The reactivity of diarylmethyl carbocations within non-protic zeolites

The generation of diarylmethyl carbocations within non-protic zeolites (LiY, NaY, KY, RbY, CsY, and NaX) was carried out via laser excitation (266 nm or 266 nm / 308 nm) of diarylacetic acids. Rapid loss of CO2, followed by photochemical oxidation of the diarylmethyl radicals resulted in the formation of the diarylmethyl carbocations. The reac- tivity of the diarylmethyl carbocations was found to be highly dependent on the alkali metal counterion and the Si/Al ratio of the zeolite framework. Furthermore, the effect of various para-electron donating substituents (4-H, 3-CH3, 4-CH3, 4,4'- CH3, and 4-OCH3) on the diarylmethyl carbocation reactivity was investigated. Comparison of the Hammett plots con- structed for the reactivity of the diarylmethyl carbocations in zeolites with those in solution (CH3CN-H2O (1:2) and TFE) showed decreased sensitivity of the electrophilic species towards para-electron donating substituents in the heterogeneous media. The leveling effect observed in the zeolite Hammett plots has been attributed to the presence of an isokinetic rela- tionship, specifically, a low isokinetic temperature for the reaction of diarylmethyl carbocations with the aluminosilicate zeolite framework.