Steric Demand and Rate‐determining Step for Photoenolization of Di‐ ortho ‐substituted Acetophenone Derivatives

Laser flash photolysis of ketone 1 in argon‐saturated methanol yields triplet biradical 1BR ( τ = 63 ns) that intersystem crosses to form photoenols Z ‐1P ( λ max = 350 nm, τ ~ 10 μs) and E ‐1P ( λ max = 350 nm, τ > 6 ms). The activation barrier for Z ‐1P re‐forming ketone 1 through a 1,5‐H shift was determined as 7.7 ± 0.3 kcal mol −1 . In contrast, for ketone 2, which has a less sterically hindered carbonyl moiety, laser flash photolysis in argon‐saturated methanol revealed the formation of biradical 2 BR ( λ max = 330 nm, τ ~ 303 ns) that intersystem crosses to form photoenol E ‐2P ( λ max = 350 nm, τ > 42 μs), but photoenol Z ‐2P was not detected. However, in more viscous basic H‐bond acceptor ( BHA ) solvent, such as hexamethylphosphoramide, triplet 2 BR intersystem crosses to form both Z ‐2P ( λ max = 370 nm, τ ~ 1.5 μs) and E ‐2P. Thus, laser flash photolysis of ketone 2 in methanol reveals that intersystem crossing from 2 BR to form Z ‐2P is slower than the 1,5‐H shift of Z‐2P, whereas in viscous BHA solvents, the 1,5‐H shift becomes slower than the intersystem crossing from 2 BR to Z‐2P. Density functional theory and coupled cluster calculations were performed to support the reaction mechanisms for photoenolization of ketones 1 and 2 .