This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Review pubs.acs.org/CR 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.