Theoretical study on the consecutive 1,2-hydroboration and 1,1-organoboration reactions of alkyn-1-yl(vinyl) silane with borane

The detailed pathway of the consecutive 1,2-hydroboration and 1,1-organoboration reactions of alkyn-1-yl( vinyl) silane with borane were studied theoretically by DFT calculations. It is found that the 1,2-hydroboration will occur at the C = C moiety when the alkynyl end is substituted, and give the anti-Markovnikov adduct as a result of steric hindrance. From the 1,2-hydroboration intermediate, the intramolecular 1,1-organoboration is a concerted asynchronous process, in which the, C-Si cleavage precedes 1,2-alkyl migration, with activation energy about 25 kcal/mol. Calculations reveal the electronic property of the substituent at the alkyne end has quite limited effect on the 1,1-organoboration activation energy. The ring constrain is found to influence the weak Si-H-B interactions in the 1-silacyclopent-2-ene product most, and similar C-H-B interactions were predicted for the corresponding carbon analogues. (C) 2008 Elsevier B. V. All rights reserved.