GB2187737A - Preparation of 2, 6-dihalo-4-cyanophenol esters - Google Patents

Abstract

A compound of formula I <IMAGE> wherein R is alkyl, and X is bromine or iodine is prepared by reacting a compound of formula II in a two phase system, said system comprising an aqueous phase and an organic phase comprising a water immiscible solvent, with an acylating agent of formula III or IV in the presence of a base catalyst. <IMAGE>

Description

SPECIFICATION Process This invention relates to a process for the preparation of organic compounds useful as herbicides.

In particularthis application relates two a process for preparation of compounds offormula I

wherein R is an alkyl group and Xis bromine or iodine Use of certain esters of bromoxynil (4-hydroxy-3,5- dibromobenzonitrile) and ioxynil (4-hydroxy-3,5diiodobenzonitrile) as herbicides has been known for many years. Examples of such derivatives are disclosed in Australia Patent No 288035.

The "Pesticide Manual" (CR Worthing, Editor; the British Crop Protection Council, 6th Edition (1 979)) describes bromoxynil octanoate (2,6-dibromo 4-cyanophenyl octanoate) as a herbicide which has been used commercially for many years forthe post-emergence control of broad leaved weeds in cereal crops.

Despite the considerable process advantages which would be afforded by direct preparation ofthese compounds from 4-hydroxybenzonitrile, it has generally been thought that, due tothe incompatability ofthe reagents and solvents for halogenation and acylation, compounds offormula I could not be prepared without first isolating the dihalodihydroxybenzonitrile intermediate.

Recently Published Australian Patent Application 25250/84 describes a process in which bromoxynil esters are formed from 4-hydroxybenzonitrile in a single halogenated reaction solvent, halogenated solvents being compatible with both halogenating agents and acylating agents.

Although this procedure overcomes the need to isolate the intermediate, the process has several disadvantages resulting in partfrom the requirement that the reaction solvent be resistant to halogenation (e.g., a halogenated alkane).

We have found that it is advantageous to carry out the reaction using a solvent which may be used in the preparation of herbicidal formulations. This overcomes the time-consuming process of isolating the product and redissolving it in a suitable solvent. However, the solvents which are most useful in formulating herbicides (e.g. aromatic hydrocarbons) are generally susceptibleto halogenation.

We have developed an acylation procedure which allows the compounds offormula I to be prepared without the need to isolate the intermediate 4-hydroxy-3,5-dihalobenzonitrile and which allows the compounds to be prepared in a suitable formulation solvent.

We have found that the intermediate 4-hydroxy-3,5-dihalobenzonitrile may be acylated in a two phase system comprising an aqueous phase and an organic phase, in the presence of a catalystseiected from the group consisting of nitrogen-containing bases and phase transfer catalysts.

Accordingly we provide a processforthe preparation of a compound offormula I

wherein R is alkyl and Xis bromine oriodine,which process comprises reacting a compound offormula II with an acylating agent of formula III or IV in a two phase system comprising an aqueous phase and an organic phase comprising a water-immiscible solvent and in the presence of a catalyst selected from the group of nitrogen containing bases and phase transfer catalysts.

Most preferablyX is bromine.

Preferably R is chosen from the group ofC2to C15 alkyl.

More preferably R is chosen from C4to C12 alkyl.

Preferablythe acylating agent is offormula Ill.

It is surprising that the esters offormula I may be prepared in the presence of an aqueous solution as it is well known that acid halides oranhydrides hydrolyse in the presence of water and the prior art procedures use dry solvents.

The acylation procedure we have developed has the further advantage of allowing the halogenation to be conducted in an aqueous mixture.

It will be understood that the use of waters a halogenation medium provides economic and handling advantages which are not present with non-aqueous systems.

Consequently, it is preferred that the hereinabove-described acylation procedure be used in combination with a halogenation process conducted in an aqueous mixture. However, it will be understood that other solvents such as acetic acid may also be used if desired.

Thus, in a preferred embodiment of the present invention, there is provided a process for the preparation of a compound offormula I which process comprises: halogenating 4-hydroxybenzonitrile in an aqueous phase to form a product offormula II; and reacting the product in a two phase system, said system comprising the aqueous phase and an organic phase comprising a water-immiscible solvent, with an acylating agentofformula III or IV in the presence of a catalystselectedfrom the group of nitrogen containing bases and phase transfer catalysts.

Halogenation may be effected by known methods of bromination or iodination such as using molecular halogen (i.e. bromine or iodine optimally in the presence of a Lewis acid catalyst such as ferric bromide; by using an alkali metal hypobromite; or by using another halogenating agent such as bromine chloride.

In the process of the present invention, we have found that it is particularly advantageous to use a halogenation procedure in which a halogenating agent is formed by the in situ oxidation of halide ion-containing materials.

Said halide ion-containing materials may be chosen from bromide or iodide salts of organic and inorganic cations.

Examples of organic and inorganic cations maybe chosen from cations of alkali metals and alkaline earth metals, and ammonium and quaternary ammonium ions.

Preferred halide ion-containing materials may be chosen from the bromide or iodide salts of cations selected from the group of sodium, potassium, magnesium and calcium.

More preferred halide ion-containing materials are the bromide salts of sodium and calcium.

In particularwe have found that advantages are provided when a halogenating agent is generated in situ by reaction of molecular chlorine with halide ion-containing material.

Accordingly, in a particularly preferred of embodimentthe present invention, there is provided a process forthe preparation of a compound offormula I which process comprises reacting an aqueous phase comprising 4-hydroxybenzonitrile and a halide ion-containing material with molecular chlorine to form a productofformula II, and reacting the product offormula II with an acylating agent offormula lil in atwo phase system comprising the aqueous phase and an organic phase comprising a water-immiscible organic solvent and in the presence of a catalyst selected from the group consisting of nitrogen-containing bases and phase transfer catalysts.

In somecaseswe have fond itto be particularly advantageous to use bromide or iodide salts of alkaline earth metal cationsand in particular calcium bromide (when X isto be bromine).

For example, we have found that the use of calcium bromide generally reduces the amount offlocculation, making separation ofthe phases and handling in general much easier. Furthermore, calcium bromide is generally less expensive than alternatives such as sodium bromide.

Examples ofsuitable water-immiscible solvents may include hydrocarbons, chlorinated hydrocarbons such as chloroform, and aromatic hydrocarbons such as benzene, xylene, toluene and mesitylene.

As herein before discussed, the process of the present invention has the advantage of allowing the compounds of formula I to be prepared in solvents suitable forthe particular herbicideformulation. Solvents useful in this regard are selected from aliphatic and aromatic hydrocarbons and particularly preferred solvents are xylene and toluene. However, it will be understood by those skilled in the art that the present invention will allow a certain flexibility in the choice ofthe solvent, and other solvents such as chlorinated hydrocarbons may be used if desired.

The catalyst may be selected from nitrogen-containing bases and phase transfer catalysts. Examples of nitrogen-containing bases include tertiary amines, nitrogen-containing heteroaromatics alkyl-substituted nitrogen containing aromatics. Examples of phase transfer catalysts include quaternary ammonium salts phosphonium salts, crown ethers, and cryptates. Preferred phase transfer catalysts are quaternary ammonium salts and phosphonium salts.

Preferred catalysts include: nitrogen-containing heteroaromatics of 5 or 6 constituent ring members optionally substituted with one or more alkyl groups; and compounds offormula selected from the groups of formulas V, VI and VII

wherein R1, R2 and R3 are independently selected from aliphatic hydrocarbon radicals offrom 1 to 20 carbon atoms, phenyl-substituted C1 to C10 alkyl and pyridyl substituted C1 to C10 alkyl.

R4 is selected from C1 to C6 alkyl and benzyl and Xis an organicorinorganicanion.

Preferred nitrogen-containing heteroaromatics include pyridine optionally substituted with from 1 to 3 C1 to C6 alkyl groups.

Preferred catalysts offormula V and VI are those compounds wherein R1 is selected from the group consisting of C1 to C20 alkyl and benzyl; R2, R3 and R4 are selected from C1 to C6 alkyl and benzyl and Xis bromine or chlorine.

Specific examples of catalysts include tri-n-butylamine, pyridine, N,N-dimethylaminopyridine, benzyltributylammonium chloride, tetra-n-butyl-ammonium chloride, decyldimethylbenzylammonium chloride, cetyltrimethylammonium chloride and tri-n-butylmethylphosphonium chloride.

Generallythe amount of catalystwill be in the range 10 4 to 0.5 mole equivalent of catalyst based on halogenated 4-hydroxybenzonitrile (i.e. compound offormula II) and preferably 0.001 to 0.1 mole equivalent.

We have found that 0.05 equivalent or less is generally adequate to provide an efficient reaction procedure under mild conditions. However, if desired, larger amounts of base catalyst, (as much as 0.5 mole equivalent or more) may be used.

The amounts of reagents used in the process of the present invention may be determined by those skilled in the art having reference to the stoichiometry of the reactions involved and/or by monitoring the formation of halogenated, and subsequently acylated, products using known analytical techniques such as thin layer chromatography or G.L.C.

For example, where bromination is effected by in situ oxidation of a metal bromide using molecular chlorine, the stoichiometry for conversion of 1 mole of 4-hydroxybenzonitrilewill involve 2 mole of bromide ion, 2 mole of molecular chlorine and one mole ofacylating agent.

In general it is preferred to use a slight excess of reagents (for example, 2.2 molar equivalents of bromide ion and molecular chlorine and 1.1 molar equivalents ofacylation agent) although larger excesses may be used if desired.

The conditions required to effectthe reactions involved in the process of the present invention will vary according to the nature of the reactants and the solvent used. The reactions may be carried out at ambient temperature; however, heat may be applied during the reaction or afterthe addition of the acylating agent.

Usually the reaction temperature will be in the range of from 0 to 200"C (preferably O to 1000) and a reaction time will generally be between 0.5 and 20 hours. However, higher or iower reaction temperatures and/or shorterorlongertimes may be employed if desired. In general,the reaction time necessary maybe determined by monitoring the course of the reactions.

Generally we have found that the reaction proceeds more efficiently if the aqueous solution is made alkaline priortothe addition ofthe acylating agent, for example by the addition ofgreaterthan 2 mole equivalents based on 4-hydroxybenzonitrile of sodium hydroxide.

In particular, in many cases we have found that good results are obtained where the aqueous mixture comprising 4-hydroxybenzonitrile and halide ion-containing material further comprises at least two mole equivalents based on 4-hydroxybenzonitrile of a metal hydroxide. (e.g. sodium hydroxide) on In a typical example of the process of the present invention, chlorine gas (approx. 2.2 mole equivalents) is bubbled through an aqueous solution comprising an alkali metal bromide (approx. 2.2 mole equivalents) or alkaline earth metal bromide (approx. 1.1 mole equivalents) and 4-hydroxybenzonitrile (1 mole equivalent), and the mixture is stirred until formation of bromoxynil is substantially complete. The acid generated during the halogenation is neutralised and an excess of base is added (e.g. 1 mole excess).Awater-immiscible organic solvent (e.g. xylene) is added and an acid chloride (1.1 mole equivalents) and catalyst are also added.

The mixture is then stirred until the reaction is substantially complete. In a preferred alternative, the initial aqueous solution comprises 3 mole equivalents of base (e.g. sodium hydroxide), obviating the need to add base during the course ofthe reaction.

It may be preferred to add the acid chloride slowly forexample overa period in the range of 5 minutesto5 hours. Unlike previously used procedures for preparation of bromoxynil esters such as bromoxynil octanoate, the present invention enables preparation from 4-hydroxybenzonitrile withoutthe need to isolate and dry the bromoxynil intermediate. This offers a considerable cost and time saving for large scale preparation.

The in situ halogenation procedure provides several time and cost saving advantages.

The reaction provides maximum utilisation of bromine (or iodine) and it avoids many ofthe of othertechniqueswhich require noxious bromine gas orthe dual handling of both bromine gas and chlorine gas. Furthermore, unlike many halogenation techniques, the brominating oriodinating agent is generated in situ and hence preformation of the agent is not required.

We believe this procedure for halogenation of 4-hydroxybenzonitrile is novel. Hence in a further embodiment of this invention there is provided a process for the halogenation of 4-hydroxybenzonitrile which process comprises reacting an aqueous mixture comprising 4-hydroxybenzonitrile and halide ioncontaining material as hereinbefore defined with molecular chlorine.

For the preparation of bromoxynil (ioxynil) the molar ratios of bromide (iodide) to 4-hydroxybenzonitrile and chlorine to 4-hydroxybenzonitrile are generally in the range 1.9:1 to 2.5:1. Good results are generally obtained when ratio is about 2.2:1.

The invention is now demonstrated but not limited tothefollowing examples - Example I A mixture of 3,5-dibromo-4-hydroxybenzonitrile (27.69 g) sodium hydroxide (4.08 g), sodium chloride (43.4 g) and water (125 ml) was stirred at room temperature for 15 minutes. Toluene (30 ml) and benzyltributyl ammonium chloride (1.57 g)was added, and a solution ofoctanoyl chloride (17.06 g) in toluene (30 ml)was slowly added to the stirred mixture over a half an hour while the temperature was maintained at 20"C. After addition was completed, the mixture was stirred for 1 hour and the phases allowed to separate.After removing the aqueous phase the organic phase was washed with water, then with 5% aqueous Na203 and again with water. The solution was dried over Na204 and evaporated to dryness, yielding 2,6-dibromo-4cyanophenyl octanoate (40.08 g, 99.5%) as a brown crystalline solid.

Example 2 To a 2 Lflaskfitted with a condenser, mechanical stirrer, condenser, gas inlet port, and a sodium hydroxide scrubberwas charged 4-hydroxybenzonitrile (62.5 g, 525 mmol), calcium bromide monohyd rate (125.9 g, 577 mmol), and water (700 mls). Into the resulting suspension was bubbled chlorine gas (85 g, 1197 mmol) at such a rate that the temperature did not rise above 50"C and that bromine fumes were not visibie. After chlorine addition was complete, the thick suspension was stirred for one hour. To the suspension was charged a 46%w/w sodium hydroxide solution (140 g, 1610 mmol), and after half an hourofstirring,toluene (300 mix) and benzyltributylammonium chloride (8.25 g, 26 mmol) were added.Octanoyl chloride (89.6 g, 551 mmol) was added to the rapidly stirred solution over half an hour, while the temperature was kept below 30"C. After addition was complete, stirring was continued for one hour. Stirring was stopped and the phases separated. Thetoluene layerwaswashed in orderwith 15% brine solution (200 mls), 5% bicarbonate (200 mls), and 15% brine solution (200 mls) and was dried by azeotropic distillation of water give 430 g of a brick red solution of bromoxynil octanoate.

Analysis by g.l.c. showed; purity + 95%, assay 46.5% w/wto give a yield of 94.7%.

Example 3 Procedure identical to the above exceptthatthe calcium bromidewas replaced by sodium bromide (1199, 1.15 mmoles). A good conversion to bromoxynil octanoate was observed.

Example 4 Preparation of (2,6-Dibromo-4-cyanophenyl) octanoate To a 1 Lflaskfitted with a condenser, mechanical stirrer, condenser, gas inlet port, and a sodium hydroxide scrubberwas charged 4-hydroxybenzonitrile (62.5 g, 525 mmol), calcium bromide monohydrate (125.9 g, 577 mmol), and water (350 mls). To the resulting suspension was slowly added a 46%w/w sodium hydroxide solution (160 g, 1838 mmol) such thatthe temperature did not rise above 30"C. Chlorine gas (85 g, 1197 mmol) was bubbled into the suspension at such a ratethattheteinperature did not rise above 50"C and thatthe bromine fumes were not visible. When the chlorine addition was complete, the suspension was stirred for one hour. To the suspension was charged toluene (300 mls), and octanoyl chloride (89.6g, 551 mmol)was then added to the rapidly-stirred solution over half an hour, while the temperature was kept below30 C.

When addition was complete, stirring was continued for an hour. Stirring was stopped, the phases were separated and thetoluene layerwas washed with 15% brine solution (200 mls).Thetoluene layer was dried by azeotropic distillation of water give 330 g of a brick red solution of bromoxynil octanoate.

Analysis by g.l.c. showed; purity 92% w/w; assay 57%w/wto give a yield of 89%.

Example 5 Preparation of 3,5-Dibromo-4-hydroxybenzonitrile The procedure of Example 3 was repeated, except that after chlorine addition and one hour of stirring, the suspension was acidified with 6% w/w hydrochloric acid solution until pH 3 or lower ways achieved. The resulting solid was filtered off and washed several times with warm water. It was then dried in a vacuum oven to give 192 g of crude product; 98% yield. Purity unknown.

Example 6 Preparation of 3,5-Diiodo-4-hydroxybenzonitrile The procedure of Example 4 was repeated except that calcium bromide was replaced with sodium iodide (173.7 g, 577 mmol). Thefinal resultwas 1 54 g of crude ioxynil, 80%yield. Purity unknown.

Example 7 Ta 2 Lflaskfitted with a condenser, mechanical stirrer, gas inlet port and a sodium hydroxide scrubberwas charged 4-hydroxybenzonitrile (62.5 g, 525 mmol), calcium bromide monohydrate (125.9 g, 577 mmol), and water 1200 ml). Chlorine gas (85 g, 1197 mmol)was bubbled into the suspension at such a ratethatthe temperature did not exceed 50"C and brominefumeswere notvisible. When the reaction was complete, sodium metabisulphite solution (5%, 100 ml) was added to the mixture and the suspension stirred for 15 minutes. The crude bromoxynil was filtered and washed well with water.

Yield after drying 141.5 9.

Example 8 To a mixture comprising sodium hydroxide (1.55 g), water (45 ml) and 3,5-dibromo-4-hydroxybenzonitrole (10 g)was added benzyltributylammonium chloride (0.57 g) and xylene (10.8 ml). The mixture was stirred and calcium chloride (14.1 g) was added.This was followed bythe slow addition (over 30 minutes) ofa mixture ofoctanoyl chloride (6.16 g) and xylene (10.8 ml). The mixture was stirred for 1 hour and the phases allowed to separate. The organic phase was washed with brine (15%) and dried to provide a good yield of bromoxynil octanoate.

Examples 9-12 The procedure of Example 8 was repeated substituting other solvents and catalysts forthe xylene and the benzylbutylammonium ofthat example according to the folowing table: Example Organic Solvent *Catalyst 9 toluene pyridine 10 toluene tri-n-butylamine 11 xylene [C16-C18 alkyl] tri- methylammonium chloride 12 xylene decyldimethyl-benzyl- ammonium chloride * The amount of catalyst used was 0.05 molar equivalents based on bromoxynil.

In all cases, a good conversion to bromoxynil octanoate was observed.

Claims (25)

1. A processforthe preparation ofa compound offormula I

wherein R is alkyl and Xis bromine or iodine which process comprises reacting a compound offormula II in a two phase system, with an acylating agent offormula Ill or IV in a two phase system comprising an aqueous phase and an organic phase comprising awater-immiscible solvent and in the presence of a catalyst selected from the group consisting of nitrogen containing bases and phase transfer catalysts

2. A process according to claim 1 wherein R is C3to C15 alkyl

3. A process according to claim 1 or claim 2 wherein R is C4to C12 aikyl.

4. A process according to any one of claims 1 to 3 inclusive wherein Xis bromine

5. A process according to claim 1 wherein the acylating agent is a compound offormula Ill.

6. A process according to anyone of claims 1 to 5 inclusive wherein the compound offormula I is prepared by halogenating 4-hydroxybenzonitrite in an aqueous phase.

7. A process according to claim 6 wherein halogenation is effected using a halogenation agent selected from the group consisting of molecular bromine, molecular iodine, alkali metal hypohalite and bromine chloride.

8. A process according to claim 6 or claim 7 wherein halogenation is effected using a halogenating agent formed bythe in situ oxidation of halide ion-containing materials.

9. A process according to claim 8 wherein said halide ion containing materials are selected from the groups of bromide and iodide salts of organic and inorganic cations said cations being selected from the group consisting of alkali metal cations, alkaline earth metal cations, ammonium cations, and quaternary ammonium cations.

10. A process according to claim 9 wherein said cations are selected from the group consisting of sodium, potassium, magnesium and calcium.

11. A process accordingto any one of claim 8 to 10 inclusive wherein the halogenating agent is formed by oxidation of halide ion-containing material with molecular chlorine.

12. A process for preparation of a compound offormula 1 according to claim 1 ,the process comprising: reacting an aqueous phase comprising 4-hydroxybenzonitrile and a halide ion containing material with molecular chlorine to form a product offormula II and reacting the product offormula II with an acylating agent offormula Ill in a two phase system comprising the aqueous phase and an organic phase comprising a water-immiscible organic solvent and in the presence of a catalyst selected from the group consisting of nitrogen containing bases a n d and phasetransfercatalysts.

13. A process accoridng to claim 11 or claim 12 wherein the molar ratios of halide ion to 4-hydroxybenzonitrile and chlorine to 4-hydroxybenzonitrole are both in the range 1.9:1 to 2.5:1.

14. A process according to any one of claims 1 to 13 inclusive wherein the water immiscible solvent is selected from the group consisting of aliphatic, hydrocarbons aromatic hydrocarbons and chlorinated hydrocarbons.

15. A process according to claim 1 4wherein the solvent is selected from xylene and toluene.

16. A process according to any one of claims 1 to 15 inclusive wherein the catalyst is selected form the group consisting of tertiary gamines, nitrogen containing heteroaromatics, alkyl substituted nitrogen containing aromatics, quaternary ammonium salts, phosphonium salts, crown ethers and cryptates.

17. A process according to claim 16 wherein the catalyst is selected from the group consisting of: nitrogen containing heteroaromatics of 5 or 6 constituent ring numbers optionally substituted with one or more alkyl groups; and compounds offormula selectedfromthe group offormulasV, VI and VII

wherein: R1, R2 and R3 are independently selected from the aliphatic hydrocarbon radicals of from 1 to 20 carbon atoms, phenyl-substituted C1 to C10 alkyl, and pyridyl-substituted C1 to C10 alkyl; R4 is selected from C1 to C5 alkyl and benzyl; and Xis an inorganic or organic anion.

18. Aprocess according to claim 17 wherein the catalyst is selected from the group consisting of: pyridine optionally substituted with from 1 to 3 C1 to Ce alkyl groups and compounds offormulas V and VI wherein: R1 is selected from C1 to C20 alkyl and benzyl; R2, R3 and R4 are selected from C1 to Ce alkyl and benzyl; and Xisselectedfrom bromine and chlorine.

19. A process according to any one of claims 1 to 18 inclusive wherein the catalyst is present at a concentration in the range of 1 x 104to 0.5 mole equivalents based onthecompound offormula Il

20. A process according to claim 19 wherein the range is 0.001 to 0.1 mole equivalents.

21. A process according to any one of claims 5 to 20 wherein the compound offormula I is prepared from 4-hydroxybenzonitrile wherein the process is carried out in a single reaction-vessel.

22. A process according to claim 1 wherein the aqueous phase is derived from an aqueous phase used in halogenation of 4-hydroxybenzonitrile according to claim 5.

23. A process according to any one of claims 1 to 16 wherein the temperature is in the range 0 to200 C.

24. A process for halogenating 4-hydroxybenzonitrilewhich process comprises reacting an aqueous mixture comprising 4-hydroxybenzonitrile and halide ion containing material as hereinbefore defined with reference to claim 9 orclaim 10, with molecular chlorine.

25. A process according to claim 1 substantially as herein described with reference to the Examples.