Process for manufacturing (all-rac)-alpha-tocopherol

Abstract: A process for the manufacture of (all- rac )-α-tocopherol by the catalyzed reaction of trimethylhydroquinone with isophytol or phytol is characterized by carrying out the reaction in the presence of a bis(perfluorinated hydrocarbyl sulphonyl)imide or a metal salt thereof as the catalyst in an organic solvent. In addition to said metal salt a Bronsted acid, e.g. sulphuric acid, phosphoric acid or p-toluenesulphonic acid, may be used as a cocatalyst. The product of the process is the most active member of the vitamin E group.

Description: The present invention is concerned with a novel process for the manufacture of (all- rac )-α-tocopherol by the acid-catalyzed reaction of trimethylhydroquinone (TMHQ) with isophytol (IP) or phytol (PH) in a solvent. As is known, (all- rac )-α-tocopherol (or as it has mostly been denoted in the prior art, "d,l-α-tocopherol") is a dia-stereoisomeric mixture of 2,5,7,8-tetramethyl-2-(4',8',12'-trimethyl-tridecyl)-6-chromanol (α-tocopherol), which is the most active and industrially most important member of the vitamin E group. Many processes for the manufacture of "d,l-α-tocopherol" (referred to as such in the literature reviewed hereinafter) by the reaction of TMHQ with IP or PH in the presence of a catalyst or catalyst system and in a solvent or solvent system are described in the literature. These processes go back to the work of Karrer et al., Bergel et al. as well as Smith et al. [see Helv. Chim. Acta 21 , 520 et seq. (1938), Nature 142 , 36 et seq. (1938) and, respectively, Science 88 , 37 et seq. (1938) and J. Am. Chem. Soc. 61 , 2615 et seq. (1939)]. While Karrer et al. carried out the synthesis of d,l-α-tocopherol from TMHQ and phytyl bromide in the presence of anhydrous zinc chloride (ZnCl 2 ; a Lewis acid), not only Bergel et al. but also Smith et al. used TMHQ and PH as starting materials. In the following years mainly modifications, e.g. alternative solvents and Lewis acids, were developed. From the work of Karrer et al. there was developed in the year 1941 a technically interesting process for the manufacture of d,l-α-tocopherol which was based on the reaction of TMHQ with IP in the presence of the catalyst system ZnCl 2 /hydrochloric acid (HCl) (US Patent 2 411 969). Later publications, e.g. Japanese Patent Publications (Kokai) 54380/1985, 64977/1985 and 226979/1987 [Chemical Abstracts (C.A.) 103 , 123731s (1985), C.A. 103 , 104799d (1985) and, respectively, C.A. 110 , 39217r (1989)], describe this reaction in the presence of zinc and/or ZnCl 2 and a Bronsted (protonic) acid, such as a hydrohalic acid, e.g. HCl, trichloroacetic acid, acetic acid and the like, especially ZnCl 2 /HCl, as the catalyst system. Disadvantages of these and further published processes featuring ZnCl 2 in combination with a Bronsted acid are the corrosive properties of the acids and the contamination of the waste water with zinc ions as a result of the large amount of ZnCl 2 required for the catalysis. The manufacture of d,l-α-tocopherol by the reaction of TMHQ with phytyl chloride, PH or IP in the presence of boron trifluoride (BF 3 ) or its etherate (BF 3 ·Et 2 O) is described in German Patents 960720 and 1015446 as well as in US Patent 3 444 213. However BF 3 too has corrosive properties. Also, the reaction of TMHQ with IP or PH in the presence of a Lewis acid, e.g. ZnCl 2 , BF 3 or aluminium trichloride (AlCl 3 ), a strong acid, e.g. HCl, and an amine salt as the catalyst system is described in European Patent Publication (EP) 100471. In an earlier patent publication, DOS 2606830, the IP or PH is pretreated with ammonia or an amine before the reaction with TMHQ in the presence of ZnCl 2 and an acid is effected. In both cases corrosion problems occur. A further interesting method for the manufacture of d,l-α-tocopherol from TMHQ and IP comprises using an isolated TMHQ-BF 3 or -AlCl 3 complex and a solvent mixture featuring a nitro compound (DOS 1909164). This process avoids to a large extent the formation of undesired by-products because it involves mild reaction conditions. The yield of d,l-α-tocopherol, based on IP and the use of the solvent mixture methylene chloride/nitro-methane, is given as 77%. However, the use of such a solvent mixture is disadvantageous. The manufacture of d,l-α-tocopherol by the reaction of TMHQ with IP using cation exchange resin complexes of metal ions (Zn 2+ , Sn 2+ and Sn 4+ ) is disclosed in Bull. Chem. Soc. Japan 50 , 2477-2478 (1977); amongst other disadvantages it gives the product in unsatisfactory yields. The use of macroreticular ion exchangers, e.g. Amberlyst® 15, as the catalyst for the reaction of TMHQ with IP is described in US Patent 3459773. However, the d,l-α-tocopherol could not be obtained in the requisite purity. EP 603695 describes the manufacture of d,l-α-tocopherol in liquid or supercritical carbon dioxide by the reaction of TMHQ with IP or PH in the presence of acidic catalysts, such as ZnCl 2 /HCl and ion exchangers. The reported yields are unsatisfactory. The reaction in the presence of a catalyst system which consists of iron(II) chloride, metallic iron and HCl gas or aqueous solution is described in DOS 2160103 and US Patent 3789086. The formation of less by-products is advantageous compared with the aforementioned process using ZnCl 2 /HCl. However, corrosion problems and chloride contamination are equally disadvantageous. An interesting alternative for the reaction of TMHQ with IP to d,l-α-tocopherol comprises using trifluoroacetic acid or its anhydride as the catalyst (EP 12824). Although in this process the avoidance of HCl is achieved, the catalyst is relatively expensive. The use of the heteropoly acid 12-tungstophosphoric or 12-tungstosilicic acid as the catalyst for the reaction of TMHQ with IP was described for the first time in React. Kinet. Catal. Lett. 47 (1), 59-64 (1992). d,l-α-Tocopherol could be obtained, using various solvents, in about 90% yield. A further process described in the literature [EP 658552; Bull. Chem. Soc. Japan 68 , 3569-3571 (1995)] for the synthesis of d,l-α-tocopherol is based on the use of a various lanthanide trifluoromethanesulphonates (triflates), e.g. scandium trifluoromethanesulphonate, as the catalyst for the reaction. With up to about 10% excess of IP this process gives yields up to 98%. The use of ion-exchanged bentonite, montmorillonite or saponite through treatment with e.g. scandium chloride and other metal salts (yttrium, lanthanum, etc.) as the catalyst for the reaction of TMHQ with IP or PH has as a disadvantage the need for a large amount of catalyst [EP 677520; Bull. Chem. Soc. Japan 69 , 137-139 (1996)]. According to the Examples of EP 694541 the reaction of TMHQ with IP to α-tocopherol can be achieved in high yields and with a high product purity when such solvents as carbonate esters, fatty acid esters and certain mixed solvent systems are employed, the exemplified catalysis being effected by ZnCl 2 /HCl. Disadvantages in this process are, in addition to the contamination of the waste water by zinc ions, the usual large "catalyst amount" of ZnCl 2 used. According to WO 97/28151 the acid-catalysed reaction of TMHQ with IP can be performed in a cyclic carbonate or α-lactone as the solvent. The preferred catalyst is a mixture of orthoboric acid and oxalic, tartaric or citric acid, or boron trifluoride etherate. WO 98/21197 describes the manufacture of d,l-α-tocopherol from TMHQ and IP using bis(trifluoromethylsulphonyl)imide or a metal salt thereof optionally together with a strong Bronsted acid, as catalyst in such types of aprotic solvents as aliphatic and cyclic ketones or esters, and aromatic hydrocarbons. Using the same kind of bis(trifluoromethylsulphonyl)imide catalyst it has been shown in EP 1000940 that the dl-α-tocopherol manufacturing process can also be realized in supercritical carbon dioxide or nitrous oxide as the solvent. From the forgoing review it is evident that most of the previously known processes have considerable disadvantages. Thus, corrosion problems occur in all processes in which such acid catalysts as boron trifluoride are used. Toxicity problems with the boron trifluoride adducts also occur, and when iron or zinc is used there is a contamination of the waste water with the metal ions which is today no longer acceptable. In some processes the formation of undesired by-products, e.g. phytyltoluene and chlorophytols, is an especially serious problem. In most cases the yields are unsatisfactory. The object of the present invention is to provide a process for the manufacture of (all- rac )-α-tocopherol by the reaction of trimethylhydroquinone with isophytol or phytol in the presence of a catalyst and in a solvent which does not have the disadvantages of previously known procedures. In this respect, it is necessary that the catalyst used has no, or at least a much reduced, corrosive action, is non-toxic, does not contaminate the environment, e.g. with chlorinated by-products or heavy metal ions, and catalyzes the desired reaction as selectively as possible and in high yields. Furthermore, the catalyst should display its activity in small, really catalytic, amounts and should be readily separable and re-usable several times. This object of the present invention is achieved by carrying out the reaction of trimethylhydroquinone with isophytol or phytol in the presence of a bis(perfluorinated hydrocarbyl sulphonyl)imide or a metal salt thereof, as will be defined hereinafter, in an organic solvent. The reaction itself is represented in the following Reaction Scheme, showing the reaction with IP only. Accordingly, the process in accordance with the present invention for the manufacture of (all- rac )-α-tocopherol by the catalyzed reaction of trimethylhydroquinone with isophytol or phytol is characterized by carrying out the reaction in the presence of a bis(perfluorinated hydrocarbyl sulphonyl)imide or a metal salt thereof of the general formula [(R 1 SO 2 ) 2 N] x R 2 wherein each R 1 , independently, signifies a perfluoroalkyl group C n F 2n+1 or pentafluorophenyl, or both symbols R 1 together signify a poly-difluoromethylene group -(CF 2 ) m -, with the proviso that both symbols R 1 cannot simultaneously signify trifluoromethyl, R 2 signifies a proton or a metal cation selected from the group consisting of boron, magnesium, aluminium, silicon, scandium, titanium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, rhodium, palladium, silver, tin, lanthanum, cerium, praseodymium, neodymium, europium, dysprosium, thulium, ytterbium, hafnium, platinum and gold, each in the cationic form, m signifies an integer from 2 to 4, n signifies an integer from 1 to 10 and x signifies the corresponding valency of the proton (1) or metal cation (1,2,3 or 4), as the catalyst in an organic solvent. Not only some of the above-defined bis(perfluorinated hydrocarbyl sulphonyl)imides but also some of their metal salts amongst the catalysts of formula I are known compounds. Those catalysts of formula I which may still not be known can be produced by methods analogous to the published methods for producing bis(trifluoromethylsulphonyl)imide and its metal salts (excluded from the scope of the catalysts of formula I by the proviso) and the higher members of these sulphonimides and their metal salts: see, for example, EP 364340/US Patent 5256821, Japanese Patent Publications (Kokai) 246338/1995, 064238/1996 (with the US counterpart US Patent 5650244), 057110/1997, 169690/1997) 176063/1997, 176171/1997, 241184/1997, 230166/1998, 330314/1998 and 209338/1999, DOS 4217366/US Patent 5502251, DOS 19533711/US Patent 5723664, Chemiker Zeitung 96 , 582-583 (1972), Chem. Lett. 1995, 307-308, Synlett 1996, 171-172, 265-266 and 839-841, Inorg. Chem. 35 (7), 1918-1925 (1996), J. Power Sources 68 , 307-310 (1997) and Cat. Today 36 (1), 81-84 (1997) as well as the further literature references mentioned in this state of the art. For example, many of the salts can be produced from the appropriate bis(perfluorinated hydrocarbyl sulphonyl)imide of formula I in which R 2 signifies a proton and the metal acetates, oxides, hydroxides and alcoholates featuring the desired metal cation. In the case of the aluminium, zinc and various other metal salts these can also be produced using the corresponding alkylmetal or dialkylmetal hydride, e.g. diethylzinc or triethylaluminium or, respectively, diisobutylaluminium hydride. In some cases the metal salts can be present in monomeric or polymeric form and, accordingly, formula I is intended to embrace all such forms. Further, these catalysts can be used in isolated form or produced in situ. Examples of a catalyst of formula I in which the symbols R 1 together signify a poly-difluoromethylene group -(CF 2 ) m - are 4,4,5,5,6,6-hexafluoro-(1, 3, 2)dithiazinane-1,3-dioxide and its silver salt. In the case of the metal salts amongst the above-defined catalysts of formula I these can be used together with a strong Bronsted acid as a co-catalyst in the process of the present invention. The Bronsted acid present in such a catalyst system can be an inorganic or organic acid, examples of which are sulphuric acid, phosphoric acid and p-toluenesulphonic acid. Solvents which can be used in the scope of the present invention are polar or non-polar organic solvents. Suitable classes of polar solvents include aliphatic and cyclic ketones, e.g. diethyl ketone and isobutyl methyl ketone and, respectively, cyclopentanone and isophorone; and aliphatic and cyclic esters, e.g. ethyl acetate and isopropyl acetate, and, respectively, γ-butyrolactone, ethylene carbonate and propylene carbonate. As suitable classes of non-polar solvents there may be mentioned aliphatic hydrocarbons, e.g. hexane, heptane and octane, and aromatic hydrocarbons, e.g. benzene, toluene and the xylenes. The reaction can be effected in a single solvent phase, e.g. in toluene alone as the solvent, or in a biphasic solvent system, e.g. in ethylene or propylene carbonate and heptane. The process is conveniently effected at temperatures from about 50°C to about 150°C, preferably from about 90°C to about 125°C, and most preferably from about 105°C to about 120°C. Furthermore, the molar ratio of trimethylhydroquinone to isophytol/phytol present in the reaction mixture conveniently extends from about 1.3 : 1 to about 2.5 : 1, preferably from about 1.5 : 1 to about 2.2 : 1, and is most preferably about 2 : 1. The amount of catalyst of formula I used is such that the molar ratio of catalyst to the educt (trimethylhydroquinone or isophytol/phytol) which is in the lesser molar amount (usually the isophytol or phytol rather than the trimethylhydroquinone) is conveniently about 0.01 : 100 to about 4 : 100, i.e. the amount of catalyst is conveniently from about 0.01 mole % to about 4 mole % of the amount of educt in the lesser molar amount. Where a catalyst system (combination of a metal salt of formula I and a strong Bronsted acid) is used, the amount of metal salt is conveniently about 0.1 mole % to about 4 mole % and the amount of Bronsted acid is conveniently about 0.01 mole % to about 0.5 mole %, in each case based on the amount of educt in the lesser molar amount. Conveniently about 10- 100 ml, preferably about 30 - 60 ml, of organic solvent are used per 10 mmol of isophytol or phytol, whichever is employed. If the process is carried out in in a biphasic solvent system, especially one consisting of a polar solvent, e.g. a cyclic carbonate such as ethylene or propylene carbonate, and a non-polar solvent, e.g. an aliphatic hydrocarbon such as heptane, then the volume ratio of the non-polar solvent to the polar solvent is conveniently in the range from about 0.3 : 1 to about 5 : 1, preferably from about 1 : 1 to about 3 : 2. Moreover, the process is conveniently carried out under an inert gas atmosphere, preferably gaseous nitrogen or argon. The actual reaction generally lasts for about 0.2 to about 20 hours, preferably about 0.5 to about 1 hour. The process in accordance with the invention can be carried out batchwise or continuously, preferably continuously, and in general operationally in a very simple manner, for example by adding isophytol or phytol, as such or in solution, portionwise to a suspension or solution of the trimethylhydroquinone and the catalyst. The rate at which the isophytol or phytol is added is not critical. Conveniently, isophytol/phytol is added continuously over a period of about 0.2 to about 5 hours. After completion of the isophytol/phytol addition and an appropriate subsequent reaction period the working-up is effected by procedures conventionally used in organic chemistry. If desired, the obtained (all- rac )-α-tocopherol can be converted into its acetate, succinate, poly(oxyethylene)succinate, nicotinate and further known application forms by standard procedures. The process in accordance with the invention enables the catalyst used to be separated readily and to be reused several times. Advantages in the use of the catalyst in the process in accordance with the invention are, in addition to high yields of (all- rac )-α-tocopherol, the avoidance of corrosion, the avoidance of waste water contamination with heavy metal ions, the high selectivity as well as the enabled ready isolation of the produced (all- rac )-α-tocopherol from the mixture after reaction. Furthermore, the amount of dehydration products, so-called phytadienes, which tend to result from the action of acids on tertiary allylic alcohols such as isophytol and phytol, is kept to an acceptable minimum in the process of the present invention, as also the amount of furane derivatives which tend to be produced as by-products in dl-α-tocopherol manufacture [see, for example, Bull. Chem. Soc. Japan 68 , 3569-3571 (1995)]. The process in accordance with the invention is illustrated by the following Examples: Example 1 7.69 g (50 mmol) of trimethylhydroquinone were suspended or dissolved in 50 ml of toluene or diethyl ketone or in 40 ml of γ-butyrolactone or ethylene carbonate or propylene carbonate and 50 ml of heptane, whereafter 0.1 mole % (based on the amount of isophytol to be used) of the catalyst of formula I was added. Then the mixture was heated to a temperature in the range 50 - 150°C and 10 g (11.9 ml; 33 mmol) of isophytol were added portionwise to the mixture over a period of about 20 minutes, all under argon. Subsequently, the reaction mixture was stirred under argon for a further 30 minutes at reflux temperature and monitored by thin layer chromatography to follow the progress of the reaction. After the establishment of completed conversion to (all- rac )-α-tocopherol this product was recovered from the reaction mixture by cooling it to about 60 - 80°C, separating the phases (if appropriate) and distilling off the solvent under reduced pressure.. Unambiguous identification of the product was effected by comparison of gas chromatographic retention times with those of an authentic sample. The results are presented in the following Table 1. Results of the use of the catalyst (R 1 SO 2 ) 2 NH in the manufacture of (all- rac )-α-tocopherol using various solvents Catalyst Solvent (Amount in ml) Yield R 1 R 1 C 2 F 5 C 2 F 5 Toluene (50) 89.6% C 2 F 5 C 4 F 9 " 90.2% CF 3 C 6 F 5 Diethyl ketone (50) 83.8% CF 3 C 6 F 5 γ-Butyrolactone (40)/Heptane (50) 87.4% CF 3 C 4 F 9 Ethylene carbonate (40)/Heptane (50) 94.0% C 2 F 5 C 2 F 5 " 93.6% C 3 F 7 C 3 F 7 " 94.5% C 4 F 9 C 4 F 9 " 87.0% C 6 F 5 C 6 F 5 " 87.2% CF 3 C 8 F 17 Propylene carbonate (40)/Heptane (50) 86.4% C 4 F 9 C 4 F 9 " 94.0% C 4 F 9 C 8 F 17 " 91.1% C 8 F 17 C 8 F 17 " 85.6% CF 3 C 6 F 5 " 92.6% C 4 F 9 C 6 F 5 " 93.3% C 8 F 17 C 6 F 5 " 68.9% C 6 F 5 C 6 F 5 " 84.9% Analogously, 4,4,5, 5, 6, 6-hexafluoro-(1,3,2)dithiazinane-1,3-dioxide (formula I, in which both symbols R 1 together signify tri-difluoromethylene, x signifies 1 and R 2 signifies a proton) was used as the catalyst in the biphasic solvent system ethylene carbonate (40 ml) and heptane (50 ml), which resulted in a 94,5% yield of (all- rac )-α-tocopherol. Example 2 The procedure of Example 1 was repeated with the differences that 1.0 mole % of a metal containing catalyst was used, and the solvent was in all cases the biphasic solvent system 40 ml of ethylene carbonate and 50 ml of heptane. The results are presented in the following Table 2. Results of the use of the catalyst [(R 1 SO 2 ) 2 N] x R 2 in the manufacture of (all- rac )-α- tocopherol Catalyst Yield R 1 R 1 R 2 x C 4 F 9 C 4 F 9 Ni 2 91% C 4 F 9 C 4 F 9 Y 3 78% CF 3 C 8 F 17 Ag 1 86% C 4 F 9 C 4 F 9 Ag 1 93% C 8 F 17 C 8 F 17 Ag 1 89% C 8 F 17 C 6 F 5 Ag 1 89% C 4 F 9 C 4 F 9 Tm 3 90% Analogously, the silver salt of 4,4,5,5,6,6-hexalluoro-(1,3,2)dithiazinane-1,3-dioxide (formula I, in which both symbols R 1 together signify tri-difluoromethylene, x signifies 1 and R 2 signifies the silver cation) was used as the catalyst, which resulted in a 88% yield of (all- rac )-α-tocopherol.

Claims: A process for the manufacture of (all- rac )-α-tocopherol by the catalyzed reaction of trimethylhydroquinone with isophytol or phytol characterized by carrying out the reaction in the presence of a bis(perfluorinated hydrocarbyl sulphonyl)imide or a metal salt thereof the general formula [(R 1 SO 2 ) 2 N] x R 2 wherein each R 1 , independently, signifies a perfluoroalkyl group C n F 2n+1 or pentafluorophenyl, or both symbols R 1 together signify a poly-difluoromethylene group -(CF 2 ) m -, with the proviso that both symbols R 1 cannot simultaneously signify trifluoromethyl, R 2 signifies a proton or a metal cation selected from the group consisting of boron, magnesium, aluminium, silicon, scandium, titanium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, rhodium, palladium, silver, tin, lanthanum, cerium, praseodymium, neodymium, europium, dysprosium, thulium, ytterbium, hafnium, platinum and gold, each in the cationic form, m signifies an integer from 2 to 4, n signifies an integer from 1 to 10 and x signifies the corresponding valency of the proton (1) or metal cation (1,2,3 or 4), as the catalyst in an organic solvent. A process according to claim 1, wherein an aliphatic or cyclic ketone, an aliphatic or cyclic ester, or an aliphatic or aromatic hydrocarbon, is used as the organic solvent. A process according to claim 2, wherein the solvent is diethyl ketone, isobutyl methyl ketone, cyclopentanone, isophorone, ethyl acetate, isopropyl acetate, γ-butyrolactone, ethylene carbonate, propylene carbonate, hexane, heptane, octane, benzene, toluene or xylene. A process according to any one of claims 1 to 3, wherein the amount of bis(perfluorinated hydrocarbyl sulphonyl)imide or metal salt thereof of the formula I used as the catalyst is from about 0.01 mole % to about 4 mole % based on the amount of trimethylhydroquinone or isophytol/phytol, which is in the lesser molar amount. A process according to any one of claims 1 to 4, wherein in addition to a metal salt of the formula I a Bronsted acid is used as a co-catalyst, preferably sulphuric acid, phosphoric acid or p-toluenesulphonic acid. A process according to any one of claims 1 to 5, wherein about 10 - 100 ml, preferably about 30 - 60 ml, of organic solvent are used per 10 mmol of isophytol or phytol, whichever is employed. A process according to any one of claims 1 to 6, wherein the reaction is effected at temperatures from about 50°C to about 150°C, preferably from about 90°C to about 125°C, most preferably form about 105°C to about 120°C. A process according to any one of claims 1 to 7, wherein the molar ratio of trimethylhydroquinone to isophytol/phytol present in the reaction mixture extends from about 1.3 : 1 to about 2.5 : 1, preferably from about 1.5 : 1 to about 2.2 : 1, and is most preferably about 2 : 1. A process according to any one of claims 1 to 8, wherein isophytol or phytol, as such or in solution, is added portionwise to a suspension or solution of the trimethylhydroquinone and the catalyst.

Inventors: BONRATH WERNER, HAAS ALOIS, HOPPMANN EIKE, PAULING HORST, BONRATH, WERNER, HAAS, ALOIS, HOPPMANN, EIKE, PAULING, HORST
Applicants: ROCHE VITAMINS AG
Application year: 2001
Main classification: C07D311
IPC codes: C07D311