Photo-Aryl Coupling and Related Reactions. III. The Transannular Dehydrogenation of [2.2]Metacyclophane Induced by Iodine

The ultraviolet irradiation of [2.2]metacyclophane (I) with iodine in a cyclohexane solution was shown to afford 4,5,9,10-tetrahydropyrene (II), which, on further dehydrogenation, was led to 4,5-dihydropyrene and pyrene. The transannular reaction leading to II occurred much more rapidly than the subsequent dehydrogenation reactions. Thus II was obtainable in an optimum yield of 62% by the irradiation of I in the presence of an equimolar amount of iodine and an excess of sodium bicarbonate. By ESR and UV spectral studies, 253.7 mμ light was found to be responsible for the transannular reaction.