Solution-Phase Photodecarbonylation of the Bicyclohexan-3-one System

Article abstract of DOI:10.1021/ja00414a046

Direct irradiation of solutions containing (-)-thujone (6) or (+)-isothujone (7) affords trans- and cis-5-methylene-6-methylhept-2-ene (8 and 9) in quantitative yield.The epimeric ketones give identical produc mixtures, which are somewhat temperature dependent.The product ratio remains constant throughout irradiation, and appropriate control experiments indicate that the observed diene mixture does not represent a rapidly established photostationary state.Limited epimerization of the ketones is observed although this process is much less efficient than extrusion of carbon monoxide.Quenching studies employing piperylene indicate that the majority of the decarbonylation products originate from a short-lived triplet state.Direct irradiation of solutions containing exo- or endo-2-methylbicyclo<3.1.0>hexan-3-one (10 or 11) yields trans- and cis-hexa-1,4-diene (12 and 13) in high yield.Again the epimeric ketones give identical product mixtures, which are slightly temperature dependent.Direct irradiation of solutions containing exo,exo-, exo,endo-, or endo,endo-2,4-dimethylbicyclo<3.1.0>hexan-3-one (16, 15, or 14) affords similar, but not identical, mixtures of trans,trans-, trans,cis-, and cis,cis-hepta-2,5-diene (17, 18, and 19) in high yield.Each of these product ratios is somewhat temparature dependent.These data are explained in terms of a mechanism involving biradical intermediates.Excitation followed by intersystem crossing leads to a short-lived triplet state which undergoes Norrish I cleavage toward the more substituted α position.The diene products are formed upon synchronous cleavage of the cyclopropane bond and loss of carbon monoxide.If internal rotation in the intermediate is rapid with respect to decay, the observed product ratios may be rationalized.Inefficient reclusure of the biradical species leads to limited epimerization of the bicyclo<3.1.0>hexan-3-ones.