Latent photochemical hydrogen abstraction reactions realized in crystalline media

Abstract

The photochemistry of four closely related compounds possessing the 2-cyclohexen-1-one chromophore has been investigated in solution as well as in the solid state. Each enone has two relatively low energy conformational isomers, one of which (A) is ideally arranged for hydrogen abstraction through a five-membered transition state, while the other (B) has a structure that favors hydrogen atom abstraction through a six-membered transition state. In solution, despite the fact that conformer A may predominate in the conformational equilibrium, all four enones give products resulting from the much faster six-membered transition state abstraction process. In the solid state, because each enone is frozen in its lower energy conformation, the process followed is conformation-specific. Two of the enones studied crystallize in conformation B and therefore give the same photoproducts as observed in solution. The other two enones crystallize in conformation A, and in these cases, the latent five-membered transition hydrogen abstraction process is realized.