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 behavior 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.