Electrosynthesis of Phenyl-2-Propanone

Procedure

The invention relates to a process for the electrosynthesis of ketones by electrochemical reduction of organic halides in the presence of organic acid derivatives, which process is employed in an electrolysis cell in an organic solvent medium containing a supporting electrolyte.

In Chemistry Letters, 1977, page 1021-1024, Shono describes the electrosynthesis of benzyl ketones by electrochemical reduction of benzyl chlorides in the presence of carboxylic acid chlorides in an acetonitrile or N,N-dimethylformamide (DMF) medium. The cell necessarily comprises two compartments separated by a ceramic diaphragm, and the anode is made of carbon. The concentration of the supporting electrolyte is high (in the region of 1M), in a manner which is inherent to the process. In acetonitrile, the yields of the isolated benzyl ketones vary between 29 and 73%, depending on the products. The use of DMF instead of acetonitrile leads to a considerable drop in this yield in every case. The electrochemical yields are always very low, taking into account that a quantity of current corresponding to 4 faradays per mole of benzyl chloride are passed through.

The aim of the present invention is, in particular, to simplify such a process and to improve the reaction yields. The Applicant Company has now found that, unexpectedly, such an aim is attained when an organic acid anhydride is used as the organic acid derivative and a sacrificial anode made of a metal chosen from the group consisting of magnesium, zinc, aluminium and their alloys is used, in combination.

The process according to the invention for the electrosynthesis of ketones by electrochemical reduction of organic halides in the presence of organic acid derivatives in an electrolysis cell fitted with electrodes in an organic solvent medium containing a supporting electrolyte is characterized in that a sacrificial anode is used which is made of a metal chosen from the group consisting of magnesium, aluminium, zinc and their alloys and the organic acid derivatives are organic acid anhydrides. When compared to the abovementioned process, which constitutes the most closely related state of the art: In this manner, higher mass and electrochemical yields are obtained, for a given solvent, while the process can be applied to the electrosynthesis of numerous ketones and is very considerably simpler to utilize, insofar as it can be carried out in a single-compartment electrolysis cell, without any diaphragm or sinter, which is an important advantage, especially on an industrial scale. Similarly, the possibility of carrying out the electrolysis at constant current instead of at a controlled potential, also simplifies this utilization. The concentration of the supporting electrolyte can be much lower. No deterioration of the solvent is observed at the anode, as is the case when an inert anode is used.

The electrosynthesis may be carried out in the presence of a catalyst, an organometallic complex of a transition metal such as nickel or palladium. This complex may be bi- or polymetallic. The complex NiBr2(bipyridine) is preferably used. When the halide is difficult to reduce or when the anhydride is easily reducible, it is found that the use of such a complex improves the yield very appreciably.

According to the process which is the subject of the present invention, an anode is used which is made of a metal chosen from the group consisting of magnesium, aluminium, zinc and their alloys. "Their alloys" means any alloy containing at least one of the three above-mentioned metals, namely magnesium, aluminium and zinc. This anode may be of any shape and, in particular, may be of any of the conventional shapes of metal electrodes which are well known to the man skilled in the an (twisted wires flat bar, cylindrical bar, renewable bed, balls, cloth, grid, and the like). Preferably, a cylindrical bar is used, whose diameter is adapted to the dimensions of the cell. For example, for a cell why total capacity is between approximately 50 cm3 and approximately 500 cm3, the bar diameter is of the order of 1 cm.

Before use, the surface of the anode is preferably cleaned chemically (using dilute HCl, for example), or mechanically (using a file or emery cloth, for example) in order, in particular, to remove the metal oxide which is often present on the surface of the metal. The cathode is made of any metal such as stainless steel, nickel, platinum, gold, silver, or of carbon. It preferably consists of a grid or a cylindrical plate arranged concentrically around the anode. The electrodes are supplied with direct current by means of a stabilized supply.

The organic solvents used within the scope of the present invention are any weakly protic solvents which are usually employed in organic electrochemistry. Examples which may be mentioned are DMF acetonitrile, tetramethylurea (TMU), tetrahydrofuran (THF) and mixtures of tetrahydrofuran with hexamethylphosphorotriamide. DMF is preferably used. The supporting electrolytes which are used may be those usually employed in organic electrochemistry. There may be mentioned, for example, the salts in which the anion is a halide, a carboxylate, an alcoholate, a Perchlorate or a fluoroborate, and the cation a quaternary ammonium, lithium, sodium, potassium, magnesium, zinc or aluminium. Among these salts, special mention may be made of tetraalkylammonium tetrafluoroborates (tetrabutylammonium tetrafluoroborate, for example) tetrabutylammonium perchlorate, tetraalkylammonium halides (for example tetrabutylammonium chloride or tetrabutylammonium iodide) and lithium perchlorate. The concentration of the supporting electrolyte in the organic solvent is preferably between 0.01 M and 0.5 M. Also preferably, the concentration of organic halides in the organic solvent is between 0.2 M and 2 M. The ratio of the concentration of the organic acid anhydride to the concentration of the organic halide in the organic solvent may have any value. An excess of anhydride and, in particular, a concentration ratio of between 1 and 20 will preferably be used.

The electrolysis is carried out:

• In a conventional electrolysis cell, well known to the man skilled in the art, comprising only a single compartment.
• At a temperature which is generally between –20°C. and +80°C. preferably between –10°C. and +40°C, advantageously in the region of 0°C.
• At a cathode current density which preferably varies between 0.1 and 10 A/dm2. In general, and preferably, the operation is carried out at a constant current, but it is also possible to operate at a constant voltage, at a controlled potential or with variable current and potential.
• While the solution is being stirred, for example by means of a bar magnet, after the solution has been degassed by bubbling an inert gas, for example nitrogen or argon.

After the passage of a quantity of current corresponding to approximately 2 Faradays (2 x 96,500 Q per mole of organic halides or, if appropriate, until the latter have been completely converted, the electrolysis is discontinued. The principle constituents of the mixture, namely the unreacted organic halide, the required products, and certain reaction byproducts are then determined in an aliquot portion of the solution, using gas chromatography (GCS in a manner known to the man skilled in the art. The required products are then extracted, isolated and purified in a conventional meaner. The reaction solvent and the volatile compounds may, for example, be evaporated off under vacuum, and then the remaining residue may be hydrolysed, for example with dilute hydrochloric acid.

The ketones are extracted, for example with ether. After evaporation of the extraction solvent a crude product is isolated, which is identified from its IR and NMR spectra and whose purity or composition is determined by (CC). The product or products are purified, if appropriate, for example by distillation.

The invention is illustrated by the following examples, which are not limiting in nature. To produce these examples, a conventional electrolysis cell was used, with a total capacity of approximately 250 cm3, comprising only a single comfit and equipped with pipes permitting the entry acrd the exit of the inert gas, sampling of the solution, if appropriate„ during the electrolysis, and the passage of electricity. The anode consists of a cylindrical bar, 1 cm in diameter. It is introduced into the cell through a central tube and is thus situated in an approximately axial position relative to the cell. The cathode consists of a cylindrical metal felt arranged concentrically wound the anode. The working surface of the cathode is of the order of 1 dm2. The cell is immersed in a thermostat bath controlled at the selected temperature. The specific operating conditions (nature of the electrodes, of the supporting electrolyte, of the solvent employed, the reaction temperature, and the like) are shown in detail, furthermore, for each example.

EXAMPLES 1 to 7

Synthesis of benzyl methyl ketone (phenylacetone) from benzyl chloride and acetic anhydride. In these examples, the anode is made of magnesium or aluminium, the cathode of nickel, the solvent is DMF (110g) and the supporting electrolyte is tetrabutylammonium fluoroborate (2 g, i.e. 6 mmol). After the electrolysis (2.2 faradays per mole of benzyl chloride), the remaining benzyl chloride, the toluene which is a byproduct of the reduction of benzyl chloride, and benzyl methyl ketone, both in free form and in the form of its enol acetate, are present in the solution. After the DMF has been evaporated off and the residue has been hydrolysed with hot dilute HCl, benzyl methyl ketone is isolated by extraction with ether. The specific operating conditions of the electrolysis in each Example, and the results obtained, are collated in Table I.

EXAMPLE 9

Synthesis of 3,4-dimethoxyphenylacetone from 3,4-dimethoxybenzyl chloride and acetic anhydride.The specific conditions in this example are the same as those in Example 4, but benzyl chloride is replaced by 3,4-dimethoxybenzyl chloride. The yield is 25%.

Table 1

1. As P2P formed, based on initial benzyl chloride
2. As P2P formed, based on initial benzyl chloride converted
3. Including 20 in the form of P2P enol acetate
4. Including 14 in the form of P2P enol acetate
5. As pure P2P isolated, based on initial benzyl chloride
6. Including 22 in the form of P2P enol acetate