GB2118177A - N-substituted- tetrahydrothiazines and their use pesticides - Google Patents

Abstract

N-substituted 2- (nitromethylene)-tetrahydro-2H-1,3- thiazines of the following formula have pesticidal, especially insecticidal, activity: <IMAGE> in which R represents an alkyl or alkenyl group of up to 26 carbon atoms; or an alkyl group substituted by one or more halogen atoms, or by an alkoxy, alkoxycarbonyl, alkanoyloxy, oxobicycloalkyl, cycloimido, optionally substituted phenyl or phenoxy, or trialkylsilyl group, or by a quaternary ammonium group of formula: Q<->R3<1>N<+>- in which each group R<1> is an alkyl group and Q<-> is a monovalent anion; and n is 0 or 1.

Description

SPECIFICATION N-substituted-tetrahydrothiazines, process for their preparation, and their use as pesticides This invention relates to N-substituted nitromethylene-tetrahydrothiazines, to processes for their preparation and to their use as pesticides, in particular against insect pests.

2-Nitromethylene-tetrahydro-2H-1 3-thiazines and their use as pesticides are known from UK Patent 1.513.951. However, although these thiazines possess a high level of insecticidal activity they suffer from the disadvantage of being unstable to light and, thus, show little persistence under field conditions. The Applicant has now found that this light-stability problem can be overcome by employing a new class of insecticidally-active thiazines.

Accordingly the invention provides N-substituted 2-(nitromethylene)-tetrahydro-2H-1 3-thiazines of formula:

in which R represents an alkyl or alkenyl group of up to 26 carbon atoms; or an alkyl group substituted by one or more halogen atoms or by an alkoxy, alkoxycarbonyl, alkanyloxy, oxobicycloalkyl, cycloimido, optionally-substituted phenyl or phenoxy, or triaikylsilyl group, or by a quaternary ammonium group of formula: Q- R31N± in which each group R1 is an alkyl group and Q is a monovalent anion; and n is O or 1.

Preferably R represents an alkyl group of up to 20 carbon atoms, for example, methyl, ethyl, propyl, butyl, isobutyl, pentyl, methylpentyl, hexyl, heptyl, octyl, nonyl, dodecyl, tetradecyl, hexadecyl or (trimethylbutyl)trimethyloctyl; an alkenyl group of up to 6 carbon atoms, for example, methylpropenyl; an alkyl group of up to 4 carbon atoms, e.g. methyl, ethyl or propyl, substituted by up to three fluorine, chlorine, bromine or iodine atoms, by an alkoxy, alkoxycarbonyl or alkanoyioxy group of up to 6 carbon atoms, for example, ethoxy, methoxycarbonyi or acetoxy, by an oxobicycloalkyi group of up to 10 carbon atoms, for example dimethyl-oxobicycloheptyl, by a phthalimide group, by a phenyl or phenoxy group in turn optionally substituted by one or more selected from the group consisting of halogen atoms (fluorine, chlorine or bromine) and alkyl, alkoxylcarbonyl and trihalomethyl groups containing up to 6 carbon atoms (e.g. methyl, methoxycarbonyl, ethoxycarbonyl and trifluoromethyl), by a trialkylsilyl group in which each alkyl is of 1 to 6 carbon atoms, for example trimethylsilyl, or by a quarternary ammonium group of formula:: Q-R31N± in which each group R1 is an alkyl group of 1 to 6 carbon atoms, for example ethyl and Q' is a halide ion, for example, chloride, bromide or iodide; and n is O or 1.

More specifically the preferred compounds according to the invention have the general formula I in which R represents an alkyl group containing 1 to 20 carbon atoms, an alkenyl group containing 2 to 4 carbon atoms, a haloalkyl group containing 1 to 4 carbon atoms and 1 to 3 fluorine, chlorine, bromine or iodine substituents, an alkoxyalkyy group containing in total 2 to 4 carbon atoms, an alkoxycarbonylalkyl or alkanoyloxyalkyl group containing in total 3 to 5 carbon atoms, a camphoryl group, a phthalimidoalkyl group in which the alkyl group contains 1 to 4 carbon atoms, a phenylalkyl group in which the alkyl group contains 1 to 4 carbon atoms, a halobenzyl group containing 1, 2 or 3 chlorine, fluorine or bromine atoms, a trifluoromethylbenzyl group, a methylbenzyl group, an alkoxycarbonylbenzyl group in which the alkoxycarbonyl group contains 2 to 5 carbon atoms, a halophenoxyalkyi group where the halogen is chlorine, fluorine or bromine and the alkyl group contains 1 to 4 carbon atoms, a trimethylsilylalkyl group in which the alkyl group contains 1 to 4 carbon atoms, or an alkyl-trialkylammonium halide group wherein each alkyl group contains 1 to 3 carbon atoms; and n=O or 1.

It will be appreciated that the compounds of formula I are capable of existing in different geometrically isomeric forms. The invention includes both the individual isomers and mixtures of such isomers.

The invention includes a process for the preparation of the N-substituted 2-(nitromethylene)tetrahydro-2H-1 ,3-thiazines of formula I in which n is 0, which comprises reacting the compound of formula:

with a sulphonyl halide of formula R-502-Hal in which R is as defined above, and Hal represents a halogen, preferably chlorine atom in the presence of a base. The base is preferably an organic base such as a tertiary amine, for example a trialkylamine, triethylamine being particularly preferred. The reaction is carried out at a temperature of OOC or below, for example at a temperature from 750 to --100C. The reaction is suitably carried out in an organic solvent, for example, a chlorinated hydrocarbon such as dichloromethane, an ether such as tetrahydrofuran or dimethylformamide.

It may be more convenient to prepare certain of the compounds of formula I in which R is a substituted alkyl group by alternative known procedures involving the replacement of one substituent by another. Examples of such replacement reactions are given in Examples 34 and 35 below, for the replacement of chlorine by iodine, and of iodine by trialkylammonium, respectively.

The compounds of formula I in which n is 1 may be prepared by oxidising the corresponding derivative in which n is 0. It may be carried out using conventional oxidising agents, for example peracids such as m-chloroperbenzoic acid or potassium permanganate, or potassium hydrogen persulphate. Conveniently the derivative to be oxidised is dissolved in a suitable solvent, for example a chlorinated hydrocarbon solvent such as chloroform or dichloromethane, or a liquid alkanol such as ethanol.

As mentioned above the N-substituted 2-(nitromethylene)-tetrahydro-2H- 1 ,3-thiazines of the invention are of interest as pesticides particularly against insect pests. They exhibit activity against such pests as the larval caterpillar or worm forms of insects, for example, of the genus Spodoptera and of the genus Heliothis. The compounds also exhibit acceptable stability towards light and oxidation.

Accordingly the invention includes pesticidal compositions comprising a substituted 2 (nitromethylene)-tetrahydro-2H-1 ,3-thiazine of the invention together with a 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 comprise individual isomers or mixture of isomers. The invention further provides a method of combating pests, particular insect pests at a locus infested, or liable to infestation, by such pests, which comprises applying to the locus a pesticidally effective amount of compound or composition according to the present invention.

The active ingredient is suitably formulated with gt least one carrier-to facilitate its application to the locus, for example plants, seeds or soil, to be treated, or to facilitate storage, transport or handling.

The carrier in a composition of the invention may be a solid or a liquid, including a material which is normally gaseous but which has been compressed to form a liquid. Any of the carriers commonly used in the formulation of pesticides may be used in the compositions of the invention. Suitable solid carriers include natural and synthetic materials. 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 kerosine and light mineral oils; chlorinated hydrocarbons, for example carbon tetrachloride, perchloroethylene and trichloroethane. Mixtures of different liquids are often suitable.

Preferably a composition of the invention contains at least two carriers, at least one of which is a surface-active agent. The surface-active agent may be an emulsifier, a dispersing agent or a wetting agent. It may be non-ionic or ionic. Pesticidal compositions are generally formulated and transported in a concentrated form which is subsequently diluted by the farmer or other user before application. A surface-active agent facilitates this process of dilution.

Preferably compositions according to the invention contain 0.5 to 95% by weight of active ingredient, the balance being made up by the carrier or carriers.

The composition of the invention may, for example, be formulated as a wettable powder, a dust, granules, a solution, an emulsifiable concentrate, an emulsion, a suspension concentrate or an aerosol.

The composition may have controlled release properties, or may be suitable for use as a bait.

Wettable powders usually contain 25, 50 or 75% of active ingredient and may contain, in addition to inert solid material, 31 0 /Ow of a despersing agent and, where necessary, C#;i 0%w of a stabilizer, a penetrant and/or a sticker. A dust is usually formulated as a dust concentrate having a composition similar to that of a wettable powder but without a dispersant, and is diluted in the field with further solid carrier to give a composition usually containing 21 1 0%w of active ingredient; Granules usually have a size in the range of from 10 to 100 BS mesh (1.676-0.152 mm) and may be manufactured by agglomeration or impregnation techniques.Generaliy, granules will contain ±75%w active ingredient and 0-1 0%w of additives, such as stabilizers, surfactants, slow release modifiers and binding agents.

Emulsifiable concentrates usually contain, in addition to a solvent, and, when necessary, co-solvent, 1050% w/v active ingredients, 2-20% w/v emulsifier and 0-20% w/v of other additives, for example a stabilizer, a penetrant and/or a corrosion inhibitor.A suspension concentrate is a stable, nonsedimenting, flowable product and usually contains 1 0-75%w active ingredient, 0.5-1 5%w of dispersing agent, 0.1-1 0%w of suspending agent, for example protective colloid and/or a thixotrnpic agent, and 0-1 0%w of other additives including, for example, a defoamer, a corrosion inhibitor, a stabilizer, a penetrant and/or a sticker, and as dispersant, water or an organic liquid in which the active ingredient is substantially insoluble; certain organic additives and/or inorganic salts may be dissolved in the dispersant to assist in preventing sedimentation or as anti-freeze for water.

The aqueous dispersions and emulsions formed by diluting a wettable powder or an emulsifiable concentrate of the invention with water, also lie within the scope of the present invention. Such dispersions and emulsions may be of the water-in-oil or of the oil-in-water type, and may have a thick "mayonnaise"-like consistency.

A composition of the invention may also contain other ingredients, for example, one or more other compounds possessing pesticidal properties. Further insecticidal compounds may be included, particularly such compounds having a different mode or spectrum of activity.

It is also been found that the thermal stability of the compounds and compositions of the invention may be improved by the addition of stabilizing amounts, usually 10-1 00%w based on the compound of certain organo nitrogen compounds such as urea, dialkylureas, thiourea or guanidine salts or alkali metal salts of weak acids such as bicarbonates, acetates or benzoates.

The invention is illustrated further in the following Examples.

EXAMPLE 1 3-methylsu lphonyl-2-nitromethylene-tetrahydro-2H-1 ,3-thiazine Procedure A Methanesulphonyl chloride (20 g) in methylene chloride (150 ml) was added dropwise over 55 mins. to a mixture of 2-nitromethylenetetrahydro-2H-1 ,3-thiazine (20 g) and triethylamine (40 ml) in methylene chloride (1 50 ml) stirred at -300C under nitrogen. The reaction mixture was stirred for a further 30 mins at -300C then washed with 2% HCI. The aqueous layer was washed twice with methylene chloride and the combined organic fractions dried over sodium sulphate, concentrated to about 50 ml and cooled to OOC. The crystalline product was filtered off and the crystals triturated several times with cold methylene chloride.

Chromatography of the mother liquor (SiO2 (600 g)/CH2CI2:CH3OH 99:1) afforded a further portion of production (0.77 g).

m.p. 144-1450C (with decomposition) NMR (60 MHz d-6 DMSO) (external reference) ppm 3.4 (3H, S, CH3), 7.8 (1 H, S, CHNO2) Analysis CoHxoN2o4s2 Calc. 30.3% C 4.2% H 11.8% N Found 30.3 C 4.3% H 11.8% N Procedure B 2-Nitromethylene-tetrahydro-2H-1 ,3-thiazine (3 moles; 480 g) was dissolved in dichloromethane (3 litres). The solution was cooled to -700C and triethylamine (6 mol; 606 g) at -700C was added.

This solution was stirred at -650 to -700C under an N2 blanket while a solution of methylsulphonyl chloride (6 mol; 687 g) dissolved in dichloromethane was added dropwise over 6 hours. A mixture of the desired product precipitated out to form a thick slurry and this was filtered off at --700C, and dried at ambient temperature. The cake formed was washed with water to remove the triethylamine hydrochloride, and the product (446 g) remained as a crystalline solid.

EXAMPLES 2 TO 27 The following additional compounds were prepared from appropriate starting materials according to procedures similar to that described in procedure A of Example 1 above. The melting point and analyses of these compounds are set out in Table 1 below.

EXAMPLE 28 3-methylsulphonyl-2-nitromethylene-tetrahydro-2H-1 ,3-thiazine-1 -oxide To a solution of the compound of Example 1 (1.56 g) in methylene chloride (70 ml) at -1 50C under nitrogen was added m-chloroperbenzoic acid (1.56 g) in methylene chloride (50 ml) over 5 mins.

The mixture was then stirred at room temperature for 1 hour. Solid sodium carbonate (5 g) was added, the mixture stirred for 5 mins and then filtered through sodium carbonate. The solvent was removed under reduced pressure. The product was recrystallised from methylene chloride to yield the sulphoxide (1.44g).

m.p. 108-1110C Analysis C6H10N2o5S2 Calc. 28.4% C 3.9% H 11.0%N Found 29.5% C 4.0% H 10.4% N EXAMPLE 29 3-(3-chloropropyl)sulphonyl-2-nitromethylene-tetrahydro-2H-1,3-thiazine-1-oxide was prepared as an oil in a similar manner to Example 12.

TABLE 1

Example Molecular Analysis No. R mp/ C Formula %C %H %N 2 CH3CH2 88-89 C7H12N204S2 Calc. 33.3 4.8 11.1 Found 33.2 4.9 11.0 3 CH3CH2CH2 109-110 C8H,4N204S2 Calc. 36.1 5.3 10.5 Found 36.1 5.4 10.4 4 CH3(CH2)2CH2 78-79 CgH16N204S2 Calc. 38.6 5.7 10.0 Found 38.4 5.8 9.8 5 CH3(CHQ6CH2 104-105 C'3H24N204S2 Calc. 46.4 7.1 8.3 Found 47.0 7.6 8.0 6 CH3(CH2)10CH2 116 C17H32N204S2 Calc. 52.0 8.2 7.1 Found 52.1 8.2 7.1 7 CH3(CH2)14CH2 '114-11 6 14--16 C2,H40N204S2 Calc. 56.3 8.9 6.3 Found 56.3 8.9 6.1 8 CH2CICH2CH2 98-99 C8H13N204S2CI Calc. 32.0 4.3 9.3 Found 32.0 4.0 9.5 9 C6H5CH2 136 C12H14N2o4S2 Calc. 45.9 4.5 8.9 Found 46.1 4.5 8.7 10 camphoryl* 152-153 Ca5H22N2o5S2 Calc. 48.1 5.9 7.5 Found 48.3 6.2 7.5 11 CH3OCO(CH2)3 75-76 C1oH16N2o6S2 Calc. 37.0 4.9 8.6 Found 36.9 4.7 8.8 12 CH30CO(CH2)2 109-110 CgN14N206S2 Calc. 34.8 4.6 9.0 Found 34.8 4.5 9.0 13 (CH3)2CHCH2 109-111 C9H16N204S2 Calc. 38.6 5.7 9.9 Found 38.6 5.7 10.0 14 (CH3)2CH(CH2)3 104 C11H2oN2o4S2 Calc. 42.5 6.6 8.9 Found 42.9 6.5 9.1 15 C2H50(CH2)2 67 CgHa6N205S2 Calc. 36.2 5.5 9.8 Found 36.5 5.4 9.5 16 CH3(CH2),3 114 C1gH36N204S2 Calc. 54.3 8.6 6.7 Found 56.7 9.5 5.9 17 C6H5(CH2)3 41 C14Hr8N204S2 Calc. 48.2 5.0 8.0 Found 49.1 5.3 8.2 18 (CH3)3Si(CH2)3 71 C11H22N204S2Si Calc. 38.9 6.5 8.3 Found 39.2 6.8 7.6 * 7,7-dimethyl-2-oxobicyclo[2,2,1]hept-1-ylmethyl TABLE 1 (continued)

Example Molecular Analysis No. R mp/OC Formula %C %H %N 19 CH2=C(CH3)CH2 oil CgH14N204S2 Calc. 38.8 5.0 10.1 Found 38.7 4.9 10.1 20 2-fluorophenylmethyl - C12H13N2o4S2F Calc. + O Found 21 CH2)2 150 C15H17N3o6S2 Calc. 45.6 3.8 10.5 Found 45.3 3.8 10.6 o 22 4o CH2 - C23H44N204S2 Calc. + Found + 23 CF3CH2 oil C7HgN204S2F3 Calc. 27.5 2.9 9.2 Found 28.0 3.0 9.0 24 CH3(CH2)6 112 C12H22N204S2 Calc. 44.7 6.8 8.7 Found 44.7 6.9 8.7 25 ilCH2)3 133 C16H1gN3o6S2 Calc. 46.7 4.1 10.2 Found 48.9 4.1 10.0 o 26 CH3COO(CH2)3 114 C1oH16N2o6s2 Calc. 37.0 4.9 8.6 Found 36.8 5.0 8.5 27 Cl O(CH2)3 135 Cl4Hl,N205S2Ci Calc. 42.8 4.3 7.1 Found 42.1 4.5 7.0 + = insufficient product available for analysis.

EXAMPLE 30 3-(2,6-dichlorophenylmethyl sulphonyl)-2-(nitromethylene)-tetrahydro 1 ,3-thiazine 2-(Nitromethylene)-tetrahydro-2H-1 ,3-thiazine (3.2 g) and N,N-di-isopropylethylamine (5.2 g) were dissolved in dimethylformamide. The solution was cooled to -40 to --500C and 2,6dichlorobenzylsulphonyl chloride (7.8 g) was added to it in small portions of about 0.1 g each over a period of one hour. The mixture was allowed to warm to ambient temperature, stirred for a further period of one hour and then poured into 500 ml of an ice/water mixture, to which a few drops of low HCI had been added. The solid formed was filtered off, washed with water and taken up in dichloromethane. The solution was dried (mgSO4) and the solvent was then removed under reduced pressure.The residue was recrystallised from ethyl acetate to yield the required product as a yellow crystalline solid m.p. 170 C.

Analysis Calculated for C12H12N2S204Cl2 C 37.6%; H 3.1%; 7.3% Found C 37.8%; H 3.1%; N 7.1% EXAMPLES 31-43 Further compounds were prepared following procedures similar to that of Example 30. These compounds are identified, and their melting points and analyses are given in Table 2.

TABLE 2

Example Molecular Analysis No. R mp/OC Formula %C %H %N 31 2-chlorophenylmethyl 148 C12H13N2S204CI Calc. 41.3 3.7 8.0 Found 42.2 3.7 7.4 32 3-chlorophenylmethyl 133 C12H13N2S204CI Calc. 41.3 3.7 8.0 Found 41.4 3.8 8.0 33 2-(ethoxycarbonyl)- 137-138 C15H18N2S206 Calc. 46.6 4.7 7.3 phenylmethyl Found 46.5 4.7 7.1 EXAMPLE 34 3-(3-iodopropylsulphonyl)-2-(nitromethylene)-tetrahydro-2H- 1 ,3-thiazine The compound of Example 8 (10 g) in acetone (700 ml) was heated under reflux for 16 hours with sodium iodide (17.5 g). The solvent was then removed under reduced pressure and the residue was partitioned between water and dichloromethane. The organic phase was dried (MgSo4) and the solvent was then removed under reduced pressure.The residue was recrystallised from dichloromethane to yield the required product as a yellow crystalline solid m.p. 106-1 090C.

Analysis Calculated for C8H13N282041 C24.5%; H 3.3%; N 7.1% Found C 25.3%; H 3.4%; N 7.2% EXAMPLE 35 3-(2-(nitromethylene)-tetrahydro-2 H-l ,3-thiazin-3-ylsulphonyl)-propyl-triethylammonium iodide The compound of Example 34 (4.0 g), and triethylamine (1.3 g) in ethyl acetate were heated under reflux for 53 hours. The reaction mixture was cooled and the precipitate formed was filtered off and recrystallised from ethanol to yield the desired product as a yellow crystalline solid m.p. 150--1570C.

Analysis Calculated for C14H28N3S2o4l C 34.1%; H 5.7%; N 8.5% Found C 34.0%; H 5.8%; N 8.6% EXAMPLES 36-43 Further compounds were prepared, the compounds of Example 36-39 and 43 being prepared by the method of Example 30 and the compounds of Examples 40-42 by the method of Example 1. These compounds are identified, and their melting points and analyses are given in Table 3.

TABLE 3 Example Molecular Analysis No. R mp/OC Formula %C %H %N 36 4-chlorobenzyl 144 C12H'3N2S204CI Calc. 41.3 3.7 8.0 Found 41.2 3.7 7.8 37 4-methylbenzyl 1 55-6 C13H16N2S2o4 Calc. 47.6 4.9 8.5 Found 47.3 4.7 8.5 38 3-trifluoromethyl- 132 C13H'3N2S204F3 Caic. 40.8 3.4 7.3 benzyl Found 40.6 3.2 7.8 39 4-trifluoromethyl- 1 52 Ct3Hl3N2S204F3 Calc. 40.8 3.4 7.3 benzyl Found 40.6 3.1 7.4 40 CH3(CH2)4 107 CaoH18N2s2o4 Calc. 40.8 6.1 9.5 Found 40.7 6.2 9.5 41 CH3(CH2)8 112 C14H26N2S2o4 Calc. 48.0 7.4 8.0 Found 47.3 7.2 7.4 42 CH3(CHz)5 101 C11H2oN2S2o4 Calc. 42.9 6.5 9.1 Found 42.9 6.5 8.9 43 4-bromobenzyl 157-8 C12H'3N2S204Br Calc. 36.6 3.3 7.1 Found 36.6 3.4 7.1 EXAMPLE 44 Pesticidal Activity - The insecticidal activity of the compounds was demonstrated against the pest Spodoptera littoralis Egyption cotton leaf-worm. The test method employed appears below.

Solutions or suspensions of the compound were made up over a range of concentrations in 10% acetone/water containing .025% Triton X100 ("Triton" is a registered trade mark). These solutions were sprayed using a logarithmic spraying machine onto petri dishes containing a nutritious diet on which the Spodoptera littoralis larvae had been reared. When the spray deposit had dried each dish was infested with 10 2nd instar larvae. Mortality assessments were made 7 days after spraying.

In each test an LC60 for the compound was calculated from the mortality figures and compared with the corresponding LCso for ethyl parathion in the same tests. The results expressed as toxicity indices = LC50'(parathion) x 100 LC50 (test compound) and are set out in Table 4 below.

TABLE 4 Compound Toxicity Index against of Ex. No. Spodoptera littoralis after 7 days 1 150 2 92 3 63 4 150 5 36 6 110 7 57 8 56 9 130 10 59 11 140 12 83 13 91 14 57 15 16 120 17 16 18 34 19 41 20 52 21 47 22 92 23 45 24 C* 25 19 26 150 27 41 28 43 29 12 TABLE 4 (continued) Compound Toxicity Index against of Ex. No. Spodoptera littoralis after 7 days 30 44 31 46 32 52 33 46 34 55 35 76 36 62 37 68 38 80 39 62 43 160 * "C" indicates very low mortality but in the case of Compound 24 the activity is much higher when on-leaf testing is carried out - see the results in Table 5.

EXAMPLE 45 Pesticidal Activity - On Leaf Tests This series of tests illustrates the insecticidal activity of the compounds according to the invention by infesting sprayed leaves of chinese cabbage with the larvae of Spodoptera littoralis and calculating the toxicity indices in the same manner as the previous example. The test method employed was as follows: The compounds were prepared for spraying a,s solutions or suspensions of technical material in 10% aqueous acetone solution, containing 0.025% Triton X-1 00 as a wetting agent. A range of dilutions of each test compound was sprayed onto a series of petri dishes, each containing a 9 cm diameter disc cut from a chinese cabbage leaf. The leaf material was placed with the under-surface - uppermost to receive the spray.After being allowed two dry, each dish was infested with 10 foliage-fed, early fourth instar Spodoptera larvae and maintained under laboratory conditions until mortalities were assessed 24 hours later.

The activity in the form of Toxicity Index for the compounds tested is presented in Table 5 below.

TABLE 5 Toxicity Index against Spodoptera Littoralis larvae on Compound of Chinese Cabbage Leaves 24 hours Example No. after spraying (Mean value) 1 120 9 130 11 120 12 148 19 110 21 89 23 82 24 176 26 159 30 103 31 114 32 84 33 112 43 92 EXAMPLE 46 Light Stability Tests This example illustrates the light stability of the compounds according to the invention and compares the results with one of the known closely-related thiazines disclosed in Example 1 of U.K.

Patent 1.513.951.

Asample of each compound (10 mg) was dissolved in pure acetone (10 cm3) and a portion (1 cm3) transferred to a quartz tube (10 cm in length 1 cm internal diameter). A thin film was formed in the tube by blowing-off the solvent with a stream of air while continuously agitating the tube. The tube was stoppered and exposed in a solar-simulator cabinet alongside tubes covered with aluminium foil which were used as controls. After exposure for 5, 20 or 50 hours the contents of the tube were dissolved in pure acetonitrile and analysed by reverse phase h.p.l.c. The solar simulator had a spectral distribution similar to daylight and an intensity about 0.5 to 0.3 that of bright summer daylight in the south of England.

The results of these tests in terms of the percentage recovered compound after exposure for 5, 20 and 50 hours are shown in Table 6.

It will be seen from the results in Table 6 that the compounds according to the invention have a light-stability far in excess of the compounds of the prior art as exemplified by Example 1 of U.K.

1.513.951.

TABLE 6 Light Stability Tests Compound of % Recovery after Exposure Example No. 5 hrs. 20 hrs. 50 hrs.

1 99 85 64 2 60 34 3 87 53 4 - 28 22 5 96 25 6 85 28 8 - 40 8 9 92 74 10 80 46 11 - 80 18 12 - 76 33 35 85 50 40 Known Compound 2-nitromethylenetetra hydro-2H-1 ,3-thiazine 26 0 (Example 1 of U.K.

Patent 1.513.951).

Claims (9)

1. N-substituted 2-(nitromethylene)-tetrahydro-2H-1 ,3-thiazines of formula:

in which R represents an'alkyl or alkenyl group of up to 26 carbon atoms; or an alkyl group substituted by one or more halogen atoms, or by an alkoxy, alkoxycarbonyl, alkanoyloxy, oxybicycloalkyl, cycloimido, optionally substituted phenyl or phenoxy, or trialkylsilyl group, or by a quaternary ammonium group of formula: Q P31N± in which each group R1 is an alkyl group and Q is a monovalent anion; and n is O or 1.

2. N-substituted 2-(nitromethylene)-tetrahydro-2H-1 ,3-thiazines according to claim 1, characterised in that R represents an alkyl group of up to 20 carbon atoms; an alkenyl group of up to 6 carbon atoms; an alkyl group of up to 4 carbon atoms substituted by up to 3 fluorine, chlorine or iodine atoms, by an alkoxy, alkoxycarbonyl or alkanoyloxy group of up to 6 carbon atoms, by an oxobicycloalkyl group of up to 10 carbon atoms, by a phthalimide group, by a phenyl or phenoxy group optionally substituted by one or more selected from the group consisting of halogen atoms and alkyl, alkoxycarbonyl and trihaloalkyl groups containing up to 6 carbon atoms; by a trialkyisilyl group in which each alkyl is of 1 to 6 carbon atoms; or by a quaternary ammonium group of formula Q- R31N± in which each group R1 is an alkyl group of 1 to 6 carbon atoms and 0- is a halide ion; and n is O or 1.

3. N-substituted 2-(nitromethylene)-tetrahydro-2H, 1 ,3-thiazines according to claim 1 or 2 characterised in that R represents an alkyl group containing 1 to 20 carbon atoms, an alkenyl group containing 2 to 4 carbon atoms, a haloalkyl group containing 1 to 4 carbon atoms and 1 to 3 fluorine, chlorine, bromine or iodine substituents, an alkoxyalkyl group containing in total 2 to 4 carbon atoms, an alkoxycarbonylalkyl or alkanoyloxyalkyl group containing in total 3 to 5 carbon atoms, a camphoryl group, a phthalimidoalkyl group in which the alkyl group contains 1 to 4 carbon atoms, a phenylalkyl group in which the alkyl group contains 1 to 4 carbon atoms, a halobenzyl group containing 1,2 or 3 chlorine, fluorine or bromine atoms, a trifluoromethylbenzyl group, a methylbenzyl group, an alkoxycarbonylbenzyl group in which the alkoxycarbonyl group contains 2 to 5 carbon atoms, a halophenoxyalkyl group where the halogen is chlorine, fluorine or bromine and the alkyl group contains 1 to 4 carbon atoms, a trimethylsilylalkyl group in which the alkyl group contains 1 to 4 carbon atoms, or an alkyltrialkylammonium halide group wherein each alkyl group contains 1 to 3 carbon atoms; and n=O or 1.

4. N-substituted 2-(nitromethylene)-tetrahydro-2H-1 ,3-thiazines according to claim 1, specifically described herein.

5. A process for the preparation of N-substituted 2-(nitromethylene)-tetrahydro-2H-1 ,3-thiazines according to claim 1 characterised in that a compound of formula:

is reacted with a sulphonyl halide of formula: R-S02-Hal (Ill) in which R and n are defined in claim 1 and Hal represents a halogen atom, in the presence of a base, and optionally oxidising the resulting product to reduce the compound in which n=1.

6. A process according to claim 5 characterised in that the reaction is carried out at a temperature at or below OOC.

7. A pesticidal composition comprising an N-substituted 2-(nitromethylene)-tetrahydro-2H-1 ,3- thiazine according to any one of the claims 1 to 4 together with a carrier.

8. A composition according to claim 7 characterised in that a thermal stabilizer is also present.

9. A method of combating pests at a locus infested, or liable to infestation by such pests, which comprises appiying to the locus a pesticidally effective amount of an N-substituted 2-(nitromethylene) tetrahydro-2H-1,3-thiazine according to any one of claims 1 to 5 or a composition according to claim 8 or9.