Quantum Yield of Photo-rearrangement of Nitrones

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Abstract
Fifteen nitrones, N-butylidenecyclohexylamine N-oxide, N-benzylidenemethylamine N-oxide, N-diphenylmethylenemethylamine N-oxide, N-benzylidenaniline N-oxide, and its derivatives, were irradiated with light in solution, each at its longest-wavelength absorption band. Each nitrone isomerized to the corresponding oxazirane. The quantum yield of this photo-rearrangement, Φ, was measured. For each nitrone, the value of Φ is larger in cyclohexane solution than in ethanol solution. In the series of N-benzylidenaniline N-oxide and its para-substituted derivatives, the substituent lowers Φ, and the more electron-donating or the more electron-withdrawing the substituent is, the smaller is Φ. These facts are explained on the basis of a hypothesis that π should increase with the change in the π-bond order between the carbon and oxygen atoms of the nitrone group upon the lowest-energy electron excitation, ΔPC,O, calculated by the simple MO method.