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.

Vapor-phase photochemistry of methyl- and cyanopyridines: deuterium labeling studies.

The three isomeric methylpyridines and the three isomeric cyanopyridines each constitute a photochemical triad. Irradition of each methylpyridine or each cyanopyridine in the vapor phase at 254 nm results in the formation of the other two isomers as primary photoproducts. Dideuteration of the 2-substituted or 3-substituted methyl or cyanopyridines expanded each triad to a pentad.

Photochemistry of phenyl-substituted 1,2,4-thiadiazoles. 15N-labeling studies.

Irradiation of 5-phenyl-1,2,4-thiadiazole (6) resulted in the formation of benzonitrile (5), 3-phenyl-1,2,4-thiadiazole (4), phenyl- and diphenyl-1,3,5-triazines (7 and 8), and a trace quantity of diphenyl-1,2,4-thiadiazole (9). The formation of 4,5, 7, and 8 can be explained in terms of photoinduced electrocyclic ring closure resulting in the formation of an intermediate 4-phenyl-1,3-diaza-5-thiabicyclo[2.1.

Photochemistry of Phenyl-Substituted 1-Methylpyrazoles.

Direct irradiation of 1-methyl-4-phenylpyrazole (2) in methanol results in regiospecific phototransposition to 1-methyl-4-phenylimidazole (4) and in photocleavage to (E)/(Z)-3-(N-methylamino)-2-phenylpropenenitrile (5) and (E)/(Z)-2-(N-methylamino)-1-phenylethenyl isocyanide (6). Deuterium labeling confirms that the phototransposition occurs via the P(4) permutation pathway. Separate experiments show that 5 and 6 undergo (Z) --> (E) isomerization and photocyclization to imidazole 4.