Nucleophilic displacement reactions at carbon, phosphorus and sulphur centres: reaction of aryl methanesulphonates with ethoxide; change in mechanism with change in leaving group

The reactions of ethoxide ion with aryl methanesulphonate esters (1a–c) in anhydrous ethanol at 25 °C have been investigated in order to determine the effect of leaving group nucleofugality on the balance between substitution and elimination pathways. The reactions of p-nitrophenyl-(1a), m-nitrophenyl-(1b) and p-trifluoromethylphenyl-(1c) methanesulphonates have been examined by means of kinetic studies, sulphene trapping experiments, and deuterium exchange experiments. It is concluded that the para-nitre-substituted ester reacts predominantly by an Elcb-type elimination mechanism via a sulphene intermediate, with nucleophilic substitution as a minor concurrent pathway. Conversely, the meta-nitro-substituted ester reacts predominantly by substitution, with elimination as a minor concurrent pathway. The evidence available indicates that the para-trifluoromethyl-substituted ester reacts solely by substitution. Thus, the mechanism of reaction changes from nucleophilic substitution to elimination–addition as leaving group nucleofugality increases.