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. Quantum yields for these reactions show that the sequence 2-->6-->4 is a major pathway for the P-4 phototransposition of 2-->4. Isocyanides were also detected as intermediates in the P-4 phototransposition of a variety of other pyrazoles confirming the generality of this pathway in pyrazole photochemistry. Direct irradiation of 1-methyl-5-phenylpyrazole (3) resulted in the formation of 1-methyl-5-phenylimidazole (7), 1-methyl-2-phenylimidazole (8), and 1-methyl-4-phenylimidazole (4). Deuterium labeling revealed that these products were formed by P-4, P-6, and P-7 permutation pathways, respectively. (E)/(Z)-3-(N-methylamino)-3-phenylpropenenitrile (9) and (E)/(Z)-2-(N-methylamino)-2-phenylethenyl isocyanide (10) photocleavage products were also formed in this reaction. Irradiation of 3 in furan solvent did not result in phototransposition but led to the formation of endo and exo adducts formed by Diels-Alder reaction of furan with 4-phenyl-5-methyl-1,5-diazabicyclo[2.1.0]pent-2-ene. This constitutes the first direct evidence for the formation of a 1,5-diazabicyclo[2.1.0]hex-2-ene from photolysis of a pyrazole and is consistent with the electrocyclic ring closure-heteroatom migration mechanism suggested for the P-6 and P-7 phototranspositions.