The solvolysis of tri-p-tolyl[2-13C]vinyl bromide in 70% HOAc – 30% H2O

Abstract

Tri-p-tolyl[2-13C]vinyl bromide (3-Br-2-13C) was solvolyzed in 70% HOAc – 30% H2O and the scrambling of the label from C-2 to C-1 in the reaction product was found to decrease with increasing concentrations of added NaOAc. For example, the 18.5% scrambling found for the solvolysis of 3-Br-2-13C in 70% HOAc in the absence of any NaOAc was decreased to 7.7% when the reaction was carried out in the presence of 4770 mM NaOAc. Common-ion rate depression as well as isotopic exchange between [82Br]Et4NBr and the organic substrate, 3-Br, were observed, indicating the formation of the dissociated tri-p-tolyivinyl cation. Apparently, the tri-p-tolylvinyl cation, similar to the trianisylvinyl cation, is of sufficient stability to allow it to discriminate between different nucleophiles. Thus the presence of increasing concentrations of the acetate ion would favor the product-forming capture of the tri-p-tolylvinyl cation at the expense of the 1,2-shift process, resulting in a decrease in the extent of scrambling. From the observed scrambling data, selectivity ratios such as KOAc/KSOH and KOAc/Kr(Tol) were calculated.