Photodecarbonylation of cis- and trans-2,7-Dimethyl-3,5-cycloheptadienones: Applicability of Orbital Symmetry Theory to Photochemical Cheletropic Fragmentations

Article abstract of DOI:10.1021/ja00310a048

A stereochemical test was conceived to determine if the photochemical extrusion of carbon monoxide from 3,5-cycloheptadienones to give 1,3,5-hexatrienes is a concerted process, and thus an example of a photochemical cheletropic fragmentation.The cis- and trans-2,7-dimethyl derivatives of this ring system were synthesized.Spectroscopic data could not distinguish unambiguously between the two materials, and configurations were therefore assigned to the two dienones based on the NMR spectra of the adducts of the dienones and the reactive dienophile PTAD and an X-ray crystal structure of the cis adduct.The several isomeric 2,4,6-octatrienes were prepared and their photoisomerization was studies preliminary to study of the photolysis of the dienones.Since no conditions could be found which afforded fragmentation of the dienones without secondary isomerization of the trienes, the composition of the product mixture was deterrmined as a function of time on excitation of the dienones at 313 nm.It was found that the isomeric dienones indeed gave different triene photoproducts, the cis-dienone affording the (E,Z,Z)-octatriene and the trans-dienone affording the (Z,Z,Z,Z)-octatriene.In both cases, these products were the predominant if not exclusive initial products, with other trienes appearing soon afterward.It is concluded that these photochenmical reactions proceed by a concerted reaction path, involving conrotatory opening of the cycloheptadienone ring system.The same course of reaction was observed some time ago for thermal cleavage of the analogous cyclic sulfones.Various theoretical rationales for this behavior are discussed.The photochemical reactions clearly do not follow the predictions for a linear cheletropic reaction, and a nonlinear path is a distinct possibility.Howeverr, the formation of the relatively strained (Z,Z,Z)-triene from the trans-2,7-dimethylcycloheptadienone is difficult to rationalize on the basis of any concerted mechanism except by invoking the principle of least nuclear motion.Thus, reaction of the trans-dienone appears to proceed selectively from the conformation in which both methyls are pseudoaxial, which according to MM2 calculations is less stable than the diequatorial conformation in the electronic ground state by 2.7 kcal/mol.It is suggested that orbital symmetry may not play a major role in controlling the course of this type of photochemical cheletropic reaction.