LU81911A1 - PHENOXYPROPIONIC ACID DERIVATIVES, PROCESS FOR THEIR PREPARATION, HEBICIDAL COMPOSITIONS CONTAINING THEM AND METHOD FOR CONTROLLING WEEDS USING THE SAME - Google Patents

Description

’-6-1S i ί iAND-DUCHË DE LUXEMBOURG J). 5o.841 ^ Patent N ° ........................................... ... ^ du i§ [^ jDV € a ^ re_ _ 197 {· Minister, Æiwïïfi of the National Economy and the Middle Classes

Book title: ................................... iSK

Industrial Property Service

Z / 77] LUXEMBOURG

here ^ u - li '. [fj i "\ <TO Application for Patent of invention I. Request ..La .... S..Q.Ci, é.fcé ..... ölte.: .. .... SHELL ..... INEEM & TZQNALE lŒSiÂRÇH .. ^ ............ {1) BV, ..... Carel van Bvlandtlaan 3o, ..... DEN HAAG # ..... Netherlands, ..... represented 4 by Mr. Jacques de Muyser acting as (2) agent ..................... .................................................. .................................................. .................................................. .................................................. ..................

deposits ........ this ...... nineteen November 19oo ..... seventy-nine ................... .. {3) at ......................... hours, at the Ministry of National Economy and the Middle Classes, in Luxembourg: „1 the present request for obtaining a patent of invention concerning: "Derivatives ..... of phenoxyphenoxypropionic acid, process for ................... ...... φ their preparation, herbicidal compositions containing them and ....... process for ..... combating weeds ..... using them ".

declares, assuming responsibility for this declaration, that the inventor (s) is (are): ..yoir ... m ... ye.rso .............. .................................................. .................................................. .................................................. .................................................. ..... (5) 2. the delegation of power, dated .......... Dell Haag ..................... .... le ... 16— 3. description in language ...... French ........................... .......... of the invention in two copies; 4 ............... LL .......... drawing boards, in two copies; 5. the receipt of the fees paid to the Registration Office in Luxembourg on November 19. 1979 claims for the above patent application the priority of one (or more) application (s) of (6) ...... ...............Patent.................................. .............. filed ^ in (7) ........ G ^ nde ^ Bretagne ................. ...........................................

*. November 21 ..... 1978 (No. 45374/78) .................................. ......................................... _ (8) c.

in the name of the applicant .............................................. .................................................. .................................................. ......................................... (9) takes up residence for him (she ) and, if appointed, for its representative, in Luxembourg ...................................... ..

3 5, ^ 73 ^ al (10) requests the issue of a patent for the subject described and represented in the abovementioned appendices, - with postponement of this issue to ........ ° .. .................................. months.

| Le î ^ andat ^ ire y .............. \ ....... {X .................. ......... v ...... X 'X v "- II. Statement of Deposit

The aforesaid patent application has been filed with the Ministry of National Economy and the Middle Classes, Industrial Property Service in Luxembourg, dated: November 19, 1979 pr i] y [injstre | 3 p.m. the Nation Economy ^^^ es Middle Classes, I A 6S007_ ~ _ / f_ ID. 5σ.841

CLAIM OF PRIORITY

of the patent application / dpp'pp \ pè In GREAT BRITAIN Of November 21, 1978 3d Brief Description filed in support of a request for

PATENT

at

Luxembourg on behalf of: Shell internationale Research maatschappij b.v.

Îpour: "Phenoxyphenoxypropionic acid derivatives, process for their preparation, herbicidal compositions containing them | and process for controlling weeds using them".

* I

»\

The present invention relates to phenoxyphenoxypropionic acid derivatives, processes for their preparation, herbicidal compositions containing them and a method of controlling weeds using them.

It has been described that certain derivatives of 4-phenoxy-phenoxy-alpha-propionic acid have an excellent selective herbicidal effect against weeds in cultivated plants. The Applicant has now found that certain optical isomers isolated from these compounds exhibit a much stronger herbicidal effect against weeds than racemic mixtures. Furthermore, particularly surprisingly, the phytotoxic effects of these isomers isolated against cultivated plants are not notably stronger than the effects of racemic mixtures. The isolated optical isomers therefore have a markedly improved selectivity between weeds and cultivated plants compared to racemic mixtures.

The invention relates to the R isomer of a compound of the general formula: 25 <RV ®> n 0 CH y --- y - Γ r2_c__6i - o- / o \ —o— / oy 30, in which n represents 0 or an integer of /

1 I

Γ * 2 1 to 3; η * represents Ο or an integer from 1 to 3; each R independently represents a halogen atom, a methoxy group or an alkyl group having from 1 to 4 carbon atoms; each R independently represents a halogen atom or a methyl group; and R represents a group of the general formula OR ^, Sr \ NHR ^ or NR ^ R ^, where R ^ represents an equivalent of a salt-forming cation, a hydrogen atom, an alkyl group having from 1 to 10 carbon atoms, a trichloroethyl group, an alkenyl group having from 2 to 4 carbon atoms, a cyclohexyl group optionally substituted by one or more methyl groups, a cyclopentyl group, a phenyl group! possibly substituted by one or two atoms of ha- ?! 4! 15 logenes or a benzyl group; R represents a group! alkyl having 1 to 6 carbon atoms, a group! phenyl optionally substituted with one or two halogen atoms, or a benzyl group; R represents a halogen atom, an alkyl group having from 1 to 4 * 20 carbon atoms, a phenyl group optionally | substituted by one or more substituents chosen from S among halogen atoms and groups -CF ,, Ä 6 ^} -0CF2CF2H and -C02CHj or a benzyl group; R represents an alkyl group having from 1 to 4 carbon atoms J and R ^ represents an alkyl or alkoxy group having from 1 to 4 carbon atoms · \ The R isomer of a compound of the formula general (I) is the isomer whose absolute configuration is:

3 ° COR2 (R1) “1 (RL

î "η η 'n. H ΒΒΒ ^ - £ - "β 0 - / o \ - 0 - (ÿ) 35 CH3" 3

Preferably, any halogen atom present in a compound of the general formula (i) is a chlorine atom and preferably any "alkyl" portion has from 1 to 4 carbon atoms. If R repre-5 has an alkenyl group , it is preferably an allyl group. Preferred salts, that is to say the compounds of general formula (I) in which R represents an equivalent of a salt-forming cation, are alkali metal salts , especially 10 of sodium and potassium.

Preferably, n represents 0, Preferably, n represents 1 or 2 and R or each R represents a halogen atom, especially a chlorine atom.

Preferably, R represents a group of formula 33 OR or R represents an alkyl group, especially an alkyl group having from 1 to 4 carbon atoms, for example a methyl or isobutyl group.

We have found that R- (methyl 2- (4- (2,4-dichlorophenoxy) phenoxy) propionate) and R- (isobutyl-2- (4- (4-chlorophenoxy) phenoxy) propionate) have properties especially useful and they are therefore the particularly preferred isomers according to the invention.

The R isomers according to the invention can be prepared by the flow of racemic mixtures of compounds of general formula (I) by methods analogous to the methods known in the art, for example by chromatography on an optically active column, or by reaction with an optically active reagent, subsequent separation of the resulting geometric isomers and regeneration of the compound of formula (i).

Preferably, however, an R isomer according to the invention is prepared by the chain of two constituents, one of which is in the form of a 35. isolated optical isomer. For example, we can

It “i 4: prepare an R isomer according to the invention by reacting a compound of the general formula; 'iiy ^ 5 AO - / ô \ -0— / ô "\ (II) 1 1 in which R, R, n and n have the meanings indicated for the general formula (I) and A represents an atom of hydrogen or an alkali metal ion, with the S isomer of a compound of the general formula: 0 CH,

It is 3 R2-C-CH-X (III) j in which R2 has the meaning indicated in connection with the general formula (i) and group X is a group which leaves suitable, the reaction being carried out in the presence of 'a base if A represents a hydrogen atom; and optionally by then transforming the R isomer resulting from a compound of the gene-20 (i) line formula into the R isomer of another compound of! general formula (I) by a suitable method whatever conch.

The S isomer of a compound of the general formula (III) has the absolute configuration: 25 ’COR2.

f I H ... mu c 1111 · · · * x 1 5 ° CH3 1 The chain reaction between the compounds of formulas (II) and (III) takes place with the inversion of the optical configuration of the asymmetric carbon atom in the compound of formula 5 III. It is obviously important that the reaction conditions are such that no racemization of the compound of general formula (III) or of the compound resulting from general formula (I) occurs during the reaction.

The group which leaves X can be for example a group of the general formula -rO.SC ^ .Q or -O.CO.Q, where Q is a hydrocarbyl group, for example an alkyl or aryl group having up to 10 10 carbon atoms, a group of the general formula -O.CH (OH) .CHal ^, wherein each.Hal is a halogen atom, conveniently a chlorine, fluorine or bromine atom; or a halogen atom, preferably a chlorine atom. Preferably, the group leaving X is a group -O.SO ^ .Q, especially a methanesulfonyl group or a paratoluene sulfonyl group.

If A represents a hydrogen atom in the compound of the general formula (II), the reaction according to the invention must be carried out in the presence of a base.

This base should be chosen so that racemization does not occur. Preferably, the base is inorganic; it can be, for example, an alkali metal carbonate or bicarbonate, for example sodium carbonate, or an alkali metal alcoholate, for example sodium methylate. If A represents an alkali metal ion, the compound of general formula (II) can be prepared from the corresponding free phenol by reaction with a base, for example an alkali metal alcoholate, for example sodium methylate.

The chain reaction between the compounds of formulas (II) and (III) is conveniently carried out at an elevated temperature, preferably at a temperature between 50 and 200 ° C. Generally, reactions 6 in which A represents a hydrogen atom require a slightly higher temperature than reactions in which the starting material is a "phenolic alkali metal salt formed in advance. If 5 A represents an ion of alkali metal, the reaction temperature is preferably in the range of 70 to 100 ° C; if A represents a hydrogen atom, the reaction temperature is preferably in the range of 110 to 170 ° C .

If one of the reactants is a liquid at the reaction temperature, the reaction can be carried out without added solvent. However, a suitable solvent can be used, for example an optionally substituted aromatic compound, preferably a hydrocarbon, such as toluene or xylene.

The S-isomer of a compound of general formula (III) used as a starting material for the chain reaction can be prepared by replacing the group -OH labeled * in an S-isomer of a compound of general formula : COR2 • * H - C - OH (IV) CH, .5 2 ^ in which R has the meaning indicated with respect to the general formula (I), by the group which leaves X, 30 under reaction conditions such that 'there is no racemization. This replacement can be done for example by reacting a corn- *

Imposed of the general formula (IV) with thionyl chloride and decomposing the resulting compound 35 so as to obtain a compound of the general formula Ü Λ fi Λ ^ * t * 7 (Eli) in which an X represents an atom of chlorine; by esterifying the -OH group in a compound of the general formula (IV) using for example an acid halide or an acid anhydride so as to produce a compound of the general famula (III) in which X represents a group -O.SO ^ .Q or -O.CO.Q; or by reacting a compound of the general formula (IV) with a halogenated aldehyde of general form CHal ^ .CHO so as to produce a compound of the general formula (III) in which X represents a group -O.CH (OH ) .CHalj.

The S isomer of a compound of general formula (IV) has the absolute configuration: COR2 15 1

HT11111 C | | (I I I I. OH

ch3 20

For certain meanings of R2, it may be advantageous to prepare an R isomer of a compound of the general formula (I) and then transform this isomer into the R isomer of another compound of the general formula (I) . For example, it may be advantageous to use S - (+) - lactic acid, that is to say the compound of general formula (IV) in which represents a hydrogen atom, as a material for initially due to its easy availability, and then to esterify the resulting compound of general formula (I) by conventional techniques so as to form a compound of general formula (I) in which R represents an alkoxy group. <.-

The R isomers according to the invention have a very selective herbicidal activity against bad plants! · 8 herbs, including wild oats, in cereal fields, such as wheat and especially barley. The R isomer is preferably used as a herbicide in the form of a herbicidal composition. The invention therefore also contemplates a herbicidal composition which comprises an R isomer according to the invention at the same time as a vehicle. A mixture of two or more vehicles can be used.

The invention also relates to a method for selectively controlling weeds in cereal fields, especially barley, according to which an R isomer according to the invention or a herbicidal composition according to the invention is applied to the cultivated area.

Throughout the present description, the expression "cereal fields" should be understood to include cereals other than oats.

A vehicle in a composition according to the invention is any material with which the active ingredient is mixed to facilitate its application to the place to be treated, which can be for example a plant, seeds or soil or for facilitate its storage, transport or handling. A vehicle can be a solid or a liquid, including one which is normally gaseous, but which has been compressed to form a liquid, and any of the vehicles normally used in the preparation can be used. ration of herbicidal compositions.

Solid vehicles which can be used include natural and synthetic clays and silicates, for example natural silicas such as diatomaceous earth; magnesium silicates, for example, * talcs; magnesium and aluminum silicates, for example attapulgites and vermiculites; of

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9 aluminum silicates, for example kaolinites, montmorillonites and micas; calcium carbonate; calcium sulfate; synthetic hydrated silicon oxides and synthetic calcium 5 or aluminum silicates; elements, for example carbon and sulfur; natural and synthetic resins, for example coumarone resins, polyvinyl chloride and styrene polymers and copolymers; solid polychlorophenols; bitumen ; waxes, 1Ό for example beeswax, paraffin wax and chlorinated mineral waxes; and solid fertilizers, for example superphosphates.

Liquid carriers which can be used include water, alcohols, for example isopropanol and glycols; ketones, for example acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; ethers; aromatic and araliphatic hydrocarbons, for example benzene, toluene and xylene; petroleum fractions, for example kerosene and light mineral oils; chlorinated hydrocarbons, for example carbon tetrachloride, perchlorethylene and trichloroethane. Mixtures of different liquids are often used.

The herbicidal compositions are often made up and transported in a concentrated form which is then diluted by the user before application. The presence of small amounts of a vehicle which is a surfactant facilitates this dilution. Thus, preferably, at least one vehicle in a composition according to the invention is a surfactant.

A surfactant can be an emulsifying agent, a dispersing agent or a wetting agent; it can be non-ionic or ionic. Examples of usable surfactants include sodium or calcium salts of polyacrylic acids and lignin sulfonic acids; condensation products of fatty acids or aliphatic amines or amides containing at least 12 carbon atoms 5 in the molecule with ethylene oxide and / or propylene oxide; fatty acid esters of glycerol, sorbitan, sucrose or pentaerythri-tol; condensation products of these esters with ethylene oxide and / or propylene oxide; condensates of fatty alcohols or alkylphenols, for example p-octylphenol or p-octylcresol, with ethylene oxide and / or propylene oxide; sulfates or sulfonates of these condensation products; alkali or alkaline earth metal salts, preferably sodium salts, sulfuric acid esters or sulfonic acid containing at least 10 carbon atoms in the molecule, for example sodium lauryl sulfate, dry sodium alkyl sulfates, sodium salts of sulfonated castor oil and sodium alkyl sulfonates such as sodium docecylbenzene sulfonate; and polymers of ethylene oxide and copolymers of ethylene oxide and propylene oxide.

The compositions according to the invention can be prepared for example in the form of wettable powders, dusting powders, granules, solutions, emulsifiable concentrates, emulsions, suspended concentrates and aerosols. Wettable powders * usually contain 25, 50 or 75% by weight of active ingredient and usually contain, in addition to an inert solid vehicle, from 5 d to 10% by weight of a dispersing agent, and when necessary, from 0 to 10% by weight of one or more stabilizers and / or other additives such as penetrating agents or adhesives. The sprinkle powders are usually composed in the form of a powder concentrate having a composition similar to that of a wettable powder, but without dispersant, and they are diluted in their place of use. 5 use by means of additional solid vehicle so as to give a composition usually containing from 0.5 to 10% by weight of active ingredient. The granules are usually prepared to be between 0.152 and 1.676 mm in size and can be produced by agglomeration or impregnation techniques. Generally, the granules will contain from 0.5 to 25% by weight of active ingredient and from 0 to 10% by weight of additives such as stabilizers, binding release modifiers and binders.

Emulsifiable concentrates usually contain, in addition to a solvent and, when necessary, a co-solvent, from 10 to 50% by weight / volume of active ingredient, from 2 to 20% by weight / volume of emulsifiers and from 0 to 20% w / v of other additives such as stabilizers, penetrating agents and corrosion inhibitors. Suspended concentrates are usually composed so as to obtain a stable, non-settling fluid product and usually contain from 10 to 75% by weight of active ingredient, from 0.5 to 15% by weight of dispersing agents, from 0.1 to 10% by weight of suspending agents such as protective colloids and thixotropic agents, from 0 to 10% by weight of other additives such as demulsifiers, corrosion inhibitors , stabilizers, penetrating agents and adhesives and water or an organic liquid in which the active ingredient is substantially insoluble; certain organic solids or certain organic salts may

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12! j be present dissolved in the composition to help prevent sedimentation or as antifreeze for water.

Aqueous dispersions and emulsions, for example compositions obtained by diluting a wettable powder or a concentrate according to the invention with water, are also included in the general scope of the present invention. These emulsions can be water-in-oil or oil-in-water type and can have a thick consistency of "mayonnaise".

The compositions according to the invention can also contain other ingredients, for example other compounds having pestici-! 15 of herbicides or fungicides.

The following examples illustrate the invention.

Example 1

Synthesis of R - (- 0- (methvl 2- (4- (2,4-dichlorophenoxy) nhenoxy) nronionate 20 Du 4- (2,4-dichlorophenoxy) phenol (5.10 g, I2.0 x 10 '2 mole), S - (-) - ethyl lactate mesylate (ajp s -70.17 ° · (without solvent)) (5.64 g, 2.0 x 10 ”2 mole) and Na ^ CO ^ (1.16 g, 1.1 x 10 mol) are heated at 150 ° C. for 8.5 hours The conversion * 2t5 to products is measured by gas chromatography as being 40%, 71% and 87% after 1,; 4 and 8.25 hours, respectively.

The mixture is then treated with toluene (25 cm 3) and then washed with water, with a solution. 30 2 M NaOH and with water until neutral. The mixture I is dried azeotropically to give R - (+) -! (methyl 2- (4- (2,4-dichlorophenoxy) phenoxy) propionate)! in the form of an oil (4.92 g, corresponding to a yield of 72%), the optical rotation ajp of a solution at 2% by weight of the product in ethanol (13 is +24, 7. The structure of the product is confirmed using NMR and IR spectroscopy and by elementary analysis.

Example 2 Preparation of R-isobutyl (2- (4-chlorophenoxy) phenoxy) propionate

4- (4-chlorophenoxy) phenol (6.61 g, 0.03 mole) potassium hydroxide (1.68 g, 0.03 mole) and ethanol are heated at reflux for 1 hour. The iO solution is evaporated to a volume of about 50 cm ^, toluene (500 cm ^) is added and the mixture is distilled until the temperature is 110 ° C. At this time, a fine solid suspension separated from the refluxed solution and stirred. S-isobutyl lactate mesylate (6.72 g, 0.03 mole) was added dropwise over 1 hour and the mixture was stirred and heated to reflux for 18 hours. The mixture is washed with water and the toluene layer is evaporated to give a brown oil which is distilled under reduced pressure to give the desired compound.

The first fraction, boiling at 178-180 ° C at a pressure of ~ 0.25 mm Hg, has an optical rotation “25 = + 18.896 ° and the second fraction, boiling at 180-182 ° C at a pressure of 0 , 25 mm 25 Hg, has an optical rotation = +22.4890. Crystals are deposited from these fractions by abandonment and the supernatant oils have the following rotations s

First fraction ï ajp s 26.0 ° 30 Second fraction: ocjp = 31.0 °

Example 3

Selective herbicide activity? R - (+) - (methyl (4- (2,4-dichlorophenoxy) phenoxy) propionate

The active ingredients tested in this example 35 are the racemic mixture R, S - (-) - (methyl (4- (2,4- I * 14 <* dichlorophenoxy) -phenoxy) propionate) and 1 * isolated isomer R- ( +) - (methy1 (4- (2,4-d ichlorophenoxy) phenoxy) propionate) and the experimental plants are barley, Hordeum vulgare and cultivated oats, Avena 5 sativa; cooked oats, Avena sativa. ocrane is a species of sqpâi- menlatiai instead of wild oats, Avena fatua, because Avena sativa is easier to grow and test; experience has shown that the results obtained using Avena sativa are * 10 applicable also to Avena fatua.

The active ingredients tested are incorporated into emulsifiable concentrates containing 20% by weight of active ingredient, 10% by weight of mineral oil, emulsifiers and xylene as solvent.

Each concentrate is diluted with water and the experimental plants are treated by spraying at various doses corresponding to the following: 20 oats Avena sativa I Racemate applied at 3.0, 1.0 and 0.3 kg / ha

R isomer applied at 1.5, 0.5 and 0.15 kg / ha barley Hordeum vulgare

Racemate applied at 4.0, 2.0 and 0.4 kg / ha 25 Isomer R applied at 2.0, 0.6 and 0.2 kg / ha IOne four identical tests are carried out at each dose. After 14 days, the effects of the active ingredients are visually evaluated, for barley on a (scale of 0 to 9, where 0 indicates zero damage and 9 30 indicates death of the plant, and for oats on i a scale from 0 to 100, where 0 indicates the same growth as for the untreated control plants and 100 indicates that there was no visible growth after the treatment.

After 35 days, the fresh weights of the ί / 15 barley plants are measured relative to the untreated controls and after 36 days the fresh weights of the oat plants are measured relative to the untreated controls.

5 The results obtained are subjected to a "probit" analysis, a calculator estimating the parameters a and b in a relation of the form:

Probit {% response) s a + b log (dose)

Using the resulting curve, the doses corresponding to a given level of response can be calculated. In this example, a dose GID ^ q, i.e. the dose required to produce a 50% response to the active ingredient, is calculated for each species, as shown in Table 1. Table 1 also indicates the relative selectivity of each active ingredient between oats and barley. Table 1 Q Selectivity 20 Ingredient ™ 0ID active barley GID50 GID50 50 [(kg / ha) (kg / ha) gID ^ q oats

Visual evaluation 25 Mixture

Racemic 0.65 3.54 5.4

R isomer 0.46 12.72 27.7

Fresh weight Mixture 30 Racemic 1.26 11.25 8.9

R-isomer 0.59 8.62 14.6

It is seen from Table 1 that the R isomer of methvl (4- (2,4-dichlorophenoxy) phenoxy) propionate is significantly more active against oats than the racemic mixture. In addition, the Raurp isomer selecti- /.- I · ί ί! I 16 quickly greatly improved between oats and barley. Example 4

Selective herbicide activity

The active ingredients tested in this example 5 are the racemic mixtures and the R (+) isomers of the following compounds: methyl 2- (4- (2,4-dichlorophenoxy) phenoxy) propionate! (Compound 1)] 2? (4- (4- (4-chlorophenoxy) phenoxy) isobutyl propionate 1 10 (Compound 2), i 1 The ingredients tested are incorporated into emulsifiable concentrates as in Example 3.

4

The doses used are as follows: ί Wheat: racemate, 2.87 kg / ha d 15 isomer, 1.43 kg / ha

Wild oats, Avena fatua: racemate, 1.77 kg /

Iha isomer, 0.89 kg / ha

Three identical tests are performed at each dose. After 8, 14 and 28 days, the average phytotoxicity for the plants is noted. The results are presented in Table 2.

Table 2 j. 25 Average phytoxicity,%

Crazy Wheat - Oats try _______ 8 14 28 8 14 28 | days days days average days days average day

Compound 1 30 mixture 33 41 36 37 60 69 77 69 racemic: l

Compound 1

Isomer R 15 17 17 16 52 67 73 64

Compound 2 d mixture 35 racemic 21 21 24 22 30 32 30 31. / 17

Compound 2

Isomer k 9 16 4 10 38 37 39 38

The results clearly show the improved selectivity of the R isomer of each compound compared to the racemic mixture.

/ - 'Λ:

Claims (15)

1. The R isomer of a compound of the general formula s 5 (R) _ J - H3. o _ <g> - <»10 in which n represents 0 or an integer from 1 to 3; n represents 0 or an integer from 1 to 3 5 each R independently represents a halogen atom, a methoxy group or an alkyl group having from 1 to 4 -1 carbon atoms; each R independently represents p 15 a halogen atom or a methyl group; and R represents a group of the general formula OR, SR, NHR or NR R, where R represents an equivalent of a salt-forming cation, a hydrogen atom, an alkyl group having from 1 to 10 carbon atoms, a trichloroethyl group, an alkenyl group having from 2 to 4 carbon atoms, a cyclohexyl group optionally substituted by one or more methyl groups, a cyclopentyl group, a phenyl group optionally substituted by one or two atoms of halogen or a benzyl group; R ^ represents an alkyl group having from 1 to 6 carbon atoms, a phenyl group optionally substituted by one or two halogen atoms, or a benzyl group; R represents a hydrogen atom, an alkyl group having from 1 to 4 carbon atoms, a phenyl group optionally substituted by one or more substituents chosen from halogen atoms and the groups -CF 1, -OCF 2CF 2 H and -CO2CH2 or a benzyl group; R ^ represents an alkyl group ^ / _ • ·: 19 j η having from 1 to 4 carbon atoms; and R represents an alkyl or alkoxy group having from 1 to 4 carbon atoms. 2. An isomer according to claim 1, Λ 5 characterized in that R represents 0, n represents 1 or 2 and R or each R represents a halogen atom. 3. An isomer according to one of claims p 1 and 2, characterized in that R represents a group of formula OR ^ where R ^ represents an alkyl group. 4. R- (methyl 2- (4- (2,4-dichlorophenoxy) phenoxy) propionate). 5. R- (isobutyl 2- (4- (4-chlorophenoxy) | phenoxy) propionate). 6. A process for the preparation of a compound as defined in one of claims 1 to 5, characterized in that a compound of the general formula 2 ° <| ^ n | AO —- 0- 1 1 in which R, R1, n and ηΊ have the meanings! 25 indicated in connection with the general formula (I) and A, represents a hydrogen atom or an alkali metal ion, with the S isomer of a compound of the general formula: 0 CH, w i 3 R2 - C - CH - X (III) '2 in which R has the meaning indicated with respect to the general formula (I) and group X is a group which leaves suitable, the reaction being carried out in the presence of a base if A represents a hydrogen atom; and optionally transforming the “R” isomer resulting from a compound of the general formula (I) into the R isomer of another compound of the general formula (I) by any suitable process . 7. A method according to claim 6, characterized in that X represents a group of the 1 ”general formula -O.SO2.Q or -0.C0.Q, where Q is a hydrocarbyl group; a group of the general formula -O.CH (OH) .CHal ,, where each Hal is a halo atom! 10 gene; or a halogen atom, preferably a chlorine atom. 8. A process according to claim 7, “characterized in that X represents a methane-sulfonyl group or a para-toluenesulfonyl group. 9. A method according to claim 8, characterized in that it is carried out at a temperature between 50 and 200 ° C. 10. An isomer according to claim 1,! characterized in that it is prepared by a process 20 according to one of claims 6 to 9 · 11. A herbicidal composition characterized 1. in that it comprises an isomer as defined in one of claims 1 to 5 and 10 together with a vehicle. 12. A composition according to claim 11, characterized in that it comprises at least two vehicles, at least one of which is a surfactant. 13. A method for selectively controlling weeds in cereal fields, characterized in that a compound as defined in one of claims 1 to 5 and 10 or a composition such as that applied to the cultivated area as defined in one of claims J 35 11 and 12. / ~ \ Λ "* '21 14. A method according to claim 13, characterized in that the cultivated cereal is barley. 15. A method according to claim 13, characterized in that R- (methyl 2- (4- 5 (2,4-dichlorophenoxy) phenoxy) propionate) is applied to a cultivated area carrying barley.