- Reactivity of Nucleophilic Nitrogen Compounds towards the Nitroso Group
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We discuss the reactivity of 43 nucleophilic nitrogen compounds towards the nitroso group of N-methyl-N-nitrosotoluene-p-sulfonamide (MNTS), and in some cases with alkyl nitrites.The series of nucleophiles considered is structurally very varied, includes members exhibiting the alpha effect, and covers 8 pKa units and a range of reactivities of almost five orders of magnitude.The values of solvent isotope effects and activation parameters have been measured and throw light on the structure of the transition states involved.Reactivities do not correlate well with thebasicity of the nucleophile, largely owing to the behaviour of primary amines, ammonia and nucleophiles with an alpha effect.Application of the curve crossing model suggests a relationship with vertical ionization potentials.The relationship with Ritchie's N+ scale is discussed, and interesting correlations with the reactivities of the same nucleophiles in various other chemical processes are noted.
- Garcia-Rio, Luis,Iglesias, Emilia,Leis, J. Ramon,Pena, M. Elena,Rios, Ana
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- The mechanism of the nitrosation of α-amino acids: Evidence for an intramolecular Pathway
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The results of a kinetic study of the nitrosation of sarcosine, proline, cysteine, and the ethyl ester of sarcosine are reported.Under the conditions in which sarcosine and proline were studied, both first- and second-order terms were found for nitrite dependence.The experimental results are interpreted by a mechanism which involves NO(1+) and N2O3 as direct nitrosating agents, while the effect of acidity changes on the firts-oredr term for nitrite shows that the N-nitroso compound is also formed in a parallel reaction in which a slow intramolecular rearrangement follows the attack of NO(1+) on the carboxylate group of the amino acid.This pathway is confirmed by the absence of sucy a parallel reaction in the nitrosation of the ethyl ester of sarcosine.The influence of acidity on the nitrosation of cysteine shows that S-nitrosation is brought about by the reaction of NO(1+) with both the N-protonated, HSCH2-CH(NH3(1+))-COOH, and zwitterion, HSCH2-CH(NH3(1+))-COO(1-), forms of the amino acid.The values obtained for the bimolecular rate constants are compatible with diffusion control, the difference between the values for both forms of the amino acid explicable in terms of charges of the reagents.
- Casado, Julio,Castro, Albino,Leis, J. Ramon,Mosquera, Manuel,Pena, M. Elena
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p. 1859 - 1864
(2007/10/02)
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