James W. Pavlik

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.

Photo-ring contraction reactions of 4-hydroxypyrylium cations

Abstract 4-Hydroxypyrylium cations undergo photo-ring contraction upon irradiation of 50 percent H 2 SO 4 . Dihydroxycyclopentenones are intermediates in these reactions.

Chapter 2 Photochemical isomerizations of some five-membered heteroaromatic azoles

Publisher Summary The photochemistry of N -substituted pyrazoles and of isothiazoles has been of considerable interest since the first report that 1- methylpyrazole undergoes photoisomerization to 1-methylimidazole. At that time, it was suggested that the isomerization occurred by way of an initial photo-ring contraction to an undetected 2-( N -methylimino)-2H-azirine intermediate and subsequent ring expansion to the observed product, 1-methylimidazole. Although the intermediacy of acylazirines has been adequately demonstrated in the analogous isoxazole-to-oxazole phototransposition, such iminoazirines have not been detected in a pyrazole to imidazole isomerization, and thus, this mechanistic suggestion has never been experimentally substantiated. More recent work in this laboratory has confirmed that 1-methylpyrazoles phototranspose to 1-methylimidazoles and has shown that they undergo two different photocleavage reactions leading to enaminonitriles and to enaminoisocyanides respectively. Although both types of photocleavage products can be spectroscopically detected and isolated, upon further irradiation these compounds also undergo photocyclization to 1-methylimidazoles.