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Crystallographic studies related to the solid state photochemistry of [alpha]-cycloalkylacetophenones Evans, Stephen Vincent Ashley


The crystal and molecular structures of fifteen α-cycloalkylacetophenones and propiophenones have been determined with the use of X-ray crystallography. All of the compounds are known to undergo the Norrish type II reaction in the solid state, which involves abstraction of a favourably oriented Ƴ-hydrogen atom to yield a biradical which can undergo either cleavage or cyclization. The compounds were studied to examine the effect of variation in aromatic ring substituents and cycloalkyl ring size on the nature and surroundings of the reaction site, and, if possible, to develop structure-reactivity relationships for reactions in the solid state. Almost all of the acetophenone derivatives with simple cycloalkyl rings studied assume a common conformation in the solid state, with the acetophenone moiety occupying an equatorial position on the cycloalkyl ring, and with the Ƴ-hydrogen atom most likely to be abstracted utilizing a boatlike abstraction geometry. The effect of α-methyl substitution of the α-cycloalkylacetophenones (to produce the corresponding propiophenones) is to rotate the cycloalkyl moiety by approximately 100° about the acyl-carbon α-carbon bond. The conformations of the cycloalkyl rings are in accord with those expected. The cycloheptyl rings assume twist-chairs (though somewhat disordered), while the cyclooctyl ring all display boat-chair conformations. Four α-adamantylacetophenones assume similar conformations in the solid state (with chair like abstraction geometries), but small differences in conformation were found to account for large differences in photochemical behaviour. Structural studies of a 3-methyladamantyl derivative yielded information as to the preferred pathways for cyclobutanol formation in the solid state, and provided an opportunity for producing an optically active product mixture from an achiral starting.

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