EFFECTS OF BRIDGEHEAD SUBSTITUTION ON STRUCTURE AND REACTIVITY OF THE7-NORBORNYL CATION

A systematic investigation was undertaken of the effect of bridgehead substitution on the solvolytic reactivity of 7-norbornyl triflates in aqueous TFE. Methyl substitution increases the rate 60-fold and the ef fect of multiple substitution is additive. Chlorine, on the other hand , exhibits a combined inductive and resonance effect and decreases the reactivity 1700 times. The observed normal secondary P-deuterium isot ope effect of the 1,4-d(2) derivative can be rationalized by assuming a tilted C-s geometry for the cationic transition structure which is o nly slightly higher in energy than the nonclassical intermediate catio n of C-1 symmetry. Bridgehead methyl-d(3) kinetic isotope effects are similar to those associated with the nonmigrating methyl group in neop entyl ester solvolyses. These results are in agreement with the recent ly published structure of the 7-norbornyl cation calculated at the hig hest ab initio level which has confirmed its nonclassical nature as or iginally proposed by Winstein.