The photochemistry of some [Beta], [gamma]-allenic and acetylenic ketones

The photochemistry of 3,3, 5, 5-tetramethylhepta-l, 6-diyn-4-one (4), 3, 3-dimethylhexa-4, 5-dien-2-one (5) and 3,3, 5, 5-tetramethylocta-1,6,7-trien-4-one (6) were investigated. The synthesis of 4 was carried out by alkali cleavage of the bistrimethylsilylated precurser which resulted from the reaction of phosgene and the Grignard reagent from l-trimethylsilyl-3-methyl-3-chloro-l-butyne. Ketones 5 and 6 were prepared by reacting 2, 2-dimethylpenta-3,4-dienal with MeMgl and 3-methylbut-2-enyl magnesium chloride respectively to form the corresponding alcohols, which were then oxidized to the ketones by pyridinium chlorochromate. Upon direct irradiation, 4 gave the expected CQ coupling hydrocarbons in a ratio similar to that obtained from thermolysis of the analogous azo compound. No turnaround (1, 3-acyl shift) product was observed. Acetone sensitized irradiation of 4 gave the same hydrocarbons in a different ratio and three unidentiüed minor components. The difference in the ratio of CQ hydrocarbons was explained by a secondary process involving acetone. None of the photoreactions of 5 appeared to be very interesting. Except for the formation of polymers, the only isolable product was 4,4,7-trimethyl -5-methyleneocta-1,2,6-triene. The direct and sensitized irradiation of J5 gave the same thirteen products, which indicates that intersystem crossing in 6 is more efficient than in normal acyclic & , X -unsaturated ketones. The cL-cleavage process may also arise from the T state in phased on this observation. Three of the products were C^Q hydrocarbons from dimerization of 1,1-dimethylallyl radical and another was 2,5-hexanedione resulting from the photoreaction of acetone with itself. Two [2+2] cycloaddition products, tricyclo[3,1, l]-2,2,4,4-tetramethyl-6-methyleneheptan-3-one and bicyclo[3,2,]-2,2, 4, 4-tetramethyl-6-methyleneheptan-3-one, were also identified