Kinetics and Mechanism of the Aminolysis of Phenyl and 4-Nitrophenyl Ethyl Thionocarbonates

Abstract

The reactions of the title substrates (PTOC and NPTOC, respectively) with secondary alicyclic amines are subjected to a kinetic study in aqueous solution at 25.0 °C, ionic strength 0.2 M (KCl). Under amine excess, pseudo-first-order rate coefficients (k obsd) are found throughout. The order in amine is one for the reactions of piperidine but is of intermediate order between 1 and 2 for the reactions of the other amines. The kinetic results can be accommodated by a reaction scheme with two hypothetical tetrahedral intermediates: a zwitterionic (T±) and an anionic (T - ) one, whereby amine catalysis (deprotonation of T± to give T - ) is kinetically important. Both the pK a of T± and the rate coefficient for proton transfer (k 3 ca. 1010 s-1 M-1) are estimated. The values of the other rate microcoefficients of the scheme are found by a nonlinear least-squares fitting, and these values are compared with those exhibited in the aminolysis of phenyl thionoacetate (PTOA), and S-phenyl and S-(4-nitrophenyl) O-ethyl dithiocarbonates (PDTC and NPDTC, respectively). The Brönsted type plots for amine basicity have slopes βN ca. 0.2 for rate-determining amine attack (k 1) and βN ca. 0.8 for amine expulsion from T± (k - 1), in accord with the βN values found in similar aminolyses. The general base catalysis by amine found in the aminolysis of NPTOC, in contrast with the lack of such catalysis in the aminolysis of 4-nitrophenyl methyl carbonate, is explained by a smaller rate coefficient for expulsion of 4-nitrophenoxide (k 2) from T± (which competes with amine deprotonation of T±) relative to the same expulsion from the analogous oxy intermediate.

Kinetics and mechanism of the aminolysis of 4‐nitrophenyl and 2,4‐dinitrophenyl 4‐methylphenyl carbonates in aqueous ethanol

“…Therefore, the k 3 value for 1 and 2 should be ca. 10 10 s −1 M −1 in water 13a, 23, 24 and 2 × 10 9 s −1 M −1 for piperazinium ion and 4 × 10 9 s −1 M −1 for the other secondary alicyclic amines in 44 wt% ethanol–water 5, 25.…”

Kinetics and mechanism of the aminolysis ofp-nitrophenylN-phenylcarbamates

“…The k 3 step is the rate-determining step for the right-hand side path of Scheme 1 since breakdown of T ± to products should be fast. 6 The proton transfer is fast in water but not in MeCN. As a result, the k 3 step may become competitive with k 2 .…”

“…For example, the effects of substituent in nucleophile, X, on the rate of deprotonation is complex since the effect of the catalyst benzylamine should be opposite to that of benzylamine within T ± . Kinetic deuterium isotope effects involving amine protons on the second-(k 2 ) and third-order (k 3 ) rate constants are studied incorporating deuterium isotopes (i) in the substrate amino group, YC 6 Tables 3 and 4. Reference to Table 3 reveals that the isotope effects involving substrate amine protons are small (k H /k D ≈ 1·10) for k 2 and negligible (ca 1·00) for k 3 .…”

Kinetic Investigation of the Condensation Reaction of Activated Ester–Amides Containing Oligo(oxyethylene) Segments

“…[24][25][26][27][28][29][30] The transformation of T AE in reaction products may occur by action of a second amine molecule of an external base, with formation of T À , a very reactive anionic intermediate that quickly decomposes to amide. If the T AE concentration is small and constant within time (stationary state), then the reaction rate is determined by [Equation (3)].…”