PHOTOTRANSPOSITION CHEMISTRY OF 1-METHYLPYRAZOLE - DEUTERIUM, METHYL, AND FLUORINE SUBSTITUTION

1-Methylpyrazole (1) was observed to undergo photo-ring cleavage to 3-(N-methylamino)propenenitrile (17) and phototransposition to 1-methylimidazole (3). Although upon prolonged irradiation 17 is also converted to 3, the efficiency of the 1 --> 17 --> 3 pathway is low and cannot account for a significant fraction of 3 observed upon short-duration irradiation. Under these conditions, deuterium-labeling studies show that 1 phototransposes to 3 by the P4, P6, and P7 permutation patterns in a ratio of 4.8:6.5:1.0. These scrambling patterns are consistent with mechanisms involving ring contration-ring expansion (P4) and electrocyclic ring closure followed by one (P6) or two (P7) sigmatropic shifts of nitrogen. Methyl and fluorine substitution on the 1-methylpyrazole ring reduces reactivity via the P6 and P7 pathways. Thus, 1,5-dimethylpyrazole transposes by these pathways in a ratio of 3.5:1.8:1.0, whereas 5-fluoro-1-methylpyrazole isomerizes only by the P4 and P6 pathways in a ratio of 9.7:1.