The Photochemistry of Aromatic - N . - Ylides . Rearrangement and Fragmentation Patterns

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 photocheinistry of N-iminopyridinium ylides has been studied in detail, sinceihe photoinduced rearrangement pattern leads in high yield to the isomeric l,2—diazepines. Overall quantum yield of this ring enlargement process is low. In order to interpret the uv spectra of these l-iminopyridiniuni ylides, their dichroism has been measured using EggerTs stretched film technique. Solvent effects and a Pariser—Parr--Pople model point to Tr.Tr* transitions for the photoactive band, possibly followed by intramolecular charge transfer. 1,2—Diazepines are thought to occur photochemically via 1,7-. diazanorcaradienes and proved to be excellent synthons for the buildup of new polycyclic molecules.