The present invention relates to a process for the preparation of a compound of general formula I:
In WO 9710199 the prior art on the production of certain herbicidal nitro substituted diphenyl ethers is reviewed and it is concluded that none of the prior art methods are particularly satisfactory for use on an industrial scale because they all have the common problem that the processes yield a mixture of the required product and other nitrated isomers. Nitrated isomers of diphenyl ether compounds are often extremely difficult to separate from one another and the quantity of other isomers is often too high for the final product to fulfil the requirements of the regulatory authorities for herbicides. The problem tends to be further exacerbated if the nitrated product is an intermediate in the synthesis of a herbicide rather than the required herbicide itself because the mixture of nitrated compounds means that larger quantities of other reagents must be used than would be necessary if the nitrated isomers could be separated satisfactorily. It is therefore important to ensure that the nitration process is controlled to avoid over nitration and also produces a product mixture containing the highest possible proportion of the desired isomer.
There is disclosed in WO9710199 a process for the preparation of a compound of general formula I:
(Formula Removed)
N02 I wherein:
R1 is hydrogen or C1-C6 alkyl, C2-C6 alkenyl or C2-C6 alkynyl, any of which may optionally be substituted with one or more substituents selected from halogen and OH; or COOR4, COR6, CONR4R5 or CONHSO2R4;
R4 and R5 are each independently hydrogen or C1-C4 alkyl optionally substituted with one or more halogen atoms;

R6 is a halogen atom or a group R4; R2 is hydrogen or halo;
R3 is C1-C4 alkyl, C2-C4 alkenyl or C2-C4 alkynyl, any of which may optionally be substituted with one or more halogen atoms; or halo;
the process comprising reacting a compound of general formula II:
(Formula Removed)
wherein R1, R2 and R3 are as defined for general formula I;
with a nitrating agent comprising nitric acid or a mixture of nitric and sulphuric acids in the presence of an organic solvent and in the presence of acetic anhydride, characterised in that the molar ratio of acetic anhydride to compound of general formula II is from about 1 : 1 to 3:1.
These reaction conditions reduced over nitration and therefore gave an improvement in the portion of the required isomer compared to earlier processes.
It has now surprisingly been found that when both nitric and sulphuric acids are used in the process it is possible to further reduce the level of over nitration by adding these acids sequentially to the reaction mixture.
Accordingly, there is provided a process for the preparation of a compound of general formula I:
(Formula Removed)

wherein:
Rl is hydrogen or C1-C6 alkyl, C2-C6 alkenyl or C2-C6 alkynyl, any of
which may optionally be substituted with one or more substituents
selected from halogen and OH; or COOR4, COR6 CONR4R5 or
CONHSO2R4;
R4 and R5 are each independently hydrogen or C1-C4 alkyl optionally
substituted with one or more halogen atoms;
R6 is a halogen atom or a group R4;
R2 is hydrogen or halo;
R3 is C1-C4 alkyl, C2-C4 alkenyl or C2-C4alkynyl, any of which may
optionally be substituted with one or more halogen atoms; or halo;

the process comprising reacting a compound of general formula II:

(Formula Removed)
wherein R1, R2 and R3 are as defined for general formula I with a nitrating agent comprising nitric and sulphuric acids in the presence of an organic solvent and in the presence of acetic anhydride, the molar ratio of acetic anhydride to compound of general formula II is from 1:1 to 3: 1 characterised in that the acids are added sequentially to the reaction mixture and the organic solvent is perklone.
It has been found particularly advantageous to add the sulphuric acid to a mixture of the compound of formula II and acetic anhydride in the chosen organic solvent, followed by addition of the nitric acid.
The nitric acid is preferably greater than about 90% strength.
The molar ratio of sulphuric acid : compound of general formula II used in the reaction will generally be up to 1.5 : 1, however a ratio of sulphuric acid : compound of general formula II of from 0.1 : 1 to 0.3 : 1 is preferred.
The improved control of nitration results in an increase in yield of the desired product and a reduction in operating costs.
In the context of the present invention, compounds of general formula I are designated 4-nitro isomers. The 2-nitro isomers referred to above have the general formula:
(Formula Removed)

Other mono-nitro isomers which may be produced in the nitration reaction include the 6-nitro isomer:
(Formula Removed)
There are also three different dinitro isomers which may be present. Trinitro isomers may also be formed.
In the context of the present invention, the term "C1-C6 alkyl" refers to a saturated straight or branched hydrocarbon chain containing from 1 to 6 carbon atoms. Examples include methyl, ethyl, n-propyl, t-butyl, n-pentyl and n-hexyl. The term "C1-C4 alkyl" is a subset of C1-C6 alkyl and refers to an alkyl group having up to 4 carbon atoms.
The term "C2-C6 alkenyl" refers to a straight or branched hydrocarbon chain containing from 2 to 6 carbon atoms and having at least one double bond. Examples include ethenyl, allyl, propenyl and hexenyl. The term "C2-C4 alkenyl" is a subset of C2-C6 alkenyl and refers to an alkenyl group having up to 4 carbon atoms.
The term "C2-C6 alkynyl" refers to a straight or branched hydrocarbon chain containing from 2 to 6 carbon atoms and having at least one triple bond. Examples include ethynyl, propynyl and hexynyl. The term "C2-C4 alkynyl" is a subset of C2-C6 alkynyl and refers to an alkynyl group having up to 4 carbon atoms.
The term "halogen" refers to fluorine, chlorine, bromine or iodine and the corresponding term "halo" refers to fluoro, chloro, bromo or iodo.
The reaction conditions of the present invention are particularly advantageous since they minimise the over nitration reactions and maximise the amount of the required 4-nitro isomer in the product mixture.
The preferred temperature range for the process of the present invention is from about -15 to 15°C , more preferably -10 to 10°C.
The weight ratio of solvent to compound of formula II (including any isomers present) in the process is preferably no greater than 4.25 : 1 and it is preferred that the ratio is from 1 : 1 to 2.5 : 1.
The reaction may be carried out in any suitable solvent and examples of solvents which may be used include halogenated solvents such as dichloromethane (DCM), ethylene

dichloride (EDC), chloroform, tetrachloroethylene (perklone) and dichlorobenzotrifluoride (DCBTF). Alternatively, solvents such as acetic acid, acetonitrile, ethers such as tetrahydrofuran (THF) or dioxane, sulpholane, nitrobenzene, nitromethane, liquid sulphur dioxide or liquid carbon dioxide may all be used successfully in the reaction.
Perklone is the preferred solvent.
After the sulphuric acid has been added, the nitric acid is typically added to the reaction mixture over a period of about 30 minutes to 15 hours. The rate of addition will, however vary according to the reaction solvent which is chosen with addition over about 1 to 6 hours, or preferably 2 to 4 hours, being appropriate for many solvents, for example EDC and DCM.
When the reaction is conducted in perklone, however, the rate of reaction is usually somewhat lower than for reactions conducted in other solvents such as EDC or DCM and so it is often advantageous to add the nitric acid more slowly, for example over a period of from 2 to 15 hours, or, more preferably, 2 to 6 hours.
Although the process of the invention may be used for the preparation of any compound of general formula I, it is especially preferred that R2 is chloro and R3 is trifluoromethyl. Particularly preferred compounds of general formula I are those in which R1 is COOH or CONHSO2CH3. These compounds are 5-(2-chloro-α,α,α-trifluoro-4-tolyloxy)-2-nitrobenzoic acid (acifluorfen) and 5-(2-chloro-a,a,a-trifluoro-4-tolyloxy)-N-methanesulphonyl-2-nitrobenzamide (fomesafen), both of which are potent herbicides.
In addition to being a herbicide in its own right, acifluorfen may also serve as an intermediate in the synthesis of fomesafen. The acifluorfen may be converted to the acid chloride which may then be reacted with methane sulphonamide to give fomesafen. Both of these steps may be carried out by conventional methods, for example as set out in EP-A-0003416.
The invention will now be further described by way of the following examples in which the following abbreviations are used:
pph - parts per hundred;
Ac2O Acetic anhydride;
HPLC - high performance liquid chromatography.

In the comparative examples, the term "mixed acid" refers to a mixture containing 33.6% nitric acid and 66.4% sulphuric acid. The molar quantities given are the moles of nitric acid in the mixture.
Comparative Example A
General method for nitration of 3-(2-chloro-α,α,α-trifluoro-4-tolvloxy)benzoic acid in
perklone to yield acifluorfen
a) Nitration
Ac20 (see Tables I and II for amounts) was added to 3-(2-chloro-α,α,α-trifluoro-4-tolyloxy)benzoic acid (I, R1 is COOH, R2 is chloro, R3 is trifluoromethyl) (20 g, 0.063 mol) in perklone (54 g, 0.635 mol) and the mixture stirred and heated to 40°C to dissolve the starting material. The mixture was then cooled to the appropriate reaction temperature (during which time any crystallisation of the starting material was observed). Mixed acid (13 g, 0.069 mol) was added dropwise over 2 hours and the reaction monitored by HPLC for the completion of the reaction. Further additions of mixed acid were made to reduce the level of starting material to about 1 pph.
b) Work-Up
The reaction mixture was washed three times as follows: wash 1 - water (30 ml) was added and the mixture washed at approximately 38°C and the aqueous layer separated;
wash 2 - water (25 ml) was added and the mixture washed at approximately 38°C and the aqueous layer separated;
wash 3 - water (25 ml) was added and the mixture washed at approximately 38°C and the aqueous layer separated.
Water (80 ml) was then added and the mixture heated to 38°C and sodium hydroxide (47% solution, 6.4 g, 0.076 mol) added to basify the mixture to pH 10-11. The mixture was heated to distil off the perklone in order to afford a solution of acifluorfen sodium salt. The solution was cooled to room temperature and transferred with the aid of a minimum amount of water to a bottle in order for the solution to be weighed and analysed.
The results for Experiments 1.1 to 2.3 which were conducted according to the general methods of Comparative Example A are set out in Tables I and II below. In each of these experiments, 20 g crude starting material was used containing 84.3% 3-(2-chloro-

α,α,α-trifiuoro-4-tolyloxy)benzoic acid. In each of the experiments described in Table I, the amount of solvent used was 54.0 g but for the experiments detailed in Table II, the quantity of solvent was varied. In all the Tables the term "reactant" refers to 3-(2-chloro-α,α,α-trifluoro-4-tolyloxy)benzoic acid.
TABLE I
(Table Removed)

TABLE II
(Table Removed)

Example 1
Nitration of 3-(2-chloro-α,α,α-trifluoro-4-tolyloxy)benzoic acid sodium salt
3-(2-Chloro-α,α,α-trifluoro-4-tolyloxy) benzoic acid sodium salt (550 g, of a 36.4% solution) was added to a nitration vessel followed by perklone (734 g). The mixture was
heated to 80°C with agitation and 98% sulphuric acid (63 g) added slowly to give a pH of 2.. The mixture was allowed to separate and the aqueous layer removed. The solvent layer was washed with water (l00g) at 80°C and after separation was dried by azeotropic removal of water up to a batch temperature of 120°C.
The solution of 3-(2-chloro-a,a,a-trifluoro-4-tolyloxy)benzoic acid in perklone was cooled to 60°C and Ac20 (195 g) and 98% sulphuric acid (63 g) added with stirring. The mixture was cooled to 0°C and 90% nitric acid (53 g) was added at 0°C over 3lA hours with vigorous agitation. The resulting mixture was stirred for 1 hour at 0°C and then water (190 g) was added. The mixture was heated to 80°C and then allowed to settle. The aqueous layer was removed and the solvent layer washed with water (162 g) at 80°C. Approximately 360 g of perklone was removed by direct distillation and the remainder removed by azeotropic distillation with added water. Once all the perklone had been removed the resulting mixture of molten acifluorfen and water was treated with sodium hydroxide (158 g of a 15% solution) at 90°C. Once the addition was complete the mixture was cooled to 45°C and sodium hydroxide (39 g of a 15% solution) added to give a pH of 7-9. The resulting 40% solution of acifluorfen sodium salt in water was the allowed to cool, yield 85%.
A sample of the product was acidified and the precipitated acifluorfen acid was filtered, washed and dried and gave the following analysis by HPLC using authentic standards for analysis of acifluorfen and the nitrated isomers:
acifluorfen 76.5%
Impurities expressed as % weight x 100 / % acifluorfen:
2'-nitro isomer 7.5
6'-nitro isomer 3.9
reactant 1.5
total dinitro isomers 1.1
trinitros 0.4

Example 2
Nitration of 3-(2-chloro-α,α,α-trifluoro-4-tolyloxy)benzoic acid sodium salt
Further experiments for the nitration of 3-(2-chloro-a,a,a-trifluoro-4-tolyloxy)benzoic acid sodium salt in perklone to yield acifluorfen were performed according to the following general procedure.
A solution of 3-(2-chloro-α,α,α-trifluoro-4-tolyloxy)benzoic acid sodium salt (100% wt, 1 mol/mol) and perklone (2.26 ml/g of the sodium salt) were charged to a reaction vessel and heated to 80°C. 98% sulphuric acid was then added to adjust the pH to 1.7. The upper aqueous phase was then removed and the residual water removed by distillation. The perklone that co-distilled was returned to the vessel. The mixture was cooled to 30-40°C and acetic anhvdride (1.8-3.0 mol/mol) and 98% sulphuric acid (0.1-1.0 mol/mol) added. The mixture was cooled to 0-5°C and 98% nitric acid (approx 1.21 mol/mol) was added over 2-3 hours while maintaining the temperature of the batch at 0-5°C. The reaction was sampled and checked for completion and additional nitric acid added if required. The mixture was quenched with water and the upper aqueous layer removed at 80°C. The reaction mass was then washed with water at 80°C and the upper aqueous layer removed to leave a crude acifluorfen-perklone mixture. The perklone was distilled azeotropically and the residual perklone and acetic acid removed by steam stripping to afford crude acifluorfen melt which was neutralised at 90°C using 15% aqueous sodium hvdroxide to form the acifluorfen salt solution.
The results are shown in Table III

TABLE III
(Table Removed)

WE CLAIM :
1. A process for the preparation of diphenyl ether compound of general
formula I
(Formula Removed)

wherein:
Rl is hydrogen or C1-C6 alkyl, C2-C6 alkenyl or C2-C6 alkynyl, any of
which may optionally be substituted with one or more substituents
selected from halogen and OH; or COOR4, COR6 CONR4R5 or
CONHSO2R4;
R4 and R5 are each independently hydrogen or C1-C4 alkyl optionally
substituted with one or more halogen atoms;
R6 is a halogen atom or a group R4;
R2 is hydrogen or halo;

R3 is C1-C4 alkyl, C2-C4 alkenyl or C2-C4 alkynyl, any of which may optionally be substituted with one or more halogen atoms; or halo;
the process comprising reacting a compound of general formula II:
(Formula Removed)
wherein R1, R2 and R3 are as defined for general formula I with a nitrating agent comprising nitric and sulphuric acids in the presence of an organic solvent and in the presence of acetic anhydride, the molar ratio of acetic anhydride to compound of general formula II is from 1:1 to 3: 1 characterised in that the acids are added sequentially to the reaction mixture and the organic solvent is perklone.
2. A process as claimed in claim 1, wherein the sulphuric acid is added to a mixture of the compound of formula II and acetic anhydride in the organic solvent, followed by addition of the nitric acid,

3. A process as claimed in claim 1 or claim 2 wherein the nitric acid is greater than 90% strength.
4. A process as claimed in any one of the preceding claims wherein the reaction is performed at a temperature of from -15°C to 15°C.
5. A process as claimed in claim 4, wherein the reaction is performed at a temperature of from -10°C to 10°C.
6. A process as claimed in as claimed in any one of the preceding claims wherein, in the compound of general formula I, R2 is chloro and R3 is trifluoromethyl.
7. A process as claimed in as claimed in any one of the preceding claims wherein the compound of general formula I is 5-(2-chloro-a,a,a,-trifluoro-4-tolyloxy)-2-nitrobenzoic acid (acifluorfen) or 5-(2-chloro-a,a,a,-trifluoro-4-tolyloxy)-N-methanesulphonyl-2-nitrobenzamide (fomesafen).
8. A process as claimed in claim 1, wherein the compound of formula I when has R1 as CONH SO2 CH3; "the compound is fomesafen.

9. A process substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.