear products.581 The same group also demonstrated that the regioselectivity of the copper- catalyzed borylation of internal alkynes, with the same masked diboron(4) compound, was dependent on the ligand present.582 Alternative copper sources were reported with copper(0) powder used to enable the addition of a boryl group 583 from B2pin2 to internal and terminal alkynes in ethanol, while Garcia and co-workers demonstrated that magnesia- aPMHS = polymethylhydrosiloxane. supported copper or iron oxide was capable of mediating similar transformations with terminal alkynes.363 Over the past few years, the substrate scope has been lectivity in these reactions were conducted by the same group ff using DFT calculations.570 Additionally, these reactions were extended to incorporate many di erent substituted alkynes. utilized by Aggarwal and co-workers in the total synthesis of the Thioacetylenes underwent hydroboration with either HBpin or α 571 B2pin2 via copper(I) catalysis, with a similar diverging reactivity universal mating hormone 1 from the fungus Phytophthora. 580,584 Recently, the reaction was made more accessible by Santos and shown, as previously reported by Tsuji et al. If B2pin2 was present in the reaction, it would result in the boryl group co-workers by using an aqueous Cu(II)-based catalytic system β 584 under aerobic conditions to afford β-boryl-α,β-unsaturated adding at a -position (with respect to the thioether group). 572 The monoborylation of silylalkynes was carried out efficiently esters, regio-, chemo-, and stereoselectively. 585 586 Hoveyda and co-workers used CuCl−NHC catalysts for the by both Yun and co-workers and by Kubota et al., while − − − silyl directing groups also facilitated the borylation of internal internal borylation of terminal aryl , amino , and alkoxy 587 alkynes,573 while Carretero and ArrayasutilizedCuCĺ − propargylic silylalkynes. Additionally, recent studies have phosphine catalysts in the highly regioselective borylation of shown the excellent reactivity of ynamides with B2pin2 to α β 588 propargyl-substituted alkynes.574,575 Their protocols proved to generate both , -disubstituted (Z)-alkenamides boronates β 589 be tolerant of a wide range of functional groups including and (E)- -alkenylamide boronates. Interestingly, McQuade ethers, esters, thioethers, sulfones, amines, and alcohols. and co-workers noted that selectivity in the borylation of propargyl-substituted alkynes could be altered by modifying the Further investigations expanded this work to show that the 590,591 Cu-catalyzed borylation could be carried out in conjunction steric bulk of the NHC ligand (Scheme 72). Two more with a subsequent allylic alkylation to form a variety of tris- recent publications have utilized copper catalysts in the substituted vinyl boronates.576 Recently, the borylation of both borylation of unsymmetrical alkynes to synthesize a range of unsymmetrical and terminal alkynes with B pin was used to dideuterated β-borylated α,β-unsaturated styrenes from alkynyl 2 2 592 sample the catalytic properties of several different copper carboxylic acids and a range of enynylboronates from the complexes. Belestkaya and co-workers reported the effective- selective borylation of one carbon−carbon triple bond in 593 ness of Cu(I) catalysts with diethoxyphosphoryl-1,10-phenan- conjugated diynes. throlines,577 while Prabusanker et al. investigated homoleptic Recently, investigations showed that slight modifications to copper(I) imidazoline-2-chalcogenone.578 the conditions could alter whether α-orβ-hydroboration Ma and Yuen showed that copper(I) phosphine complexes occurred. Cazin and co-workers observed that β-borylation of are highly selective for catalytic boryl additions to internal internal alkynes with B2pin2 could be carried out with excellent alkynes.579 Conversely, Tsuji and co-workers documented that selectivity in the presence of a copper(I) NHC complex and 594 the use of B2pin2/MeOH for hydroboration of internal alkynes more surprisingly in air. Using similar conditions and the with a copper(I) catalyst gave complementary regioselectivity, same catalyst, α-hydroboration was observed when HBpin was 594 compared with the corresponding hydroboration reaction with used, instead of B2pin2. Furthermore, Prabhu et al. HBpin, under nearly identical reaction conditions (Scheme demonstrated that in the palladium-catalyzed borylation of 580 ff 71). The authors postulated the formation of di erent active terminal alkynes with B2pin2, the regioselectivity could be catalysts, depending on the borylation agent used (L−Cu−H switched depending on the ligand present.595 − − for HBpin and L Cu B for B2pin2) to be responsible for the Alternative metals to copper have also been established as divergent reactivity. Furthermore, Yoshida and co-workers went excellent catalysts in the boryl addition reactions to alkynes. A on to establish that BpinBdan could be used in the copper(I)- silver(I)−NHC catalyst was shown to catalyze the β-selective

Scheme 71. Divergent Regioselectivity in Copper-Catalyzed Hydroborations with HBpin and B2pin2

9120 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 72. Ligand Effect on the Regioselectivity of the Copper(I)-Catalyzed Borylation of Propargyl-Substituted Alkynes

hydroboration of a range of terminal alkynes as well as several Scheme 73. Asymmetric Borylation of 1,1-Disubstituted 596,597 internal alkynes and allenes with B2pin2. Magnetically Alkenes separable iron nanoparticles or ferric chloride also catalyzed a similar transformation with terminal alkynes.598 It was observed that the catalyst could be recovered by using an external magnetic field, with the catalysts reused at least 6 times without a major effect detected on the catalytic activity.598 Recently, Takita, Uchiyama, and co-workers published a report detailing the zinc(II)-catalyzed borylation of alkynes and arynes in the presence of NaOtBu.599 Alternatively to quenching the reaction with water, which generated the borylation product, the boryl− zinc addition products could be quenched with allyl bromide or fi iodine, resulting in allylboration or iodoboration, respectively. Recently, the copper-catalyzed borylation of terminal ole ns, Transition-metal-free borylation of alkynes has also been in the absence of a ligand, has been investigated further to establish that different selectivity is observed depending on the reported. Sun and co-workers observed the borylation of 609 terminal and internal alkynes using an NHC-precursor/base substrate. When allyl arenes are present in the reaction, the combination with methanol, which was effective for the Markovnikov alkylboronate product is detected; however, the reaction under mild conditions and open to the atmosphere.557 opposite regioselectivity is observed if a styrene derivative is used.609 Similar products were also detected in the ferrous Song and Yang generated alkylboronates from arylacetylenes or 610 600 chloride-catalyzed hydroboration of aryl alkenes. Addition- vinyl arenes with B pin under similar conditions. 2 2 ally, a copper-catalyzed borylation of these substrates was 4.3. Alkenes combined with an ortho-cyanation reaction followed by a silver- 611 Borylation reactions of alkenes are also known.566,591 catalyzed cyclopentannulation to form a range of indanones. fi The borylation of strained alkenes was also explored by Stoichiometric hydroboration reactions of B2H6 with ole ns were shown to work successfully by Brown in 1957,601 while several groups. Tortosa reported a novel copper-catalyzed borylation of cyclopropenes to afford cyclopropylboronates, Sadighi and co-workers were able to isolate a copper(I) 612 boroalkyl species resulting from alkene insertion into a enantio- and stereoselectively (Scheme 74). The reaction copper−boron bond.351,602 However, interest into this scope was extended further by investigating the hydroboration of benzonorbornadienes and other analogous strained alkenes particular copper-catalyzed hydroboration reaction with dibor- 613 on(4) compounds increased following Hoveyda and co- with B2pin2 in the presence of a copper catalyst. The same workers’ publication in 2009, in which they reported the group also established that if MeOH was exchanged for MeI asymmetric β-borylation of vinyl arenes catalyzed by copper- (I)−NHC complexes and NaOtBu in the presence of 2 equiv of Scheme 74. Diastereo- and Enantioselective Hydroboration methanol.603,604 This approach was applied to 1,1-disubstituted of Cyclopropenes vinyl arenes, the hydroboration products of which are otherwise difficult to synthesize with good enantiopurity (Scheme 73).605 The same group expanded upon this work by establishing that under similar conditions, aryl- and alkyl-substituted vinylsilanes could undergo borylation to form a range of borosilanes.606 A similar system was recently used to examine the catalytic viability of dinuclear Cu(I) complexes and silver analogues in the hydroboration of styrene.607 The β-borylation of vinyl arenes and 1-hexene was also observed in the presence of a copper/magnetite catalyst, albeit sluggishly.494 Deng, Zeng, and co-workers developed a mild and efficient hydroboration reaction for aryl alkenes with B2pin2 in methanol using Cu2O 608 and PPh3.

9121 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 75. Regioselective Copper-Catalyzed Borylation of Enynes

Scheme 76. (a) Differing Regioselectivity in the Borylation of Allenes, Based on the Size of the NHC Ligand; (b) Olefin- Directed Borylation of Allenes

Scheme 77. (a) Borylation and Generation of Alcohol Trifluoroborate Salts from Aldehydes and Their Subsequent Protection and Suzuki−Miyaura Coupling Reaction; (b) Enantioselective Copper-Catalyzed Borylation of Aldehydes To Afford α- Alkoxyorganoboronate Esters

then a successful carboboration reaction would be carried out et al. reported similar investigations into the same copper- (these types of reactions are discussed further in section 5.1).613 catalyzed borylation of p-quinone methides; however, the use An enantiodivergent hydroboration was reported for similar of a different ligand enabled them to expand the reactivity 616 substrates, catalyzed by a copper salt and (R,R)-Taniaphos.614 scope and improve the enantioselectivity of the reaction. When B2pin2 was present in the reaction, the (S)-enantiomer 4.4. Dienes, Enynes, and Allenes was generated; however, the opposite was obtained when 614 Ito and co-workers studied the asymmetric copper(I)-catalyzed HBpin was used. borylation of cyclic 1,3-dienes and found that the regioselec- As an interesting aside, borylative addition of Bpin to the tivity of the reaction could be altered through modification of electron-deficient alkenes, p-quinone methides, was facilitated the reaction conditions.617 The same group demonstrated that by an enantioselective copper catalyst to generate the optically 1,3-enynes could be borylated with high regioselectivities,618 active gem-diarylmethine boronates.615 Subsequently, Tortosa but this selectivity was dependent on substrate structure. When

9122 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 78. Borylation as Part of the Total Synthesis of Bortezomib (Velcade)

the alkene functionality is monosubstituted, borylation took catecholborane generated in situ produced the reduced N- place selectively at the terminal alkene carbon. In cases where borylamine. A computational study supports the proposed the alkene functionality is more sterically hindered, catalyst mechanism and suggested that less bulky substrates may favor selection was extremely important for the regioselectivity of the diboration in the rhodium-catalyzed reaction.630 Sulfinyl addition (Scheme 75). As an interesting aside, Ito and co- aldimines were borylated in a diastereoselective fashion with − 631 workers used a similar copper(I) catalytic system to generate a B2pin2 and a copper(I) NHC catalyst. This reaction was range of enantioenriched heterocycles from the borylative exploited for the preparation of Bortezomib (Velcade), a dearomatization of indoles.619 boronic acid-containing protease inhibitor approved for the Copper(I) phosphine-catalyzed borylations of aryl allenes treatment of cancer (Scheme 78).632,633 have been reported to be moderately regio- and stereoselective, Similar copper-catalyzed borylation reactions of N-sulfinyl with the addition taking place at the terminal double bond and aldimines and ketimines in the presence of NaH and a catalytic the boryl group ending up on the central carbon of the allene amount of a carbene precursor were reported.634 This protocol functionality.620 In the presence of a monodentate ligand, such is faster and proceeds under milder conditions than the 631 as P(C6H4OMe-p)3, the reaction occurred at the more previously reported procedure. A later report showed that N- substituted CC bond, while bulkier bidentate ligands caused tert-butyl sulfinyl imines could be stereoselectively borylated a reversal in regioselectivity, with the more hindered internal with B2pin2 using low loadings of a copper(II) catalyst formed double bond undergoing borylcupration (and subsequent in situ from CuSO4 and an added ligand in the presence of a protonation by MeOH). Hoveyda and co-workers also noted catalytic amount of base in a 5:1 toluene:water solution in that there was a trend of catalyst selectivity based on the steric air.635 Aldimines could be borylated asymmetrically using t bulk of the carbene used in the borylation of allenes (Scheme B2pin2 and a catalyst derived from CuCl, NaO Bu, and a bulky 76a).621 It was observed that with bulky NHC ligands, the chiral imidazolium salt NHC precursor,636 while Liao and co- internal double bond underwent the addition, while with workers established that a boryl group could be selectively smaller variants, the addition took place at the terminal double added to N-Boc-imines in the presence of CuCl and a chiral bond. Further work by the same group extended the substrate sulfoxide−dialkylphosphine ligand, affording α-aminoboronate scope of the borylation of 1,1-disubstituted allenes to afford the esters with high enantioselectivities.637 Furthermore, N- alkenyl boronate esters, resulting from addition to the internal phosphinylimines were shown to undergo successful copper- double bond, with the boryl group added to the central carbon catalyzed enantioselective electrophilic borylation to form the of the allene in high enantioselectivity.622 Similar trends based desired α-aminoboronate ester required in an alternative on ligand size were noted by Tsuji and co-workers for the pathway to Bortezomib (Velcade).638 Sun and co-workers borylation of allenes and dienes.623 Additionally, they went on reported a metal-free variation of the reaction using an NHC to report on a related system which generates useful 2-boryl- precursor/base combination in methanol (for more informa- 1,3-butadiene building blocks efficiently via the copper- tion see section 4.2).557 Finally, Tang and co-workers also α 624 α catalyzed reaction of -alkoxy allenes with B2pin2. Amide- established that -aryl amides can undergo hydroboration using substituted allenes have also been borylated via a copper- rhodium catalysts to synthesize novel chiral α-amino tertiary catalyzed route in a highly regio- and stereoselective fashion,625 boronate esters.639 while a similar catalytic system was used to facilitate the 4.6. Ring-Opening Reactions enantioselective borylcupration of allenylsilanes.626 Further- Lithium salts of epoxides can undergo stereoselective ring more, a regio- and stereoselective palladium-catalyzed hydro- 640 boration of allenes using an olefin as a directing group has been opening with B2pin2. Remarkably, reaction of the same reported by Backvall̈ and co-workers (Scheme 76b).627 lithium salt with BpinSiPhMe2 produced the opposite stereo- isomer (Scheme 79). These reactions demonstrate an efficient 4.5. Aldehydes and Imines way to construct elaborate CF3-containing organic molecules Simple aldehydes could be borylated in methanol by a which would otherwise be difficult to access. copper(I) catalyst as part of an approach to prepare chiral A copper(I) catalyst was found to be effective for the ring secondary alcohols (Scheme 77a).628 Protection of the alcohol opening of α,β-unsaturated epoxides, allowing for the group of the trifluoroborate salt followed by Suzuki cross- coupling afforded the corresponding benzyl ethers. Ito and co- Scheme 79. Ring Opening of a Lithium Epoxide workers went on to develop this reaction to establish a novel enantioselective copper-catalyzed borylation of the CO bond ff α in aliphatic and aromatic aldehydes with B2pin2 to a ord - alkoxyorganoboronate esters (Scheme 77b).629 Monoborylated products have also been isolated from the rhodium-catalyzed reaction of B2cat2 with ketimines (Scheme 49).459 These reactions produce a mixture of N-boryleneamine and N-borylamines through a selective β-hydride elimination step. Subsequent hydroboration of the imine with the

9123 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 80. Borylative Ring Opening of α,β-Unsaturated Cyclopropanes, Epoxides, and Aziridines

diastereoselective synthesis of complex syn-andanti-1,4- none-containing enynes,665 and dienynes (Scheme 81).666 A diols.641 A palladium pincer complex has been utilized by recent publication by Backvall̈ et al. should be noted as chiral Szabóand co-workers in the borylative ring opening of substituted vinylcyclopropanes and vinyl aziridines with Scheme 81. Borylative Cyclizations of (a) Allenynes, (b) tetrahydroxydiboron (Scheme 80a),642 while recently the Enallenes, (c) Enynes, (d) 2-Alkynylarylisocyanates, and (e) same group demonstrated that allenyl boronates and alkenyl 2-Alkynylanilines diboronates could be generated from the copper-catalyzed borylative ring opening of propargyl cyclopropane, epoxide, aziridine, and oxetane.643 Similar nickel-catalyzed ring-opening reactions of vinylcyclopropanes644 and aryl cyclopropyl ketones645 have also been reported. Borylative ring opening can occur when α,β-unsaturated aziridines and epoxides are reacted with B2pin2 in the presence of a zerovalent nickel catalyst (Scheme 80b).646,647 Recently, Fernandeź and co- workers reported metal-free borylative ring-opening reactions of epoxides and aziridines with an in situ formed methoxy- bis(pinacolato)diboron adduct (Scheme 80c).219,229,232 The allylic boronate was isolated in good yield with subsequent oxidation to the trans-diol or reaction with benzaldehyde to generate a 1,3-diol.229,647 4.7. Borylative Cyclizations and Related Intermolecular Reactions In related chemistry, Cardenas̀ and co-workers presented a borylative cyclization reaction of 1,6-enynes catalyzed by palladium(II) complexes.648 The reaction proceeds in a similar fashion to the borylation reaction, but here the preorganization of the substrate by palladium results in the formation of a new carbon−carbon bond. Reductive elimination subsequently ff a ords the borylative cyclization product. A similar intra- phosphoric acids were used as a cocatalyst with Pd(OAc)2 to molecular borylative cyclization reaction was observed for help facilitate the enantioselective cyclization taking place, allenyl ketones,649 2-alkynylaryl isocyanates,650 a 2,3-dienylbu- which generates borylated carbocycles in high enantiomeric tadienamine,651 2-alkynylanilines,652 enynes,648,653,654 ene- excess (Scheme 81b).661 − diynes,655,656 allenynes,657,658 enallenes,659 661 2-alkenylpheny- In a reaction analogous to the transition-metal-catalyzed lisocyanides,662 1,7-enynes,663 enone diones,664 cyclohexadie- allylboration of carbonyls,434,438,667,668 the borylative coupling

9124 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 82. Boracarboxylation of Alkynes Catalyzed by a Copper(I)−NHC Complex

Scheme 83. Borylstannylation of Diphenyl Acetylene (right) and α,β-Unsaturated Esters (left)

Scheme 84. Aminoboration of (a) trans-β-Methyl Styrene and (b) Unactivated Terminal Alkenes

Scheme 85. Enantioselective Multicomponent Reaction Reported by Hoveyda et al.

α β of alkynes with , -unsaturated ketones was catalyzed by a BpinBdan, is used instead of B2pin2, the inverse in nickel(0) phosphine complex.669 Hoveyda and co-workers were regioselectivity is observed, with the Bdan moiety present also able to apply their metal-free boryl addition methodology instead of the Bpin, in the desired product.674 Borylstannylation to the coupling of α,β-unsaturated ketones with benzalde- of activated alkenes was also conducted, under similar hyde,214,215 while a copper-catalyzed version of the borylative conditions, to form a simple route to obtain a range of vic- reaction has also been published.670 Furthermore, Ito et al. borylstannylalkanes (Scheme 83, left).384,675 Furthermore, Liao reported the copper-catalyzed exo-borylative cyclization of and co-workers went onto establish the synthesis of chiral alkenyl aryl ketones to obtain syn-1-aryl-2-(borylmethyl)- organostannanes from a range of different vinyl arenes in the cyclobutanol products with good selectivity.671 Borylative presence of copper(I) chloride and a chiral sulfinylphosphine coupling has been accomplished using alkynes, carbon dioxide, ligand.676 and a copper(I) N-heterocyclic carbene catalyst, the net result At a similar time, Miura and co-workers introduced a method being the boracarboxylation of an alkyne (Scheme 82).672 This for aminoborylation of alkenes, wherein the amine acts as the 677 route provides access to highly functionalized alkenes using electrophile and the boryl group serves as the nucleophile. readily available, affordable starting materials. This methodology is accomplished through the use of O- benzoyl-N,N-dialkylhydroxylamines as electrophiles and bis- 4.8. Boron-Element Additions Across Mutiple Bonds (pinacolato)diboron along with a copper(I)−phosphine A catalyst system derived from copper(II) acetate/PCy3 has catalyst (Scheme 84a). This method has since been extended been used to effect the borylative coupling of alkynes with tin to include methylenecyclopropanes678 and was also adapted to alkoxide species, resulting in a net borylstannylation (Scheme enable stereoselective aminoboration of bicyclic alkenes, 83, right).386,387,673 These boryl stannane products could be including oxa- and azabenzonorbornadienes.679 It was observed subjected to successive Stille and Suzuki coupling reactions, that some of the amino-borated products were very reactive allowing for the synthesis of a wide variety of substituted when Bpin was the boryl substituent. Therefore, B2pin2 was alkenes. Interestingly, if the masked diboron(4) reagent, exchanged for BpinBdan and the reactions were carried out

9125 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 86. (a) Three-Component Coupling of an Alkene, B2pin2, and Benzyl Chloride; (b) Carboboration of Alkynes Reported by Brown et al.; (c) Alkylboration of Alkenes Controlled by the Ligand Present in the Copper-Catalyzed Reaction

successfully under the same conditions, affording products 5. BORYL SUBSTITUTIONS containing the masked boryl moiety Bdan that were easier to 679 Nucleophilic boryl groups derived from diborons have been handle. Furthermore, unactivated terminal alkenes have been shown to displace a wide variety of leaving groups from various established as excellent substrates in the copper-catalyzed 680 substrates. These substitution reactions can involve the aminoboration reaction. It was noted that when a xantphos- formation of a new B−C bond in combination with the loss based copper catalyst was used, one regioisomer was observed, of a leaving group. This section also covers the somewhat more while in the presence of an NHC-based IPrCuBr catalyst and complex but incredibly useful substitutions of aryl C−X BpinBdan, the opposite regioisomer was formed (Scheme functionalities. 680 84b). 5.1. Coupling Reactions Different three-component transformations have also been applied to other synthetic pathways. Hoveyda and co-workers The coupling reactions and cyclizations discussed in this reported that 2-Bpin-substituted homoallylic alkoxides could be section involve the elimination of a leaving group, as opposed to coupling reactions discussed in section 4.8.385,387 The utility formed in a one-pot synthesis from the reaction of B2pin2, allenes, and aldehydes or ketones.681 Subsequently, they of combining borylation and coupling reactions, resulting in net applied the same concept to combine 1,3-enynes, aldehydes, carboboration, was highlighted in 2000 by Cheng and co- workers in the palladium-catalyzed borylative coupling of and B pin in the presence of copper(I) chloride and a chiral − 2 2 allenes with acid chlorides.685 687 A decade later, both bis-phosphine complex to form a 1,3-diol after oxidative ’ ’ workup (Scheme 85).682 Recently, Procter et al. demonstrated Tortosa s and Yoshida s groups reported their investigations into the copper(I)-catalyzed borylative coupling of al- a similar copper-catalyzed borylative cross-coupling reaction kynes688,689 or alkenes688 with alkyl halides (Scheme between allenes, B pin , and imines to afford branched α,β- 385−387,566 2 2 86a). Yoshida went on to expand this work by substituted-γ-boryl homoallylic amines, regio-, chemo-, and 683 reporting the carboboration of disubstituted alkenes and diastereoselectively. Lam and co-workers investigated the extending the different carbon electrophiles that could be copper-catalyzed borylative couplings of vinylazaarenes, B2pin2, utilized in the reaction, such as sterically congested 2,4,6- and N-Boc imines to form the boronic ester initially, before triisopropylbenzyl chloride and 1-naphthylmethyl chloride, as subsequent oxidation to generate azarene-containing amino 386,690 684 well as methyl iodide and cyclopropylmethyl bromide. alcohols. Brown et al. expanded the repertoire of these reactions when 4.9. Diagram Summarizing Section 4 his group showed that a similar system could be applied to facilitate the carboboration of alkynes and allenes with aryl iodides and B2pin2 to synthesize a range of vinyl boronate esters (Scheme 86b).691 Furthermore, Fu, Xiao, and co-workers established that changing the ligand altered the regioselectivity of the copper-catalyzed carboboration reaction between 692 alkenes, alkyl halides, and B2pin2 (Scheme 86c). If a combination of CuCl and Xantphos was used, the anti- Markovnikov product was obtained; however, if Cy-Xantphos was used, the slight change in ligand facilitated the formation of the Markovnikov product.692 Cooperative palladium and copper catalytic systems were also shown to be very useful in several different carboboration reactions of alkenes. Nakao and Semba reported the arylboration of vinyl arenes and methyl crotonate with B2pin2 and a range of aryl halides to obtain 2-boryl-1,1-diarylethanes

9126 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 87. (a) Borylative Cyclization with Phosphate Elimination; (b) Borylative Coupling Using Allenes, B2pin2, and Allyl Phosphates

and an α-aryl-β-boryl ester,693 while Brown et al. used the Scheme 88. (a) Alkylboration of Alkynes to Form Cyclic syn synergistic catalytic system to couple alkenes and B2pin2, under Alkenylboronates with Excellent Regio- and -Selectivity; high levels of diastereocontrol, with either aryl or vinyl (b) Domino Heck/Borylation Route To Form Indolinone-3- bromides694 or 1,2-disubstituted styrenes.695 This was methyl Boronate Esters expanded on further by Liao and co-workers to discover the first example of catalytic enantioselective intermolecular 696 allyboration of styrenes with B2pin2 and allyl carbonates. Very recently, the arylboration of bicyclic alkenes was reported, continuing the growing number of groups investigating these remarkable synthetic pathways to multifunctionalized al- kenes.697 In these reactions, an alternative route to β-aryl alkylboronates was observed involving a palladium-catalyzed arylboration of norbornene or norbornadiene with B2pin2 and a range of aryl bromides or iodides.697 Intramolecular versions of this reaction have also been 705 reported, resulting in cyclizations.566,698 Ito and co-workers copper-catalyzed oxyboration of unactivated terminal alkenes examined the copper-catalyzed reaction of allylic phosphates or a cascade reaction with an initial borylation reaction followed with B pin , resulting in boryl addition, cyclization to give a 3- by an ortho-cyanation and then a Cope rearrangement to give 2 2 706 membered ring, and elimination of the phosphate group the desired product. A similar cascade reaction has also been 699 reported for the synthesis of 1,2-bisfunctionalized arenes via the (Scheme 87a). The reaction is believed to proceed through a 707 borylcuprate intermediate, with the CCbondofthe borylation and ortho-amination of aryl iodides. substrate inserting into the copper−boron bond, followed by 5.2. Allylic and Propargylic Substitutions an intramolecular nucleophilic substitution of the organo- Allylic acetates can be displaced through nucleophilic attack by cuprate for the phosphate group. More recently, Tsuji et al. a boryl group in a palladium-catalyzed borylation reaction, reported the borylative allyl−allyl coupling of allenes with although in the initial report allyl−allyl coupling was also 708 B2pin2 and allyl phosphates under similar conditions (Scheme observed (Scheme 89a). The reaction is, nonetheless, 700 87b). It is proposed that an allyl copper species generated in synthetically useful, as the addition of an aldehyde or imine the reaction reacts with the allyl phosphate present to form to the reaction mixture results in near quantitative yields of the 700 various boryl-substituted 1,5-dienes. This combination of allylboration product and no allyl−allyl coupling, as the reagents was also utilized by Hoveyda and co-workers to borylation product is consumed as it is generated.709 The use synthesize trisubstituted chiral alkenes initially, before using the of a chiral diboron source allowed for moderate enantiose- methodology as part of the total syntheses of both rottnestol lectivity,710 while a more recent publication reported the use of 701 and herboxidene. Furthermore, the conditions were adapted zerovalent nickel or palladium(II) catalysts in the same to build a new subset of reactions to facilitate the carboboration reaction.711 Borylation of oxo-2-alkenyl acetates resulted in of alkynes with B2pin2 and allyl phosphates to obtain a range of the generation of oxo-2-alkenyl boranes, which can undergo an boron-substituted 1,4-dienes.702 intramolecular cyclization reaction (Scheme 89b).712 In 2015, Ito and co-workers reported further borylative Conjugate additions resulting in the displacement of an cyclizations by using silicon-tethered alkynes to facilitate an acetate have also been shown to occur using unsaturated intramolecular alkylboration, in the presence of a copper acetates.713 These products can be converted in situ to bench- catalyst to obtain a range of cyclic alkenylboronates (Scheme stable trifluoroboronates or added to an aldehyde, resulting in 88a).703 An interesting report by Eycken et al. combined a allylboration (Scheme 90).714,715 The use of a chiral diboron or Heck and borylation reaction to provide a synthetic route to a chiral auxiliary group resulted in low degrees of indolinone-3-methyl boronate esters, which could overcome enantioselectivity. several hurdles, such as the direct Miyaura reaction of the Copper(I)-catalyzed conjugate additions of boryl groups to product of the hydroarylation of the alkene (Scheme 88b).704 allyl carbonates are also possible, including asymmetric variants, Two reports have recently been published which demonstrate resulting in elimination of the carbonate group and the that similar reactions can be conducted to carry out either a generation of an enantiomerically enriched allylboronate

9127 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 89. (a) Palladium-Catalyzed Substitution of an Allylic Acetate for a Boryl Group; (b) Cyclization of Oxy-2-Alkenyl Acetates via Boryl Group Substitution

Scheme 90. Allylboration via Boryl Group Substitution

Scheme 91. (a) Reaction of Allylacetates with B2pin2, Followed by Allylboration and Oxidation; (b) Copper-Catalyzed Borylation of Propargylic Carbonates To Synthesize Multisubstituted Allenyl Boronates

− product.716 718 A more user-friendly system was subsequently borylation of allenes with a coupling reaction between reported involving catalytic CuCl and stoichiometric NaOtBu propargyl carbonates and aryl iodides. It was postulated that along with an auxiliary ligand, instead of a catalytic amount of the cross-coupling proceeds through an alkenyl boronic acid CuOtBu, which is difficult to handle due to its extreme intermediate.726 sensitivity to air and moisture.719 A palladium-catalyzed Allyl chlorides and acetates can be displaced with a boryl 30,31,711 variation of this reaction was used in the total synthesis of group in a palladium-catalyzed reaction with B2pin2. the Phytophthora universal mating hormone α1(section Allyl boronates prepared in this fashion have been used for the 4.2).571 Additionally, aldehydes have been added to the development of a new allyl−propargyl coupling reaction to reaction mixture, allowing for one-pot asymmetric desymmet- prepare 1,5-enynes.711 Furthermore, 1,6-enynes have also been rization of meso-2-alkene-1,4-diols and generation of asym- synthesized by the borylation of allylic carbonates containing an 720 654 metric homoallylic alcohols (Scheme 91a). Recently, Ito, additional alkyne group. If Pd(OAc)2 and an NHC, Sawamura, and co-workers extended this work in a novel IMesHCl, is used, the reaction will form the desired 1,6- α γ approach to form -chiral linear or carbocyclic (E)-( - enyne; however, if the ligand is changed to PCy3 then an alkoxyallyl) boronates via the borylation of allyl acetals with unprecedented borylative cyclization reaction is observed, 721 B2pin2. which is similar to the cyclization reactions discussed previously A reversal of regioselectivity was observed with γ-silylated (section 4.7.)654 allylic carbonate substrates due to a favorable interaction Ito, Kunii, and Sawamura published a report on the between the organocuprate and the silyl groups.722 The reversal enantioconvergent synthesis of substituted cyclic hydro- in regioselectivity leads to an intramolecular cyclization, carbons.727 A racemic mixture of a starting material reacted producing cyclopropane derivatives. Nucleophilic attack by a with B2pin2 in the presence of a chiral copper(I) catalyst. The boryl group on (E)- and (Z)-homoallylic sulfonates also leads chiral catalyst promotes facial selectivity, resulting in excellent to a stereoselective intramolecular cyclization reaction, which enantioselectivity in the product (Scheme 92). Alternatively, can even produce cyclobutane derivatives.723 Additionally, benzaldehyde could be added to the reaction mixture, resulting propargylic carbonates were investigated by Ito and co-workers in an asymmetric allylboration reaction. in a similar system to obtain allenyl boronates in good to McQuade and co-workers used a similar concept, working excellent yield (Scheme 91b).724 The utility of these substrates with their bulky copper(I)−NHC catalyst to promote facial was further investigated by Szabóand co-workers for the synthesis of allenyl and propargylic boronate esters.725 By using Scheme 92. Enantioconvergent Synthesis of Cyclic Boronate a combination of Pd and Cu or Pd and Ag in the absence of a Esters strong base, an allenyl boronate could be obtained stereo- ′ selectively via an SN2 mechanism, while the formation of propargylic boronate esters was observed when the copper salt was exchanged from CuI to CuCl.725 The same group went on to extend the reactivity of the reactions by incorporating the

9128 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 93. Stereoconvergent Synthesis of Boryl Alkenes through Phenolic Substitution

Scheme 94. (a) Boryl Substitution of Allylic Alcohols Catalyzed by a Palladium(II)-Pincer Complex; (b) Further Reactivity of the Initially Synthesized Allyl Boronates; (c) Borylative Substitution of an α,β-Unsaturated Alcohol and Ring-Closing Metathesis

selectivity in the borylation of E/Z mixtures of allyl aryl ethers, experimental studies revealed that transmetalation of B2pin2 eliminating the phenoxy group in the stereoconvergent with a palladium(II) allyl intermediate is shown to be slow and synthesis of chiral allylboronates (Scheme 93).728 The to proceed with high stereoselectivity, while the reductive mechanism was subsequently studied in more detail, and a elimination step to afford the allyl−Bpin product is fast.735 The primary kinetic isotope effect was observed for MeOH, generation of allylboronic acids in this fashion can be further consistent with a transition state involving a proton transfer exploited in the synthesis of homoallyllic alcohols through from methanol to the aryloxy leaving group.729 Furthermore, allylboration of α-amino acids via a 4-component coupling the same system was used in the first step of the asymmetric reaction.732 Using a SCS−palladium(II) pincer complex for the reaction to synthesize, selectively, the syn- or anti-1,2-diols.730 same reaction, the authors discovered that different solvent Szabóand co-workers discovered that a boryl group from mixtures can result in a complete reversal of regioselectivity 736 B2(OH)4 or B2pin2 substituted for allylic alcohols in a (Scheme 94b). This was rationalized by further rearrange- − palladium-catalyzed reaction (Scheme 94a and 94b).731 734 ment of the alkoxyborate species in the absence of methanol The authors found that the solvent mixture was key to (this species reacted readily with methanol). Thus, allyl promoting the reaction. Some methanol is required for high boronates generated could also be coupled with hydrolyzed yields and activity, but too much methanol resulted in acetals resulting in regio- and stereochemically pure homo- decomposition of the substrate, suggesting a mechanism allyllic alcohols.737 The allylboration of unsaturated aldehydes involving the alcohol. In some cases the authors added a or acetals left two carbon−carbon double bonds in the product, catalytic amount of acid to promote the reaction. Further which could undergo ring-closing metathesis using a Hoveyda−

9129 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Grubbs catalyst (Scheme 94c).738 This was expanded to step taking place and therefore was not as affected by the steric incorporate ketones by initial synthesis and isolation of allyl hindrance present in the substrate.749 boronic acids from allylic alcohols and B2(OH)4 followed by In 2009 Marder, Steel, Liu, and co-workers demonstrated the reaction with a range of ketones at ambient temperature in first copper-catalyzed borylation of primary and secondary alkyl 739 THF or CHCl3. halides and tosylates for boryl groups using diboron(4) In 2016, a transition-metal-free borylation of tertiary allylic compounds.750 The reaction of 1-bromo-6-chlorohexane alcohols was reported by Fernandeź and Szabo.́740 It was showed selective substitution of the bromide using 1.5 equiv − − postulated that the Lewis base acid adduct [B2pin2OMe] of B2pin2, while 3 equiv along with slightly more harsh reaction once more played an important role in the allylic borylation conditions and NBu4I yielded the bis-borylated product. reaction (vide supra). Interestingly, they also demonstrated that Interestingly, the authors note that 6-bromohex-1-ene under specific conditions, 1,2,3-polyborated products were singularly underwent borylative cyclization rather than generated through tandem allylic borylation and diboration substitution (Scheme 96b). 740 reactions. Nearly simultaneously, Ito and co-workers published similar 5.3. Alkyl Substitutions results with a chiral copper(I) catalyst system, which afforded 751 In the presence of a zerovalent palladium catalyst, a phosphine high diastereoselectivities in some cases. Ito went on to report that alkyl halides bearing terminal carbon−carbon ligand, and potassium acetate, Miyaura and co-workers 752 discovered that allyl and benzyl halides could be borylated double bonds could undergo selective boryl substitution. It 30,31,741 was observed that by altering the catalytic system for this with B2pin2 (Scheme 95). Zerovalent palladium-based reaction, the selectivity of the reaction could be changed from 698 Scheme 95. Borylative Substitution of Alkyl Halides borylative cyclization previously observed (section 5.1)to boryl substitution.752 Deng et al. also showed that amino acids could be used as ligands in the copper-catalyzed borylation of primary and secondary alkyl bromides.753 Copper nanoparticles have also been shown to catalyze the borylation of primary and 754 secondary alkyl halides with B2pin2 by both the Chung and 755 catalysts continued to be investigated in the borylation of the Xu groups. primary alkyl halides by Biscoe and co-workers,742 while Shi et Alkyl chlorides are still very unreactive in many copper- al. recently published the direct borylation of benzyl alcohols in catalyzed borylation reactions; however, one method has been the absence of a base.743 In this case, it was proposed that the previously reported for the borylation of primary alkyl chlorides 3 reaction went through a previously unobserved sp C−O bond by Gong, Jiang, and Fu using B2pin2, a rhodium catalyst and 2 activation facilitated by the palladium catalyst present.743 equiv of NaOtBu, proving that these substrates under the right 756 Molander and co-workers further demonstrated the utility of conditions undergo borylation successfully. these reactions in the borylation of ethers744,745 and hetero- The list of inexpensive metals available to catalyze this cycles746,747 (Scheme 96a). A similar reaction was published by reaction has been growing with both zinc- and iron-catalyzed Fu et al. with a nickel(II) bromide diiminopyridine catalytic borylation reactions of alkyl halides being investigated. In 2014, system, which included tertiary alkyl halides (although not Marder and co-workers reported the first zinc-catalyzed unactivated alkyl chlorides).748 The reactivity order in the borylation of primary, secondary, and some tertiary alkyl reaction between primary, secondary, and tertiary alkyl halides halides.757 This was the second system that worked successfully was shown to be the opposite of that previously observed in to borylate tertiary alkyl halides, with only the nickel system nickel-catalyzed alkyl−alkyl cross-coupling.748,749 This interest- discussed above having been reported previously.748,757 ing reaction observation was studied in more detail using DFT Following this, tertiary alkyl halides were also shown to 749 758 calculations by Lin and Marder et al. It was discovered that undergo borylation by Cook et al. In this reaction, Fe(acac)3 in the cross-coupling reaction, reductive elimination was rate and TMEDA were observed to facilitate the borylation of limiting, as the steric hindrance from the tertiary alkyl halides activated and unactivated alkyl halides using an excess of both increased the barrier in this particular step. However, the rate in B2pin2 and ethyl magnesium bromide. Furthermore, benzylic the borylation reaction was determined by the atom transfer and allylic chlorides, tosylates, and mesylates also underwent

Scheme 96. (a) Synthetic Route To Forming Potassium 2-(Trifluoroboratomethyl)-2,1-borazaraonaphthalenes and Subsequent Cross-Coupling Reaction with Aryl Chlorides; (b) Boryl Substitution of Alkyl Halides (right) and Borylative Cyclization of 6- Bromo-hex-1-ene (left)

9130 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review borylation, affording moderate to good yields of the desired has proven to be a valuable and widely used synthetic product.758,759 A few months later, Bedford and co-workers also methodology, the reaction can suffer from poor regiocontrol. reported a similar methodology using an iron(II)−chloride- To overcome this issue there has been considerable interest in based catalyst with phosphine ligands instead to borylate a the boryl substitutions of other functional groups on an range of alkyl halides with B2pin2 and exchanging the Grignard aromatic ring using B2pin2. for tBuLi.233 The same group recently reported that cyclic 5.5.1. Aryl Halides. Aryl halides are widely available and sulfamidates act in a similar way to alkyl halides and are generally affordable reagents and thus were an excellent target successfully borylated under slightly altered conditions to for the borylation reaction. Borylations using basic organo- obtain (β- and γ-aminoalkyl)boronate esters, extending the metallic reagents, such as organolithium and Grignard reagents, substrate scope of the well-established copper-catalyzed are well known; however, these reactions suffer from poor 760 borylation reactions. functional group tolerance. In rare cases, B2pin2 is used as the 5.4. Alkenyl Substitutions boron reagent in stoichiometric reactions with organolithium 769,770,782 In the early 2000s, Miyaura reported the first borylations of species (see Scheme 98 for example, section 5.4). The fi alkenyl triflates and halides using palladium(II) chloride rst report of the borylation of aryl halides using diboron(4) − phosphine catalysts and a base additive (Scheme 97).761 764 compounds was from Miyaura and co-workers in 1995 (Scheme 100)783 in which a palladium complex catalyzed the Scheme 97. Borylation of Alkenyl Halides and Triflates reaction in the presence of KOAc. The reported borylations of aryl iodides and bromides proved to have much more functional group tolerance than the synthesis of aryl boronates using organometallic reagents, with aldehyde, ketone, ester, imine, or nitrile functional groups being tolerated. This reaction has found widespread use in synthetic chemistry for the production of a vast array of aryl boronate esters. Experimental783 and computational784 studies of this reaction Later, the borylation of alkenyl triflates was used in the have suggested that the catalytic cycle begins with oxidative preparation of 4-aryl-tetrahydropyridines765 and tetrasubsti- addition of the C−X bond at the zerovalent palladium center. tuted alkenes.766 Xue et al. recently adapted this further to Palladium(II) catalyst precursors can be used for the reaction; provide a more efficient route to forming 3,6-dihydro-2H- however, they are reduced in situ by the diboron and base pyran-4-boronic acid pinacol esters in a one-pot synthesis.767 additive to catalytically active zerovalent palladium com- 785 Alkenyl carbamates have been borylated using [NiCl2(PCy3)2] plexes. Salt metathesis with KOAc generates an aryl as a catalyst in the presence of K3PO4 and two additional palladium acetate species which subsequently undergoes 768 − equivalents of tricyclohexylphosphine. transmetalation with B2pin2. Finally, the aryl Bpin product is Lithium salts generated from terminal alkenyl halides and generated through reductive elimination (Scheme 101). The dihalides were borylated using a variety of diborons, producing presence and nature of the base is vital to achieving high yields 769,770 gem-diboryl alkenes. The gem-diboryl alkenes could be and selectivities. The use of stronger bases was found to stereoselectively coupled with aryl halides sequentially, − 771 promote Suzuki Miyaura coupling of the aryl boronate ester producing tetrasubstituted alkenes (Scheme 98). Another product with the aryl halide starting material (a symmetrical report found that gem-diboryl alkenes reacted with 1-bromo-1- − 772 Suzuki Miyaura coupling reaction). As an interesting aside, lithioethene to form 2,3-bisboryl-1,3-dienes. Braun and co-workers also demonstrated that the oxidative The borylation of a vinyl bromide-containing heterocycle has addition products could be obtained from the initial been used in the preparation of a novel cyclic amino acid.773 stoichiometric reaction of SF5-functionalized aryl bromides Other nitrogen-containing heterocycles were also tolerant of i and iodides with Pd(P Pr3)2, followed by the reaction of the the reaction conditions required for the borylation of alkenyl corresponding palladium fluoride complex with B pin , 774 fl 2 2 halides. Substitution of Bpin for a tri ate group has been generating the desired aryl boronate ester.786 used in the construction of a polymer containing a novel This borylation reaction has been used to convert polymer- extended π-conjugated system.775 Alkenyl iodides could be 123 bound aryl halides to aryl boronates, which have been used for borylated with B2neop2 and subsequently reacted with Na Ito 123 776 the preparation of polymer-bound biaryls that could sub- form I-substituted alkenes. In a similar fashion, vinyl sequently be cleaved from the polymer.787,788 Lee and Kelly sulfonates and phosphates of substituted tetrahydropyridines borylated aryl halides and then oxidized the product, replacing and other N-heterocyclic compounds were borylated and used 789 − 777,778 the boryl group with an hydroxyl group, whereupon the in Suzuki Miyaura coupling reactions (Scheme 99). phenols were loaded onto a resin for solid-state synthesis. 5.5. Aromatic Substitutions Giroux and co-workers reported a two-step, one-pot Given their utility in Suzuki−Miyaura cross-coupling reactions, synthesis of biaryl species through the addition of an aromatic the selective synthesis of aryl boronate esters is extremely electrophile (aryl halide or pseudohalide) and sodium 779−781 − important. While borylations of C H bonds with B2pin2 carbonate following completion of the borylation reaction

Scheme 98. Borylative Route to Tetrasubstituted Alkenes via Borylation and Sequential Suzuki Cross-Coupling Reactions

9131 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 99. Borylation and Suzuki−Miyaura Coupling of Substituted Tetrahydropyridines

Scheme 100. Palladium-Catalyzed Borylation of Aryl Halides et al. reported a study using a similar method, which facilitated the synthesis of several α,ω-pinacol boronates.811 The Suzuki− Miyaura reaction could subsequently be carried out without any purification steps required. In comparison with the analogous Stille coupling reaction, a one-pot borylation/Suzuki−Miyaura reaction resulting in the Scheme 101. Proposed Mechanism for the Palladium- polymerization of bis-bromo thiophene-based monomers was Catalyzed Borylation of Aryl Halides found to be superior, producing higher quality, more homogeneous polymers.812 The borylation/Suzuki−Miyaura sequence outperformed alternative coupling methodologies in the synthesis of hippadine and pratosine, two naturally occurring biologically active molecules, although HBpin was ff found to be a more e ective borylating agent than B2pin2 in this case (Scheme 103).813 Additionally, the one-pot borylation of aryl halides and subsequent symmetrical Suzuki−Miyaura cross-coupling has proven to be a useful synthetic technique.814 However, − (Scheme 102).790 The one-pot borylation/Suzuki−Miyaura unintended symmetrical Suzuki Miyaura reactions have reaction could also be used to generate polymers with occurred during the borylation of 8-bromoquinoline derivatives by [PdCl2(dppf)], even using KOAc, which is normally not a − sufficiently strong base for the Suzuki−Miyaura coupling Scheme 102. One-Pot Borylation/Suzuki Miyaura Coupling 815 Reaction without Added Catalyst reaction. Zhang and co-workers successfully averted the symmetrical Suzuki−Miyaura reaction to allow the borylation n of 8-bromo- and chloroquinolines by using Pd2(dba)3/ BuPAd2 (1:1.5 ratio) as the catalyst, DMAc as the solvent, and KOAc as the base additive.816 Under these conditions, the desired boronic acid was isolated following an aqueous workup or used 816 dihaloarenes.791 The borylation of aromatic C−X bonds has in a one-pot Suzuki−Miyaura coupling. Firooznia and co- been exploited to prepare substituted porphyrin derivatives workers demonstrated that the chiral center in phenylalanine (including polymers).792,793 Indeed, a BINAP/BINOL polymer derivatives was preserved through one-pot borylation and has also been prepared through borylation and subsequent Suzuki−Miyaura coupling reactions.817 Microwave heating Suzuki−Miyaura coupling.794 This ligand scaffold proved could also be used to promote the borylation of aryl halides effective for tandem asymmetric catalysis. A related BINAP- catalyzed by a palladium−NHC catalyst.818 based polymer was found to be similarly useful.795 Linking While the borylation of ortho-substituted aryl halides was other ligands, such as bis-terpyridine, can be accomplished initially found to be much more difficult than with less hindered using the palladium-catalyzed borylation of aryl halides with substrates, progress has been made by use of a Pd biphenyl 796−805 B2pin2 or B2neop2. Likewise, arylpyridines have been dicyclohexyl phosphine catalyst system, which was shown to borylated and subjected to Suzuki−Miyaura coupling.806 In outperform palladium species with more labile ligands, 807 808 819 related studies, thiopeptides and indazoles, dityrosine, and including PPh3. Some bulky aryl iodides can undergo 809,810 820 other polytyrosines were prepared using this borylation catalytic borylation using a [PdCl2(dppf)] catalyst system, and Suzuki−Miyaura coupling methodology. More recently, Jin while the Bedford palladacyclic precursor, in combination with

Scheme 103. One-Pot Synthesis of Hippadine via Borylation/Suzuki−Miyaura Cross-Coupling/Lactamization

9132 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 104. Synthesis of Biaryls by Borylation and Subsequent Suzuki−Miyaura Reactions, Catalyzed by a Palladacycle

biaryl phosphine ligands, gave a highly active catalyst system for was reported by Nechaev et al. for both the Miyaura borylation the borylation of bulky aryl bromides.821 and the one-pot, two-step homocoupling of aryl halides.841 Additional palladacyclic catalyst precursors have been shown Palladium nanoparticles could be employed as the catalyst for to be effective catalysts for the borylation of aryl halides and borylation and one-pot borylation/Suzuki−Miyaura cou- − subsequent Suzuki−Miyaura reactions (Scheme 104).822 826 pling.842,843 Palladium nanocrystals ligated by phosphines, Notably, two reports also demonstrated that aryl bromides can generated in supercritical CO2, proved to be good catalysts be generated and subsequently borylated in situ with N- for the borylation of aryl halides using 4-bromoanisole as a test bromosuccinimide as the brominating agent.823,824 substrate.844 Tricyclohexylphosphine and biaryl-dicyclohexyl- The use of [PdCl2(PCy3)2], instead of [PdCl2(dppf)], greatly phosphine-ligated nanocrystals proved to be most active. Solid- accelerated borylation reactions of aryl bromides and triflates supported palladium(II) bound to a silica surface through a and, notably, allowed the use of aryl chlorides as sub- phosphine ligand has likewise been used as a catalyst for the − strates.68,827 Buchwald later reported that if XPhos was used borylation of aryl halides.845 847 Although activity was high as the ligand, the palladium-catalyzed borylation of aryl even with bulky chloroaryl substrates, attempts to reuse the chlorides could be carried out at room temperature.828 This catalyst were unsuccessful.845 Palladium(II) bound to a enhanced reactivity is attributed to the bulk of the biaryl diphenyl phosphine-based silica support was found to be effective for the borylation of aryl halides in batch and flow phosphine ligand and the ability of KOAc to displace the − palladium−arene interaction. Kwong and co-workers later reactor conditions.848 850 reported a PPh2-based phosphine ligand which, when Tetrahydroxydiboron (B2(OH)4) is an alternate boron combined with Pd2(dba)3, is a highly active and general source for the borylation of aryl halides, as demonstrated by catalyst system for the borylation of aryl chlorides and Molander and co-workers (Scheme 105), using a palladium(II) subsequent one-pot Suzuki−Miyaura coupling.829 Recently, Yu and co-workers tried to overcome some of the limitations in Scheme 105. Borylation Using B2(OH)4 and Conversion to these systems, such as high catalyst loading or further addition More Stable Trifluoroboronates of the catalyst to facilitate the subsequent coupling reaction. They established that some of these problems could be controlled by the use of 2-aryl indenyl phosphine ligands, which showed a very broad reaction scope in the palladium-catalyzed borylation/Suzuki cross-coupling reaction.830 Alternatively, monophosphine ligands have also been investigated to afford aryl boronates in high yields from the respective aryl chlorides.831 Furthermore, microwave radiation was useful in promoting the borylation of bulky aryl chlorides832 and accelerating reactions of aryl bromides.818,833 In 2003, Zhang and co-workers showed that palladium acetate, as the catalyst precursor for the borylation of aryl bromides, negated the requirement for phosphine ligands altogether by facilitating the formation of a palladium aryl acetate intermediate.834 A subsequent Suzuki−Miyaura cross- coupling was also carried out using this same catalyst system. phenethylamine catalyst precursor.851,852 Importantly, the use 835 836 Ionic liquids and polyethylene glycol have been used as of B2(OH)4 reduces the cost of the diboron reagent and means alternative solvent systems for the borylation reaction. that pinacol will not need to be chemically removed and Palladium-catalyzed borylation of a wide range of aryl bromides separated from the product following the reaction. Even aryl could be done in water at room temperature using a surfactant chlorides were subject to borylation using this protocol, and to protect the catalyst.837,838 A biphasic system was employed products were readily converted to aryl boronate esters or successfully for the borylation and subsequent Suzuki−Miyaura trifluoroborate salts. Interestingly, similar conditions were used coupling, showing enhanced reactivity for substrates containing as a part of a synthetic route to form MK-8876, which is a non- electron-withdrawing groups, heterocycles, or bulky substitu- nucleoside inhibitor developed to help treat the Hepatitis C ents.839 Different length chains of poly(ethylene)glycol have virus (HCV).853 One-pot borylation Suzuki−Miyaura coupling also been used to form novel NHCs, altering the behavior of reactions could also be achieved using this palladium(II) the ligands, which were investigated in both borylation and catalyst.854 Further research into these systems was also Suzuki−Miyaura catalysis.840 Recently, a solvent-free system conducted using a modified Pd catalyst and ethylene glycol as

9133 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review fi Scheme 106. (a) Generation and Borylation Reactions of B2(diol2) Species in Situ; (b) Borylation and Transesteri cation Reaction To Produce a Chiral Dienophile for Diels−Alder Reactions

the additive. The presence of the ethylene glycol led to a metal options have increased in popularity over the past decade. reduction in the amount of diboron required as well as faster A synergistic catalytic system utilizing palladium and copper reaction times and a larger substrate scope.855 A catalytic catalysts was reported by Kiatisevi and co-workers.863 This system comprising of [NiCl2(dppp)] (dppp = 1,3-bis- system, which includes added phosphine ligand and excess base ff (diphenylphosphino)propane) and 2 equiv of PPh3 proved to in the form of Cs2CO3, was found to be e ective for the be more tolerant of carbonyl groups and functioned at lower borylation of aryl iodides with B2pin2 in air at room temperatures than the corresponding palladium systems.856 An temperature. The palladium catalyst is believed to activate the alternative nickel-based catalyst, [NiCl(o-tolyl)(TMEDA)], was aryl carbon−iodide bond, while the copper catalyst serves as a also investigated under similar conditions in the reaction transmetallating reagent. However, Marder and co-workers ff between aryl bromides and B2(OH)4,a ording a moderate to reported a pure copper-catalyzed borylation of aryl bromides 231 good yield of the desired aryl boronate ester, thus indicating and iodides with B2pin2 and B2neop2 at room temperature. A n t that nickel catalysts are a good alternative to the previously copper(I) salt (10 mol %), P Bu3 (13 mol %), and KO Bu (1.5 used palladium complexes.857 equiv) make up the catalytically active mixture. IPrCuOtBu also n The Molander group also demonstrated the use of catalyzes the reactions but is slower than the P Bu3 system. A tetrakis(dimethylamino)diboron (B2(NMe2)4) as the borylating ligand-free variation of this method has also been published, agent for aryl chlorides and bromides.858 Additionally, which demonstrated the borylation of aryl bromides and B2(NMe2)4 was also used as the starting diboron(4) compound iodides, as well as benzyl halides, though yields for these in the borylation of aryl bromides by an in situ generated reactions were only moderate.864 Ishizuka and co-workers 859 B2(diol)2 (Scheme 106a). Complete conversion to the continued to investigate NHC ligands and showed that a − B2(diol)2 from B2(NMe2)4 and 2 equiv of diol was observed in bicyclic NHC CuCl complex could successfully borylate a solution, and the reactivity of the borylated products in a range of aryl halides but at slightly decreased yields compared subsequent (one-pot) Suzuki−Miyaura reaction differed, based to the seminal copper−phosphine system reported.865 on the nature of the diol used. An elegant synthesis of a 2- Nickel has already been discussed briefly in the borylation of boronoacrylanilides from Kennedy and Hall utilized the aryl halides with B2(OH)4 but has also been shown to catalyze 860 borylation of an aryl bromide with B2neop2. The aryl the borylation of a large range of aryl chlorides using B2pin2, fi 866 boronate ester could undergo transesteri cation with chiral [NiCl2(PMe3)2], TMSOCH2CF3, and CsF. Furthermore, diols. Several subsequent steps produced monosubstituted nickel catalysts have been utilized in several reactions by Darcel terminal alkenes (Scheme 106b), which showed low degrees of et al., forming aryl boronates in moderate yields.867,868 diastereoselectivity as dienophiles in ensuing Diels−Alder However, due to the toxic properties of nickel, alternative reactions. The borylation of aryl iodides with B2neop2 was transition metals were investigated further. reported to be a useful step in the preparation of isotopically The groups of Takita and Uchiyama discovered the zinc(II)- enriched aryl iodides861 by subsequently reacting the aryl catalyzed borylation of aryl iodides and aryl bromides using 123 123 boronate ester with Na I to produce the I-enriched aryl pyrophoric Et2Zn as catalyst with B2pin2,B2cat2, and B2neop2 in iodide. the presence of excess NaOtBu, with THF as the solvent.599 Nonsymmetrical diboron(4) compounds have also been Marder and Bose applied a similar method to their previously investigated in the borylation of aryl halides. Li et al. published discussed zinc-catalyzed borylation of alkyl halides (section 5.3) the use of BpinBdan in the borylation of both aryl bromides in the borylation of aryl halides,757 thus providing a novel zinc- and chlorides in the presence of a palladium catalyst. The catalyzed reaction with B2pin2 under much milder condi- formation of the ArBdan product illustrated the potential of the tions.869 During their investigations, an interesting subset of Bdan group to form a range of masked boronic acids.862 reactions was discovered when the ligand was changed to 4,4′- Palladium catalysts have facilitated a myriad of borylation di-tert-butyl-2,2′-bipyridine (dtbpy). This ligand facilitated the reactions; however, alternative more affordable and abundant borylation of not only the C−X group but also the adjacent C−

9134 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

H group to form a range of 1,2-bis(Bpin) benzenes (Scheme 4-boryltetrafluoropyridine is obtained.877 Subsequently, Zhang 107).870,871 et al. reported that N-heterocyclic-substituted polyfluoroarenes could also undergo ortho-selective C−F bond borylation with Scheme 107. Zinc-Catalyzed Borylation of Aryl Halides To B2pin2 using [Rh(COD)2]BF4. Mechanistic studies suggested III Afford 1,2-Bis(Bpin) Benzenes that a rhodium(III) hydride complex, [(H)Rh Ln(Bpin)], was a key intermediate in the catalytic cycle.878 Several research groups have also exchanged the rhodium catalysts for nickel species. Martin’s and Hosoya’s groups simultaneously reported the borylation of monofluoroarenes (Scheme 108b). Martin et al. used a combination of Ni(COD)2 fl and PCy3 with B2neop2 to borylate aryl uorides (Scheme 108b, right), while Hosoya and co-workers used the same catalyst but with the additional help of CuI to borylate fl 879,880 mono uoroarenes with B2pin2 (Scheme 108b, left). As an interesting aside, several reports have shown that B2pin2 can also be used as a cocatalyst in copper-catalyzed Interestingly, several reports have recently been published on fluorination/fluoroalkylation reactions. Szabóet al. reported metal-free borylations of aryl electrophiles. Initially, Zhang and − fl that B2pin2 may accelerate the C H tri uoromethylation of co-workers published results on the borylation of aryl iodides quinones,881 while a similar effect was observed by Zhang and but not aryl bromides or chlorides with B2pin2, mediated by fl 882 872 ̃ co-workers in the amino uorination of styrenes. Further- Cs2CO3 in methanol. More recently, Muniz et al. established more, Ding, Wang, and co-workers observed that stoichio- that diaryliodonium salts could also be combined with B2pin2 in metric amounts of B2pin2 were required to facilitate the methanol to provide a metal-free borylation route to aryl transition-metal-catalyzed C−F activation in the iridium- boronate esters in moderate to good yield.873 Furthermore, Li ffi fl catalyzed synthesis of symmetrical diaryl ethers from reported an e cient continuous ow photolytic borylation of fl 883 874 uorarenes. aryl iodides and bromides in aqueous solution. Several − ff − ff 5.5.2. Aryl C O Electrophiles. Di erent C O bond- di erent diboron sources were examined including B2pin2, containing aryl electrophiles, such as aryl triflates, have been B2neop2, BpinBdan, and B2(OH)4, with the product from the successfully used as an alternative to aryl halides in borylation latter diboron source converted directly with KHF2 to generate fi 874 reactions. The rst such report came from Miyaura and co- the potassium aryl trifluoroborate. Subsequently, Larionov et 884 workers and conditions employed varied slightly from those al. showed that the photoinduced borylation of haloarenes, fl used initially in the halide substitution reaction, as dioxane was including electron-rich uoroarenes, with B2(OH)4 could be 875 used as a solvent and extra dppf ligand was required to prevent conducted under metal- and additive-free conditions. Under 68 catalyst deactivation and decomposition to palladium black. the same conditions, quaternary arylammonium salts were also 875 Two-step, one-pot reactions have been reported, which shown to undergo borylation. 790,884 814 incorporate an unsymmetrical or symmetrical Suzu- Finally, Marder, Perutz, and co-workers published a rare ki−Miyaura cross-coupling reaction after the borylation step. example of stoichiometric C−F borylation of pentafluoropyr- Symmetrical Suzuki−Miyaura coupling of naphthyl triflates has idine using a rhodium(I) complex. The reaction occurred at the been used as part of an approach to the synthesis of crisamicin 2- and 4-positions, and full conversion required 2 equiv of A, a compound which has previously demonstrated antiviral B cat to produce the borylated pyridine and a triborylrhodium − 2 2 properties.885 887 The use of B neop as the borylating agent, complex. Braun, Macgregor, and co-workers were able to 2 2 instead of B pin , was found to assist the borylation of ortho- develop a closely related catalytic reaction pathway for C−F 2 2 substituted aryl triflates.888 A 2-phenyl-4-oxazolylboronate bond activation using a rhodium(I) boryl catalyst (Scheme 876 could be prepared by borylation of the corresponding 108a), while further investigations reported several years triflate.889 Aryl triflates have been reported to be converted later showed that the regioselectivity of the borylation reaction to aryl iodides through borylation with B neop and subsequent of pentafluoropyridine is governed by the choice between 2 2 reaction with NaI and chloramine-T.890 B pin and HBpin. When B pin is used, the pyridine is 2 2 2 2 As an extension to this chemistry, aryl tosylates and borylated at the 2-position, while in the presence of HBpin, the mesylates can also be borylated. These reagents are particularly useful in that they allowed for the borylation of a large family of Scheme 108. (a) Borylation of an Aromatic C−F Bond; (b) alcohol substrates through an initial sulfonation step. Aryl Two Synthetic Pathways to Monosubstituted Aryl Boronate mesylates and tosylates also tend to be significantly more stable Esters and easier to isolate than their triflate counterparts. Kwong and co-workers first demonstrated that aryl mesylates and tosylates can be borylated with B2pin2, Pd(OAc)2, a phosphine ligand, and KOAc (Scheme 109).891 Other diboron reagents were used in this type of reaction; however, reactions with B2pin2 produced the highest yields. One-pot Suzuki−Miyaura coupling was also possible with this system. A 2011 report from Shi and co-workers documented the ability of bis(tricyclohexylphosphine)nickel(II) chloride to 768 catalyze the borylation of aryl carboxylates with B2neop2. t Stoichiometric amounts of NaO Bu or K3PO4 and two additional equivalents of tricyclohexylphosphine were required

9135 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Scheme 109. Palladium-Catalyzed Borylation of Aryl Mesylates and Tosylates

for optimal reactivity. Aryl carbamates were found to be metric reactions of several isolated anionic Lewis base adducts especially active, partly due to their stability to hydrolysis.768 with different substituted aryl diazonium salts.232 The adducts Additionally, aryl 2-pyridyl ethers892 and aryl pivalates893 were reacted successfully with the pure aryl diazonium salts, showing successfully borylated by diboron(4) reagents using a rhodium that the in situ-formed anionic adducts could play a vital role as catalyst. the nucleophilic boryl source in the reaction.232 Furthermore, Finally, the simplest reagent in the phenol series was also several research groups attempted to improve the reaction established as a good alternative to aryl halides. In 2014, Martin conditions by one group observing that the borylation could be 908 and co-workers observed that aryl ethers could undergo ipso- conducted in water at room temperature with B2(OH)4, borylation in the presence of a nickel catalyst, Ni(COD)2, and a while another group showed that the reaction with either 894 909 phosphine ligand, PCy3, while a similar system was used by bisboronic acid or B2pin2 could function in pure methanol. Chatani et al. to form a one-pot synthesis in a range of As an interesting aside, Toste and co-workers have recently symmetrical biaryls.895 In these reactions, the phosphine ligand shown that an aryl diazonium salt can also be used, alongside α- fi has been exchanged for an NHC, and it is noteworthy that no ole ns and B2pin2, in enantioselective 1,1-arylborylation of biaryl homocoupling product was observed under identical alkenes to synthesize chiral benzylic boronate esters.910 conditions to those used by Martin and co-workers.895 In 2014, Tobisu, Nakamura, and Chatani established a 5.5.3. Aryl C−N Bonds. Perhaps not surprisingly, aryl synthetic pathway to aryl boronate esters from N-aryl amides fl 911 diazonium tetra uoroborates serve as electrophiles for the and B2neop2 (Scheme 111a). The reactions required a palladium(II)-catalyzed borylation reaction.896 The reaction was found to proceed in the absence of a base at 60 °C,896 Scheme 111. Nickel-Catalyzed Borylation of N-Aryl Amides though addition of an NHC ligand allowed the reaction to (a) and Aryl Ammonium Salts (b) with Diboron(4) proceed efficiently at room temperature.897 The reaction was Compounds later found to work in water at room temperature with CuBr as the catalyst and is tolerant of a wide variety of functional groups.898 More recently, aryl diazonium salts have been used in a transition-metal-free borylation reaction, mediated by PPh3, and conducted under mild conditions.899 The availability and relatively low cost of aryl amines compared to aryl halides gave an alternative substrate to be used in aryl borylation reactions.900 Wang and co-workers reported one-pot, two-step transformations of aryl amines to aryl boronates, which proceeded through the generation of the diazonium salt in a Sandmeyer-like fashion, followed by metal- free borylation.900,901 The reaction was optimized and shown to be quite general, while the proposed mechanism is believed to relatively high loading of Ni(COD)2 (10 mol %), imidazolium involve a single electron transfer step from a boryl anion salt (10 mol %), and NaOtBu (20 mol %) and heating at 160 911 formed in situ from the reaction of KOAc with B2pin2 (Scheme °C in toluene for 20 h. However, borylation of tertiary and 110).902,903 The same group continued to improve the benzylic amines remained unexplored until an investigation by Itami et al., in which they established that aryl ammonium salts Scheme 110. Metal-Free Borylation of Aryl Amines could undergo borylation in the presence of Ni(COD)2 and tri- n-butylphopshine in dioxane (Scheme 111b).912 Furthemore, the reaction scope was expanded to incorporate benzylic ammonium salts using a slightly different nickel catalyst, · 912 Ni(NO3)2 6H2O. Subsequently, a similar set of reactions was synthetic route by showing that aryl borylation can be reported by Shi and co-workers that could be used to borylate conducted on a larger scale and used this method to generate the same sp2 and sp3 C−N bonds.913 Indoline and 1,2,3,4- aromatics substituted with both stannyl and boryl function- tetrahydroquinoline were also investigated under the same alities, allowing for one-pot sequential Stille and Suzuki− conditions and shown to undergo ring opening via borylative − Miyaura coupling reactions.904 906 cleavage of the C−N bond.913 Additionally, enantioenriched Furthermore, diazonium salts were also generated in situ benzylic boronates were briefly mentioned in the same during the borylation of aryltriazene to form the respective aryl publication but extended upon in greater detail by a following boronate ester. Under identical conditions, the aryl triazene was report from Watson and co-workers.914 They demonstrated replaced by the aryl diazonium salt to obtain the same product that the catalytic combination of Ni(COD)2 and PPh3 could as previously synthesized.907 Additionally, Yamane and Zhu facilitate the borylation of secondary benzylic ammonium salts − proposed the formation of an [B2pin2F] adduct, which was to generate highly enantioselective benzylic boronates. necessary to facilitate the reaction.907 These findings were 5.5.4. Aryl Nitriles. Chatani, Tobisu, and co-workers analyzed further when Marder et al. examined the stoichio- pioneered the development of aryl nitrile borylation with

9136 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

915,916 B2neop2. In the presence of a rhodium catalyst, 2 equiv of Biographies B2neop2, and 1 equiv of base, aryl nitriles bearing a wide variety of functional groups can be selectively borylated (Scheme 112). Emily Neeve was born in Canterbury, United Kingdom. She received her MSci (Hons) degree in Chemistry from the University of Bristol, U.K., in 2009 and her Ph.D. degree at the same university in 2013 with Scheme 112. Rhodium-Catalyzed Borylation of Aryl Nitriles Professor Robin B. Bedford, working on iron-catalyzed cross-coupling reactions. During her Ph.D. studies she also spent 1 month at the École Normale Superieuré in Paris, France, with Professor Anny Jutand. Since 2013, she has been a postdoctoral fellow in Professor Todd Marder’s group at the Julius-Maximilians-UniversitatWü ̈rzburg, Germany, examining the chemistry of anionic diboron adducts and their role in borylation reactions. Steve Geier is a native of Sackville, New Brunswick. He completed his The proposed mechanism involves transmetalation of B2neop2 with the rhodium complex, followed by nitrile insertion into the B.Sc. degee at Mount Allison University and went on to pursue his rhodium−boron bond, E/Z isomerization of the imine, and Ph.D. degree at the University of Windsor under the supervision of Dr. then β-arene elimination. The mechanism of this reaction was Douglas W. Stephan. His Ph.D. work focused on “Frustrated Lewis recently investigated in greater detail by Liu et al.917 Pair” chemistry. He accompanied Dr. Stephan in his move to the 5.6. Diagram Summarizing Section 5 University of Toronto. Following completion of his Ph.D. degree in 2010, he spent 2 years as an NSERC Postdoctoral Fellow in the lab of Dr. Jeffrey R. Long at the University of California, Berkeley. He then returned to Sackville to help Stephen A. Westcott and has been a Research Associate of Dr. Stephen A. Westcott’s group at Mount Allison University since January 2013. His research interests include all things boron. Ibraheem Mkhalid was born in Jeddah, Saudi Arabia, in 1976. He received his B.Sc. degree from King Abdul Aziz University (1999) and his Ph.D. degree from Durham University (2006) for work on transition-metal-catalyzed borylation of C−H bonds carried out under the supervision of Professor Todd B. Marder. In 2006, he returned to King Abdul Aziz University as Assistant Professor in the Chemistry Department. Now, he is an Associate Professor at the same department. Steve Westcott was born in the 1960s somewhere around Tecumseh and received his Ph.D. degree from the University of Waterloo under the joint supervision of Drs. Todd B. Marder (now at Universitaẗ ̈ 6. CONCLUSION Wurzburg) and R. Tom Baker (now at the University of Ottawa) working on metal-catalyzed hydroborations. He was an NSERC PDF, While just synthetic inorganic curiosities for ca. 70 years, since where he spent 1 year at Emory University in Atlanta with Dr. Lanny the mid-1990s the profound impact of diboron(4) reagents on Liebeskind and more than 1 year working with Dr. Maurice Brookhart modern synthetic chemistry is undeniable. Further adding to at the University of North Carolina at Chapel Hill, NC. He has been at their synthetic utility of these reagents are the straightforward Mount Allison University since August 1995 and is currently a Canada methods to prepare these remarkable synthons, which are now Research Chair in Boron Chemistry. His research interests include commercially available in bulk quantities. With the develop- − − − catalysis and the synthesis and development of biologically active ment of the Suzuki Miyaura reaction, other C C and C boron and transition-metal compounds. element coupling reactions, as well as numerous functional group transformations, organoboronates have become ex- Todd Marder received his B.Sc. degree in Chemistry from M.I.T. tremely important synthetic building blocks. In addition to (1976) and his Ph.D. degree from the University of California at Los metal-catalyzed diborations, β-borylations, and C−H and C−X Angeles (1981), where he was a University of California Regents borylations, important recent developments include metal-free Intern Fellow. Following postdoctoral research at the University of catalytic borylations and new catalyst systems which can Bristol in England, he spent 2 years as a Visiting Research Scientist at function in water under an atmosphere of air. These user- DuPont Central Research in Wilmington. He joined the faculty at the friendly advances as well as the continuing development of University of Waterloo, Canada, in 1985 and in 1995 was awarded the entirely new applications will certainly help to ensure an Rutherford Memorial Medal for Chemistry of the Royal Society of important place for diboron(4) compounds in synthesis for Canada. He moved to the University of Durham in England in 1997 to many years to come. take the Chair in Inorganic Chemistry previously held by Ken Wade. In 2008, he received the RSC Award in Main Group Element AUTHOR INFORMATION Chemistry. In 2010, he was awarded a JSPS Invitation Fellowship, a Corresponding Authors Humboldt Research Award, and a Royal Society Wolfson Research Merit Award. In 2012, he accepted a Chair in Inorganic Chemistry at *E-mail: [email protected]. the University of Würzburg, Germany, a major center for boron and *E-mail: [email protected]. organometallic chemistry. In 2015, he was elected to the Bavarian Notes Academy of Sciences and was the recipient of the RSC Award in The authors declare no competing financial interest. Organometallic Chemistry. He holds or has held Visiting/Honorary/

9137 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161 Chemical Reviews Review

Distinguished Professorships in the United Kingdom, France, Hong (21) Simpson, P. G.; Folting, K.; Lipscomb, W. N. The Molecular − Kong, mainland China, and Japan and was the 2014 Craig Lecturer at Structure of i-B18H22. J. Am. Chem. Soc. 1963, 85, 1879 1880. ANU. He has served on the editorial boards of Organometallics, (22) Lewin, R.; Simpson, P. G.; Lipscomb, W. N. Molecular and Inorganic Chemistry, The Journal of Organometallic Chemistry, Crystal Structure of C2H5NH2B8H11NHC2H5. J. Chem. Phys. 1963, 39, − Polyhedron, Inorganica Chimica Acta, Applied Organometallic Chemistry, 1532 1537. (23) Corcoran, E. W.; Sneddon, L. G. Transition-Metal-Promoted The Canadian Journal of Chemistry, Crystal Engineering, etc. His diverse Reactions of Boron Hydrides. 4. A One-Step Synthesis of the Coupled research interests include synthesis, structure, bonding, and reactivity ′ − Cage Borane 1,2 -[B5H8]2. Inorg. Chem. 1983, 22, 182. of organometallic and metal boron compounds, homogeneous (24) Soderquist, J. A.; Brown, H. C. Simple, Remarkably Efficient ff catalysis, small molecule triggers of stem cell di erentiation, Route to High Purity, Crystalline 9-Borabicyclo[3.3.1]nonane (9- luminescence, nonlinear optics, liquid crystals, and crystal engineering. BBN) Dimer. J. Org. Chem. 1981, 46, 4599−4600. (25) The 2010 Nobel Prize was shared equally with Richard F. Heck ACKNOWLEDGMENTS and Ei-ichi Negishi. ̈ (26) Welch, C. N.; Shore, S. G.; Boron Heterocycles, V. Preparation T.B.M. thanks the DFG and the University of Wurzburg for and Characterization of Selected Heteronuclear Diboron Ring support. S.A.W. thanks Chris Vogels for helpful discussions and Systems. Inorg. Chem. 1968, 7, 225−230. NSERC of Canada for support. (27) Biffar, W.; Nöth, H.; Pommerening, H.; Wrackmeyer, B. 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9161 DOI: 10.1021/acs.chemrev.6b00193 Chem. Rev. 2016, 116, 9091−9161