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

The reaction of triallylborane (All(3)B, 1) with various 1-alkynylsilanes o f the type Me3Si-C=CR1 [R-1 = H (2a), Me (2b), Ph (2c), C=C-SiMe3 (2d), SiM e3 (2e)], Ph3Si-C=CPh (3) MeC=C-SiMe2SiMe2-C=CMe (4) and Me2Si(Cl)-C=CPh (5 ) was studied. Triallylborane 1 turned out to be much more reactive than ot her triorganoboranes R3B (e.g. R = Et, Ph). In the cases of 2 and 5, the pr oducts are organometallic-substituted alkenes 6 and 11, respectively, with the boryl and silyl group in cis-positions as the result of selective 1,1-a llylboration (via cleavage of the Si-CE 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 forme d selectively (twofold 1,1-allylboration). The alkyne 3 did not react with 1. The products were characterised by H-1-, B-11-, C-13- and Si-29-NMR spec troscopy. (C) 1999 Elsevier Science S.A. All rights reserved.