CYCLOADDITION REACTIONS INITIATED BY PHOTOCHEMICALLY EXCITED PYRYLIUM-SALTS

Several pyrylium, thiapyrylium, and pyridinium salts have been synthesized and used as sensitizers for photochemically induced electron-transfer (PET) reactions. The salts have been tested in the mixed cycloaddition reactions of styrenes 9 with 1,3-cyclohexadiene (8) or 1,1'-dicyclohexenyl (23). In the case of the PET [4 + 2] cycloaddition of styrene (9a) to 1,3-cyclohexadiene (8), the reaction takes place via the cation radical of the diene. When chloroform instead of dichloromethane is used as the solvent, only [2 + 2] cycloaddition products are obtained. In contrast, if 1,3-cyclohexadiene (8) is replaced by 1,1'-dicyclohexenyl (23), the key step of the reaction seems to be the oxidation of styrene (9). The product ratios depend on the sensitizers used. If solvent-separated ion pairs are formed, styrene reacts as a diene to give 1-cyclohexenyloctahydrophenanthrene derivatives 28; cycloaddition via contact ion pairs leads to the Diels-Alder product with styrene acting as the dienophile.