Vinylic Cations from Solvolysis. 30. Bromide Exchange as a Reactivity-Selectivity Probe in Vinylic Solvolysis

Article abstract of DOI:10.1021/ja00545a020

Simultaneous exchange with radioactive Br(-) and solvolysis of 9-(α-bromo-p-methoxybenzylidene)anthrone (3a), its p-methyl analogue (3b), and three α-anisyl-β,β-diarylvinyl bromides (5a, 5b, and 5c) in AcOH/NaOAc/Et4NBr were studied.The reactions proceed via free vinyl cations and the dependence of k_t on the NaOAc concentration for 3a and 5a indicates the rare S_N2(C+) mechanism.The selectivity constants of the derived cations toward bromide and acetate ions α = k_Br/k_OAc increase with the reactivity of RBr, both in the family of the α-aryl-substituted (3) and in the family of the β-aryl-substituted (5) derivatives.Consequently, the reactivity-selectivity relationship rather than Ritchie's constant selectivity relationship applies for solvolytically generated free triarylvinyl cations.The exchange-solvolysis method is compared with the method based on common ion rate depression as reactivity-selectivity probes for the vinyl cations formed in the solvolysis.The exchange-solvolysis method is more accurate and measures the selectivity and reactivity in independent processes.Caution should be exercised when comparing k₁ and α values which are measured by the two different methods, due to unknown salt effects on the two parameters.