Photochemical electron transfer initiated oxidative fragmentation of aminopinacols. Factors governing reaction rates and quantum efficiencies of C-C bond cleavage

The fragmentation reactions of the cation radicals of a series of remote diamine-substituted pinacols have been investigated. The cation radicals were generated upon photooxidation with excited 2,6,9,10-tetracyanoanthracene in acetonitrile. The products are consistent with cleavage of the central C-C bond. The rate constants for fragmentation were determined both from steady-state quantum yield studies and from time-resolved measurements. In general, the rate constants for fragmentation increase with increasing stability of the radical and cation products. However, the results of temperature dependence studies clearly demonstrate that conformational effects play an important role in the transition state. In some cases, these conformational effects can result in changes in the reactivity order expected from purely thermodynamic considerations.