Photochemistry of Phenyl-Substituted 1-Methylpyrazoles

Abstract

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.

Pyrazole Synthesis Using a Titanium‐Catalyzed Multicomponent Coupling Reaction and Synthesis of Withasomnine

“…The crude product was purified by column chromatography using 20% ethyl acetate/ hexanes on neutral alumina; yield: 280 mg (35%); mp 98-99 8C (Lit. [30] mp: 100-101 8C …”

The photochemistry of some pyranopyrazoles

“…The photochemistry of N-substituted pyrazoles has also been extensively studied and these compounds have been found to undergo photoisomerization to imidazoles [15][16][17][18][19][20][21] and photocleavage to yield enaminonitriles and enaminoisocyanides. [18][19][20][21] Although the photochemistry of the individual ring systems have been studied, we are unaware of any reports concerning the photochemistry of the pyranopyrazole ring system.…”

“…[18][19][20][21] Although the photochemistry of the individual ring systems have been studied, we are unaware of any reports concerning the photochemistry of the pyranopyrazole ring system. Because of our interest in these compounds, we have undertaken a study of the photochemistry of 1,3,6-trimethylpyrano [2,3-c] pyrazole-4(1H)-one (1) and of 3,6-dimethyl-1-phenylpyrano [2,3-c] pyrazole-4(1H)-one (2).…”

Synthesis of novel fused azaheterocycles by photostimulated intramolecular SRN1 reactions with nitrogen nucleophiles

“…It is important to notice that in all reactions with pyrazole derivatives remained substrate were not found, and the dehalogenation was complete (100% of I À ions), which indicated the total conversion of the substrate. As the nucleus of pyrazole is quite sensitive to light, 21 we believe that precursor 15 -and/or products 16 are partially destroyed by the irradiation. Nevertheless, yields of 54-56% of fused azaheterocycles 17 were obtained.…”