1,3-Rearrangements of some allylic sulphones

Abstract

The 1,3-rearrangement of a number of acyclic and cyclic allylic p-tolyl sulphones has been found to occur on treatment (a) with benzoyl peroxide (BPO) in CCl₄ under reflux or (b) with sodium toluene-p-sulphinate in aqueous acetic acid at 110 °C. Rearrangement is only successful in cases where the product sulphone is thermodynamically more stable than the starting material, e.g. prenyl† sulphone (10) is obtained from dimethylallyl sulphone (9). A radical chain addition–elimination mechanism involving arenesulphonyl radicals is proposed.

More heavily substituted cyclic allylic sulphones such as 1-methylcyclohex-2-enyl p-tolyl sulphone (21; R = Me) rearranged only sluggishly under the BPO–CCl₄ conditions but underwent smooth isomerisation to (22; R = Me) on being heated in AcOH–water (3 : 2) at 110 °C. Evidence is presented in favour of an ion-pair dissociation-recombination mechanism in these cases.