Novel Keto−Enol Systems: Cyclobutane Derivatives

3H-Perfluorobicyclo[2.2.0]hexan-2-one
(3) has been synthesized from hexafluorobenzene and
equilibrated
with its enol form (4). In carbon tetrachloride
Ke/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.