In this work, we use computational approaches to determine the effects of substituents on the formation and cyclization of 2’-arylbenzaldehyde oxime ether radical cations. Recent experimental work has demonstrated that these reactive intermediates, formed through photoinduced electron transfer with the photosensitizer 9,10-dicyanoanthracene, undergo intramolecular cyclization to yield substituted phenanthradines. The experimental study further showed correlations between the yield of cyclized products and the Hammett σ<sup>+</sup> parameter of the substituent on the aryl group, with both strongly electron-withdrawing and strongly electron-donating substituents shown to reduce the yield of cyclized product. Using thermally averaged ionization potentials, calculated at the ωB97M-V/def2-QZVPP// ωB97X-D/cc-pvdz level of theory, we provide an explanation for these experimental observations. In particular, we show that electron-withdrawing substituents impair the formation of radical cations by increasing the energetic cost of oxidation whereas strongly electron-donating substituents alter the site of oxidation and hence directly affect the energetics of radical cation cyclization. We then extend our analysis to computationally screening an exhaustive number of 2’-arylbenzaldehyde oxime ether compounds with substituents on the central benzene ring in addition to the aryl group. In doing so, we make predictions about the overall scope of this photo-oxidative route towards substituted phenanthradines and identify which compounds merit future experimental investigation.

Computational investigation of the formation and intramolecular cyclization of 2’-arylbenzaldehyde oxime ether radical cations

2019 Spring National Meeting

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