Phototransposition of 1-Methylpyrazole. Deuterium, Methyl, and Fluorine Substitution

Article abstract of DOI:10.1021/jo00022a019

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 mechanims involving ring contaction-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.