2562 J. Am. Chem. Soc., Vol. 118, No. 11, 1996
Lindner et al.
determined in open capillary tubes with a Thomas-Hoover “Uni-Melt”
and are uncorrected.
and the reaction vessel was connected to a vacuum line. The solution
was treated with concentrated sulfuric acid (0.4 mL, 8 mmol) at ambient
temperature. The immiscible liquids were vigorously strirred for 1 min
and then the products were passed through a series of two traps. The
first trap at -13 °C (ethylene glycol, CO2(s)) collected the solvent and
the second trap cooled to -78 °C (2-propanol, CO2(s)) collected the
enol. The enol (95% pure, 0.5 g, 72% yield) was statically transferred
to a silated glass bulb and stored at -78 °C. 19F NMR (CCl4): Φ
-116.5 (m, 2F); -120.2 (m, 2F); -139.0 (s, 1F). 1H NMR (CCl4): δ
5.8 (s).
All solvents and reagents used in this work were reagent grade.
Carbon tetrachloride was distilled from P2O5 before use in equilibration
studies. Zinc was activated by stirring with 1 N hydrochloric acid for
5 min, washing with water, ethanol, and diethyl ether, and drying under
vacuum; it was then stored under nitrogen. NMR tubes were silated
by adding a few drops of N,O-bis(trimethylsilyl)acetamide and heating
the tube with a heat gun in the hood while the liquid made contact
with all inner surfaces; the tube was washed out with acetone and dried
under vacuum. Fluorine gas in helium (20/80) was purchased from
Matheson and was passed over a bed of sodium fluoride before use.
Perfluorobenzene was purchased from Fairchild Chemical and chlo-
rotrifluorethylene came from PCR. Perfluorocyclobutene was prepared
by the method of Tatlow,33 and should be handled in a well-ventillated
hood due to its high toxicity.
2-Hydroperfluorocyclobutanone Oxime Etherate (13).16 To ethyl
ether (120 mL) at 0 °C was added freshly prepared hydroxylamine34
(10.6 g, 0.32 mol) in a three-necked round-bottom flask equipped with
a dry ice condenser. The solution was stirred under nitrogen for 10
min to dissolve as much as possible of the hydroxylamine. Perfluo-
rocyclobutene (26 g, 0.16 mol) was condensed into the reaction vessel
over a 15-min period. After 8 h of additional stirring, the yellow
precipitate was removed by filtration and the ether removed under
reduced pressure. The oxime distilled at 32-42 °C/12 Torr (lit. bp 47
°C/12 Torr) complexed with 0.5 equiv of ether. A total of 12 g, 42%,
was isolated. 19F NMR(CDCl3): Major isomer Φ -115.4, -116.6 (AB
q, J ) 234 Hz, 2F); -122.2, -131.9 (AB q, J ) 226 Hz, 2F); -200.0
(d, J ) 46 Hz, 1F). Minor isomer Φ -133.3, -122.4 (AB q, J ) 226
Hz, 2F); -118.7 (s, 2F); -195.6 (d, J ) 46 Hz, 1F). 1H NMR
(CDCl3): δ 1.22 (t, 3H); 3.60 (q, 2H); 5.65 (m, 1H); 12.2 (s, 1H).
2-Hydroperfluorocyclobutanone-1,1-diol Etherate (14).6 To a
100-mL round-bottom flask equipped with a magnetic stir bar was
added 50 mL of concentrated hydrochloric acid. Oxime 13 (13.3 g,
80 mmol) was added dropwise over the course of 10 min. After 20 h
of stirring at ambient temperature the solid was removed by filtration
and the filtrate was diluted with 30 mL of distilled water. The product
was extracted with three 40-mL portions of diethyl ether and dried
over MgSO4. The solvent was removed under reduced pressure and
the product distilled at 40-45 °C/0.5 Torr to give a clear, colorless
liquid. 19F NMR(CD2Cl2): Φ -120.4, -134.1 (AB q, J ) 231 Hz,
2F); -127.9, -132.6 (AB q, J ) 215 Hz, 2F); -213.9 (d, J ) 47 Hz,
1F). 1H NMR (CD2Cl2): δ 1.22 (t, 3H); 3.60 (q, 2H); 5.01 (s, 2H);
5.06 (d of multiplet, J ) 47 Hz, 1H).
Equilibration of Ketone 1 and Enol 2. Ketone 1 was statically
transferred at -23 °C (30 mTorr) to a cold NMR tube (-196 °C)
containing 2.0 mL of dry CCl4 and 1 µL (8.7 × 10-6 mol) of
hexafluorobenzene as an internal standard. After the tube had been
carefully warmed under a nitrogen atmosphere, integration of the ketone
and hexafluorobenzene signals revealed 2.6 × 10-6 mol of 1 (1.3 mM).
A 1.3-µL aliquot of a 0.02 M solution of 1-methyl-2-pyrrolidone (2.6
× 10-8 mol) in CCl4 was added via syringe. The 19F NMR spectra
were recorded every 6 h.
5,6-Dibromoperfluorobicyclo[2.2.0]hex-2-ene (8). A 250-mL three-
necked round-bottom flask containing 19.5 g of 94% pure perfluoro-
(Dewar benzene) (5, 0.098 mol) in 40 mL of Freon 11 was placed in
an ice bath. Bromine (5.0 mL, 0.098 mol) was added dropwise from
a pressure-equalizing dropping funnel with good stirring, while the
reaction vessel was irradiated with a 75-W sunlamp. A water-cooled
condenser connected to a bubbler was attached to the flask. The
bromine color was discharged almost immediately after entering the
solution. Its presence is an indication of the end point of the reaction.
The solvent was short-path distilled, and fractional distillation through
a 10 cm Vigreux column of the remaining material under reduced
pressure (60-65 °C/40 Torr) afforded 31.7 g (90% yield) of a colorless
liquid. This fraction was found to be 96% dibromoolefin 25 (0.088
mol, 2.33/1 exo, cis/trans) by gas chromatography (25 m methylsilicone;
exo,cis tR ) 4.17; trans tR ) 3.59 min). The dibromo adducts were
separated by preparative gas chromatography. 19F NMR (CDCl3): exo,
cis Φ -113.3 (s, 2 F); -115.7 (d, 2F); -173.9 (s, 2 F); trans Φ -102.1
(s, 1F), -108.1 (s,1F); -115.1 (s, 1F); -117.9 (s, 1F); -171.7 (s, 1F);
-191.8 (s, 1F). IR (thin film): exo,cis 1760, 1370, 1125, 970, 850,
750 cm-1; trans 1760, 1370, 1140, 970, 850, 800, 770, 695 cm-1. MS
+
(same for both isomers) (m/e): 346(M+), 265 (C6F6Br+), 186 (C6F6
,
base), 155 (C5F5+), 117 (C5F3+).
2,3-Dibromoperfluorobicyclo[2.2.0]hexane (9). To 45 g of di-
bromoolefin 8 (93% pure, 0.12 mol) was added 450 mL of Freon 11
in a 500-mL heavy-walled Pyrex cylinder with 2-holed Teflon cap for
gas inlet and outlet tubes. The temperature of the reaction was kept at
-23 °C by immersing the reaction vessel in a dewar flask containing
methanol cooled by means of an extermal refrigerating unit. Fluorine
gas (F2/He, 20/80) was bubbled through the solution (40 mL/min) for
22 h. The fluorine flow was stopped and the system was flushed with
argon for 2 h. The solvent was removed by short-path distillation and
the product was fractionally distilled through a 10-cm Vigreux column
(48-53 °C/60 Torr) to recover 41.2 g of 85% pure dibromo compounds
9 (0.091 mol, 76%). The products were isolated pure by preparative
gas chromatography. 19F NMR (CDCl3): exo, cis Φ -119.5 (m, 2F);
-119.5, -124.5 (AB q, J ) 230 Hz, 2F); -171.3 (s, 2F); trans Φ
-104.1 (m, 1F); -119.7, -126.9 (AB q, J ) 225 Hz, 2F); -119.1,
-125.5 (AB q, J ) 225 Hz, 2F); -171.4 (d, 1F); -189.3 (d, 1F). IR
(gas phase): exo,cis 1390, 1310, 1280, 1220, 1080, 1000, 860, 710
cm-1; trans 1380, 1310, 1270, 1210, 960, 870, 710 cm-1. MS (same
for both isomers) (m/e): 303 (C6F8Br+), 222 (C4F5Br+), 191 (C3F4-
Br+, base), 155 (C5F5+), 124 (C4F4+). Anal. Calcd for C6F8Br2: C,
18.76; Br, 41.64; F, 39.60. Found trans: C, 18.80; Br, 41.36; F, 39.29.
Mixture of isomers: C, 18.89.
2-Hydroperfluorocyclobutanone (1).6 Concentrated sulfuric acid
(130 mL, 2.4 mol) was added to a dry 250-mL three-necked round-
bottom flask attached to a 25-mL U-trap equipped with stopcocks at
each port. The acid was heated to 40 °C and 2-hydroperfluorcyclobu-
tanone-1,1-diol-0.5 diethyl ether (19 g, 0.09 mol) was added to the
reaction vessel via syringe. After 20 min of magnetic stirring at 55
°C the pressure was reduced as the volatile components were dynami-
cally removed and trapped at -196 °C. Distillation of the reaction
mixture through a 10 cm Vigreux column at 42 °C under an argon
atmosphere produced 7.2 g (50%) of 3 as a clear, colorless liquid. 19
F
NMR(CCl4): Φ -122.5, -130.6 (AB q, J ) 273 Hz, 2F); -128.7,
-135.6 (AB q, J ) 234 Hz, 2F); -205.8 (d, J ) 47 Hz, 1F). 1H
NMR (CCl4): δ 5.80 (d of multiplet, J ) 47 Hz, 1H).
2-Hydroperfluorocyclobutanone-1,1-diol. Ketone 1 (0.5 g, 3
mmol) was statically transferred to a round bottom flask containing 50
µL (3 mmol) of distilled water. After the mixture had warmed to room
temperature, the quantitative formation of white crystals was observed.
The solid was sublimed at 40 °C/2 Torr, to give clear, colorless crystals
that were free of ether. Mp 52.5-54.5 °C. 19F NMR (CD2Cl2): Φ
-120.6, -133.7 (AB q, J ) 231 Hz, 2F); -127.7, -132.8 (AB q, J )
215 Hz, 2F); -213.5 (d, J ) 47 Hz, 1F). 1H NMR (CD2Cl2): δ 3.60
(s, 1H); 3.82 (s, 1H); 5.12 (d of multiplet, J ) 47 Hz, 1H).
Perfluorobicyclo[2.2.0]hex-2-ene (7). To a 250-mL three-necked
round-bottom flask was added 120 mL of DMSO and 30.0 g of activated
zinc (0.46 mol). A pressure equalizing dropping funnel and 50-mL
U-trap cooled to -196 °C were connected to the reaction vessel. The
pressure was reduced to 150 Torr and 11.6 g (0.026 mol) of 86%
dibromoolefin was added dropwise over a 2-h period. A total of 4.7
g (0.021 mol, 81% yield) of 100% pure perfluoroolefin (7) was
recovered (GC, 12 m methylsilicone; tR ) 0.6 min). 19F NMR
(CDCl3): Φ -116.6 (d, 2F); -115.1, -126.1 (subsplit AB quartet, J
) 219.2 Hz, 2F); -196.8 (s, 2F). IR (gas phase): 1750, 1370, 1330,
1-Hydroxyperfluorocyclobut-1-ene (2).6 A 25-mL three-necked
round-bottom flask was charged with benzyl enol ether 12 (1g, 4 mmol)
and 1,2,4-trichlorobenzene (10 mL). A magnetic stir bar was added
(33) Buxton, M. W.; Ingram, D. W.; Smith, F.; Stacey, M.; Tatlow, J.
C. J. Chem. Soc. 1952, 3830.
(34) Hurd, C. D.; Audrieth, L. F.; Nalefshi, L. A. Inorg. Synth. 1937, 1,
87.