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

Date

2018

Authors

Das, Anushree
Thomas, Suma S.
Garofoli, August A.
Chavez, Kevin A.
Krause, Jeanette A.
Bohne, Cornelia
Gudmundsdottir, Anna D.

Journal Title

Journal ISSN

Volume Title

Publisher

Photochemistry & Photobiology

Abstract

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 2BR (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 2BR 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 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 .

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Citation

Das, A., Thomas, S. S., Garofoli, A. A., Chavez, K. A., Krause, J. A., Bohne, C., & Gudmundsdottir, A. D. (2018). Steric Demand and Rate-determining Step for Photoenolization of Di-ortho-substituted Acetophenone Derivatives. Photochemistry and Photobiology, 95(1), 154-162. https://doi.org/10.1111/php.12996.