Type II photoreaction of benzoyl‐ and (benzoylmethyl)cyclohexanones: cyclohexanone ring inversion and other factors determining their photoreactivities

X‐ray crystallographic structure analysis showed that a molecule of 2‐benzoylcyclohexanone ( 2 ), which undergoes an efficient Type II reaction in solution but not in the crystalline state, is in a keto and chair form with an equatorial benzoyl group. Irradiation of 3‐benzoylcyclohexanone ( 3 ) gives 7‐phenylhepten‐4,7‐dione ( 4 , 61%) and a mixture of cyclization products 5 (25%). High photoreactivity of 2 and 3 is caused by the efficient cyclohexanone ring inversion. Upon irradiation, 2‐(benzoylmethyl)cyclohexanone ( 6 ) gives acetophenone ( 7 ), 2‐cyclohexenone ( 8 ) and the cyclobutanol 9 in 59, 59 and 35% yield, respectively, and 1‐phenyl‐3‐propylpentan‐1,4‐dione ( 10 ) gives 7 and the cyclobutanol 11 in 73 and 4% yield, respectively. The quantum yields for the disappearance of 6 and 10 are 0.68 and 0.67, respectively. Irradiation of 2‐(benzoylmethyl)‐2‐ethoxycarbonylcyclohexanone ( 12 ) gives 7 and the cyclobutanol 13 in 46 and 51% yield, respectively. The quantum yield for the disappearance of 12 is 1.00, so that reverse hydrogen transfer is suppressed by intramolecular hydrogen bonding in the 1,4‐biradical intermediate. When 6 is irradiated on a dry silica gel surface, the Type II reaction occurs to give 7 , 8 and 9 in 38, 38 and 43% yield, respectively. The cyclization in the photoreaction of (benzoylmethyl)cyclohexane also increases from 68% in benzene to 81% on a dry silica gel surface. Restriction of conformational change in the 1,4‐biradical is an important factor for the cyclization. Copyright © 2005 John Wiley & Sons, Ltd.