Solvent-promoted E1cB/E2 reactions competing with stepwise solvolysis through homoallylic carbocations

Abstract

The water-promoted elimination reactions of (R,S)-1-(1-X-ethyl)indene 1-X [X = Cl, Br, I, OBs (4-bromophenylsulfonyloxy)] and the corresponding (R,R) isomers 2-X in 25 vol% acetonitrile in water exhibit non-stereospecific and stereospecific 1,2 elimination, respectively. The reactions are E1cB and E2 type reactions; the conclusion is based upon measured large kinetic deuterium isotope effects and Brønsted β parameters. The rate of the competing solvolytic substitution, which occurs through homoallylic carbocations, increases with the leaving group in the order Br^–, I^– and BsO^–. The spontaneous reactions in methanol are slow; the anti stereochemistry increases with increasing basicity of added base and is favored by negative charge of the base.