Study on guanidine-based task-specific ionic liquids as catalysts for direct aldol reactions without solvent

Abstract

In this work, the effect of the anions of tetramethylguanidine-based ionic liquids on the catalytic activity in direct aldol reactions was studied. In solvent-free conditions at room temperature, [TMG][Ac] (1,1,3,3-tetramethylguanidinium acetate) was found to have the highest activity among the ionic liquids including [TMG][Pr] (1,1,3,3-tetramethylguanidinium propionate), [TMG][n-Bu] (1,1,3,3-tetramethylguanidinium n-butyrate), [TMG][i-Bu] (1,1,3,3-tetramethylguanidinium isobutyrate), [TMG][Lac] (1,1,3,3-tetramethylguanidinium lactate) and [TMG][TFA] (1,1,3,3-tetramethylguanidinium trifluoroacetate). On the basis of this preliminary result, a series of direct aldol reactions was performed using [TMG][Ac] as the catalyst, and satisfactory results with high yields and good chemo- and regioselectivity were obtained. From combination of the reaction results with UV–Vis spectroscopy studies, it was deduced that the reactions proceeded through an enamine intermediate. The anion effect was believed to operate though its interaction with the cation and the special structure of the enamine based on tetramethylguanidine may be the reason for the unusual regioselectivity, which is different from that of aldol reactions catalyzed by proline and its derivatives in organic solvents.