A Photoisomerization Reaction of Cyclic Ketones in the Liquid Phase

It has been found that six-, seven-, and eight-membered cyclic ketones, when irradiated as pure liquids with light of 3130 Å., undergo a ring contraction reaction to give the 2-methyl derivative of the cyclic ketone with one less carbon. The reaction accounts for 6% of the cyclohexanone that undergoes photolysis but only 0.2 % in cy-cloheptanone. A variant of this reaction gives 2-n-propylcyclopentanone in the photolysis of cyclooctanone. In cyclohexanone the reaction has a quantum yield of 0.03. This value decreases in solutions of cyclohexanone in cyclohexane. At a given concentration, the addition of cyclohexene has no effect on the yield. Substitution of the ring by a methyl group in the 2-position seems to obscure the reaction, while in the 3-position only one of the two possible dimethylcyclopentanones is formed. In the photolysis of cyclohexanone-2, 2, 6, 6-d4, the product corresponded to COCH(CHD2)CH2CH2CD2. The reaction is believed to involve the transfer of a proton from the 3- to the 2-position in a configuration in which the ring is substantially intact. © 1965, American Chemical Society. All rights reserved.