Kinetic and mechanistic studies on nitrosation of enols

Abstract

Kinetic and mechanistic studies involving nitrosation reactions of some carbonyl compounds were undertaken. The nitrosation of the carbonyl compounds proceeded via their corresponding enol forms and the products of the reactions were the oximes. Significant nucleophilic catalysis by chloride, bromide, thiocyanate ions and thiourea was observed in all cases. Most of the kinetic results are consistent with a mechanism involving a rate limiting reaction between H(_2)N0(_2)(^+)/ N0(^+) or the NOX species (in presence of nucleophile X') and the enol. The general mechanistic features of the nitrosation of the enols fitted in well with the pattern now well established in nitrosation at N, S, 0, and other C sites. The reactions of the enol form of ethylacetoacetate with the different nitrosating species were not encounter controlled indicating that the presence of the electron withdrawing group reduces the reactivity of the enol, relative to that derived from acetone. Kinetic studies on nitrosation of dimedone and 1,1,1-trifluoropentane 2-4-dione revealed that reactions proceeded not only via the neutral enol but also via the enolate ion. The reactions of the latter with all the nitrosating species occurred at the encounter rate. The values of the rate constants suggest that the enolate ion is one of the most reactive species studied in nitrosation. Its high reactivity therefore makes it an excellent potential nitrite trap. The mechanism of nitrosation of Meldrum's acid has not been completely elucidated. Some of the kinetics are complicated by mixed order reactions involving rate limiting enolisation and nitrosation. However there is kinetic evidence which points to reactions proceeding via both neutral enol and enolate ion.