Trifluoromethyl-substituted phenylsilanes: the regiochemical course of their metalation dictated by buttressing effects

Triethyl[(2-trifluoromethyl)phenyl]silane reacts with the superbasic LIC-KOR mixt. of butyllithium and potassium tert-butoxide exclusively at the 4-position ("meta-metalation") and not at all at the 3-position ("ortho-metalation"). Two further substrates which simultaneously contain two trifluoromethyl groups, triethyl[2,4- and 2,5-bis(trifluoromethyl)phenyl]silane, undergo deprotonation at the 5- and 4-position, resp. Thus, a buttressing effect blocks the attack of metalating agents on the ortho position of any trifluoromethyl group which is neighbored, on the other side, by a trialkyl substituent. [on SciFinder (R)]