Org. Synth. 1967, 47, 49
DOI: 10.15227/orgsyn.047.0049
β,β-DIFLUOROSTYRENE
[1,1-Difluoro-2-phenylethylene)]
Submitted by Samuel A. Fuqua
1, Warren G. Duncan
2, and Robert M. Silverstein
2.
Checked by John J. Miller, Herbert Aschkenasy, and William D. Emmons.
1. Procedure
In a 250-ml. two-necked flask fitted with a reflux condenser, a drying tube, a magnetic stirrer, and a heated dropping funnel with a pressure-equalizing side arm (Note 1) are placed 23.1 g. (0.088 mole) of triphenylphosphine, 8.5 g. (0.081 mole) of benzaldehyde, and 10 ml. of anhydrous 2,2'-dimethoxydiethyl ether (diglyme) (Note 2). A solution of 18.3 g. (0.12 mole) of dry sodium chlorodifluoroacetate (Note 3) is prepared by stirring the finely divided salt in 50 ml. of anhydrous diglyme at 70° for about 5 minutes. This warm solution is placed in the dropping funnel which is heated to 60°. The system is purged with dry nitrogen. The solution in the flask is stirred and heated in an oil bath held at 160°, while the contents of the dropping funnel are added dropwise over a period of 1.5–2 hours (Note 4). The diglyme and product are flash-distilled at 1 mm and a bath temperature of 100° into a receiver cooled with dry ice. The distillate is fractionated through a spinning-band column (18 in. × 6 mm. I.D.); the yield of product collected at a head temperature of 52–54° (40 mm.) is 7.6–8.9 g. (67–79%) (Note 5), n20D 1.4939 (Note 6).
2. Notes
1.
All glassware is
oven-dried. The dropping funnel is wrapped with heating tape, and a
thermometer is inserted between the funnel and the tape.
2.
Triphenylphosphine is available from M and T Chemicals, Inc. Benzaldehyde is distilled immediately before use. Diglyme is refluxed for 4 hours over
calcium hydride and distilled under reduced pressure.
3.
Sodium chlorodifluoroacetate is prepared from
chlorodifluoroacetic acid (K & K Laboratories) as follows: To a cooled, stirred solution of
60.7 g. (1.52 moles) of sodium hydroxide in
700 ml. of methanol is slowly added a solution of
198 g. (1.52 moles) of chlorodifluoroacetic acid in
300 ml. of methanol, the temperature being kept below 40°. The
methanol is removed under reduced pressure at 40°. The salt, which is pulverized and dried overnight at room temperature at 1 mm., is obtained in essentially quantitative yield. The salt is again dried in the same way immediately before use.
4.
During the development of this procedure, evolution of
carbon dioxide was monitored with a wet-test meter. At the bath temperature given (160°),
sodium chlorodifluoroacetate eliminates
carbon dioxide as rapidly as it is added over a period of 1.5–2 hours. If the bath temperature is allowed to drop, there is danger of buildup of
sodium chlorodifluoroacetate followed by violent exothermic decomposition. Addition of the
sodium chlorodifluoroacetate solution should not be started until the flask contents are equilibrated with the oil bath.
It is quite feasible to run the reaction at a bath temperature of 90–95° by adding all reagents to the flask initially; a quantitative evolution of carbon dioxide occurs over a period of about 18 hours. The reaction can also be carried out in refluxing 1,2-dimethoxyethane (Arapahoe Chemicals, Inc.) over a period of about 50 hours (yield 40–55%), or in triethylene glycol dimethyl ether (Ansul Chemical Company) at a bath temperature of 160° over a period of 2 hours (yield 64%).
5.
The distilled product gave a single symmetrical peak on gas chromatography under the following conditions: 25% LAC on Chromosorb W, 6 ft. × ¼ in., 110°,
helium flow 41 ml./min., elution time 14.2 minutes. The checkers used LB 5-50 on Fluoropak 80 and obtained a single peak. Gas chromatography of the flash distillate before fractionation showed an actual yield of
10.6 g. (
95%). The product fumes in moist air, and some etching of
glass containers was noted. This is presumably due to elimination of
hydrogen fluoride. Samples in open glass containers deposit a small amount of solid on standing; the solid is probably a product of the glass-hydrogen fluoride reaction.
6.
Care must be taken to clean and dry the refractometer prisms before and after use in order to prevent etching of the prisms.
3. Discussion
The literature preparation
3 of
β,β-difluorostyrene consists of seven steps from
sodium difluoroacetate, the last step involving pyrolysis at 600°; the overall yield was 5%.
4. Merits of the Preparation
The method described is a general synthesis for compounds containing the -CH=CF
2 moiety. There is no other simple general route to such compounds. Aromatic, aliphatic, and heterocyclic aldehydes to which this procedure has been applied are:
p-fluorobenzaldehyde (65%),
p-methoxybenzaldehyde (60%),
heptanal (43–51%), and
furfural (75%).
4 The method is also applicable to ketones
5 and to α-perfluoroketones.
6 Substitution of
lithium chlorodifluoroacetate for the sodium salt has been advocated.
7This preparation is referenced from:
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
2,2'-dimethoxydiethyl ether (diglyme)
methanol (67-56-1)
sodium hydroxide (1310-73-2)
nitrogen (7727-37-9)
carbon dioxide (124-38-9)
hydrogen fluoride (7664-39-3)
benzaldehyde (100-52-7)
Furfural (98-01-1)
1,1-Difluoro-2-phenylethylene,
β,β-Difluorostyrene (405-42-5)
sodium chlorodifluoroacetate (1895-39-2)
chlorodifluoroacetic acid (76-04-0)
sodium difluoroacetate
lithium chlorodifluoroacetate
Heptanal (111-71-7)
calcium hydride (7789-78-8)
1,2-dimethoxyethane (110-71-4)
helium (7440-59-7)
triphenylphosphine (603-35-0)
triethylene glycol dimethyl ether (112-49-2)
p-methoxybenzaldehyde (123-11-5)
p-fluorobenzaldehyde (459-57-4)
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