Process for racemization of optically active 1-phenylethylamine derivatives

Abstract: This invention provides an excellent process for racemization of an optically active 1-phenylethylamine derivative represented by the following formula [1]: wherein R 1 represents a phenyl group substituted at least at the ortho-position, which comprises reacting the optically active 1-phenylethylamine derivative [1] with an aldehyde compound represented by the formula [2]: R 2 -CHO wherein R 2 represents an optionally substituted alkyl group or an optionally substituted phenyl group to form an optically active imine represented by the formula [3]: wherein R 1 and R 2 are as defined above, reacting the imine with an alkaline metal tert-alkoxide in an aprotic polar solvent or a mixture of the aprotic polar solvent and an aprotic nonpolar solvent, and then hydrolyzing the resultant racemic imine.

Description: The present invention relates to a process for racemization of certain optically active 1-phenylethylamines. Optically active 1-phenylethylamines are useful compounds as production intermediates of pesticides such as fungicides (a certain racemate is described in JP-A-2-76846, and a certain optically active substance is described in JP-A-2-11550), and reagents for optical resolution. As the production process thereof, for example, a process of producing certain optically active 1-phenylethylamines by optical resolution of a racemate is described in JP-A-2-306942. In such a process of producing an optically active substance by optical resolution, however, effective utilization of the undesirable antipode remained after isolating the useful optically active substance, for example, reutilization of the antipode through racemization becomes an important industrial object. As the process for racemization of optically active 1-(4-chlorophenyl)ethylamine, a process of reacting the amine with potassium tert-butoxide in dimethyl sulfoxide has hitherto been described in JP-A-4-275258. It has been found that, according to the process, the racemization reaction of optically active 1-(2,4-dichlorophenyl)ethylamine does not proceed at all and the process can not be applied for racemization of optically active 1-phenylethylamines having a substituent at the ortho-position. On the other hand, as a process of converting optically active 1-phenylethylamines into a corresponding imine and then racemizing the imine, a process of dehydration-condensing optically active 1-(4-chlorophenyl)ethylamine with acetophenone, followed by reacting with potassium tert-butoxide has been described in JP-A-7-188120. This process had a problem that a large amount of an undesirable amine is formed as a by-product after hydrolysis because isomerization (migration of double bond) of the imine proceeds simultaneously with the racemization reaction. It is an object of the present invention to provide a process for racemization of an optically active 1-phenylethylamine derivative having substituent(s) at the ortho-position, which is represented by the following formula [1]. This object would be achieved on the basis of the finding that the optically active 1-phenylethylamine derivative can be efficiently racemized by reacting it with an aldehyde compound represented by the following formula [2], to form an optically active imine represented by the following formula [3], reacting the imine with an alkaline metal tert-alkoxide in an aprotic polar solvent or a mixture of the aprotic polar solvent and an aprotic nonpolar solvent, followed by hydrolyzing. That is, the present invention provides a process for racemization of an optically active 1-phenylethylamine derivative represented by formula [1]: wherein R 1 represents a phenyl group substituted at least at the ortho-position, which comprises reacting the optically active 1-phenylethylamine derivative [1] with an aldehyde compound represented by formula [2]: R 2 -CHO wherein R 2 represents an optionally substituted alkyl group or an optionally substituted phenyl group to form an optically active imine represented by formula [3]: wherein R 1 and R 2 are as defined above (hereinafter referred to as "reaction 1"), reacting the imine with an alkaline metal tert-alkoxide in an aprotic polar solvent or a mixture of the aprotic polar solvent and an aprotic nonpolar solvent(hereinafter referred to as a "reaction 2"), and then hydrolyzing the resultant racemic imine (hereinafter referred to as a "reaction 3"). In the above formulas [1] and [3], one preferred embodiment of R 1 is a group represented by formula [4]: wherein X represents a halogen atom (e.g. a chlorine atom) or a C 1 -C 4 alkyl group (e.g. a methyl group), and Y represents a halogen atom (e.g. a chlorine atom), a C 1 -C 4 alkyl group (e.g. a methyl group) or a hydrogen atom. A more preferred embodiment of R 1 is a 2,4-dichlorophenyi group. In the above formulas [2] and [3], one preferred embodiment of R 2 is a C 1 -C 6 alkyl group (e.g. a tert-butyl group, isopropyl group or isobutyl group). The present invention also provides an optically active or racemic imine compound represented by formula [5]: wherein R 21 represents a C 1 -C 6 alkyl group (e.g. a tert-butyl group, isopropyl group or isobutyl group). According to the process of the present invention, formation of a compound represented by formula [6]: wherein R 1 and R 2 are as defined above, which is an isomerized product wherein a double bond of the optically active imine of the above formula [3] has migrated, can be inhibited and, therefore, formation of an amine as the by-product represented by formula [7]: R 2 -CH 2 -NH 2 wherein R 2 is as defined above which is obtained by hydrolyzing the isomerized product [6] can be inhibited. The present invention will now be explained in detail. First, reaction 1 will be explained in detail. The optically active 1-phenylethylamine derivative represented by formula [1], used as the starting material may be in the R-form or S-form, or a mixture containing excess of one of them. The substituent in the phenyl group substituted at least at the ortho position which is represented by R 1 of the formula [1], may be any one which does not inhibit the reaction, and is not specifically limited. The substituent in the optionally substituted alkyl group or optionally substituted phenyl group which is represented by R 2 of the formula [2], used as the other starting material may be any one which does not inhibit the reaction, and is not specifically limited. Examples of R 2 include a C 1 -C 6 alkyl group (e.g. an isobutyl group, a neopentyl group, an isopropyl group, a sec-butyl group, a sec-pentyl group, an isopentyl group and a tert-butyl group), a phenyl group, a 4-methylphenyl group, a 4-chlorophenyl group and a 2,4-dichlorophenyl group. Specific examples of the aldehyde compound represented by formula [2], include isovaleraldehyde, 3,3-dimethylbutyraldehyde, isobutyraldehyde, 2-methylbutyraldehyde, 2-ethylbutyraldehyde, 2-methylvaleraldehyde, pivalaldehyde, benzaldehyde, 4-methylbenzaldehyde, 4-chlorobenzaldehyde, and 2,4-dichlorobenzaldehyde. The amount of the aldehyde compound [2] used is normally from 0.5 to 5 mol, preferably from 0.95 to 2 mol, based on 1 mol of the optically active 1-phenylethylamine derivative represented by formula [1]. The reaction is normally conducted in a solvent in the presence of a catalyst, and can also be conducted without using the solvent. When using the solvent, the solvent used may be any one which does not inhibit the reaction. Examples thereof include aromatic hydrocarbon solvents such as toluene, benzene and xylene; ether solvents such as dioxane and methyl tert-butyl ether; aliphatic hydrocarbon solvents such as hexane and heptane; and halogenated hydrocarbon solvents such as dichloroethane, chloroform and chlorobenzene. The amount of the solvent used is normally from 1 to 20 parts by weight, preferably from 3 to 10 parts by weight, based on 1 part by weight of the optically active 1-phenylethylamine derivative represented by formula [1]. Examples of the reaction catalyst which is optionally used include sulfonic acids such as benzenesulfonic acid and p-toluenesulfonic acid. The amount of the catalyst is normally from 0.001 to 0.1 mol, preferably from 0.005 to 0.05 mol, based on 1 mol of the optically active 1-phenylethylamine derivative represented by the formula [1]. The reaction temperature is normally within the range from about 70 to 180°C and the reaction time is normally within the range from 0.5 to 24 hours. It is preferred to react while the formed water is removed out of the system by using a water isolator. The formed optically active imine represented by the general formula [3], may be used for the reaction 2 as it is after the reaction solution has been cooled and optionally washed with water to remove the catalyst, or the imine may be used for the reaction 2 after it has been isolated by low-boiling fraction distillation. The imine may also be used for the reaction 2 after it has been purified by means such as distillation, recrystallization and/or various chromatographies. Next, reaction 2 will be explained in detail. Examples of the alkaline metal tert-alkoxide used include alkaline metal tert-C 4 -C 5 alkoxides such as potassium tert-butoxide, sodium tert-butoxide, potassium tert-pentyloxide and sodium tert-pentyloxide. The amount of the alkaline metal tert-alkoxide used is normally from 0.01 to 2 mol, preferably from 0.05 to 0.3 mol, based on 1 mol of the optically active imine of the general formula [3]. It is advantageous to use potassium tert-butoxide or sodium tert-butoxide as the alkaline metal tert-alkoxide in view of industrial availability. As the aprotic polar solvent used, those having a dielectric constant of not less than 22 are preferred, and examples thereof include N,N-dimethylformamide, dimethyl sulfoxide, N,N-dimethylacetamide, N-methylpyrrolidone, 1,3-dimethyl-2-imidazolidinone, sulfolane, nitromethane, acetonitrile and N,N'-dimethylpropyleneurea. Preferred examples thereof include N,N-dimethylformamide and dimethylsulfoxide. When using only the aprotic polar solvent as the solvent, the amount of it is normally from 0.5 to 20 parts by weight, based on 1 part by weight of the optically active imine of the general formula [3]. When using a mixture of the aprotic polar solvent and an aprotic nonpolar solvent as the solvent, examples of the aprotic nonpolar solvent include aromatic hydrocarbon solvents such as benzene, toluene and xylene; halogenated hydrocarbon solvents such as chlorobenzene; ether solvents such as diethyl ether and methyl-tert-butyl ether; and aliphatic hydrocarbon solvents such as hexane and heptane. The amount of the solvent used varies depending on the kind of the solvent used, but is normally from 0.5 to 100 parts by weight, preferably from 1 to 10 parts by weight, based on 1 part by weight of the optically active imine of formula [3]. The amount of the aprotic polar solvent used is normally from 0.5 to 2 parts by weight, based on 1 part by weight of the optically active imine of the formula [3]. It is advantageous to use a mixture of the aprotic polar solvent and the aprotic nonpolar solvent as the solvent and to use an aromatic hydrocarbon solvent as the aprotic nonpolar solvent from the industrial point of view. The reaction temperature and reaction time vary depending on the kind or amount of the alkaline metal tert-alkoxide and kind or amount of the solvent. The reaction temperature is normally within the range from 0°C to the boiling point of the solvent or 150°C, preferably from 20 to 100°C. The reaction time is normally within the range from 0.5 to 48 hours. Proceeding of the reaction can be followed by a process of collecting aliquot of the reaction mixture and measuring a rotation angle, or subjecting to a high performance liquid chromatography with an optically active column after hydrolysis. The formed racemic imine of formula [3] can be used for reaction 3, for example, after the aprotic polar solvent is removed from the reaction mixture by washing with water or an aqueous solution containing an inorganic salt such as sodium chloride. For example, the racemic imine may be used for reaction 3 after it has been isolated by e.g. low-boiling fraction distillation, or it may be used for reaction 3 after it has been optionally purified by means such as distillation, recrystallization and/or various chromatographies. Next, reaction 3 will be explained in detail. Reaction 3 can be conducted under normal hydrolysis reaction conditions,and one embodiment thereof is as follows. The reaction is conducted, for example, in the presence of acids such as dilute hydrochloric acid and sulfuric acid. In this case, acids are normally used in an amount within the range from 1 to 100 mol, preferably from 1.05 to 20 mol, based on 1 mol of the racemic imine of the formula [3]. Water is normally used in an amount within the range from 1 to 1000 mol, preferably from 20 to 200 mol, based on 1 mol of the racemic imine of the formula [3]. In the reaction, an organic solvent may be used. When using an organic solvent, the organic solvent may be any one which does not inhibit the reaction. Examples thereof include alcohol solvents such as methanol and ethanol; aliphatic hydrocarbon solvents such as hexane and heptane; halogenated hydrocarbon solvents such as dichloroethane and chloroform; ester solvents such as ethyl acetate; ether solvents such as diethyl ether and methyl-tert-butyl ether; and aromatic hydrocarbon solvents such as toluene, xylene and chlorobenzene. The amount of the organic solvent used is normally from 0.1 to 5 parts by weight, based on 1 part by weight of the racemic imine of the formula [3]. The reaction temperature and reaction time vary depending on the kind or amount of acids. The reaction temperature is normally within the range from 0°C to the boiling point of the solvent or 100°C, preferably from 50 to 90°C. The reaction time is normally within the range from about 10 minutes to 5 hours. When reaction 3 is conducted, for example, in the presence of an acid,a water-soluble salt of a racemate of the 1-phenylethylamine derivative of the formula [1] and the acid, and the aldehyde compound of formula [2] are formed. When using no organic solvent, the objective racemate of the 1-phenylethylamine derivative of formula [1] can be isolated by adding a water-insoluble solvent to the reaction solution after the completion of the reaction to extract and isolate the aldehyde compound of formula [2], and other impurities in the organic layer, alkalifying the aqueous layer with an aqueous alkaline solution such as an aqueous sodium hyroxide solution, extracting this with a water-insoluble solvent and then concentrating the resultant organic layer under reduced pressure. When using a water-soluble solvent such as an alcohol solvent, the treatment is conducted according to the same manner as that described above, after distilling off the water-soluble solvent. When using a water in-soluble solvent, the treatment may be conducted according to the same manner as that described above except that the reaction mixture is partitioned as it is to extract the aldehyde compound of the formula [2] in the organic layer. The objective derivative can also be isolated by steam-distilling the reaction mixture to isolate the aldehyde compound of formula [2] and other non-basic organic impurities, alkalifying the resultant mass with an aqueous alkaline solution such as an aqueous sodium hydroxide solution, extracting this with a water-insoluble solvent and then concentrating the resultant organic layer under reduced pressure. In the above post-treatment, the aldehyde compound of formula [2] extracted in the organic layer or the aldehyde compound distilled and removed from the reaction solution can be optionally isolated from impurities using an operation such as distillation and reused in the reaction 1. The following Examples further illustrate the present invention in detail. In the Examples, "% s'" are by weight unless otherwise stated. The conditions of gas chromatography in the following Examples are as follows. Apparatus: Shimadzu GC-14A Column: DB-17 30 m, megabore: 0.53 mm, film thickness: 1 µm Injection temperature: 250°C Column temperature: raised from 100°C (0 minute) to 250°C (5 minute) in a rate of 5°C/minute Carrier gas and flow rate thereof: He, 5 ml/minute Detection: FID In the following Examples, all optical isomer ratios were determined by high performance liquid chromatography using an optically active column and conditions thereof are as follows. Apparatus: Shimadzu SPD-6A (UV detector) LC-6A Column: SUMICHIRAL™ OA-4100, 5 µm, 4.6 mm in diameter x 25 cm in length Mobile phase: Hexane:ethanol:CF 3 CO 2 H = 240:10:1 (v/v/v) 1 ml/minute Detection: UV 254 nm Example 1 (1) To a mixture of optically active 1-(2,4-dichlorophenyl)ethylamine (optical isomer ratio: S-form/R-form = 62.5/37.5) (60 g), pivalaldehyde (2,2-dimethylpropanal) (54.2 g) and toluene (600 g) was added p-toluenesulfonic acid (0.6 g), and then the mixture was heated under reflux while the formed water was removed out of the system by using a water isolator for 5 hours. The reaction solution was cooled to 20°C and washed with water (300 g). Toluene was distilled off from the organic layer after partitioning to obtain 79.0 g of optically active N-neopentylidene-α-(2,4-dichlorophenyl)ethylamine as a pale yellow oil. The yield thereof after correction of purity by gas chromatography was 92.5%. 1 H-NMR (CDCl 3 /TMS) δ value (ppm): 1.1 (s, 9H), 1.4 (d, 3H), 4.7 (q, 1H), 7.2-7.7 (m, 4H) (2) To this optically active N-neopentylidene-α-(2,4-dichlorophenyl)ethylamine (10 g) were added toluene (100 g) and N,N-dimethylformamide (10 g) and then sodium tert-butoxide (0.74 g), and the mixture was stirred at 75°C for 9 hours.At this time, an aliquot of the reaction mixture was collected and the isomerization rate of N-neopentylidene-α -(2,4-dichlorophenyl)ethylamine was measured by gas chromatography. The ratio of N-(α-methyl-2,4-dichlorobenzylidene)-neopentylamine as the isomerized product was as follows: N-neopentylidene-α-(2,4-dichlorophenyl)ethylamine:N-(α-methyl-2,4-dichlorobenzylidene)-neopentylamine = 93.4:6.6. This reaction mixture was cooled to 25°C and poured into water (100 g), followed by stirring at 25°C for 5 minutes. After the mixture was allowed to stand and partitioned, the aqueous layer was extracted with toluene (50 g) and then mixed with the organic layer obtained by the partition above . (3) To this organic layer was added aqueous 20% hydrochloric acid (100 g) and, after stirring at 80°C for 3 hours, the mixture was allowed to stand and partitioned and then the mixture layer was cooled to 25°C. To this aqueous layer was added an aqueous 20% sodium hydroxide solution (101.7 g) to adjust the pH value to not less than 12, followed by extracting twice with toluene (100 g). Then, this organic layer was concentrated to obtain 6.58 g of nearly racemized 1-(2,4-dichlorophenyl)ethylamine as a colorless liquid. The yield thereof after correction of purity by gas chromatography was 92.4%. The optical isomer ratio was as follows: S-form/R-form = 52.2/47.8 Example 2 (1) To a mixture of optically active 1-(2,4-dichlorophenyl)ethylamine (optical isomer ratio: S-form/R-form = 62.5/37.5) (10 g), isobutyraldehyde (2-methylpropanal) (5.67 g) and toluene (100 g) was added p-toluenesulfonic acid (0.1 g), and then the mixture was heated under reflux while the formed water is removed out of the system by using a water isolator for 5 hours. The reaction mixture was cooled to 20°C and washed with water (50 g). Toluene was distilled off from the organic layer after partitioning to obtain 12.3 g of optically active N-isobutylidene-α-(2,4-dichlorophenyl)ethylamine as a pale yellow oil. The yield thereof after correction of purity by gas chromatography was 94.6%. 1 H-NMR (CDCl 3 /TMS) δ value (ppm): 1.1 (2d, 6H), 1.4 (d, 3H), 2.5 (m, 1H), 4.7 (q, 1H), 7.1-7.7 (m, 4H) (2) To this optically active N-isobutylidene-α-(2,4-dichlorophenyl)ethylamine (10 g) were added toluene (100 g) and N,N-dimethylformamide (10 g) and then sodium tert-butoxide (0.79 g), and the mixture was stirred at 80°C for 8 hours. At this time,an aliquot of the reaction mixture was collected and the isomerization rate of N-isobutylidene-α-(2,4-dichlorophenyl)ethylamine was measured by gas chromatography. The ratio of N-(α-methyl-2,4-dichlorobenzylidene)isobutylamine as the isomerized product was as follows: N-isobutylidene-α-(2,4-dichlorophenyl)ethylamine:N-(α-methyl-2,4-dichlorobenzylidene)-isobutylamine = 97.8:2.2. This reaction mixture was cooled to 25°C and poured into water (100 g), followed by stirring at 25°C for 5 minutes. After the mixture was allowed to stand and partitioned, the aqueous layer was extracted with toluene (50 g) and then mixed with the organic layer obtained by the partition above. (3) To this organic layer was added aqueous 20% hydrochloric acid (120 g) and, after stirring at 80°C for 5 hours, the mixture was allowed to stand and partitioned and then the aqueous layer was cooled to 25°C. To this aqueous layer was added an aqueous 20% sodium hydroxide solution (120.7 g) to adjust the pH value to not less than 12, followed by extracting twice with toluene (100 g). Then, this organic layer was concentrated to obtain 7.31 g of nearly racemized 1-(2,4-dichlorophenyl)ethylamine as a colorless liquid. The yield thereof after correction of purity by gas chromatography was 80.5%. The optical isomer ratio was as follows: S-form/R-form = 55.3/44.7. Example 3 According to the same manner as that described in Example 1 except for changing the optical isomer ratio of optically active 1-(2,4-dichlorophenyl)ethylamine to as follows: S-form/R-form = 87/13) in Example 1 (1), using N-neopentylidene-α-(2,4-dichlorophenyl)ethylamine (6.7 g), toluene (41 g), dimethyl sulfoxide (6.7 g) and potassium tert-butoxide (0.58 g) in place of N-neopentylidene-α-(2,4-dichlorophenyl)ethylamine (10 g), toluene (100 g), N,N-dimethylformamide (10 g) and potassium tert-butoxide (0.74 g) and changing the reaction temperature to 30°C, the reaction was conducted to obtain 4.43 g of nearly racemized 1-(2,4-dichlorophenyl)ethylamine. The yield thereof after correction of purity by gas chromatography was 85.0%. The optical isomer ratio was as follows: S-form/R-form = 51.3/48.7. Aliquot of the reaction mixture was collected before hydrolysis and the isomerization rate was measured by gas chromatography. The results are as follows: N-neopentylidene-α-(2,4-dichlorophenyl)ethylamine:N-(α-methyl-2,4-dichlorobenzylidene)-neopentylamine = 90.9:9.1. Reference Comparative Example 1 A mixture of optically active 1-(2,4-dichlorophenyl)ethylamine (optical isomer ratio: S-form/R-form = 8/92) (5 g), dimethyl sulfoxide (50 g) and potassium tert-butoxide (1.21 g) was heated with stirring at 80°C for 10 hours. After cooling to 25°C, aliquot of the reaction solution was collected and the optical isomer ratio of 1-(2,4-dichlorophenyl)ethylamine was measured by a high performance liquid chromatography. As a result, a ratio of the S-form to the R-form was 8:92 and the racemization reaction did not proceed at all.

Claims: A process for racemization of an optically active 1-phenylethylamine derivative represented by formula [1]: wherein R 1 represents a phenyl group substituted at least at the ortho-position, which comprises reacting the optically active 1-phenylethylamine derivative [1] with an aldehyde compound represented by formula [2]: R 2 -CHO wherein R 2 represents an optionally substituted alkyl group or an optionally substituted phenyl group to form an optically active imine represented by formula [3]: wherein R 1 and R 2 are as defined above, reacting the imine with an alkaline metal tert-alkoxide in an aprotic polar solvent or a mixture of the aprotic polar solvent and an aprotic nonpolar solvent, and then hydrolyzing the resultant racemic imine. The process according to claim 1, wherein the aprotic polar solvent is N,N-dimethylformamide or dimethyl sulfoxide and the aprotic nonpolar solvent is an aromatic hydrocarbon solvent. The process according to claim 1 or 2, wherein the alkaline metal tert-alkoxide is sodium tert-butoxide or potassium tert-butoxide. The process according to claim 1, 2 or 3, wherein R 1 is a group represented by the general formula [4]: wherein X represents a halogen atom or a C 1 -C 4 alkyl group, and Y represents a halogen atom, a C 1 -C 4 alkyl group or a hydrogen atom, and R 2 is a C 1 -C 6 alkyl group. The process according to claim 1, 2 or 3, wherein R 1 is a 2,4-dichlorophenyl group and R 2 is a C 1 -C 6 alkyl group. An optically active or racemic imine compound represented by formula [5]: wherein R 21 represents a C 1 -C 6 alkyl group. The imine compound according to claim 6, wherein R 21 is a tert-butyl group, an isopropyl group or an isobutyl group.

Inventors: GOTO HIDEYUKI, HAGIYA KOJI, NAGATA SHINICHIRO, YAMADA YOSHIMI, GOTO, HIDEYUKI, HAGIYA, KOJI, NAGATA, SHINICHIRO, YAMADA, YOSHIMI
Applicants: SUMITOMO CHEMICAL CO, SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1996
Main classification: C07C209
IPC codes: C07B55, C07C209, C07C211, C07C251