PROCESS FOR PRODUCING INDOLES

Abstract: A process for producing an indole by catalytic reaction of a 2-(o-aminoaryl)ethanol, said reaction being carried out by heating in a liquid phase in the presence of an alkali metal us­ ing a catalyst containing a metal selected from the group con­ sisting of a platinum-group metal, nickel and copper. The process is industrially advantageous in that indoles are produced in high yield.

Description: The present invention relates to a process for producing indoles in high yields from 2-(o-aminoaryl)ethanols. Indoles are useful as starting materials for the industrial production of perfumes, amino acids such as tryptophan, and polymer stabilizers. German Patent 606,027 discloses a process for producing indolines and/or indoles by catalytic gas-phase reaction of 2-(o-aminoaryl)ethanols. A list of the catalysts usable in this process includes Cu, Co, Ni, Ag, Fe, Pd, Pt, Mo, W, Pb, Mn, Al, Ti, Zr, Cr and Th, and CuCO₃ is mentioned as a specific example capable of producing indole as the main product. However, none of these catalysts are completely satisfactory for industrial purposes primarily because their activity is reduced during run. Methods for producing indoles from indolines by dehydrogenation reaction are also known and they include the use of a Raney nickel catalyst [Acta Chim. Acad. Sci. Hung., 54 , 167 (1967)], the use of a chromium or copper-chromium catalyst [Roczniki, Chem., 38 , 507 (1964)] and the use of a manganese dioxide catalyst [J. Org. Chem., 29 , 1540 (1964)]. However, these catalysts have the disadvantages of low indole yields and selectivities. Unexamined Published Japanese Patent Application No. 142063/77 discloses a method of producing indoles by heating indolines or 2-(o-aminoaryl)ethanols together with at least one acidic compound selected from the group consisting of nitric acid, nitrous acid, an acidic sulfate salt and an acidic sulfite salt. This method is not industrially feasible, either, since it consumes a large amount of catalyst. Japanese Patent Publication No. 7619/85 proposes a one-step production process of indole by heating o-(2-nitrophenyl)ethanol in the liquid state in the presence of hydrogen and a reducing catalyst, in order to improve the industrial disadvantage of reduced catalytic activity inevitable in a gas-phase process for producing indole from o-(2-nitrophenyl)ethanol in a single step (Japanese Patent Publication No. 20778/74). According to the Publication, the catalyst used in this process experiences a relatively small activity loss even if a large amount of the starting material is processed. However, a large quantity of catalyst is necessary for the purpose of cyclic use and the yield of indole produced per unit amount of catalyst is low. The present inventors repeated this prior art method and found that it is not industrially feasible because of the very low yield of indole obtained (see Referential Examples 1 to 3 given later in this specification). GB-A-205 1065 discloses a process, wherein an o-aminoaryl alcohol hydrochloride is converted into an indole in the presence of a solvent such as water at elevated temperatures. Optionally this reaction can be carried out in the presence of a platinum metal group catalyst. A reaction in which a hydrochloride is used at elevated temperatures is, however, not very suitable as an industrial method, because it causes problems regarding the quality of the material for the reactor. It is the object of the present invention to provide an industrially advantageous process for producing high yields of indoles from readily available and inexpensive materials at low catalyst costs. The present invention therefore provides a process for producing an indole by catalytic reaction of a of a 2-(o-aminoaryl)ethanol, said reaction being carried out by heating at a temperature of 130 to 300 o C in a liquid phase in the presence of an alkali metal and a catalyst containing a metal selected from nickel and copper, wherein the alkali metal and the catalyst are used in an amount of 10⁻⁴ to 10g per g of the starting material. The process of the present invention is industrially advantageous in that it requires a small catalyst loading and achieves high indole yield at an increased reaction rate. Starting material : The 2-(o-aminoaryl)ethanol used in the process of the present invention is a compound represented by the following formula I: wherein X is hydrogen, a C₁₋₁₂ alkyl group, a C₁₋₁₂ alkoxy group, a halogen atom or a hydroxyl group; R¹ and R² which may be the same or different each is a substituent selected from the group consisting of hydrogen, a C₁₋₄ alkyl group, a C₅₋₆ cycloalkyl group and a C₆₋₁₀ aryl group. Examples of the 2-(o-aminoaryl)ethanols represented by the above formula are as follows: 2-(o-aminophenyl)ethanol, 2-(2-amino-5-isopropylphenyl)ethanol, 2-(2-amino-5-tert-butylphenyl)ethanol, 2-(2-amino-5-fluorophenyl)ethanol, 2-(2-amino-5-hydroxyphenyl)ethanol, 2-(2-amino-3-methoxyphenyl)ethanol, 2-(2-amino-6-methoxyphenyl)ethanol, 1-methyl-2-(o-aminophenyl)ethanol, 1-ethyl-2-(o-aminophenyl)ethanol, 1-isopropyl-2(o-aminophenyl)ethanol, 1-cyclohexyl-2-(o-aminophenyl)ethanol, 2-(2-amino-6-benzyloxyphenyl)ethanol, 2-(2-amino-5-benzyloxyphenyl)ethanol, 1-phenyl-2-(o-aminophenyl)ethanol, 2-methyl-2-(o-aminophenyl)ethanol, 2-pnenyl-2-(o-aminophenyl)ethanol, and 1-phenyl-2-(2-amino-5-methoxyphenyl)ethanol. Catalyst : The catalyst used in the process of the present invention contains a metal selected from the group consisting of nickel and copper and is associated with at least one alkali metal such as lithium, sodium, potassium, rubidium or cesium. A catalyst containing a metal selected from the group consisting of nickel and copper associated with either potassium or sodium as an alkali metal is preferred from the viewpoint of indole yield. The alkali metal may be either incorporated in the catalyst or directly added to the reaction system. The alkali metal is used in an amount ranging from 10⁻⁴ to 10 g, preferably 5 x 10⁻⁴ to 1 g, per gram of the 2-(o-aminoaryl)ethanol used as the starting material. The catalyst containing nickel associated with the alkali metal may be used as it is in the indole-producing reaction but, if desired, the catalyst may be supported on a carrier. Illustrative carriers include metal oxides, active carbon, carbon black, graphite, metal sulfate salts, metal carbonate salts, and metal phosphate salts. Specific examples are active carbon, carbon black, alumina, silica, titania, diatomaceous earth, and mixed oxides such as silica-alumina, titania-silica, titania-magnesia, silica-magnesia and alumina-titania. Diatomaceous earth is preferred for use with nickel. Other preferred catalysts are Raney nickel and Raney copper. The state of the metal and alkali metal present as the active components of the catalyst used in the process of the present invention is not limited to any particular form, no matter whether the catalyst is virgin, in the course of run or no longer used. For example, nickel may be either elemental or in a slightly oxidized (e.g., surface-oxidized) form; copper may be in an elemental form; and alkali metals may be in the form of metal oxides, metal carbonate salts, metal sulfate salts, metal halides or metal phosphate salts. The catalyst used in the present invention may contain a third component such as various metals and metal compounds that will not appreciably impair the catalytic activity. Examples of this third component include alkaline earth metals, metals of IB group such as copper and silver, metals of VIII group such as iron, cobalt and nickel, metals of VII group such as manganese, metals of VI group such as chromium, molybdenum and tungsten, metals of V group such as vanadium, metals of IV group such as tin and lead, metals of IIB group such as zinc and cadmium, metals of III group such as boron, and the metal compounds thereof. The catalyst described above are used in an amount of 10⁻⁴ to 10 g, preferably 5 x 10⁻⁴ to 1 g, per gram of the starting 2-(o-aminoaryl)ethanol in general. A catalyst having nickel supported on diatomaceous earth in the virgin state contains 30 to 80 wt%, preferably 40 to 70 wt%, of nickel in an elemental form. Nickel and copper catalysts of Raney type may be PrePared by activating Al-Ni alloy, and Al-Cu alloy, respectively, with an aqueous solution of sodium hydroxide in accordance with conventional techniques, for example, by the method described in "Shokubai Binran (Catalyst Handbook)", ed. by the Catalysis Society of Japan, Chijin Shokan, pp. 482-486. According to this method, 1 volume of the specific alloy is charged bit by bit into 1.3 to 1.5 volume of a 20 to 25% aqueous solution of sodium hydroxide at 50 to 60°C while cooling under thorough agitation, with care taken to remove any hydrogen evolved; thereafter, the solution is stirred at 50 to 100°C for 30 to 120 minutes in order to remove aluminum. If the agitation is inadequate, the catalyst is insufficiently activated to provide the desired activity. The catalyst recovered from the NaOH solution is usually washed by decantation with 8 to 9 times the volume of the alloy of water until the pH of the washings comes to be within the range of 8 to 9. Reaction conditions : In accordance with the process of the present invention, the desired indoles may be produced in high yield by simply heating the starting 2-(o-aminoaryl)ethanols in the Presence of the catalyst described above and an alkali metal. The temperature at which the reaction system is heated generally ranges from 130 to 300°C, preferably from 150 to 250°C. Above 300°C, side reactions will occur to an undesirable extent. Below 130°C, the reaction rate is slowed down to prolong the reaction period. The reaction time is not limited to any particular range but is generally set between about 0.1 and about 20 h. The reaction should be carried out at a pressure sufficient to maintain at least a part of the 2-(o-aminoaryl)ethanol in the liquid phase. Atmospheric pressure is generally used but the reaction may be performed under reduced pressure or under pressure if at least a part of the 2-(o-aminoaryl)ethanol can be maintained in the liquid phase. It is preferable to stir the reaction solution either by using conventional agitating blades or by blowing nitrogen or other inert gases into the reaction system. Stirring the reaction solution is effective for the purpose of attaining improved selectivity for indoles. The reaction will proceed satisfactorily in the absence of a solvent. The indoles produced may be continuously recovered while water and hydrogen forming as by-products are removed from the reaction system. In this case, the 2-(o-aminoaryl)ethanol is preferably fed into the reactor in a continuous manner. If desired, a solvent may be used in performing the reaction and preferred solvents are those which will boil at higher temperatures than the product indoles. The following examples, comparative examples and referential examples are provided for the purpose of further illustrating the process of the present invention. The yields of the products obtained in the following examples and comparative examples are indicated in terms of mol% of the 2-(o-aminoaryl)ethanols used as starting materials. COMPARATIVE EXAMPLE 1 A Pyrex flask equipped with a Dimroth condenser was charged with 1.37 g (10 mmol) of 2-(o-aminophenyl)ethanol and 0.95 g (7 mmol) of acidic potassium sulfate. After putting a magnetic stirrer into the flask, reaction was conducted at 220°C for 1 h. The reaction mixture was neutralized with an aqueous solution of sodium carbonate and subjected to extraction with ether. Gas chromatographic analysis of the ether layer revealed that an indole was obtained in a yield of 6.2%. The indoline yield was 8.2%. EXAMPLE 1 A Pyrex flask equipped with a single-distillator (distillation head, condenser, adapter and receptacle) and a N₂ gas blowing pipe was charged with 4.0 g (29.2 mmol) of 2-(o-aminophenyl)ethanol, 0.015 g of "N-103" (the nickel/diatomaceous earth catalyst with 49 to 52% Ni available from Nikki Chemical Co., Ltd.) and 0.06 g (0.4 mmol) of potassium carbonate. After putting a magnetic stirrer into the reactor, reaction was performed for 7 h on an oil bath (190 to 200°C) while nitrogen gas was blown into the reactor. The conversion of 2-(o-aminophenyl)ethanol was 100% and the yield of indole was 91.8%. The indoline yield was 5.6%. EXAMPLE 2 The procedures of Example 1 were repeated except that reaction was carried out for 4 h using 2 g (14.6 mmol) of 2-(o-aminophenyl)ethanol and 0.45 g of Ni-K/diatomaceous earth as a catalyst. This catalyst was prepared by the following method: 0.7 g of "G-49B" (Ni/diatomaceous earth catalyst with 55% Ni available from Nissan-Girdler Catalyst Co., Ltd.) was added to a solution of 0.035 g of potassium carbonate (K₂CO₃) in 4 ml of water, and after leaving the mixture to stand overnight, the water was distilled off under reduced pressure. The so prepared catalyst contained 2.8 wt% of potassium on the basis of "G-49B". The results of the reaction were as follows: the conversion of 2-(o-aminophenyl)ethanol was 100%, and the yield of indole was 87.1%. The indoline yield was 7.0%. The catalyst was recovered from the reaction solution and used in the second run of reaction. The conversion of 2-(o-aminophenyl)ethanol was 100% and the yield of indole was 88.5%. The indoline yield was 6.4%. COMPARATIVE EXAMPLE 2 The procedures of Example 1 were repeated using 0.015 g of the catalyst, "N-103", except that reaction was continued for 14 hours on an oil bath heated at 210°C. The conversion of 2-(o-aminophenyl)ethanol was 97.3% and the yield of indole was 96.1%. The indoline yield was 0.6%. COMPARATIVE EXAMPLES 3 TO 5 The first run of the reaction was carried out as in Example 2 except that the catalyst was changed to those indicated in Table 1. The results of the reaction are also shown in Table 1. Run No. Catalyst Amount of Catalyst (g) OAPE Conversion (%) Indole Yield (%) Indoline Yield (%) 3 Ni/Cu/Co - silica (57.3 wt% NiO, 5.6 wt% CuO, 6.4 wt% CoO) 0.3 36.9 17.6 15.8 4 Ni - Al (50.9 wt% NiO with the balance of Al₂O₃) 0.3 12.3 2.2 2.5 5 Cu - Al (12.5 to 15 wt% CuO with the balance of Al₂O₃) 0.3 43.5 4.4 36.6 EXAMPLE 3 A Pyrex flask equipped with a single-distillator (Claisen type connected pipe, condenser, adapter and receptacle) and a N₂ gas blowing pipe was charged with 5 g (36.5 mmol) of 2-(o-aminophenyl)ethanol, 0.1 g of "NDT-90" (Raney nickel catalyst of Kawaken Fine Chemical Co., Ltd.) and 0.075 a of potassium carbonate (K₂CO₃). After putting a magnetic stirrer into the flask, the latter was placed over an oil bath heated at 195 to 200°C. Reaction was carried out for 5 h with nitrogen gas being blown into the flask. After completion of the reaction, gas chromatographic analysis of the product showed that the conversion of 2-(o-aminophenyl)ethanol and the yield of indole were 93.0% and 86.0%, respectively. More 5 g of 2-(o-aminophenyl)ethanol was added to the reaction solution and another run of indole production was carried out in succession: the overall conversion of 2-(o-aminophenyl)ethanol was 86.7% and the yield of indole was 80.5%. EXAMPLE 4 The first run procedures of Example 3 were repeated except that 2 g (14.6 mmol) of 2-(o-aminophenyl)ethanol, 3.1 g of "CDT-60" (Raney copper catalyst of Kawaken Fine Chemical Co., Ltd.) and 0.03 g of potassium carbonate (K₂CO₃) were used. The conversion of 2-(o-aminophenyl)ethanol and the yield of indole were 98.8% and 93.6%, respectively. COMPARATIVE EXAMPLE 6 The first run procedures of Example 3 were repeated except that 0.15 g of "NDT-90" was used without K₂CO₃ and reaction was continued for 9 h. The conversion of 2-(o-aminophenyl)ethanol was 12.5% and the yield of indole was 10.8%. COMPARATIVE EXAMPLE 7 The procedures of Example 4 were repeated except that 0.1 g of "CDT-60" was used without K₂CO₃. The conversion of 2-(o-aminophenyl)ethanol was 28.6% and the yield of indole was 24.3%. REFERENTIAL EXAMPLE 1 A 120-ml autoclave was charged with 4.45 g (26.6 mmol) of 2-(o-nitrophenyl)ethanol, 15 ml of dioxane and 0.54 g of 5% Ru - C catalyst of Nippon Engelhard Ltd. After purging the atmosphere in the autoclave with hydrogen, the pressure in the autoclave was increased to 20 kg/cm² and reaction was conducted at 200°C for 1 h with agitation. Gas chromatographic analysis of the product revealed that although the conversion of 2-(o-nitrophenyl)ethanol was 100%, the yield of indole on the basis of 2-(o-nitrophenyl)ethanol was only 13.8%. REFERENTIAL EXAMPLE 2 The procedures of Referential Example 1 were repeated except that the catalyst and the reaction time were changed to 0.59 g of 5% Pd - C of Nippon Engelhard Ltd. and 1.5 h. The conversion of 2-(o-nitrophenyl)ethanol was 100% and the yield of indole was 39.2%. REFERENTIAL EXAMPLE 3 A 120-ml autoclave was charged with 4.53 g (27.1 mmol) of 2-(o-nitrophenyl)ethanol, 15 ml of dioxane and 0.45 g (as metal) of "NDT-90" (Raney nickel catalyst of Kawaken Fine Chemicals Co., Ltd.). After purging the atmosphere in the autoclave with hydrogen, the pressure in the autoclave was increased to 30 kg/cm² and reaction was conducted at 200°C for 1 h with agitation. The results were as follows: 100% conversion of 2-(o-nitrophenyl)ethanol, with an indole yield of 1.1%.

Claims: A process for producing an indole by catalytic reaction of a 2-(o-amioaryl)ethanol, said reaction being carried out by heating at a temperature of 130 o C -300 o C in a liquid phase in the presence of an alkali metal and a catalyst containing a metal selected from nickel and copper, wherein the alkali metal and the catalyst are used in an amount of 10⁻⁴ to 10g per g of the starting material. The process according to claim 1, wherein the 2-(o-aminoaryl) ethanol is a compound represented by the formula I. wherein X is hydrogen, a C₁₋₁₂ alkyl group, a C₁₋₁₂ alkoxy group, a halogen atom or a hydroxyl group; R¹ and R² which may be the same or different each is a substituent selected from the group consisting of hydrogen, a C₁₋₄ alkyl group, a C₅₋₆ cycloalkyl group and a C₆₋₁₀ aryl group. The process according to claim 1 wherein the nickel-containing catalyst contains both nickel and diatomaceous earth. The process according to claim 1 wherein the catalyst containing nickel or copper is selected from the group consisting of Raney nickel and Raney copper. The process according to claim 1 wherein said alkali metal is sodium or potassium.

Inventors: IMANARI, MAKOTO, IWANE, HIROSHI, KUZIRA, KATSUHUMI, SETO, TAKATOSHI
Applicants: RESEARCH ASSOCIATION FOR UTILIZATION OF LIGHT OIL
Application year: 1985
Main classification: C07D209
IPC codes: C07D209, C07B61, B01J23