Mechanisms of elimination reactions. 36. Stereochemistry and transition-state structure in eliminations from primary alkyltrimethylammonium salts

A study of stereochemistry of elimination in from primary allsyltrimethylammonium salts reveals that syn elimination can become the major reaction path when radicals are both bulky groups such as aryl or branched alkyl. The results can be rationalized by a simple conformational argument in which steric interactions between bulky β-substituents and the leaving trimethylammonio group destablizie the transitions tate for anti elimination. Primary β-tritium, secondary α-tritium, and primary α-14C isotope effects were determined on the (2,2-diphenylethyl)trimethylammonium ion and compared with similar data on the (2-phenylethyl)trimethylammonium ion, which eliminates by an exclusively anti mechanism. The extent of proton transfer in the transition state seems not to differ widely between the two systems, but the extent of C-N cleavage appears less in the 2,2-diphenylethyl system. Hammett rho values are smaller in the 2,2-diphenylethyl system, though their interpretation presents ambiguities. 6 tables