Hydroboration of Alkyne‐Functionalized 1,3,2‐Benzodiazaboroles

Abstract

Reactions of equimolar amounts of dicyclohexylborane (DCB) with a series of 2‐alkynyl‐1,3‐diethyl‐1,3,2‐benzodiazaboroles R–C≡C–B(NEt)2C6H4 (1: R = nBu, 2: R = tBu, 3: R = Ph, 4: R = p‐Me2NC6H4, 5: R = p‐MeOC6H4) regioselectively afforded the 1,1‐diborylalkenes (Z)‐R(H)C=C{(B{NEt}2C6H4)B(c‐C6H11)2} (7a–11a) as the result of a cis‐addition of the BH bond of the borane to the C≡C triple bond in compounds 1 to 5. In contrast to this, reaction of Me3Si–C≡C–B(NEt)2C6H4 (6) with HB(c‐C6H11)2 yielded a 2.5:1 mixture of the 1,2‐diborylated alkene (Z)‐Me3Si{(c‐C6H11)2B}C=CHB(NEt)2C6H4 (12b) and the 1,1‐regioisomer (Z)‐Me3Si(H)C=C{B(NEt)2C6H4}{B(c‐C6H11)2} (12a). These results were rationalized by the differing π‐acceptor qualities of the benzodiazaborolyl and trimethylsilyl substituents at the C≡C triple bond in compounds 1 to 6. The novel products 7–12 were characterized by elemental analysis, mass spectrometry and NMR spectroscopy (1H, 11B, and 13C NMR). The molecular structures of 9b and 12b were substantiated by single‐crystal X‐ray diffraction analysis.

Unsymmetrical 1,1-Bisboryl Species: Valuable Building Blocks in Synthesis

“…Molecules 2020, 24, x FOR PEER REVIEW 26 of 30 In 2013, Weber et al reported the hydroboration of 2-alkyl-and 2-aryl-ethynyl-1,3,2benzodiazaboroles with dicyclohexylborane (DCB) under metal-free conditions at room temperature, affording the cis-1,1-bis(boryl)alkene 82 as a major regioisomer, with quantitative yields [50]. Interestingly, they also observed the hydroboration of 2-silylethynyl-1,3,2-benzodiazaborole with DCB, which afforded 1,1-bis(boryl)alkene (82) as a minor regioisomer and trans-1,2-bis(boryl)alkene as a major regioisomer (83), with quantitative yields (Scheme 33) [50]. Scheme 33.…”

Cobalt-Catalyzed 1,1-Diboration of Terminal Alkynes: Scope, Mechanism, and Synthetic Applications

“…Two-step procedures starting from terminal alkynes (Scheme 2B) 16–18 and methods involving stoichiometric amounts of organolithium reagents (Scheme 2C) have also been reported. 19,20 However, these approaches suffer from low volumetric throughput and poor functional group compatibility, respectively.…”

Synthesis of 1,1‐Diborylalkenes through a Brønsted Base Catalyzed Reaction between Terminal Alkynes and Bis(pinacolato)diboron

“…[1] Several synthetic methods for accessing 1,1-diborylalkenes have been developed. [1][2][3][4] Specifically,4 0years ago Matteson reported the synthesis of 1,1-diborylalkenes through an addition reaction of triborylmethyllithium to carbonyl compounds (Scheme 1a). [2] More recently,Shimizu, Hiyama, and co-workers reported ar eaction between bis(pinacolato)di-boron and 1-bromo-1-lithioalkenes,w hich were prepared from 1,1-dibromoalkenes by means of Br-Li exchange (Scheme 1b).…”