Substituent effects on the photogeneration and selectivity of triaryl vinyl cations

The photochemical reactions of a series of triaryl vinyl halides 1X in acetic acid and in acetonitrile have been studied using product analysis as a function of the time of irradiation. The quantum efficiencies of formation of the products derived from the photogenerated vinyl cations 1(+) depend on the alpha-aryl substituent, the beta-aryl substituent, the leaving group X (= bromide or chloride), and the temperature at which the irradiations are carried out. Hammett correlation or noncorrelation of the alpha-aryl substituent effects with (excited-state) substituent constants indicates that the ions 1(+) are formed directly from the excited states of 1X by heterolytic cleavage of the carbon-halogen bond. Homolytic cleavage, yielding radicals 1(*), is a parallel process: the partitioning into ion and radical occurs in the excited state. This conclusion is corroborated by the leaving group effect and the temperature effect. By performing the photochemical reactions of 1X in the presence of HOAc and/or NaOAc as well as the labeled common halide ion (82)Br(-) or (36)Cl(-), the relative reactivities of the cations 1(+) toward these nucleophiles were determined. The selectivities follow the Reactivity-Selectivity Principle. The temperature effect data show that the reactions of the cations with the anionic nucleophiles are (de)solvation controlled and their reactions with the neutral nucleophile activation controlled.