1,3-Diketonatoboron difluoride sensitized cation radical reactions

Abstract

Dibenzoylmethanatoboron difluoride (DBMBF2) and allied BF2 complexes interact from their singlet excited state with trans-anethole (t-A), quadricyclene (QC), and norbornadiene (NBD) by electron transfer to generate the corresponding cation radicals, which undergo the reported reactions. By sensitization, t-A undergoes dimerization to form the anti head-to-head and syn head-to-head dimers with retention of stereochemistry. The formation is reversible under sensitization conditions, leading to accumulation of the more stable anti isomer. However, irradiation of the absorption band of the DBMBF2 – t-A ground state complex did not lead to dimerization of t-A. By DBMBF2 sensitization, QC is cleanly converted to NBD while NBD is not affected. The calculation shows QC+• possesses higher energy than NBD+• by 7.5 kcal/mol, hence an irreversible rearrangement. Other sensitizers (e.g., cyanoaromatics and tetrachlorobenzoquinone) also promote these cation radical reactions but not as cleanly as DBMBF2. Key words: photosensitization by boron complexes, cation radical rearrangement, cation radical cycloaddition, electron transfer sensitization, photoreaction of ground state complexes.