Dehalogenation reactions of vicinal dihalides. Part II. Substituent and leaving-group effects in the reactions of 1,2-dihalogeno-1,2-diphenylethanes with iodide, bromide, and chloride ions

Abstract

The I^–-, Br^–-, and Cl^–-promoted dehalogenations of several meso- or erythro-1,2-dibromo-1-(4-R¹-phenyl)-2-(4-R²-phenyl)ethanes and the I^–- and Br^–-promoted dehalogenations of erythro-1-bromo-2-chloro-1,2-di-phenylethane have been investigated. The reactions with I^– have been carried out in MeOH and dimethylformamide, those with Br^– and Cl^– in dimethylformamide. Both leaving-group and substituent effects indicate that these reactions occur by a completely concerted mechanism of elimination with a large amount of double bond developed in the transition state. It has been observed that the order of nucleophilic reactivity in dimethylformamide is I^– > Br^– > Cl^–, which is consistent with an attack of the nucleophile on one bromine atom of the substrate. A peculiar result has been obtained in the case of the dinitro-derivative (R¹= R²= NO₂) which is, with Br^– and Cl^–, much more reactive than expected. A modification of the usual transition-state structure is probably required in this case.