Photoreaction of 1-(1-Cycloalkenyl)-4-phenyl-1-butanone under Neutral and Acidic Conditions

UV irradiation of 1-(1-cycloalkenyl)-4-phenyl-1-butanones gives differing products depending on the reaction conditions, neutral or acidic. In benzene 1-(1-cyclopentenyl)-4-phenyl-1-butanone gives 4-phenylspiro[4,4]-nonan-1-one, formed by the intramolecular abstraction of hydrogen, and 4-cyclopentylidene-4-phenylbutanal, formed by Norrish type I fission of the spiro ketone. 1-(1-Cyclohexenyl)-4-phenyl-1-butanone does not exibit this behavior. Both ketones give the reduction products, 1-cyclopentyl-4-phenyl-1 -butanone and 1-cyclohexyl-4-phenyl-1-butanone, under the action of trifluoroacetic acid, and cyclized products, 7-phenylperhydro-4-indenone and 4-phenylperhydro-1-naphthelenone, under the action of boron trifluoride etherate. The reaction processes under acidic conditions have both an ionic and a free radical nature. The results have lead to the proposition of the mechanism involving electron transfer, proton transfer, and hydrogen abstraction.