FIELD OF THE INVENTION This invention relates to the synthesis of aryl boronic acids.

BACKGROUND OF THE INVENTION The use and effectiveness of Grignard reagents in ethereal solvents to alkylate or arylate non-metal compound was established in the first half of the Twentieth Century. See, Kharasch, M. S., Reinmuth, O., "Grignard Reactions of Nonmetallic Substances", Prentice-Hall, New York (1954).

Known aryl boronic acid synthesis typically entails concurrent or dual addition of substantially equimolar amounts of a trialkyl borate ester and an aromatic Grignard reagent to a reaction vessel at a temperature of less than-60°C, wherein a reaction mixture containing an aryl boronic acid ester is produced in a 30% to 50% yield. The ester is separated and hydrolyzed to provide the desired aryl boronic acid.

Aryl boronic acids are now in substantial commercial use to synthesize biaryl compounds by the palladium catalyzed cross-coupling"Suzuki"reaction.

See, e. g., A. Suzuki, et al., Syn. Commun. (1981) 513-

519; Y. Young, et al., Acta Chem. Scand. (1939) 47: 221- 230; G. B. Smith, et al., J. Org. Chem. (1994) 598: 8151- 8156; S. Sengupta, et al., J. Orra. Chem. (1997) 62: 3405- 3406. Accordingly, there is a need for an improved aryl boronic acid synthesis.

SUMMARY OF THE INVENTION It has been found, contrary to the prior art, that neither temperatures below-60°C nor control of reactant mole ratio is necessary during the synthesis of the aryl borate alkyl diesters. Pursuant to this invention, the synthesis of aryl borate diesters is accomplished at a temperature of from about-10° to 0°C by direct addition of the Grignard to the trialkyl borate or conversely in any desired mole ratio.

Hydrolysis of the aryl borate diesters may be accomplished in situ in synthesis reaction mixture with an aqueous mineral, preferably, aqueous sulfuric acid, in known manner. This invention typically provides aryl boronic yields approximately 20-40% greater than those achieved by the prior art. More specifically, the invention may provide aryl boronic acid yields of from about 50% to about 70% based on the trialkyl borate reactant.

DETAILED DESCRIPTION OF THE INVENTION The invention may, but need not be, practiced as a one pot method. A Grignard reagent is prepared in known manner in an appropriate ethereal solvent, preferably at the greatest concentration resulting in a

solution. Any ethereal solvent compatible with Grignard formation may be used. Useful solvents may be ethers which are cyclic, e. g., tetrahydrofuran, or which have the formula ROR, in which R is an alkyl group preferably having 2 to 6 carbon atoms.

Pursuant to the invention, an aryl boronic acid triester is synthesized at a temperature of-10°C to 0°C as illustrated by equation 1: Yield = 50-70%.

The Grignard aryl (Ar) groups may be substituted at one, some, or all available positions by any halogen (chlorine, bromine, fluorine or iodine), or by any straight or branched chain, substituted or unsubstituted alkyl group, or by any functional group compatible with or protected by standard methods to be compatible with the Grignard function. Phenyl, 4-fluorophenyl and naphthyl groups are typical Grignard aryl groups.

The alkyl borate reactant (B (OR3)) is preferably used in an amount greater than stoichiometric with respect to the ArMgX reagent. For example, from about 1.1 to 2.0, preferably about 1.5 moles, of B (OR3) may be used for each mole of ArMgX.

Each of the R groups may be any alkyl group.

Appropriate R groups have one to ten carbon atoms. R is preferably methyl.

The hydrolysis of the aryl boronic acid diester (equation 2) may be performed in known manner, e. g., by combining a mineral acid, preferably aqueous H2SO4, directly with the Grignard reaction mixture: ArB (OR) 2+H2sO4laq) ArB (OH) 2+2ROH (2) EXAMPLE 1 (General Example) The Grignard reagent is prepared in known manner in an appropriate ethereal solvent, preferably at the greatest concentration resulting in a solution.

A flask of appropriate size to accommodate the reaction volume as well as the volume of the aqueous acid hydrolysis solution is set up under a sweep of inert atmosphere and equipped for reflux and cooling.

Trialkyl borate, e. g., trimethyl borate diluted with an equal weight of an ethereal solvent, preferably THF, is charged to the flask in an amount to provide a mole ratio of ArMgX : B (OCH3) 3 of 1: 1. 5. The flask is cooled with dry ice: acetone to-10°C and addition of Grignard reagent begun, wherein a reaction mixture containing an aryl borate dialkyl ester is produced.

Upon completion of the Grignard addition, the reaction mixture is stirred to room temperature (1-2 hours). The dialkyl ester is hydrolyzed in situ by directly adding a 10% w/v H2SO4 aqueous solution in an amount equal to 2 mole H2SO4 with vigorous stirring for 30 minutes. The reaction mixture is then allowed to settle for 30 minutes. The hydrolysis reaction mixture

forms an upper organic layer and a lower aqueous layer.

The organic layer is separated, and then dried over sodium sulfate. Solvent is distilled until solids begin to precipitate. Hexanes are added, and the precipitated product collected by filtration.

The method described by this general example has yielded the results reported in Table 1.

TABLE 1

Grignard Grignard Solvent Product Quantity Yield of % Name Concen-Name (s) Reacted Product Yield tration (Mole) (Mole) Phenyl 2M THF Phenyl 5 2 67 MgCl boronic acid; Benzene boronic acid Phenyl 3M Ether Phenyl 3 1.9 65 MgBr boronic acid; Benzene boronic acid

4-Fluoro 1.5M THF 4-Fluoro 3 1.65 55 phenyl phenyl MgBr boronic acid; 4- Fluoro- benzene boronic acid 4-Fluoro 2M Ether 4-Fluoro 3 1.71 57 phenyl phenyl MgBr ; 4-acid Fluoro- benzene boronic acid 4-Methyl 2M THF 4-Methyl 3 1.65 55 phenyl phenyl MgCl boronic acid; 4- Methyl benzene boronic acid 4-Methyl 2M Ether 4-Methyl 3 1.62 54 phenyl phenyl MgBr boronic acid; 4- Methyl benzene boronic acid

Naphthyl 1M THF 1-28.9 14.7 51 MgBr Naphthyl boronic acid 4-Chloro 2M Ether 4-Chloro 2 1.24 62 phenyl phenyl MgBr boronic acid; 4- Chloro- benzene boronic acid 4-Bromo 2M Ether 4-Bromo 2 1.12 56 phenyl phenyl MgBr boronic acid; 4- Bromo- benzene boronic acid 4-1.2M THF 4-3 1.65 55 Methoxy Methoxy phenyl benzene MgBr boronic acid 4-2M Ether 4-3 1.8 60 Methoxy Methoxy phenyl benzene MgBr boronic acid

The results reported in Table 1 were obtained in the laboratory with the exception that the synthesis of 1-naphthyl boronic acid was achieved in a pilot plant.

EXAMPLE 2 (Phenyl Boronic Acid) A reaction flask was charged with trimethyl borate (1.5 moles, 155.7g) and tetrahydrofuran (300g). The mixture was cooled to a temperature of-5°C to 0°C.

One mole of phenyl magnesium bromide was added slowly with maintenance of the temperature between-5°C and 0°C. The reaction mixture was stirred for thirty minutes.

Ten percent (10%) aqueous sulfuric acid was directly added to the Grignard reaction mixture which contained the methyl ester of phenyl boronic acid. The resulting hydrolysis reaction mixture was stirred for thirty minutes and then allowed to settle for thirty minutes. The mixture separated into an upper layer and a lower layer. The upper layer was separated, dried over sodium sulfate (lOg), and filtered. The volume of the filtrate was reduced by distillation of THF, 300 ml of hexane was added, and the distillation continued until most of the THF was removed. The remainder of the reaction mixture was cooled and filtered to recover solids which dried in vacuum. Yield-65% to 70% phenyl boronic acid.