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 (tau = 63 ns) that intersystem crosses to form photoenols Z-1P (lambda(max) = 350 nm, tau similar to 10 mu s) and E-1P (lambda(max) = 350 nm, tau > 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 2BR (lambda(max) = 330 nm, tau similar to 303 ns) that intersystem crosses to form photoenol E-2P (lambda(max) = 350 nm, tau > 42 mu s), but photoenol Z-2P was not detected. However, in more viscous basic H-bond acceptor (BHA) solvent, such as hexamethylphosphoramide, triplet 2BR intersystem crosses to form both Z-2P (lambda(max) = 370 nm, tau similar to 1.5 mu s) and E-2P. Thus, laser flash photolysis of ketone 2 in methanol reveals that intersystem crossing from 2BR 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 2BR to Z-2P. Density functional theory and coupled cluster calculations were performed to support the reaction mechanisms for photoenolization of ketones 1 and 2.