Coplanar vs. orthogonal bimolecular substitution at a benzylic carbon

Abstract

Bimolecular substitution at the benzylic centre in dibenzylethylsulphonium fluoroborate (8) with thiocyanate anion is 8000 times faster than the corresponding reaction in the cyclic sulphonium fluoroborate (4) in which the attacking and leaving groups must remain in approximately the plane of the benzene ring thereby precluding conjugation in the transition state; it is estimated that a ca. thousand-fold increase in rate is attributable to conjugation in unconstrained benzylic substitution.