Rearrangement studies with 14C. XLIV. Ion-pairs and dissociated ions in the solvolysis of triphenyl[2-14C]vinyl and trianisyl[2-14C]vinyl bromides in 70% acetic acid – 30% water

Abstract

In the solvolysis of triphenyl[2-14C]vinyl bromide (1-Br-2-14C) in 70% HOAc – 30% H2O, the presence of up to 30 equiv. of added NaOAc or 25 equiv. of added Et4NBr did not significantly change the extent of scrambling of the label from C-2 to C-1 in the reaction product. The mean scrambling value observed for 1-Br-2-14C under a variety of conditions was 14.7 ± 0.7%. On the other hand, in the similar solvolysis of trianisyl[2-14C]vinyl bromide (2-Br-2-14C), the extent of scrambling in the product was found to decrease with increasing amounts of added NaOAc. The observed scramblings varied from a mean of 27.7% to 5.7%, respectively, for reactions without and with 10 M added NaOAc. With 1-Br-2-14C, the possibility is considered that ionization proceeds only to an ion-pair in which the scrambling via 1,2-phenyl shift is not affected by the presence of added OAc−. In the case of 2-Br-2-14C, further dissociation to the free trianisylvinyl cation took place and in the dissociated ion, competition between the scrambling process and capture reactions with all the nucleophiles present resulted in a decrease in the extent of scrambling with the presence of the more nucleophilic OAc−. Using the extents of scrambling as well as product data, various ratios including kSOH/kr, kOAc/kr, [Formula: see text], and kOAc/kHOAc, are evaluated.