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Recent Synthesis Developments of Organoboron Compounds Via Metal-Free Catalytic Borylation of Alkynes and Alkenes

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Recent Synthesis Developments of Organoboron Compounds Via Metal-Free Catalytic Borylation of Alkynes and Alkenes molecules Review Recent Synthesis Developments of Organoboron Compounds via Metal-Free Catalytic Borylation of Alkynes and Alkenes Yanmei Wen, Chunmei Deng, Jianying Xie and Xinhuang Kang * Faculty of Chemistry and Environmental Science, Guangdong Ocean University, Zhanjiang 524088, China; [email protected] (Y.W.); [email protected] (C.D.); [email protected] (J.X.) * Correspondence: [email protected]; Tel.: +86-759-2383728 Academic Editor: Igor V. Alabugin Received: 28 October 2018; Accepted: 19 December 2018; Published: 28 December 2018 Abstract: Diboron reagents have been traditionally regarded as “Lewis acids”, which can react with simple Lewis base to create a significant nucleophilic character in one of boryl moieties. In particular, bis(pinacolato)diboron (B2pin2) reacts with simple Lewis bases, such as N-heterocyclic carbenes (NHCs), phosphines and alkoxides. This review focuses on the application of trivalent nucleophilic boryl synthon in the selective preparation of organoboron compounds, mainly through metal-free catalytic diboration and the β-boration reactions of alkynes and alkenes. Keywords: organoboron compound; metal-free catalytic boration; nucleophilic boron; alkyne; alkene 1. Introduction Organoboron compounds play an important role in organic chemistry as they can be converted into a wide variety of functional groups [1–13]. Thus, the synthesis of different types of organoboron compounds continues to be an important research field [14–20]. Conventional methods for their preparation are based on either reactive organometallic reagents or transition-metal-mediated processes [21–37]. Recently, great effort has been made towards the development of transition-metal-free methods with high chemoselectivity or regioselectivity and environmental sustainability [38–48]. The transition-metal-free catalyzed borylation reaction of alkynes and alkenes offers an attractive method for generating valuable organoboron compounds. The interaction of a diboron reagent, in particular bis(pinacolato)diboron (B2pin2), with a simple Lewis base, such as N-heterocyclic carbenes (NHCs), phosphines and alkoxides, generates Lewis acid-base adducts, in which the formally intact sp2 boryl moiety becomes the important source of nucleophilic boryl species, resulting in an appropriate approache towards transition-metal-free selective preparation of organoboron compounds. This review focuses on the application of trivalent nucleophilic boryl synthon in the selective preparation of organoboron compounds, mainly through diboration and β-boration reactions of alkynes and alkenes. 2. Organoboron Compounds via Transition-Metal-Free Catalytic Diboration of Unsaturated Hydrocarbons In recent years, the preparation of 1,2-diborated or 1,1-diborated products has rapidly grown due to the addition of diboryl compounds to unsaturated organic compounds in the transition metal-free catalysis. The first diboration reaction of non-activated alkenes in the absence of transition metal complexes was reported by Fernández and coworkers (Scheme1a) [ 49]. The diboration of non-activated olefins, such as terminal and internal alkenes, allenes and vinylarenes, can be produced via the formation of a 1,2-diborated product in a tetrahydrofuran (THF) solvent at 70 ◦C for ~6–16 h. Molecules 2019, 24, 101; doi:10.3390/molecules24010101 www.mdpi.com/journal/molecules Molecules 2018, 23, x 2 of 16 Molecules 2018, 23, x 2 of 16 Molecules 2019, 24, 101 2 of 15 activated olefins, such as terminal and internal alkenes, allenes and vinylarenes, can be produced via the formation of a 1,2-diborated product in a tetrahydrofuran (THF) solvent at 70 °C for ~6–16 hours. The interaction of B2pin2 (1) and a methoxide anion, derived from the combination of Cs2CO3 and The interaction of B22pin2 ((11)) and a methoxide anion, derived fromfrom thethe combinationcombination ofof CsCs22CO3 and MeOH, can generate in situ the adduct [B2pin2OMe]−−. This adduct reacts with the unsubstituted MeOH, can generate inin situsitu thethe adductadduct [B[B2pin2pin22OMe]OMe]−. .This This adduct adduct reacts reacts with with the unsubstitutedunsubstituted 1 carbon atom (C 1)) of of the the C=C C=C double double bond bond via via the the transition transition state state (TS1), (TS1), which which is is the the overlap overlap between between the strongly polarized B-B B-B σ orbital of the activated diboron reagent reagent and and the the C–C C–C ππ* orbital of the 2 olefin.olefin. When the B–B bond weakens, the negative charge density onon thethe CC22 of the C=C double bond increases. The The negatively negatively charged charged olefin-boryl olefin-boryl olef olefin-B(pin)in-B(pin) fragment fragment attacks attacks the the boron boron atom atom with with an electrophilican electrophilic character character to form to form the thesecond second transition transition state state (TS2), (TS2), which which then then protonates protonates to form to form the diborated product (Scheme 2). The adduct [B2pin2OMe]− has− been fully characterized by both 11B- diboratedthe diborated product product (Scheme (Scheme 2). 2The). The adduct adduct [B2pin [B 22pinOMe]2OMe]− has beenhas beenfully fullycharacterized characterized by both byboth 11B- NMR11B-NMR spectroscopy spectroscopy and and ESI-MS. ESI-MS. A Alater later report report extended extended the the methodology methodology using using unsymmetrical Bpin–Bdan (dan(dan represents represents 1,8-diaminonaphthalene) 1,8-diaminonaphthalene) (2) as(2 the) as diboron the diboron reagents, reagents, where the where methoxide the methoxideselectively selectively interacted interacted with the stronger with the Lewisstronger acid Lewis Bpin acid moiety. Bpin moiety. The Bdan The moiety Bdan moiety appears appears in the ininternal the internal position position (Scheme (Scheme1b) [50 ].1b) [50]. B(OR)2 1.1 equiv B2(OR)2 B(OR)2 1.1 equiv B2(OR)2 B(OR) 15 mol% Cs2CO3 B(OR)2 15 mol% Cs2CO3 2 a) 5.0 equiv MeOH a) 5.0 equiv MeOH THF, 70 oC, 6 h THF, 70 oC, 6 h O O O O O O O O B OR = B B B B B OR = B B B B OR O O O O O1 O O O 1 Isolated yield: 71% 59% 56% 82% Isolated yield: 71% 59% 56% 82% -OMe -OMe NH O NH O B B OMe B B NH OMeO O NH B O NH 2 O B B O NH O b) NH 2 B O NH O O b) N -OMe HN B B O Cs CO , MeOH NH - HN B B Cs2CO3, MeOH H OMe 70 2oC, 123 h 70 oC, 12 h Isolated yield: 38% Isolated yield: 38% Scheme 1. (a) Transition-metal-free diboration reaction of non-activated olefins; (b) Mixed Scheme 1.1.( a)(a Transition-metal-free) Transition-metal-free diboration diboration reaction re ofaction non-activated of non-activated olefins; (b) Mixedolefins; organocatalytic (b) Mixed organocatalytic diboration of non-activated olefins. organocatalyticdiboration of non-activated diboration olefins.of non-activated olefins. O O O O BB BB - O O O O O - O O O O O BB BB O O MeO- O O R MeO- R O O O - base + O O O - base -H+ BB -H BB TS1 O O TS1 O O - - R H R H O O O MeOH O O O MeOH B + O B B + O B O O O R O O O B O R O B - O - - O O O - B + O O O O B H+ O O O BB O H O O B O BB R B O O O R O B O HO O B H R TS2 R TS2 R R Scheme 2. Suggested catalytic cycle for the dibo diborationration of non-activated olefins.olefins. Scheme 2. Suggested catalytic cycle for the diboration of non-activated olefins. Molecules 2019, 24, 101 3 of 15 Molecules 2018, 23, x 3 of 16 Molecules 2018, 23, x 3 of 16 In 2015, Sawamura reported the formation of α,β-diboryl acrylates through trialkylphosphine In 2015, SawamuraSawamura reportedreported thethe formationformation ofof αα,,ββ-diboryl-diboryl acrylatesacrylates throughthrough trialkylphosphinetrialkylphosphine catalyzed anti-selective diboration of the C≡C triple bond (Scheme 3a) [51]. Through cross-coupling catalyzed anti-selectiveanti-selective diboration ofof thethe CC≡≡CC triple triple bond bond (Scheme 33a)a) [[51].51]. Through cross-coupling reaction of the differentiated heteroatom substituents of the α,β-diboryl acrylates in a stepwise reaction of the differentiateddifferentiated heteroatom substituents of the α,,β-diboryl acrylates in a stepwisestepwise manner, a diverse array of unsymmetrical tetrasubstituted alkenes can be accessed. Very recently, manner, a diverse array of unsymmetrical tetrasubstituted alkenes can can be be accessed. Very Very recently, recently, Santos employed an unsymmetrical Bpin–Bdan 2 as a diboron reagent for the direct diboration of Santos employed an unsymmetrical Bpin–Bdan 22 as a diboron reagent for the direct diboration of alkynamides, which display broad functional group tolerance. The transition-metal-free catalyzed alkynamides, which display broad functionalfunctional groupgroup tolerance.tolerance. The transition-metal-freetransition-metal-free catalyzed diboration of alkynamides with Bpin–Bdan, where the Bdan and Bpin moiety install α- and β-carbon diboration of alkynamides with Bpin–Bdan, where the Bdan and Bpin moiety install α-- and β-carbon atoms respectively. (Scheme 3b) [52]. atoms respectively. (Scheme(Scheme3 3b)b) [ 52[52].]. 10 mol% PBu3 Ph Bpin Isolated yield 1.010 equivmol% BPBupin3 Ph Bpin Isolated yield a) 1.0 equiv B2pin2 a) Ph CO2Et o 2 2 Ph CO2Et THF, 80 C, 8 h 64% THF, 80 oC, 8 h pinB CO2Et 64% pinB CO2Et 1.4 equiv pinB-Bdan O 1.4 equiv pinB-Bdan O 1.1 equiv NaH Bpin O 1.11.0 equiv NaH15-crown-5 Bpin O N Me 1.0 equiv 15-crown-5 Me 67% b) N Me Ph N Me 67% b) Ph H THF, RT, Ph N Ph H THF,0.5 h-1 RT, h Bdan H 0.5 h-1 h Bdan H Scheme 3.
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