Photolysis dynamics of diastereoselectivity in the photocyclization of o-ethoxybenzophenone and o-2,2,2-trifluoroethoxybenzophenone

The photochemical reactions of o-ethoxybenzophenone (R-1) and o-2,2,2-trifluoroethoxybenzophenone (R-2) in cyclohexane solutions of 0.7 M pyridine have been investigated using time-resolved laser flash photolysis to understand diastereoselectivity in the photocyclization of the two compounds. Whereas the formation times of biradical intermediates (7 ns) are the same for the two compounds, the lifetime of the biradical intermediate of R-2 (200 ns) is eight times larger than that of R-1 (25 ns), explaining why diastereoselectivity is much lower in the photocyclization of R-2.