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-Dipp/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-withdrawing 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.