Nitrosation of acetylacetone (pentane-2,4-dione) and some of its fluorinated derivatives

Abstract

Nitrosation of acetylacetone (AA)(pentane-2,4-dione), 1,1,1-trifluoropentane-2,4-dione (TFA), and 1,1,1,5,5,5-hexafluoropentane-2,4-dione (HFA) occurs readily in acetonitrile using t-butyl nitrite under mildly acidic conditions. In this solvent the reactants exist primarily in their enol forms and the equilibrium constants K_E have been estimated using ¹H n.m.r. Kinetic studies at different acidities show that whilst AA reacts entirely via the enol form, TFA reacts via both the enol and enolate forms and HFA reacts only via the enolate anion. The results demonstrate both the deactivating effect of the CF₃ group to electrophilic addition and also its acid strengthening effect. In all cases the rate-limiting step is the reaction of the nitrosating agent (probably NO⁺) with the enol or enolate. Similar results are reported for the reaction of TFA in dilute aqueous acid using nitrous acid, and also for the reactions catalysed by Cl^–, Br^–, and SCN^–; again reactions occur simultaneously by way of the enol and enolate forms.