CA1158662A - Process for the preparation of vinylphosphonic acid diesters - Google Patents

Abstract

Abstract of the disclosure:

Process for preparing vinylphosphonic acid di-esters of the formula in which R denotes C1-C4 alkyl, by heating dialkyl 2-acetoxyethanephosphonates of the formula in which R has the abovementioned meaning, to 150 to 270°C
in the presence of acid or basic catalysts and reacting the resulting reaction product with orthoesters of the general formula R'C(OR)3 in which R' denotes hydrogen, or C1-C4-alkyl or C1-C4-alkoxy and R has the abovementioned meaning, at 30 to 200°C.

Description

11586~2

- 2 -It is known that vinylphosphonic acid derivatives can be prepared from 2-chloroethanephosphonic acid deriva-tives. However, the synthesis of these 2-chloroethane-phosphonic acid derivatives is technically involved. A
S technically simpler route is via 2-acetoxyethanephos-phonic acid diesters, which can be prepared from vinyl acetate and dialkyl phosphites (German Offenlegungsschrift 2,127,821). ~imethyl vinylphosphonate can then be obtained in a yield of only 50% from dimethyl 2-acetoxyethanephosphon-ate by elimination of acetic acid at 550C lM. Yamagamiet al., Nippon Kagaku Kaisha 10, 1991 (1972)l. A new pro-cess is therefore sought which would make it possible to prepare vinylphosphonic acid diesters from 2-acetoxy-ethanephosphonic acid diesters in a higher yield.
It has now been found, surprisingly, that vinyl-phosphonic acid diesters of the general formula O
CH2 = CH - P(OR)2 in which R denotes alkyl groups having 1 to 4, preferably - 1 to 2, carbon atoms, can be prepared in a simple and economical way by heating dialkyl 2-acetoxyethanephosphon-ates of the general formula CH3COOCH2CH2P(OR)2 ~.

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in which R has the abovementioned meaning, at 150 to 270C, preferably 170 to 230C, in the presence of acid or basic cat;alysts and reactin~ the resulting reaction product with orthoesters of the general formula R'C(OR)3 in which ~' denotes hydrogen or C1-C4-alkyl or C1-C4-alkoxy and R denotes alkyl groups having 1 to 4, prefer-ably 1 to 2, carbon atoms, at 30 to 200~C.
It is surprising that in this process the ortho-estersr-are not added onto the vinylphosphonic acid group.
For it is known that orthoesters can be added onto un-saturated compounds, such as, for example, vinyl ethers - or vinyl esters, in the presence of acid catalysts (Houben-Weyl, Methoden der Organischen Chemie [Methods of Organic Chemistryl, Volume VI/3, page 247-248, Georg Thieme Verlag Stuttgart, 1965).
Examples of possible starting materials are the dimethyl, diethyl, diisopropyl and di-n~butyl ester of 2-acetoxyethanephosphonic acid. Dimethyl 2-acetoxyethane-phosphonate is particularly preferable. Examples of orthoesters which are particularly suitable are trimethyl orthoformate, triethyl orthoformate, trimethyl ortho-acetate, triethyl orthoacetate, triethyl orthopropionate and tetramethyl orthocarbonate.

Numerous compounds are possible as acid or basic catalysts. Acid catalysts used can be:
A) sulfuric acid or phosphoric acid B) halogen-containing carboxylic acids having a PKa 11586i,Z

value ~ 2.5, such as dichloroacetic acid, trichloroacetic acid or trifluoroacetic acid . .
C) aromatic sulf`onic acids having a P value ~ 2.5, Ka such as benzenesulfonic acid or p-toluenesulfonic acid D) preferably phosphinic acids having 2 to 18 carbon atoms, such as dimethylphosphinic acid, methylethylphos-phinic acid, dioctylphosphinic acid, methylphenylphosphinic acid or diphenylphosphinic acid E) particularly preferably phosphonic acids having 1 to 18 carbon atoms and their half~esters having 1 to 4 carbon atoms in the alcohol radical, such as methanephosphonic acid, propanephosphonic acid, propanephosphonic acid mono-methyl ester, octadecanephosphonic acid, 2-acetoxyethane-phosphonic acid, 2-acetoxyethanephosphonic aci.d monomethyl ester, vinylphosphonic acid, vinylphosphonic acid mono-methyl ester, vinylphosphonic acid monoethyl ester or benzenephosphonic acid F) likewise particularly preferably pyrophosphonic acids or their half-esters, such as methanepyrophosphonic acid, benzenepyrophosphonic acid, vinylpyrophosphonic acid or vinylpyrophosphonic acid monomethyl ester G) acid reaction mixtures which are produced in the pro-cess according to the invention are also highly suitable.
Basic catalysts used can be:
A) Tertiary aliphatic and aromatic amines and phosphines having 3 to 18 carbon atoms, such as trimethylamine, tri-propylamine, tributylamine, triphenylamine, trimethylphos-phine, triethylphosphine, tripropylphosphine, tributyl-phosphine, triphenylphosphine and tris-(p~diethylamino-- 5 ~1 S8~ ~

phenyl)-phosphine and the corresponding mixed amines, phosphines, phospholanes and phospholenes, such as dimethylethylaminc, di~thylbutylamine, N-dimethylaniline, 4-methyl-~-dimethylaniline, N-diethylaniline, N,N-tetra-methylphenylàiamine or N-methylpyrrolidine; methyldiethyl-phosphine, di~ethylpropylphosphine, diethylbenæylphosphine, 1-methylphosphol-3-ene and 1-ethyl-3-methylphosphol-3-ene.
B) Quaternary ammonium salts and phosphonium salts having

3 to 18 carbon atoms, such as tetramethylammonium chloride, tetramethylam~onium bromide or tetraethylphosphonium chloride, trimethylbenzylammonium chloride, triethylbenzyl-ammonium chloride, triethylbenzylammonium bromide, tri-methylbenzylphosphonium chloride or triphenylethylphos-phonium 2,4-diaminobenzenesulfonate.
C) Heterocyclic compounds having aromatic charactcr, such as pyridine, quinoline, their various al~yl and dialkyl, pre~erably methyl or dimethyl derivatives, imidazole, N-vinylimidazole, benzothiazole, 2-amino-6-ethoxybenzo-thiazole, and also phosphabenzoles.
D) Acid amides, such as dimethylformamide, N-dirnethyl-acetamide, N-diethylpropionamide, N-dimethylbenzamide, N-methylpyrrolidone or N,N'-tetramethylterephthalic acid diamide or ureas, such as tetramethylurea or trimethyl-phenylurea.
E) Other nitrogen compounds or phosphorus compounds having a higher valency of one N atom or P atom than 3, such as pyridine-N-oxide, trimethylphosphine oxide, tri-butylphosphine oxide, trihexylphosphine oxide, triphenyl-phosphine oxide, dimethylphenylphosphine oxide, dimethyl-- 6 - ~ ~58~2 phenylphosphine sulfide, dimethylchloromethy~phosphine oxide, dimethy]eicosylphosphine oxide, dimethyldodecyl-phosphine oxide, dimethylphosphine oxide, dimethylpyrrolid-inyl-1-methylphosphine oxide, triphenylphosphine dichlor-i~e, dimethy~dodecylphosphine sulfide, triphenylphosphine-imine, dimethylchloromethylphosphine dichloride, N-2-dimethylphosphinylethylmethylacetamide or N-2-dirnethyl- -phosphinylethylmethylamine, or-phospholene oxide, such as 1-methylphosphol-1-ene oxide or 1-ethyl-3-methylphosphol-1-ene oxide.
F) Amides of phosphinous and phosphonous acid and of phosphinic and phosphonic acids and also their thio analogs, such as ethanephosphonic acid bis-diethylamide, methanebutanephosphinous acid dimethylamide or diethyl-15 phosphinous acid isobutylamide. Also triamides of phos-phoric and of thiophosphoric acid, such as hexamethylphos-phoric acid triamide.
The catalysts are used in arnounts of 0.01 to 10, preferably 0.1 to 5, % by weight. When vinylphosphonic acid, monoalkyl esters thereof or acid reaction mixtures already obtained are used~even larger amounts of 10 to 50%
by weight can be used.
The process is in general carried out by mixing the starting material with the catalyst and raising the 25 mixture to the required reaction temperature of 150 to 270C, preferably 170 to 230C.
Higher temperatures are possible, but they do not yield any benefit. The danger of an increased formation of by-products, and also of polymerization, then arises.

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_ 7 _ ~15~2 This reaction eliminates an alkyl acetate and essentially produces a vinylphosphonic acid half-ester.
The alkyl acetate is distilled off together with small amounts of an alkanol and of a dialkyl ether. The dis-tillation is carried out under atmospheric pressure, ifappropriate~with the aid of an inert gas, such as, for example, nitrogen. However, in particular cases it may be advantageous to distil off in vacuo. The eli,mina-tion of the alkyl acetate is complete after 2 to about 20 hours. It can be advantageous to continue stirring thereafter for another 1 to 4 hours at the reaction tem-perature. The process can alsobe carried outin acontinuous manner. To prevent polymeri~ation, it is advantageous to add correspondinginhibitors, such as, forexample, hydro-5 quinone, hydro~uir~one monom~thyl ether orphenothiazine.lf 2-acetoxyethanephosphonic acid diesters which are contaminated from their preparation with small amounts of the corresponding monoester are used as a starting material5a further addition of a catalyst is not necessarily required. It is here advantageous to start the reaction at about 250C. When the acid reaction product which actually also acts as a catalyst for the elimination has been formed to a sufficient extent~the process can be con-tinued at lower temperatures, for example at 180 to 220C.
2~ The crude vinylphosphonic acid half-esterproduced in the first stage is reacted with the orthoesters at 30 to 200C, and carboxylates or carbon dioxide and the corres-ponding alcohols are formed at the same time. In the reaction to give the carboxylates and alcohols, it is iX

- 8 - l~S86~2 advantageGus to operate within a temperature range which is such that the carboxylates and alcohols distil o~f after they have been formed. As a rule at least one mole, preferably 1.5 to 2 moles, of orthoester are used per mole of vinylphosphonlc acid half-ester. Excesses which are greater yield no essential benefit. A particularly advan-tageous method of carrying out the reaction is to mix the half-ester with approximately the same amount of fully formed vinylphosphonic acid diester, in the state in which it is obtained as reaction product, and to leave the mix-ture for about 1 hour at an elevated temperature, for example at 160C, and thereafter to react this reaction mixture with the orthoester. In this procedure, the amount of dialkyl ether ~Ihich is otherwise necessarily obtained is small, and the exploitation of the alkylating potential of the orthoester is very high. It is also possible to mix the crude vinylphosphonic acid half-ester with the orthoester, that is initially to introduce the orthoester and to meter in the vinylphosphonic acid half-ester, or vice versa, and to allow the resulting mixture to react to completion at the particular reaction temperature required. When using orthocarbonates, this reaction tem-perature is at about 30 to 90C, while in the case of orthoformate and the higher orthocarboxylates a tempera-ture of about 30 to 160C is required. The process canalso be carried out in a continuous manner.
The vinylphosphonic acid diesters obtained in the present process are purified by distillaticn. As a rule, they contain relatively sm~l amounts of trialkyl phosphates which, .,~".~` .

~Sl~G~2 _ 9 _ if it is desired, can be separated off by suitable methods, for example by distillation methods.
Dialkyl vinylphosphonates have long been well known intermediates in the preparation of, for exa~ple, flame-retardants, flotation auxiliaries and dyestuffs.
They are also ~nown as comonomers in the preparation of numerous synthetic polymers.
Example 1 100 g of dimethyl 2-acetoxyethanephosphonate were heated with stirring at 220 - 230C. A mixture of 200 g of dimethyl 2-acetoxyethanephosphonate and 3 g of 4-(dimethylamino)-pyridine were added dropwise in the course of 6 hours and 112 g of methyl acetate distilled off over the same time period. 6 g of dimethyl ether were collec-~5 ted in a cold trap downstream of the apparatus. 171 g ofcrude monomethyl vinylphosphonate remained. 80 g of this reaction product and 93 g of trimethyl orthoacetate were mixed, and the mixture was heated gradually, in the course of 4.5 hours, and with stirring to an internal temperature of 150C while methyl acetate and methanol distilled off.
Another 35 g of trimethyl orthoacetate were then added at room temperature (molar ratio of monomethyl vinylphosphon-- ate to orthoester was about 1:1.6) and the mixture was - again gradually heated to 150C while methyl acetate dis-tilled off. A total of 90 g of a mixture of methyi acetate and methanol were obtained. The residue was dis-tilled under 0.5 mm Hg. 86 g of dimethyl vinylphosphonate were obtained which, according to the 31P-NMR spectrum, contained 7% o~ trimethyl phosphate. The distillation - 10 - ~586~Z
residue was 7 g. The yield of pure dimethyl vinylphos-phonate was 83.5% of theory, relative to the amount of dimethyl 2-acetoxyethanephosphonate started with.
Example 2 276 g of crude monomethyl vinylphosphonate, which had been prepared as in Example 1, were mixed with 376 g of trimethyl orthoformate, and the mixture was gradually heated in the course of 8 hours with stirring to 150C
while methyl formate and methanol distilled off. Another 100 g of trimethyl orthoformate (molar ratio of monomethyl vinylphosphonate to orthoester was about 1:2) were then added at room temperature, and the temperature was gradu-ally increased in the course of 13 hours to 150C while methyl formate and methanol distilled off. The residue was distilled under 0.5 mm Hg. 273 g of dimethyl vinyl-phosphonate were obtained which, according to the 31P-NMR
spectrum, contained 6% of trimethyl phosphate. The dis-tillation residue was 47 g. The yield of pure dimethyl vinylphosphonate was 76% of theory, relative to the amount of crude monomethyl vinylphosphonate started with.
Example 3 150 g of tetramethyl orthocarbonate were added dropwise with stirring at room temperature to 80 g of crude monomethyl vinylphosphonate, which had been prepared as in Example 1. The temperature increased slightly to 35C and carbon dioxide was eliminated at the same time.
The temperature was then gradually increased in the course o~ about 2.5 hours to 87C while carbon dicxide continued to be eliminated. The mixture was then distilled under ~2 0.5 mm Hg. This produced 74 g of dimethyl vinylphosp'non-ate and, in a cold trap downstream of the apparatus, a mixture of about 30 g of methanol and 77 g of unconverted tetramethyl orthocarbonate. The reaction of the 80 g of - 5 crude monomethyl vinylphosphonate used as starting mater-ial thus consumed 73 g of tetramethyl orthocarbonate (molar ratio of monomethyl vinylphosphonate to orthoester was 1:0.8).
Example 4 25 g of vinylphosphonic acid and 70 of diethyl 2-- acetoxyethanephosphonate were heated with stirring to 180C. 252 g of diethyl 2-acetoxyethanephosphonate were then slowly added dropwise over 14 hours while the temperature gradually increased to 200C and ethyl acetate distilled off. The mixture was then stirred for 9 hours at 190C.
190 g of crude monoethyl vinylphosphonate were obtained.
Distillation produced 128 g of ethyl acetate and addition-ally 8 g of light ends which were trapped in a cold trap downstream of the apparatus. 95 g of the crude monoethyl vinylphosphonate and 124 g of triethyl orthoformate were refluxed for 8 hours at about 75C. The temperature was then gradually increased in the course of 5.5 hours to 150C while ethyl acetate and ethanol distilled off.
A further 100 g of triethyl orthoformate were then added at room temperature and the mixture was gradually heated again to 150C while light ends distilled off. A cold trap downstream of the apparatus collected less than 1 g. The mixture is then distilled, 9~.5 g of dimethyl vinylphos-phonate were obtained. This corresponded to a yield of i2 about 70%, relative to the amount of vinylphosphonic acid and diethyl 2-acetoxyethanephosphonate started with.
During the reaction with the orthoester, about 160 g dis-tilled off which contained in addition to about 60% of ethyl formate, 28% of ethanol and about 10% of diethyl ether.
Example 5 80 g of crude monomethyl vinylphosphonate as obtained in ~xample 1 were mixed with 80 g of the dimethyl vinylphosphonate obtained according to Example 1, and the mixture was kept at 160C for 2 hours. The resulting mix-ture was add~od dropwise with stirring in the course of 6 hours to 148 g of trimethyl orthoformate at 100C. During this period 93 g of a mixture of methyl acetate and methanol distilled off. No light ends were observed in a cold trap downstream of the apparatus. The mixture was then heated to 145C and finally distilled. 153 g of dimethyl vinylphosphonate and 22 g of distillation residue were obtained. The cold trap of the distillation appara-tus contained 22 g of essentially trimethyl orthoacetate.73 g of dimethyl vinylphosphonate were obtained.
Example 6 30 g of vinylphosphonic acid and 80 g of diethyl 2-acetoxyethanephosphonate were mixed, and the mixture was 25 heated with stirring to 175 - 180C. 500 g of diethyl 2-acetoxyethanephosphonate were then added dropwise in the course of 4 hours at this temperature, 13 hours at 185C
and 6 hours at 190C, while 187 g of ethyl acetate dis-tilled off. 2 g of light ends were collected in a cold - 13 - ~58~2 trap downstream of the apparatus. The resulting reaction mixture (305 g) was maintained for a further 1 hour at 200C. 295 g of crude monomethyl vinylphosphonate were then obtained. 81 g of this product were mixed with 106 g of triethyl orthoformate, and the mixture was heated with stirring for 7 hours up to 150C while ethyl formate and ethanol distilled off. A further 60 g of trimethyl ortho-formate were then added at room temperature, and the resulting mixture was again heated for 2 hours up to 150C.
A total of 109 g of ethyl formate together with ethanol and diethyl ether were distilled off. Distillation of the reaction mixture produced 81 g of diethyl vinylphosphonate which contained relatively small amounts of triethyl phosphate.

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Claims (2)

Patent Claims:

1. A process for preparing vinylphosphonic acid di-esters of the formula in which R denotes C1-C4-alkyl, which process comprises heating dialkyl 2-acetoxyethanephosphonates of the formula in which R has the abovementioned meaning, at 150 to 270°C
in the presence of acid or basic catalysts and reacting the resulting reaction product with orthoesters of the formula R'C(OR)3 in which R' denotes hydrogen, or C1-C4-alkyl or C1-C4-alkoxy and R has the abovementioned meaning, at 30 to 200°C.

2. The process as claimed in claim 1, wherein the dialkyl 2-acetoxyethanephosphonate is heated at a tempera-ture of 170 to 230°C.