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Cycloaddition Reactions of Carbonyl Compounds Possessing High Energy Content

Turro, Nicholas J.

Detailed structure and kinetic analyses of the photocycloaddition of alkanones to (a) α,β-unsaturated nitriles and (b) enol ethers have been performed. The excited singlet states of alkanones were found to add stereospecifically and regiospecifically to 3-unsaturated nitriles. Alkanone triplets do not add to unsaturated nitriles but instead transfer triplet excitation (at the diffusion controlled rate) and thereby sensitize dimerization and cis-trans isomerization of the nitrile. Even when all the alkanone singlets are quenched by a ground state nitrile, oxetane formation is relatively inefficient. The possibility is put forward that singlet exciplex of alkanone and nitrile is formed directly, but then dissociates (undergoes internal conversion) about ten times faster than it collapses to oxetane. In contrast, both S1 and T1 of alkanones add to enol ethers and effect both oxetane formation and cis-trans isomerization. Evidence is produced in support of singlet and triplet biradicals as intermediates to oxetanes. The possibility of a precursor to the biradicals is considered. Alkanone fluorescence quenching, by ethylenes known to form oxetanes, is used as a probe of the stereoelectronjc requirements of the quenching process and to study excited state conformations.


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Pure and Applied Chemistry

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August 2, 2010