Abstract

Abstract Rearrangement and fragmentation patterns, which have been observed during uv, irradiation of the following pyridinium ylides, will be discussed: pyridinium methylides, N-iminopyridinium ylides and pyridine-N-oxides. As a rule these mesoionic compounds undergo two competing reactions: i) photolytic cleavage via a triplet state, leading to the corresponding pyridines and to carbenes, nitrenes and oxene respectively; ii) photochemical rearrangement via an excitet singlet state, leading either to ring contraction, to ring enlargement or to ring cleavage. The photochemistry of N-iminopyridinium ylides has been studied in detail, since the photoinduced rearrangement pattern leads in high yield to the isomeric 1, 2-diazepines. Overall quantum yield of this ring enlargement process is low. In order to interpret the uv spectra of these 1-iminopyridinium ylides, their dichroism has been measured using Egger's stretched film technique. Solvent effects and a Pariser-Parr-Pople model point to π-π* transitions for the photoactive band, possibly followed by intramolecular charge transfer.

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