Photochemistry of trifluoromethyl substituted-1-methylpyrazoles

Abstract

This research involves the exploration in the photochemical reactionof trifluoromethyl substituted- l-methylpyrazoles. The 3-,4-, and5-(trifluoromethyl)-1-methylpyrazoles were synthesized and theirphotochemical behaviors were investigated. The photoreactions were carriedout with appropriate light sources and monitored by GC-FID and GC-MS. Theproduct identification was performed by the comparison of theirchromatographic and mass spectroscopic data with the authentic samples.Some anticipated products, such as 2-,4-, and5-(trifluoromethyl)-1-methylimidazoles, were also synthesized. Uponirradiation of 1-methyl-3-(trifluoromethyl)pyrazole, the result indicatedthe generation of 1-methyl-2(trifluoromethyl)imidazole and1-methyl-4-(trifluoromethyl)imidazole via electrocyclic ring closure. Thelatter was assumed to be the secondary product arising from1-methyl-2-(trifluoromethyl)imidazole. The photoreaction of1-methyl-4-(trifluoromethyl)pyrazole afforded only1-methyl-4-(trifluoromethyl)imidazole. In thisreaction the photocleavageintermidiates, which were identified as cis-and trans-isomers of3-(~iN~i-methylamino)-2-(trifluoromethyl)propen enitrile and(~iN~i-methylamino)-1-(trifluoromethyl)ethenylisocyanide, were detected by('1)H-NMR and infrared spectroscopy. When1-methyl-5-(trifluoromethyl)pyrazole was irradiated, it underwentphotocleavage to 1-methyl-5-(trifluoromethyl)imidazole, as well aselectrocyclic ring closure to 1-methyl-2-(trifluoromethyl)imidazole and1-methyl-4-(trifluoromethyl)imidazole. By using ('1)H-NMR spectroscopictechnique, it could be concluded that the photocleavage intermediates ofthis reaction were cis- and trans- isomers of(~iN~i-methylamino)-3-(trifluoromethyl)propenenitrile and2-(~iN~i-methylamino)-3-(trifluoromethyl)ethenylisocyanide