An investigation of the yang photocyclization reaction in the solid state:Asymmetric induction studies and crystal structure-reactivity relationships

The Norrish/Yang type II photochemistry of 16 ketones having the basic cis- 4-tert-butyl-1-benzoylcyclohexane or 2-benzoyladamantane structure has been investigated in the solid state and solution. In both media, ketones beari ng methyl substituents alpha to the benzoyl group undergo stereoselective Y ang photocyclization to afford endo-arylcyclobutanols. Quantum yield and qu enching studies in solution show that the reactions are efficient triplet-m ediated processes. Asymmetric induction studies were carried out by providi ng the reactants with carboxylic acid substituents to which "ionic chiral a uxiliaries" were attached through salt formation with optically active amin es. Irradiation of the salts (17 in total) in solution gave racemic cyclobu tanols, but in the crystalline state, moderate to near-quantitative enantio meric excesses were obtained. Single crystal X-ray diffraction studies were successfully performed on 10 neutral ketones and four salts. This allowed the reactive gamma-hydrogen atoms to be identified and the distance and ang ular parameters associated with their abstraction to be tabulated. For the 14 compounds whose crystal structures were determined, the average value of d, the C=O ... H abstraction distance, was 2.61 +/- 0.07 Angstrom, and the values of omega (the gamma-hydrogen out-of-plane angle), Delta (the C=O .. . H, angle) and a (the C-H-gamma... O angle) were 53 +/- 11, 84 +/- 7, and 115 +/- 2 degrees, respectively. In a similar manner, the geometric paramet ers associated with the ring closure reactions of the intermediate 1,4-hydr oxy biradicals were estimated from the crystallographic data. This indicate s that the biradicals are 66.5 +/- 9.8 and 32.7 +/- 3.2 degrees out of alig nment for cleavage, but that they are well oriented for cyclization, with r adical separations (D) of 3.08 +/- 0.09 Angstrom. For one of the salts in t he adamantane series, the solid-state photoreaction was shown to be topotac tic; that is, a single crystal of the reactant salt was transformed quantit atively into a single crystal of the corresponding cyclobutanol salt. Since the absolute configuration of the ionic chiral auxiliary in this case was known, this permitted the absolute steric course of the reaction to be mapp ed by X-ray crystallography and the abstracted gamma-hydrogen to be identif ied unequivocally. In addition, the crystal structure of a partially reacte d crystal containing 60% product and 40% reactant was successfully determin ed. This showed that the two compounds in the mixed crystal have nearly ide ntical shapes and orientations, thus accounting for the single crystal natu re of the process. Finally, it was found that ketones lacking methyl substi tuents alpha to the benzoyl group an either photochemically unreactive or u ndergo Norrish type II cleavage. Possible reasons for this difference in be havior are presented and discussed.