Elimination and addition reactions. Part XX. Elimination of phenoxide from β-substituted ethyl phenyl ethers. Evidence for the E1cB mechanism

Abstract

Evidence has been obtained from three sources that the base-induced elimination of phenoxide from β-substituted aryl ethyl ethers occurs by the E1cB mechanism.

Rates of elimination from β-phenoxyethyldimethylsulphonium iodide and methyl β-phenoxyethyl sulphoxide in aqueous sodium hydroxide have been measured. Rate constants for the corresponding bis-β-deuterio-substrates in deuterium oxide are about 1·5 times larger. Consideration of the solvent and secondary deuterium isotope effects leads to values for the primary isotope effects of around unity.

In reactions of aryl 2-aryloxyethyl sulphones and sulphoxides and 2-aryloxyethyldimethylsulphonium salts in ethanolic sodium ethoxide, nuclear substitution in the leaving group affects rates to an extent which is very little dependent on the nature of the activating group. Nuclear substitution in the activating group (sulphoxides and sulphones) gives Hammett plots with ρ values of +1·7 and +2·1 respectively. The ratio of these values is similar to that found for the relative effects of these groups on acid dissociations. The value for the sulphone is close to that found for γ-elimination in aryl 3-chloropropyl sulphones leading to arylsulphonylcyclopropanes, a reaction which is also considered to have the carbanion mechanism.

Rates of elimination from the substrates in which the activating group is arylsulphonyl, acetyl, and ethoxy-carbonyl have been compared for reactions in which the base is ethoxide and alkanethiolate. Reactions with thiolate are slower and there is little alteration in the thiolate:ethoxide rate ratio as the activating group is changed. For thiolate reactions, rates are inversely proportional to the concentration of free thiol.