ELECTRONIC EFFECTS IN E2 REACTIONS: II. t-BUTOXIDE-INDUCED ELIMINATIONS OF 2-METHYL-3-PENTYL ARENESULFONATES

Abstract

As part of a study of the influence of electronic effects imposed by the leaving group on orientation in E2 reactions, a study of elimination in a series of 2-methyl-3-pentyl arenesulfonates (I) has been carried out. The compositions of the olefin mixtures resulting from reaction of selected members of this series with potassium t-butoxide in t-butanol (1a, Id), 50% (by volume) t-butanol – dioxane (Ia–Ig), 25% (by volume) t-butanol – dioxane (Id–If), and 25% (by volume) t-butanol – dimethyl sulfoxide (Ia, Id, If, Ig), at 50 °C, have been determined using vapor phase chromatography. Second-order rate constants were measured for these reactions in t-butanol (Ie), and 50% t-butanol – dioxane (Ia, Ic–Ig) at 50 °C and in 25% t-butanol – dimethyl sulfoxide (Ia, Id, Ie) at 25 °C.The compositions of the olefin mixtures vary in a fairly regular way with changes in the electronic nature of the leaving group, the fraction of 2-methyl-2-pentene and ratio of trans-to cis-4-methyl-2-pentene for the most part increasing with increasing electron withdrawal. The E2 reactions in 25% t-butanol – dimethyl sulfoxide produce trans- and cis-4-methyl-2-pentene in ratios (20 to 35) considerably higher than those in an equilibrium mixture (ca. 6).The results are discussed in terms of recent views on the E2 transition state.