CA1248141A - Process for the preparation of tetraarylborates - Google Patents

Abstract

TITLE
Process for the Preparation of Tetraarylborates ABSTRACT OF THE DISCLOSURE

Process for the preparation of an alkali metal salt of a tetraarylborane by reacting an alkali metal, an aryl halide and a borate ester at a ratio of halide to ester in the range of about 4.0/1 to 6.0/1 at a temperature in the range 90-130°C in an inert organic solvent.

Description

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TITLE
Process for the Preparation of Tetraarvlborates BACKGROUND OF THE INVENTION
S Field of the Invention _ The present invention is directed to an im-proved process for prepariny tetraarylborates by reacting an alkali metal, an aryl halide and a borate ester at specific ratios of halide to ester and within a range of temperatures.
Description of the Prior Art A variety of methods have been emplcyed to prepare organo substituted boranes. U.S. Patent

2,880,242 discloses a process for preparing trisubsti-tuted boranes by reacting an alkali metal, an organichalide and a boron halide in dry ethereal solution.
The preparation of organo-boron compounds by reacting an organo-alkali metal with a boron trihalide or an ester of boric acid in an inert liquid reaction medium to produce the corresponding organo-boron halide or organo-boric acid ester is disclosed in U.S. Patent

3,199,857. U.S. Patent 3,187,054 discloses reacting boron trifluoride, a boron ester or a boron carbon compound with an organo-sodium compound in an inert hydrocarbon solvent and also teaches that reactions of organo-sodium with boron esters tend to favor production of compounds having one or two boron-carbon bonds~
U.S. Patent 3,311,662 discloses a method for preparing tetraarylboron compounds which comprises reacting a preformed aryl sodium compound in an organic solvent with a boron compoùnd such as boron tri-chloride and particularly discloses the reaction of boron trichloride with phenyl sodium to produce PI-0340 35 sodium tetraphenylboron.

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The reaction of an alkali metal, an organic halide and an orthoborate ester to produce triaryl-boranes wherein the reaction products are contacted with water to form the hydroxide salt of the substituted borane and methods for optimizing the recovery of the thus prepared triarylborane from the aqueous hydroxide solution are described in U.S. Patents 4,046,815;

4,045,495 and 4,076,756.
Boranes of the types produced by the process of the present invention are disclosed for a wide variety of uses including cure-promoters for epoxy and the like resins, eOg., in U.S. Patent 3,637,572, in copper complexes as photo-sensitive materials, e.g., in ~.S. Patent 3,927,955, in the recovery of cesium values by forming insoluble complexes therewith, e.g., in U.S. Patent 3,114,716 and in the preparation of deuterobenzene by the reaction of an appropriate pyridine hydrohalide with a water soluble alkali metal tetraphenylboron in the presence of heavy water, e.g., in U.S. Patent 3,132,188.
SUMMARY OF THE INVENTION
The present invention is a process for the preparation of an alkali metal salt of a tetraaryl-borane, e.g., sodium tetraphenylborate which process comprises reactiny an alkali metal, e.g., sodium; an aryl halide, e.g., chlorobenzene, and a borate ester, e.g., isopropylorthoborate while employing a ratio of halide to ester in the range of 4.0/1 to ~.0/1, preferably 4.5/1 to 5.5/1 and a temperature in the range 90-130C, prefera~ly 110-125C, in an inert organic solvent. The preferred method of product recovery is to contact the reaction mixture with water to thereby obtain an aqueous solution of the sodium salt of the tetraarylborane. Unwanted impuri-ties can be removed before recoverin~ the salt.

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DETAILED DESCRIPTION OF THE INVENTION
Numerous organics can be employed as theinert organic solvent-reaction medium in the present process so long as the reactants are sufficiently soluble in the organic at reaction temperature but do not react with it. It is pre~erred to employ a solvent which has a boiling point at atmospheric pressure near the desired rea~tion temperature in order to simplify equipment and fac}litate heat re-moval by reflux of the solvent during the reaction.Examples of suitable solvents include, either singly or mixed, branched or unbranched alkanes having the

5-8 carbon atoms, e.g., pentane, hexane, octane, heptane and 3-methylpentane and cycloalkanes having S-8 carbon atoms, e.g., cyclohexane, methylcyclohexane, cyclooctane and cyclopentane. Other suitable solvent-reaction medium will be apparent to one skilled in the art in view of the foregoing discussion. Cycloheptane and mixtures ofC8 10 isoalkanes (mostly C8) having a boiling point in the range 116-14~C are preferred because the compounds boil at atmospheric pressure near the optimum temperature range for the conduct of the present process.
Alkali metals which are operable in the present process include lithium, potassium, etc. with sodium being preferred. Preferably the alkali metal is introduced in a suspension of fine part~cles (1-20~) in the reaction solvent. If the reaction is conducted at temperatures of above 100C, e.g., by employing a cycloheptane solvent, sodium can be introduced in the molten state directly into the reaction medium. In a preferred embodiment, the reaction rate and conse-quently the temperature is controlled by continuously metering the sodium dispersion or molten sodium to the reaction or by staged addition o~ the sodium. This ,::

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technique minimizes the cooling requirements for this highly exothermic react~n.
The arylhalide which is one of the reactants o the present invention can be any halogen substituted organic which is compatible in the system and wherein the halogen i5 available for reaction while other sites are essentially inert. The arylhalides may be the same or different depending upon the substituent groups desired on the final tetra~orane. Aryl and substituted arylhalides wherein the aryl group has 6-10 carbon atoms are preferred. Substituent groups can be the same or different alkyl groups having 1-8 carbon atoms;
alkenyl groups having 2-8 carbon atoms; aryl groups having 6-10 carbon atoms; alkoxy groups having 1-8 carbon atoms and amino groups having the formula -NR2 wherein R is hydrogen or the above-mentioned substi-tuent groups except halogen. Preferably the total number of carbon atoms in the arylhalide does not exceed 12. Specific examples of arylhalides include chlorobenzene, bromobenzene, 4-chlorobiphenyl, 2-, 4-chlorotoluene, dichlorobenzene and bromobenzene.
Chlorobenzene is preferred. Other halo aromatics should be apparent to one skilled in the art. ~lthough the amount of halo aromatic that is required to pro-duce the tetra-substituted boranes can vary depending upon its reactivity, it is necessary in most cases to maintain a ratio of arylhalides to borate ester of at least 4.0/1 and usually not greater than 6.0/1 while ratios of 4.5/1 to 5.5/1 are preferred.
The borate esters which are operable in the present invention include those which are deri~ed from an alcohol containing 1-10 carbon atoms and are repre-sented by the formula B(OR)3 wherein R is selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, sec bu-tyl, sec-amyl, methyl-isobutyl, ~ 4 . :

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octyl, cyclohexyl, cyclopentyl and phen~l The R may be the same or different. Orthoborate esters derived from the lower secondary alkyl alcohols, i.e., those having 3-8 carbon atoms are especially preferred since the metahorates and pyroborates give lower yields.
The tetraborate can be recovered and purified by several methods. In one method an aqueous solution of sodium tetraphenylborate is obtained by quenching the reaction mixture with water. The sodium tetra-phenylborate may then be isolated as a precipitateby adding salt to the aqueous solution. This solid may be further purified by recrystallization from acetone or some other suitable solvent. Another method involves the addition of salt and tetrahydrofuran to the aqueous solution. The sodium tetraphenylborate enters the THF phase while undesirable inorganic salt impurities remain in the aqueous phase. The THF
phase is then isolated, mixed with water and the THF
stripped out by heating, affording an aqueous solution of the purified sodium tetraphenylborate. This latter method avoids problems associated with solid handling.
The following examples are presented to illustrate but not to restrict the present invention.
Parts and percentages are by weight unless otherwise indicated. Reagents were at least CP grade. The yields reported are based on the ester employed.
Example l The apparatus employed consisted of a 250 ml 4-necked round botto~ flask with 4 vertical indentations to increase the effectiveness of mixing. The necks were, in turn, fitted with a mechanical stirrer, an air-cooled reflux condenser, a thermometer and a rubber septum. The apparatus was flushed thoroughly with dry 35 nitrogen and then connected to a mineral oil bubbler ~ 5 ,~i lZ~8~1 to maintain a static nitrogen atmosphere in the re-actor at a slight positive pressure. After the apparatus had b~en thoroughly flushed with dry nitroqen, approximately 5.7 grams of dry sodium particles (10-20~) were dispersed in 55.1 grams of cycloheptane which had been dried for 16 hoursin contact with a 4-A molecular sieve and the dispersion injected into the reactor.
The contents of the reactor were heated to 118C fol-lowing which a solution containing 5.0 grams of iso-propylorthoborate, 13.17 grams of chlorobenzene and25.9 grams of dry cycloheptane was introduced into the reactor using a syringe pump over a period of 50 minutes while maintaining the contents of the vessel at 118C. The ratio of ester to halide to sodium was 1/4.~/9.3. After the introduction of the aforementioned solution, samples of the contents of the reactor were withdrawn over a 3-hour period and analyzed for sodium tetraphenylborate. The analysis showed that the reaction was essentially complete after the addition of the ester and arylhalide. The yield to tetraphenyl-borate was 80%. The sodium salt of tetraphenylborane can be recovered in aqueous solution by injecting water into the reactor after the contents are per-mitted to cool to room temperature.
Examples 2-9 The procedure of Example 1 was repeated ex-cept that the solvent, temperature and relative amount of reactants were varied. The results are reported in Table I. In Example 5 the borane was recovered by salting out from aqueous solution and in Example 7 using tetrahydrofuran as described hereinabove.

TABLE I
Ester, Halide, TempSodium Na~4B
Example Solvent(C)Mole Ratio Yield 2 Cy~lohexane 82 1/4.4/9.3 35%
3 Cyclohexane 82 I/5.6/11.8 47%
4 Methylcyclo-hexane 1011/4.4/9.3 70%
Methylcyclo-hexane 1011/S.6/11.8 75

6 Methylcyclo-hexane 1011/5.6/11.8 76

7 Methylcyclo-hexane 1011/5.6/11.8 75%

8 Isoalkane* 125 1/4.4/9.3 7396

9 Cyclooctane 152 1/4.4/9.3 69%
*Mixtures of about 65~ C8, 30% Cg and 5% C10 isoalkanes having a boiling range 116-149C and no signi~icant amount of alkenes or aromatics.

Example 10 Approximately 3.7 grams of dry sodium parti-cles (10-20~) were~dispersed in 44.5 grams of dry cycloheptane and the dispersion was charged t~ the ; ~ apparatus prepared as described in Example 1. Stirring 25 ~was~commenced~and t~e reactor contents gently heated to approximately 40C following which a solution of 8.52 grams of dry chlorobenzene in 12.1 grams of dry cyclo-heptane were fed to the reactor over a 25-minute ~ interval while maintaining the contents of the reactor ; 30 at a temperature in the range 45-50C. The cont~nts of the reactor were then cooled for a period of 30 minutes to approximately 29C. After cooling, the pot temperature was then~increased to 117C and a ~solution of 33.23 grams of isopropylorthoborate in 12.1 grams of dry cycloheptane was introduced to the reactor over a~42-minute period. Analysis of the reactor contents`indicated that the yield of approxi-~ `mately 61~ of sodium tetraphenylborate was obtained : ~ 7 ~ :

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essentially upon completion of the introduction of thereactants into the reactor. The contents of the record were then cooled to room temperature and water injected to obtain an aqueous solution of sodium tetraphenylborate.
Example 11 Example 10 was repeated except that cyclo-hexane was used as a solvent and the reaction tempera-ture was maintained at about 82C. The yield to sodium tetraphenylborate was 55%.
When cyclooctane was used as a solvent and the reaction temperature maintained at about 150C, the yield to sodium tetraphenylborate decreased to 13~.

Claims (6)

1. A process for the preparation of an alkali metal salt of a tetraarylborane which comprises reacting an alkali metal, an aryl halide and a borate ester at a ratio of halide to ester in the range of about 4.0/1 to 6.0/1 at a temperature in the range 90-130°C in an inert organic solvent.

2. The process of Claim 1 wherein the ratio of halide to ester is maintained in the range of 4.5/1 to 5.5/1.

3. The process of Claim 1 wherein the borate ester is an orthoborate ester derived from an alcohol having 1-10 carbon atoms.

4. The process of Claim 2 wherein the borate ester is an orthoborate ester derived from a secondary alkyl alcohol having 3-8 carbon atoms.

5. The process of Claim 2 wherein the re-action is conducted at a temperature in the range 110-125°C.

6. The process of Claim 1 wherein the re-action is conducted at a temperature in the range 110-125°C.