Improved process for producing n-alkyl-carbamates

Abstract: In the process for preparing N-alkyl-carbamates by directly reacting low-molecular-weight alkyl isocyanates, in particular methyl isocyanate, and a substituted phenolic precursor, by operating in solution, as the catalysts basic compounds are used, which belong to the class of the bicyclic amidines or to the class of bicyclic guanidines.

Description: The present invention relates to an improved process for producing N-alkyl-carbamates by means of the reaction of a phenol or of a naphthol with an alkyl isocyanate. N-alkyl-carbamates are valuable products and many of them display an useful activity as plant disease control agents, such as, e.g., 2,3-dihydro-2,2-dimethyl-benzofuran-7-yl N-methyl-carbamate (known as CARBOFURAN), 1-naphthyl N-methyl-carbamate (known as CARBARYL) and 2-isopropoxy-phenyl N-methyl-carbamate (known as PROPOXUR). N-alkyl-carbamates are obtained, according to the prior art, by reacting an alkyl isocyanate, in particular methyl isocyanate, with a phenol or a naphthol, by operating in an inert organic solvent and in the presence of a basic catalyst. For this purpose, reference is made to R.J. Kuhr and H.D.Dorough, "Carbamates Insecticides; Chemistry, Biochemistry and Toxicology", CRC Press (1977) and to the disclosure of US-A 4 390 710 and of EP-A. 200 249. The catalysts used in said reaction have a basic nature, and are normally selected from tertiary amines (e.g., triethylamine and triisopropylamine); heterocyclic bases (e.g., pyridine, 4 -dimethyl-amino-pyridine and N-methyl-imidazole); alkali-metal alkoxides (e.g., sodium methoxide and sodium ethoxide); alkali-metal carbonates (e.g., sodium carbonate); and derivatives of such metals as tin and titanium (e.g., dibutyl-tin dilaurate and dibutyl-tin diacetate). The preferred catalysts are the tertiary amines and tin derivatives. In practice, the reaction between a phenol or a naphthol and an alkyl-isocyanate is carried out in the presence of said catalysts, by operating at relatively low temperatures, in an organic solvent, and using a mutual stoichiometric ratio of the reactants, or a slight excess of the phenol or naphthol over the stoichiometric amount. Under these conditions, the reaction proceeds regularly, with high yields to the useful reaction product. The drawback which affects said reaction consists in the long times required for completing, or substantially completing, the same reaction, in particular in order to reduce to negligibly low values (less than 100 ppm - parts per million by weight) the residual alkyl isocyanate content in the reaction medium. Such a matter of fact constitutes a disadvantage from both standpoints of process cheapness, and of the extremely high toxicity of alkyl isocyanates in general, and of methyl isocyanate in particular. The present Applicant has found now that the drawbacks affecting the prior art can be overcome by means of the adoption of particular catalysts which enable unexpectedly high reaction speeds between the phenol or the naphthol and the alkyl-isocyanate to be reached. Therefore, according to the present invention, it results possible the concentration of the alkyisocyanate to be reduced to negligibly low values, by operating with short reaction times, in systems using stoichiometric, or approximately stoichiometric, amounts of the reactants. In accordance therewith, the present invention relates to a process for preparing N-alkyl-carbamates by catalytically reacting an alkyl-isocyanate with a phenol, or a naphthol, in an inert organic solvent characterized in that said reaction is carried out at a molar ratio of the phenol or the naphthol to the alkylisocyanate of from 1:1 to 1,1:1, at a temperature of from 0°C to 50°C, and in the presence of an amount of from 0,0001 mol to 0,05 mol per mol of the phenol or the naphthol concerned, of a catalyst selected from the bicyclic amidines, having the general formula: or from the bicyclic guanidines containing a stretched tertiary nitrogen bonded to a ketoimine carbon atom, having the general formula: wherein: R₁ and R₂ are, independently, C₃-C₅ unsubstituted alkylenes or substituted alkylenes, the substituents whereof are one or more C₁-C₅ alkyls or phenyls; R₃ and R₄ are, independently, C₃ or C₄ unsubstituted alkylenes or substituted alkylenes, the substituents whereof are one or more C₁-C₅ alkyls or phenyls, and R₅ is selected from the C₁-C₅ alkyls or the alkylaryls. Specific examples of catalysts are: 1,5-diazabicyclo (4.3.0) non-5-ene 3-phenyl-1,5-diazabicyclo (4.3.0) non-5-ene 1,8-diazabicyclo (5.4.0) undec-7-ene 7-methyl-1,5,7-triazabicyclo (4.4.0) dec-5-ene 1,8-Diazabicyclo (5.4.0) undec-7-ene can be prepared by making caprolactam and acrylonitrile react with each other (Michael reaction) and catalytically reducing the so obtained reaction product, as described by H. Oediger and H. Moller in Angew. Chem. 79, 53 (1967). 1,5-Diazabicyclo (4.3.0) non-5-ene can be prepared in a similar way by starting from butyrolactam. Other amidines useful for the intended purpose are described in Tetrahedron Letters, 51, 5175-7 (1967). The reaction between the phenol or naphthol and the alkyl-isocyanate can be represented by the following equation: The alkyl-isocyanates (R-N=C=O) useful for the intended purpose are those wherein R represents an alkyl group containing from 1 to 4 carbon atoms, and preferably a methyl group. Examples of R¹OH compounds, useful for the intended purpose, are phenol; substituted phenol, with from 1 to 3 substituents, which may be either equal to, or different from, one another, selected from alkyl, oxy-alkyl, thio-alkyl, amino-alkyl, alkylene-oxy-alkyl, alkylene-thio-alkyl, alkylene-amino-alkyl groups, wherein the alkyl group, which may be either linear or branched, contains from 1 to 5 carbon atoms (preferably from 1 to 3 carbon atoms) and the alkylene group contains 1 or 2 carbon atoms (it is preferably methylene); 1-naphthol; 2-naphthol; 2,3-dihydro-2,2-dimethyl-benzofuran-7-ol; 2,2-dimethyl-1,3-benzodioxol-4-ol; and 2-(1,3-dioxolan-2-yl)-phenol. Examples of preferred R¹OH compounds for the purposes of the present invention are: 3,5-xylenol; 3,4-xylenol; 2-isopropyl-phenol; 2-isopropoxy-phenol; 2-(ethylthiometyl)-phenol; 2-cresol; 3-isopropyl-5-methyl-phenol; 4-methyl-thio-3,5-dimethyl-phenol; 4-dimethylamino-3-methyl-phenol; 1-naphthol; 2,3-dihydro-2,2-dimethylbenzofuran-7-ol; and 2,2-dimethyl-1,3-benzodioxol-4-ol. According to the process of the present invention, the reaction is carried out in solution in an inert organic solvent and the solvents suitable for the intended purpose can be selected from aromatic hydrocarbons, such as benzene, toluene and xylene; ketones, such as acetone, methyl-ethyl-ketone and methyl-isobutyl-ketone; esters, such as ethyl acetate, dimethyl carbonate and diethyl carbonate; chlorinated aliphatic hydrocarbons, such as chloroform, methylene chloride, carbon tetrachloride and dichloroethane; and ethers, such as diethyl ether and tetrahydrofuran. The reaction temperatures are within the range of from 0° to 50°C. Furthermore, the reaction is carried out with an equimolar or nearly equimolar ratio of the phenol or naphthol reactant to the alkyl-isocyanate reactant, i.e. with a molar ratio within the range of from 1/1 to 1.1/1. The amount of catalyst used for the reaction is within the range of from 0.0001 to 0.05 mol per each mol of the phenol or naphthol compound. The preferred catalyst amounts are comprised within the range of from 0.0005 to 0.02 mol per each mol of the phenol or naphthol compound. The way how the reactants are placed into contact is not critic, however, in the preferred form of practical embodiment the reaction is carried out by dissolving the phenol or naphthol, besides the catalyst, in the selected organic solvent, and to the so obtained solution the alkyl isocyanate is gradually added until a molar ratio of the phenol or naphthol to said alkyl-isocyanate is reached, which is comprised within the above stated range of values. A technique of such a kind is disclosed, e.g., in US-A. 4 659 845. By means of the catalysts of the present invention, rates of reaction between the phenol or naphthol and the alkyl-isocyanate are reached, which are generally from 100 to 1,000 times higher than those which can be obtained by means of the catalysts known in the prior art, with the molar catalyst concentration, and the other reaction conditions being the same. In order to better evidence the activity of the catalysts according to the present invention, the values can be compared of the constants of rate of carbamate formation (Kabs) from 2,3-dihydro-2,2-dimethyl-benzofuran-7-ol with methyl isocyanate, which are obtained by operating at 25°C in toluene as the solvent, by using various concentrations of: 1,8-diazabicyclo (5,4,0) undec-7-eme (DBU), 1,5-diazabicyclo (5,4,0) non-5-ene (DBN) and 7-methyl-1,5,7-triazabicyclo (4,4,0) dec-5-ene (MTBD) and of the catalyst known from the prior art triethylamine (TEA). DBU Kabs 2 . 10⁻³ mol/litre 5,63 litres.mol⁻¹.min⁻¹ 8 . 10⁻³ mol/litre 22.2 litres.mol⁻¹.min⁻¹ 2 . 10⁻² mol/litre 27.7 litres.mol⁻¹.min⁻¹ DBN Kabs 1 . 10⁻³ mol/litre 4.5 litres.mol⁻¹.min⁻¹ 8 . 10⁻³ mol/litre 15.5 litres.mol⁻¹.min⁻¹ MIBD Kabs 3.4 . 10⁻⁴ mol/litre 12.5 litres.mol⁻¹.min⁻¹ 2.3 . 10⁻³ mol/litre 31.0 litres.mol⁻¹.min⁻¹ TEA Kabs 2.6 . 10⁻³ mol/litre 6.89.10⁻³ litres.mol⁻¹.min⁻¹ 8.15. 10⁻³ mol/litre 2.9 .10⁻² litres.mol⁻¹.min⁻¹ 1.3 . 10⁻² mol/litre 5.0 .10⁻² litres.mol⁻¹.min⁻¹ The following experimental examples are illustrative and not limitative of the present invention. Example 1 The equipment reported in the figure of the hereto attached drawing table is used, which comprises a reactor, equipped with a stirrer, which operates at atmospheric pressure and is connected through a condenser to a demister. The operating temperature is 20°C. Methyl isocyanate is fed as a liquid (-10°C), at a flow rate of 6.93 g/hour, to the reactor, to which: 2,3-dihydro-2,2-dimethyl-benzofuran-7-ol (63.2 g), toluene (250 g) and 1,8-diazabicyclo (5.4.0) undec-7-ene (0.045 g) were previously charged. The methyl isocyanate stream is fed for 3 hours. After this time period, the flow is discontinued and the reactor is separated from the feed line. The residual amount of methyl isocyanate in the solution is determined at regular time intervals, by means of the chromatography of a constant nitrogen stream saturated, at room temperature, with methyl isocyanate and toluene (the reactor's vapour phase). By means of a standard curve, the concentration of methyl isocyanate in the liquid phase is obtained. The values of residual methyl isocyanate concentrations, expressed as parts per million parts by weight (MIC ppm) starting from the time of interruption of reactor feed are: Time (minutes) MIC ppm 0 1,600 10 I 160 15 I 57 20 I 21 By operating as previously disclosed, but using triethylamine as the catalyst, fed in an amount of 0.45 g (i.e., one order of magniture larger than of 1,8-diazabicyclo (5.4.0) undec-7-ene), the following results are obtained: Time (minutes) MIC ppm mol/litre 0 25,500 0.402 10 I 22,000 0.347 15 I 20,100 0.318 20 I 18,800 0.296 EXAMPLE 2 The process is carried out as described in Example 1, by charging to the reactor: 2,3-dihydro-2,2-dimethyl-benzofuran-7-ol: 63.2 g toluene : 250 g 1,5-diazobicyclo (4.3.0) non-5-ene : 0.036 g The values of residual methyl isocyanate concentration are: Time (minutes) MIC ppm mol/litre 0 1,120 0.017 5 I 150 2.37.10⁻³ 10 I 30 4.37.10⁻⁴ EXAMPLE 3 The process is carried out as described in Example 1, by charging to the reactor: - 1-naphthol 55.5 g - toluene 500 g - 1,8-diazabicyclo (5.4.0) undec-7-ene 0.174 g The values of residual methyl isocyanate concentration are: Time (minutes) MIC ppm mol/litre 0 1,380 0.022 5 I 352 5.5.10⁻³ 10 I 120 1.9.10⁻³ 15 I 40 6.10⁻⁴

Claims: Process for preparing N-alkyl-carbamates by catalytically reacting an alkyl-isocyanate with a phenol, or a naphthol, in an inert organic solvent characterized in that said reaction is carried out at a molar ratio of the phenol or the naphthol to the alkylisocyanate of from 1:1 to 1,1:1, at a temperature of from 0°C to 50°C, and in the presence of an amount of from 0,0001 mol to 0,05 mol per mol of the phenol or the naphthol concerned, of a catalyst selected from the bicyclic amidines, having the general formula: or from the bicyclic guanidines containing a stretched tertiary nitrogen bonded to a ketoimine carbon atom, having the general formula: wherein: R₁ and R₂ are, independently, C₃-C₅ unsubstituted alkylenes or substituted alkylenes, the substituents whereof are one or more C₁-C₅ alkyls or phenyls; R₃ and R₄ are, independently, C₃ or C₄ unsubstituted alkylenes or substituted alkylenes, the substituents whereof are one or more C₁-C₅ alkyls or phenyls, and R₅ is selected from the C₁-C₅ alkyls or the alkylaryls. Process according to Claim 1, wherein the catalyst is selected from: 1,5-diazabicyclo-(4.3.0)-non-5-ene; 3-phenyl-1,5-diazabicyclo-(4.3.0)-non-5-ene; 1,8-diazabicyclo-(5.4.0)-undec-7-ene, and 7-methyl-1,5,7-triazabicyclo-(4.4.0)-dec-5-ene. Process according to Claim 1, wherein the amount of the catalyst is from 0,0005 mol to 0,02 mol of the catalyst concerned per mol of the phenol or the naphthol concerned. Process according to Claim 1, wherein said phenol or naphthol is selected from: phenol, substituted phenols carrying from 1 to 3 substituents, equal to, or different from each other and selected from: alkyls, oxyalkyls, thioalkyls, aminoalkyls, alkylenoxyalkyls, alkylenethioalkyls, and alkyleneaminoalkyls, the alkyl of the latter being a linear or a branched C₁-C₅ alkyl, the alkylene being a C₁-C₂ alkylene; 1-naphthol; 2-naphthol; 2,3-dihydro-2,2-dimethyl-benzofuran-7-ol; 2,2-dimethyl-1,3-benzodioxol-4-ol, and 2-(1,3-dioxolan-2-yl)phenol. Process according to Claim 4, wherein said phenol or naphthol is selected from 3,4-xylenol, 3,5-xylenol, 2-isopropyl-phenol, 2-isopropoxy-phenol, 2-(ethylthiomethyl)-phenol, 2-cresol, 3-isopropyl-5-methyl-phenol, 4-methyl-thio-3,5-dimethyl-phenol, and 4-dimethylamino-3-methyl-phenol. Process according to Claim 1, wherein the alkyl of said alkyl isocyanate is a C₁-C₄ alkyl. Process according to Claims 1 and 6, wherein the alkylisocyanate is methyl isocyanate. Process according to Claim 1, wherein said inert organic solvent is selected from the aromatic hydrocarbons, the ketones, the esters, the chlorinated hydrocarbons and the ethers.

Inventors: MIZIA FRANCO, RIVETTI FRANCO, ROMANO UGO, MIZIA, FRANCO, RIVETTI, FRANCO, ROMANO, UGO
Applicants: ENICHEM SINTESI, ENICHEM SYNTHESIS S.P.A.
Application year: 1988
Main classification: C07C271
IPC codes: C07C67, B01J31, C07C271, C07C239, C07D317, C07D307, C07B61