Photoreaction of 2‐benzoylcyclohexanones on a silica gel surface: Deviation from their solution photochemistry

On irradiation on a dry silica gel surface, 2-benzoylcyclohexanones which have the lowest n, π* state and are devoid of methyl substituents on their cyclohexanone rings, undergo an oxidative cleavage to give adipic acid and subsituted benzoic acid along with the Norrish Type II product. Irradiation of 2-chlorocylohexanone and cyclohexane-1,2-dione on silica gel also gives adipic acid. The cyclohexanonyl radical on the surface, which is produced from the α-cleavage of the 2-benzoyl group of the 2-benzoylcyclohexanones or the cleavage of the CCl bond of 2-chlorocyclohexanone, is suggested as the precursor of adipic acid; the radical is probably converted into cyclohexane-1,2-dione, which undergoes a secondary photoreaction to give adipic acid on the surface.