Process for the preparation of diethers

Abstract: Preparation of diethers of the formula wherein R is an alkyl, cycloalkyl, aryl, or aralkyl radical, by formation of an unsaturated aldehyde of the formula wherein R is as defined above, by means of an aldol condensation reaction, reduction of said aldehyde to form the corresponding saturated compound, transformation of the saturated compound into the corresponding diol, and consequent methylation of the diol into diether.

Description: The present invention concerns a process for the preparation of diethers of the formula where R is a linear or branched C₁₋₁₈ alkyl, C₃₋₁₈ cycloalkyl, C₆₋₁₈ aryl or C₇₋₁₈ arylalkyl group; preferably R is isopropyl. It is already known how to prepare diethers of the formula (I) according to methods described in published European application EP-A-361 493, corresponding to US Serial 413,409. Said methods comprise an aldol condensation reaction carried out in the presence of an alkaline-metal hydroxide or an alkaline-earth-metal hydroxide at high temperature according to the description in German patent DE-C-643341. For many reasons, however, the above mentioned methods are not very adequate for commercial application. For example, as hereinafter shown, the method described in said German patent gives high amounts of high-boiled by-products in the case of the synthesis of certain aldehydes. For example, the patent EP-application mentioned above describes the preparation of 2-isoamyl-2-isopropyl-1,3-dimethoxypropane in four steps with a total yield of 12%. Moreover, the final methylation of the diol is obtained using extremely expensive reagents. By operating according to the process of this invention, on the other hand, one can obtain the ethers (I) in high yield with a simplified process, using unexpensive reagents. The essential features of the process of this invention are defined in claim 1. Some specific embodiments of the process of this invention are defined in dependant claims 2 and 3. The reactions which occur in the process are represented as follows: The aldol condensation is carried out in the presence of an ion-exchange resin with an extremely basic function, for instance quaternary ammonium hydroxide. An example of such resin is Amberlite IRA 910 (produced by Rohm & Haas). It is also possible to use a resin in the form of quaternary ammonium chloride mixed with an equivalent amount of NaOH. Operating under these conditions it is possible to limit the formation of polycondensates. In fact, as shown in Comparative Examples 2 and 3, if one uses NaOH, even when operating at very low conversions, the quantity of undesired polycondensates expressed in percentage on the aldehyde (II) obtained, are respectively 25.7% and 23.5%, compared to 5.8% of Example 1 (where the resin used was in the OH form), and 8.7% of Example 4 (where the resin used was in the form of Cl⁻ + NaOH). The resin can be recycled repeatedly without appreciable loss of activity. Aldehyde (II) is selectively reduced with Pd on charcoal catalyst to (III) with practically quantitative yields. Reduction times, with equal quantities of catalyst, are noticeably reduced by adding small quantities of aqueous solutions of alkaline bicarbonates. As shown in Example 5 and Comparative Example 6, the reduction times after adding an aqueous saturated solution of NaHCO₃ go from 80 hours to 1 hour. In the methylation of the diol (IV) the ether (I) obtained will not mix with the dimethyl sulfoxide: thus the separation and recovery of the solvent are simplified. A preferred embodiment of the process of the invention is carried out in the following manner: a) isovaleraldehyde is heated in the presence of an ion-exchange resin with an extremely basic functionality (form OH⁻), or, as an alternative, a resin in the form of quaternary ammonium chloride with the same amount of NaOH, to a temperature between 70°C and 120°C, removing the water continuously as it forms. The aldehyde (II) is separated from the resulting reaction mixture by means of fractional distillation; b) (II) is hydrogenated in the presence of a solvent at room temperature and at a pressure from 10⁵ to 10⁶ Pa (from 1 to 10 ata), preferably from 2 × 10⁵ to 3 × 10⁵ Pa (from 2 to 3 ata), in the presence of a palladium on charcoal catalyst and small quantities of an aqueous solution of alkaline bicarbonate (1-10% by weight of saturated solution). Solvents suitable for use in this process are alcohols, such as CH₃OH, ethanol, isopropanol and butanol. Preferably methanol and ethanol are used. The concentration of (II) in the solvent can vary from 10 to 50%, preferably it is from 15 to 20%. The quantity of catalyst can vary from 0.2 to 3 g Pd/Kg of (II). The hydrogenation rate (at equal pressure and temperature) is proportional to the quantity of catalyst used. The alcohol solution of (III), after removal of the catalyst by filtration, is used directly in the following step without further purification. c) The alcohol solution of (III) with a CH₂O aqueous solution is heated in the presence of an inorganic base, and refluxed until the reaction is complete. The alcohol is recovered by distillation and the diol(IV) is separated from the aqueous phase wherein it is scarcely soluble, anhydrated and it is then used in the following step without further purification. Bases suitable for the reaction are NaOH, KOH, Na₂CO₃ and K₂CO₃. The CH₂O is added in a molar ratio of at least 2:1 with the aldehyde (III), preferably in a ratio of 3:1. d) The unpurified diol (IV) is dissolved in dimethyl sulfoxide (DMSO) and methylated with CH₃Cl in the presence of NaOH, at atmospheric pressure and a temperature from 30 to 40°C. The use of finely divided NaOH is preferred, since it allows for a faster reaction. The ether (I) is insoluble in the DMSO and after filtration of the salts, it is easily separated from the solvent and rectified. After dehydration (addition of toluene and azeotropic distillation of the toluene/H₂O mixture for example), the DMSO can be recycled. The following examples are given in order to illustrate and not limit the invention. Example 1 Preparation of 2-isopropyl-5-methyl-2-hexenal [II, R = (CH₃)₂CH]. 2070 g of isovaleraldehyde and 260 cc of Amberlite IRA 910 resin (produced by Rohm & Haas) in the form of OH⁻ (prepared from the Cl⁻ form by washing with NaOH until no Cl⁻ appears in the wash and then with H₂O until it is neutralized) are heated in reflux. The water that forms is continuously removed with a Marcusson distiller. After 3 hours about 260 cc of H₂O are collected, and the temperature in the boiler goes from the initial 79°C to 115°C. The resin is separated by filtration and the reaction product is fractioned under vacuum. The following products are obtained: 444 g of isovaleraldehyde which can be recycled, an intermediate fraction of 50 g, a fraction of 1300 g made up essentially (II) having boiling point 85-90°C/2666 Pa (85-90°C/20 mmHg) and 75g of nondistillable residue of polycondensate. The gas chromatographic titre of (II) is 97% (cis and trans isomers in an approximate 20:80 ratio.). The quantity of high boiling point by-products is 5.8% with respect to the aldehyde (II) obtained. Example 2 (Comparative Example) The procedure and ingredients of Example 1 are used except that 20.7 g of NaOH dissolved in 250 cc of H₂O are used instead of the resin. After fractionation, 1233 g of aldehyde (II) and 317 g of residue are obtained. The percentage of residue with respect to the product (II) is 25.7%. Example 3 (Comparative Example) The procedure and ingredients of Example 2 are used except that 11.3 g of NaOH dissolved in 350 cc of H₂O are used. The reaction mixture is maintained in reflux for 5 hours (maximum temperature of the boiler 85°C). The organic phase is separated and distilled under vacuum. The results are 980 g of light product, mainly consisting of isovaleraldehyde, 811 g of (II) and 191 g of residue. The percentage of residue is 23.5% with respect to the desired product (II). Example 4 1400 g of isovaleraldehyde and 150 cc of wet Amberlite IRA 910 resin in Cl⁻ form, and 5 g of NaOH dissolved in 100 cc of H₂O are heated. They are heated in reflux distilling off, with a Marcusson apparatus, the water that forms during the process. After 3 hours the internal temperature reaches 90°C. Another 3 g of NaOH dissolved in 30 cc of H₂O are added and the distillation continues again for 5 hours until a temperature of 120°C is reached. The reaction mass is then cooled to 80°C and diluted with 200 cc of H₂O. The aqueous phase and the resin are then separated, and the unpurified reaction product is distilled. The results are 110 g of light products constituted primarily of isovaleraldehyde, 1025 g of aldehyde (II) and 90 g of residue. The percentage of residue is 8.7% with respect to the desired product (II). Example 5 Preparation of 2-isopropyl-5-methylhexanale (III, R = (CH₃)₂CH⁻]. Into a glass flask with two necks equipped with agitator, are introduced 10 g of the aldehyde of Example 1 dissolved in 70 cc of ethanol, 1 cc of a saturated NaHCO₃ aqueous solution and 0.25 g of 10% Pd on charcoal. The apparatus is fluxed with nitrogen, then with hydrogen, and connected to a graduated burette filled with hydrogen. The mixture is maintained under agitation at room pressure and temperature. The absorption of hydrogen stops at the stoichiometric value after 60 min. Example 6 (Comparative Example) Example 5 is repeated, but without adding the NaHCO₃ solution. Absorption is complete only after 80 hours. Example 7 A solution of 1300 g of aldehyde [II, R=(CH₃)₂CH] in 7 liters of ethanol and 80 cc of saturated NaHCO₃ solution are hydrogenated in a 20 liters autoclave in the presence of 25 g of 10% Pd on charcoal. The operation takes place at an initial H₂ pressure of 3 × 10⁵ Pa (3 Ata) reintegrating the gas as soon as its pressure decreases to 1.5 × 10⁵ Pa (1.5 Ata). The absorption is complete after 1 hour. The catalyst is filtered and the ethanol solution is used for the next reaction. The content of aldehyde [III, R=(CH₃)₂CH] calculated by gas chromatography is 97%. Example 8 Preparation of 2-isopropyl-2-(3-methylbuthyl)-1,3-dihydroxypropane [IV, R=(CH₃)₂CH]. The aldehyde solution of Example 7 is heated in reflux with a solution of 690 g of K₂CO₃ in 1700 cc of H₂O and 2200 cc of CH₂O aqueous at 40% for 7 hours. The ethanol is distilled by bringing the temperature in the boiler at 97-98°C. The organic phase is separated, washed with hot water until it is neutralized and the oil phase is dehydrated by means of azeotropic distillation with toluene. After evaporating the toluene under vacuum, 1455 g of a colorless oil [IV, R=(CH₃)₂CH] are obtained. The product does not have to be further purified and is used as is in the following reaction. Example 9 Preparation of 2-isopropyl-2-(3-methylbutyl(-1,3-dimethoxypropane [I, R=(CH₃)₂CH]. The diol obtained in Example 8 is dissolved in 3.5 litres of dimethyl sulfoxide in a flask equipped with a bubble valve for release of gas, a mechanical blade agitator, thermometer and tube for the introduction of gases. 460 g of finely divided NaOH are added while agitating and cooling with an external bath in order to keep the temperature from going above 30°C. CH₃Cl is added into the well stirred mass while maintaining the temperature between 30 and 35°C and regulating the addition so that the CH₃Cl does not escape from the discharge valve. After 2.5 hours, an additional 290 g of finely divided NaOH are introduced and after 5 hours another 415 g, of said NaOH. After about 8 hours the absorption of CH₃Cl ceases. The total CH₃Cl used is 925 g. After 10 hours a gas chromatographic control of the resulting reaction mixture shows the presence of 98.8% of diether [I, R=(CH₃)₂CH] and only 0.1% of monoether. The reaction mass is filtered and the salts are washed with hexane. The upper phase is separated from the DMSO, the hexane coming from the wash is added, it is washed repeatedly with H₂O, and dehydrated on anhydrous Na₂SO₄. After eliminating the hexane, one obtains, from the distillation, a single fraction of 1335 g of (I), with boiling point of 105-6°C/2000 Pa (105-6°C/15 mmHg]. The content of diether established by gas chromatography is 99.5%. The yield on the three stages, reduction, Cannizsaro and methylation, is 77.7%. After azeotropic dehydration with toluene the DMSO can be recycled. Example 10-12 Condensation, hydrogenation, Cannizzaro condensation, and etherification reactions on the feedstock shown in the Table are carried out, respectively, with the same process as described in Examples 1, 7, 8, and 9. Aldehyde (g) II (g) Aldehyde recovered (g) III (g) IV (g) I (g) CH₃(CH₂)₂CHO (1730) 1000 370 980 1050 940 CH₃(CH₂)₅CHO (2745) 1770 588 1720 1900 1700 C₆H₁₁-CH₂CHO (3030) 1900 640 1850 2030 1816

Claims: A Process for the preparation of a diether of the formula wherein R is C₁₋₁₈ alkyl, C₃₋₁₈ cycloalkyl, C₆₋₁₈ aryl or C₇₋₁₈ aralkyl radical, comprising the following steps: a) formation of an unsatured aldehyde of the formula wherein R is as defined above, by means of an aldol condensation reaction, wherein said reaction is carried out in the presence of an ionic exchange resin with an extremely basic function or in the presence of a resin in the form of quarternary ammonium chloride mixed with an equivalent amount of NaOH; b) reduction of said unsaturated aldehyde to a saturated compound of the formula wherein R is as defined above, and said reduction is carried out with hydrogen using Pd on charcoal catalyst in the presence of small quantities of aqueous solutions of alkaline bicarbonates; c) transformation of said saturated compound into a diol of the formula wherein R is as defined above, by means of a crossed Cannizzaro reaction with formaldehyde; d) methylation of said diol into the corresponding di-ether, wherein the said methylation is carried out in the presence of dimethyl sulfoxide. The process of claim 1, wherein R is selected from the group consisting of isopropyl, ethyl, butyl, isobutyl, cyclohexyl, cyclopentyl. The process of claim 1 or claim 2, wherein the reduction of the unsaturated aldehyde is carried out using, as a catalyst, Pd on charcoal suspended in an alcoholic solution.

Inventors: BORSOTTI GIAMPIETRO, BORSOTTI, GIAMPIETRO
Applicants: MONTELL NORTH AMERICA INC, MONTELL NORTH AMERICA INC.
Application year: 1991
Main classification: C07C43
IPC codes: C07C43, C07C47, C07C31, C07C45, C07C29, C07C41