1,4-hydroxybiradical behavior revealed through crystal structure - Solid-state reactivity correlations

Structure- reactivity correlations for triplet 1,4-hydroxybiradicals in solution are made difficult by the presence of multiple reactive conformers and the possibility of conformation-dependent intersystem crossing. These problems can be overcome by working in the crystalline state, where the conformations of the 1,4-hydroxybiradicals are fixed and determinable by X-ray crystallography of the parent ketones, assuming that hydrogen atom abstraction occurs with little or no change in conformation. This approach is applied to 15 bi- and tricyclic ketones designed to have slightly different biradical conformations, so that the effect of small and incremental changes in geometry on biradical behaviour can be tested. The results indicate that, while geometry does have a strong influence on 1,4-hydroxybiradical partitioning between cyclization, cleavage, and reverse hydrogen transfer, a full understanding of the results requires that the strain involved in forming the cyclization products be taken into account.