MXPA00011305A - Process for the selective oxidation of alcohols using easily separable nitroxyl radicals - Google Patents

Abstract

Selective oxidation of alcohols to ketones with an alkali hypohalide comprises using an insoluble heterogeneous oxidation catalyst or a 4-oxy-2,2,6,6-tetramethylpiperidin-1-oxyl 4-oxy-bound to a Merrifield polymer. Selective oxidation of alcohols to ketones with an alkali hypohalide under alkaline conditions comprises using an insoluble heterogeneous oxidation catalyst of formulas (I), (II) or (III) or a 4-oxy-2,2,6,6-tetramethylpiperidin-1-oxyl 4-oxy-bound to a Merrifield polymer;n=3-3000. An Independent claim is included for the compounds per se.

Description

PROCEDURE FOR SELECTIVE OXIDATION OF ALCOHOLS WITH NITROXYL RADICALS OF EASY DISSOCIATION The invention relates to a process for the selective oxidation of alcohols to obtain aldehydes and ketones, with a heterogeneous, easily dissociated oxidation catalyst, based on a nitroxyl radical, using an alkali metalhalogenite as the oxidation agent. Alcohols are among the most important organic synthesis components. A wide range of preparatory methods for their production make the primary and secondary alcohols the ideal precursors for the synthesis of aldehydes, ketones and carboxylic acids. The usual oxidation agents are heavy metal reactants such as chromium (VI), lead (IV) and ruthenium, manganese and vanadium compounds, peracids, activated dimethylsulfoxide (DMSO) and hypervalent iodine compounds. The selectivity plays a preponderant role in these oxidations. Usually, under the selected conditions, the other functional groups present in the molecule, such as double bonds, will not be affected. Often the targeted oxidation of secondary alcohol functions is also desired along with primary and vice versa, without affecting the respective other function respectively. In the synthesis of aldehydes from primary alcohols, carboxylic acids are frequently obtained as by-products of the oxidation reaction (over-oxidation), the oxidation of 1,2-diols or α-hydroketones often leads to C-C cleavage reactions. Another disadvantage of many oxidants consists of their obtaining or handling often laborious or difficult; In addition, mainly reagents containing heavy metals generally have high toxicity and damage the environment. Finally, the costs of an oxidation process play a decisive role especially for an industrial application. It is known that the primary and secondary alcohols can be converted into the corresponding carbonyl compounds in the presence of catalytic amounts of organic nitroxyl radicals with aqueous sodium hypochlorite solution (AEJ from Nooy, AC Besemer, H. van Bekkum, Synthesis, 1996 , 1153). So far, these reactions have been carried out mainly in a homogeneous phase, in particular using 2, 2, 6, 6-tetramethylpiperidin-l-oxyl (TEMPO). The reactions are carried out either stoichiometrically or catalytically in relation to the TEMPO or its resulting oxidation product. In the above, the processing of the reaction products is often laborious, since the catalyst and its accompanying products must be dissociated with difficulty. On the other hand, oxidations with immobilized or easily dissociated nitroxyl compounds have not been described to date. It has now been surprisingly found that alcohols, using as catalysts certain high molecular weight 2,2,6,6-piperidin-1-oxylenes, or oligomeric or polymer-bound, with sodium hypochlorite as an oxidant, can be used as a catalyst. react with good yields to obtain the corresponding carbonyl compounds. The 1,3-aliphatic diols, in the presence of an aldehyde or of a ketone, under suitable reaction conditions in basic medium, can be reacted directly to obtain the corresponding cyclic acetals, or, ketals (1,3-dioxanes) . Depending on the reaction conditions, the 1,5-diols are converted into tetrahydropyran-2-ols, or their ethers or into tetrahydropyran-2-ones (d-valerolactones). The hydroxyl functions in position a with respect to carboxyl functions are not affected. In this process, an addition of bromide can be eliminated without any disadvantage, which, in the case of oxidation carried out homogeneously with the hypochlorite / TEMPO system, produces a considerable acceleration of the reaction (SD Rychnovsky, R. Vaidyanathan, J. Org. Chem., 1999, 64, 310). The advantages of the present process consist in the simplified processing of the reaction loads, in the repeated reuse of the catalyst and in the elimination of bromide as the accelerating substance of the reaction. An object of the invention is a process for the selective oxidation of alcohols to obtain ketones or aldehydes with an alkali metalhalogenite under alkaline conditions, characterized in that the oxidation is carried out in the presence of a heterogeneous oxidation catalyst insoluble in the reaction medium, selected from of the group containing the compounds of formulas (I), (II), (III) where n is a number from 3 to 3000; or a 4-oxy-2, 2,6,6,6-tetramethylpiperidin-1-oxyl bound to a Merrifield 4-oxy polymer. Preferably, n is a number from 10 to 1000, in particular from 10 to 500 and especially from 10 to 100. Preferred is a process characterized in that LiOCl, NaOCl, KOC1, LiOBr, NaOBr or KOBr are used as the alkali metalhalogenite. Especially preferred are LiOCL, aOCl and KOC1, in particular NaOCl. Preferably, the oxidation agent is added as an aqueous solution to the alcohol to be oxidized. The concentration can vary within wide limits and is preferably between 5 and 20% by weight of active chlorine, in particular from 10 to 15% by weight, based on the alcohol to be oxidized. Together with the oxidation agent, the aqueous solution can be adjusted alkaline with a base. Preferred bases are aqueous solutions of alkali metal or alkaline earth metal hydroxides, or the corresponding hydrocarbons. Alkalihydrocarbonates, in particular sodium hydrocarbonate, are especially preferred. The pH of the aqueous oxidation solution, after adding the desired base, is preferably in the range of 8 to 12, especially in the range of 9 to 11 and in particular in the range of 9 to 10. Alcohol by oxidising it can be liquid or solid.

In the case of liquid alcohols, the reaction can be carried out without addition of other solvents, however, it may be convenient to oxidize at stronger dilutions. In any case, the solid alcohols require a suitable organic solvent. Suitable organic solvents or solvent mixtures are those not miscible with water. Examples are aliphatic hydrocarbons, aromatic hydrocarbons, chlorinated hydrocarbons or mixtures of these solvents with ketones, amides or esters. Preferred solvents are aromatic hydrocarbons or mixtures thereof with ketones. Preferred examples are benzene, toluene or the xylene isomers, which may optionally be mixed with acetone.

The mixing ratio can be from 10: 1 to 2: 1, preferably from 5: 1 to 2: 1. A toluene / acetone mixture in the ratio of 3: 1 is particularly preferred. A process in which a 4-oxy-2, 2,6,6-tetramethylpiperidin-1-oxyl bound to a Merrifield 4-oxy polymer is used is preferred. The so-called Merrifield polymers are known to the person skilled in the art and can be obtained commercially. It is chlormethylated polystyrene, which is partially crosslinked with divinylbenzene and, thus, is insoluble in the usual organic solvents. The degree of crosslinking can be, for example, from 1 to 5%, usually from 1 to 2%. The size of the particles can vary within wide limits, being normally from 100 to 400 mesh. The chlorine content is, for example, 0.2 to 5 mmoles / g, the customary polymers contain 0.6 to 4 mmoles / g. The Merrifield polymer and the exchange of the chlorine atom can be described schematically as shown below: -CH-O- N-O. Preferably, the heterogeneous oxidation catalyst is used in amounts of 0.1 to 20% by weight, in particular 1 to 10% by weight and in particular 2 to 6% by weight, based on the alcohol used. A process in which a two-phase solvent system is used, wherein one phase is aqueous and contains the oxidizing agent, is preferred. The solvents, or mixtures of suitable solvents and their preferences have already been described above. Preferably, the reaction is carried out at a temperature of less than 10 ° C. A temperature range of about 0 ° C to 10 ° C is especially preferred. The preparation of the compounds of the formulas (I) to (III) and the preparation of the modified Merrifield polymer are carried out according to methods known per se, according to the following reaction scheme. In a first step, the compound 3 (4-hydroxy-2, 2,6,6-piperidin-1-oxyl (4-hydroxy-TEMPO)) is reacted with sodium hydride. The 4-hydroxy-TEMPO itself can be obtained commercially.

According to the desired final product, the compound 4 is still reacted according to reaction 2, 3, 4 or 5.

The compounds of the formulas (II) and (III) are novel and also constitute an object of the invention. Another object of the invention is the use of a compound of the formula (I), (II), (III) or a 4-oxy-2, 2,6,6-tetramethylpiperidin-1-oxyl bound to a Merrifield polymer, as a catalyst for the selective oxidation of alcohols to obtain ketones, with an alkalihipohalogenite under alkaline conditions. The following examples illustrate the invention.

Example A: Obtaining the oxidation catalysts EXAMPLE To the Obtaining of Compound 6. In a 500 ml agitation vessel, 20 g (113.6 mmol) of 4-hydroxy-TEMPO 3 are dissolved in 200 ml of anhydrous toluene and then 3.34 g (139.2 mmol) of sodium hydroxide are added in small portions. NaH The reaction mixture is stirred for approximately 12 h at room temperature and a solution of 4.75 g (25.75 mmoles) of cyanide chloride 5 in approximately 50 ml of toluene is subsequently added dropwise. First, the solution is stirred 2.5 h at room temperature and then 72 h at about 70 ° -90 ° C. After cooling to room temperature, the organic phase is washed three times with respectively 100 ml of a 10% aqueous solution of Na 2 CO 3 and subsequently dried by Na 2 SO 4. All the volatile components are evaporated at 10"2 Torr under vacuum and the residual red oil is recrystallized from ethyl acetate to obtain 9.3 g (61%) of product in the form of fine red-orange needles. fusion 164 to 166 ° C. The product is insoluble in H20 but soluble in CH2C12, CHC13, C6H5C1, toluene and ethyl acetate.

EXAMPLE A2 Obtaining compound 8. In a 500 ml agitation vessel, 20 g (113.6 mmol) of 4-hydroxy-TEMPO 3 are dissolved in 200 ml of anhydrous THF and then 3.34 g (139.2 mmol) of solution are added in small portions. NaH The reaction mixture is stirred for approximately 12 h at room temperature and subsequently a solution of 6.26 g (18 mmol) of hexachlorocyclotriphosphase 7 in approximately 50 ml of THF is added dropwise and the reaction mixture is heated for 24 h under reflux at 70 ° C. . After cooling to room temperature, the volatile components are evaporated in vacuo (approximately 10 2 Torr) and the residue is dissolved in 100 ml of CH 2 C 12, the organic phase is washed twice with respectively 50 ml of a 10% aqueous solution. NaOH and then respectively three times with about 50 ml of H20.The organic phase is dried with Na2SO4 and the volatile components are evaporated in vacuo (about 10"2 Torr). A powdery red-orange solid is obtained, which is insoluble in H20 and hexanes and of relatively good solubility in THF, CHC13 and CH2C12.

Example A3 Preparation of compounds 10. In a 500 ml stirring vessel, 29.62 g (168.3 mmoles) of 4-hydroxy-TEMPO 3 are dissolved in 200 ml of anhydrous THF and then 5.0 g (21.1 mmoles) of NaH are added. It is left stirring 12 h at room temperature and subsequently this reaction mixture is dripped into a solution of 5 g (43.1 mmoles) of poly (dichlorophosphates) 9 in 100 ml of THF. The reaction mixture is left stirring for 12 h at room temperature and then heated for two hours under reflux. Then, the reaction mixture is concentrated to about 10% of the volume and poured into ice water. The polymer that precipitates is filtered and re-treated with ice water. The pulverulent salmon colored precipitate is filtered, then washed with a mixture of THF and H20 (20/80) and then with hexane. The powder that forms is dried for approximately 12 h under vacuum. 16 g are obtained (95.7%) of the product 10; melting point > 180 ° C. Compound 10 is soluble in CH2C12, CHC13, acetone, THF and toluene, Mw approximately 25'000.

Example A4 Preparation of compound 12: In a 500 ml stirring vessel, 7 g (40.63 mmoles) of 4-hydroxy-TEMPO 3 are dissolved in 120 ml of DMF or THF (freshly distilled). At 0 ° C, 1.6 g (66.67 mmoles) of NaH are added to the solution. Allow to warm to room temperature and stir 1 h. Then, the reaction mixture is cooled to 0 ° C in an ice bath and 3.5 g of polymer are added (Merrifield polymer from Fluka company, 200-400 mesh, 1% divinylbenzene, 1.7 mmole Cl / g). Stir 30 minutes at 0 ° C and then allow to warm to room temperature. The load is stirred 1 to 4 days. It is then diluted with ice water, stirred and filtered. The residue is washed with ice water until the filtrate is colorless. Subsequently, the product is suspended in toluene and stirred 1 to 2 h in order to remove unfixed 4-hydroxy-TEMPO. Then it is filtered again and the yellow powder is dried in the air jet. It contains 0.9 mmoles of N-oxyl / g.

Examples B: Oxidation of alcohols General prescription for the oxidation of a primary or secondary alcohol. a) Obtaining the NaOCl / NaHC03 solution: 4 ml of a saturated solution of NaHCO3 are mixed with 2 ml of a NaOCl solution (13-14%). The solution is stored at 0 ° C in a closed bottle. b) Oxidation requirement: 1.0 g of the alcohol and 0.25 g of the compound (12) of Example A4 are placed in a 250 ml round flask and then suspended in a mixture of 5 ml of acetone and 15 ml of toluene. The load is stirred vigorously for 5 to 10 minutes at room temperature, until the alcohol has dissolved and the resin swollen. The reaction vessel is cooled to 0 ° C. Under intense agitation, 6 ml of the NaOCl / NaHCO3 solution obtained in point 1) are added and left stirring 0.5 h at 0-5 ° C. After the usual processing, the ketone is obtained in quasi-quantitative yield. c) Recovery of the catalyst If the catalyst is to be used in several successive reactions, it is filtered after the reaction (fried G3), washed briefly with acetone and reused immediately. After use number ten, no loss of activity was observed. The catalytic activity of the compound (12) can also be maintained after storage. For this, the filtered catalyst is washed several times with acetone / toluene, then with water, again with acetone and finally with acetone / toluene (elimination of NaOCl, educt and product). The storage is made wet in a hermetic container.

Examples Bl and B2 Oxidation of simple alcohols: 13 14 OH O ^ 1 ^ NaOCI / cat. ^ Jl ^ 17 18 The tests are performed as described in the general prescription. In this, a-hydroxyketone 14 from 13 and acetone 18 from isopropanol 17 are obtained in almost quantitative yield.

Example B3 Obtaining a-hydroxyacetophenone from phenylglycol 19 14 Phenylglycol 19 is reacted as described in the general prescription with NaOCl in the presence of 5 weight percent of 12 as a catalyst, to obtain α-hydroxyacetophenone (20). The product 20 is isolated as a crystalline solid in almost quantitative yield.

Example B4 Obtaining acetylacetone: In a 250 ml round flask, 1.0 g of alcohol 21 and 0.25 g of compound 12 are placed, then 20 ml of toluene are added. The reaction vessel is then cooled to 0 ° C. Under vigorous stirring, 6 ml of NaOCl / NcHC03 described above are added and the mixture is left stirring at 0-5 ° C for half an hour. The 1,3-diketone 22 is obtained as a reaction product in almost quantitative yield.

Example B5 Obtaining 2, 2, 4, 6-tetramethyl-l, 3-dioxane (23): 21 23 The reaction is carried out as described in example 4, with the difference that the reaction is carried out instead of with 20 ml of toluene, in a mixture of 15 ml of toluene and 5 ml of acetone. As the reaction product, 2, 2, 4,6-tetramethyl-1,3-dioxane (23) is obtained in almost quantitative yield by gas chromatography.

Example B6 Obtaining 2-ethyl-2,4,6-trimethyl-l, 3-dioxane (24): The reaction is carried out as described in example B4, with the difference that instead of 20 ml of toluene a mixture of 15 ml of toluene and 5 ml of 2-butanone is used. As the reaction product, 2-ethyl-2,4,6-trimethyl-1,3-dioxane (24) is obtained in almost quantitative yield.

Example B7 Obtaining tetrahydropyran-2-ol (21) and tetrahydropyran-2-one (22) 26 27 1,5-pentanediol is oxidized as described in example B4 with basic NaOCl in the presence of compound 12 as a catalyst, with the difference that the oxidizing agent sodium hypochlorite (13% active chlorine) is used stoichiometrically . After 30 minutes of reaction time, a 75% mixture of 26 and 25% of 27 is obtained.

Example B8 Obtaining tetrahydropyran-2-one (27) 26 27 The mixture of 26 and 27 obtained in example B7 is reacted as described with an excess of a NaOCl solution adjusted to a pH of 9.0-9.5. In the above, 26 is completely oxidized to 27.

Example B9 Obtaining 2- (5-hydroxy-pentoxy) -tetrahydropyran (26) 28 26 The reaction described above is carried out with a fourfold excess of diol 25 relative to NaOCl. After the usual processing, a mixture of 50% of acetal 28 and 50% of tetrahydropyran-2-ol is obtained.

Example B10 Obtaining benzyl (29) from benzoin: . 0 g (23.5 mmoles) of benzoin (28) are reacted as described in the general prescription with sodium hypochlorite in the presence of 1 g of compound 12, at 0-5 ° C, in 50 ml of toluene / acetone 3: 1. During the addition of the hypochlorite, the solid that is in the reaction mixture dissolves. After the addition is complete, stirring is continued for another 45 minutes. The revision by thin layer chromatography shows a total and selective reaction. After the usual processing and subsequent recrystallization of the crude product from n-hexane, 4.6 g of benzyl (29) (93% of theory) are obtained.

Claims (8)

1. A process for the selective oxidation of alcohols to obtain ketones or aldehydes with an alkali metalhalogenite under alkaline conditions, characterized in that the oxidation is carried out in the presence of a heterogeneous oxidation catalyst insoluble in the reaction medium, selected from the group containing the compounds of the formulas (I), (II), (III): where n is a number from 3 to 3000; or a 4-oxy-2, 2,6,6,6-tetramethylpiperidin-1-oxyl bound to a Merrifield 4-oxy polymer.

2. A process according to claim 1, characterized in that LiOCl, NaOCl, KOC1, LiOBr, NaOBr or KOBr are used as alkalihipohalogenite.

3. A process according to claim 1, characterized in that a 4-oxy-2,4,6,6-tetramethylpiperidin-1-oxyl bound to a Merrifield 4 -oxi polymer is used.

4. A process according to claim 1, characterized in that the heterogeneous oxidation catalyst is added in an amount of 0.1 to 20% by weight, based on the alcohol used.

5. A process according to claim 1, characterized in that a two-phase solvent system is used, being an aqueous phase and containing the oxidation agent.

6. A process according to claim 1, characterized in that the reaction is carried out at a temperature of less than 10 ° C.

7. Compounds of formulas II and III according to claim 1.

8. The use of a compound of the formulas (I), (II), (III) or a 4-oxy-2, 2, 6, 6 -tetramethylpiperidin-1-oxyl bound to a Merrifield 4-oxy polymer, as a catalyst for the selective oxidation of alcohols to obtain ketones, with an alkalihipohalogenite under alkaline conditions. SUMMARY OF THE INVENTION The invention relates to a process for the selective oxidation of alcohols to obtain ketones or aldehydes with an alkali metalhalogenite under alkaline conditions, characterized in that the oxidation is carried out in the presence of a heterogeneous oxidation catalyst insoluble in the reaction medium, selected from the group containing the compounds of the formula (I): where xi is a number from 3 to 3000; or a 4-oxy-2,2,6,6-tetramethylpiperidin-1-oxyl bound to a Merrifield 4-oxy polymer. Other objects of the invention are the compounds of the formulas (II) and (III) as well as the use of the aforementioned oxidation catalysts for the oxidation of alcohols. . "Ia l..¿: >