Reaction of 1-alkynylsilanes with triallylborane—competition between 1,1- and 1,2-allylboration

The reaction of triallylborane (All₃B, 1) with various 1-alkynylsilanes of the type Me₃Si–CCR¹ R¹=H (2a), Me (2b), Ph (2c), CC–SiMe₃ (2d), SiMe₃ (2e)], Ph₃Si–CCPh (3) MeCC–SiMe₂SiMe₂–CCMe (4) and Me₂Si(Cl)–CCPh (5) was studied. Triallylborane 1 turned out to be much more reactive than other triorganoboranes R₃B (e.g. R=Et, Ph). In the cases of 2 and 5, the products are organometallic-substituted alkenes 6 and 11, respectively, with the boryl and silyl group in cis-positions as the result of selective 1,1-allylboration (via cleavage of the Si–C bond) or mixtures of such and other alkenes 7 or 8 because of competition between 1,1- and 1,2-allylboration (the composition of these mixtures depends on the polarity of the solvent). In the case of 4, the 1,2-dihydro-1,2-disilaborepine derivative 12 is formed selectively (twofold 1,1-allylboration). The alkyne 3 did not react with 1. The products were characterised by ¹H-, ¹¹B-, ¹³C- and ²⁹Si-NMR spectroscopy.