IL101830A - Process for the preparation of triaryl phosphates - Google Patents

Description

REF : 2213/92 PROCESS FOR THE PREPARATION OF TRIARYL PHOSPHATES 2213/92 PROCESS FOR THE PREPARATION OF TRIARYL PHOSPHATES Field of the invention This invention relates to the preparation of triaryl phosphates from phenols or substituted phenols.

Background of the invention Triaryl phosphates are used mainly as plasticizers, lubricants and flame retardants. They are generally prepared on an industrial scale by the thermal condensation of phosphorus oxychloride with the respective phenol in the presence of a catalyst (e.g. EP 278353). The drawbacks of thermal condensations are: - relatively high temperatures, 160-250°C, are required; - discolored products are often produced; relatively long reaction time, viz. several hours, are required, so that the use of heat-sensitive alkyl-phenols as starting product is problematic; - the HC1 removed during the reaction may entrain significant amounts of the volatile POCl3; - the reaction system is highly corrosive due to the presence of HC1 at high temperatures; - the product is usually contaminated with the metallic salts, such as AICI3 or MgCl2, which are used to catalyse the reaction.

Attempts have been made in the past to find alternative ways for preparing triaryl phosphates at lower temperatures: The Schotten-Baumann type reaction between aryloxide salts and POCI3 in aqueous media, using a relatively small proportion of an inert organic solvent, is disclosed in U.S. Patent 1,837,176. However, when using -2- 101830/2 2213/92 said method, the product either precipitated together with the co-produced NaCl from which it had to be separated by extraction, involving an additional filtration step, followed by freezing out or distilling, or separated as a liquid phase contaminated with the solvent, from which the product had to be recovered by the aforesaid steps. The numerous operations involved make for an overly costly process.

U.S. Patents 4,267,127 and 4,290,977 disclosed modifications of the aforesaid process, which consist in increasing the amount of solvent used and performing the operation in two or more reaction stages, in the presence or absence of a PTC reagent, or in regulating the quantity of solvent used so as to obtain a 15-30% (w/w) solution of the product. Neither modification eliminates the complication of product recovery from the solvent.

The processes described above have also been employed by V.K. Krishnakumar in Synthesis, 558, (1983) and in Synth. Comm. 14, 189 (1984). The latter study also showed that the dry (i.e. non-aqueous) sodium salt of the phenol can be reacted with POCI3 in the solvent, but the authors themselves do not recommend this mode of operation for industrial application.

All of the above processes are quite complicated and are wasteful of energy.

Summary of the invention It has now been unexpectedly found that triaryl phosphates can be prepared much more simply and in high yields by contacting an aqueous alkaline solution of a phenol with POCI3 in the presence of an organic solvent which has been pre-saturated with the triaryl phosphate product. 2213/92 The reaction may be performed either in the presence or absence of a phase transfer catalyst (PTC). This latter may serve to help in the separation of the phases. The product separates as a third, usually solid phase which is readily separated from the reaction medium, e.g. by filtration. The resulting aqueous phase, containing dissolved alkali chloride, is discarded, after extracting (if necessary) any dissolved reagent/product with make-up solvent. The organic phase, together with make-up solvent is recycled for re-use. Excellent yields are obtained in an energy saving process.

The reason for the improvement obtained is not clearly understood. A skilled person would have expected that operating with solvent pre-saturated with product might lead to the premature precipitation of partially arylated phosphoryl chlorides and their hydrolysis by the water present. This possibility is suggested by U.S.P. 1,837,176, Col. 1, lines 22-31. Possible hypotheses are that the presence of the dissolved product serves to catalyze the desired reaction, or to provide better contact between the reactants, or preferentially co-dissolve POCI3 thereby minimizing its hydrolysis while the additional triaryl phosphate, which is formed, precipitates, or to disproportionate with POCI3 in the organic phase to produce partial aryl phosphoryl chlorides which then react with the alkali phenolate. However it may be that none of the above hypotheses is correct ,and the applicant does not wish to be bound to any explanation of the unexpected results of the invention.

Detailed description of preferred embodiments In the practice of this invention, the phenol or substituted phenol - which may be e.g. phenol, cresol, a propylphenol, a butylphenol, or a higher alkylphenol, an halogenated phenol, such as a chlorinated or brominated phenol, a nitrophenol, a bisphenol, such as bisphenol-A or bisphenol-F, or 2213/92 the like, or a mixture thereof is reacted with a stoichiometric amount, or a slight excess, of an aqueous alkaline solution, containing NaOH, KOH, Na2CC>3, K2CO3, or the like, which may also contain dissolved inorganic salts such as NaCl, and added to the reactor containing an inert organic solvent in which the product is at least partially soluble, such as acetonitrile, or a halogenated organic solvent, e.g. chlorobenzene, chloroform, dichloroethane, methylene dichloride, dibromomethane, or a hydrocarbon e.g. toluene, heptane, xylene, or the like, oir mixtures thereof, saturated with respect to the triaryl phosphate to be produced. "At least partially soluble" should be construed as meaning soluble to the extent of at least 1% at 40°C.

A phase transfer catalyst chosen for example from the class of quaternary ammonium salts, such as tetrabutylammonium bromide, tricaprylylmethylammonium chloride, tetrabutylammonium bisulfate, quaternary phosphonium salts such as tetraphenylphosphonium bromide or chloride, etc., or polyethylene glycol, may also be added, in the concentration range of 1-15% based on the weight of reagents.

The reaction temperature lies between that at which one or more of the components freezes or separates out of the liquid in which it is contained and the boiling point of the solvent employed. The preferred temperature lies between -15°C and 50°C, most preferably between -10°C and 40°C.

The mixture is stirred and kept at the above temperature, while the POCl3 (neat or dissolved in some of the pure reaction solvent) is added, in an approximately equimolar amount or slightly less relative to the phenol. The time of addition of the POCI3 is within 5-120 minutes. 2213/92 The triaryl phosphate formed precipitates out, and is separated from the mother liquor by filtration, centrifugation or other means of solid-liquid separation. The cake is washed, and dried. The mother liquor is composed of two liquid phases, the aqueous phase, which contains primarily the alkali metal chloride, and the organic phase, which is recycled without further treatment. A bleed may be required to prevent the build-up of impurities, such as the bis(aryl)phosphoric acid (briefly designated as "bis-acid") which may be co-produced in small amounts in the organic phase and the concentration whereof may increase on recycling. Means, hereinafter described, have been found to separate the bis-acid by-product, and to hydrolyse it, whereby the phenolic moiety is recovered, and recycled as well. This may be an important feature with respect to the overall economics of the process.

The facile separation and hydrolysis of the bis-acid is based on the discovery that treatment of the organic phase mother liquor in which it is contained with aqueous alkali induces the separation of that phase into a bis-acid rich bottom phase, essentially free of triaryl phosphate product but which contains the PTC, and a bis-acid depleted phase containing all the triaryl phosphate. For the purpose of this separation, it is desirable to use mild conditions to minimize the extent of hydrolysis of the product. Thus, dilute alkali, preferably below 10%, and low temperatures, preferably below ambient but above the freezing point of the mixture, are also recommended. Several minutes have been found to suffice.

The hydrolysis of the bis-acid is effected by refluxing the bis-acid rich phase with excess concentrated aqueous alkali (preferably 20-40%) to completion of the reaction. 2213/92 The production of the triaryl phosphates may be performed in the batch or continuous mode. In the latter case, the process can be carried out in standard equipment, such as the pumped loop or cascade reactors described in columns 5 and 6 of U.S.P. 4,290,977 in conjunction with continuous filters or centrifuges to remove the product.

EXAMPLE 1 2,4-Dibromophenol, 75.6 g. was introduced into a l l. R.B. flask provided with a mechanical stirrer, thermowell and condenser. An aqueous solution saturated with NaCl, 180 ml., was added to the reactor, followed by 13.6 g. NaOH, and 450 ml. acetonitrile (MeCN) pre-saturated with tris(2,4-dibromophenyDphosphate (TRIS). The mixture was stirred thoroughly at 20°C and 14 g. POCl3 was added during one hour. The product, TRIS, which formed, precipitated out as a colorless, crystalline solid. After filtration, the product was washed with water until Cl-free and dried. A total of 59 g. of pure TRIS, m.p. = 108-9°C was obtained, corresponding to an 88% yield based on unrecovered dibromophenol. The filtrate was separated into the aqueous and the MeCN solutions. The latter contained the unreacted phenol and ~5 g. of the by-product bis(2,4-dibromophenyl) phosphoric acid (bis-acid).

EX MPLE 2 Example 1 was repeated at 40°C using 4 g. tetrabutylammonium bisulfate as catalyst. The results obtained were essentially the same as those described above.

EXAMPLE S The reaction was performed in a 5 1. double-wall glass reactor cooled to 0 -5°C and stirred at 800 rpm. The reactor was charged with 378 g. of 2,4- 2213/92 dibromophenol (DBP), 60 g. NaOH, 900 ml. water, 19 g. tetrabutylammonium bromide and a solution made up of 800 ml. dichloromethane (DCM) saturated with TRIS. To this mixture was added during 25 minutes, 70 g. POCl3 dissolved in 160 ml. DCM. The initial pH of 12.5 did not drop below 10 during the reaction. The mixture was filtered, the precipitate, washed with water and dried, whereby 255 g. of TRIS was obtained. The organic filtrate was separated from the aq. filtrate, and replaced in the reactor.

To the reactor were added 315 g. of 2,4-dibromophenol, 50 g. NaOH, 770 ml. water and 1.9 g. tetrabutylammonium bromide. To the cooled and stirred mixture was added during 1/2 hour, another 70 g. POCI3 dissolved in 160 ml. DCM. The reaction mixture yielded after filtration, washing and drying of the precipitate, another 210 g. of product. The organic filtrate contained 400 g. TRIS, 43 g. bis-acid and 58 g. unreacted DBP. The conversion of the phenol in these two cycles was 92%, and the yield of TRIS was 90% based on unrecovered DBP. Bis-acid was co-produced to an extent of 6% (same basis).

EXAMPLE 4 Example 3 was repeated using instead of 2,4-dibromophenol, a mixture consisting of 10% (w/w) 2,4,6-tribromophenol and 90% (w/w) 2,4-dibromophenol and an equivalent amount of KOH. The wet precipitate which was obtained was washed with a 1% KOH solution followed by water, then dried overnight at 80°C/20 mm. Hg. The colorless product obtained, analysed by HPLC was found to contain 97% TRIS, approximately 3% of the mixed ester bis (2,4-dibromophenyl)-(2,4,6-tribromophenyl) phosphate, and only 70 ppm of free dibromophenol. 2213/92 EXAMPLE 5 An aq. Na-dibromophenolate solution (Na-DBP) was prepared from 378 g. of 2,4-dibromophenol, 63.5 g. NaOH and 675 g. water. Into a l l. double-wall reactor were introduced: 224 g. of the Na-DBP solution, 450 g. of a saturated solution of TRIS in DCM and 3.9 g. of tetrabutylammonium bromide (TBAB). To the mixture, which was stirred and cooled to 0°C was added during 1/2 hour a solution of 15.3 g. POCI3 in 30 ml. DCM. The precipitated product was removed, and the organic phase was recycled to a subsequent reaction step to which were added fresh amounts of the Na-DBP solution, POCI3/DCM and only 10% of the amount of TBAB used in the first cycle. The results obtained for 5 cycles are summarized in Table I: Table I Cycle POC13 DBP T R I S No. (mole) (mole) (mole) Yield* 1 0.1 0.30 0.082 88% 2 0.1 0.27 0.086 98% 3 0.1 0.30 0.078 87% 4 0.1 0.30 0.075 80% 5 0,; 0.30 0.078 94% ♦Yields are based on unrecovered DBP and represent the amounts formed in each cycle, less the amounts introduced from the previous cycle.

EXAMPLE β Example 5 was repeated in the absence of TBAB. The results obtained are summarized in Table II. Although the yields were satisfactory, the phase separations were poorer: 2213/92 Table Π Cycle POC13 DBP T R I S No. (mole) (mole) (mole) Yield* 1 0.1 0.30 0.088 100% 2 0.1 0.30 0.080 96% 3 0.1 0.30 0.070 84% 4 0.1 0.28 0.071 81% 5 0.1 0.28 0.073 73% 6 0.1 0.25 0.071 81% *Yields are based on unrecovered DBP and represent the amounts formed in each cycle, less the amounts introduced from the previous cycle EXAMPLE 7 Two hundred grams of an organic phase obtained after 15 reaction cycles, which contained: 14.5% TRIS, 20% bis-acid, 4.5% DBP and 0.9% TBAB was mixed at 0°C for 10 minutes with 200 g. of an aq. 5% NaOH solution, and with 30 ml. DCM. The mixture separated into three distinct phases: - Bottom phase, 106 g. which contained, besides the solvent, all the bis- acid and TBAB present, most of the DBP, and no TRIS - Middle phase, 125 g. which contained, besides the solvent, only TRIS, and small amounts of DBP - Top phase, 205 g. of an aq. NaOH solution.

The bottom phase, was treated in order to hydrolyze the bis-acid and recover the DBP: 100 g. was mixed with 72 g. of a 20% aq. NaOH solution, heated to strip out the dissolved DCM, and then refluxed for one hour. The homogeneous phase obtained, 144 g., was free of bis-acid, containing instead, 27% DBP and the TBAB, which were recycled. 2213/92 EXAMPLE 8 The middle phase obtained in Example 7 was recycled to a new reaction cycle. The following were introduced into a one liter glass reactor, stirred at 700 rpm and cooled to 0°C: - 500 g. of an organic phase purified according to Example 7, containing 19.9% TRIS, 0.4% DBP - 140 g. of a hydrolyzed bottom phase, obtained according to Example 7, which contained TBAB catalyst and 21% DBP (0.18 mole) - 46.2 g. additional DBP (0.12 mole) - 2 g. NaOH/44 ml. water.

POCl3, 15.3 g. was added during 1/2 hour. After filtration of the solid which was produced, the precipitate was washed with 1% aq. NaOH, water and dried. The liquid phases were separated into an organic phase which was recycled, and an aqueous phase. The following results were obtained: 76.1 g. solid TRIS + 82.5 g. TRIS in Mother Liquor = 158.6 g., an 87% yield based on unrecovered DBP. No detectable amounts of bis-acid formed.

EXAMPLE 9 Example 3 was repeated using instead of DCM as solvent, other solvents saturated with TRIS. Polyethylene glycol 600 was added as catalyst. The results obtained are summarized in Table III: Table III Solvent Yield Of TRIS* Chlorobenzene 89% 1 ,2-Dichloroethane 80 Dibromomethane 83% Chloroform 79% Toluene** 84% *Based on unrecovered DBP ** at -10°C 2213/92 EXAMPLE 10 Example 1 was repeated using 4-bromophenol instead of 2,4-dibromophenol in the presence of tricaprylylammonium chloride as catalyst. A yield of 85% tris (4-bromophenyl) phosphate was obtained.

EXAMPLE 11 Example 10 was repeated using phenol instead of bromophenol. The MeCN solvent used was saturated with triphenyl phosphate. The reaction was performed at -5°C whereby triphenyl phosphate was produced to an extent of 69% EXAMPLE ½ Into a l l. double- wall reactor were introduced: a solution of 315 ml. heptane and 35 ml. toluene pre-saturated with TRIS, 337 g. of a Na-dibromophenolate solution prepared as in Example 5, and 5.8 g. of tetrabutylammonium bromide. To this mixture which was stirred and cooled to -5°C, was added during 1/2 hour, 23 g. of POCI3. The product was filtered, washed with aq. NaOH, water, and dried. Fifty g. of pure TRIS was obtained.

The filtrate was separated into the aqueous phase which contained the NaCl formed, and the organic phase saturated with the product which contained < 0.2% of bis-acid.

Claims (1)

1. 2213/92 C LA I M S 1 - Process for the preparation of triaryl phosphates, comprising contacting with POCI3 an aqueous alkaline solution of a phenol in the presence of an organic solvent, which has been pre-saturated with the triaryl phosphate product. 2 - Process according to claim 1, performed in the presence of a phase transfer catalyst. 3 - Process according to claim 1, wherein the phenol is a substituted phenol. 4 - Process according to claim 3, wherein the phenol is chosen from among phenol, cresol, propylphenols, butylphenols, higher alkylphenols, halogenated phenols, nitrophenols, bisphenols, and mixtures thereof. 5 - Process according to claim 1 , wherein a stoichiometric amount or a slight excess of the aqueous alkaline solution is used. 6 - Process according to claim 1, wherein the aqueous alkaline solution contains an alkali chosen from among NaOH, KOH, Na2C03, and K2CO3. 7 - Process according to claim 1, wherein the aqueous alkaline solution contains dissolved inorganic salts. 8 - Process according to claim 7, wherein the inorganic salt is sodium chloride. 2213/92 9 - Process according to claim 1, wherein the organic solvent is a solvent in which the product is soluble to the extent of at least 1% at 40° C. 10 - Process according to claim 9, wherein the organic solvent is chosen from among acetonitrile, halogenated organic solvents, aliphatic and aromatic hydrocarbon solvents, and mixtures thereof. 11 - Process according to claim 10, wherein the organic solvent is chosen from among dichloromethane, chlorobenzene, 1 ,2-dichloroethane, dibromomethane, ΟΗΟΙ , toluene, heptane and their mixtures. 12 - Process according to claim 2, wherein the catalyst is chosen from among quaternary ammonium salts, quaternary phosphonium salts or polyethylene glycol. 13 - Process according to claim 12, wherein the catalyst is a quaternary ammonium salt chosen from among tetrabutylammonium bromide, tricaprylylmethylammonium chloride and tetrabutylammonium bisulfate, 14 - Process according to claim 12, wherein the catalyst is a quaternary phosphonium salt chosen from among tetraphenylphosphonium bromide or chloride. 15 - Process according to claim 2, wherein the catalyst is used in a concentration 1-15% based on the weight of reagents. 16 - Process according to claim 2, wherein the reaction is carried out at temperatures between that at which one or more of the components freezes ' 2213/92 or separates out of the liquid in which it is contained and the boiling point of the solvent employed. 17 - Process according to claim 1, wherein the reaction is carried out at temperatures between -15°C and 50°C and preferably between -10°C and 40°C. 18 - Process according to claim 1 , wherein the POCI3 is used in an approximately equimolar amount, or slightly less, relative to the phenol. 19 - Process according to claim 1, wherein the triaryl phosphate is tris(2,4-dibromophenyl)phosphate and the phenol is 2,4-dibromophenol. 20 - Process according to claim 1, comprising separating the bis-acid byproduct from the triaryl phosphate product and hydrolyzing it, whereby to recover and recycle the unreacted phenolic moiety. 21 - Process according to claim 20, wherein the organic phase mother liquor containing the bis-acid is treated with aqueous alkali, whereby said phase separates into a bis-acid rich bottom phase, essentially free of triaryl phosphate product and a bis-acid depleted phase containing all the triaryl phosphate. 22 - Process according to claim 20, wherein the treatment of the organic phase mother liquor is effected with dilute alkali and at low temperatures. 23 - Process according to claim 20, wherein the hydrolysis of the bis-acid is effected by refluxing the bis-acid rich phase with excess concentrated aqueous alkali. 2213/92 - Process according to claim 1, performed in the batch mode. 25 - Process according to claim 1, performed in the continuous mode. LUZZATTO & LUZZATTO