GB2109790A

GB2109790A - Preparation of fluoro- substituted benzoyl fluorides

Info

Publication number: GB2109790A
Application number: GB8229164A
Authority: GB
Inventors: Andreas Gunther; Manfred Lenthe; Fritz Doring; Gerhard Dankert; Frank Doscher
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1981-10-13
Filing date: 1982-10-12
Publication date: 1983-06-08
Also published as: DE3140634A1

Abstract

Benzoyl fluorides which are fluorinated in the nucleus and which are intermediates in the production of plant protection agents, are prepared by reacting a benzoyl halide, which is chlorinated in the nucleus, with potassium fluoride at an elevated temperature and at the boiling pressure of the reaction mixture, and the benzoyl fluoride which is fluorinated in the nucleus is distilled off during the reaction.

Description

SPECIFICATION Process for the preparation of benzoyl fluorides which are fluorinated in the nucleus The present invention relates to an unobvious process for the preparation of certain benzoyl fluorides which are fluorinated in the nucleus.

It has already been disclosed that benzoyl fluorides which are fluorinated in the nucleus can be obtained from the corresponding benzoyl fluorides which are chlorinated in the nucleus, or mixtures of benzoyl fluorides and chlorides, by reaction with potassium fluoride at elevated temperature and elevated pressure, as a rule in a solvent (see DOS [German Patent Specification] 2,931,688). Pentafluorobenzoyl fluoride, for example, is obtained in a 10% yield according to this Offenlegungsschrift (Published Specification), by reacting pentachlorobenzoyl fluoride with 7 times the molar amount of potassium fluoride in tetramethylenesulphone at 2500C and under a nitrogen pressure of approximately 14 bar.A 29% yield of pentafluorobenzoyl fluoride is obtained, according to the same Offenlegungsschrift (Published Specification) when a mixture of pentachlorobenzoyl fluoride and chloride is first reacted with 7 times the molar amount of potassium fluoride, under similar reaction conditions, and the reaction products, after they have been separated off, are then reacted once again with 1.5 times the molar amount of potassium fluoride.

The known process has, in particular, the disadvantages of a low yield and of the relatively poor availability of the benzoyl fluorides employed, which are prepared from the corresponding benzotrifluorides according to DOS (German Published Specification) 2,931,688.

The present invention now provides a process for the preparation of a benzoyl fluoride which is fluorinated in the nucleus of the general formula

in which each R independently represents a radical selected from hydrogen, chlorine, C1 to C4 alkyl, CH, SH, alkoxy, alkylmercapto, amino, monoalkylamino and dialkylamino, and n is 1, 2, 3, 4 or 5, in which a benzoyl halide which is chlorinated in the nucleus of the general formula

in which R and n have the meanings given above, and Hal represents a halogen atom, is reacted with a potassium fluoride in a diluent and, if appropriate, in the presence of a catalyst, at an elevated temperature and at the boiling pressure of the reaction mixture, and the benzoyl fluoride which is fluorinated in the nucleus of formula (I) is distilled off during the reaction.

It is to be regarded as particularly surprising that the process for the preparation of benzoyl fluorides which are fluorinated in the nucleus from the corresponding benzoyl halides which are chlorinated in the nucleus, and potassium fluoride under a relatively low pressure and with the product being distilled off leads to much higher yields than the fluorination of benzoyl fluorides, which are chlorinated in the nucleus, under high pressure in autoclaves, as described in DOS (German Published Specification) 2,931,688.

Thus, for example, pentafluorobenzoyl fluoride is obtained in one step in a 50% yield from pentachlorobenzoyl chloride under approximately 1 bar by the process according to the invention, while according to DOS (German Published Specification) 2,931,688, a yield of only 10% is obtained from pentachlorobenzoyl fluoride under approximately 1 5 bar.

The process according to the invention has, in particular, the advantages of a substantially better yield and of substantially more readily obtainable starting materials. Further advantages are the relatively low pressure stage of the reactor, and the possibilities, extended by the choice of the reaction pressure, of obtaining products in which only some of the nucleus-bonded chlorine atoms have been replaced by fluorine.

Benzoyl fluorides in which all nucleus-bonded chlorine atoms have been substituted by fluorine as well as the benzoyl fluorides in which only some of the nucleus-bonded chlorine atoms have been replaced by fluorine are obtainable by the process according to the invention. 2,4,6trifluoro-3,5-dichlorobenzoyl fluoride, 2,3,4,6tetrafluoro-5-chlorobenzoyl fluoride and pentafluorobenzoyl fluoride, for example, can be obtained from pentachlorobenzoyl chloride by the process according to the invention, depending on the process conditions chosen (amount of potassium fluoride, reaction temperature and reaction pressure).

Any of the substituted benzoyl halides which carry at least one chlorine atom in the nucleus, for example pentachlorobenzoyl fluoride, are suitable starting materials. In many cases, these compounds can readily be obtained by catalytic chlorination of the nucleus of the benzoyl halides which are unsubstituted in the corresponding positions, for example pentachlorobenzoyl chloride from benzoyl chloride.

Preferred starting compounds of the formula (II) are those in which R represents a hydrogen or chlorine atom, a C, to C4 alkyl, C, to C4 alkoxy, C, to C4 alkylthio, NH2, CH, SH, C, to C4 monoalkylamino or C, to C4 dialkylamino group.

The benzoyl chlorides which are chlorinated in the nucleus are preferably employed as starting materials. Pentachiorobenzoyl chloride is particularly preferred.

The reaction is generally carried out as follows: the benzoyl halide which is chlorinated in the nucleus, the potassium fluoride, the solvent and, if appropriate, the catalyst are introduced into a stirred vessel, a paddle reactor, or another reactor which effects thorough mixing, and the mixture is heated up to the reaction temperature. By evacuation, by forcing in an inert gas, for example nitrogen, or by the choice of the reaction temperature, the pressure is adjusted so that the product evaporates from the reaction mixture. The product is distilled off over a column, either continuously or with interruptions.In this process, starting material and any resulting intermediate products in which the desired number of chlorine atoms are not yet substituted by fluorine remain in the reaction mixture, since the less fluorinated compounds always have lower vapour pressure.

After the end of the reaction, unreacted intermediate products and the solvent can be separated off from the potassium chloride and potassium fluoride by distillation or filtration, and can be recycled to the process. Under certain circumstances, it may also be advantageous to carry out the reaction in several stages, for example firstly to distil off an intermediate product in a rapidly proceeding reaction step, and then to prepare the product therefrom in the manner described, in a slower, second reaction step, so that a higher adduct concentration prevails in this step.

The potassium fluoride is generally employed in stoichiometric amounts. In the cases in which it is intended to substitute all nucleus-bonded chlorine atoms by fluorine, a low excess of potassium fluoride is advantageous, preferably 10 to 50%.

Suitable diluents are inert organic solvents which have a lower vapour pressure than the product, for example tetramethylenesulphone for the fluorination of pentachlorobenzoyl chloride.

The amount of solvent is chosen so that the reaction mixture remains stirrable until the end of the reaction.

The reaction rate and the yield are favourably influenced, in some cases, by catalytic amounts, 0.001 to 0.1 mol per mol of benzoyl halide, or rubidium, caesium, quaternary ammonium halides or crown ethers, for example by caesium fluoride, caesium chloride, triethylbenzylammonium chloride or 1 8-crown-6.

The reaction temperature is generally between 10 and 3000C. In some cases the type of product can be influenced by the choice of the reaction temperature, a higher temperature implying a higher degree of fluorination. For example, 2,4,6trifluoro-3,5-dichlorobenzoyl chloride is already formed from pentachlorobenzoyl fluoride from 1600C, and 2,3,4,6-tetrafluoro-5-chlorobenzoyl fluoride and pentafluorobenzoyl fluoride are only formed from 2200C, and decomposition occurs from 2700C. It can be advantageous to heat the mixture up to the maximum reaction temperature only in the course of the reaction, in order to reduce the formation of by-products.

The column should have as small an operating hold-up as possibly, since the proportions of intermediate products contained in the column are withdrawn from the reaction. Its capacity should be adapted to the particular reaction rate and its separating effect should be adapted to the particular product mixture in order to achieve optimum results. In the ideal case, the product is distilled off at the same rate at which it is formed in the reaction mixture. In reality, the product is distilled somewhat more rapidly, and the distillation is slowed down or interrupted when the top temperature shows that the desired product purity is no longer being achieved. In this process, the individual interruptions are each preferably 0 to 5% of the reaction time.

The benzoyl fluorides which are fluorinated in the nucleus and which are obtainable by the process according to the invention are valuable intermediate products for the preparation of plant protection agents, for example according to DE OS (German Published Specification) 2,658,074.

The Examples which follow further illustrate the process according to the invention.

Example 1 In a stirred vessel, 391 g (1.25 mol) of pentachlorobenzoyl chloride, 654 g (11.25 mol) of potassium fluoride and 1,350 g of tetramethylenesulphone are heated for 2 hours at 2200C and then to 2500C. After the mixture has reached 2500C, the product pentafluorobenzoyl fluoride is distilled off continuously over a column, under a pressure of 1 bar. By varying the reflux ratio, a product purity of 80% is maintained. The pressure is reduced to 0.45 bar in the course of the reaction. Finally, the less fluorinated benzoyl fluorides are also distilled off. The distillate contains 1 34 g (50% yield) of pentafluorobenzoyl fluoride, in addition to 58 g of 2,3,4,6-tetrafluoro-5-chlorobenzoyl fluoride and 1 5 g of 2,4,6trifluoro-3,5-dichlorobenzoyl fluoride. These three products are obtained in pure form by distillation over a column under reduced pressure.

Example 2 625 g (2 mol) of pentachlorobenzoyl chloride, 581 g (10 mol) of potassium fluoride and 1,200 g of tetramethylenesulphone are heated to 1 7000 in a stirred vessel. After the mixture has reached the temperature of 1700C, the product of 2,4,6trifluoro-3,5-dichlorobenzoyl fluoride is distilled off under a pressure of 0.2 bar. During the distillation, the temperature at the bottom is increased from 170"C to 2200C in the course of 5 hours. By varying the reflux ratio, a 95% pure product is taken off continuously. The distillate contains 410 g (83% yield) of 2,4,6-trifluoro-3,5dichlorobenzoyl fluoride, in addition to 9 g of 2,3,4,6-tetrafluoro-5-chlorobenzoyl fluoride and 1 3 g of tetramethylenesulphone.The vessel content is then filtered, the filter cake is washed with acetone, and the filtrate is distilled. 95% of the tetramethylenesulphone employed is recovered.

Example 3 40.8 kg (165 mol) of 2,4,6-trifluoro-3,5-dichlorobenzoyl fluoride, 28.8 kg (496 mol) of potassium fluoride and 60 kg of tetramethylenesulphone are heated to 2500C in a paddle reactor, and are kept at this temperature for 1 hour. A column of 2 m length and 4 cm diameter is mounted on the reactor. The column is filled with wire mesh rings of 6 mm size. A little nitrogen is continuously fed in upstream from the condensor, and the nitrogen pressure is released via the downstream pressure-regulating valve. After the 1 hour has elapsed, the product pentafluorobenzoyl fluoride is distilled off at a constant bottom temperature of 2500C and under a pressure of 2 bar. By varying the reflux ratio between 1:1 and 10:1, a 90% pure product is continuously taken off. The pressure is decreased to 0.45 bar in the course of the reaction.Finally, the less fluorinated benzoyl fluorides are also distilled off. The distillate contains 21.1 kg (60% yield) of pentafluorobenzoyl fluoride, in addition to 3.4 kg of 2,3,4,6-tetrafluoro-5-chlorobenzoyl fluoride and 0.4 kg of starting material.

Example 4 865 g (3.5 mol) of 2,4,6-trifluoro-3,5dichloro-benzoyl fluoride, 447 g (7.7 mol) of potassium fluoride and 925 g of tetramethylenesulphone are heated to 2500C in a stirred vessel, and are kept at this temperature for 30 minutes.

After 30 minutes have elapsed, the product pentafluorobenzoyl fluoride is distilled off under a pressure of 2 bar at a constant bottom temperature of 2500C for 0.5 hour at a time, and the distillation is then interrupted for 0.5 hour at a time. The pressure is reduced to 0.45 bar in the course of the reaction. Finally the less fluorinated benzoyl fluorides are also distilled off. The distillate contains 375 g (50% yield) of pentafluorobenzoyl fluoride, in addition to 161 g of 2,3,4,6-tetrafluoro-5-chlorobenzoyl fluoride and 43 g of starting material.

Comparative Example 4a This example shows that the yield of the process according to DOS (German Published Specification) 2,931,688 is substantially lower.

865 g (3.5 mol) of 2,4,6-trifluoro-3,5-dichlorobenzoyl fluoride, 447 g (7.7 mol) of potassium fluoride and 925 g of tetramethylenesulphone are kept at 2500C for 10 hours in a stirred vessel. A pressure of 6 bar is established by forcing in nitrogen. After the reaction time has elapsed, the benzoyl fluorides are distilled off. 1 50 g (20% yield) of pentafluorobenzoyl fluoride are obtained, in addition to 250 g of 2,3,4,6-tetrafluoro-5chlorobenzoyl fluoride and 208 g of starting material.

Claims

1. A process for the preparation of a benzoyl fluoride which is fluorinated in the nucleus of the general formula

in which each R independently represents a radical selected from hydrogen, chlorine, C1 to C4 alkyl, OH, SH, alkoxy, alkylmercapto, amino, monoalkylamino or dialkylamino, and nis 1,2,3,4of5, in which a benzoyl halide which is chlorinated in the nucleus of the general formula

in which R and n have the meanings given above, and Hal represents a halogen atom, is reacted with potassium fluoride in a diluent at an elevated temperature and at the boilingpressure of the reaction mixture, and the benzoyl fluoride which is fluorinated in the nucleus of formula (I) is distilled off during the reaction.

2. A process according to claim 1, in which a benzoyl halide of formula (II) in which R represents a hydrogen or chlorine atom, a Ct to C4 alkyl, C1 to C4 alkoxy, C1 to C4 alkylthio, NH2, OH, SH, C1 to C4 monoalkylamino or C1 to C4 dialkylamino group, is used.

3. A process according to claim 2, in which the benzoyl halide of formula (II) is pentachlorobenzoyl chloride.

4. A process according to any one of the claims 1 to 3, in which the reaction is carried out in the presence of a catalyst.

5. A process according to claim 4, in which the reaction is carried out in 0.001 to 0.1 mol per mol of benzoyl halide of a catalyst selected from rubidium, caesium and quaternary ammonium halides and crown ethers.

6. A process according to claim 1 , substantially as described in any one of Examples 1 to 4.

7. A benzoyl fluoride of formula (I) as defined in claim 1 , whenever produced by the process according to any of claims 1 to 6.

8. A plant protection agent whenever produced from a benzoyl fluoride according to claim 7.