Process for the dehalogenation of 1,3-dioxolanes

A process for the dehalogenation of 1,3-dioxolanes of formula: ##STR1## wherein X.sub.1 and X.sub.3, like or different from each other, are Cl or Br, X.sub.2 and X.sub.4, like or different from each other, are F or H. Said process consists in contacting the dioxolane of formula (I) with metallic zinc at a temperature ranging from +30.degree. to 130.degree. C. The corresponding dioxoles are obtained.

The present invention relates to a process for the dehalogenation of 
1,3-dioxolanes to obtain the corresponding dioxoles. 
More in particular, the present invention relates to a process for 
dehalogenating halogenated 1,3-dioxolanes of formula: 
##STR2## 
wherein: X.sub.1 and X.sub.3, like or different from each other, are Cl or 
Br, X.sub.2 and X.sub.4, like or different from each other, are F or H, by 
removal of the two vicinal halogens X.sub.1 and X.sub.3, and the 
consequent obtainment of the dioxoles of formula: 
##STR3## 
wherein: X.sub.2 and X.sub.4 are the same as defined hereinbefore. 
The growing demand for the abovesaid fluorodioxoles, for the use thereof as 
monomers in the preparation of fluoropolymers, justifies the interest in 
perfecting the processes for the dehalogenation of the corresponding 
dioxolanes. 
In fact, as is apparent from the prior art, the dehalogenation reactions of 
1,3-dioxolanes are affected by problems concerning both reproducibility 
and yield. 
U.S. Pat. Nos. 3,865,845 and 3,978,030 describe a process for 
dehalogenating fluorinated dioxolanes in an organic solvent in the 
presence of magnesium. However, the yields of this reaction are low and 
not reproducible. There is also described the debromination reaction, with 
metal zinc, of perfluoro-2,2-dimethyl-4,5-dibromo-1,3-dioxolane, prepared 
by treating perfluoro-2,2-dimethyl-1,3-dioxole with bromine. There is 
obtained the starting dioxole with low yields. 
U.S. Pat. No. 4,393,227 describes an improved dechlorination process with 
magnesium, a mercury salt or metallic mercury, iodine and tetrahydrofuran, 
by means of which process the reproducibility of the yields is improved. 
The drawback of this process, however, resides in that the molar ratios 
among the metals shall be exact and defined, since even slight variations 
result in a drastic reduction of the dioxol yield. 
Using this type of magnesium-based reagent, in U.S. Pat. Nos. 4,485,250 and 
4,499,264 perfluoro-1,3-dioxole was prepared starting from the 
corresponding 4,5-dichlorodioxolane. 
In U.S. Pat. No. 4,535,175 
2,2-bis(trifluoromethyl)-4,4,5-trichloro-5-fluoro-1,3-dioxolane is 
dechlorinated either with metal zinc and zinc chloride, or with magnesium, 
mercury chloride, iodine and tetrahydrofuran. The dioxole yield is much 
higher when the magnesium-based reagent is utilized. 
Analogously, in U.S. Pat. No. 4,810,806 
2-trifluoromethyl-2-cyanodifluoromethyl-4,4,5-trichloro-5-fluoro-1,3-dioxo 
lane is dechlorinated in the presence of zinc, providing low dioxole 
yields. The corresponding dioxolane, in which a Cl in position 4 is 
substituted by a F, is dechlorinated with the magnesium-based reagent; 
only very low amounts of the desired dioxole are obtained. 
Lastly, in U.S. Pat. No. 4,908,461 there are used LiAlH.sub.4 and 
TiCl.sub.3 or TiCl.sub.4 in tetrahydrofuran in particular molar ratios and 
according to a quite complex methodology. 
From the examination of the prior art it is therefore apparent that in 
order to dechlorinate 2,2-difluoro-1,3-dioxolanes it was necessary to 
operate according to complicated methodologies in the presence of complex 
and not easily utilizable reagents, not always obtaining, however, high 
and constant yields. 
Thus, it is object of the present invention to provide a perfected process 
for the dehalogenation of 2,2-difluoro-1,3-dioxolanes, which is easy to 
carry into effect, utilizes simple reagents and provides high and constant 
yields. 
This object is achieved, according to the invention, by a process for the 
dehalogenation of 1,3-dioxolanes of formula: 
##STR4## 
wherein: X.sub.1 and X.sub.3, like or different from each other, are Cl or 
Br, X.sub.2 and X.sub.4, like or different from each other, are F or H, 
characterized in that the dioxolane of formula (I) is brought into contact 
with metallic zinc in an at least stoichiometric amount and that, on 
conclusion of the reaction, the dioxole of formula: 
##STR5## 
wherein X.sub.2 and X.sub.4 are the same as defined hereinbefore, is 
isolated. 
The reaction temperature usually ranges from +30.degree. to +130.degree. 
C.; preferably it ranges from +50.degree. to +100.degree. C. 
The dioxolanes of formula (I) are preparable according to the methods 
described in the prior art, for example in U.S. Pat. Nos. 3,865,845 and 
4,485,250 and in Italian patent application 20578/A 90 in the name of the 
Applicant. 
Among the utilized dioxolanes there are preferred the ones in which X.sub.2 
and X.sub.4 are F. Particularly preferred are 
4,5-dichloro-tetrafluoro-1,3-dioxolane and 
4,5-dibromo-tetrafluoro-1,3-dioxolane. Starting from these reagents, 
perfluoro-1,3-dioxole is obtained from the dehalogenation reaction. 
The zinc/dioxolane molar ratio can vary over a relatively wide range. 
Generally it is higher than 1, preferably it ranges from 1.5 to 3.0. 
In a preferred embodiment of the invention, the starting dioxolane is fed 
to the reaction vessel maintained at the reaction temperature, said 
reaction vessel containing the metallic zinc along with an optional 
solvent and, preferably, little amounts of sodium or potassium iodide and 
of sodium or potassium carbonate. 
On conclusion of the reaction, the reaction products are collected in a 
trap maintained at a lower temperature than the boiling temperature of 
said products. 
To favour this operation it is preferable to provide a nitrogen gas flow in 
the reaction vessel. 
If a solvent is utilized, it must be inert under the reaction conditions, 
and preferably it is selected from amides (such as dimethylformamide), 
ethers (such as dioxane) and sulphoxides (such as dimethylsulphoxide). 
Usually it is operated at about atmospheric pressure. However it is 
possible to use both reduced pressures and pressures higher than 1 
atmosphere. 
The reaction time is not a critical parameter; usually, the reaction is 
complete in a few minutes. 
The resulting dioxoles can be utilized as monomers for preparing copolymers 
and homopolymers as is described for example in published European 
application No. 80,187 and U.S. Pat. Nos. 4,535,175 and 3,978,030. 
The abovesaid polymers are used, besides, as anticorrosive coating material 
or as sheaths for optical fibers.

For a better understanding of the possible embodiments of the present 
invention, a few illustrative but not limitative examples are given 
hereinafter. 
EXAMPLE 1 
Dehalogenation of 4,5-dichloro-tetrafluoro-1,3-dioxolane 
Operating in a nitrogen atmosphere, Zn (4.4 g), KI (180 mg), K.sub.2 
CO.sub.3 (300 mg) and dimethylformamide (DMF) (7 ml) were introduced into 
a 50 ml flask equipped with thermometer, dropping funnel, distillation 
column and magnetic stirrer. 
Then, after having brought the mixture to 60.degree. C., 2.75 g of 
4,5-dichloro-tetrafluoro-1,3-dioxolane (synthesized according to the 
method described in Italian application No. 20578/A 90) dissolved in 2.5 
ml of DMF were very slowly dropped in one hour into said mixture. During 
dropping, the mixture was heated up to 100.degree. C. and 
perfluoro-1,3-dioxol was distilled off as it formed. 
In the collecting flask there were condensed 1.76 g of distillate, which, 
subjected to gas chromatographic analysis is (column sp 1000, from 
50.degree. C. to 180.degree. C., 10.degree. C./min.), resulted to be 
composed of perfluoro-1,3-dioxole (82.2%), 
4-chloro-2,2,4,5-tetrafluoro-1,3-dioxolane (3%) and 
4,5-dichloro-2,2,4,5-tetrafluoro-1,3-dioxolane (15%). 
The mixture was then distilled in a conventional vacuum line through four 
traps cooled to -80.degree. C., -130.degree. C., -150.degree. C. and 
-196.degree. C. 
1.370 g of perfluoro-1,3-dioxole were collected in the trap cooled to 
-150.degree. C. 
The isolated perfluoro-1,3-dioxol yield, calculated on the converted 
product, was equal to 82%. 
EXAMPLE 2 
Dehalogenation of 4,5-dibromo-tetrafluoro-1,3-dioxolane 
Into a three-neck flask equipped with reflux cooler, an inlet for a slight 
carrying nitrogen flow, and a rubber bottom, there were charged 2 ml of 
previously anhydrified dimethylformamide, 0.53 g of previously activated 
zinc powder, 0.05 g of KI, 0.07 g of K.sub.2 CO.sub.3. The zinc powder had 
been previously washed with diluted hydrochloric acid, then with water and 
methanol and lastly it was dried under vacuum. 
The reflux cooler, the top of which had been connected with a collecting 
trap maintained at -78.degree. C., was cooled to 0.degree. C. and the 
reaction flask was maintained at +60.degree. C. 
Then, with the reaction mixture being maintained under stirring, through 
the bottom there were injected 0.53 g (1.71 mmols) of 
4,5-dibromo-tetrafluoro-1,3-dioxolane, synthesized by adding bromine to 
perfluoro-1,3-dioxole. The reaction was concluded in a few minutes and in 
the trap at -78.degree. C. there were collected 1.48 mmols of liquid 
tetrafluoro-1,3-dioxole; its .sup.19 F-NMR and IR spectra were 
corresponding to the ones of an authentic sample. 
The yield of this reaction was 86%. 
EXAMPLE 3 (COMATIVE TEST) 
Dehalogenation of perfluoro-2,2-dimethyl-4,5-dibromo-1,3-dioxolane 
Operating in like manner as in example 2, 0.69 g (1.7 mmols) of 
perfluoro-2,2-dimethyl-4,5-dibromo-1,4-dioxolane, synthesized by adding 
bromine to perfluoro-2,2-dimethyl-1,3-dioxole, were charged into the 
reaction vessel. 
0.25 mmols of perfluoro-2,2-dimethyl-1,3-dioxole were collected. 
The reaction yield was equal to 15%.