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 diene to give 1‐cyclohexenyloctahydro‐phenanthrene derivatives 28 ; cycloaddition via contact ion pairs leads to the Diels‐Alder product with styrene acting as the dienophile.