| Abstract
| - Photochemical electron transfer (PET) and chemicalelectron transfer (CET) studies have been conductedin solution and within zeolite cavities for thebicyclo[2.1.0]pentanes (2a−j),prepared by direct photolysis of thecorresponding azoalkanes 1. The advantage of the CEToxidations is that they proceed catalytically in a cleanmanner to afford the rearranged cyclopentenes 3 in excellentyields. A complete reversal in the regioselectivityofthe 1,2 migration has been observed for the unsymmetrical derivativesof bicyclo[2.1.0]pentane, namely 2b(methylsubstitution) versus 2c,i (phenylsubstitution). Both in solution and in the zeolite cavities, theless substitutedcyclopentene 3b‘ is obtained for the methyl derivative2b and the more substituted cyclopentenes3c,i for the phenylcases 2c,i. This unexpected fact isrationalized in terms of delocalization of the positive charge into thearomaticring for the phenyl-substituted radical cation, as corroborated by AM1calculations. Furthermore, the electron transferresults of stereolabeled housanes demonstrate that for thedeuterium-labeled bridgehead dialkyl-substituted housane2e(D) also the stereochemical memory effectoperates. In contrast, for the methyl-labeled housanesanti- and syn-2h, exclusively hydrogen migration occurs. Thisdiffering behavior is interpreted in terms of facile ringinversionof the syn-2h•+radical cation to the more stable anti isomer and subsequentpreferential migration of the pseudo-axial hydrogen atom. Moreover, the heterogeneous PET chemistry ofthe bicyclopentanes 2 in the zeolitesestablishesconvincingly that tailor-made, encapsulated electron transferphotosensitizers serve as effective electron acceptorson optical excitation. In spite of the inherent diffusion problemsin such solid sensitizers, quite efficient PET activityis observed compared to that in the homogeneous phase.Unfortunately, the steric confinement imposed by thezeolite support is not sufficient for the small bicyclopentanes, whichpenetrate into the zeolite interior, to promoteselective rearrangements of the radical cationintermediates.
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