Nucleophilic Substitution Reactions of Diethyl 4-Nitrophenyl Phosphate Triester: Kinetics and Mechanism

The reactions of diethyl 4-nitrophenyl phosphate (1) with a series of nucleophiles: phenoxides, secondary alicyclic (SA) amines, and pyridines are subjected to a kinetic study. Under excess of nucleophile, all the reactions obey pseudo-first-order kinetics and are first order in the nucleophile. The nucleophilic rate constants (k(N)) obtained are pH independent for all the reactions studied. The Bronsted-type plot (log k(N) vs. pK(a) nucleophile) obtained for the phenolysis is linear with slope beta = 0.21; no break was found at pK(a) 7.5, consistent with a concerted mechanism. The Bronsted-type plots for the SA aminolysis and pyridinolysis are linear with slopes beta = 0.39 and 0.43, respectively, also suggesting concerted processes. The concerted mechanisms for the latter reactions are proposed on the basis of the lack of break in the Bronsted-type plots and the instability of the hypothetical pentacoordinate intermediates formed in these reactions. (C) 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 708-714, 2011