Reactivity of nitrogen nucleophiles towards S-nitrosopenicillamine

We report the results of a kinetic study of the reactions of a number of ni trogen nucleophiles with the nitrosothiol S-nitrosopenicillamine (SPEN). Th e range of nucleophiles includes primary, secondary and tertiary aliphatic amines, together with hydrazine, hydroxylamine, azide ion, ammonia, semicar bazide, thiomorpholine and S-methylcysteine. Secondary amines form N-nitros amines quantitatively. As expected, reaction occurs via the free base forms of the nucleophiles and consequently most of the reactions take place read ily only at relatively high pH. Experiments were carried out with [nucleoph ile] much greater than [RSNO], and for many reactions, plots of the first o rder rate constant vs. [nucleophile] were linear. For ammonia and the prima ry amines, however, this plot tended to level off at high [nucleophile] and an explanation is offered involving the reversible formation of an inactiv e RSNO-amine complex, for which there is spectral evidence, in parallel wit h the main reaction. For the secondary amines there is a reasonably good Br onsted plot with a beta value of approximate to 0.2. The much greater react ivities of S-methylcysteine and thiomorpholine, compared to those of primar y amines and morpholine respectively are consistent with initial attack at the sulfur atom, followed by an internal rearrangement. Over the whole rang e of nucleophiles studied there is a reasonable correlation with the Ritchi e N+ parameter, and not with the Pearson n scale. Comparisons are made with the corresponding reactions of alkyl nitrites and N-methyl-N-nitrosotoluen e-p-sulfonamide (MNTS).