Reaction Mechanism in Crystalline Solids: Kinetics and Conformational Dynamics of the Norrish Type-II Biradicals from α-Adamantyl-p-Methoxyacetophenone
G. Kuzmanich, C.S. Vogelsberg, E.F. Maverick, J.C. Netto-Ferreira, J.C. Scaiano, and M.A. Garcia-Garibay, “Reaction Mechanism in Crystalline Solids: Kinetics and Conformational Dynamics of the Norrish Type-II Biradicals from α-Adamantyl-p-Methoxyacetophenone”, J. Am. Chem. Soc., 134 (2), 1115-1123 (2012); JAN 2012
In an effort to determine the details of the solid-state reaction mechanism and diastereoselectivity in the Norrish type II and Yang cyclization of crystalline α-adamantyl-p-methoxyacetophenone, we determined its solid-state quantum yields and transient kinetics using nanocrystalline suspensions. The transient spectroscopy measurements were complemented with solid-state NMR spectroscopy spin–lattice relaxation experiments using isotopically labeled samples and with the analysis of variable-temperature anisotropic displacement parameters from single-crystal X-ray diffraction to determine the rate of interconversion of biradical conformers by rotation of the globular adamantyl group. Our experimental findings include a solid-state quantum yield for reaction that is 3 times greater than that in solution, a Norrish type II hydrogen-transfer reaction that is about 8 times faster in crystals than in solution, and a biradical decay that occurs on the same time scale as conformational exchange, which helps to explain the diastereoselectivity observed in the solid state.