- Photochemistry of Phenyl-Substituted 1-Methylpyrazoles
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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-methylaimno)-1-phenylethenyl isocyanide (6). Deuterium labeling confirms that the phototransposition occurs via the P4 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 P4 phototransposition of 2 → 4. Isocyanides were also detected as intermediates in the P4 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 P4, P6, and P7 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 P6 and P7 phototranspositions.
- Pavlik, James W.,Kebede, Naod
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p. 8325 - 8334
(2007/10/03)
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