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



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} (7a11a) 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 712 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.