US Patent 4,529,824 describes in general certain complexes of vanadium, niobium and tantalum and their use in hydroxylating aromatic hydrocarbons either as reactant or catalyst. Additionally, Moiseeva et al. Kinet. Katal 29(4), 970-4 (1988) describe oxidation of benzene with vanadium compounds albeit not selectively for phenol (oxidation proceeds at least in part to quinone).

It has now surprisingly been found that oxidation of 1,4-dichlorobenzene to 2,5- dichlorophenol can be effected with high yield and selectivity when carried out in the presence of certain metal derivatives and in the presence of formic or an alkanoic acid.

The present invention therefore provides a process for the preparation of 2,5-dichloro- phenol which comprises selectively oxidizing 1 ,4-dichlorobenzene using an oxidizing agent and a peroxo-, hydroperoxo-, superoxo- or alkylperoxo-metal species wherein said metal is selected from scandium, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, selenium, tellurium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, zinc, aluminum, gallium, indium, tin, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and uranium or mixtures thereof in the presence of an acid selected from formic or an alkanoic acid.

The oxidation may be effected using an oxidation agent. Suitable oxidation agents included organic peroxides such as peroxycarboxylic acids, RCO3H, alkyl hydroperoxides, ROOH, and dialkylperoxides, ROOR, for example peroxyacetic acid, peroxybenzoic acid, peroxyformic acid, t-butyl hydroperoxide, and di-t-butylperoxide and inorganic peroxides such as peroxydisulfuric acid. The preferred peroxide for use in the invention however, is hydrogen peroxide, especially in aqueous solution. The present process can be carried out at dilutions of 3% to 90% with almost complete utilization of H202 and quantitive yields.

However, ca 50% aqueous H202 is preferred. The proportions of H202 to 1,4- dichlorobenzene may vary between 0.2 and 5, e.g. between 0.5 and 2. It has been determined that particularly high efficiency may be achieved using approximately equimolar amounts of H202 and 1 ,4-dichlorobenzene, e.g. ca 0.9 to ca 1.1 molar proportion of H202 per mole of 1 ,4-dichlorobenzene.

The peroxo-, hydroperoxo-, superoxo- or alkylperoxo-metal species may be and preferably is generated in situ by adding the desired metal either as pure metal or in the form of a suitable oxide, salt or other derivative. Alternatively, the desired derivative e.g. an acetoacetonate may be formed prior to reaction and added to the reaction mixture.

Suitable metal forms include oxides, anhydrides, acetylacetonates, alkoxymetal derivatives, sulfates, nitrates, halides, oxyhalides, alkylthiocarbamates or metalates with other cations, (e.g. ammonium metavanadate NH4VO3).

Preferred metals include molybdenum, titanium, vanadium, tungsten, rhenium, selenium, niobium, tantalum and tellurium with vanadium being particularly preferred.

Preferred metal forms include oxides, acetylacetonates, alkoxymetal oxides and oxychlorides.

A discussion of peroxo-, hydroperoxo-, superoxo- and alkylperoxo-metal species can be found in Conte et al. in Organic Peroxides Ed. W. Ando, John Wiley & Sons (1992) (pp 559-598).

Discussion of hydroperoxide oxidising agents and metal derivatives may also be found in US Patents 3,350,422; 3,351,635; 3,360,584; 3,360,585; and 3,662,006.

A particular advantage of the present process is that the peroxo-, hydroperoxo-, superoxo- or alkylperoxo-metal species can be and preferably are free of complex ligands.

The metal derivatives can be present in amount equivalent to between 0.001 and 100 mol % of metal. Preferably catalytic amounts of 0.1 to 15 mol % are employed.

When employed as a catalyst the metal derivative may be recycled and regenerated.

For example when an oxide such as vanadium pentoxide is used the spent catalyst can be heated in air (cf Polish Patent PL 73-165695; C.A. 87.91418) or treated with hydrogen peroxide.

The acid component of the process according to the invention is selected from formic or an alkanoic acid. Preferred alkanoic acids are lower alkanoic acids containing e.g. 2 to 6 carbon atoms such as acetic, propionic or butyric acid. The preferred acid according to the invention is acetic acid.

The use of solvents may also have a beneficial effect on the process according to the invention. Examples of such solvents include aprotic solvents such as chlorinated solvents e.g. 1 ,2-dichloroethane, dichloromethane and chloroform, nitrated solvents, e.g.

nitromethane; nitriles e.g. acetonitrile, propionitrile and benzonitrile; ketones, e.g. acetone and methylethylketone; alkanes, e.g. hexane; esters e.g. ethylacetate; alcohols e.g.

methanol, ethanol and isopropanol; or mixtures thereof.

Reaction temperatures lie between 0 and 150C and are preferably in the range of room to elevated (ca 90) temperature e.g. when employing acetic acid and a vanadium derivative. The reaction is preferably carried out in the substantial absence of water. In certain cases removal of water further increases yield. This is the case for example where the oxidising agent is used in aqueous solution or where the water is generated during the reaction. Removal of water can be achieved for example by carrying out the reaction in the presence of a drying agent such as activated molecular sieves (3A or 4Å), calcium sulphate, magnesium sulphate or acetic anhydride, etc.

In certain cases pH will affect the course of the reaction and therefore the addition of a Lewis acid or a Brnsted acid (cf Kirk Othmer Encyclopedia of Chemical Technology; 4th Edition; Wiley Interscience) such as p-toluenesulfonic or methanesulfonic acid can increase the yield.

Addition of a salt of the chosen formic or alkanoic acid e.g. dry sodium acetate with acetic acid can increase the yield in certain cases.

The reaction mixture may be mono- or multi- e.g. bi-phasic. Examples of such systems can be found in Bianchi et al. J. Mol. Catal. 83 (1993) 107-116; Bonchio et al. J.

Org. Chem. 54 (1989) 4368-4371.

Typically the substrate to be oxidised (1 ,4-dichlorobenzene) is dissolved in the chosen acid (e.g. acetic acid) and the metal derivative (e.g. a high valency oxide) added followed by the slow addition of the oxidising agent (e.g. aqueous H202). The metal derivative may be deposited on a carrier such as silica, alumina, aluminosilicates, zeolites, coals, titanium oxide, quartz, etc.

The starting material 1 ,4-dichlorobenzene has the formula and is a known, commercially available substance.

The desired end product 2,5-dichlorophenol has the formula and is useful as an intermediate in the preparation of the commercial herbicide dicamba in high isomeric purity (e.g. in substantial absence of 3,5-dichloro-o-anisic acid).

This process involves carboxylation of the 2,5-dichlorophenol to give 2-hydroxy-3,6- dichlorobenzoic acid (a.k.a. 3,6-dichlorosalicylic acid) and methylation of this substrate with subsequent saponification to give high purity dicamba which may be isolated in free acid, salt or ester form (cf e.g. US Patent 3,013,054 for reaction from 2,5-dichlorophenol to dicamba).

In addition, the end product 2,5-dichlorophenol is also useful as an intermediate in the preparation of the commercial acaricide lufenuron.

This process involves either (1 a) nitration of the 2,5-dichlorophenol to give the 2,5-dichloro- 4-nitrophenol and (2a) hydrogenation of the nitro group to give the required intermediate 2,5-dichloro-4-aminophenol, or (1 b) haloalkylation of the 2,5-dichlorophenol to give the 2,5- dichloro-haloalkoxybenzene, (2b) nitration of the benzene ring to give the 2,5-dichloro-4- nitro-haloalkoxybenzene and (3b) subsequent reduction of the nitro group to give the required intermediate 2,5-dichloro-4-amino-haloalkoxybenzene.

The resulting 2,5-dichloro-4-amino-haloalkoxybenzene is then transferred to the corresponding isocyanate derivative, for example with phosgene, and reacted with halobenzamide, for example 2,6-difluorobenzamide, to give lufenuron.

The preparation of lufenuron from 2,5-dichloro-4-nitrophenol and 2,5-dichloro-4-amino- haloalkoxybenzene respectively is described in EP-A-0 179 021 and EP-A-0 179 022.

The relevant portions of publications and other patent documents cited herein are hereby also incorporated by reference.

The following examples illustrate the invention. Temperatures are in degrees centigrade.

EXAMPLE 1 To a solution of 1.47 g (10 mmol) of 1,4-dichlorobenzene dissolved in 20 mL of acetic acid is added with stirring 0.182 g (1 mmol) vanadium(V) oxide followed by the gradual infusion of 2mL of 50% aqueous hydrogen peroxide (30 mmol) over 8 hours. After 24 hours at 209C the solid is removed by filtration, and washed with methanol. The combined filtrates are treated dropwise with concentrated aqueous sodium bisulfite to destroy any residual hydrogen peroxide and the solvents are removed in vacuo to give the desired product. This may be purified e.g. by vacuum distillation (b.p. 70gC at 2 mm pressure) to give 2,5- dichlorophenol, m.p. 57-59C.

EXAMPLE 2 To a solution of 1.47 g (10 mmol) of 1,4-dichlorobenzene dissolved in 20 mL of acetic acid is added with stirring 1.0 g of 3.5 weight percent vanadium(V) oxide deposited on silica (0.2 mmol vanadium catalyst) the reaction is heated to 45C followed by the gradual infusion of 0.67 mL of 50% aqueous hydrogen peroxide (10 mmol) over 8 hours. After 16 hours at 45C the solid is removed by filtration, and washed with.methanol. The combined filtrates are treated dropwise with concentrated aqueous sodium bisulfite to destroy any residual hydrogen peroxide and the solvents are removed in vacuo to give the desired product. This may be purified e.g. by vacuum distillation (b.p. 709C at 2 mm pressure) to give 2,5-dichlorophenol, m.p. 57-59C.

EXAMPLE 3 To a solution of 1.47 g (10 mmol) of 1,4-dichlorobenzene dissolved in 20 mL of acetic acid is added with stirring 265 mg (1 mmol) vanadyl acetylacetonate followed by the gradual infusion of 2.0 mL of 50% aqueous hydrogen peroxide (30 mmol) over 8 hours. After 16 hours at 209C the solid is removed by filtration, and washed with methanol. The combined filtrates are treated dropwise with concentrated aqueous sodium bisulfite to destroy any residual hydrogen peroxide and the solvents are removed in vacuo to give the desired product. This may be purified e.g. by vacuum distillation (b.p. 709C at 2 mm pressure) to give 2,5-dichlorophenol, m.p. 57-59C.

Example 4: 2,5-Dichloro-(1 , 1 ,2,3,3,3-hexafluoropropoxy)-benzene Molten 2,5-dichlorophenol (1.0 mol) is dissolved in acetonitrile, potassiumhydroxide (0.2 mol) and water. To the solution, hexafluoropropylene (1.025 mol) is added at 25-30°C over 8 hours. Dilute hydrochloric acid (10% water solution) is added simultaneously to hold the pH-value of the solution at 8.5-9.5. After the reaction is completed (controlled by gaschromatography) 500 ml of acetonitrile/water is distilled off at 60°C/600 mbar. Finally the lower layer is separated and the product is distilled at 80"C and 10 mbar.

Example 5: 2,5-Dichloro-(1 , 1 ,2,3,3,3-hexafluoropropoxy)-4-nitrobenzene 2,5-Dichloro-(l,l ,2,3,3,3-hexafluoropropoxy)-benzene (1.0 mol, Example 4) is dissolved in concentrate sulfuric acid. Then a mixture of fuming nitric acid (1.15 mol) and concentrated sulfuric acid (0.75 mol) is added over 3 hours at 25-40°C. To complete the reaction, the mixture is stirred for a further 3 hours (conversion control by gaschromatography). Then the mixture is poured into cold water and neutralized with sodium hydroxide. The mixture is extracted with toluene. Finally the nitro-product is isolated by evaporation of the toluene at 50-60"C and 100-200 mbar.