ITMI970433A1 - PROCEDURE FOR THE SYNTHESIS OF AROMATIC CARBAMATES FROM NITROAROMATIC COMPOUNDS - Google Patents

Description

MINISTRY OF UNIVERSITY AND SCIENTIFIC AND TECHNOLOGICAL RESEARCH

Description

The present invention relates to a process for the preparation of aromatic carbamates by catalytic reductive carbonylation of nitroaromatic compounds with carbon monoxide and a compound containing at least one hydroxyl group which uses suitable catalysts.

Aromatic carbamates (or urethanes) are valuable intermediates useful for the production of pesticides, dyes, pharmaceutical compounds and aromatic isocyanates used in the preparation of polyurethanes. Among the carbamates those of greatest interest, from a commercial point of view, are phenyl urethane and 2,4-toluene-urethane used in the preparation of methylenediphenyl diisocyanate (MDI) and toluene diylsocyanate (TDI) which are currently industrially produced by phosgenation of the corresponding diamines.

It is known in the art to prepare aromatic carbamates by reductive carbonylation of nitroaromatic compounds with carbon monoxide and an alcohol in the presence of suitable catalytic systems.

Thus, for example, patent US-4,186,269 describes a process for the preparation of aromatic carbamates which uses a catalytic system consisting of a noble metal (Pd), a Lewis acid (generally FeCl3 or SnCl4) and a tertiary amine (typically triethylamine or pyridine) as co-catalysts. This process substantially suffers from the disadvantages deriving from corrosion problems and from those deriving from the recovery of the products and the catalytic system. .

In order to overcome these drawbacks, alternative synthesis methods have been proposed, which are based on the use of catalytic systems deriving from the combination of:

1) a metal salt belonging to the eighth group of the periodic table, preferably palladium;

2) a mono or bidentate base, generally consisting of an alkyl or cycloalkyl hydrocarbon symmetrically substituted by two dialkylphosphinic or diphenylphosphinic groups or by two groups containing at least one nitrogen atom; and 3) an acid whose conjugate base is not a palladium ligand.

Such catalytic systems are, for example, described in patents EP-86.281, EP-231.045, EP-296 .686, EP-100.109 and US 5.177.036, and allow to obtain aromatic carbamates with good yields and selectivity.

These processes of the known art, however, have the drawbacks deriving from the fact that the catalytic systems require the presence of strong acids as cocatalysts (for example CF3COOH, p-toluene sulphonic acid) which create problems of a technological nature, for example corrosion of the autoclaves. Furthermore, operating according to these known processes, high molar ratios of palladium with respect to the starting nitroaromatic compound (EP-86.81, EP-231.045, EP-296.686) or nitrogen binder and / or acid cocatalyst are required (US 5.177.036), to obtain good yields and selectivity in the useful product. This has technical and economic disadvantages relating to the recovery of the products and the recycling of the catalytic system, especially when operating on an industrial scale.

Alternative processes have recently been proposed in the art for the preparation of aromatic carbamates which are substantially based on the use of preformed palladium complexes which contain two nitrogenous bidentate chelators per palladium atom and two non-esterifiable or hardly esterifiable, non-coordinating, non-labile anions. , having the formula [Pd (chel) 2] [A] 2, where chel is a nitrogenous ligand, generally phenanthroline or its derivatives, and A is the anion of an acid selected from PFG-, BF, -, OTf- ( Alessio et al. J. of Molecular Catalysis, 42, 67-80 (1987); P. Wehman et al., J. Chem. Soc., Chem. Commun. 1996, pages 217-218).

These processes are also not free from problems, as they require high molar ratios of palladium with respect to the nitroaromatic compound and / or high amounts of acid. Furthermore, the decomposition of the metal palladium catalyst is observed which co-precipitates together with the reaction products, as occurs in the reductive carbonylation of 2,4-dinitrotoluene, or it can deposit on the reactor walls.

It has now been found that it is possible to overcome the drawbacks of the prior art described above by means of the process of the present invention which uses palladium carboalkyloxy of general formula (I) as catalyst.

This catalyst is stable under operating conditions. In fact, no metallic palladium formation is observed at the end of the carbonylation reaction.

By operating according to the process of the present invention, it is possible to prepare aromatic mono and dicarbamates with high yields and selectivities, using reduced quantities of the catalytic system.

Accordingly, in a first aspect, the present invention relates to a process for the synthesis of aromatic carbamates by reductive carbonylation of nitroaromatic compounds with carbon monoxide and a compound containing at least one hydroxyl group in the presence of:

(a) a catalyst of general formula (I)

where: chel represents a nitrogenous or phosphorated bidentate ketant and R represents a C 1 -C 2 alkyl group;

b) a free nitrogen or phosphorus chelator and c) a cocatalyst of general formula (II)

where is it :

chel 'represents a nitrogenous bidentate chelator equal to or different from chel and A <'> an anion of an acid with pKa less than 2.

Examples of nitrogenous bidentate chelators can be chosen from those of general formula (III)

where: X and Y, the same or different from each other, represent a bridged organic group having at least three atoms in the bridge of which at least two are carbon atoms.

When, in addition to the carbon atoms, the X and Y groups contain other atoms these are preferably nitrogen or oxygen atoms.

Preferred nitrogenous bidentate chelators according to the present invention are those in which the bridging groups X and Y are the same and contain from 3 to 10 atoms at least two of which are carbon atoms.

Examples of nitrogen chelators are:

1,10-phenanthroline (phen); 4-methyl-1,10-phenanthroline; 5-methyl-1,110-phenanthroline; 4,7-dimethyl-1,110-phenanthroline; 3,8-dimethyl-1,110-phenanthroline; 4,7-diphenyl-1,10-phenanthroline; 4,7-dichloro-1,10-phenanthroline; 3,4,7,8-tetramethyl-1, 10-phenanthroline,

4,4'-dimethyl-5, 5'-dioxazole.

Phosphorated bidentate chelators are selected from those of general formula (IV):

where: R represents an alkyl radical with a number of carbon atoms from 2 to 4, a cycloalkylidene radical with a number of carbon atoms from 2 to 10 or an orthophenylene radical; R1-R4 the same or different from each other, each represent a C1-C10 alkyl radical, C3-C10 cycloalkyl radical or a C6-C12 aromatic radical optionally substituted with a C1-C4 alkyl or alkoxy radical.

Non-limiting examples of phosphorated bidentate chelators suitable for the purposes of the present invention are selected from 1,3-bis (diphenylphosphino) propane, 1,3-bis (di-4-methoxy-phenylphosphine) propane, 1,4-bis (dicyclohexylphosphine ) butane and 1,2-bis (diphenylosphine) eielohexane.

In the catalysts of general formula (I), R is preferably a methyl group.

The catalysts of general formula (I) according to the present invention can be prepared by the process described by Hanson, B.E. et al. (Organometallics, 12, 568, 1993).

In the cocatalyst of general formula (II), A <“> is an essentially non-coordinating and non-esterifiable anion selected from hexafluorophosphate, tetrafluoroborate and hexafluoroantimoniate. Preferred is <'> hexafluorophosphate (PF6 <~>).

In the preferred embodiments of the process of the present invention the cocatalyst [phenH '] [PF6] is used.

Co-catalysts can be synthesized as white solids according to the following reaction:

The amount of catalyst (I) used in the process of the present invention can vary within wide limits. Conveniently, a quantity of catalyst is used such as to give a concentration of gram atoms of palladium per mole of nitroaromatic compound comprised between 1.10 <-2> and 1 .10 <-5> preferably between 2.10 <-3> and 2.5.10 <-4 >.

The free aromatic chelator (chel) is used in an amount such as to give a molar ratio chel / Pd comprised between 0.1: 1 and 20: 1, preferably between 1: 1 and 16: 1.

Advantageous results are obtained by using molar ratios between cocatalyst (II) and palladium ranging from 0.1: 1 to 20: 1, preferably from 1: 1 to 16: 1.

Nitroaromatic compounds usable in the process of the present invention can be selected from compounds containing at least one aromatic group in which an N02 group is directly linked to a carbon atom of the aromatic ring. Examples of said compounds are: nitrobenzene, alkyl or alkoxy nitrobenzene, optionally substituted, dinitrobenzenes and polynitrobenzenes.

Particularly useful nitroaromatic compounds in the process of the present invention are nitrobenzene, m-dinitrobenzene, 2,3-4-trinitrotoluene, 2,4-dinitrotoluene, 2,6-dinitrotoluene and 4,4'-dinitrodiphenylmethane.

Compounds containing at least one hydroxyl group can be represented by the general formula R (OH) m where m is an integer between 1 and 4 and R represents an achyllum, aryl, alkylaryl, arylachyl, cycloaliphatic group with a number of carbon atoms up to 20.

Compounds according to the general formula R (OH) m are primary, secondary or tertiary mono or polyhydroxylated alcohols, or mixtures of these, possibly substituted with oxygen or one or more halogen atoms.

Examples of monohydroxylated alcohols are ethanol, methanol, cicohexanol, CF3-CH2-OH, 2,6-dimethyl-4-heptanol, tert-amyl alcohol, tert-butyl, n-amyl, n-butanol, iso-butanol, n- propanol, iso-propanol, benzyl alcohol, chlorobenzyl, methoxy benzyl, methoxy ethanol, butoxy ethanol, phenol and cresols.

Examples of polyhydroxylated alcohols are selected from ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerol and trimethylolpropane.

Preferred for the purposes of the present invention are methanol and ethanol.

Alcohol can be used in stoichiometric quantities with respect to the nitrogroups present in the nitroaromatic compound, but it is preferable to use alcohol in slight excess as it also acts as a solvent. An inert organic solvent can be used in combination with alcohol.

According to an embodiment of the process of the present invention, the reaction can be carried out in the presence of an anhydrifying agent. Examples of such agents are selected from 2,2'-dimethoxy propane, trialkyl orthoformates, zeolites. In this case the drying agents are used in quantities ranging from 1 to 10% by weight with respect to the alcohol.

According to a further embodiment of the process of the present invention, the carbonylation reaction can be carried out in the presence of 2-methyl-5-nitro-aniline. This advantageously allows to improve the conversion and selectivity of the reaction.

The process of the present invention can be carried out at temperatures between 25 and 20 ° C and at a pressure between 1 and 100 bar. Conveniently, it operates at a temperature between 100 and 150 C and at a pressure not exceeding 60 bar.

The reaction time is a function of temperature and pressure. However, reaction times ranging from 1 to 5 hours are adequate.

The process according to the present invention can be carried out in batch, continuous or semi-continuous. At the end of the reaction, the product is recovered by usual separation techniques. For example, the carbamate can be isolated by crystallization after removal of the catalyst and excess alcohol from the reaction mixture.

The catalyst present in solution, after separation of the carbamate, can be removed from the reaction medium by absorption using silica, alumina or carcoal.

In order to better understand the present invention, some illustrative but non-limiting examples are given.

Example 1

Preparation of the Pd (phen) complex (COOMe) -.

To a suspension consisting of 1 g of Pd (Phen) Cl. and 0.1 g of phenanthroline (0.55 mmoles) in 25 ml of methanol, kept under stirring, 11.5 ml of a 0.55 M MeONa solution in methanol are added dropwise. The resulting mixture is placed under CO; after 20 hours at room temperature, the solvent is distilled and the residue is extracted with methylene chloride. After evaporation of the solvent, the recovered solid is washed with ethyl ether and dried under vacuum. 0.94 g of complex are obtained (83% yield).

Examples 2-4

Effect of temperature

In a 500 ml autoclave in Hastelloy <R> C, equipped with a magnetic stirrer and heat exchange means, the following are loaded:

Molar ratio: Substrate / Pd = 500.

After eliminating the air present in the autoclave with two CO washes, 60 bar of CO are loaded and the temperature is brought to the desired temperature, stirring for 2 hours. At the end of this period, the autoclave is cooled to room temperature and the unreacted gas is vented. The reaction mixture is analyzed by chromatographic analysis. The results are reported in table 1.

Table 1

Examples 5-8

The aim of the experiment was to study the effect of the variation of the molar ratio between the palladium complex, phen and phenH in the carbonylation of DNT to TDU.

In a 500 ml autoclave in Hastelloy ^, equipped with a magnetic stirrer and heat exchange means, the following are loaded:

Molar ratio: Substrate / Pd = 500.

After eliminating the air present in the autoclave with two CO washes, 60 bar of CO are loaded and brought to 135 "C, stirring. After 2 hours, the autoclave is cooled to room temperature and the unreacted gas The reaction mixture is analyzed by chromatographic analysis The results obtained by varying the molar ratio between Pd / phe and phenH <*> are shown in table 2.

Table 2

Example 10

In a 500 ml autoclave in Hastelloy <R> C, equipped with a magnetic stirrer and heat exchange means, the following are loaded:

1 ml of 2,2'-di-methoxy-propane (DMP)

The reaction is carried out under the same operating conditions as in Examples 5-8. After 2 hours, the DNT conversion is 100%, with a TDU yield of 71%.

Example 11

In a 500 ml autoclave in Hastelloy <a> C, equipped with a magnetic stirrer and heat exchange means, the following are loaded:

The reaction is carried out under the same operating conditions of example 10, but for 1 hour at 145 <°> C. After this period, the DNT conversion is 100%, with a yield in TDU of 63%.

Example 12

In a 500 ml autoclave in Hastelloy <R> C, equipped with a magnetic stirrer and heat exchange means, the following are loaded:

Substrate molar ratio: Pd = 1000; Pd / phen / PhenH '= 1: 8: 8

The reaction is carried out under the same operating conditions as in Examples 5-9. After 2 hours, the DNT conversion is 100%, with a TDU yield of 40%.

Example 13

In a 500 ml autoclave in Hastelloy <R> C, equipped with a magnetic stirrer and heat exchange means, the following are loaded:

Substrate molar ratio: Pd = 1000; Pd / phen / PhenH '= 1: 8: 8

The reaction is carried out under the same operating conditions as in Examples 5-9. After 2 hours, the conversion of nitrobenzene is 99%, with a selectivity in methyl-N-phenylcarbamate of 89%.

Example 14

In a 500 ml autoclave in Hastelloy <R> C, equipped with a magnetic stirrer and heat exchange means, the following are loaded:

The reaction is carried out under the same operating conditions as in example 13. After 2 hours, the conversion of nitrobenzene is 100%, with a selectivity in methyl-N-phenylcarbamate of 92%.

Example 15

In a 500 ml autoclave in Hastelloy <R> C, equipped with a magnetic stirrer and heat exchange means, the following are loaded:

The reaction is carried out under the same operating conditions of example 13. After 2 hours, the nitrobenzene conversion is 50%, with a selectivity in methyl-N-phenylcarbamate of 79%.

Claims (27)

Claims 1. Process for the synthesis of aromatic carbamates by reductive carbonylation of nitroaromatic compounds with carbon monoxide and a compound containing at least one hydroxyl group which comprises carrying out the carbonylation reaction in the presence of: (a) a preformed catalyst of general formula (I) where: chel represents a nitrogenous or phosphorated bidentate chelator, and R represents a Cj-C ,, alkyl group (b) a free chelator and (c) a cocatalyst chosen from compounds definable by the general formula (II) where: A an anion of an acid with pKa less than 2 and chel 'represents a nitrogenous bidentate chelator equal to or different from chel. 2. The process according to claim 1, which is carried out in the presence of 2-methyl-5-nitro-aniline. 3. The process according to claim 1, wherein the nitrogenous bidentate chelators are selected from those of general formula (III) where X and Y, the same or different from each other, represent bridged organic groups each having at least three atoms in the bridge, of which at least two are carbon atoms. 4. The process according to claim 3, where when, in addition to the carbon atoms, the groups X and Y contain other atoms these are selected from oxygen or nitrogen. The process according to claim 4, wherein the nitrogenous bidentate chelators have the same bridging groups X and Y and contain from 3 to 10 atoms at least two of which are carbon atoms. 6. The process according to claim 3, wherein the nitrogenous bidentate chelators are selected from 1,10-phenanthroline, 4-methyl-1, 10-phenanthroline, 5-methyl-1, 10-phenanthroline, 4,7-dimethyl-1 , 10-phenanthroline, 3,8-dimethyl-1, 10-phenanthroline, 4,7-diphenyl-1, 10-phenanthroline, 4,7-dichloro-1,10-phenanthroline, 3,4,7,8-tetramethyl --1,10-phenanthroline and 4,4'-dimethyl-5,5'-dioxazole. 7. The process according to claim 1, wherein the phosphorated bidentate chelators are selected from those of general formula (IV): where: R represents an alkyl radical with a number of carbon atoms from 2 to 6, a cycloalkylidene radical with a number of carbon atoms from 2 to 10 or an orthophenylene radical; R1-R4, the same or different from each other, each represent a C1-C10 cycloalkyl radical C3-C10 or a C6-C, 2 aromatic radical optionally substituted with a C1-C4 alkyl or alkoxy radical. 8. The process according to claim 7, wherein the phosphorated bidentate chelators are selected from 1,3-bis (diphenylphosphino) propane; 1.3-bis (di-4-methoxy-phenylosphino) propane; 1.4-bis (dicyclohexylphosphino) butane and 1,2-bis (diphenylphosphino) cyclohexane. 9. The process according to claim 1, where A <"> is selected from hexafluorophosphate, tetrafluoroborate and hexafluoroantimoniate. 10. The process according to claim 9, where A is hexafluorophosphate. 11. The process according to claim 1, where the cocatalyst is [phenH <*>] [PF6-]. 12. The process according to claim 1, characterized in that it additionally contains an anhydrifying agent selected from zeolites, 2, 2'-di-methoxy-propane and tri-alkyl-orthoformate. 13. The process according to claim 12 characterized in that the anhydrifying compound is 2,2'-di-methoxy-propane. 13. The process according to claim 1, wherein the nitroaromatic compound is selected from among the compounds containing at least one aromatic group in which an N02 group is directly bonded to a carbon atom of the aromatic ring. 14. The process according to claim 13, wherein the nitroaromatic compound is selected from optionally substituted nitrobenzene, alkyl or alkoxy nitrobenzene, dinitrobenzenes, such as 2,4-dinitrotoluene and 2, 6-dinitrotoluene, 4,4'-dinitrodiphenylmethane and polynitrobenzenes. 15. The process according to claim 13, wherein the nitroaromatic compound is selected from nitrobenzene, m-dinitrobenzene, 2,3-4-nitrotoluene, 2,4-dinitrotoluene, 2,6-dinitrotoluene and 4,4'-dinitrodiphenylmethane. 16. The process according to claim 1, where the compounds containing at least one hydroxyl group are selected from those represented by the general formula R (OH) m where m is an integer comprised between 1 and 4 and R represents an akyl, aryl, alkylaryl group , arylachyl, cycloaliphatic with a number of carbon atoms up to 20 possibly substituted. 17. The process according to claim 16, where the compounds of general formula R (OH) m are primary, secondary or tertiary mono or polyhydroxylated alcohols, or mixtures of these, optionally substituents with oxygen or one or more halogen atoms. 18. The process according to claim 17, where the monohydroxylated alcohols are selected from ethanol, methanol, cicohexanol, CF3-CH2-OH, 2.6-dimethyl-4-heptanol, tert-amyl alcohol, tert-butyl, n-amyl alcohol, n -butanol, iso-butanol, n-propanol, iso-propanol, benzyl alcohol, chlorobenzyl, methoxybenzyl, methoxy ethanol, butoxy ethanol, phenol and cresols. 19. The process according to claim 17, wherein the polyhydroxylated alcohols are selected from ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerol and trimethylolpropane. 20. The process according to claim 1, characterized in that the quantity of the catalyst (I) is such as to give a concentration of gram atoms of palladium per mole of nitroaromatic compound comprised of 10 <-2> and 10 <-5>. 21. The process according to claim 20, characterized in that the quantity of the catalyst (I) is such as to give a concentration of gram atoms of palladium per mole of nitroaromatic compound comprised between 2x10 <-3> and 2.5x10 22. The process according to claim 1, characterized in that the quantity of cocatalyst (II) is comprised between 1 and 50 moles per gram of palladium. 23 The process according to claim 22, characterized in that the quantity of cocatalyst is comprised between 1 and 18 moles per gram of palladium. 24. The process according to claim 1, where the amount of free chelant (b) is comprised between 1 and 50 moles per gram of palladium. 25. The process according to claim 24, characterized in that the quantity of free chelant is comprised between 1 and 16 moles per gram of palladium. 26. The process according to claim 1, characterized in that the reductive carbonylation reaction is carried out at a temperature between 25 ° and 200 ° C and at a pressure between 1 and 100 bar. 27. The process according to claim 26, characterized in that the temperature is between 100 and 180 ° C and the pressure does not exceed 60 bar.