Enrichment of optical purity of 2-(4-aryloxyphenoxy) propionic acids by crystallization as hydrates

Abstract: The present invention concerns a process for enrighing the optical purity of an optically active 2-(4-aryloxyphenoxy)propionic acid by formation of the hydrate of the enantiomer in excess and its removal from solution.

Description: The present invention is directed to enriching the optical purity of optically active 2-(4-aryloxyphenoxy)propionic acids. More specifically, the present invention is directed to enriching the optical purity of optically active 2-(4-aryloxyphenoxy)propionic acids by crystallization as hydrates. The herbicidal activity of 2-(4-aryloxyphenoxy)propionic acids and derivatives thereof is well known in the art. Furthermore, optical isomers are often known to exhibit enhanced herbicidal activity over the corresponding racemates. For example, U.S. Patent No. 4,531,969 discloses that the R-enantiomers of certain 2-(4-aryloxyphenoxy)propionic acids and certain derivatives thereof are distinguished by a considerably enhanced herbicidal action compared to the racemic modifications. Since reduced quantities of herbicide are required to achieve comparable levels of control, the application of mixtures enriched in the more efficacious R-enantiomer offers both economical and environmental advantages. To exploit the agronomic benefits of these advantages, it is necessary to efficiently resolve racemic mixtures of herbicides that are normally produced industrially. Various methods for obtaining high concentrations of individual enantiomers are known. For example, individual enantiomers can be obtained by direct synthesis employing appropriate optically active starting materials. Alternatively, a racemate can be resolved by conversion with an optically active reagent into a mixture of diastereomers which can subsequently be separated on the basis of their different physical properties. Theoretically, one can obtain essentially 100 percent of the desired enantiomers by these methods. In practice, however, the products typically contain from 70 to 95 percent of the desired enantiomer and, correspondingly, from 5 to 30 percent of the other optical isomer. Such products are then said to possess an optical purity of 40 to 90 percent, i.e., from 40 to 90 percent of the mixture is the desired enantiomer and from 10 to 60 percent is a racemic mixture. An efficient technique for the continued enrichment of mixtures already containing an excess of a particular enantiomer is resolution by entrainment, otherwise known as preferential crystallization. Unlike other resolution methods, entrainment does not require an auxiliary chiral compound such as, for example, an optically active reagent to form diastereomers or an optically active solvent, in order to achieve an effective resolution. The principles of enantiomer resolution by direct crystallization, particularly by entrainment, have been reviewed by J. Jacques et al. in Enantiomers, Racemates, and Resolutions , John Wiley & Sons, Inc., New York, N.Y., 1980, Chapter 4, pages 223-241. U.S. Patent 3,857,889 discloses a general process for the isolation of an enantiomer in excess in a partially resolved mixture of enantiomers by treating the mixture with a crystallization solvent and a solubilizing agent so that only the enantiomer in excess passes into the solution. It has now been found that, by the process of the present invention, it is possible to enrich the optical purity of a 2-(4-aryloxyphenoxy)propionic acid already containing a preponderance of the herbicidally more efficacious R-enantiomer. The method, which is based on the preferential formation of the hydrate of the enantiomeric acid in excess, advantageously requires neither an auxiliary chiral reagent or solvent nor any additional additives with the exception of water. More specifically, the present invention is directed to a process for enriching the optical purity of an optically active 2-(4-aryloxyphenoxy)propionic acid of Formula (I) wherein: Ar is a substituted phenyl or pyridinyl group substituted with one or two substituents independently selected from halogen, -CF₃ or -CN which comprises contacting an organic solution comprised of the 2-(4-aryloxyphenoxy)propionic acid having an excess of one enantiomer of from 60 to 99 percent and an aromatic hydrocarbon or a halogenated aliphatic or aromatic hydrocarbon with at least one equivalent of water to form a hydrate of the enantiomer in excess, cooling, and separating the hydrate from the solution. The 2-(4-aryloxyphenoxy)propionic acids of Formula (I) to which the present process may be applied are disclosed, for example, in U.S. Patent Nos. 4,046,553, 4,332,960, 4,332,961, 4,477,276, 4,523,017, 4,531,969, 4,550,192, 4,565,568, 4,600,432, 4,609,396 and 4,629,493 and European Patent Application Publication Nos. 483, 1,473 and 3,890. Particularly valuable examples of 2-(4-aryloxyphenoxy)propionic acids of Formula (I) to which the present process may be applied are of Formula (II)-(III): wherein: Y is halogen, CF₃ or CN; and Z is H or halogen; and wherein: W is CF₃ or halogen; and Z is as previously defined. For compounds of Formula (II), Y is preferably CF₃, Br, I or CN and Z is preferably F or Cl. For compounds of Formula (III), W is preferably CF₃, Cl, Br or I and Y is preferably H, F or Cl. This does not imply that the present process works equally well with all of the compounds disclosed above. Differences in physical properties may allow hydrate formation and separation to vary from compound to compound resulting in significantly different efficiencies. Compounds of Formulas (II) and (III) wherein Y is halogen, Z is hydrogen or halogen and W is CF₃ or halogen are well-suited for the present enrichment process. As used herein, the term "halogen" refers to F, Cl, Br or I. The excess of a particular enantiomer in the original optically active 2-(4-aryloxyphenoxy)propionic acid to be enriched can range from 60 to 99 percent, i.e., optical purities of from 20 to 98 percent. The present method is particularly effective for enriching the optical purity of samples which are already of relatively high optical purity, e.g., from 90 to 98 percent optically pure. In this range, not only is an (aryloxyphenoxy)propionic acid of very high optical purity obtained, but the propionic acid is recovered in relatively higher yield than when less substantial excesses of a particular enantiomer are employed. In a typical operation, the optically active 2-(4-aryloxyphenoxy)propionic acid can be dissolved in an organic solvent which is capable of dissolving and preferably crystallizing the propionic acid and which should be, to a great extent, immiscible with water. Aromatic hydrocarbons and halogenated aliphatic or aromatic hydrocarbons are particularly useful solvents for the present process. Once dissolution is complete, anhydrous racemate or material of lesser optical purity than that initially employed can optionally be removed by cooling and subsequent filtration. To isolate the hydrate of the enantiomer in excess, at least one equivalent of water based on the amount of acid present can be added to the solution with agitation and the mixture can be cooled. The material that crystallizes as the hydrate can be isolated by routine techniques such as filtration or centrifugation which are typically used to recover solid products. The anhydrous 2-(4-aryloxyphenoxy)propionic acid can be recovered from the hydrate by conventional dehydration procedures. Example 1 Enrichment of the Optical Purity of R-2-[4-(2'-fluoro-4'-bromophenoxy)phenoxy]prop ionic acid by Crystallization as the Hydrate A dry solution of 100 grams (g) of R-2-[4-(2'-fluoro-4'-bromophenoxy)phenoxy]propionic acid (96 percent R) in 300 milliliters (mL) of toluene was prepared by heating the mixture to reflux and azeotropically removing the water from the solution. The solution was cooled to about 10°C and the solid which formed was removed by filtration. Water 20 grams (g) was then added to the solution which was stirred at 10°C for one hour (hr). The resulting crystals (53 g) of monohydrate were isolated by filtration. Enantiomeric analysis indicated an optical purity of 99.4 percent R-enantiomer. Example 2 Enrichment of the Optical Purity of R-2-{4-[(3'-fluoro-5'-trifluoromethyl-2'-pyrid inyl)oxy]phenoxy}propionic acid by Crystallization as the Hydrate The methyl ester of R-2-{4-[(3'-fluoro-5'-trifluoromethyl-2'-pyridinyl)oxy]phenoxy}propionic acid (100 g; R/S = 95/5) was added to 278 g of 5 percent NaOH solution and the mixture was heated to 40°C. When hydrolysis was complete, the pH was adjusted to approximately 1.0 to precipitate the acid from solution. The acid was isolated by filtration and was dissolved in 400 mL of perchloroethylene. Residual water was removed by distillation and the solution was cooled to 15°C. About 17 g of anhydrous acid having an R to S ratio of 73/27 was collected by filtration. The filtrate was treated with 40 mL of water and was kept at 15°C for 2 hr. A total of 82 g of monohydrate having an optical purity of 99.5 percent was collected. Example 3 Preparation of R-2-{4-[(3'-chloro-5'-trifluoromethyl-2'-pyridinyl)oxy]phenoxy}propionic acid 2,3-Dichloro-5-trifluoromethylpyridine (43.2 g) and R-2-(4-hydroxyphenoxy)propionic acid (36.4 g; R/S = 97/3) were added to 200 mL of dimethylsulfoxide. Potassium carbonate (66.4 g) was added to the solution and the temperature was raised to 85°C and maintained there for 3 hr. After cooling to ambient temperature, the solids were removed by filtration and 400 mL of water were added to the filtrate. The pH of the mixture was adjusted to 1.0 and the product was extracted from the aqueous mixture with methyl iso-butyl ketone. After evaporation of the solvent, the crude product was dissolved in 80 mL of perchloroethylene. Water (20 mL) was added to the solution which was chilled to -4°C overnight. A total of 22 g of hydrate having an optical purity greater than 99.8 percent was collected by filtration. Example 4 Preparation of R-2-(4-(2'-fluoro-4'-cyanophenoxy)phenoxy)propionic acid 3,4-Difluorobenzonitrile (6.9g) and R-2-(4-hydroxyphenoxy)propionic acid (9.2g R/S = 97/3) were dissolved in 50 mL of dimethylsulfoxide. Potassium carbonate (16g) was added to the solution and the temperature was raised to 85°C and maintained there for 2 hr. The dimethyl sulfoxide was evaporated under reduced pressure and the residue was dissolved in 100 mL of water. The aqueous solution was gradually acidified with aqueous HCl, so that the product acid was slowly precipitated. A total of 13.5 g of wet material was collected by filtration. The wet acid was dissolved in 40 mL of hot toluene (enough water was present to form a distinct layer). The mixture was slowly cooled to ambient temperature and crystals slowly formed. The crystals (9.5 g) were collected by filtration and air dried. The product was found to contain 6.7 percent water of hydration and had a R/S = 99.3/0.7.

Claims: A process for enriching the optical purity of an optically active 2-(4-aryloxyphenoxy)propionic acid for Formula (I) wherein: Ar is a substituted phenyl or pyridinyl group substituted with one or two substituents independently selected from halogen, -CF₃ or -CN which comprises contacting an organic solution comprised of the 2-(4-aryloxyphenoxy)propionic acid having an excess of one enantiomer of from 60 to 99 percent and an aromatic hydrocarbon or a halogenated aliphatic or aromatic hydrocarbon with at least one equivalent of water to form a hydrate of the enantiomer in excess, cooling, and separating the hydrate from the solution. The process of Claim 1 wherein Ar represents a substituted phenyl group of the formula wherein: Y is halogen, CF₃ or CN; and Z is hydrogen or halogen. The process of Claim 1 wherein Ar represents a substituted pyridinyl group of formula wherein: W is CF₃ or halogen; and Z is hydrogen or halogen. The process of Claim 2 wherein Y is halogen or -CN and Z is halogen. The process of Claim 2 wherein Y is Br, I or CN and Z is F or Cl. The process of Claim 3 wherein W is CF₃ and Z is hydrogen or halogen. The process of Claim 3 wherein W is CF₃, Cl, Br or I and Y is H, F or Cl. The process of Claim 1 wherein the optical purity of the 2-(4-aryloxyphenoxy)propionic acid to be enriched is from 90 to 98 percent.

Inventors: KERSHNER LARRY D, TAI JIMMY J, KERSHNER, LARRY D., TAI, JIMMY J.
Applicants: DOWELANCO
Application year: 1989
Main classification: C07C59
IPC codes: C07C59, C07C51, C07C253, C07C255, C07D213, C07D263, C07B57, C07D241