The first tetraaminoethylene was prepared by Pruett in 1950, 1 but the systematic study of molecules of this type began a decade later when Wanzlick reported the synthesis of 1 (R ) Ph). 2,3

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.

2H-Perfluorocyclopentanone (1k) and its enol (1e) have been independently synthesized and equilibrated. In carbon tetrachloride, the enol is the only detectable form at equilibrium. In addition to its high relative stability, this enol displays interesting reactivity, including reversible bromination and hydrolysis reactions. Replacing the vinyl fluorine of 1e with hydrogen changes the relative enol stability dramatically as the enol is only present to the extent of 13% in carbon tetrachloride under equilibrating conditions. In Lewis basic solvents, however, the enol is the only detectable form because of its strength as a hydrogen bond donor. Quantum mechanical calculations on both systems suggest that ketone destabilization, but not enol stabilization, by fluorination is responsible for the remarkable relative stability of the enols.

Recent reports from this laboratory have revealed that highly fluorinated 4- and 5-membered-ring enols are comparable in stability to, or more stable thermodynamically than, the corresponding ketones, even in non-Lewis-basic media. Work on perfluorinated keto−enol systems has now been extended to 2H-perfluorocyclohexanone plus its enol and to a series of acyclic analogues. In carbon tetrachloride, K E/K = 0.33 (22 °C) for the six-ring system, but only enol is detectable in Lewis-basic solvents. This shift is attributable to strong hydrogen-bond formation between the enol and Lewis base. A perfluoroenol has been shown to form significantly stronger hydrogen bonds than the potent hexafluoroisopropyl alcohol. Acyclic systems (e.g., 3H-perfluoro-2-butanone and its enol) contrast sharply with the cyclic, as no enol is detectable at equilibrium even in powerfully Lewis-basic media. Ab initio quantum mechanical calculations indicate that it is principally the enols, not the ketones, that are responsible for the difference between the two types of keto−enol systems, i.e. acyclic perfluoroenols are strongly destabilized relative to cyclic counterparts.

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