Photochemical Reduction of 2‐Bromo‐4,4‐dimethyl‐2‐cyclohexenone

The photochemical behaviour of 2‐Bromo‐4,4‐dimethyl‐2‐cyclohexenone (1) was studied in 2‐propanol and cyclohexane. In both solvents (n‐π*)‐excitation followed by intersystem crossing leads to population of a low‐lying triplet (T 1 ) state, which can be quenched by 1,3‐cyclohexadiene but does not undergo chemical transformation efficiently. (π‐π*)‐Excitation affords 4,4‐dimethyl‐2‐cyclohexenone ( 2 ) as the only product. While in 2‐propanol 2 is formed in 60% from the S 2 ‐state and in 40% from the T 2 ‐state, in cyclohexane reduction occurs exclusively from this upper triplet state. The T 2 ‐state can also be populated via energy transfer using acetone or benzene as sensitizer. The mechanistic dissimilarities for the reduction of excited 1 in either 2‐propanol or cyclohexane are discussed.