Mechanistic study of syn- and anti-elimination from diastereoisomeric halogenosulphonylethanes

Abstract

The reactions of erythro- and threo-1-chloro- and -1-bromo-1,2-diphenyl-2-p-tolylsulphonyiethanes (I_H)–(IV_H) and of the 1,2-dideuterio-derivatives (I_D)–(IV_D) with methoxide ion in methanol and with triethylamine in benzene have been followed kinetically. For methoxide ion the reactions of the erythro-isomers (anti-elimination) are much faster than those of the threo-derivatives (syn+anti-elimination). The sensitivity to the leaving group is rather modest for both isomers whereas reaction rates are markedly influenced by isotopic substitution. In the reactions with amine the k_erythro : k_threo ratios become much smaller and k_Br : k_Cl values close to unity are observed for the latter isomer. Isotope effects are higher for syn- than for anti-elimination. These and related observations are evaluated in terms of an E2 mechanism involving a small degree of C–X breakage in the transition state or an E1cB mechanism of irreversible type.