1,3-MIGRATION OF A PHENYL GROUP IN THE CONVERSION OF (ME3SI)(2)(PHME2SI)CSIMEPHX INTO (ME3SI)(2)(PH2MESI)CSIME2Y SPECIES

The finding that compounds of the type (Me3Si)(2)(PhMe2Si)CSiMePhX rea ct with electrophiles to give very predominantly rearranged products ( Me3Si)(2)(Ph2MeSi)CSiMe2Y, which would be expected to be thermodynamic ally disfavoured, can be rationalized in terms of a mechanism in which the anchimerically-assisted departure of X- gives the Ph-bridged cati on [(Me3Si)(2)CSiMe2(mu-Ph)SiMePh](+) which is attacked by the nucleop hile at the less hindered centre bearing two Me groups rather than tha t bearing one Me and one Ph group, with the outcome determined by kine tic rather than thermodynamic factors. Both (Me3Si)(2)(Ph2MeSi)CSiMe2B r and its isomer (Me3Si)(2)(PhMe2Si)CSiMePhBr react with AgBF4 in CH2C l2 or Et2O to give > 95% of the fluoride (Me3Si)(2)(Ph2MeSi)CSiMe2F. R eaction of the bromide (Me3Si)(2)(PhMe2Si)CSiMePhBr with AgO2CCF3 in E t2O, and that of the hydride (Me3Si)(2)(PhMe2Si)CSiMePhH with ICl in C Cl4, likewise give > 95% of the rearranged (Me3Si)(2)(Ph2MeSi)CSiMe2O2 CCF3 and (Me3Si)(2)(Ph2MeSi)CSiMe2Cl, respectively. (C) 1998 Elsevier Science S.A. All rights reserved.