Process for the selective reduction of 2-quinoxalinol-4-oxides to 2-quinoxalinols

Abstract: Certain 2-quinoxalinol-4-oxides are selectively reduced to form 2-quinoxalinols employing hydrogen as a reducing agent by reacting such 2-quinoxalinol-4-oxides with a catalyst selected from the group consisting of sulfided platinum, sulfided palladium, sulfided rhodium and sulfided ruthenium.

Description: Field of the Invention This invention is directed to an improved process for the selective reduction of certain 2-quinoxalinol-4-oxides to form 2-quinoxalinols employing sulfided platinum as catalyst and employing hydrogen as the reducing agent. Background of the Invention 2-quinoxalinol compounds, such as 6-chloro-2-hydroxyquinoxaline, are well known intermediates for the production of pharmaceutically and agriculturally effective chemicals. These compounds are generally prepared by the selective reduction of 2-quinoxalinol-4-oxides. Selective reduction is necessary so that excessive reduction, resulting in the formation of compounds such as 3,4-dihydro-2-quinoxalinol, can be avoided. This is particularly a problem with halogenated 2-quinoxalinol compounds wherein excessive hydrogenation will eliminate the halogen substituent. United States Patent No. 4,620,003 to Ishikura discloses a process for the reduction of 2-quinoxalinol-4-oxide compounds to 2-quinoxinol compounds, which process involves reacting a 2-quinoxalinol-4-oxide with hydrazine in the presence of (i) a Raney catalyst (especially Raney nickel or sulfided Raney nickel) and (ii) a alkali metal hydroxide, an alkaline earth metal hydroxide, or ammonium hydroxide. While this process reduces the 2-quinoxalinol-4-oxides in desirable efficiencies, a major drawback in the commercial use of the Ishikura process is the relatively high cost of hydrazine relative to other potential reducing agents, in particular hydrogen. While United States Patent No. 4,636,562 to Davis discloses a process for preparing 2-chloro-6-haloquinoxaline compounds from the corresponding 4-halo-2-nitroaniline, which process includes a step involving the reduction of 6-chloro-2-hydroxyquinoline-4-oxide to 6-chloro-2-hydroxyquinoline employing hydrogen as a reducing agent (in the presence of an aqueous alkali metal hydroxide solvent and a transition element metal hydrogenation catalyst, preferably Raney nickel), this Davis process is also not desirably commercialized. This is because Davis indicates that hydrogen pressures of 1-4 atmospheres are effective with pressures of 1-2 atmospheres being preferred. It is well known to those of ordinary skill in the art that processes involving gaseous hydrogen at such low pressures are inherently dangerous because there is a risk of air leaking into the system. If hydrogen and oxygen contact the hydrogenation catalyst together, water is formed in an explosive manner. Sakata et al, "The Facile One Pot Synthesis of 6-Substituted 2(1H)-quinoxalinones", Heterocycles , Vol. 23, No. 1, (1985), 143-151, disclose a process for the reduction of 6-chloro-2-quinoxalinol-4-oxide to 6-chloro-2-quinoxalinol employing hydrogen as a reducing agent and palladium as a catalyst. However, Sakata et al indicate that an undesirable amount of overly reduced by-product is formed. Consequently, it would be desirable to possess a process for the selective reduction of 2-quinoxalinol-4-oxides to 2-quinoxalinols safely and efficiently employing hydrogen as a reducing agent. Accordingly, it is an object of this invention to provide a method of selectively reducing 2-quinoxalinol-4-oxides to 2-quinoxalinols employing hydrogen as a reducing agent. It is a further object of this invention to provide a high pressure method of selectively reducing 2-quinoxalinol-4-oxides to 2-quinoxalinols employing hydrogen as a reducing agent such that the danger of air leaking into the system is minimized. The above objects and additional objects will become more fully apparent from the following description and accompanying examples. Description of the Invention The present invention is directed to a process for preparing a 2-quinoxalinol compound of the formula: wherein R is hydrogen, halogen or trihalomethyl; which process comprises reducing a 2-quinoxalinol-4-oxide compound of the formula: wherein R is as defined above, with hydrogen at a pressure of 6 x 10 5 to 15 x 10 5 Nm -2 (6 to 15 atmospheres) at a temperature of 20 to 60°C and in the presence of an amount of 0.5 to 1.36 parts of sulfided platinum per 100 parts of said oxide in a stirred autoclave. The 2-quinoxalinol-4-oxide compounds employed as starting materials in the process of this invention are of the formula: wherein R is hydrogen, halogen or trihalomethyl. Preferably, R is hydrogen, chlorine, fluorine, trichloromethyl or trifluoromethyl. A particularly preferred starting material is 6-chloro-2-hydroxyquinoxaline-4-oxide, which is subsequently reduced to 6-chloro-2-hydroxyquinoxaline. These starting materials may be in isolated form or may be present as a reaction product. Thus, for example, 6-chloro-2-hydroxyquinoxaline-4-oxide may be present as the product of the reaction of 4-chloro-2-nitroacetoacetanilide with a hydroxide (such as sodium hydroxide or potassium hydroxide). The process of this invention is catalyzed by sulfided platinum as hydrogenation catalyst. The catalyst is employed in amounts of 0.5 to 1,36 parts by weight per 100 parts by weight of 2-quinoxalinol-4-oxide. When the starting material employed in the process of this invention is the reaction product of a nitroacetoacetanilide and a hydroxide, the reaction of this invention is conducted in a solution containing at least one compound selected from the group consisting of alkali metal hydroxides, alkaline earth metal hydroxides and ammonium hydroxide. Water is preferred as a reaction solvent but organic solvents (such as alcohols or hydrocarbons) or solvent mixtures of water with an organic solvent may also be employed. The process of this invention is carried out at pressures between (6 and 15 atmospheres) 6 x 10 5 and 15 x 10 5 Nm -2 . Reaction temperatures may vary widely, with temperatures ranging between 20° and 60°C. The reaction temperatures and pressures which are most preferably employed (e.g. about 60°C and 8 to 15 x 10 5 Nm -2 (8 to 15 atmospheres)) will vary in accordance with a number of factors including the starting material employed; the catalyst concentration; and the like. As is apparent to one skilled in the art, reaction pressures and temperatures which are too high will reduce the selectivity of the reaction and produce products which may be too greatly reduced. Conversely, reaction temperatures and pressures which are too low will reduce the activity of the catalyst. However, one of ordinary skill in the art may readily determine the optimum reaction conditions for a particular set of reactants by routine experimentation. By employing methods well known to those skilled in the hydrogenation art, the process of this invention may be employed in a continuous or in a batch manner. Because the process of this invention employs hydrogen as a reducing agent, it is greatly more economical than prior art processes employing relatively expensive chemicals (such as hydrazine) as reducing agents. Moreover, because the process of this invention may be run at high pressures, the danger of an explosion occurring due to the leaking in of air is greatly reduced. Examples The following Examples are intended to further illustrate the invention. Examples 1-2 and Comparative Experiments A and B and C3 to C5 To a 300 ml autoclave containing agitation means was added 125 ml of a stock solution containing 9.2 grams of 6-chloro-2-hydroxyquinoxaline-4-oxide (which had been produced by the reaction of 482.4 grams of 4-chloro-2-nitroacetoacetanilide with 813 grams of 86.9 percent potassium hydroxide in 3200 grams of water) in a 17.6 weight percent aqueous potassium hydroxide solution. The amount and type of catalyst indicated in Table I below was added and the autoclave sealed. In this regard, it is to be noted that, with the exception of Comparative Experiment B (NiS), all catalysts were 5 weight percent on carbon. The autoclave was purged, first with nitrogen and subsequently with hydrogen. Hydrogen was added until the pressure was as indicated in Table I below. The autoclave was heated with agitation for the indicated reaction time, and was then cooled and depressurized. The reaction product was removed with water, filtered through celite (to remove catalyst), and dried. The reaction product was analyzed, and the yield (based upon the complete cyclization, reduction, neutralization, filtration and drying steps) calculated. The results of such analyses are presented in Table I. Reduction of 6-Chloro-2-Hydroxyquinoxaline-4-oxide to 6-Chloro-2-Hydroxyquinoxaline Employing H 2 as the Reducing Agent Example or Comparative Experiment Catalyst Type Catalyst Amount(g) Reaction Temperature (°C) Reaction Pressure 10 5 Nm -2 (psig) Reaction Time (hours) Percent H 2 Absorbed Percent Yield 1 Sulfided Pt 0.125 60 7-14 (100-200) 5.0 100 79 2 Sulfided Pt 0.125 60 7-14 (100-200) 4.0 100 78 C3 Sulfided Pd 1.9 20 7-14 (100-200) 5.8 117 43 C4 Sulfided Rh 2.0 20 41-48 (600-700) 2.0 114 61 C5 Sulfided Ru 1.9 60 43-50 (620-720) 5.9 157 24 A Pd 1.0 30 31-48 (450-700) 0.08 232 <0.5 B Sulfided Ni 3.8 60 46-52 (660-760) 5.0 113 19 The above data indicate the unexpectedly improved yield of selectively reduced product achieved by the process of this invention which employs a sulfided platinum catalyst relative to catalysts of the type or similar to the type employed in the prior art.

Claims: A process for preparing a 2-quinoxalinol compound of the formula: wherein R is hydrogen, halogen or trihalomethyl; which process comprises reducing a 2-quinoxalinol-4-oxide compound of the formula wherein R is as defined above, with hydrogen at a pressure of 6 x 10 5 to 15 x 10 5 Nm -2 (6 to 15 atmospheres) at a temperature of 20°C to 60°C in the presence of an amount of 0.5 to 1.36 parts of sulfided platinum per 100 parts of said oxide in a stirred autoclave. A process in accordance with claim 1 wherein said 2-quinoxalinol-4-oxide is in a solution further comprising at least one compond selected from the group consisting of alkali metal hydroxides, alkaline earth metal hydroxides and ammonium hydroxide. A process in accordance with claim 2 wherein said solution is an aqueous solution. A process in accordance with claim 1 wherein R is selected from the group consisting of hydrogen, chlorine, fluorine, trichloromethyl and trifluoromethyl. A process in accordance with claim 1 wherein the starting material is 6-chloro-2-hydroxyquinoxaline-4-oxide. A process in accordance with claim 1 wherein the catalyst is 5 weight percent sulfided platinum on carbon. A process in accordance with claim 1 for preparing 6-chloro-2-hydroxyquinoxaline from 6-chloro-2-hydroxyquinoxaline-4-oxide which process comprises reacting 6-chloro-2-hydroxyquinoxaline-4-oxide with hydrogen in the presence of (a) a catalytically effective amount of sulfided platinum and (b) an aqueous solution of an alkali metal hydroxide at between 20°C and 60°C and at from 6 x 10 5 Nm -2 to 15 x 10 5 Nm -2 . A process in accordance with claim 7 wherein said reaction occurs at about 60°C and at from 8 x 10 5 to 15 x 10 5 Nm -2 (8 to 15 atmospheres).

Inventors: FEICCABRINO JOSEPH ALBERT, MALZ RUSSELL EDWARD JR, SARGENT JOHN WINTHROP, FEICCABRINO, JOSEPH ALBERT, MALZ, RUSSELL EDWARD, JR., SARGENT, JOHN WINTHROP
Applicants: UNIROYAL CHEM CO INC, UNIROYAL CHEMICAL COMPANY, INC.
Application year: 1988
Main classification: C07D241
IPC codes: B01J27, C07B61, C07D215, C07D241