Reaction mechanisms in ionic liquids: the kinetics and mechanism of the reaction of O,O-diethyl (2,4-dinitrophenyl) phosphate triester with secondary alicyclic amines

Abstract

The reactions of O,O-diethyl 2,4-dinitrophenyl phosphate triester (1) with secondary alicyclic (SA) amines in the ionic liquids [Bmim]BF₄ and [Bmim]DCA were subjected to a kinetic study. Eyring plots were obtained for the title reactions in the above ionic liquids (ILs) and also in aqueous ethanol (44 wt% ethanol). Two different reaction pathways were observed in [Bmim]BF₄: nucleophilic attack at the phosphoryl center, S_N2(P), and at the C-1 aromatic carbon, S_N(Ar), where the product distribution remained constant and independent of the amine nature. In contrast, in [Bmim]DCA only the S_N2(P) pathway was found. From the kinetic analysis of the S_N2(P) pathway in both ILs, curved upwards plots of k_obsdvs. 1-formylpiperazine concentration were obtained. Based on the kinetic behavior, a change in the mechanism of the S_N2(P) pathway is proposed for the aminolysis of 1, from a concerted process in aqueous ethanol to a stepwise mechanism, through a zwitterionic pentacoordinate intermediate, when [Bmim]BF₄ and [Bmim]DCA are used as the solvents of the reaction.