A REINVESTIGATION OF THE REACTIONS OCCURRING UPON PHOTOEXCITATION OF CYCLOHEXENONE IN THE PRESENCE OF TRIETHYLAMINE

It had been previously reported that the distribution of products from irradiation of cyclohexenone (CH) in the presence of triethylamine (TEA) required the intermediacy of a triplet excimer intermediate derived from the enone. Reinvestigation of this reaction has given results removing the necessity of postulating such an intermediate. Specifically, the reaction results in the products previously reported, namely cyclohexanone, photodimers (two major, two minor), and a pair of 1:1 enone-amine adducts, as well as previously unobserved products from reductive dimerization of the enone (3-(3-oxocyclohexyl)cyclohexanone) and from condensation of two enone and one amine moieties. The photodimer yields decreased dramatically as the amine concentration was increased, and the product ratios were dependent on the CH concentration, findings which are contrary to the earlier report but understandable if there is a competition between TEA and ground-state CH for capture of monomeric CH triplets. The mechanism for formation of the products derived from interaction of TEA involves an initial electron-transfer step to give a pair of radical ions, followed immediately by proton transfer to a ketyl and an alpha-aminoalkyl radical, which yield the observed products by subsequent radical combination and H-abstraction reactions. These results are consistent with published laser flash kinetic data from this laboratory, and earlier studies of the photochemical behavior of other ketones in the presence of amines.