Photochemistry of Ketones in Solution. 80. Irradiation of 3,4-Dimethyl-4-(trichloromethyl)-2,5-cyclohexadienone in Methanolic Solutions

Article abstract of DOI:10.1021/jo00392a027

The photochemistry of the title compound in methanolic solutions has been studied as a function of acidity, temperature and dienone concentration.A large number of products are formed under these conditions, 12 of which have been isolated and characterized spectroscopically and, in some cases by comparison with products previously isolated from irradiations in other solvents.Relatively minor variations in the temperature and acidity of the reaction medium had a profound effect on the product distribution.The relative yield of all but one product (18) increased with temperature, and the formation of a subset of products was facilitated in more highly acidic solutions.The yields of two of the products, 11 and 16, increased as a function of starting dienone concentration, which is attributed to reaction via a triplet excimer leading to a radical ion pair, the proposed source of these products.Naphthalene quenches formation of all the products with the same efficiency, indicating a common triplet precursor for all products.Differential quenching by 1,3-cyclohexadiene is attributed to quenching of both the monomeric and excimeric triplet states, the latter having too low a triplet excitation energy to allow quenching by naphthalene.The unique temperature dependence and quenching of formation of 18 prompted a study to determine if this product was indeed a product of the n,?^* triplet state of the starting dienone.A dioxetane which would lead to this triplet was prepared by photooxygenation of an appropriate triene.This dioxetane was unstable even at -70 deg C in methanol, under which conditions the rearrangement product 18 was indeed formed.Mechanisms for formation of most of the products are proposed in this and a previous paper, and an overall mechanistic scheme is presented emphasizing the points at which there is partitioning of the reaction pathway; i.e., formation of the proposed triplet excimer is directly competitive with the route leading to 3,5-bridged zwitterions, and there is an additional bifurcation with one set of products derived directly from the well-established zwitterion intermediate and another set upon protonation and nucleophilic trapping of the zwitterion.All in all, the rearrangement and fragmentation pathways available to this system are more numerous and complex than had been anticipated.