RU2393272C1 - Electrochemical method of oxidising of alcohols to carbonyl compounds - Google Patents

Abstract

FIELD: chemistry. ^ SUBSTANCE: method involves preparation of a reaction mixture at room temperature consisting of the alcohol to be oxidised, sodium bicarbonate, an organic solvent and a nitroxyl radical. Electrolysis is carried out on platinum electrodes with current of 1 A and temperature of 20-25C. Potassium iodide is added to the reaction mixture. The organic solvent used is dichloromethane and the nitroxyl radical used is 4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxyl of formula: with ratio of alcohol to nitroxyl radical equal to 10:1. ^ EFFECT: invention ensures high output of end products a within short period of time and less expenses on electricity using a high-technology method. ^ 2 ex

Description

The invention can find application as a general method for the oxidation of alcohols to carbonyl compounds and can be used in pharmaceuticals.

A known electrochemical method for the oxidation of alcohols in the KJ-water-cosolvent system (Tetrahedron Letters, 1979, No. 2, pp. 165-168, T. Shono, Y. Matsumura, J. Hayashi and M. Mizoguchi). In this case, primary alcohols are oxidized to acid esters, and secondary alcohols to ketones.

For example, in a cell equipped with a stirrer and a cooling jacket and platinum electrodes, a solution of 2.49 g (0.015 mol) of potassium iodide in 15 ml of water, 0.06 mol of 2-methylcyclohexanol and 15-20 ml of a co-solvent (tert-butyl alcohol or hexane). The process is conducted at constant current (current density 0.02 A / cm ² ) and terminated after 20 F / mol of electricity is passed. The organic layer was separated, and the aqueous layer was extracted three times with ether; organic extracts are combined, washed with water and aqueous sodium thiosulfate. Then the solvent is removed, and the product is purified by distillation and identified by comparison with a known sample. The yield of 2-methylcyclohexanone is 52% by substance and 6% by current.

The disadvantage of this method is the high specific consumption of electricity, because per mole of the final product, it is necessary to pass up to 20 F / mol of electricity, in addition, primary alcohols under these conditions are not oxidized to aldehydes, but to acid esters.

The closest is the method of oxidation of alcohols in a two-phase system of water-toluene in the presence of crystalline iodine and a nitroxyl radical. (Ross A. Miller and R Scott Hoerrer. Organic Letters, 2003, V.5, No. 3, pp 285-287).

For example, in a 100 ml flask equipped with a magnetic stirrer, 0.75 g (3.98 mmol) of cinamoyl alcohol, 10 ml of toluene and a solution of 1 g (11.94 mmol) of sodium bicarbonate in 10 ml of distilled water are placed. Then, while stirring, 0.06 g (0.398 mmol) of 2,2,6,6-tetramethylpiperidin-1-oxyl and 2.02 g (7.96 mmol) of crystalline iodine are added. The mixture is stirred for 16 hours at 20 ° C. The mixture was then cooled to 5 ° C, treated with ethyl acetate (10 ml) and an aqueous solution of sodium sulfate (0.501 g of Na ₂ SO ₄ in 5 ml of distilled water) was added. Then the mixture is transferred to a separatory funnel and the aqueous layer is separated. The organic layer is washed with 10 ml of potassium bicarbonate solution, dried with anhydrous sodium sulfate and evaporated in vacuo to a volume of 10 ml, and then in an inert gas medium to a volume of 10 ml. The precipitate formed is filtered off, washed with toluene (3 ml) and dried in vacuo. As a result, 0.69 g (85%) of the corresponding aldehyde is isolated.

The disadvantage of the above method is the high consumption of iodine (1 mol of alcohol requires 2 mol of iodine), which is quite expensive, as well as the formation of a by-product of potassium iodide.

The objective of the invention is the improvement of the process of oxidation of alcohols to carbonyl compounds in an energy-economic relation.

The problem is achieved by preparing the reaction mixture at room temperature, consisting of oxidizable alcohol, water, sodium bicarbonate, an organic solvent and a nitroxyl radical, methylene chloride is used as an organic solvent, and 4-acetylamino-2 is used as a nitroxyl radical, 2,6,6-tetramethylpiperidin-1-oxyl of the formula:

when the ratio of alcohol and nitroxyl radical is 10: 1, potassium iodide is added, and electrolysis is carried out on platinum electrodes at a current strength of 1 A and a temperature of 20-25 ° C. Moreover, the cost of electrical energy for the oxidation of 1 mol of alcohol is 5 times less. Also, in contrast to the prototype and the analogue, instead of crystalline iodine, potassium iodide is used in the synthesis, which is not consumed during the oxidation process, which allows the oxidation of 1 mole of alcohol to use 8 times less amount of potassium iodide than crystalline iodine. Moreover, primary alcohols are oxidized only to aldehydes, and not acids, which is very important, because it is not always possible to stop the process at this stage. In addition, the proposed method uses catalytic amounts of a nitroxyl radical - 4-acetylamino-2,2,6,6-tetramethylpiperidin-1-oxyl (100 times less than in the analogue), which, however, does not interfere with a high output by substance and current.

The process is carried out in a plateless electrolyzer on platinum electrodes, in a two-phase system, methylene chloride - an aqueous solution of potassium iodide, in the presence of a nitroxyl radical. Iodine is formed on the anode, oxidizing the nitroxyl radical to the oxoammonium salt, which acts as a one-electron oxidizing agent of alcohol to the corresponding carbonyl compound. In this case, the oxoammonium salt is converted to hydroxylamine, which is easily oxidized to the initial nitroxyl radical, thus closing the catalytic cycle. In this case, 1 mol of the target product requires from 2 to 4 F of electricity.

These conditions increase the manufacturability of the process, reduce the time to obtain the target product, because oxidation is much faster and requires less electricity.

The proposed method allows the oxidation of primary and secondary alcohols by electrically generated iodine in the presence of 4-acetylamino-2,2,6,6-tetramethylpiperidin-1-oxyl, which is a mediator of the oxidation process. Electrogenated iodine has an advantage over other reagents, as during oxidation, they do not affect other easily oxidizing groups.

Example 1

Oxidation of secondary alcohols by the example of the oxidation of cyclohexanol to cyclohexanone

In a 150 ml diaphragmless electrolyzer equipped with a mechanical stirrer, a water cooling jacket and two platinum electrodes (plates, anode area of 20 cm ² , cathode 15 cm ² ), 4.5 ml (0.042 mol) of cyclohexanol are dissolved, which is dissolved in 30 ml of methylene chloride. Then 5 g (0.06 mol) of NaHCO ₃ and 2 g (0.01 mol) of KJ dissolved in 70 ml of distilled water are added. 1.1 g (0.005 mol) of 4-acetylamino-2,2,6,6-tetramethylpiperidin-1-oxyl are added last. Electrolysis conditions: temperature 20-25 ° C, current strength 1 A (density 0.05 A / cm ² ). The synthesis is completed after passing 4 F / mol of electricity. After electrolysis, the contents of the electrolyzer are drained, the excess of iodine is neutralized with an alkali solution or by adding crystalline Na ₂ S ₂ O ₃ , after which the organic layer is separated. The aqueous layer was extracted with methylene chloride (2 × 50 ml), the organic extracts were combined and dried with anhydrous sodium sulfate. According to TLC, the reaction mixture contains two substances - the starting alcohol and the product. The content of cyclohexanone and cyclohexanol is determined by GLC. The degree of conversion of cyclohexanol to ketone according to GLC was 85%. To isolate the ketone, methylene chloride was removed, and the residue was distilled in vacuo, collecting a fraction of 48 ° C (7 mm Hg). The yield of cyclohexanone is 3.05 g (78%).

Example 2

Oxidation of primary alcohols by the example of the oxidation of phenylethyl alcohol to α-toluyl aldehyde.

In a 150 ml diaphragmless electrolyzer equipped with a mechanical stirrer, a cooling water jacket, and two platinum electrodes (plates with an area of 20 cm ² , cathode 15 cm ² ), 4.5 ml (0.042 mol) of phenylethyl alcohol dissolved in 30 ml of methylene chloride. Then add 5 g (0.06 mol) of NaHCO ₃ and 2 g (0.01 mol) of KJ dissolved in 70 ml of distilled water. 1.1 g (0.005 mol) of 4-acetylamino-2,2,6,6-tetramethylpiperidin-1-oxyl are added last. Electrolysis conditions: temperature 20-25 ° C, current strength 1 A (density 0.05 A / cm ² ), intensive mixing. The synthesis is completed after passing 4 F / mol of electricity. After electrolysis, the contents of the cell are drained, the excess of iodine is restored by crystalline Na ₂ S ₂ O ₃ , after which the organic layer is separated. The aqueous layer was extracted with methylene chloride (2 × 50 ml), the organic extracts were combined and dried with anhydrous sodium sulfate. According to TLC, the reaction mixture contains two substances - the starting alcohol and the product. The content of α-toluyl aldehyde and phenylethyl alcohol is determined by GLC. The degree of conversion of phenylethyl alcohol to α-toluyl aldehyde according to GLC was 90%. To isolate the aldehyde, methylene chloride was removed, and the residue was distilled in vacuo, collecting a fraction of 41-42 ° C (7 mmHg). The yield of α-toluyl aldehyde 4.01 g (80%).

Claims (1)

The electrochemical method for the oxidation of alcohols to carbonyl compounds involves the preparation of a reaction mixture at room temperature consisting of an oxidizable alcohol, water, sodium bicarbonate, an organic solvent and a nitroxyl radical, characterized in that the electrolysis is carried out on platinum electrodes at a current strength of 1A and a temperature of 20-25 ° С, potassium iodide is added to the reaction mixture, methylene chloride is used as an organic solvent, and 4-acetylamino-2,2,6,6-tetramethylpip is used as a nitroxyl radical eridin-1-oxyl of the formula:

with a ratio of alcohol and nitroxyl radical of 10: 1.