Method for Preparing 3-Alkyl-2,6-Dichloroacylanilides by Electrolytic Debromination of 3-Alkyl-4-Bromo-2,6-Dichloroacylanilides

The invention describes a method for preparing 3-alkyl-2,6-dichloroacylanilides by electrolytic debromination of 3-alkyl-4-bromo-2,6-dichloroacylanilides.

Alkylacylanilides are valuable intermediates for the preparation of physiologically active compounds (U.S. Pat. No. 5,006,656, U.S. Pat. No. 4,937,350).

3-alkyl-2,6-dichloroacylanilides are not accessible by direct chlorination of 3-alkylacylanilides, since in the process the corresponding 4-chloro-substituted product is also formed at the same time. On the other hand, 3-alkyl-4-bromo-2,6-dichloroacylanilide can be obtained in high yield by bromination and subsequent chlorination.

Reductive dehalogenations on aromatic rings are described in the literature. These require toxic reagents such as triphenyltin hydride (J. Org. Chem. 1963, 28, 2332) or expensive reagents such as, e.g. lithium aluminum hydride (J. Chem. Res. 1990, 190), which makes these methods unsuitable for industrial purposes. The catalytic dehalogenation by means of hydrogen proceeds with low selectivity.

There was therefore a great need for a method which makes 3-alkyl-2,6-dichloroacylanilides accessible in high yield and purity.

This object is achieved by a method for preparing a 3-alkyl-2,6-dichloroacylanilide of the formula I ##STR3## in which R.sup.1, R.sup.2 are, independently of one another, (C.sub.1 -C.sub.4)-alkyl, which comprises bromoalkylacylanilides of the formula II ##STR4## in which R.sup.1, R.sup.2 have the above-specified meaning, being electrolytically debrominated.

The method is particularly important for the compounds of formula I in which R.sup.1 and R.sup.2 are both methyl.

The reaction apparatus used for the electrolysis is an appliance which can be divided, by a diaphragm or a membrane, into an anode compartment and a cathode compartment, the debromination taking place at the cathode. The electrolysis is carried out at room temperature or slightly elevated temperature.

In many cases it has proved useful to employ, as the reaction medium on the anode side, an aqueous solution of an acid or an alkali metal halide having adequate conductivity. The anode material is selected in such a way that it is inert under reaction conditions. If an aqueous acid such as, e.g. sulphuric acid is used as the anolyte, a dimensionally stable anode or a platinum anode is preferably employed. If an aqueous alkali metal bromide solution is used as the anolyte, it has proved useful to employ an anode based on carbon, such as, e.g., graphite.

On the cathode side, lower alcohols are used as the solvent. Preferably, methanol is employed. The cathode material used is an electrically conductive material which has adequate stability with respect to halide-containing solutions and is distinguished by a hydrogen overpotential which is sufficiently high for the debromination of the compounds I. Preferably carbon-based materials are used such as, e.g., graphite.

To increase the hydrogen overpotential, the catholyte can be admixed with catalytic amounts of a salt of the following elements: Cu, Ag, Au, Zn, Cd, Hg, Sn, Pb, Ti, Zr, Bi, V, Ta, Cr, Ce, Co, Ni. If salts are employed to increase the hydrogen overpotential, they are preferably salts of the elements Pb, Sn, Bi, Zn, Cd, Cr. In particular, Pb salts are employed as a catholyte additive. The preferred anions of the said salts are chloride, sulfate, nitrate or acetate. The salts can be admixed to the electrolyte directly or alternatively, prepared in the solution by the addition of oxides or carbonates. The salt concentration of the electrolyte is expediently from 10.sup.-6 to 10% by weight, preferably from 10.sup.-5 to 10.sup.-1 % by weight, especially from 10.sup.-4 to 4.times.10.sup.-2 % by weight, in each case based on the total amount of the electrolysis solution.

The conductivity of the electrolyte in the cathode compartment is provided by a supporting electrolyte which is inert under the reaction conditions and which is proportioned in such a way that adequate conductivity is achieved. For example, a carboxylic acid salt which is soluble in the electrolyte, such as sodium acetate, can be employed for this purpose.

The advantage of the invention compared to other methods for debromination consists in the highly selective removal of the bromine atom from the molecule, without adversely affecting other functional groups.

Monitoring of the reaction can be effected by common analytical methods such as, e.g., gas chromatography. The conversion can also be calculated from the electric charge transferred and the amount of hydrogen formed on the cathode, since reactions other than the debromination of the compounds I and generation of hydrogen are not observed. In particular, the chlorine atoms in compounds of the type I are not eliminated reductively if the electric charge required for complete debromination is exceeded.

The synthesis of the starting compounds can be carried out in a simple manner by methods known in the literature, starting from alkylanilines. For example, the 4-bromo-2,6-dichloro-3-methylacetanilide employed in the following examples can be obtained by reacting m-toluidine with acetic anhydride. The 3-methylacetanilide thus obtained is then reacted with bromine, which leads to selective bromination in the 4-position. The subsequent reaction with an excess of chlorine gives the 4-bromo-2,6-dichloro-3methylacetanilide in good yield and with good selectivity.

Starting material: 23.1 g of 4-bromo-2,6-dichloro-3-methylacetanilide

Catholyte: 300 ml of methanol, 6.83 g of sodium acetate, 30.degree.-35.degree. C.

Anolyte: 300 ml of 10% strength sulphuric acid, 30.degree.-35.degree. C.

Cell: divided flow cell, platinum anode (14 cm.sup.2), graphite cathode (14 cm.sup.2), Nafion 417 membrane

Current: 1400 mA

Electric charge: 4.17 Ah

Conversion: 71%

Starting material: 23.1 g of 4-bromo-2,6-dichloro-3-methylacetanilide

Catholyte: 300 ml of methanol, 6.83 g of sodium acetate, 200 mg of lead(II) acetate trihydrate, 30.degree.-35.degree. C.

Anolyte: 300 ml of 10% strength sulphuric acid, 30.degree.-35.degree. C.

Cell: divided flow cell, platinum anode (14 cm.sup.2), graphite cathode (14 cm.sup.2), Nafion 417 membrane

Current: 1400 mA

Electric charge: 4.17 Ah

Conversion: 79%

Starting material: 23.1 g of 4-bromo-2,6-dichloro-3-methylacetanilide

Catholyte: 300 ml of methanol, 6.83 g of sodium acetate, 30.degree.-35.degree. C. 200 mg of lead(II) acetate tihydrate, 30.degree.-35.degree. C.

Anolyte: 300 ml of 30% strength sodium bromide solution, 30.degree.-35.degree. C.

Cell: divided flow cell, graphite anode (14 cm.sup.2), graphite cathode (14 cm.sup.2), Nafion 417 membrane

Current: 1400 mA

Electric charge: 5.63 Ah

Conversion: quantitative

Work-up: The pH of the catholyte is set to 4 by means of dilute sulphuric acid. The solution is concentrated on a rotary evaporator to about half of its original volume and the reaction product is isolated by filtration. 14.3 g of 2,6-dichloro-3-methylacetanilide are obtained.