56525-81-6Relevant articles and documents
Kinetic Study of the Aminolysis and Pyridinolysis of O-Phenyl and O-Ethyl O-(2,4-Dinitrophenyl) Thiocarbonates. A Remarkable Leaving Group Effect
Castro, Enrique A.,Cubillos, Maria,Aliaga, Margarita,Evangelisti, Sandra,Santos, Jose G.
, p. 2411 - 2416 (2007/10/03)
The reactions of a series of secondary alicyclic (SA) amines with O-phenyl and O-ethyl O-(2,4-dinitrophenyl) thiocarbonates (1 and 2, respectively) and of a series of pyridines with the former substrate are subjected to a kinetic investigation in water, at 25.0 °C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, all the reactions obey pseudo-first-order kinetics and are first-order in amine. The Broensted-type plots are biphasic, with slopes (at high pKa) of β1 = 0.20 for the reactions of SA amines with 1 and 2 and β1 = 0.10 for the pyridinolysis of 1 and with slopes (at low pKa) of β2 = 0.80 for the reactions of SA amines with 1 and 2 and β2 = 1.0 for the pyridinolysis of 1. The pKa values at the curvature center (pK a0) are 7.7, 7.0, and 7.0, respectively. These results are consistent with the existence of a zwitterionic tetrahedral intermediate (T?) and a change in the rate-determining step with the variation of amine basicity. The larger pKa0 value for the pyridinolysis of 1 compared to that for 2 (pKa0 = 6.8) and the larger pKa0 value for the reactions of SA amines with 1 relative to 2 are explained by the greater inductive electron withdrawal of PhO compared to EtO. The larger pKa0 values for the reactions of SA amines with 1 and 2, relative to their corresponding pyridinolysis, are attributed to the greater nucleofugalities of SA amines compared to isobasic pyridines. The smaller pKa0 value for the reactions of SA amines with 2 than with O-ethyl S-(2,4-dinitrophenyl) dithiocarbonate (pKa0 = 9.2) is explained by the greater nucleofugality from T? of 2,4-dinitrophenoxide (DNPO-) relative to the thio derivative. The stepwise reactions of SA amines with 1 and 2, in contrast to the concerted mechanisms for the reactions of the same amines with the corresponding carbonates, is attributed to stabilization of T? by the change of O- to S-. The simple mechanism for the SA aminolysis of 2 (only one tetrahedral intermediate, T?) is in contrast to the more complex mechanism (two tetrahedral intermediates, T? and T-, the latter formed by deprotonation of T? by the amine) for the same aminolysis of the analogous thionocarbonate with 4-nitrophenoxide (NPO-) as nucleofuge. To our knowledge, this is the first example of a remarkable change in the decomposition path of a tetrahedral intermediate T? by replacement of NPO- with DNPO- as the leaving group of the substrate. This is explained by (i) the greater leaving ability from T? of DNPO- than NPO- and (ii) the similar rates of deprotonation of both T? (formed with DNPO and NPO).
Kinetics and mechanism of the aminolysis of phenyl and 4-nitrophenyl ethyl thionocarbonates
Castro, Enrique A.,Cubillos, Maria,Santos, Jose G.
, p. 3501 - 3505 (2007/10/03)
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 (kobsd) 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 pKa of T± and the rate coefficient for proton transfer (k3 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 Broensted type plots for amine basicity have slopes βN ca. 0.2 for rate-determining amine attack (k1) 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 (k2) 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 O-Ethyl S-Phenyl Dithiocarbonate in Aqueous Ethanol
Castro, Enrique A.,Cabrera, Mauricio,Santos, Jose G.
, p. 49 - 58 (2007/10/02)
The reactions of secondary alicyclic amines with the title substrate (PDTC) are subjected to a kinetic study in 44 wt.percent aqueous ethanol, 25.0 deg C, ionic strength 0.2 M (KCl).Pseudo-first-order rate coefficients (kobs) are found under amine excess.Linear plots of /kobs against 1/, where N is the free amine, are obtained for the reactions with piperidine, piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine.The reaction with 1-formylpiperazine exhibits a linear plot of kobs against 2.These results are interpreted through a mechanism consisting of two tetrahedral intermediates: a zwitterionic (T+/-) and an anionic (T-), where the amine catalyzed proton transfer from T+/- to T- is partially rate determining for the four former reactions and is fully rate determining for the reaction of 1-formylpiperazine.The rate microcoefficients involved in the reaction scheme are either determined experimentally or estimated.Comparison with the corresponding microcoefficients reported for the same reactions in water reveals that the rate coefficient for formation of T+/- from reactants (k1) is smaller and that for the reversal of this (k-1) is larger in aqueous ethanol compared to water, in agreement with the expected structure of the corresponding transition state.Broensted-type plots are obtained for k1, k-1, and K1 (= k1/k-1) with slopes ca. 0.4, -0.6, and 1.0, respectively.Comparison of the present stepwise reactions with the concerted ones found in the same aminolysis of O-ethyl 2,4,6-(trinitrophenyl) dithiocarbonate indicates that T+/- is so destabilized by the change of PhS by the 2,4,6-trinitrobenzenethio group that T+/- no longer exists and becomes a transition state.
