Keto–Enol Thermodynamics of Breslow Intermediates

Breslow intermediates, first postulated in 1958) are pivotal intermediates in carbene-catalyzed umpolung. Attempts to, isolate and characterize these fleeting amino enol species first met with success in 2012 when we found that saturated bis-Dip/Mes imidazolidinylidenes readily form isolable, though reactive diamino enols with aldehydes and enals. In contrast, triazolylidenes, upon stoichiometric reaction with aldehydes, :gave exclusively the keto tautomer, and no isolable enol. Herein, we present the synthesis of the "missing" keto tautomers of imidazolidinylidene-derived diamino enols, and computational thermodynamic data for 15 enol ketone pairs derived, from various carbenes/aldehydes. Electron-with-drawing substituents on the aldehyde favor enol formation, the same holds for N,N'-Dipp [2,6-di(2-propyl)phenyl] and N,N'-Mes [2,4,6-trimethylphenyl] substitution on the carbene component. The latter effect rests on stabilization of the diamino enol tautomer by Dipp substitution, and could be-attributed to dispersive interaction of the 2-propyl groups with the enol moiety. For three enol-ketone pairs, equilibration of the, thermodynamically disfavored tautomer was attempted, with acids and bases but could not be effected, indicating kinetic inhibition of proton transfer.