Uncommon aqueous media for nitrilimine cycloadditions. I. Synthetic and mechanistic aspects in the formation of 1-aryl-5-substituted-4,5-dihydropyrazoles

A number of 1-aryl-5-substituted-4,5-dihydropyrazoles 4 have been synthesised by 1,3-dipolar cycloaddition of variously substituted nitrilimines 2 onto the appropriate alkenyl dipolarophiles 3 in aqueous media and in the presence of a surfactant. Under these conditions, uncommon for the large majority of [3 + 2] cycloadditions, the electronic features of both the cycloaddends strongly dictate the reaction outcome. Clean and fast cycloadditions were observed between electron-rich nitrilimines and electron-poor dipolarophiles, while the reversal of the electronic features of the reactants gave poor results. Changes in surfactant concentration leads to some novel mechanistic insights.