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

TitleAn investigation of the yang photocyclization reaction in the solid state: Asymmetric induction studies and crystal structure-reactivity relationships
Publication TypeJournal Article
Year of Publication1998
AuthorsLeibovitch, M, Olovsson, G, Scheffer, JR, Trotter, J
JournalJournal of the American Chemical Society
Date PublishedDec
Type of ArticleArticle
ISBN Number0002-7863

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 bearing methyl substituents alpha to the benzoyl group undergo stereoselective Yang photocyclization to afford endo-arylcyclobutanols. Quantum yield and quenching studies in solution show that the reactions are efficient triplet-mediated processes. 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 (17 in total) in solution gave racemic cyclobutanols, but in the crystalline state, moderate to near-quantitative enantiomeric 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 angular 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 parameters associated with the ring closure reactions of the intermediate 1,4-hydroxy biradicals were estimated from the crystallographic data. This indicates that the biradicals are 66.5 +/- 9.8 and 32.7 +/- 3.2 degrees out of alignment for cleavage, but that they are well oriented for cyclization, with radical separations (D) of 3.08 +/- 0.09 Angstrom. For one of the salts in the adamantane series, the solid-state photoreaction was shown to be topotactic; that is, a single crystal of the reactant salt was transformed quantitatively 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 mapped by X-ray crystallography and the abstracted gamma-hydrogen to be identified unequivocally. In addition, the crystal structure of a partially reacted crystal containing 60% product and 40% reactant was successfully determined. This showed that the two compounds in the mixed crystal have nearly identical shapes and orientations, thus accounting for the single crystal nature of the process. Finally, it was found that ketones lacking methyl substituents alpha to the benzoyl group an either photochemically unreactive or undergo Norrish type II cleavage. Possible reasons for this difference in behavior are presented and discussed.

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