Ratio of 1,3- and 1,5-dialkyl-substituted pyrazoles obtained from chlorovinyl alkyl ketones and alkylhydrazines, 3(5)-pyrazoles and alkyl bromides

Abstract

Unsymmetrically substituted 1,3- and 1,5-dialkylpyrazoles and 1,3-dialkyl-5-chloropyrazoles were prepared. 2-Chlorovinyl ketones react with C1-C8-alkylhydrazines along two routes giving mixtures of 1-R′-3-R- and 1-R′-5-R-pyrazoles with the prevalence of 1,3-isomer. The proportion of 3-substituted 1-alkylpyrazole in the isomers mixture grows from 60 to 94–97% with growing length of the alkyl chain in the hydrazine. The alkylation of 1-unsubstituted 3(5)-alkylpyrazoles with alkyl bromides afforded predominantly (by 16–50%) 1,3-disubstituted pyrazoles. 1,3-Dialkyl-5-chloropyrazoles form from 2,2-dichlorovinyl alkyl ketones and functionalized alkylhydrazines. It was suggested to synthesize 1,3-dialkyl-substituted pyrazoles by dechlorination of unsymmetrically substituted 1,3-dialkyl-5-chloropyrazoles in the presence of complex nickel catalyst and water as the proton source.