Novel Keto−Enol Systems:  Cyclobutane Derivatives

Abstract

3H-Perfluorobicyclo[2.2.0]hexan-2-one (3) has been synthesized from hexafluorobenzene and equilibrated with its enol form (4). In carbon tetrachloride K e/k = 0.07 ± 0.01 (25 °C), but in Lewis basic solvents (e.g. acetonitrile, ether, and tetrahydrofuran) only enol is detectable at equilibrium because of its strength as a hydrogen bond donor. In the monocyclic counterpart of this keto−enol system, 2H-perfluorocyclobutanone (1) and perfluorocyclobut-1-enol (2), the enol is more stable yet. Here ketone is undetectable under equilibrating conditions in all media examined, including carbon tetrachloride. Among unhindered and unconjugated enols, 2 and 4 are more stable relative to their ketones than any others that have been reported. Ab initio quantum mechanical calculations support the conclusion that destabilization of the ketones, but not stabilization of the enols, by fluorine substitution is responsible for the unique relative stability of these enols.

A Theoretical Study of Substituted Cyclobutanones and Their Enols

“…Thus the difference in the heat of formation is reduced to 2.319 kcal/mol as compared to the parent (without perfluorination, 10.638 kcal/mol). Therefore the perfluorination process causes a destabilisation of the ketone and a stabilisation of the enol, which agrees with the work of Lindner et al [14] and with the large basis set of B3LYP [15]. This is supported by calculation of the Gibbs free energy (∆G), which is found to be 1.639 kcal/mol and less than that of the parent (without fluorination, 7.792 kcal/mol).…”

“…But the order of stability reverses at a larger basis set, that is, B3LYP/6-311++G * * //B3LYP/6-31+G * at which enol becomes ∼ 1.0 kcal/mol more stable than ketone. This fact agrees with the thermodynamic equilibrium constant between ketone and enol [14]. In the case of 2,2H-Perfluorocyclobutanone it was found that the ketone is more stable than its enol counterpart [15].…”

“…perfluorination destabilized ketone. In contrast, the HF method [14] could not predict this keto-enol equilibrium in the right direction. B3LYP calculations [15] for ketones (2H-Perfluorocyclobutanone and 2,2H-Perfluorocyclobutanone) and their enol counterparts were carried out at various levels of the theory.…”

“…They found amazingly stable enols, resisting the influence of powerful acids such as sulfuric acid. Lindner et al found experimentally that 2H-perfluorocyclobutanone (all the hydrogen atoms are replaced by fluorine atoms except one hydrogen atom at the α-position of the carbonyl group) is less stable than perfluorocyclobut-1-enol (enol counterpart) [14], i.e. perfluorination destabilized ketone.…”

Functionalization of saturated fluorocarbons with and without light

“…(6)). Protonation of the resulting enolate (33) gave the enol (34) [21], which we found some years ago to be lower in energy than its keto form [22]. Under our conditions, perfluorocyclohexane (30) was reduced very slowly if at all even by potassium benzophenone ketyl.…”