CA1185262A - Process for preparing vinylphosphonic acid - Google Patents

Abstract

Abstract of the disclosure:
Process for preparing vinylphosphonic acid, by heating dialkyl 2-acetoxyethanephosphonates of the general formula

Description

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HOE ~1/~ 125 It is known that pure vinylphosF~honic acid can be obtained from vinylphosphonic acid dichloride. However, the synthesis o~ ?ure vinylphosphonic acid dichloride is technically elaborate. A simpler process to prepare vinylphosphonic acid is therefore desirable.
It has now been founc` that vinylphosphonic acid ean be prepared in a simple and economical manner by heating dialkyl 2-acetoxyethanephocphonates of the formula C~l3CCH2CH2~(OR)2 in which ~ denotes allcyl grou?s having 1 to 4. preferably 1 to 2, carbon atoms, at 150 to 270C, preferably 170 to 230C, in the presence of acid or basic catalysts, react-ing the resulting reaction product with orthoesters of the general formula R'C(OR)3 in which R' denotes hydrogen, C1-C~-alkyl or C1-C4-allcoxy ancl R denotes alkyl groups having 1 to ~, preferably 1 to

2, carbon atoms, at 30 to 200C and hydrolyzing the resultinp, vinylphosphonic acid diester with t~ater at tem-peratures between 130 and 230C, preferably between 140and 175C, while simultareously distillirg off the alco-hols formed.
It is surprisine that in this process the ortho-35;~6~

esters 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 tHouben-~eyl, ~ethoden der Organischen Chemie [Methods of Organic Cher~istryl, Volume VI/3, pages 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 tetramet;-yl orthocarbonate.
Numerous compounds are possible as acid or basic catalysts. Acid catalysts us d can be:
- A) sulfuric acid or phosphoric acid B) halo~en-containing carboxylic acids having a PKa value ~ 2.5, such as dichloroacetiC acid, trichloroacetic acid or trifluoroacetic acid C) aromatic sulfonic acids having a P value c 2.5, such as benz~rlesulfonic acid or p-toluenesulfonic aci~
D) preferably phospninic acids having 2 to l& carbon atoms, such~s ~ir,lethylpho~phin_c acid, rnethylethylphos-phinic acid, dioctylphosphinic acid, methylphenylphosphinic acid or diph~nylphosphinic acid E) particularly preferably phosphon,c acids having 1 to 1~ carbon atoms and their half-esiers havin~ 1 to 4 carbon atoms ln the alcohol radical, such as methanephosphonic .acid~ propanephosphonic acid, propanephosphonic acid mono-methyl ester, octadecanephosphonic acid, 2-acetoxyethane-phosphonic acid, 2-acetoxyethanephosphonic acid monomethyl ester, vinylphosphonic acid, vinylphosphonic acid mono-methyl ester, vinylphosphonic acid monoethyl ester or benzenephosphonic acid F) likewise p~rticularly preferably pyrophosphonic aeids or their half-esters, such as methanepyrophosphonic aeid, 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 a i~h~tic and aromatic amines and phosphines having 3 to 1& carbon atoms, such as trimethylamine, tri-propylamine, tributylamine, triphenylamine, trimethylphos-phine, triethylphosphine, tripropylphosphine, tributyl-phosphine, triphenylphosphine and tris-(p-diethylamino-phenyl)-phosphine and the corresponding mi~ced amines, .phosphines, pho$pholanes and phospholenes, such as dimethylethylamine, diethylbutylamine, N-dimethylaniline, 4-methyl-N-dirr.ethylalliline, N-diethylaniline, N,l~-tetra-rnethylphenylcliamine or N-methylpyrrolidine; methyldiethyl-phospiline, dim~thylpropylphosphine, diethylben~ylphosphine,1-methylphosphol-3-ene and 1-ethyl-3-methylphosphol-3-ene.
B) Quaternary ammonium salts and phosphonium salts having

3 to 18 carbon atoms, su~h as tet-amethylammonium chloride, tetramethylammonium brornide or tetraethylphosphoniu~n ~l852~

chloride, trimethylbenzylam~nonium chloride, triethylbenzyl-~nMcnium chloride, triethylben ylammonium bromide, tri-methylbenzylphosphonium chloride or triphenylethylphos-phonium 2,4-diaminoben~enesulfonate.
C) Heterocyclic com?ounds having aromatic character, such as pyridine, quinoline, their various alkyl and dialkyl, preferably methyl or dimethyl derivatives, imidaæole, N-vinylimidazole, benzothiazole, 2-amino-~-ethoxybenzo-thiazole, and also phosphabenæenes.
D) Acid amides, such as dimethylfor,.,amide, N-dimethyl-acetamide, N-diethylpropionamide, N-dimethylbenzamide, N-methylpyrrolidone or N,N'-tetr2methylterephthalic acid diamide or ure~s, such as tetramethylurea or trimethyl-phenylurea.
15 E) Other nitroge-~ compGunds or phosphorus compounds having a higher valency of one N atom or P atom than 3, such as pyridine-N-oxi~e, trimethylphosphine oxide, tri-butylphosphine oxide, trihexylphosphine oxide, triphenyl-phosphine oxide,.dimethylphenylphosphine oxide, dimethyl-phenylphosphine sulfide, dirnethylchloromethylphosphineoxide, dimethyleicosylphosphine oxide, dimethyldod~cyl-phosphine oxide, dimethylphosphine oxide, dimethylpyrrolid--.
inyl-1-methylphosphine oxide, tri?henylphosphinc dichlor-ide, dimcthyldod~cylphosphine sulfide, triphcnylphosphine-25 imine, dimet~ylchloromethylphosphine dic)-loride, N-2-dimethylphos?hinylethylmethylacetamide or N-2-climethyl-phosphinylethylnethylamine, or phospholene oxide, such as 1-met)~ylphosphcl-1-ene oxide or 1-ethyl-3-methylphosphol--1-ene oxide.

2~

F) Amides of phosphinous and phosphonous acids and of phosphinic and phosphonic acids and also their thio analogs, such as ethanephosphonic acid bis-diethylamide, methanebutanephosphinous acid dimethylamide or diethyl-phosphinous acid isobutylamide. Also triamides of phos-phoric and olf thiophosphoric acid, such as hexamethylphos-phoric acid triamide~
The catalysts are used in amounts of 0.01 to 10, preferably 0.1 to 5, /0 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 call be used.
The process is in general carried out by mixing the starting material with the catalyst and raising the mixture to the required reaction temperatu.e OI 150 to 270C, preferably 170 to 230C.
Higher temperatures are possible, but they do not yield any benefit. The danger o~ an increased formation of by-products, ~.d also of polymerization, then arises.

This reaction eliminates an alkyl acetate and essentially produces a vinylphosphonic acid half-ester.
The alkyl acetate is distilled off together ~Jith small amounts of an alkanol and of a dialkyl ether. The dis-tillation is carried out under atmospheric pressure, if appropriate ~/ith 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 elimina-tion of the alkyl acetate is cornplete 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. Theprocesscan alsobecarriedoutirlacontinuous manner. To prevenipolymeri~ation,it is advantageous to add correspondinginhibitors, such as, forexample,hydro-5 quinone,hydro~uino~le monomethyl etherorphenothia~ine.
If 2-acetoxyethanephosphonic acid diesters which are contaminated from their preparation with small amounts of the corresponding monoester are used as a starting material,a further addition of a catalyst is not necessarily 10 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 extent7the process can be con-tinued at lower tem?eratures, for example at 180 to 220C.
The crud~ vi~ylph~sph~n_c aci~alf-esterproducedin 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 20 advantageous to operate within a temperature range which is such that the carboxylates and alcohols distil off after they have been formed. As a rule at least one mole, preferably 1.5 to 2 moles, of orthoester are used peI` mole Or vinylphosphonic acid half-ester. Excesses which are 25 grcater yie~d no cssential benefit. A particularly advan-tageous method of carrying out the reaction is to mix the half-ester with a?proximately the same amount of fully formed vinylphos?honic acid diester, iT) the state in which it is obtained as reaction product, and to leave the mix-:1~85Z6~ `

t~re for about 1 hour at an elevated temperature, foreY~ampl~'at lbOC, and thereafter to react this reaction mixture with the orthoester. In this procedure, the àmount of dialkyl ether which is other~ise 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 hal~-ester, or vice versa, and to allow the resuiting 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 ort~.ocarboxylates a tempera-ture of about 30 to i6GC is required. Tne process canalso be carried out in a continuous manner.
The vinylphosphonic acid diesters obtained in the presentprocess are puri~ied, if desired, by distillation. As a n~e, they contain relati~ely sr~l a~ounts of trialkyl phosphates which, if it is desired, can be separated off by suitable methods, for example by distillation methods.
The dialkyl vinylphosphonates prepared in this process step and any trialkyl phosphates present are then reacted with water at the reactioll temperature required 2~ and the resulting alcollcl is advantageously distilled off via a column. Srnall amounts of a dialkyl ether and ole-fins can be formed in this reaction. Th~ reaction with water is complete when no mcre alcohol is split orf. Here it can be advantageous to add relatively large amGunts of ~ ~52~;~
g water toward the er.d of the reaction and to distil of~
some of the unreacted water together ~Yith the alcohol.
The press~:re to be used in this process is not critical, but the process is preferably carried out under approxi-mately a-tmospheric pressure. In this process stage also it is ad~antageous to admix an amount of final pro-duct, namely vinylphosphor.ic acid, to the dimethyl vinyl-phosphor.ate before the hydrolysis.
The reaction temperatures in ~his process step are betweer. 130 and 230C. The reaction is preferably carried out within a temperature range of 140 to 175C. The reac-tion with water can also be carried out in a continuous manner. lhe resu~ting vinylphosphonic acid can be freed from relat-vely small amounts of dissolved water in vacuo at an elevated temperature. Tne acid contains phosphoric acid as a secondar" constituent if the trialkyl phosphate was not separated from the vinylphosphonic acid diester before the hydrolysis. It is possible to separate the phosphoric acid from the vinylphosphcnic acid via the salts.
Vinylphosphor,ic acid is suitable for use as a corrosion inhibitor in aqueous systems and, in particular, also as intermediate in the preparation of f]ame-retardants.
Vinylpllosphonic ?~cids can also be converted into poly-vinylphosphonic acid for whic~ there are many inductrialuses.

ExamDle 1 __ _ lO0 g of dimethyl 2-acetoxyethanephosphollate were heated with stirring at 220 to 230C. A mixture c~f 200 g ~5;~ Ei2 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-ted in a cold trap do~rnstream o~ the apparatus. 171 g ofcrude monomethyl vinylphosphonate remained. This quan-. . .
ti~y was rrixed with 200 g of methyl orthoacetate, and -the resultin~ mix-ture was gradually heated in the course of

4.5 hours with stirring to an internal temperature o-f l~ 150C while a mixture of methanol and methyl acetate dis-tillecl off. A further 70 g of trimethyl orthoacetate werethenadded at room temperature (molar ratio of mono-m~thyl vinylphosphonate to orthoester was about 1 : 1.6) and the mixture was again gradually heated to 150C while methyl acetate ænd methællol distilled off. A total of 192 g of a mixture of methanol and methyl acetate were obtained. The residue was distilled off under 0.5 mm ~g.
172 g of dimethyl vinylphosphonate were obtained which, according to a 31P-NMR spectrum, contained 7% of trimethyl phosphate. These 172 g o~ dimethyl vinylphosphonate were mixed with 17 g of vinylphosphonic acid, and the mixture was heated with stirring at 160 to 175C. Water was metered in at the same time, while rr.ethanol dis-tillecl off via a cclumn with a silver-coated jacket. 12 g of dimethy] ether were collected in a cold trap downstream of the apparatus. After about 7 hours, the water con-tainecl in the reaction mix-ture~lasdistilled cf~ a-t 90C
and under C.5 mm ~Ig. 153 g of vin~lphosphonic acid rernained which hacl a content of 7% Gf phosphcric acid and

5;~iEiZ

- 2% of the compound - O O
CH2=CH-?-OcH2cH2P(OH)2 as measured from the 31P~NMR spectrum~

Example 2 -350 g cf dimethyl vinylphosphonate, as prepared in Example 1, and 35 g of vinylphosphor.ic acid were mixed, and the mixture was heated with stirring to 145C. Water was then metered in for 14 hours while methanol distilled off via a columr with a silver-coated jacket. 3 g of dimethyl ether were eollected in a eold trap downstream of the apparatus. The water ccntaining the reaction mix-ture was then distilled off at gOC and under 0.5 mm Y.g.
310 g of vinylphosphonic acid remain~d.

Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing vinylphosphonic acid, which comprises heating dialkyl 2-acetoxyethanephosphonates of the formula in which R denotes alkyl groups having 1 to 4 carbon atoms, at 150 to 270°C in the presence of acid or basic catalysts, reacting the resulting reaction pro-duct with orthoesters of the formula R'C(OR)3 in which R' denotes hydrogen, C1-C4-alkyl or C1-C4-alkoxy and R denotes alkyl groups having 1 to 4 carbon atoms, at 30 to 200°C and hydrolyzing the resulting vinylphosphonic acid diester with water at temperatures between 130 and 230°C
while simultaneously distilling off the alcohols formed.

2. The process as claimed in Claim 1, wherein R denotes alkyl groups having 1 or 2 carbon atoms.

3. The process as claimed in Claim 1, wherein the dialkyl 2-acetoxy-ethanephosphonate is heated at a temperature of 170 to 230°C.

4. The process as claimed in Claim 1, 2 or 3 wherein the vinylphosphonic acid diester is hydrolyzed at temperatures between 140 and 175°C.