GB2306956A

GB2306956A - Oxidation of alkylaromatic compounds

Info

Publication number: GB2306956A
Application number: GB9522850A
Authority: GB
Inventors: Craig Warren Jones; Scott William Brown
Original assignee: Solvay Interox Ltd
Current assignee: Solvay Interox Ltd
Priority date: 1995-11-08
Filing date: 1995-11-08
Publication date: 1997-05-14
Also published as: GB9522850D0

Abstract

A process for the oxidation of alkylaromatic compounds comprises reacting an alkylaromatic compound comprising an alkyl hydrogen atom, and optionally a hydroxyl group, bonded to the carbon alpha to the aromatic ring with hydrogen peroxide in the presence of a catalyst. The catalyst comprises a source of bromide ions and an acid, the mole ratio of each of source bromide ions and acid to substrate being no more than 0.5 : 1. The process employs as solvent a low molecular weight, oxidation resistant nitrile or alcohol, preferably t-butanol. The process is particularly suitable for the oxidation of methylaromatic compounds to the corresponding aldehydes.

Description

Oxidation of Alkylaromatic Compounds This invention concerns a process for the oxidation of alkylaromatic compounds. More specifically, this invention concerns a catalysed process for the oxidation of alkylaromatic compounds with peroxygen compounds.

The oxidation of alkylaromatic compounds is a desirable reaction in organic chemistry because it permits the conversion of a readily available organic substrate having only iimited reactivity into a compound including more reactive functional groups and hence having greater reactivity. Oxidation of alkylaromatic compounds commonly involves the oxidation of an alkyl hydrogen which requires the use of a relatively strong oxidant. It is therefore readily apparent that such an oxidant can often readily be employed for the oxidation of other more easily oxidised functional groups such as alcohols.

Many systems have been proposed for carrying out the oxidation of alkylaromatic compounds, including the use of transition metal oxides such as manganese dioxide, potassium permanganate and chromium oxide, and other oxidants such as organic peracids and air or gaseous oxygen. One particularly desirable oxidant that has been employed comprises an aqueous solution of hydrogen peroxide. In order to successfully employ hydrogen peroxide in the oxidation of alkylaromatic compounds it is often necessary to employ some other component such as a co-reagent or a catalyst. Commonly a transition metal ion catalyst is employed. Examples of the use of co-reagents with hydrogen peroxide include the oxidation systems disclosed by European patents nos. 0 334 511 and 0 568 892, and International patent application No. WO 90/02731.European patent No. 0 334 511 discloses that diphenylmethanes can be oxidised to benzophenone by an oxidation system comprising bromine, generated in situ from at least 1 mole of HBr and 1.5 moles of H202 per mole of the diphenylmethane, under photolytic conditions in the presence of hydrophobic solvents such as hydrocarbons or chlorinated solvents. When photolysis-free conditions were employed in Comparison CA of European patent No. 0 334 511, only a very low conversion (10%) of a relatively readily oxidised substrate was achieved, despite the use of an extended reaction period compared with photolysed examples. International patent application no.WO 90/02731 discloses that 4-sulpho-2-chlorobenzyl alcohol or the corresponding aldehyde can be oxidised to the corresponding acid by bromine, generated in situ by the reaction between HBr and H202 under photolysis-free conditions, in the presence of chlorinated organic solvents. The mole ratios of HBr and H202 to substrate are substantially stoichiometric or superstoichiometric. European patent application no. 0 569 892 discloses that ditertiaryalkylmethyl phenols can be oxidised by bromine generated in situ from an oxidation system comprising HBr and of H202, with at least 1.5 moles of HBr and of H202 per mole of substrate being employed.

Photolysis-free conditions are preferably employed. The use of a chlorinated solvent is required, although comparative examples in which t-butanol is employed as solvent are disclosed. In these comparative examples, relatively poor conversions of substrate and low yields of product are achieved.

European patent application no. 0 569 892 therefore teaches that t-butanol is a poor solvent for use in the oxidation of alkylaromatic substrates by HBr as a co-reagent with hydrogen peroxide.

The use of an oxidation system in which bromine is generated in situ at a potentially high concentration has a number of disadvantages resulting from the toxic and corrosive properties of bromine. Additionally, systems in which photolysis is required can entail significant capital expenditure to provide the necessary photolysis apparatus, coupled with significant operating costs.

These problems can in theory be substantially avoided or ameliorated by the use of a catalytic oxidation system. A number of transition metal-based oxidation systems have been disclosed for the oxidation of alkylaromatic compounds with hydrogen peroxide. International patent application no. WO 93/00319 discloses that alkylaromatic compounds can be oxidised by hydrogen peroxide in the presence of a low molecular weight aliphatic acid solvent and a catalyst system comprising cobalt salts and catalytic amounts of bromide ions. International patent application no. WO 95/09139 discloses that alkylaromatic compounds can be oxidised by hydrogen peroxide in the presence of a low molecular weight aliphatic acid/anhydride solvent and a catalyst system comprising cerium salts and catalytic amounts of bromide ions.

Comparative example 3 of WO 95/091 39 discloses that when t-butanol was employed as solvent in place of the acid/anhydride, no conversion of substrate was observed, again indicating that t-butanol is a poor solvent for use in the oxidation of alkylaromatic compounds in the presence of transition metal catalysts.

Although successful oxidations of alkylaromatic compounds can be achieved employing transition metal-containing catalysts, concerns about the potential discharge of such metals into the environment, coupled with the potential costs of employing and recovering the metals means that it is desirable to identify catalytic systems for the oxidation of alkylaromatic compounds with hydrogen peroxide which do not require the use of transition metal catalysts. Additionally, the use of aliphatic acid or anhydride solvents can pose problems. The oxidation of an alkylaromatic compound can be regarded as proceeding via a number of stages i.e. alkyl o alcohol o carbonyl compound , carboxyl compound (if appropriate).The formation of an intermediate alcohol in the presence of aliphatic acid/anhydride can result in the formation of significant quantities of esters. Although this may be advantageous in certain circumstances, the esters are generally of little commercial importance. It would therefore be desirable to identify catalytic systems for the oxidation of alkylaromatic compounds with hydrogen peroxide which do not require the use of an aliphatic acid or anhydride solvent.

In addition to the stages of the oxidation of alkylaromatic compound outlined in the preceding paragraph, the presence of additional reagents may lead to the formation of other compounds, for example mono- and/or dibrominated species in the case of a bromide co-reagent. These compounds can sometimes be easily oxidised and/or hydrolysed to produce alcohols, carbonyl or carboxyl compounds. It will be recognised that once an alcohol or carbonyl compound, especially an aldehyde has been formed, the oxidising conditions can often result in this compound being further oxidised. For this reason, it is often difficult to halt the oxidation at one of the intermediate oxidation products, and on account of this, such Intermediate compounds can command a premium price. It would therefore be particularly desirable to identify an oxidation system for alkylaromatic compounds that, in addition to not requiring the use of bromine or a transition metal catalyst and/or the use of an acid or anhydride solvent, was effective at producing such intermediate oxidation products and/or which produced compounds that could be conveniently converted to such intermediate products.

It is a first object of certain aspects of the present invention to provide a process for the oxidation of alkylaromatic compounds with hydrogen peroxide that does not require the use of bromine or a transition metal catalyst and/or the use of an acid or anhydride solvent.

It is a second object of further aspects of the present invention to provide an oxidation system for alkylaromatic compounds that, in addition to not requiring the use of bromine or a transition metal catalyst and/or the use of an acid or anhydride solvent, is effective at producing intermediate oxidation products and/or which produced compounds that could be conveniently converted to such intermediate oxidation products.

According to the present invention, there is provided a process for the oxidation of an alkylaromatic substrate comprising an alkyl hydrogen atom, and optionally a hydroxyl group, bonded to the carbon alpha to the aromatic ring by contacting the substrate with hydrogen peroxide in a reaction medium comprising a catalyst and a solvent, characterised in that the catalyst comprises a source of bromide ions and an acid, the mole ratio of each of source bromide ions and acid to substrate being no more than 0.5 : 1, and that the solvent is selected from the group consisting of low molecular weight, oxidation resistant nitriles and alcohols.

According to a second aspect of the present invention, there is provided a process for the selective oxidation of methylaromatic substrates to aldehydes by contacting the substrate with hydrogen peroxide in a reaction medium comprising a catalyst and a solvent, characterised In that the catalyst comprises a source of bromide ions and an acid, the mole ratio of each of source bromide ions and acid to substrate being no more than 0.5 : 1, and that the solvent is selected from the group consisting of oxidation resistant, low molecular weight nitriles and alcohols.

The alkylaromatic compounds which can be oxidised by the process of the present invention are those which comprise at least one alkyl, preferably a (C1-C6} alkyl, substituent having at least one hydrogen atom and optionally a hydroxyl group, at the alpha position relative to the aromatic ring. Although higher alkyl substituents may be oxidised by the process of the present invention, such as those having up to 30 carbon atoms, (C1-C6) alkyl substituents are preferred. From the preferred group of substituents, straight chain (C1-C6) alkyl substituents and branched chain alkyl substituents having less than 5 carbons are most preferred.Examples of alkylaromatic compounds which can be oxidised by the present invention include alkylbenzenes, such as toluene, ethylbenzene, p-t-butyltoluene, cumene, o-, m- or p-xylenes, o-, m-or p-diethylbenzenes and polynuclear alkylaromatic compounds such as the mono-, di- and tri-alkyl naphthalenes, e.g. methyl naphthalenes, ethyl naphthalenes and dimethylnaphthalenes.

The alkyl substituent or substituents of the alkylaromatic compounds can be any substituted alkyl which may be oxidised to an alcohol, a ketone or carboxylic acid as may be appropriate. Such substituted alkyls commonly have at least one hydrogen atom, and optionally a hydroxy group, at the alpha position relative to the aromatic ring. Alkyls substituted with at least one phenyl-, hydroxy-, halo- or oxy- substituent are examples of the substitutedalkylaromatic compounds which may be oxidised by the process of the present invention. For example, when the alkyl- substituent is methyl-, the methylmay be a mono- or di-substituted methyl- of the formula -CHR1R2 where R1 and R2 are independently selected from the group consisting of H-, substituted or unsubstituted phenyl-, -OH and -hal (-hal is -F, -Cl, -Br or -I).Examples of substituted-alkylaromatic compounds are diphenylmethane and diphenylethane. Preferably, the alkylaromatic compound is a methylaromatic compound.

The alkylaromatic compounds that can be oxidised by the process according to the present invention can comprise an aromatic ring which is substituted with one or more substituents. The range of substituents that can be present will depend on the presence or absence of a hydroxy group at the position alpha to the aromatic ring, and on the position of substitution, i.e.

ortho, meta or para to the alkyl group. When a hydroxy group is present alpha to the aromatic ring, the substituent can be one or more of nitro, alkyl, halo, aldehyde, ketone, carboxylate or sulphonic acid groups, and preferably alkyl, and the substituent can be present at any position on the ring.

When a hydroxy group alpha to the aromatic ring is not present, the nature of the substituent that can be tolerated will often depend on the position of substitution. Substituents meta to the alkyl group can be selected from the above list for when the hydroxy group Is present. Substituents ortho and/or para to the alkyl group must not be such that the aromatic ring is net strongly deactivated, such as by the presence by themselves of strongly electron withdrawing groups such as nitro or aldehyde groups, although it is believed that the effect of such groups may be ameliorated by the presence of one or more activating, electron donating groups such as alkoxy or amino groups.The aromatic ring must not be net strongly activated to ring bromination, such as by the presence by themselves of alkoxy and amino groups, although it is believed that the effect of such groups can be ameliorated by the presence of one or more deactivating, electron withdrawing groups such as nitro, aldehyde or carboxyl groups. In many embodiments, substituents ortho and/or para to the alkyl group when a hydroxy is not present alpha to the aromatic ring are selected from activating or neutral groups such as alkyl and halo groups.

It will be recognised that in addition to the oxidation of alkylaromatic substrates, alternative substrates may also be oxidised in the process of the present invention, including especially secondary, aliphatic alcohols, particularly octan-2-ol and 1 , 3-dichloropropan-2-ol. Substantially the same conditions are employed for the oxidation of these alternative substrates as are employed for the oxidation by the process of the present invention of alkylaromatic compounds.

The hydrogen peroxide is preferably introduced into the reaction medium in the form of a concentrated aqueous solution, often comprising from about 25 to about 70% w/w, and frequently from about 30 to 50% w/w hydrogen peroxide. In certain embodiments of the present invention, the hydrogen peroxide can comprise an inorganic or organic source of hydrogen peroxide , such as a persalt, e.g. sodium percarbonate or sodium perborate mono or tetrahydrate, or an organic hydrogen peroxide adduct, such as urea hydrogen peroxide. Preferably, the hydrogen peroxide is introduced into the reaction medium which contains both the substrate and catalyst system, and particularly preferably it is introduced gradually, for example over a period of from 1 5 minutes to 4 hours.An addition of hydrogen peroxide over a relatively short time, such as from 0.5 to 1.5 hours may in some embodiments favour a more selective production of aldehydic products. In certain embodiments of the present invention, a plurality of hydrogen peroxide additions, with optionally a plurality of additions of source of bromide ions, at intervals during the reaction can be employed.

The hydrogen peroxide can be introduced into the reaction medium in stoichiometric, sub-stoichiometric or greater than stoichiometric amounts, based on the mole ratio of hydrogen peroxide to alkylaromatic compound. It may be preferable to employ a sub-stoichiometric amount of hydrogen peroxide when the substrate is particularly sensitive to further oxidation. In most embodiments, however, it is preferred to employ at least a stoichiometric amount of hydrogen peroxide, and often an amount in excess of the stoichiometric amount, such as up to 10 moles of hydrogen peroxide per mole of alkylaromatic, i.e. up to an excess of 9 times, and preferably from about 1.5 to about 5 moles of hydrogen per mole of alkylaromatic, i.e. an excess of about 0.5 to about 4 times above the stoichiometric amount.

The catalyst employed in the process according to the present invention comprises a source of bromide ions and an acid, each employed in no more than a catalytic amount, i.e. a substantially sub-stoichiometric mole ratio to the substrate. The mole ratio of each of bromide ions and acid to substrate is generally no more than 0.5 : 1, preferably no more than 0.4 : 1, often in the range of from 0.01 1 ? to 0.3 : 1, and commonly in the range of from 0.05 : 1 to 0.2 : 1. Sources of bromide ions that can be employed in the process according to the present invention can be selected from the group consisting of hydrogen bromide and bromide salts.Suitable bromide salts include alkali metal bromides, particularly sodium and potassium bromide, alkaline earth metal bromides such as magnesium and calcium bromide, and amine-derived salts such as quaternary ammonium bromides and ammonium bromide. It is believed that molecular bromine may also be employed as a source of bromide ions, but this would entail tolerating the disadvantageous properties of molecular bromine, and so is not preterred. In certain embodiments of the present invention, when hydrogen bromide is employed as source of bromide ions, the hydrogen bromide can also serve as the acid. When this is the case, the mole ratio of hydrogen bromide to substrate is no more than 0.5 : 1.The concentration of source of bromide ions in the total of substrate, solvent and source of bromide is often selected to be from about 0.1% w/w to about 5% w/w, with a concentration of from about 0.5% w/w to about 2.5% w/w being preferred when the source of bromide is an alkali metal or alkaline earth metal bromide.

Acids that can be employed in the process according to the present invention include one or more of mineral acids, organic sulphonic and phosphonic acids and polycarboxylic acids comprising 6 or more carbon atoms, i.e. those polycarboxylic acids which are poor at forming esters.

When a mineral acid is employed, it is conveniently selected from sulphuric acid, phosphoric acid, nitric acid or hydrochloric acid. When an organic sulphonic acid is employed, it can desirably be selected from methane sulphonic acid, trifluoromethane sulphonic acid, toluene sulphonic acid, and xylene sulphonic acid. When an organic phosphonic acid is employed, it is preferably selected from aminophosphonic acids including particularly ethylenediaminetetramethylene phosphonic acid, diethylenetriaminepentamethylene phosphonic acid, cyclohexane-1,2diaminotetramethylene phosphonic acid, and hydroxyethylidinediphosphonic acid. Examples of suitable polycarboxylic acids comprising 6 or more carbon atoms include ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid and cyclohexane-1,2-diaminotetraacetic acid.

The acid can be added to the reaction mixture prior to the commencement of the addition of hydrogen peroxide. In such a case the concentration of the acid in the reaction medium before the addition of the hydrogen peroxide is often up to 2% w/w, and is commonly from 0.1% to 1% w/w. In certain embodiments of the present invention, good results, especially high selectivities to an aldehyde product, have been achieved by adding the acid, particularly phosphoric acid, to an aqueous solution of hydrogen peroxide prior to the addition of the hydrogen peroxide solution to the reaction mixture. In such a case, the concentration of acid in the hydrogen peroxide solution is often up to about 20% w/w, and is commonly from about 1 to about 10% w/w.

The solvent employed in the process according to the present invention is selected from the group consisting of low molecular weight, oxidation resistant nitriles and low molecular weight, oxidation resistant alcohols.

Mixtures of two or more such solvents can be employed if desired. In certain embodiments, a solvent may be employed to achieve the desired solubility properties of substrate and product. For example, it may be desirable to select a solvent or solvent mixture in which the substrate is readily soluble, but in which the product is not soluble, in order to facilitate separation of the product from the reaction mixture. Examples of suitable low molecular weight oxidation-resistant nitriles include particularly C2 to C6 aliphatic nitriles including adiponitrile, isovaleronitrile, hexanenitrile, and especially acetonitrile.

Examples of suitable low molecular weight oxidation-resistant alcohols include particularly aliphatic C1 to C6 alcohols including ethanol, hexan-1-ol, and especially methanol and t-butanol. The most preferred solvent is t-butanol.

The process according to the present invention is usually carried out at elevated temperature, typically from 5O0C up to the reflux temperature of the reaction medium, and particularly from about 60 to about 850C. Particularly for substrates which boil under standard atmospheric pressure at lower temperatures than the desired reaction temperature, the reaction may be conducted at an elevated pressure selected so as to permit the desired temperature to be attained, but of course the higher boiling substrates may likewise be reacted at elevated pressure if desired.

The process according to the present invention does not require the use of photolytic conditions, for example, strong irradiation by light to form bromine radicals, and can be carried out in the substantial absence of light if desired. However, there is no requirement for light to be excluded, the process being tolerant of light.

In certain embodiments of the present invention, in addition to the oxidised alkylaromatic compounds, the process produces small, but significant, quantities of monobrominated alkylaromatic compounds. Although it is desirable to minimise the quantity of brominated compounds, their presence is not actually too detrimental to the viability of the process, especially when they can be converted into a carbonyl compound or alcohol.

Such a conversion may be effected by acid or alkali catalysed hydrolysis, followed if desired by further oxidation.

The product(s) of the process according to the present invention can be separated from the reaction medium by conventional means well known to those skilled in the art depending on the physical form of the product at the temperature the separation is to occur. If the product is a solid, separation will often be achievable by filtration or centrifugation. If the product is a liquid, separation will often be achievable by distillation, solvent extraction or an alternative method such as column chromatography. In certain aspects of the present invention, and particularly where very high selectivities to a given product or desired group of products is achieved, unreacted substrate can readily be employed in subsequent oxidations, thus avoiding unnecessary wastage of substrate.

According to a preferred aspect of the present invention, there is provided a process for the oxidation of an alkylaromatic substrate comprising an alkyl hydrogen atom, and optionally a hydroxyl group, bonded to the carbon alpha to the aromatic ring by contacting the substrate with hydrogen peroxide in a reaction medium comprising a catalyst and a solvent, characterised in that the catalyst comprises an alkali metal bromide and an acid selected from the group consisting of sulphuric acid, phosphoric acid and ethylenediaminetetramethylene phosphonic acid, the mole ratio of each of alkali metal bromide and acid to substrate being in the range of from 0.05 : 1 to 0.2 : 1, and that the solvent is t-butanol.

Having described the invention in general terms, specific embodiments thereof are described in greater detail by way of example only.

Comparison 1 4-t-butyltoluene (49, 27 mmol), and t-butanol (409) were charged to a reactor and heated to 700C with stirring. To this reaction medium was added 35% aqueous hydrogen peroxide solution (10.79, 110 mmol) over a period of 1 hour using a peristaltic pump. The reaction was continued for a period of 3 hours after completion of the hydrogen peroxide addition, and then the reaction medium analysed by gas chromatography.

The analysis showed that none of the substrate had been converted, indicating that no reaction had taken place.

Comparison 2 The general method of Comparison 1 was followed, with the addition of 0.5g sodium bromide to the reaction medium prior to heating.

The results of the analysis showed that only 6.7% of the 4-t-butyltoluene was converted, yielding 3.6% 4-t-butylbenzaldehyde with no 4-t-butylbenzylbromide being detected.

Comparison 3 The general method of Comparison 1 was followed, with the addition of 0.29 (0.5 mmol) ethylenediaminetetramethylene phosphonic acid (EDTMP) to the reaction medium prior to heating.

The results of the analysis showed that only 7% of the 4-t-butyltoluene was converted, although no 4-t-butylbenzaldehyde or 4-t-butylbenzylbromide was detected.

Example 4 The general method of Comparison 1 was followed, with the addition of 0.5g (4.9 mmol) sodium bromide and 0.29 (0.5 mmol) EDTMP to the reaction medium prior to heating.

The results of the analysis showed that 27% of the 4-t-butyltoluene was converted, yielding 18.4% 4-t-butylbenzaldehyde and 2.4% 4-t-butylbenzylbromide. This represents a selectivity to aldehyde of 68%, and a total selectivity to desired products (aldehyde and bromide) of 76.8%.

Example 5 The general method of Example 4 was followed, except that 1.7 mmol sulphuric acid (as 98% w/w aqueous solution) was employed in place of the EDTMP.

The results of the analysis showed that 27.6% of the 4-t-butyltoluene was converted, yielding 8.5% 4-t-butylbenzaldehyde and 5.7% 4-t-butylbenzylbromide. This represents a selectivity to aldehyde of 30.9%, and a total selectivity to desired products (aldehyde and bromide) of 55.1%.

Example 6 The general method of Example 4 was followed, except that 3.4 mmol ethylenediaminetetraacetic acid was employed in place of the EDTMP.

The results of the analysis showed that 26% of the 4-t-butyltoluene was converted, yielding 14.6% 4-t-butylbenzaldehyde and 6% 4-t-butylbenzylbromide. This represents a selectivity to aldehyde of 56%, and a total selectivity to desired products (aldehyde and bromide) of 79%.

Example 7 The general method of Example 4 was followed, except that 2 mmol phosphoric acid (0.229, 90% w/w aqueous solution was employed in place of the EDTMP.

The results of the analysis showed that 29% of the 4-t-butyltoluene was converted, yielding 14.6% 4-t-butylbenzaldehyde and 2.5% 4-t-butylbenzylbromide. This represents a selectivity to aldehyde of 50.5%, and a total selectivity to desired products (aldehyde and bromide) of 59.1%.

Example 8 The general method of Example 7 was followed, except that the phosphoric acid was added to the hydrogen peroxide solution prior to the addition of the hydrogen peroxide solution to the reaction medium. The addition time for the hydrogen peroxide solution was 1 hour.

The results of the analysis showed that 21.8% of the 4-t-butyltoluene was converted, yielding 18.4% 4-t-butylbenzaldehyde and 2.8% 4-t-butylbenzylbromide. This represents a selectivity to aldehyde of 84.2%, and a total selectivity to desired products (aldehyde and bromide) of 97.2%.

Example 9 The general method of Comparison 1 was followed, except that 7.3 mmol hydrobromic acid (1.289, 48% w/w aqueous solution) was added to the reaction medium prior to heating.

The results of the analysis showed that 23.5% of the 4-t-butyltoluene was converted, yielding 9.6% 4-t-butylbenzaldehyde and 5.7% 4-t-butylbenzylbromide. This represents a selectivity to aldehyde of 41 %, and a total selectivity to desired products (aldehyde and bromide) of 65.2%.

Example 10 The general method of Example 4 was followed, except that diphenylmethane (4.59, 27 mmol) was employed as the substrate.

The results of the analysis showed that 66% of the diphenylmethane was converted, yielding 54.1% benzophenone, a selectivity of 82%.

Example 11 The general method of Example 4 was followed, except that acetonitrile (409) was employed as solvent in place of the t-butanol.

The results of the analysis showed that 17.6% of the 4-t-butyltoluene was converted, yielding 6.3% 4-t-butylbenzaldehyde and 5.3% 4-t-butylbenzylbromide. This represents a selectivity to aldehyde of 36%, and a total selectivity to desired products (aldehyde and bromide) of 66.3%.

Comparison 1 2 The general method of Comparison 2 was followed, except that acetic acid (409) was employed in place of the t-butanol.

The results of the analysis showed that although 29% of the 4-t-butyltoluene was converted, the yield of 4-t-butylbenzaldehyde was only 4.4% and that of 4-t-butylbenzyibromide only 3.5%. This represents a selectivity to aldehyde of only 15%, and a total selectivity to desired products (aldehyde and bromide) of only 27%.

Example 13 The general method of Example 4 was followed, except that the reaction was conducted in the substantial absence of light.

The results of the analysis showed that 20% of the 4-t-butyltoluene was converted, yielding 12.4% 4-t-butylbenzaldehyde and 2.6% 4-t-butylbenzylbromide. This represents a selectivity to aldehyde of 62%, and a total selectivity to desired products (aldehyde and bromide) of 75%.

Example 14 The general method of Example 4 was followed, except that only 5.39 35% w/w aqueous hydrogen peroxide solution was employed.

The results of the analysis showed that 21.2% of the 4-t-butyltoluene was converted, yielding 8.5% 4-t-butylbenzaldehyde and a trace of 4-t-butylbenzylbromide. This represents a selectivity to aldehyde of 40%, and a total selectivity to desired products (aldehyde and bromide) of 40%.

Example 1 5 The general method of Example 4 was followed, except that the reaction was continued for only 2 hours after the completion of the addition of the hydrogen peroxide solution.

The results of the analysis showed that 13% of the 4-t-butyltoluene was converted, yielding 9% 4-t-butylbenzaldehyde and 4% 4-t-butylbenzylbromide. This represents a selectivity to aldehyde of 69%, and a total selectivity to desired products (aldehyde and bromide) of 99%.

Example 1 6 The general method of Example 4 was followed, except that 60g t-butanol was employed as solvent.

The results of the analysis showed that 25.8% of the 4-t-butyltoluene was converted, yielding 14.8% 4-t-butylbenzaldehyde and 3.1% 4-t-butylbenzylbromide. This represents a selectivity to aldehyde of 57.4%, and a total selectivity to desired products (aldehyde and bromide) of 69.4%.

Example 17 The general method of Example 4 was followed, except that 1.3 mmol (0.5g) EDTMP was employed.

The results of the analysis showed that 31 % of the 4-t-butyltoluene was converted, yielding 4.1% 4-t-butylbenzyl alcohol, 10.1% 4-tbutylbenzaldehyde and 3.6% 4-t-butylbenzylbromide. This represents a selectivity to aldehyde of 33%, and a total selectivity to desired products (aldehyde, alcohol and bromide) of 58%.

Example 18 The general method of Example 4 was followed, except that benzyl alcohol (27 mmol) was employed as substrate, and that only 5.39 35% w/w aqueous hydrogen peroxide solution was employed.

The results of the analysis showed that 100% of the benzyl alcohol was converted, yielding 70.4% benzaldehyde and 32% benzoic acid. This represents a selectivity to aldehyde of 70.4%, and a total selectivity to desired products of 102.4%. The fact that a selectivity of > 100% was achieved was probably due to experimental error in the analytical technique.

The results of Examples 4 to 11 and 1 3 to 1 8 clearly show the benefit of the present invention in providing a process for the oxidation of alkylaromatic compounds. The results of Examples 4 to 9, 11 and 1 3 to 1 7 were particularly advantageous in that high selectivities to the aldehyde were generally obtained. The results obtained were significantly better than those achieved in Comparisons 1 to 3, when one or both of source of bromide ions and acid were omitted. The result of Comparison 12 demonstrated that the use of acetic acid as solvent gave very poor selectivities to the desired aldehyde and bromide products compared with the use of the solvents according to the present invention. This is particularly surprising in view of the extensive teaching of the prior art concerning the use of acetic acid as a solvent for use with transition metal and bromide catalysed oxidations of alkylaromatic compounds. The result of Example 13 demonstrated that the process according to the present invention does not require photolytic conditions in order to improve acceptable results.

Claims

1. A process for the oxidation of an alkylaromatic substrate comprising an alkyl hydrogen atom, and optionally a hydroxyl group, bonded to the carbon alpha to the aromatic ring by contacting the substrate with hydrogen peroxide in a reaction medium comprising a catalyst and a solvent, characterised in that the catalyst comprises a source of bromide ions and an acid, the mole ratio of each of source bromide ions and acid to substrate being no more than 0.5 : 1, and that the solvent is selected from the group consisting of low molecular weight, oxidation resistant nitriles and alcohols.

2. A process for the selective oxidation of methylaromatic substrates to aldehydes by contacting the substrate with hydrogen peroxide in a reaction medium comprising a catalyst and a solvent, characterised in that the catalyst comprises a source of bromide ions and an acid, the mole ratio of each of source bromide ions and acid to substrate being no more than 0.5 : 1, and that the solvent is selected from the group consisting of oxidation resistant, low molecular weight nitriles and alcohols.

3. A process according to either preceding claim, characterised in that the mole ratio of bromide ions to substrate is in the range from 0.01 : 1 to 0.3 1, and preferably in the range from 0.05 : 1 to 0.2 1.

4. A process according to any preceding claim, characterised in that the mole ratio of acid to substrate is in the range from 0.01 : 1 to 0.3 : 1, and preferably in the range from 0.05 : 1 to 0.2 : 1.

5. A process according to any preceding claim, characterised in that the source of bromide ions is selected from alkali metal bromides, and preferably is sodium bromide.

6. A process according to any preceding claim, characterised in that the acid is selected from the group consisting of sulphuric acid, phosphoric acid, ethylenediaminetetramethylene phosphonic acid, and ethylenediaminetetraacetic acid.

7. A process according to any preceding claim, characterised in that concentration of source of bromide ions in the total of substrate, solvent and source of bromide is from about 0.1% w/w to about 5% w/w.

8. A process according to any preceding claim, characterised in that the concentration of the acid in the reaction medium before the addition of the hydrogen peroxide is up to 2% w/w.

9. A process according to any one of claims 1 to 7, characterised in that the acid is added to aqueous hydrogen peroxide solution prior to addition to the reaction mixture.

10. A process according to any preceding claim, characterised in that the solvent is t-butanol.

11. A process for the oxidation of an alkylaromatic substrate comprising an alkyl hydrogen atom, and optionally a hydroxyl group, bonded to the carbon alpha to the aromatic ring by contacting the substrate with hydrogen peroxide in a reaction medium comprising a catalyst and a solvent, characterised in that the catalyst comprises an alkali metal bromide and an acid selected from the group consisting of sulphuric acid, phosphoric acid and ethylenediaminetetramethylene phosphonic acid, the mole ratio of each of alkali metal bromide and acid to substrate being in the range of from 0.05 : 1 to 0.2 : 1, and that the solvent is t-butanol.