The vapor-phase phototransposition chemistry of pyridine: deuterium labeling studies

The six isomeric trideuteriopyridines and the three isomeric tetradeuteriopyridines undergo phototransposition upon S0 --> S2 (pi, pi*) excitation in the vapor phase at 254 nm. On the basis of the products formed, the six trideuteriopyridine isomers can be divided into two triads. Similarly, the three isomeric tetradeuteriopyridines also constitute a triad. Irradiation of any one member of each triad results in the formation of the other two members of that triad. These isomerizations are consistent with a mechanism involving photocyclization, nitrogen migration around the five sides of the cyclopentenyl ring, and rearomatization.