Stereoselective ultrasonically induced reductive monosilylation of geminaldibromonorcaranes. Steric effects

Sonochemical reductive silylation of 1-R-7,7-dibromonorcaranes (R = H, Me, Et, i-Pr) by magnesium produces in each case two 7-bromo-7-trialkylsilylnor caranes (alkyl = methyl or ethyl). The major isomer is exo (the trialkylsil yl group is cis to the R substituent), but the stereoselectivity of silylat ion decreases as the alkyl group size at C-l increases, 1-Phenyl-7,7-dibrom onorcarane produced a mixture of phenylcycloheptadienes and monobromonorcar anes. (C) 1999 Elsevier Science S.A. All rights reserved.