An Investigation of the Solid-State Photochemistry of α-Mesitylacetophenone Derivatives: Asymmetric Induction Studies and Crystal Structure–Reactivity Relationships

The photochemical conversion of a series of α-mesitylacetophenone derivatives into 2-indanols via δ-hydrogen abstraction has been investigated in the solid state. A correlation between solid-state reactivity and crystal structure has been established for this type of reaction. For the seven compounds whose crystal structures were determined, the average value of d (CO⋯H distance) and L (CO⋯CH3 distance) were 2.77±0.04 Å and 3.42±0.06 Å, and the value of ω (δ-H out of plane angle), Δ (CO⋯H angle) and θ (C–H⋯O angle) were 59±2°, 80±7° and 123±3°, respectively. These parameters depended mainly on the magnitude of the deviation of the carbonyl group from the fully bisecting position over the mesityl ring, which ranged from 9–14° in the case of in the solid state reactive ketones and approached 0° for the unreactive compounds. Asymmetric induction studies were carried out by providing the reactants with carboxylic acid substituents to which ionic chiral auxiliaries were attached through salt formation with optically active amines. Irradiation of the salts (13 in total) in the crystalline state gave enantiomeric excesses of up to 90%. The crystal structures of three of the salts were determined and on this basis, the reasons for the selectivity in the crystalline state are discussed.