Photoenolization of ortho-methyl-substituted acetophenones: solvent effects on the triplet state reactivity

Abstract

The initial reaction steps in the photoenolization of 2-methyl-, 2,4-dimethyl-, and 2,4,6-trimethyl-acetophenone were investigated using conventional and laser flash photolysis techniques. Internal hydrogen transfer occurs from the triplet as well as from the first excited singlet state. Only one isomeric form of the enol was observed, absorbing at about 380 nm. The first-order decay rate constant of the photoenol of 2-methylacetophenone, and hence, the rate constant of reketonization, was measured in cyclohexane (0.32 s^–1), dioxan (1.8 s^–1), and propan-2-ol (7.9 s^–1). The photoenol reacts with oxygen and maleic anhydride; the rate constants for these reactions are 2.0 × 10⁵ and 2.1 × 10⁴ dm³ mol^–1 s^–1, respectively.

The effect of solvent polarity on the 2-methylacetophenone triplet state reactivity was studied by triplet decay measurements in various organic solvents. The first order decay rate constant of triplet 2-methylacetophenone decreases from 6.6 × 10⁶ s^–1 in cyclohexane to 7.7 × 10⁵ s^–1 in propan-2-ol. The triplet state reactivity variations are interpreted as being due to solvent-induced shifts in the relative energy levels of the closely spaced (n, π*) and (π, π*) configurations; these shifts change the relative population of the ^1,3(n, π*) and ^1,3(π, π*) states and the extent of vibronic and spin-orbit interaction between these states.