GB2173188A - The preparation of fluorobenzene and fluoropyridine compounds - Google Patents

Abstract

A process for the preparation of a fluorobenzene or fluoropyridine compound comprises diazotising the corresponding aminobenzene or amino-pyridine in hydrogen fluoride and decomposing the diazonium salt in the diazotisation mixture so formed, and is characterised in that decomposition is carried out in the presence of an organic solvent miscible with and inert to hydrogen fluoride at a temperature between 50 DEG C and 100 DEG C and substantially under atmospheric pressure.

Description

SPECIFICATION Chemical process This invention relates to the preparation of fluorobenzene and fluoropyridine compounds which are useful chemical intermediates in the synthesis of, for example, agricultural chemicals such as fungicidal compounds.

It is known from Ferm et al, JACS 72 4809 [1950], that fluorobenzene and substituted fluorobenzenes can be prepared by diazotising the appropriate aminobenzene in anhydrous hydrogen fluoride and decomposing the benzenediazonium fluoride so formed. Various modifications of this method for the preparation of both fluorobenzenes and fluoropyridines have been reported, including carrying out diazotisation and decomposition in hydrogen fluoride containing a tertiary amine (US 4096196) and hydrogen fluoride solution containing ammonium ions (GB 1534841).

Decomposition is carried out by heating the diazonium salt until all nitrogen has been removed. This may occur by the time the temperature reaches the reflux temperature of the HF diazotisation mixture.

However, in the case of more stable diazonium salts higher temperatures are required necessitating heating under pressure or evaporating solvent until the desired temperature is reached. This can lead to the formation of complex mixtures of high melting solids and tars. The present invention is concerned with improving the method of decomposition.

According to the present invention, there is provided a process for the preparation of a fluorobenzene or fluoropyridine compound which comprises diazotising the corresponding aminobenzene or aminopyridine in hydrogen fluoride and decomposing the diazonium salt in the diazotisation mixture so formed, characterised in that decomposition is carried out in the presence of an organic solvent miscible with and inert to hydrogen fluoride at a temperature between 50"C and 100 C and substantially under atmospheric pressure.

The organic solvent may be any such solvent which is miscible with and not reactive with hydrogen fluoride and which is capable of raising the boiling point of the diazotisation mixture above 50"C. Suitable solvents include ethers that are miscible with hydrogen fluoride and boil above 50"C under atmospheric pressure, for example, dioxan. 1,2-Dimethoxyethane ("glyme") is ideal.

It is convenient to have all or part of the solvent present at the diazotisation stage but it is envisaged that it could be added afterwards. The amount of solvent employed is not critical provided there is sufficient present to raise the boiling point of the diazotisation mixture to 50"C or above. Suitably, the proportion by volume of solvent to hydrogen fluoride will be from 4:1 to 0.5:1, typically about 1:1.

For practical purposes, the temperature of decomposition should be at least 550C and will normally be at least 65 or 70"C. A temperature range of from 75 to 95"C is preferred, especially from 80 to 850C. When the diazotisation mixture is refluxed at about 84"C, the decomposition can be completed in 3 hours. At lower temperatures it takes longer and product yield may suffer.

Diazotisation of the aminobenzene or aminopyridine (hereinafter referred to as "the amine") may be carried out using any conventional diazotising agent. Conveniently the agent is formed in situ by dissolving an alkali metal nitrite, for example sodium nitrite, in excess hydrogen fluoride. At least 1 mole of diazotising agent per mole of the amine will be used, preferably from 1 to 1.25 moles, and typically about 1.1 moles. The temperature of diazotisation is not critical: a temperature from -5 C to 20"C is satisfactory and is conveniently room temperature. Unused diazotisation agent may be destroyed in the usual way by the addition of urea or sulphamic acid.

Hydrogen fluoride provides a source of fluorine and forms at least part of the reaction medium, depending at what stage the solvent is introduced. It will also normally be used to prepare the diazotising agent in situ by reaction with, for example, an alkali metal nitrile. At least 2 moles of hydrogen fluoride will be required and normally from 20 to 30 moles per mole of the amine. Preferably, the hydrogen fluoride is anhydrous to minimise hydrolysis of the diazotised amine. As some water is formed during diazotisation, hydrolysis may not be completely avoided.

Suitable amines for use in the process of the present invention include amines of the formula (I)

in which A is CH or N; R is halogen, preferably chlorine and fluorine; nitro; cyano; hydroxy; alkyl, preferably C, 4 alkyl; alkoxy, preferably C14 alkoxy; trifluoromethyl; hydroxycarbonyl; or alkoxycarbonyl, preferably C14 alkoxycarbonyl; m is 0, 1, 2 or 3 and, when m is 2 or 3, the substituents represented by R may be the same or different; and n is 1 or 2.

Examples of suitable aminobenzenes are o-, m- and p-toluidine aniline o-, m- and p-haloaniline 2,4- and 2,5-dimethylaniline m- and p-alkoxyaniline m- and p-aminophenol m- and p-nitroaniline m- and p-carboxyaniline and especially o-carboxyaniline (anthranilic acid) 2-methyl-5-nitroaniline o-, m- and p-trifluoromethylaniline o-, m- and p-phenylenediamine.

Illustrative compounds of the pyridine series include 2, 3 and 4-aminopyridine, diaminopyridines such as 2,6-diaminopyridine, haloaminopyridines such as 2-amino-4,5 and 6-halopyridine and 3-amino-5 and 6halopyridine, nitroaminopyridines such as 2-amino-5-nitropyridine and alkylaminopyridines such as 2amino-4,5 and 6-methylpyridine and 2-amino-4,6-dimethylpyridine.

In one illustrative method of carrying out the process, an hydros hydrogen fluoride is added cautiously, with stirring, to an equal volume of a suitable solvent, in this case glyme, at less than 10"C. The solution is cooled to -5 C and the amine added followed by sodium nitrite to effect diazotisation, the temperature being maintained by external cooling between -5 and -2"C. The diazotisation mixture is allowed to rise to room temperature and stirred for about an hour before being boiled under reflux conditions for about 3 hours at 84"C to effect decomposition of the diazotised amine.

After decomposition, the product can be isolated by pouring the reaction mixture onto ice and filtering the precipitated product. Residual product in the aqueous filtrate can be extracted with a suitable solvent such as dichloromethane and isolated by evaporation of the solvent.

The invention is illustrated by the following Example in which percentages are by weight and the following abbreviations are used: "C degress Centigrade ml millilitres g gram FEP fluorinated ethylene/propylene HPLC high performance liquid chromatography Example 1,2-Dimethoxyethane (102 ml) was poured into a 300 ml FEP-lined stainless steel reactor and stirred at less than 10 C while anhydrous hydrogen fluoride (100 ml) was slowly added with caution (vigorous exotherm). The stirring was continued while anthranilic acid (25.0g) was added at -50C, foilowed by sodium nitrite (14.0g) at between -5 to -2"C. During all those additions cooling was necessary to maintain the temperature at the desired level. The reaction solution was stirred for 1 hour at ambient temperature then refluxed for 3 hours at 83.5"C. The contents of the reactor were poured out onto ice, filtered and the aqueous filtrate extracted with methylene chloride. The solid filtered off was added to the dried methylene chloride solution and rotary evaporated. After drying in an oven, 32.0g of product were obtained, which, by HPLC analysis, was shown to contain 21.9g of 2-fluorobenzoic acid (86% yield) and 1.1g of salicyclic acid (3%).

Claims (1)

1. A process the the preparation of a fluorobenzene or fluoropyridine compound which comprises diazotising the corresponding aminobenzene or aminopyridine in hydrogen fluoride and decomposing the diazonium salt in the diazotisation mixture so formed, characterised in that decomposition is carried out in the presence of an organic solvent miscible with and inert to hydrogen fluoride at a temperature between 50"C and 100"C and substantially under atmospheric pressure.