An unexpected aromatization during the N-alkylation reaction of 3,4-dihydro-1H-pyrazole derivatives: insight into the reaction mechanism

In this letter we describe the unexpected aromatization that takes place during the N-alkylation reaction performed on several 3-(2-nitrobenzoyl)-4,5-dihydro-1H-pyrazole-5-carboxylic acid methyl esters, giving rise to a mixture of 1-alkyl-3-(2-nitrobenzoyl)-4,5-dihydro-1H-pyrazole-5-carboxylic acid methyl esters and 1-alkyl-3-(2-nitrobenzoyl)-1H-pyrazole-5-carboxylic acid methyl esters.