Selectivity and Charge Transfer in Photoreactions of α,α,α‐Trifluorotoluene with Olefins

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Abstract
In photoreactions of α,α,α‐trifluorotoluene with olefins, the mode of reaction strongly depends on the charge transfer between the starting materials. Substitution of an F‐atom is preferred if the electron transfer becomes exergonic, i.e. if ΔG 0, the olefins undergo cycloaddition onto the arene ring. In the transition area of ΔG ≈︁ 0 eV, both the electron transfer and the reaction mode can be controlled by varying the solvent polarity. For instance, 1 and 9 mainly form cycloaddition products in dioxane (ΔG = + 0.11 eV), whereas in solvents with ε > 3 (ΔG < 0), substitution is the only observed photoreaction. Moreover, in systems of endergonic electron‐transfer processes, the degree and the direction of charge transfer influence both the regio‐ and the stereochemistry of the cycloaddition.

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