Reactivity of nitrogen nucleophiles towards S-nitrosopenicillamine

We report the results of a kinetic study of the reactions of a number of nitrogen nucleophiles with the nitrosothiol S-nitrosopenicillamine (SPEN). The range of nucleophiles includes primary, secondary and tertiary aliphatic amines, together with hydrazine, hydroxylamine, azide ion, ammonia, semicarbazide, thiomorpholine and S-methylcysteine. Secondary amines form N-nitrosamines quantitatively. As expected, reaction occurs via the free base forms of the nucleophiles and consequently most of the reactions take place readily only at relatively high pH. Experiments were carried out with [nucleophile] much greater than [RSNO], and for many reactions, plots of the first order rate constant vs. [nucleophile] were linear. For ammonia and the primary amines, however, this plot tended to level off at high [nucleophile] and an explanation is offered involving the reversible formation of an inactive RSNO-amine complex, for which there is spectral evidence, in parallel with the main reaction. For the secondary amines there is a reasonably good Bronsted plot with a beta value of approximate to 0.2. The much greater reactivities of S-methylcysteine and thiomorpholine, compared to those of primary amines and morpholine respectively are consistent with initial attack at the sulfur atom, followed by an internal rearrangement. Over the whole range of nucleophiles studied there is a reasonable correlation with the Ritchie N+ parameter, and not with the Pearson n scale. Comparisons are made with the corresponding reactions of alkyl nitrites and N-methyl-N-nitrosotoluene-p-sulfonamide (MNTS).