6944-41-8Relevant academic research and scientific papers
Phenolysis and aminolysis of 4-nitrophenyl and 2,4-dinitrophenyl S-methyl thiocarbonates in aqueous ethanol
Castro, Enrique A.,Aliaga, Margarita E.,Cepeda, Marjorie,Santos, Jose G.
experimental part, p. 353 - 358 (2011/11/30)
The reactions of S-methyl O-(4-nitrophenyl) thiocarbonate (1) and S-methyl O-(2,4-dinitrophenyl) thiocarbonate (2) with a series of secondary alicyclic (SA) amines and phenols are subjected to a kinetic investigation. Under nucleophile excess, pseudo-first-order rate coefficients (kobs) are obtained. Plots of kobs against the free nucleophile concentration at constant pH are linear with slopes kN. The Brnsted plots (log kN vs. nucleophile pKa) for the reactions are linear with slope (β) values in the 0.5-0.7 range, in accordance with concerted mechanisms. Comparison of the SA aminolysis of 1 with the same one carried out in water shows that the change of solvent from water to aqueous ethanol destabilizes the zwitterionic tetrahedral intermediate, changing the mechanism from stepwise to concerted. This destabilization is greater than that due to the change from SA amines to quinuclidines. For the phenolysis reactions, the kN values in aqueous ethanol are smaller than those for the same reactions in water. Considering that the nucleophile is an anion, this result is unexpected because the anion should be more stabilized in the more polar solvent. This result is explained by the facts that the phenoxide reactant has a negative charge that is delocalized in the aromatic ring and the transition state is highly polar.
Kinetics and mechanisms of the reactions of S-methyl chlorothioformate with pyridines and secondary alicyclic amines
Castro, Enrique A.,Aliaga, Margarita,Gazitúa, Marcela,Santos, José G.
, p. 4863 - 4869 (2007/10/03)
The pyridinolysis of S-methyl chlorothioformate shows a biphasic Br?nsted-type plot, in agreement with a stepwise mechanism and a change in the rate-limiting step, from formation of a zwitterionic tetrahedral intermediate (T±) at high pKa to its breakdown at low pKa. The reaction of the same substrate with secondary alicyclic amines shows a linear Br?nsted-type plot of slope 0.23, which is in accordance with a stepwise mechanism where formation of T± is the rate-determining step for the whole pKa range examined.
