Browsing by Author "Leis, J. R."
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- Mechanism for basic hydrolysis of N-nitrosoguanidines in aqueous solutionPublication . Moreira, José; Leis, J. R.; Norberto, F.; García-Río, Luís; Araujo, E.; Ribeiro, LauraA kinetic study was carried out on the hydrolysis of two N-nitrosoguanidines, 1-nitroso-1-methyl- 3-tolylsulfonylguanidine (TSGNO) and 1-nitroso-1-methyl-3-benzoylguanidine (BCGNO). We observed an absence of buffer catalysis using H2PO4 -/HPO4 2-, H3BO3/H2BO3 -, and HCO3 -/CO3 2- regulators and a complex dependency of the rate constant on the pH. We discovered the existence of three simultaneous reaction paths: spontaneous decomposition of the neutral form of the N-nitrosoguanidine, decomposition of the monoanion, and decomposition through the form of the dianion. The analysis of the kinetic data has allowed us to obtain the acidity constant for the formation of the monoanion of the N-nitrosoguanidine, with values of pKa I ) 11.5. The reaction rate for the process through the monoanion, k2, decreases as the acidity increases. The application of the principle of nonperfect synchronization shows that the basicity and reactivity do not correlate when there exists a possibility of stabilization of the negative charge by resonance. This behavior is consistent with the mechanism E1cB whereby the stabler the negative charge, the slower the elimination reaction. When dealing with the case of the elimination through the neutral form we observe that the reaction rate increases together with the capacity of stabilization of the positive charge on the nitrogen atom adjacent to the imino group. For the reaction through the dianion we used a maximum value of k3 ) 1010 s-1 to estimate the value of pKa II for the formation of the dianion of the N-nitrosoguanidine, obtaining values of pKa II < 24.
- Nitrosation of amines in non-aqueous solvents: difference between N-N=O and O-N=O Nitroso donorsPublication . García-Río, Luís; Leis, J. R.; Moreira, José; Serantes, D.A kinetic study has been carried out on nitroso group transfer from substituted N-methyl-N-nitroso-benzenesulfonamides to different secondary amines: pyrrolidine, piperidine, Nmethylpiperazine,and morpholine in cyclohexane. The observed pseudo-first-order rate constant kobs shows a linear and quadratic dependency on the amine concentration with the existence of a primary kinetic isotope effect. Experiments carried out at different temperatures show Arrhenius-type behavior. Addition of isopropylamine(iPrNH2) to the reaction medium produces an increase in kobs. In the presence of a high iPrNH2 concentrations the influence of secondary amine concentration on kobs shows the disappearance of the quadratic dependency of kobs on the secondary amine. These results would be compatible with an addition-elimination mechanism, similar to that observed for the aminolysis of thealkyl nitrites in apolar solvents. The observed behavior in the presence of 18-crown-6 is very different, however. Addition of crown ether catalyses the reaction of aminolysis of alkyl nitrites insofar as it does not alter the rate of the nitroso group transfer from N-nitrososulfonamides. This behavior has been interpreted in terms of a concerted reaction mechanism through cyclical transition states, with four or six centers, involving one or two molecules of secondary amine. Addition of iPrNH2 to the reaction medium causes the appearance of a third reaction path, which emerges through a mixed transition state formed by a molecule of a secondary amine and a molecule of isopropylamine.
- Nitroso group transfer from N-nitrososulfonamides to thiolate ions. Intrinsic reactivityPublication . Moreira, José; García-Río, Luís; Leis, J. R.; Adam, C.The nitroso group transfer from N-nitrososulfonamides to thiolate ions was studied. Based on the results, the reaction rate is strongly dependent on the nature of the leaving group (αlg≈−1.30)(αlg≈−1.30), but virtually independent of the basicity of the thiol (βnuc≈0.10)(βnuc≈0.10). This dependence is ascribed to the presence of a nucleophile desolvation equilibrium (βd)(βd) that is followed by the attack of the thiolate ion on the nitroso group (βnuc′) via a concerted mechanism. The equilibrium constants for the loss of a nitroso group from a nitrosothiol and an N-nitrososulfonamide were used to obtain the equilibrium constants for the different reactions involved. By using rate–equilibrium correlations, the parameters αlgnorm, βdβd, and βnuc′norm were obtained.