are sufficient to promote condensation reactions with readily
enolizable carbonyl compounds, in agreement with results
noted by Wheaton and Burton.10 Our standard conditions with
1a,b,d,g,i,j, which have R-hydrogen atoms, gave the desired
difluoroalkenes. However, insoluble resins resulted from
analogous treatment of â-tetralone (4), as well as the
aldehyde 5 and ketones 6 and 7 (Figure 1). Steric hindrance
Scheme 2. Difluoromethylene Compound Cyclopropanation
modifies its reactivity. Because of the low binding energies,
difluoromethylene ylides are thought to equilibrate with their
phosphine and difluorocarbene components (Scheme 3).8
Scheme 3. Postulated Dissociation of Difluoromethylene
Ylides To Give Tetrafluoroethene
Figure 1. Substrates that did not undergo the standard difluoro-
methylenation reaction.
Difluorocyclopropanation of tetramethyethylene with a di-
fluoromethylene ylide was postulated to involve ylide
dissociation.9 Reactions of difluorocarbene with solvent
molecules (with no phosphines present) gave byproducts that
were not detected (19F NMR) in our reaction mixtures that
contained phosphines. This suggests that changes in the
association and/or reactivity of our difluorocarbene com-
plexes might result in enhanced attraction/addition with the
electron-poor difluoroolefins. The absence of reports on
analogous difluorocyclopropanation reactions might suggest
the formation of additional difluorocarbene-mercury and/
or difluorocarbene-mercury-phosphine complexes in our
systems. We lack evidence to support or disprove such
possibilities.
at the R-carbon(s) is another important factor. Tetralone (1a)
underwent difluoromethylenation readily, whereas its 2,2-
dibenzyl, 8a, and 2,2-dimethyl, 8b, derivatives were recov-
ered unchanged under identical conditions.
Our new methodology provides convenient access to the
difluoromethylene compounds 2 on a small scale. For
example, we isolated 61% of purified 2a, whereas 6-fluoro-
tetralone had been converted into its 6-fluoro-1-(difluoro-
methylene) derivative with HMPT and sodium chlorodi-
fluoroacetate in 3% isolated yield.11 Our method gave 2c in
67% yield, whereas it had been obtained previously (13%)
by a decomposition of difluoroacetate.12 We isolated diene
2f in 41% yield on a 1.65 mmol scale, whereas Burton and
co-workers had prepared 2f (30%) by treatment of cinnam-
aldehyde (1f) (20 mmol scale) with the volatile CBr2F2 plus
Zn/Ph3P13 or by the 72 h treatment of cinnamylidenetri-
phenylphosphorane (80 mmol scale) with CHClF2 (15%).10
Apart from its severe toxicity, the ready availability6 and
low cost of (CF3)2Hg and its convenient applicability for
small-scale reactions make it a useful new reagent for
difluoromethylenations.
In conclusion, we have developed a convenient new
method for conversion of carbonyl compounds into alkenes
and dienes with a difluoromethylene terminus. These selec-
tively fluorinated compounds are important starting materials
for synthesis of other classes of gem-difluoro compounds.5,14
Also, we have observed the previously unreported formation
of tetrafluorocyclopropane adducts and describe conditions
under which the difluoromethylene compounds can be
Formation of adducts 3a,b,d,g occurred within the first 2
h of heating in parallel with difluoromethylenation of
1a,b,d,g to give 2a,b,d,g [i.e., until the decomposition of
(CF3)2Hg was complete]. Longer reaction times did not
improve yields of the tetrafluorocyclopropanes 3. However,
mixtures of 2 and 3 were converted completely into 3 upon
successive cycles of purification and resubmission to the
standard reaction conditions (until 2 was no longer present).
With dienes 2f and 2h, difluorocarbene addition to either
the -HCdCH- or CdCF2 π-bond was possible. However,
only the latter addition was observed (as indicated by the
presence of only two 19F signals resulting from the planar
symmetry of 3 and the similarity of the 19F NMR data for
all of our tetrafluorocyclopropanes 3). The 19F chemical shifts
for 3 are in the range of δ 141-154 ppm, and the geminal
2
couplings are in the range of JF,F ) 170-180 Hz.
The scope of our present evaluation for efficient difluoro-
methylenation reactions includes less easily enolized alde-
hydes and ketones. Basicities of the difluoromethylene ylides
(10) Wheaton, G. A.; Burton, D. J. J. Org. Chem. 1983, 48, 917-927.
(11) Burgess, D. A.; Rae, I. D.; Snell, J. D. Aust. J. Chem. 1977, 30,
541-551.
(12) Fuqua, S. A.; Duncan, W. G.; Silverstein, R. J. Org. Chem. 1965,
30, 2543-2545.
(8) Dixon, D. A.; Smart, B. E. J. Am. Chem. Soc. 1986, 108, 7172-
7177.
(9) Burton, D. J.; Naae, D. G.; Flynn, R. M.; Smart, B. E.; Britelli, D.
R. J. Org. Chem. 1983, 48, 3618-3619.
(13) Hayashi, S.; Nakai, T.; Ishikawa, N.; Burton, D. J.; Naae, D. G.;
Kesling, H. S. Chem. Lett. 1979, 983-986.
(14) (a) Piettre, S. R. Tetrahedron Lett. 1996, 37, 2233-2236. (b) Lee,
C.-C.; Lin, S.-T. Synthesis 2000, 4, 496-498.
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