CA2174332C - Preparation of asymmetrically substituted triazines - Google Patents

Abstract

Described is a method of preparing unsymmetrically substituted triazines of formula (I), in which R1 is hydrogen, methyl or ethyl, R2 and R3, independently of each other, are an optionally substituted hydrocarbon group , by reacting a cyanoguanidine of formula (II) with a carboxylic acid derivati ve in the presence of an alcohol of formula (III), the method being characteriz ed in that the reaction is carried out with a carboxylic acid ester of formula (IV), in which R3 is as defined above and R4 is an optionally substituted hydrocarbon group, in the presence of a base or a carboxylic acid amide selected from the group comprising N,N-dialkylformamide, N,N-dialkylacetamid e and N-methylpyrrolidone and in the presence of a salt or a salt-like compoun d of one of the elements magnesium, calcium, aluminium, zinc, copper, iron, cobalt, nickel and chromium.

Description

2 ~ ~~332 Preparation of asymmetrically substituted triazines The present invention relates to a process for preparing asymmet-5 rically substituted triazines of the general formula I

N/\N
I, where R1 is hydrogen, methyl or ethyl, R2 and R3 independently of one another are a hydrocarbon radical having 1 to 6 C atoms, which can carry substituents which are inert under the reaction conditions, by reaction of a cyanoguanidine of the formula II
NH
II
/C~ /CN
HN N
H

with a carboxylic acid derivative in the presence of an alcohol of the formula III
RZ~H III.
Asymmetrically substituted triazines can be prepared in a great variety of ways, eg. starting from N-cyanoamides by reaction with Vilsmeier complexes (R.L.N. Harries, Aust. J. Chem. 34 (1981) 623), from N-cyanoimidate esters (DE-A 34 11 202; M.A. Perez, J.L. Soto, Heterocycles, 20 (1983) 463; K.R. Huffman, F.C. Schaefer, J. Org. Chem. 28 (1963) 1816) or from biguanidines (S. L. Shapiro et al, J. Org. Chem. 25 (1960) 379;
US-A 2 535 968). The reaction of guanylthiourea with dimethyl sulfate and carboxylic acid derivatives has also been published (H. Eilingsfeld, H. Scheuermann, Chem. Ber. 100 (1967) 1874;
DE-A 16 70 147; EP-A 545 149) as well as the reaction of tri-chloroacetamidinoguanidines with derivatives of trifluoroacetic acid (DE-A 40 34 078). According to all these methods, no chelate complexes are passed through as intermediates.

2 ~ ~4~3?_ Another possibility for preparing 6-trifluoromethyl-1,3,5-tri-azines according to a process known from Yakugaku Zasshi 95, (1975) 499-511 consists in converting N-cyanoguanidines into copper complexes of N-amidino-O-alkylisoureas, liberating the urea derivatives with hydrogen sulfide and then reacting them with trifluoroacetate esters according to the following reaction scheme:
N NH NH NH
III II + ROH _ 1/2 CuCl2 ~N~ ~ H2 + 1/2 CuCl2 R ~ ~ ~ ~ Hz H H

II II N/\N
+ 1/2 H2S ~ ~ ~ ~ . HC1 NaOR~
- 1/2 CuS R~ N NH2 CF3COOR R~ N NH2 H
A variant of this process is known from DD-A-252 374, the acetate of copper being used instead of copper(II) chloride.
Stoichiometric amounts of Cu salts are necessary for this pro-cess; in the absence of Cu salts, mainly guanylurea is formed instead of the N-amidino-O-alkylisourea (Kyushu Kogyo Daigaku Kenkyu Hokoku No. 12, (1962) 69-78).
The reaction of N-amidino-O-alkylisourea hydrochloride with ethyl chloroacetate in ethanol/NaOC2H5 leads to the corresponding chlo-romethyltriazine only in poor yields. The starting material has to be set free from the Cu complex beforehand.
The preparation of chelate complexes starting from cyanoguanidine using copper acetate or zinc chloride in methanol is described by R.I. Dutta and A. Syamal in Coord. Chem. Rev., Vol. 2, 1967, pp. 441-457. It is known from Chemistry of Heterocyclic Com-pounds, Vol. 25, 1989, pp. 547-550 to isolate zinc complexes of this type and then to react them with trifluoroacetic anhydride to give the triazine.

In a similar manner, zinc sulfate forms a chelate complex which after working up by boiling in water decomposes in a second reac-tion step to give the sulfate of arnidino-0-methylisourea (US-A 3,360,534, IN-A 167 500) and this can be reacted in a third reaction step with acetic anhydride (S. Lotz, G. Kiel, G. Gattow, Z. anorg. allg. Chem. 604 (1991) 53-62) or with methyl trifluoro acetate (T. Tsujikawa, Yakugaku Zasshi 95, loc. cit) to give the triazine in yields of 31 and 26% respectively.
The last-mentioned processes are always very laborious multi-stage processes which in the case of the esters lead to the desired triazines only in poor yields or in the case of the anhy-drides inevitably include the production of stoichiometric amounts of carboxylic acid which can only be recycled to give the corresponding anhydrides very laboriously. As a rule, the toxico-logically unacceptable, occasionally poorly filterable heavy metal complexes have to be isolated and then reacted with the anhydrides in an inert solvent, as reaction in alcoholic solution is prevented on account of the reaction of anhydrides with alco-hots. In addition, the processes described are disadvantageous inasmuch as unavoidably large amounts of heavy metal salts are obtained in the form of organic slurries which can only be dis-posed of with difficulty.
US-A-4,886,881 describes the single stage synthesis of 2-amino-triazines, starting from cyanoguanidine and trimethyl ortho-acetate in the presence of a Lewis acid catalyst such as zinc chloride. Dimethylformamide and acetonitrile are recommended as solvents.
It is an object of the present invention to find a process which makes available triazines of the structure I by reaction of cyanoguanidines with carboxylic acid esters which are more readily available but also less reactive than corresponding anhy-drides or orthoesters. If possible, this process is intended to be carried out without isolation of intermediates (one-pot process).
We have found that this object is achieved by a process for pre paring asymmetrically substituted triazines of the general formu la I

N~N
Ir where R1 is hydrogen, methyl or ethyl, R2 and R3 independently of one another are a hydrocarbon radical having 1 to 6 C atoms, which can carry substituents which are inert under the reaction conditions, which process comprises reacting a cyanoguanidine of the formula II:
NH
/ ~N I I
HN N
H

with a carboxylic acid ester of the formula IV:
R3-COOR4 fV
where R3 has the abovementioned meaning and R4 is a hydrocarbon radical having 1 to 6 C atoms, which optionally carries substituents which are inert under the reaction conditions such a reaction being carried out in the presence of:
i) an alcohol of the formula III

ii) a base or a carboxamide selected from the group consisting of N,N-di(C1-C~-alkyl)formamide, N,N-di(C1-C4-alkyl)acetamide and N-methylpyrrolidone and iii) a salt or a salt-like compound of an element selected from the group consisting of magnesium, calcium, aluminum, zinc, copper, iron, cobalt, nickel and chromium.
The substituted cyanoguanidine used as a starting substance is generally known. The N-methyl- and N-ethyl-substituted deriva-tives are available from cyanoguanidine and dialkyl sulfate as described by A.E. Kretov and A.S. Hespalyi in 2hur. Prik. Khimii, 4a Vol. 34, (1961) 621ff. The guanidine can also be employed in the form of an acid addition salt, in this case the acid set free during the reaction expediently being neutralized by addition of a suitable base such as sodium methoxide.
As the hydrocarbon radical in the triazine I, alighatic, cyclo-aliphatic, aromatic or araliphatic radicals having up to 8 carbon atoms such as C1-Ce-alkyl, C3-Ca-alkenyl, C3-Ce-alkynyl, C3-C8-cycloalkyl and also phenyl, benzyl or phenethyl are to be mentioned in particular for RZ. With respect to the use of the intermediates I to be prepared, R2 is, for example, C1-C4-alkyl 2 ~ ~4~3?_ such as methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl or tert-butyl; C3-C4-alkenyl such as prop-2-en-1-yl, 1-methylprop-2-en-1-yl, but-2-en-1-yl or but-3-en-1-yl; C3-C4-alkynyl such as prop-2-yn-1-yl or but-2-yn-1-yl; C3-C6-cycloalkyl such as cyclo-5 propyl, cyclobutyl, cyclopentyl or cyclohexyl; particularly pre-ferably C1-C4-alkyl such as methyl.
The radical R2 can in turn carry still further substituents which are inert under the reaction conditions, such as eg. fluorine or chlorine, phenyl, C1-C4-alkyl, C1-C4-haloalkyl or C1-C4-alkoxy.
The carboxylic acid ester radical R4 has the meaning mentioned above for R2 and is preferably identical to R2.
According to observations to date, the radical R3 is widely vari-able. It can have eg. the meanings specifically mentioned for R2.
With respect to the intended use of the triazines I, R3 is prefer-ably C1-C4-haloalkyl, in particular fluoro- or chloromethyl or -ethyl such as CC13, CF3, CF2CF3, CF2C1, CFC12, CH2C1, CHC12, CH2F
and CHFz. Perhalogenated alkyl radicals such as CC13, CF3 and C2F5 are particularly preferred.
According to the invention, the reaction of the cyanoguanidine II
with the alcohol III (or R40H, formed by hydrolysis of the ester Iv) is carried out in the presence of a base. Surprisingly, the reactivity of the metal chelate complexes can be increased by this means such that the reaction with the esters IV is possible, even though these esters are less reactive by far than corre-sponding carboxylic anhydrides or orthoesters.
Suitable bases are inorganic and organic bases. Preferred inor- -ganic bases are alkali metal and alkaline earth metal hydroxides, and preferred organic bases are tertiary amines such as C1-C4-tri-alkylamine, eg. triethylamine, pyridine or N-methylmorpholine.
The alkali metal or alkaline earth metal alkoxide of the alcohol RZOH to be reacted is expediently used as a base. This alkoxide can also be formed in situ, eg. from the corresponding alkali metal or sodium amide or sodium hydride and the alcohol III.
The amount of base is customarily from 0.1 to 2, in particular from 0.8 to 1.2, mole equivalents, based on the cyanoguanidine II. Larger amounts are possible, but as a rule provide no further advantages.
A carboxamide from the group consisting of N,N-di(C1-C4-alkyl)-formamide, N,N-di(C1-C4-alkyl)acetamide or N-methylpyrrolidone can also be used instead of the base or additionally. Examples which ~I~~~~~

may be mentioned are dimethylformamide, diethylformamide, dime-thylacetamide and diethylacetamide. The addition of carboxamides has proven particularly suitable when using heavy metals, in par-ticular copper, as chelating agents.
The amount of carboxamide is in general from 1 to 30, in particu-lar from 5 to 10, mole equivalents, based on the cyanoguanidine II. It may also be advantageous to utilize the carboxamide as a solvent.
Suitable salts or salt-like compounds of the alkaline earth metals, of aluminum or of the heavy metals are products readily soluble in the reaction medium, such as halides, eg. fluorides, chlorides or bromides, nitrates, sulfates or possibly phosphates, alkoxides or acetates. According to present knowledge, apart from good solubility the type of alkaline earth metal compound or metal compound does not matter, so that, inter alia, cost consid-erations are decisive in the choice. Examples which may be listed are the following compounds: MgCl2, Mg(OCH3)2, CaCl2, Zn(N03)2, Cu(CH3C00)2, A1C13, AlBr3, ZnCl2, ZnBr2, CuClz, NiBr2, CrCl3, CaO, Ca(N03)2, Mg(N03)2, MgO, ZnO, FeCly, FeCl3, Fe(N03)2, Fe(N03)3. The chlorides, in particular CaCl2 and ZnCl2, are particularly preferred.
one advantage of the process according to the invention is that the presence of heavy metals can be largely or completely dis-pensed with and, for ecotoxicological reasons, less unacceptable elements such as magnesium- (sic] and in particular calcium can be avoided. Soluble salts of these elements are therefore partic-ularly preferred. Very good results are also achieved, however, with zinc compounds.
The salts or salt-like compounds of the abovementioned elements can be employed in stoichiometric amounts or preferably less than stoichiometric amounts, based on the cyanoguanidine II, eg. in amounts of from 0.001 to 2 mol, in particular from 0.005 to 1.0 mol per mole of II.
When using heavy metals, their amount is kept as low as possible and only catalytic amounts, eg. under 0.6 mol, are employed per mole of cyanoguanidine. In the case of magnesium and calcium salts, the particularly preferred amount is from 0.01 to 1.0, in particular from 0.05 to 0.5 mol. Of course, more than stoichio-metric amounts, based on II, eg. from 1 to 2 mol per mole of II, can be employed, but reasons of economy are a point in favor rather of employing substoichiometric amounts.

~ ~ 7~~:~2 On the one hand, the starting substance III can be formed in situ from the carboxylic acid ester IV (RZ = R4) or is added to the reaction mixture (preferably as a solvent). When using the start-ing material III simultaneously as a solvent, it is recommended 5 to select the alcohol component in the ester IV accordingly (ie.
R2 = R4) in order to avoid by-products.
The molar ratio of cyanoguanidine II to the alcohol III is in general from 1 to 30, in particular from 5 to 10.
The carboxylic acid ester IV is expediently used in an amount of from 0.5 to 10, in particular from 1 to 5 mol, per mole of cyano-guanidine II. The esters IV to be mentioned are particularly pre-ferably the following compounds:
C1-C4-alkane- or haloalkanecarboxylic acid esters such as methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl trifluoroacetate, methyl difluoroacetate, methyl fluoro-acetate, methyl trichloroacetate, methyl dichloroacetate, methyl chloroacetate, ethyl trifluoroacetate, (m)ethyl pentafluoropro-pionate and (m)ethyl pentachloropropionate. With respect to the intended use of the triazines I as intermediates for crop protec-tion agents, methyl trifluoroacetate and ethyl trifluoroacetate are particularly preferred as starting substances IV.
The reaction of II with III and IV can be carried out in sub-stance, ie. without addition of inert solvents. Advantageously, the alcohol R20H is simultaneously used as a solvent. According to a particularly preferred embodiment, the corresponding alkoxide is then selected as a base.
The reaction temperature is from 0 to 200°C, in particular from 20 to 150°C, particularly preferably the reflux temperature of the reaction mixture.

Particular conditions with respect to the pressure are not neces-sary, in general the reaction is carried out at atmospheric pres-sure or under the autogenous pressure of the particular reaction medium.
The reaction can be carried out continuously and also batchwise.
In the continuous procedure the reaction components are prefer-ably passed through a tubular reactor or a stirred tank reactor cascade.

LI~~~~~
According to a particularly preferred embodiment of the process according to the invention, the triazine I is prepared without isolation of intermediates. For example by initially introducing the cyanoguanidine II, the ester IV and less than the stoichio-metric amounts of the alkaline earth metal salt or of the metal salt (or salt-like compounds) into the alcohol R20H as a solvent, metering in the base, eg. the alkali metal alkoxide such as NaOR2 or KORz and heating the mixture to reflux.
As a rule, the reaction is terminated in a customary manner when cyanoguanidine can no longer be detected in the reaction mixture (eg. by means of thin-layer chromatography, high pressure liquid chromatography or gas chromatography).
Working-up to the process product I is then as a rule carried out by conventional processes such as distillation, filtration, cen-trifugation or by addition of water and subsequent extraction.
The crude products obtained can be further purified if desired, eg. by crystallization, rectification or by means of chromato-graphic methods.
The triazines I which can be prepared in a simple manner by the process according to the invention are useful intermediates for the synthesis of dyestuffs, drugs and crop protection agents, in particular herbicides, as described eg. in the publications EP A 508 348, EP A 111 442 or DE A 40 38 430.
Synthesis examples Example 1 2-Amino-4-methoxy-6-trifluoromethyl-1,3,5-triazine 21 g (0.25 mol) of N-cyanoguanidine, 3.4 g (0.025 mol) of anhy-drous zinc chloride and 160 g (1.25 mol) of methyl trifluoro-acetate are initially introduced into 400 ml of methanol, the mixture is warmed to 50°C and 50 g (0.27 mol) of 30% strength sodium methoxide solution are pumped in in the course of 10 hours. The solvent is then largely removed, and the residue is washed with 250 ml of water and 250 ml of dilute hydrochloric acid and dried at 60°C/20 mbar. 43.2 g (0.22 mol, 89%) of 2-ami-no-4-trifluoromethyl-6-methoxy-1,3,5-triazine are obtained in the form of a colorless powder (HPLC: > 99% by weight) M.p.: 161-163.5°C

_ 0050/44394 Example 2 2-Amino-4-methoxy-6-trifluoromethyl-1,3,5-triazine 56.6 g (0.5 mol) of calcium chloride (98%, powdered) and 210 g (2.5 mol) of N-cyanoguanidine are initially introduced into 2 1 of methanol. The mixture is heated with stirring to reflux tem-perature and stirred under reflux for one hour, whereupon a homo-geneous solution is obtained. The mixture is then cooled to room temperature and 640 g (5.0 mol) of methyl trifluoroacetate and then, in the course of 25 minutes, 450 g (2.5 mol) of a solution of sodium methoxide (30% by weight in methanol) are added, where-upon a white precipitate deposits. After heating under reflux for 2 hours, the mixture is cooled to room temperature and a pH of about 6 is set by addition of conc. hydrochloric acid. The metha-nol is then distilled off, about 2 1 of water are added little by little, and the deposited, finely crystalline white crystals are separated off and dried in vacuo.
Yield: 402.4 g (2.07 mol; 83% of theory).
1H-NMR spectrum (270 MHz, CDC13, int. TMS, 8 (ppm)): 6.45 br (1H);
5.88 br (1H); 4.03 s (3H).
Example 3 2-Amino-4-methoxy-6-trifluoromethyl-1,3,5-triazine 84 g (1 mol) of cyanoguanidine and 100 g (0.5 mol) of copper acetate are initially introduced into 600 ml of methanol and the mixture is heated to reflux for 7 h. After cooling to 20°C, the mixture is filtered with suction and the solid copper complex is separated off and dried in vacuo. 41.4 g (0.1 mol) of this resi-due are initially introduced into 200 ml of methanol and 45 g (0.25 mol) of 30% strength sodium methoxide solution are added dropwise in the course of 15 min. 76.8 g (0.6 mol) of methyl tri-fluoroacetate are then added dropwise and the mixture is heated to reflux for 2 h. It is allowed to cool to 40°C, a red-violet solid (28.6 g) is filtered off and the mother liquor is concen-trated. The residue is washed with water and dried to give 16.4 g (84.5 mrnol, 38%) of the abovementioned triazine.
Example 4 2-Amino-4-methoxy-6-trifluoromethyl-1,3,5-triazine 41.4 g (0.1 mol) of the copper complex from Example 3 are ini-tially introduced into 300 ml of DMF and 51.2 g (0.4 mol) of methyl trifluoroacetate are added dropwise at 20°C in the course of 15 min. The mixture is then warmed at 50°C for 1 hour and at 90°C for 5 hours. The blue reaction solution is concentrated and the residue is stirred with 100 ml of water and 100 rnl of dilute . 0050/44394 hydrochloric acid. After filtering the suspension with suction, washing the filter cake and drying the residue, 23.4 g (0.12 mol, 60%) of the abovementioned triazine are obtained in the form of a colorless powder.

Example 5 2-Amino-4-ethoxy-6-trifluoromethyl-1,3,5-triazine 8.4 g (0.1 mol) of cyanoguanidine and 35.5 g (0.25 mol) of ethyl 10 trifluoroacetate are initially introduced into 46 g of ethanol, and a suspension of 8.5 g (0.125 mol) of sodium ethoxide in 39.8 g of ethanol is added in the course of 5 min. After the addition of 5.66 g (0.05 mol) of calcium chloride, the reaction mixture is heated to reflux for 7 hours. 0.5 ml of a conc. hydro-chloric acid is then added at 20°C and the ethanol is removed.
100 g of water are added and the suspension is filtered with suc-tion. The residue is washed with 50 ml of water and dried at 50°C/20 mbar. 18.6 g (0.089 mol, 89%) of 2-amino-4-ethoxy-6-tri-fluoromethyl-1,3,5-triazine are obtained in the form of a color-less powder (HPLC: 99.8 area %, m.p. 124-125°C).
Example 6 2-Amino-4-difluorornethyl-6-methoxy-1,3,5-triazine A solution of 1.9 g (23 mmol) of N-cyanoguanidine and 2.6 g (23 mmol) of calcium chloride in 50 ml of methanol is stirred under reflux for 90 min. The reaction mixture is cooled to from 20 to 25°C and 5.0 g (45 mmol) of methyl difluoroacetate are rap-idly added dropwise, then 4.1 g (23 mmol) of sodium methoxide solution (30 percent by weight in methanol) are slowly added dropwise, whereupon a white precipitate separates. After stirring under reflux for 2 hours, the volatile constituents are removed in a water-jet vacuum at a bath temperature of 40°C, the residue is partitioned between 100 ml of water and 100 ml of ethyl ace-tate, and the organic phase is separated off and dried over MgS04.
After removal of the solvent at 40°C in a water-jet vacuum, the title compound remains as a slightly contaminated oil (1.1 g, 6.3 mmol; 28% of theory) which if required can be crystallized by trituration with an ether/hexane mixture (v:v=1:3).
1H-NMR spectrum (400 MHz, CDC13, int. TMS, b (ppm)): 7.44 br (1 H); 6.97 br (1 H); 6.27 t (zJH_F=55 Hz, 1 H); 3.96 s (3 H).
isC-NMR spectrum (100 MHz, CDC13/CD3S(O)CD3, int. TMS, 8 (ppm), proton-decoupled): 171.5 s (C-OCH3); 169.8 t (C-CHF2, zJ~_F25 Hz [sic]); 168.8 s (C-NH2); 111.3 t (CHF2, 1J~_F243 Hz [sic]); 54.7 s (OC_H3).

Claims (13)

1. A process for preparing an asymmetrically substituted triazine of the general formula I:

where R1 is hydrogen, methyl or ethyl, R2 and R3 independently of one another are a hydrocarbon radical having 1 to 6 C atoms, which optionally carries substituents which are inert under the reaction conditions, which process comprises reacting a cyanoguanidine of the formula II:

with a carboxylic acid ester of the formula IV:

where R3 has the abovementioned meaning and R4 is a hydrocarbon radical having 1 to 6 C atoms, which optionally carries substituents which are inert under the reaction conditions such a reaction being carried out in the presence of:
i) an alcohol of the formula III

ii) a base or a carboxamide selected from the group consisting of N,N-di(C1-C4-alkyl)formamide, N,N-di(C1-C4-alkyl)acetamide and N-methylpyrrolidone and iii) a salt or a salt-like compound of an element selected from the group consisting of magnesium, calcium, aluminum, zinc, copper, iron, cobalt, nickel and chromium.

2. A process as claimed in claim 1, wherein the base (ii) that is used is an alkoxide or a tertiary amine.

3. A process as claimed in claim 1, wherein the base (ii) that is used is an alkali metal alkoxide or alkaline earth metal alkoxide of the alcohol III.

4. A process as claimed in claim 1, wherein the salt or salt-base compound (iii) is in the form of an halide, nitrate sulphate, alkoxide or acetate.

5. A process as claimed in claim 1, wherein the salt or salt-like compound (iii) is used in an amount of from 0.001 to 2 mol per mole of cyanoguanidine II.

6. A process as claimed in claim 1, wherein the salt or salt-like compound (iii) is used in less than a stoichiometric amount, based on the cyanoguanidine II.

7. A process as claimed in claim 1, wherein use is made of a salt or salt-like compound of the alkaline earth metal calcium or magnesium.

8. A process as claimed in claim 6, wherein the alkaline earth metal compound is used in an amount of from 0.01 to 2 mol per mole of cyanoguanidine II.

9. A process as claimed in claim 1, wherein the cazrboxylic acid ester used is a C1-C6-alkyl ester of a perfluorinated or perchlorinated C1-C3-carboxylic acid.

10. A process as claimed in claim 1, wherein methyl trifluoro-acetate is reacted in methanol in the presence of an alkali methoxide as sail base.

11. A process as claimed in claim 1, wherein methyl trifluoro-acetate is reacted with a cyanoguanidine II in the presence of an alkali methoxide and of a calcium salt.

12. A process as claimed in claim 1, wherein methyl trifluoro-acetate is reacted with the cyanoguanidine II in N,N-di-methylformamide in the presence of copper salts.

13. A process as claimed in claim 1, wherein methyl trifluoro-acetate is reacted with a cyanoguanidine II in the presence of an alkali methoxide and of a zinc salt.