Herein, the reactivity and selectivity of the reaction of O,O-diethyl 4-nitrophenyl phosphate triester (Paraxon, 1) with piperidine in ionic liquids (ILs), three conventional organic solvents (COS), and water is studied by (31)P NMR, UV-vis, and GC/MS. Three phosphorylated products are identified as follows: O,O-diethyl piperidinophosphate diester (2), O,O-diethyl phosphate (3), and O-ethyl 4-nitrophenyl phosphate diester (4). Compound 4 also reacts with piperidine to yield O-ethyl piperidinophosphate monoester (5). The results show that both the rate and products distribution of this reaction depend on peculiar features of ILs as reaction media and the polarity of COS.

The reactions of the title compounds with phenoxides, secondary alicyclic (SA) amines, and pyridines, in 44 wt% ethanol-water, at 25 • C and an ionic strength of 0.2 M, were subjected to kinetic and product studies. From analytical techniques (HPLC and NMR), two pathways were detected (nucleophilic attack at the phosphoryl center and at the C-1 aromatic carbon) for the reactions of all the nucleophiles with the phosphate (2) and for the pyridinolysis of the thionophosphate (1). Only aromatic nucleophilic substitution was found for the reactions of 1 with phenoxides and SA amines. For the dual reactions, the nucleophilic rate constants (k N ) were separated in two terms: k P N and k Ar N , which are the rate constants for the corresponding electrophilic centers. The absence of a break in the Brønsted-type plots for the attack at P is consistent with concerted mechanisms. The Brønsted slopes, β Ar 0.32-0.71, for the attack at the aromatic C-1, are in agreement with stepwise mechanisms where formation of a Meisenheimer complex is the rate-determining step.

The reactions of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates (DNPC and TNPC, respectively) and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates (DNPTC and TNPTC, respectively) with a series of benzenethiolate anions were subjected to a kinetic investigation in water, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). These reactions obey pseudo-first-order kinetics, under excess of benzenethiolate, and are first order in the latter reactant. However, comparable reactant concentrations were used in the reactions of 4-nitrobenzenethiolate anion with TNPC and TNPTC, which showed second-order kinetics. The nucleophilic rate constants are pH independent, except those for the reactions of TNPC with 4-methoxy- and pentafluorobenzenethiolates, and TNPTC with benzenethiolate and 4-chloro- and 3-chlorobenzenethiolates, which show acid dependence. The Brønsted-type plots for the nucleophilic rate constants are linear with slopes beta = 0.9, 1.0, 0.9, and 0.9 for the reactions of DNPC, TNPC, DNPTC, and TNPTC, respectively. No break in the Brønsted plot was found for the reactions of DNPC and DNPTC at pK(a) ca. 4.1 and 3.4, respectively, consistent with concerted mechanisms. TNPC is more reactive toward benzenethiolate anions than DNPC, and TNPTC more than DNPTC due to the better leaving groups involved. Comparison of the kinetic results obtained in this work with those for the concerted phenolysis of the same substrates shows that benzenethiolate anions are better nucleophiles toward carbonates than isobasic phenoxide anions. This is explained by Pearson's "hard and soft acids and bases" principle.

The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl O-ethyl thiolcarbonates with substituted phenoxide ions are subjected to a kinetic study in water, 25.0 °C, ionic strength 0.2 M (KCl). By following the reactions spectrophotometrically, pseudo-first-order rate coefficients (k obsd) are found under excess of the nucleophile. Plots of k obsd vs phenoxide anion concentration at constant pH are linear, with the slope (k N) independent of pH. The Brönsted-type plots (log k N vs pK a of the phenols) are linear with slopes β = 0.92, 0.77, and 0.61 for the reactions of the 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl derivatives, respectively. For these reactions, a concerted mechanism is proposed since the slope values are similar to those found in the concerted phenolysis of aryl acetates; the slope magnitudes are not consistent with a stepwise mechanism where the formation of a tetrahedral intermediate is rate limiting. Our results are in line with the finding that the reactions of the 2,4-dinitrophenyl and 2,4,6-trinitrophenyl derivatives with secondary alicyclic amines in water are concerted. In contrast, the reactions of the same substrates with pyridines are stepwise, which means that substitution of a pyridine moiety in a tetrahedral intermediate with a benzenethio group by a phenoxy group destabilizes the intermediate.

The reactions 4-methylphenyl 4-nitrophenyl carbonate (MPNPC), 4-chlorophenyl 4-nitrophenyl carbonate (CIPNPC), 4-methylphenyl 2,4-dinitrophenyl carbonate (MPDNPC), and 4-chlorophenyl 2,4-dinitrophenyl carbonate (CIPDNPC) with a homogeneous series of phenoxide anions are subjected to a kinetic investigation in aqueous solution (25.0 degrees C, ionic strength 0.2 M (KCI)). Under an excess of phenoxide with respect to the substrate, all of these reactions obey pseudo-first-order kinetics and are first order in phenoxide. The Brönsted-type plots for the nucleophilic rate constants (k(N)) are linear, with slopes beta = 0.48 (MPNPC), 0.67 (ClPNPC), 0.41 (MPDNPC), and 0.32 (ClPDNPC). The magnitude of these slopes and the absence of a curvature in the Brönsted plot at pK(a) = 7.1 for the CIPNPC reactions are consistent with concerted mechanisms (one step). The carbonates MPDNPC and ClPDNPC are more reactive than MPNPC and CIPNPC, respectively, toward phenoxide nucleophiles. This can be explained by the presence of a second nitro group in the nucleofuge of the dinitro derivatives, which (i) leaves their carbonyl carbon more positively charged, making them better electrophiles, and (ii) makes 2,4-dinitrophenoxide a better leaving group than 4-nitrophenoxide. The 4-chloro derivatives are more reactive than the corresponding 4-methyl derivatives. This should be due to the greater electron withdrawal of 4-chloro than 4-methyl, which makes the former carbonyl more electrophilic. Comparison of the concerted phenolysis of MPNPC with the stepwise reactions of secondary alicyclic amines with the same substrate indicates that substitution of a secondary alicyclic amine group in a zwitterionic tetrahedral intermediate by a phenoxy group greatly destabilizes the intermediate. An equation is deduced for log k(N) in terms of the basicity of the nucleophile, the nonleaving moiety, and the leaving group. This equation shows that for these reactions, the sensitivity of log k(N) to the basicity of the nonleaving moiety (beta(nlg) = -0.27) is very similar to that of the nucleofuge (beta(lg) = -0.25).