Photochemical and chemical electron transfer reactions of bicyclo[2.1.0]pentanes (housanes) in solution and in zeolite cavities

Photochemical electron transfer (PET) and chemical electron transfer (CET) studies have been conducted in solution and within zeolite cavities for the bicyclo[2.1.0]pentanes (2a-j), prepared by direct photolysis of the corresponding azoalkanes 1. The advantage of the CET oxidations is that they proceed catalytically in a clean manner to afford the rearranged cyclopentenes 3 in excellent yields. A complete reversal in the regioselectivity of the 1,2 migration has been observed for the unsymmetrical derivatives of bicyclo[2.1.0]pentane, namely 2b (methyl substitution) versus 2c,i (phenyl substitution). Both in solution and in the zeolite cavities, the less substituted cyclopentene 3b' is obtained for the methyl derivative 2b and the more substituted cyclopentenes 3c,i for the phenyl cases 2c,i. This unexpected fact is rationalized in terms of delocalization of the positive charge into the aromatic ring for the phenyl-substituted radical cation, as corroborated by AM1 calculations. Furthermore, the electron transfer results of stereolabeled housanes demonstrate that for the deuterium-labeled bridgehead dialkyl-substituted housane 2e(D) also the stereochemical memory effect operates. In contrast, for the methyl-labeled housanes anti- and syn-2h, exclusively hydrogen migration occurs. This differing behavior is interpreted in terms of facile ring inversion of the syn-2h(.+) radical cation to the more stable anti isomer and subsequent preferential migration of the pseudo-axial hydrogen atom. Moreover, the heterogeneous PET chemistry of the bicyclopentanes 2 in the zeolites establishes convincingly that tailor-made, encapsulated electron transfer photosensitizers serve as effective electron accepters on optical excitation. In spite of the inherent diffusion problems in such solid sensitizers, quite efficient PET activity is observed compared to that in the homogeneous phase. Unfortunately, the steric confinement imposed by the zeolite support is not sufficient for the small bicyclopentanes, which penetrate into the zeolite interior, to promote selective rearrangements of the radical cation intermediates.