Photooxygenation of tetramethylallene competing (2+2) cycloaddition and ene-reactions with singlet oxygen

TPP-sensitized photooxygenation of tetramethylallene (4) in carbon tetrachloride yields acetone (5), 2,4-dimethyl-4-hydroxy-1-penten-3-one (8) and 2,4-dimethyl-1,4-pentadien-3-one (9) in a ratio of 35:20:45, besides minor amounts of resinous products and carbon dioxide. Isomerization of 4 to 2,4-dimethyl-1,3-pentadiene (6) does not occur under the reaction conditions. DABCO quenches the photooxygenation, whereas 2,4,6-tri-t-butylphenol (10) enhances the oxygen consumption rate but leaves the ratio of 5:8:9 unchanged. These results indicate that 4 is oxygenated by singlet oxygen. A mechanism is proposed according to which acetone is generated via a (2+2) cycloaddition whereas 8 and 9 are formed via an ene-reaction between 4 and singlet oxygen.