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Xie F, Mao Z, Curran DP, Liang H, Dai W. Facile Borylation of Alkenes, Alkynes, Imines, Arenes and Heteroarenes with N-Heterocyclic Carbene-Boranes and a Heterogeneous Semiconductor Photocatalyst. Angew Chem Int Ed Engl 2023; 62:e202306846. [PMID: 37555790 DOI: 10.1002/anie.202306846] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2023] [Revised: 08/08/2023] [Accepted: 08/09/2023] [Indexed: 08/10/2023]
Abstract
Although the development of radical chain and photocatalytic borylation reactions using N-heterocyclic carbene (NHC)-borane as boron source is remarkable, the persistent problems, including the use of hazardous and high-energy radical initiators or the recyclability and photostability issues of soluble homogeneous photocatalysts, still leave great room for further development in a sustainable manner. Herein, we report a conceptually different approach toward highly functionalized organoborane synthesis by using recoverable ultrathin cadmium sulfide (CdS) nanosheets as a heterogeneous photocatalyst, and a general and mild borylation platform that enables regioselective borylation of a wide variety of alkenes (arylethenes, trifluoromethylalkenes, α,β-unsaturated carbonyl compounds and nitriles), alkynes, imines and electron-poor aromatic rings with NHC-borane as boryl radical precursor. Mechanistic studies and density functional theory (DFT) calculations reveal that both photogenerated electrons and holes on the CdS fully perform their own roles, thereby resulting in enhancement of photocatalytic activity and stability of CdS.
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Li G, Huang G, Sun R, Curran DP, Dai W. Regioselective Radical Borylation of α,β-Unsaturated Esters and Related Compounds by Visible Light Irradiation with an Organic Photocatalyst. Org Lett 2021; 23:4353-4357. [PMID: 34003657 DOI: 10.1021/acs.orglett.1c01270] [Citation(s) in RCA: 21] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Radical hydroboration reactions have only recently been reported and are still rare. Here we describe a photoredox radical hydroboration of α,β-unsaturated esters, amides, ketones, and nitriles with NHC-boranes that uses only an organocatalyst and visible light. The conditions are mild, the substrate scope is broad, and the α/β regioselectivity is high. The reaction requires only the organocatalyst; there is no costly metal, and there are no other additives (base, cocatalyst, initiator).
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Kawamoto T, Morioka T, Noguchi K, Curran DP, Kamimura A. Inverse Hydroboration of Imines with NHC-Boranes Is Promoted by Diphenyl Disulfide and Visible Light. Org Lett 2021; 23:1825-1828. [PMID: 33621108 DOI: 10.1021/acs.orglett.1c00230] [Citation(s) in RCA: 19] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
We describe a simple and efficient procedure for nucleophilic borylation of imines in the absence of a photoredox catalyst. Visible light irradiation of an acetonitrile solution of an imine, an NHC-borane, and diphenyl disulfide (10 mol %) provides various stable α-amino NHC-boranes in good yields. The reaction proceeds via addition of a nucleophilic boryl radical to an imine, followed by hydrogen abstraction from thiophenol, which is generated from NHC-borane and diphenyl disulfide.
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Takahashi K, Geib SJ, Maeda K, Curran DP, Taniguchi T. Radical trans-Hydroboration of Substituted 1,3-Diynes with an N-Heterocyclic Carbene Borane. Org Lett 2021; 23:1071-1075. [PMID: 33481616 DOI: 10.1021/acs.orglett.0c04284] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
Monohydroboration of substituted 1,3-diynes with an N-heterocyclic carbene borane (NHC-borane) occurs under radical conditions using an azo initiator, such as ACCN and AIBN, and a thiol as a polarity-reversal catalyst. The reaction is highly regio- and stereoselective and provides stable NHC-(E)-alkynylalkenylboranes.
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Shimoi M, Watanabe T, Maeda K, Curran DP, Taniguchi T. Berichtigung: Radical
trans
‐Hydroboration of Alkynes with N‐Heterocyclic Carbene Boranes. Angew Chem Int Ed Engl 2020. [DOI: 10.1002/ange.202014353] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
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Shimoi M, Watanabe T, Maeda K, Curran DP, Taniguchi T. Corrigendum: Radical
trans
‐Hydroboration of Alkynes with N‐Heterocyclic Carbene Boranes. Angew Chem Int Ed Engl 2020; 59:22824. [DOI: 10.1002/anie.202014353] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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7
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Bolt DA, Curran DP. Reactions of NHC‐Boranes with Dibenzoyl Peroxide and Benzoic Acid. Adv Synth Catal 2020. [DOI: 10.1002/adsc.202000147] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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8
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Studer A, Curran DP. Happy Birthday to Bernd Giese. Adv Synth Catal 2020. [DOI: 10.1002/adsc.202000065] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
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9
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Dai W, Geib SJ, Curran DP. 1,4-Hydroboration Reactions of Electron-Poor Aromatic Rings by N-Heterocyclic Carbene Boranes. J Am Chem Soc 2020; 142:6261-6267. [PMID: 32101418 DOI: 10.1021/jacs.0c00490] [Citation(s) in RCA: 39] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Reactions of N-heterocyclic carbene boranes (NHC-boranes) with electron-poor aromatic rings under photoredox conditions provide dearomatized 3-NHC-boryl-1,5-cycohexadienes, which are formally products of 1,4-hydroboration reactions. When regioisomers are possible, the more crowded (doubly ortho-substituted) product is formed preferably or exclusively. The mechanism may involve oxidation of the NHC-borane to an NHC-boryl radical, reduction of the electron-poor aromatic ring to a radical anion, coupling of the radical and the radical anion to give a cyclohexadienyl anion, and finally regioselective protonation.
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Walton JC, Dai W, Curran DP. EPR Studies on the Addition of Ligated Boryl Radicals to Carbonyl Compounds. J Org Chem 2020; 85:4248-4255. [PMID: 32056434 DOI: 10.1021/acs.joc.9b03420] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The boron-centered radicals derived from alkenyl N-heterocyclic carbene (NHC)-boranes bearing ester substituents were recently found to ring close in 5-endo mode by addition to the oxygen atoms of the ester substituents. The inference from this was that NHC-boryl radicals might add intermolecularly to carbonyl-containing substrates. Several different NHC-boryl radicals were generated by H-atom abstraction from NHC-ligated trihydroborates. Electron paramagnetic resonance (EPR) spectroscopy proved that these did indeed add to the oxygen atoms of diaryl ketones with production of the corresponding bora-ketyl radicals. The same unusual regioselectivity of addition was observed with monoaryl ketones, but no bora-ketyls were observed with dialkyl ketones. Similarly, no bora-ketyl adduct radicals were observed with esters, even esters of benzoic acid. EPR spectroscopic evidence suggested that NHC-boryl radicals were also added to the O-atoms of aromatic aldehydes. Amine-boryl and phosphine-boryl radicals were also observed to add to the O-atom of benzophenone with production of the corresponding ketyl radicals.
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Takahashi K, Shimoi M, Watanabe T, Maeda K, Geib SJ, Curran DP, Taniguchi T. Revisiting Polyfluoroarenes as Radical Acceptors: Radical C–F Bond Borylation of Polyfluoroarenes with N-Heterocyclic Carbene Boranes and Synthesis of Borane-Containing Liquid Crystals. Org Lett 2020; 22:2054-2059. [DOI: 10.1021/acs.orglett.0c00481] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
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12
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Shimoi M, Kevlishvili I, Watanabe T, Maeda K, Geib SJ, Curran DP, Liu P, Taniguchi T. The Thermal Rearrangement of an NHC-Ligated 3-Benzoborepin to an NHC-Boranorcaradiene. Angew Chem Int Ed Engl 2020; 59:903-909. [PMID: 31670450 DOI: 10.1002/anie.201912234] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/24/2019] [Indexed: 11/08/2022]
Abstract
An N-heterocyclic-carbene-ligated 3-benzoborepin with a bridged structure has been synthesized by double radical trans-hydroboration of benzo[3,4]cycloundec-3-ene-1,5-diyne with an N-heterocyclic carbene borane. The thermal reaction of the NHC-ligated borepin at 150 °C gives an isolable NHC-boranorcaradiene. Experiments and density functional theory calculations support a mechanism whereby the borepin initially rearranges to a boranorcaradiene by a thermal 6π-electrocyclic reaction. This is followed by 1,5-boron shift to give a rearranged boranorcaradiene. This shift occurs with stereoinversion at boron through a transition state with open-shell diradical character. This is the first example of the isolation of a boranorcaradiene from a thermal reaction of a borepin.
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Shimoi M, Kevlishvili I, Watanabe T, Maeda K, Geib SJ, Curran DP, Liu P, Taniguchi T. The Thermal Rearrangement of an NHC‐Ligated 3‐Benzoborepin to an NHC‐Boranorcaradiene. Angew Chem Int Ed Engl 2019. [DOI: 10.1002/ange.201912234] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
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Dai W, Geib SJ, Curran DP. Facile Synthesis of α-N-Heterocyclic Carbene-Boryl Ketones from N-Heterocyclic Carbene-Boranes and Alkenyl Triflates. J Am Chem Soc 2019; 141:12355-12361. [DOI: 10.1021/jacs.9b05547] [Citation(s) in RCA: 34] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
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15
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Shimoi M, Maeda K, Geib SJ, Curran DP, Taniguchi T. Esters as Radical Acceptors: β‐NHC‐Borylalkenyl Radicals Induce Lactonization by C−C Bond Formation/Cleavage on Esters. Angew Chem Int Ed Engl 2019. [DOI: 10.1002/ange.201902001] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
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16
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Shimoi M, Maeda K, Geib SJ, Curran DP, Taniguchi T. Esters as Radical Acceptors: β‐NHC‐Borylalkenyl Radicals Induce Lactonization by C−C Bond Formation/Cleavage on Esters. Angew Chem Int Ed Engl 2019; 58:6357-6361. [DOI: 10.1002/anie.201902001] [Citation(s) in RCA: 29] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2019] [Indexed: 01/22/2023]
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Dai W, Geib SJ, Curran DP. Ring-Opening Reactions of NHC-Boriranes with In Situ Generated HCl: Synthesis of a New Class of NHC-Boralactones. J Am Chem Soc 2019; 141:3623-3629. [PMID: 30717598 DOI: 10.1021/jacs.8b13010] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The first ring-opening reactions of ligated boriranes (boracyclopropanes) are described. Treatment of readily available NHC-boriranes bearing ester substituents on the borirane ring with HCl provides stable γ-NHC-bora-γ-lactones in isolated yields ranging from 40% to 73%. The reactions occur through 1,3-addition of HCl across a B-C bond of the NHC-borirane to form a ring-opened NHC-boryl chloride, followed by lactonization with chloride displacement. Experimental evidence suggests that both the borirane ring-opening reaction and the boralactonization reaction occur with inversion at boron.
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Dai W, Curran DP, Walton JC. EPR and Preparative Studies of 5- endo Cyclizations of Radicals Derived from Alkenyl NHC-Boranes Bearing tert-Butyl Ester Substituents. J Org Chem 2019; 84:2102-2111. [PMID: 30663305 DOI: 10.1021/acs.joc.8b03112] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Radical H atom abstraction from a set of N-heterocyclic carbene (NHC) complexes of alkenylboranes bearing two tert-butyl ester substituents was studied by EPR spectroscopy. The initial boraallyl radical intermediates rapidly ring closed onto the O atoms of their distal ester groups in 5- endo mode to yield 1,2-oxaborole radicals. Unexpectedly, two structural varieties of these radicals were identified from their EPR spectra. These proved to be two stable rotamers, in which the carbonyl group of the tert-butyl ester was oriented toward and away from the NHC ring. These rotamers were akin to the s- trans and s- cis rotamers of α,β-unsaturated carbonyl compounds. Their stability was attributed to the quasi-allylic interaction of their unpaired electrons with the carbonyl units of their adjacent ester groups. EPR spectroscopic evidence for two rotamers of the analogous methyl ester containing NHC-oxaborole radicals was also obtained. An improved synthetic procedure for preparing rare NHC-boralactones was developed involving treatment of the alkenyl NHC-boranes with AIBN and tert-dodecanethiol.
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Bolt DA, Geib SJ, Curran DP. Synthesis and characterization of N-heterocyclic carbene complexes of 1,3,2-dioxaborolane-4,5-dione (NHC-boryl oxalates). Tetrahedron 2018. [DOI: 10.1016/j.tet.2018.10.031] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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20
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Dai W, McFadden TR, Curran DP, Früchtl HA, Walton JC. 5- Endo Cyclizations of NHC-Boraallyl Radicals Bearing Ester Substituents: Characterization of Derived 1,2-Oxaborole Radicals and Boralactones. J Am Chem Soc 2018; 140:15868-15875. [PMID: 30369236 DOI: 10.1021/jacs.8b09288] [Citation(s) in RCA: 26] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
EPR studies of radical hydrogen abstraction reactions of N-heterocyclic carbene (NHC) complexes of alkenylboranes bearing two ester substituents revealed not the expected boraallyl radicals but instead isomeric 1,2-oaxborole radicals. Such radicals are new, and DFT calculations show that they arise from the initially formed boraallyl radicals by a rapid, exothermic 5- endo cyclization. These spectroscopic and computational discoveries prompted a series of preparative experiments that provided access to a novel family of robust NHC-boralactones. A one-pot procedure was developed to access the boralactones directly from an NHC-borane (NHC-BH3) and dimethyl acetylenedicarboxylate.
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Yuan Y, Dong W, Gao X, Xie X, Curran DP, Zhang Z. Visible-Light-Induced Radical Cascade Cyclization: Synthesis of (20S
)-Camptothecin, SN-38 and Irinotecan. CHINESE J CHEM 2018. [DOI: 10.1002/cjoc.201800358] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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22
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Dai W, Geib SJ, Curran DP. Reactions of N-Heterocyclic Carbene Boranes with 5-Diazo-2,2-dimethyl-1,3-dioxane-4,6-dione: Synthesis of Mono- and Bis-hydrazonyl NHC-Boranes. J Org Chem 2018; 83:8775-8779. [PMID: 30004702 DOI: 10.1021/acs.joc.8b01125] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
N-Heterocyclic carbene boranes (NHC-boranes) react with 5-diazo-2,2-dimethyl-1,3-dioxane-4,6-dione at 40 °C in dichloromethane to provide NHC-boryl hydrazone derivatives of 2,2-dimethyl-1,3-dioxane-4,6-dione. These hydrazones disproportionate to bis-hydrazones on treatment with diiodine in dichloromethane at room temperature. The mono- and bis-hydrazones are yellow solids that are stable to chromatography and storage.
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Subervie D, Graff B, Nerkar S, Curran DP, Lalevée J, Lacôte E. Difluorination at Boron Leads to the First Electrophilic Ligated Boryl Radical (NHC‐BF
2
.
). Angew Chem Int Ed Engl 2018. [DOI: 10.1002/ange.201806476] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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Subervie D, Graff B, Nerkar S, Curran DP, Lalevée J, Lacôte E. Difluorination at Boron Leads to the First Electrophilic Ligated Boryl Radical (NHC-BF 2. ). Angew Chem Int Ed Engl 2018; 57:10251-10256. [PMID: 29928777 DOI: 10.1002/anie.201806476] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/05/2018] [Indexed: 11/10/2022]
Abstract
1,3-Dimethylimidazol-2-ylidene difluoroborane (NHC-BF2 H) was prepared in a one-pot, two-step reaction from the parent ligated borane (NHC-BH3 ). The derived difluoroboryl radical (NHC-BF2. ) was generated by laser flash photolysis experiments and characterized by UV spectroscopy and rate-constant measurements. It is transient and reacts quickly with O2 . Unusually, it also reacts more rapidly with ethyl vinyl ether than with methyl acrylate. By this measure, it is the first electrophilic ligated boryl radical. Both NHC-BH3 and NHC-BF2 H serve as co-initiators in bulk photopolymerizations, converting both electron-poor and electron-rich monomers at roughly similar rates. However, the difluorinated coinitiator provides polymers with dramatically increased chain lengths from both monomers.
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Shimoi M, Watanabe T, Maeda K, Curran DP, Taniguchi T. Radical trans-Hydroboration of Alkynes with N-Heterocyclic Carbene Boranes. Angew Chem Int Ed Engl 2018; 57:9485-9490. [PMID: 29845716 DOI: 10.1002/anie.201804515] [Citation(s) in RCA: 70] [Impact Index Per Article: 11.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/17/2018] [Revised: 05/28/2018] [Indexed: 01/06/2023]
Abstract
Hydroboration of internal alkynes with N-heterocyclic carbene boranes (NHC-boranes) occurs to provide stable NHC (E)-alkenylboranes upon thermolysis in the presence of di-tert-butyl peroxide. The E isomer results from an unusual trans-hydroboration, and the E/Z selectivity is typically high (90:10 or greater). Evidence suggests that this hydroboration occurs by a radical-chain reaction involving addition of an NHC-boryl radical to an alkyne to give a β-NHC-borylalkenyl radical. Ensuing hydrogen abstraction from the starting NHC-borane provides the product and returns the starting NHC-boryl radical. Experiments suggest that the observed trans-selectivity results from kinetic control in the hydrogen-transfer reaction.
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