The Photoenolization of 2‐Methylacetophenone and Related Compounds

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Abstract
A reinvestigation of 2‐methylacetophenone (1) by ns flash photolysis has provided detailed evidence for the reaction sequence of photoenolization. The triplet reaction proceeds adiabatically from the lowest excited triplet state of the ketone, 3K (1), to the enol excited triplet state, 3E (1), which decays both to enol and ketone ground state. The Z‐ and E‐isomers of the photoenol, Z‐E (1) and E‐E (1) are formed in about equal yield by the triplet pathway, while direct enolization from the lowest excited singlet state of 1 yields (predominantly) the Z‐isomer. Intramolecular reketonization from Z‐E (1) to 1 proceeds at a rate of ca. 108s−1 in cyclohexane, but can be retarded to ca. 104s−1 in hydrogen‐bond‐acceptor solvents. The proposed mechanism is summarized in Scheme 1 and rationalized on the basis of a state correlation diagram, Scheme 2. 3,3,6,8‐Tetramethyl‐1‐tetralone (2) was used as a reference compound with fixed conformational position of the carbonyl group, and some results from a brief investigation of 2,4‐dimethylbenzophenone (3) are also reported.