Rearrangement studies with 14C. XLII. The solvolysis of triphenylvinyl-2-14C bromide in aqueous acetic acid and in 2,2,2-trifluoroethanol

Abstract

Solvolytic studies with triphenylvinyl-2-14C bromide (l-Br-2-14C) in HOAc–H2O and in 2,2,2-trifluoroethanol (TFE) containing 2,6-lutidine were carried out under a variety of conditions. Scramblings of the label from C-2 to C-l in the reaction product and in the unconsumed reactant were determined. For solvolyses in HOAc–H2O, scrambling was observed in the product, but not in the recovered reactant. The extent of scrambling was higher when the proportion of H2O in the solvent was lower. With a given solvent composition, the amount of scrambling did not change with reaction time, the average extents of scrambling being 9.6,12.7, 15.3, 16.7, and 18.4%, respectively, for solvolysis in 50, 60, 70, 80, and 90% HOAc. With 70% HOAc, the presence of about 3 equiv. of NaOAc or NaBr did not influence the extent of scrambling. For the reactions in TFE, there was scrambling in both the reaction product and the recovered reactant, indicating the occurrence of a return process, The presence of added Et4NBr did not affect the extent of scrambling, suggesting that the 1,2-phenyl shift occurred in the ion-pair stage and the return is, therefore, an ion-pair return. The amount of scrambling in the product and in the recovered reactant increased with increasing reaction time, but for a given reaction time, the extents of scrambling in the product and in the recovered reactant were the same. The mean values for the scrambling of the label from C-2 to C-l in the product and the recovered reactant, after a reaction time of 1, 2, 4, and 20 days at 150 °C, were 20.1, 30.0,40.0, and 44.1%, respectively. Mechanistic explanations for both reactions in HOAc-H2O and in TFE are discussed.