GB2202529A - Thiopyran-3,5-dione derivatives - Google Patents

Abstract

Thiopyran-3,5-dione derivatives of general formula I <IMAGE> and of general formula II <IMAGE> or salts thereof [in which R<2> represents a hydrogen atom or an optionally substituted alkyl group; R<3> represents an optionally substituted alkyl group; R<4> represents an optionally substituted alkyl or alkenyl group; and R<5> represents a hydrogen atom or an optionally substituted alkyl group] exhibit herbicidal, in particular, graminicidal activity.

Description

THIOPYRAN-3,5-DIONE DERIVATIVES This invention relates to certain oximino derivatives of various thiopyran-3,5-dione systems, a process for their preparation, herbicidal compositions containing such compounds and to their use in controlling undesired plant growth. The invention also relates to certain thiopyran-3,5-dione derivatives which are useful as intermediates in the preparation of the aforesaid oximino derivatives and to a process for the preparation of these intermediates.

EP 0164056 A2 discloses certain substituted tetrahydrothiopyran-2,4-diones which have herbicidal activity.

However, it has now been discovered that certain thiopyran-3,5-dione derivatives also possess useful herbicidal properties.

According to the present invention there is therefore provided a compound of the general forrttiila I

or a metal salt thereof, in which R represents a hydrogen atom or an optionally substituted alkyl group ; R represents an optionally substituted alkyl group ; R4 represents an optionally substituted alkyl or alkenyl group; and R5 represents a hydrogen atan or an optionally substituted alkyl group.

When the compounds of this invention contain an alkyl or alkenyl substituent group, this may be linear or branched and may contain up to 6, preferably up to 4, carbon atocs, suitable examples being methyl, ethyl, propyl and allyl, and in the case of an alkyl group may also be substituted by a halogen, especially chlorine, atan, or an alkoxy group such as a methoxy group.

When any of the foregoing substituents are designated as being optionally substituted, the substituent groups which are optionally present may be any of those custcmarily employed in the development of pesticidal compounds, and/or the modification of such compounds to influence their structure/activity, persistence, penetration or other property.

Specific examples of such substituents include halogen, especially chlorine, atoms and alkoxy, e.g. methoxy groups.

Particularly preferred compounds are those in which R represents a hydrogen atom, a methyl group or a propyl group ; R represents a methyl, ethyl or propyl group ; R4 represents a methyl, ethyl or allyl group and R5 represents a hydrogen atom or a methyl group.

The compounds of the invention of formula I exist in four tautomeric forms which are in dynamic equilibrium with each other and which can be represented by the following formulae : -

In these formulae R2, R3, R4 and R5 have the meanings given above.

It should also be appreciated that the compounds of formula I are capable of existing as different geometric and/or optical isomers. The invention thus includes both the individual isomers and mixtures of such isomers.

The invention also provides a process for the preparation of a compound of the general formula I as defined above, or a metal salt thereof, which comprises reacting a compound of the aeneral formula II

in which R2, R3 and R5 are as defined above, with a compound of the general formula III 4 R@ - O - NH2 III in which R4 is as defined above, or with an acid addition salt thereof, and if desired, converting the compound of formula I so obtained into a metal salt thereof, especially a sodium salt thereof.

If an acid addition salt of the compound of general formula III is used, the process is suitably carried out in the presence of a base. Preferably the base, if used, is an inorganic base such as sodium hydroxide, potassium hydroxide, sodium carbonate and the like or an organic base such as a tertiary amine, a triakkylarnine such as triethylamine being particularly preferred. The reaction is suitably carried out in the presence of a solvent, preferably an organic solvent. Suitable solvents include ethers, esters, halogenated hydrocarbons and alcohols, ethanol being particularly preferred. The reaction is suitably carried out at a temperature of -200 to 500C, the preferred reaction temperature being 0" to 200C.

The compounds of general formula I have been found to show interesting activity as herbicides and, in particular, as graminicides. Accordingly, the invention further provides a herbicidal composition which comprises a compound of formula I as defined above, or a salt Thereof, in association with a carrier and a method of making such a composition which comprises bringing a compound of formula (I) as defined above, or a salt thereof, into association with at least one carrier.

Such a composition may contain a single compound or a mixture of several compounds of the invention. It is also envisaged that different isomers or mixtures of isomers may have different levels or spectra of activity and thus compositions may ccmprise individual isomers or mixtures of isomers.

In accordance with another aspect of the invention there is provided the use of a compound of formula I as defined above, or a salt thereof, as a herbicide.

The invention also provides a method of combating undesired plant growth at a locus which comprises treating the locus with a compound or composition according to the invention.

Application to the locus may be pre-emergence or post-emergence.

The dosage of active ingredient used may, for example, be 0.01 to 10 kg/ha, preferably 0.05 to 5kg/ha.

A carrier in a composition according to the invention is any material with which the active ingredient is formulated to facilitate application to the locus to be treated which may for example be a plant, seed or soil, or to facilitate storage, transport or handling. A carrier may be a solid or a liquid, including a material which is normally gaseous but which has been compressed to form a liquid, and any of the carriers normally used in formulating herbicidal compositions may be used. Preferably compositions according to the invention contain 0.5 to 95% by weight of active ingredient.

Suitable solid carriers include natural and synthetic clays and silicates, for example natural silicas such as diatanaceous earths; magnesium silicates, for example talcs; magnesium aluminium silicates, for example attapulgites and verniculites; aluminium silicates, for example kaolinites, montmorillonites and micas; calcium carbonate, calcium sulphate; ammonium sulphate; synthetic hydrated silicon oxides and synthetic calcium or aluminium silicates; elements, for example carbon and sulphur; natural and synthetic resins, for example coumarone resins, polyvinyl chloride, and styrene polymers and copolymers; solid polychlorophenols; bitumen; waxes; and solid fertilisers, for example superphosphates.

Suitable liquid carriers include water; alcohols, for example isopropanol and glycols; ketones, for example acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; ethers; aromatic or araliphatic hydrocarbons, for example benzene, toluene and xylene; petroleum fractions, for example kerosene and light mineral oils; chlorinated hydrocarbons, for example carbon tetrachloride, perchloroethylene and trichloroethane. Mixtures of different liquids are often suitable.

Agricultural compositions are often formulated and transported in a concentrated form which is subsequently diluted by the user before application. The presence of small amounts of a carrier which is a surface-active agent facilitates this process of dilution. Thus, preferably at least one carrier in a composition according to the invention is a surface-active agent. For example the composition may contain at least two carriers, at least one of which is a surface-active agent.

A surface-active agent may be an emulsifying agent, a dispersing agent or a wetting agent; it may be nonionic or ionic. Examples of suitable surface-active agents include the sodium or calcium salts of polyacrylic acids and lignin sulphonic acids; the condensation products of fatty acids or aliphatic amines or amides containing at least 12 carbon atoms in the molecule with ethylene oxide and/or propylene oxide; fatty acid esters of glycerol, sorbitan, sucrose or pentaerythritol; condensates of these with ethylene oxide and/or propylene oxide; condensation products of fatty alcohols or alkyl phenols, for example Eroctylphenol or Eroctylcresol, with ethylene oxide and/or propylene oxide; sulphates or sulphonates of these condensation products; alkali or alkaline earth metal salts, preferably sodium salts, of sulphuric or sulphonic acid esters containing at least 10 carbon atoms in the molecule, for example sodium lauryl sulphate, sodium secondary alkyl sulphates, sodium salts of sulphonated castor oil, and sodium alkylaryl sulphonates such as dodecylbenzene sulphonate; and polymers of ethylene oxide and copolymers of ethylene oxide and propylene oxide.

The compositions of the invention may for example be formulated as wettable powders, dusts, granules, solutions, emulsifiable concentrates, emulsions, suspension concentrates and aerosols. Wettable powders usually contain 25, 50 or 75% w of active ingredient and usually contain in addition to solid inert carrier, 3-10% w of a dispersing agent and, where necessary, 0-108 w of stabiliser(s) and/or other additives such as penetrants or stickers. Dusts are usually formulated as a dust concentrate having a similar composition to that of a wettable powder but without a dispersant, and are diluted in the field with further solid carrier to give a composition usually containing -108 w of active ingredient.Granules are usually prepared to have a size between 10 and 100 BS mesh (1.676 0.152 mm), and may be manufactured by agglomeration or impregnation techniques. Generally, granules will contain 1/2-75% w active ingredient and 0-10% w of additives such as stabilisers, surfactants, slow release modifiers and binding agents. The so-called "dry flowable powders" consist of relatively snall granules having a relatively high concentration of active ingredient. Emulsifiable con concentrates usually contain, in addition to a solvent and, when necessary, co-solvent, 10-50% w/v active ingredient, 2-20% w/v emulsifiers and 0-20% w/v of other additives such as stabilisers, penetrants and corrosion inhibitors.Suspension concentrates are usually compounded so as to obtain a stable, non-sedimenting flowable product and usually contain 10-75% w active ingredient, 0.5-15% w of dispersing agents, 0.1-10% w of suspending agents such as protective colloids and thixotropic agents, 0-10% w of other additives such as defoamers, corrosion inhibitors, stabilisers, penetrants and stickers, and water or an organic liquid in which the active ingredient is substantially insoluble; certain organic solids or inorganic salts may be present dissolved in the formulation to assist in preventing sedimentation or as anti-freeze agents for water.

Aqueous dispersions and emulsions, for example compositions obtained by diluting a wettable powder or a concentrate according to the invention with water, also lie within the scope of the invention. The said emulsions may be of the water-in-oil or of the oil-in-water type, and may have a thick 'mayonnaise' -like consistency.

The composition of the invention may also contain other ingredients, for example other compounds possessing herbicidal, insecticidal or fungicidal properties.

According to the present invention there is also provided a compound of the general formula II

in which R2, R3 and R5 are as defined above. These compounds of formula II are useful as intermediates in the preparation of compounds of formula I and also exhibit herbicidal, particularly graminicidal, activity.

The compounds of formula II exist in three tautomeric forms which are in dynamic equilibrium with each other and which car.

be represented by the following formulae:

II IIa IIb In these formulae R, R and R5 have the meanings given above.

It should also be appreciated that the compounds of formula II are capable of existing as different optical isomers. The invention thus includes both the individual isomers and mixtures of such isomers.

The invention also provides a process for the preparation of a compound of the general formula II as defined above which comprises reacting a compound of the general formula IV

in which R2 and R5 are as defined above, with a compound of the general formula V

in which R3 is as defined in claim 1 and X represents an electron-withdrawing leaving group, in the presence of a base and in the presence of a metal salt.

Preferably, the base used is an organic base such as pyridine or a tertiary amine. In the latter case, a trialkylamine such as triethylamine is particularly preferred.

It is also preferred that the mental salt is zinc chloride or zinc cyanide.

Suitable electron-withdrawing leaving groups X include halogen atoms, especially chlorine atans.

The invention is further illustrated by the following examples.

(A) Preparation of compounds of general formula I Example 1 4-(1-Allyloximinopropyl)-2-methyl-thiopzan-3,5-dione Triethylamine (1.1g) was added to a solution of 4-propionyl-2-methyl-thiopyran-3,5-dione (2g) and 0-allylhydroxylamine hydrochloride (1.1g) in absolute ethanol (50ml) at roan temperature. After stirring for 4 hours at this temperature, the reaction mixture was evaporated in vacuo, water was added to the residue and the aqueous phase was extracted with rnethylene chloride. The extracts were then dried over anhydrous magnesium sulphate and the methylene chloride evaporated in vacuo. Chromatographic separation of the residue on a silica gel column using 5% (v/v) diethylether-methylene chloride as eluant yielded 4-(1-allyloximinopropyl)-2-methyl- thiopyran-3,5-dione (l.Og) as an orange oil. Mass spectrographic data revealed the mass/mass charge ratio M/e of the parent molecule ion to be 255 thereby confirming the molecular weight of the product to be 255.

Analysis: Calc. for C12H17NO3S : C 56.5; H 6.7; N 5.5% Found : C 56.5; H 6.7; N 5.9% Example 2 4 - (1-Al lyloximino-n-butyl) -2thyl-thiopyran- 3, 5-dione Triethylamune (0.9g) was added to a solution of 4-n-butyryl-2-methyl-thiopyran-3,5-dione (1.6g) and O-allylhydroxylamine hydrochloride (0.9g) in absolute ethanol (50ml) at roan temperature. After stirring for 4 hours at this temperature, the reaction mixture was evaporated in vacuo, water was added to the residue and the aqueous phase was then extracted with methylene chloride.The extracts were then dried over anhydrous magnesium sulphate and the methylene chloride evaporated. Chromatographic separation of the residue on a silica gel column using 5% (v/v) diethyl ether-methylene chloride as the eluant yielded 4-(1-allyloximino-n-butyl)-2- methql-thioran-3, 5-dione (0. 7g) as a red oil.

M/e Found: 269 Analysis Calc. for C13H19NO3S : C 58.0; H 7.1; N 5.2% Found : C 58.0: H 6.9; N 5.3% Examples 3 to 14 Following procedures similar to those described in Example 1, further compounds of formula I were prepared whose characteristics and analyses are set out in Table 1. In all cases, the identity of the product was confirmed by n.m.r.

analysis.

TABLE 1

Example R R R4 R5 Analysis No. C H N 3 H- C2H5- CH2=CH-CH2- H- Calc: 54.8 6.2 5.8 Found: 54.3 6.1 6.0 TABLE 1 continued..

Example R R R4 R5 Analysis No. C H N 4 CH3 C2H5- C2H5- H- Calc: 54.3 7.0 5.7 Found: 51.7 6.7 5.7 5 CH3- C2H5- CH3- H Calc : 52.4 6.5 6.1 Found : 52.2 6.6 6.2 6 H- C2H5- C2H5- H- Calc: 52.4 6.6 6.1 Found: 51.3 6.3 6.2 7 H- CH3 C2H5- H- Calc: 50.2 6.0 6.5 Found: 51.3 6.4 6.6 8 H- CH3 CH2=CH-CH2- H- Calc: 52.9 5.7 6.2 Found: 54.3 6.0 6.4 9 CH3 C3H7- C2H5- H- Calc: 56.0 7.4 5.4 Found : 55.3 7.2 5.3 10 CH3- C3H7- CH3- H- Calc : 54.3 7.0 5.7 Found: 54.6 6.8 6.5 11 H- C3H7- CH2=CH-ch2- H- Calc: 56.5 6.7 5.5 Found: 55.5 6.4 5.3 12 H- C3H7- C2H5- H- Calc: 54.3 7.0 5.8 Found: 54.1 6.7 5.0 TABLE 1 continued..

Example R R R4 R5 Analysis No. C H N 13 (CH3)2CH- C3H7- CH2=CH-CH2- H- Calc: 60.6 7.7 4.7 Found: 60.5 7.6 4.9 14 CH3- C3H7- CH2=CH-CH2- CH3- Calc. : 59.3 7.4 4.9 Found: 59.9 7.4 5.1 (B) Preparation of compounds of general formula II Example 15 4-n-butyryl-2-methyl-thiopyran-3,5-dione Zinc chloride (38g) was added carefully to pyridine (400ml). 2-methyl-thiopyran-3,5-dione (20g) was then added followed by the dropwise addition of n-butyryl chloride (12.8g) and the mixture was then refluxed for 4 hours.After cooling, the mixture was poured into 50e HC1 (800ml) and the crude product was then extracted with methylene chloride. The extracts were then dried over anhydrous magnesium sulphate and the methylene chloride evaporated. Chromatographic separation of the residue on a silica gel column using 5%(v/v) diethyl ether-methylene chloride as the eluant yielded 4-n-butyryl-2-methyl-thiopyran-3,5-dione (14g) as a red oil.

M/e Found: 214 Analysis Calc. for C10H14SO3 : C 56.0; H 6-5% Found : C 55.3; H 6.4% Example 16 4-propionyl-thiopyran-3,5-dione Zinc cyanide (9.4g) was added to a solution of propionyl chloride (5.7g) in dry acetonitrile (50ml). After stirring for 15 minutes at room temperature, thiopyran-3,5-dione (10.4g) was added followed by the dropwise addition of triethylamine (lO.lg). The reaction mixture was then refluxed for 1 hour, cooled and then treated with 2N HC1. The crude product was then extracted with diethyl ether, dried over anhydrous magnesium sulphate and the ether evaporated.Chromatographic separation of the residue on a silica gel column using 10%(v/v) methanol-methylene chloride as the eluant yielded 4-propionyl thiopyran-3 , 5-dione as a red oil (4g).

M/e Found: 186 Analysis Calc. for C8H10SO3 : C-51.6: H 5.48 Found : C 51.3: H 5.48 Examples 17 to 21 Following procedures similar to those described in Examples 15 and 16, further compounds of formula II were prepared whose characteristics and analyses are set out in Table 2. In all cases the identity of the product was confirmed by n.m.r. and mass spectometry analysis.

TALE 2

Example R R R5 Analysis No. C B 17 CH3- C2H5- H- Calc : 54.0 6.0 Found : 53.1 5.9 18 H- CH3- H- Calc : 48.8 4.7 Found : 49.6 4.9 19 H- C3H7- H- Calc : 54.0 6.0 Found : 53.3 5.9 Table 2 (continued)

Example R2 R3 R5 Analysis No. C H 20 (CH3)2CH- C3H7- H- Calc: 59.5 7.4 Found: 60.1 7.5 21 CH3 C3H7- CH3 Calc: 57.8 7.0 Found: 56.7 6.7 The e preparation of compounds of the general formula IV, which are utilised in Examples 15 to 21, is already described in the literature, (see, for example, DE 3318648 A (Bayer) and Boll. Sci. Fac. Chim. Ind. Bologna, Vol. 25 (1-2), pages 51-60).

(C) Herbicidal Activity Example 22 - To evaluate their herbicidal activity, compounds according to the invention of general formula I were tested using as a representative range of plants:maize, Zea mays (Mz); rice, Oryza sativa (R); barnyard grass, Echinochloa crusgalli (BG); oat, Avena sativa (0); linseed, Linus usitatissimum (L); mustard, Sinapsis alba (M); sugar beet, Beta vulgaris (SB) and soya bean, Glycine max (S).

The tests fall into two categories, pre-emergence and post-emergence. The pre-emergence tests involved spraying a liquid formulation of the compound onto the soil in which the seeds of the plant species mentioned above had recently been sown. The post-emergence tests involved two types of test, viz., soil drench and foliar spray tests. In the soil drench tests the soil in which the seedling plants of the above species were growing was drenched with a liquid formulation containing a compound of the invention, and in the foliar spray tests the seedling plants were sprayed with such a formulation.

The soil used in the tests was a prepared horticultural loam.

The formulations used in the tests were prepared fran solutions of the test compounds in acetone containing 0.48 by weight of an alkylphenol/ethylene oxide condensate available under theade mark TRITON X-155. These acetone solutions were diluted with water and the resulting formulations applied at dosage levels corresponding to 5 kg or 1 kg of active material per hectare in a volume equivalent to 600 litres per hectare in the soil spray and foliar spray test, and at a dosage of level equivalent to 10 kilograms; of active material per hectare in a volume equivalent to approxirrately 3,000 litres per hectare in the soil drench tests.

In the pre-emergence tests untreated sown soil and in the post-emergence tests untreated soil bearing seedling plants were used as controls.

The herbicidal effects of the test compounds were assessed visually twelve days after spraying the foliage and the soil, and thirteen days after drenching the soil and were recorded on a 0-9 scale. A rating 0 indicates grak-th as untreated control, a rating 9 indicates death. An increase of 1 unit on the linear scale approximates to a 10% increase in the level of effect.

The results of the tests are set out in Table 3 below, in which the compounds are identified by reference to the preceding examples.

TABLE 3

Compound Post-emergence of Soil drench 10/kg/ha Dosage Foliar spray Pre-emergence Ex. No. Mz R BG O L M SB S kg/ha Mz R BG O L M SB S Mz R BG O L M SB S 1 4 7 8 7 0 0 0 0 5 0 0 0 0 0 0 0 0 3 6 6 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 5 8 9 8 0 0 0 0 5 6 6 8 7 0 0 0 0 1 7 8 6 0 0 0 0 1 3 0 6 1 0 0 0 0 1 2 1 1 0 0 0 0 3 6 8 8 8 0 0 0 0 5 7 7 8 7 0 0 0 0 3 8 9 7 0 0 0 0 1 4 4 6 5 0 0 0 0 0 1 0 0 0 0 0 0 4 5 7 8 6 0 0 0 0 5 6 7 7 7 0 0 0 0 2 8 9 7 0 0 0 0 1 No spray at 1kg/ha TABLE 3 continued...

Compound Post-emergence of Soil drench 10/kg/ha Dosage Foliar spray Pre-emergence Ex. No. Mz R BG O L M SB S kg/ha Mz R BG O L M SB S Mz R BG O L M SB S 5 3 7 6 5 0 0 0 0 5 4 6 7 6 0 0 0 0 0 3 6 5 0 0 0 0 1 2 2 4 2 0 0 0 0 0 0 0 0 0 0 0 0 6 5 7 9 7 0 0 0 0 5 2 2 4 0 0 0 0 0 0 0 4 0 0 0 0 0 1 2 2 4 0 0 0 0 0 0 0 0 0 0 0 0 0 7 2 6 6 5 0 0 0 0 5 6 7 7 5 1 1 0 0 3 3 7 4 0 0 0 0 1 4 2 5 1 0 0 0 0 0 0 0 0 0 0 0 0 8 4 7 7 6 0 0 0 0 5 6 7 8 7 1 1 0 0 2 8 7 7 0 0 0 0 1 3 3 6 2 0 0 0 0 0 1 3 0 0 0 0 0 TABLE 3 continued...

Compound Post-emergence of Soil drench 10/kg/ha Dosage Foliar spray Pre-energence Ex. No. Mz R BG O L M SB S kg/ha Mz R BG O L M SB S Mz R BG O L M SB S 9 7 7 9 7 0 0 0 0 5 4 6 7 7 0 0 0 0 0 8 6 3 0 0 0 0 1 0 5 5 6 0 0 0 0 0 2 2 0 0 0 0 0 10 5 7 7 7 0 0 0 0 5 3 5 6 4 4 0 0 0 0 0 6 0 0 0 0 0 1 1 0 3 2 2 0 0 0 0 0 2 0 0 0 0 0 11 6 7 8 8 0 0 0 0 5 6 6 8 6 0 0 0 0 0 8 8 7 0 0 0 1 1 3 5 6 4 0 0 0 0 0 0 0 0 0 0 0 1 12 4 7 8 7 0 0 0 0 5 6 6 6 6 0 0 0 0 0 5 7 5 0 0 0 0 1 3 3 4 2 0 0 0 0 0 1 0 2 0 0 0 0 TABLE 3 continued...

Compound Post-emergence of Soil drench 10/kg/ha Dosage Foliar spray Pre-emergence Ex. No. Mz R BG O L M SB S kg/ha Mz R BG O L M SB S Mz R BG O L M SB S 13 7 7 6 3 0 0 2 0 5 6 6 7 6 0 0 4 4 3 8 6 4 0 0 0 0 1 3 5 6 6 0 0 2 1 1 2 1 1 0 0 0 0 14 6 8 7 7 0 0 0 0 5 7 7 9 7 2 0 4 0 5 8 7 0 0 0 0 0 1 4 4 6 5 1 0 3 0 0 4 3 0 0 0 0 0

Claims (17)

1. CLAIMS 1. A compound of the general formula I.

   or a metal salt thereof, in which R2 represents a hydrogen atom or an optionally substituted alkyl group; R3 represents an optionally substituted alkyl group; represents an optionally substituted alkyl or alkenyl group ; and R5 represents a hydrogen atom or an optionally substituted alkyl group.
2. 2. A compound according to claim 1 in which each optionally substituted alkyl or alkenyl group contains up to 4 carbon atoms.
3. 3. A compound according to claim 1 or claim 2 in which R -represents a hydrogen atom, a methyl group or a propyl group; R3 represents a methyl, ethyl or propyl group; R4 represents a methyl, ethyl or allyl group; and R5 represents a hydrogen atom or a methyl group.
4. 4. A compound according to claim 1 and as hereinbefore described in any one of Examples 1 to 14.
5. 5. A process for the preparation of a compound of the general formula I as defined in claim 1 or a metal salt thereof, which comprises reacting a compound of the general formula II

   in which R2, R3 and R5 are as defined in claim 1, with a compound of the general formula III R4 - O - NH2 III in which R4, is as defined in claim 1, or with an acid addition salt thereof, and, if desired, converting the compound of formula I so obtained into a salt thereof.
6. 6. A process according to claim 5 in which the reaction is carried out in the presence of a base.
7. 7. A process according to claim 5 carried out substantially as hereinbefore described with reference to any one of Examples 1 to 14.
8. 8. A compound of the general formula I as defined in claim 1, or a metal salt thereof, whenever prepared by a process according to claim 5 or claim 6.
9. 9. A herbicidal composition which comprises a compound according to any one of claims 1 to d and 8, or a metal salt thereof, in association with a carrier.
10. 10. A composition according to claim 9 which comprises at least two carriers, at least one of which is a surface active agent.
11. 11. A method of combating undesired plant growth at a locus which comprises treating the locus with a &alpha; compound according to any one of claims 1 to 4 and 8, or a metal salt thereof, or with a composition according to claim 9 or claim 10.
12. 12. The use of a compound according to any one of claims 1 to 4 and 8, or a salt thereof, as a herbicide.
13. 13. A compound of the general formula II

    in which R2, R3 and R5 are as defined in claim 1.
14. 14. A compound according to claim 13 and as hereinbefore described in any one of Examples 15 to 21.
15. 15. A process for the preparation of a compound of the general formula II as defined in claim 13 which comprises reacting a compound of the general formula IV

    in which R and R5 are as defined in claim 1, with a compound of the general formula V

    R@ - C - X V in which R is as defined in claim 1 and X represents an electron withdrawing leaving group, in the presence of a base and a metal salt.
16. 16. A process according to claim 15 in which the base is pyridine or a tertiary amine.
17. 17. A process according to claim 15 or claim 16 in which the metal salt is zinc chloride or zinc cyanide.

    18 A process according to claim 15 carried out substantially -as hereinbefore described with reference to any one of Examples 15 to 21.