An Investigation of the Yang Photocyclization Reaction in the Solid State:  Asymmetric Induction Studies and Crystal Structure−Reactivity Relationships

Abstract

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.

Asymmetric induction under confinement

“…15 4) Use of ionic chiral auxiliaries to form ionic crystals between a photoactive anion (or cation) and a nonphotoactive chiral cation (or anion). Scheffer and colleagues 16 obtained almost purely enantiomeric ciclobutanols by Yang photolysis of ketones carrying a carboxylate group, in order to form salts with chiral amines and hence give chiral crystals. Their approach, summarized in Scheme 3, comprises the use of ionic chiral auxiliaries and appears to be fairly general.…”

Structural Aspects and Templation of Photochemistry in Solid‐State Supramolecular Systems

“…67 Some of them are involved with intramolecular cyclization or other reactions with minimal molecular migrations. 10 photodimerization. 44 In some cases, the study of these reactions provides information about the structural effect on the neighboring molecules that do not participate in the reaction; in other cases, it provides valuable information with regard to the mechanism or course of the reaction.…”

Formation of a Four‐Membered Ring

“…It is not only the competition between cyclization and fragmentation that is determined by geometric requirements, but also the hydrogen abstraction in the first step. Recently, the structural parameters in favor of this transformation were identified by a series of transformations carried out in crystals [84].…”