The Norrish/Yang type II photochemistry of 16 ketones having the basic cis-4-tert-butyl-1benzoylcyclohexane or 2-benzoyladamantane structure has been investigated in the solid state and solution. In both media, ketones bearing methyl substituents R 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 γ-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 CdO‚‚‚H abstraction distance, was 2.61 ( 0.07 Å, and the values of ω (the γ-hydrogen out-of-plane angle), ∆ (the CdO‚‚‚H γ angle) and θ (the C-H γ ‚‚‚O angle) were 53 ( 11, 84 ( 7, and 115 ( 2°, 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°out of alignment for cleavage, but that they are well oriented for cyclization, with radical separations (D) of 3.08 ( 0.09 Å. 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 γ-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 R to the benzoyl group are either photochemically unreactive or undergo Norrish type II cleavage. Possible reasons for this difference in behavior are presented and discussed.

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Because they proceed with high selectivity, can be driven to completion without crystal melting or disintegration, and can be followed by X-ray diffraction methods, single crystal-tosingle crystal (topotactic) transformations occupy a preeminent position in the field of solid state chemistry. 1 In the present paper, we report the first example of a crystalline phase photorearrangement that is not only topotactic, but which also (i) occurs with a high degree of asymmetric induction (ca. 90% enantiomeric excess) and (ii) involves an ionic chiral auxiliary of known absolute configuration, so that X-ray diffraction studies of the same single crystal at the beginning, midpoint, and final stages of reaction allow the absolute steric course of the photochemical reaction to be mapped. 2 The reaction chosen for investigation was the Yang photocyclization 3 of ketones of general structure 1 (Scheme 1). 4 On the basis of studies with closely related compounds, 5 we were confident that irradiation of 1 would lead to cyclobutanols of general structure 2, and this proved to be the case. Thus, photolyses of ketone 1b in benzene or tert-butyl alcohol afforded essentially quantitative chemical yields of racemic 2b with quantum yields of 0.073 (benzene) and 0.17 (tert-butyl alcohol). Stern-Volmer plots (benzene, 2,5-dimethyl-2,4-hexadiene as quencher) were linear with k q τ ) 2.07 M -1 , indicating reaction through the triplet manifold. The stereochemistry of cyclobutanol 2b follows from crystallographic measurements on the corresponding salt 2c (Vide infra).Our plan was to carry out an asymmetric Yang photocyclization by preparing salts of keto acid 1a with optically active amines and irradiating the resulting crystalssthe so-called solid state ionic chiral auxiliary approach to asymmetric synthesis. 6 Crystals composed of optically active components are required to crystallize in chiral space groups, and these materials provide an asymmetric environment in which to carry out enantioselective chemical reactions. 7 For ketone 1, the two axial γ-hydrogen atoms H x and H y are enantiotopic, and two enantiomeric forms of photoproduct 2 are therefore possible.Keto acid 1a was treated with a variety of optically active amines, including R-methylbenzylamine, norephedrine, pseudoephedrine, ephedrine, and prolinol, and the resulting crystalline salts were irradiated through Pyrex under nitrogen to conversions with a range of <5% to >90%. Aqueous acidic workup followed by treatment with ethereal diazomethane afforded unreacted starting material and photoproduct in the form of their methyl esters 1b and 2b, and this mixture was analyzed for the extent of conversion by capillary gas chromatography and for enantiomeric excess by chiral HPLC (Chiralcel OD column). The results showed that the best ionic chiral auxiliaries were R-methylbenzylamine and norephedrine, which led to enantiomeric excesses in photoproduct 2b of ca. 90% at 5% conversion. The other ionic auxiliaries were much less effective, giving ee values of 20-30% at low conversion...