Perfluorinated Cyclic and Acyclic Keto-Enol Systems
J. Am. Chem. Soc., Vol. 119, No. 14, 1997 3265
M in both perfluoromethylcyclohexane and N-methylpyrrolidone was
added. The purpose of the perfluoromethylcyclohexane was to have
an area standard close in chemical shift to the ketone signal to ensure
optimal integration. After 4 h, integration of the enol (Φ -139.0)-to-
perfluoromethylcyclohexane (Φ -190.2) signals revealed a (140 ( 20)-
to-1 ratio, but the signal at Φ -205.8 ppm for ketone 1k was not
detectable. The same was true after 16 h. On the basis of the signal-
to-noise ratio of the perfluoromethylcyclohexane resonance, which had
approximately the same line width as the ketone signal, the ratio of
enol-to-ketone was clearly >250.
resolvable. The spectrum remains broad down to -50 °C. 1H NMR
(C6D6, 25 °C): 0.24 (s, 18H); 4.60 (broad d, J ) 48 Hz, 1H). MS:
m/e 422 (M+), 147 (C5H15OSi2+), 73 (C3H9Si+). Anal. Calcd for
C12H19F9O2Si2: C, 34.12; H, 4.53; F, 40.48. Found: C, 34.44; H, 4.68;
F, 40.29.
2H-Perfluorocyclohexanone (3k).8 To a 5 mL round-bottom flask
containing 63 mg of compound 10 (0.15 mmol) and 2.0 mL of carbon
tetrachloride was added 0.1 mL of concentrated sulfuric acid (2.0
mmol). The contents were well-stirred for 20 h under nitrogen, and
then all volatiles were removed at 30 mtorr to a U-trap cooled to -78
°C. The contents of the U-trap were statically transferred (25 °C, 30
mTorr) to a flame-dried NMR tube; the total volume was 1.0 mL. After
the tube had been carefully thawed under nitrogen, 1.0 µL (8.7 × 10-3
mmol) of hexafluorobenzene was added. Integration of the ketone-
to-hexafluorobenzene signals in the 19F NMR spectrum revealed 0.12
mmol of ketone 1k (80% yield). 19F NMR (CDCl3): Φ -115.2,
-139.7 (AX q, J ) 291 Hz, 2F); -124.3, -144.2 (AX q, J ) 290 Hz,
2F); -125.6, -143.8 (AX q, J ) 280 Hz, 2F); -126.2, -130.6 (AB
q, J ) 268 Hz, 2F); -226.7 (d, J ) 47 Hz, 1F).
Equilibration of 3e and 3k. To the 0.12 M solution of 3k in CCl4
prepared above was added 120 µL (0.0012 mmol) of a 0.010 M NMP
solution in CCl4. The reaction was monitored by 19F NMR spectroscopy
for 12 h, i.e. until well after the keto:enol ratio had become constant.
Starting with the ketone, KE values of 0.35 and 0.33 were obtained in
separate runs. When equilibrium was approached from the enol side,
the enol/ketone ratio was found to be 0.32, leading to an average value
Perfluorocyclohex-1-enol (3e). To a dry 50 mL round-bottom flask
containing 15 mL of 1,2,4-trichlorobenzene and a stir bar was added
3.2 g (9.1 mmol) of 1-benzoxyperfluorocyclohexene.8 The reaction
vessel was attached to the vacuum line, and two U-traps cooled to -13
°C (ethylene glycol/CO2(s)) and -78 °C (isopropyl alcohol/CO2(s))
were attached in series. Concentrated sulfuric acid (10 mL, 0.19 mol)
was added, and the heterogeneous liquids were stirred vigorously for
2 min. All volatile products were removed under reduced pressure
(30 mTorr) and collected in the U-traps. The -13 °C U-trap contained
only 1,2,4-trichlorobenzene and the -78 °C U-trap contained 1.7 g
1
(77% yield) of enol 3e (95%) and enone 15 (5%). The 19F and H
NMR spectra of both compounds were consistent with those reported
elsewhere.8
2-Bromoperfluorocyclohexane-1,1-diol (8). Freshly prepared enol
3e (3.0 g, 12 mmol) was statically transferred at 15 mTorr to a 25 mL
round-bottom flask containing 10 mL of dry acetonitrile. With the
vessel cooled in an ice bath, bromine (3 mL, 60 mmol) was added at
once, followed by 20 mL of water. Residual bromine was quenched
with 10 mL of a saturated solution of sodium thiosulfate, and the
product was extracted with 3 × 15 mL of diethyl ether. The solvents
were removed first on a rotary evaporator and then with a vacuum
pump at 1 Torr. Analysis of the 1H NMR spectrum revealed 0.6 equiv
of acetonitrile complexed to the hydroxyl groups. Otherwise, the light
yellow oil was shown to be pure by 19F NMR spectroscopy (3.5 g,
82% yield). 19F NMR (CD2Cl2): Φ -109.5, -128.8 (AX q, J ) 270
Hz, 2F); -119.8, -135.8 (AX q, J ) 278 Hz, 2F); -121.4, -140.6
(AX q, J ) 280 Hz, 2F); -122.1, -137.5 (AX q, J ) 285 Hz, 2F);
-132.2 (m, 1F). 1H NMR (CD2Cl2): δ 5.5 (bs, 2H); 2.01 (s, 3H).
2-Bromo-1,1-bis(trimethylsiloxy)perfluorocyclohexane (9). To a
dry 25 mL round-bottom flask equipped with a Teflon stir bar was
added 2 mL of N,O-bis(trimethylsilyl)acetamide (8 mmol) and 20 mL
of methylene chloride. At ambient temperature 0.90 g (2.5 mmol) of
diol 8 dissolved in 5 mL of methylene chloride was added dropwise
with stirring to the reaction vessel. After 15 min the reaction mixture
was washed with one 10 mL portion of water, and the methylene
chloride layer was separated and dried over MgSO4. The solution was
concentrated and the resulting liquid passed through a silica gel column
with methylene chloride as the eluting solvent to remove residual
silating agent. After removal of the solvent in vacuo, 1.1 g of 9 was
obtained as a clear, colorless liquid (88% yield). 19F NMR (CDCl3):
Φ -109.5, -127.8 (AX q, J ) 280 Hz, 2F); -117.2, -133.0 (AX q,
J ) 273 Hz, 2F); -120.6, -140.1 (AX q, J ) 281 Hz, 2F); -121.6,
-137.8 (AX q, J ) 277 Hz, 2F); -130.8 (m, 1F). 1H NMR (CDCl3):
δ 0.14 (s, 9H); 0.24 (s, 9H). MS: m/e 395 (C9H1081BrF8OSi+), 393
(C9H1079BrF8OSi+), 147 (C5H15OSi2+), 73 (C3H9Si+). HRMS: calcd
for C12H1879BrF9O2Si2, 499.9885; found, 499.9876.
2H-1,1-Bis(trimethylsiloxy)perfluorocyclohexane (10). A solution
consisting of 300 mg (0.600 mmol) of compound 9, 8 mL of isopropyl
alcohol, and 1 mL of acetone was irradiated in a Pyrex tube with a
450 W medium-pressure Canrad-Hanovia mercury lamp for 30 min.
Pentane (20 mL) was added, and the product was washed with 20 mL
of water to remove isopropyl alcohol and hydrogen bromide. The
pentane layer was separated, dried over MgSO4, and filtered. After
the solvent was removed by short-path distillation, a dynamic vacuum
transfer at 25 °C (30 mTorr) left behind all high-boiling material. The
volatile fraction contained 180 mg of compound 10 as a clear, colorless
oil (71% yield). The liquid was obtained analytically pure by
preparative GC (170 °C, tR ) 10.4 min). 19F NMR (C6D6, 25 °C):
-120.0, -128.0 (AB q, J ) 283 Hz, 2F); -129.0 to -134.0 (broad
m, 6F); -212.0 to -216.0 (broad s, 1F). The 19F NMR spectrum
sharpens considerably at 70 °C as chair-chair interconversion occurs
faster, but certain peaks in the -129 to -134 ppm range are still not
of 0.33 for K3e/3k
.
(E)- and (Z)-2-tert-Butoxyperfluoro-2-butene (11). With the aid
of a dry ice condenser, 5.0 g (25 mmol) of (E)- and (Z)-perfluoro-2-
butene (4:1) was distilled into a dry 100 mL three-neck round-bottom
flask containing 15 mL of freshly distilled triglyme. A 1.2 M solution
of potassium tert-butoxide in triglyme was added dropwise to the
reaction vessel with efficient stirring at 0 °C. The progress of the
reaction was monitored by 19F NMR spectroscopy. After all of the
olefin was consumed, the volatile products were removed at 20 °C (1
Torr) to a U-trap cooled to -78 °C (isopropyl alcohol/CO2(s)). The
contents of the U-trap were thawed to produce 1.5 g (24% yield) of a
50:50 mixture of the E and Z isomers of 11, which were not separated.
19F NMR (CDCl3).26 Z isomer: -64.4 (q, J ) 15 Hz, 3F); -66.8 (q,
J ) 15 Hz, 3F); -132.2 (s, 1F). E isomer: -65.6 (d, J ) 23 Hz, 3F);
-67.7 (s, 3F); -142.5 (q, J ) 23 Hz, 1F). 1H NMR (CDCl3). E and
Z isomers: 1.32 (s); 1.34 (d, J ) 1.2 Hz), respectively. Anal. Calcd
for C8H9F7O: C, 37.82; H, 3.57; F, 52.18. Found (both isomers): C,
37.53; H, 3.47; F, 52.16.
(E)- and (Z)-Perfluoro-2-buten-2-ol (5e). A 50:50 mixture of E
and Z isomers of 11 (50 mg, 0.20 mmol) was added to an NMR tube
containing 0.5 mL of CDCl3/benzene (95:5 v/v). Two drops of
concentrated sulfuric acid were added, and the NMR tube was shaken
vigorously for 1 min. The organic layer was decanted from the acid
and transferred to a dry NMR tube. The 19F NMR showed clean and
quantitative conversion to a 50:50 mixture of E and Z isomers of 5e.
19F NMR (CDCl3). Z isomer: -65.5 (q, J ) 15 Hz, 3F); -66.2 (q, J
) 15 Hz, 3F); -153.7 (s, 1F). E isomer: -66.8 (s, 3F); -68.9 (d, J
) 23 Hz, 3F); -163.5 (q, J ) 23 Hz, 1F). 1H NMR (CDCl3). E and
Z isomers: 4.6 (broad s).
Equilibration of 5e and 5k. The enol was prepared as above except
CCl4 and bibenzyl were substituted for CDCl3 and benzene, respectively.
The enol/CCl4 solution was removed from the acid by a dynamic
vacuum transfer at 20 °C (30 mTorr) with the volatiles collected in a
U-trap cooled to -78 °C. The contents of the trap were statically
transferred to an NMR tube and carefully thawed under nitrogen. To
catalyze the equilibration, 5 µL of NMP was added and the reaction
(26) Assignment of the Z and E enol ethers was established by analysis
of the 19F NMR spectrum. The trifluoromethyl groups of the Z isomer split
each other into quartets with a coupling constant of 15 Hz as a consequence
of through-space interaction of the CF3 groups. The vinyl fluorine appears
as a sharp singlet at -132.2 ppm. For the E isomer the splitting pattern is
different, as one CF3 group appears as a singlet at -67.7 ppm and the other
as a doublet (J ) 23 Hz) centered at -65.6 ppm. This doublet splitting
arises from through-space coupling between the trifluoromethyl group and
the vinyl fluorine that is cis to that CF3 group. Splitting of the vinyl fluorine
into a quartet (J ) 23 Hz) also reflects this coupling. The E and Z isomers
of the enol (5e) were assigned analogously.