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A study of the Norrish type II reaction in the solid state : structure-reactivity correlations based on X-ray crystallography Nalamasu, Omkaram


The Norrish type II reaction of 31 substrates with the general α-cycloalkyl-p-substituted acetophenone structure was investigated in solid and solution media. The difference in photoreactivity between the solid and solution states and the requirements for photochemical hydrogen abstraction have been derived from X-ray crystal structure data obtained for 17 derivatives. The difference in stereoelectronic requirements for hydrogen transfer among Norrish type II, McLafferty and Barton reactions are discussed. Six-atom abstraction geometries other than chair, namely boat and half-chair, and smooth hydrogen abstractions at O•••H distances greater than the previously supposed upper limit of 2.7Å were observed. Abstractions also occurred at abstraction angles that are as unfavorable as 62° for τ (the degree to which the hydrogen in question lies outside the mean plane of the carbonyl group) and 77° for Δ (the C-O•••H angle). The ideal values of τ and Δ are 0° and 90-120°, respectively. The effect of the solid state on photoreactivity was found to range from very small to significant. The photochemistry of α -adamantyl-p-methoxyacetophenone delineates the importance of packing effects in steering the course of a unimolecular reaction in the solid state. The difference in the solid state reactivity between the two crystal modifications of α-adamantyl-p-chloroacetophenone has been found to be a result of conformational factors rather than packing effects. No correlation was evident in comparing the solution state hydrogen abstraction rate constants with the hydrogen abstraction geometries obtained from X-ray crystallography for the cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl derivatives of α-cycloalkyl-p-chloroacetophenone. This was interpreted as indicating a significant contribution to the hydrogen abstraction rate constant in solution from non-minimum energy conformations with better hydrogen abstraction geometries than that of the minimum energy conformations present in the crystal. An absolute asymmetric synthesis with >80% optical yield was achieved by performing photolyses on the chiral crystals of the achiral starting material, α-3-methyl-l-adamantyl-p-chloroacetophenone. Finally, photolyses of α-cyclohexyl and ct-cyclopentylacetophenone as guest:host complexes with Dianin's compound (4-p-hydroxyphenyl-2,2,4-trimethyl-chroman) revealed that the selectivity achieved in this constrained medium is similar to that obtained in the solid state.

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