Reactions of mono para-substituted dibenzyl sulphides with bromine and alcohols: mechanism of the pummerer rearrangement

Abstract

REACTIONS OF MONO PARA-SUBSTITUTED OIBENZYL SULPHIDES WITH BROMINE AND ALCOHOLS: MECHANISM OF THE PUMMERER REARRANGEMENT Nine mono para-substituted dibenzyl sulphides were prepared in 63.2-96.7% yield by the reaction of p-substituted benzyl bromides and p-substituted benzyl alcohols with benzyl mercaptan. Dibenzyl sulphide reacted with bromine to yield both cleavage and Pummerer products in various proportions depending on the temperature and solvent used. In tetrachloromethane, the formation of Pummerer products was favoured while in ~he more polar solvents, dichloromethane and trichloromethane, formation of cleavage products was favoured. High temperature 'slightly fevoured -the yield ofPummerer products at the Bxpense of cleavage products. Diols reacted with dibenzyl sulphide and mono para-substituted dibenzyl sulphides to yield the corresponding benzylidene acetals. The acetals ~ formed were isolated in 20-35% yield. Monohydric .. alkanols with exception of methanol resisted reaction and were not converted to acetal. The cleavage reaction of the intermedipte mono para-substituted dibenzyl bromosulphonium bromides was shown to proceed by a synchponous SN2 mechanism when the substituent was electron withdrawing. and by mixed SN1 and SN2 mechanisms when the substituent was electron donating. A Hammett correlation of the rates of the a-bromination reaction o~ mono para-substituted dibenzyl sulphides at room temperature in tetrachloromethane with substituent constants (0) afforded a p value of 0.97+0.02. A p value of 1.27+0.02 was obtained for the a-alkoxylation reaction of mono para-substituted dibenzyl sulphides at reflux in tetrachloromethane. These.results indica~ed that the Pummerer reaction of intermediate bromosulphonium and alkoxysulphonium salts proceeds Vla an ylide intermediate;