2-mercaptopyridine reacts rapidly with nitrous acid in mildly acid aqueous solution to give an unstable S-nitroso ion (SNO⁺) in a reversible process. SNO⁺ is readily detected by two peaks in the UV spectum at 295 and 240 nm. Addition of cyclic 1,3-diones (RH) to an acidified solution of SNO⁺ resulted in quantitative nitrosocompound formation and kinetic measurements of the nitrosation of Meldrum´s acid, dimedone and barbituric acid in the presence of MP showed marked catalysis at low MP concentration. At higher MP concentration values, catalysis disappears since there is virtually complete conversion of HNO₂ to SNO⁺.

We carried out some experiments to examine the potencial of SNO⁺ cations as electrophilic nitrosating species. SNO⁺ was generated at acid pH from equimolar amounts of MP and nitrous acid and was used to react with Meldrum´s acid, dimedone and barbituric acid. The reaction kinetics were studied by following the change in absorbance at 240 nm for Meldrum´s acid, at 250 nm for barbituric acid and 305 nm for dimedone due to the nitrosocompound formation using an spectrophotometer fitted with thermostated cell holders. All kinetic experiments were performed with a great excess of the 1,3-diones over SNO⁺. The absorbance-time data always fitted the first-order integrated equation, and k_obs, the corresponding pseudo-first order rate constant, could be reproduced to within 3%.

If we accept that reaction occurs as indicated by equations (1) and (2), we can calculate a value for K_N and for the bimolecular rate constant (k).

MP + HNO₂ + H⁺ « SNO⁺ + H₂O K_N (1)

SNO⁺ + RH ®R-NO + MP+ H⁺ k (2)

Meldrum´s acid itself exists primarily in the diketo form, with a very small enol content This contrasts quite markedly with the behaviour of dimedone which exists in aqueous solution primarily as the enol tautomer.

The possibility then exists that electrophilic attack in RH can occur either via the carbanion (enolate) or the enol intermediates.

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