The present invention relates to a method for the production of carbamate compounds in which an organic amine is reacted with an organic halogen compound in the presence of a quaternary organic ammonium bicarbonate, thereby obtaining a reaction mixture comprising a carbamate compound and a quaternary organic ammonium salt.

Several chemical routes for the production of O-alkyl carbamates as precursors for isocyanates are known, e.g. by reaction of primary amines with urea as the carbonyl source and an alcohol and evolution of ammonia. Efforts have also been made to use CO2 directly as the carbonyl source which is favorable since CO2 is available as sustainable raw material and no ammonia is produced as a side-product.

Tetrahedron 1992, 48, 1515-1530 discloses that O-alkyl carbamates can be synthesized directly with CO ₂ with primary amines and an alkyl halide in presence of crown ether. However, the use of crown ether is impractical to use on a large scale for price reasons, and the reported maximum yield reached only 57%. Higher yields could be obtained under harsher conditions such as high pressure (J. Org. Chem, 1995, 60, 2820-2830) or electrochemical activation (Appl. Organometal. Chem. 2007, 21, 941- 944). However, this is also impractical to use on a large scale, since special equipment is required. A process under mild conditions is preferred.

Another example for an electrochemical process is the Italian patent application RM2014A000694, filed on November 28, 2014 which discloses an electrochemical process for preparing bis-O-alkyl- carbamates from primary diamines with CO2 as carbonyl source, characterized in that at least one alkyl halide with at least 3 C-atoms in the alkyl group is used as alkylating agent in the presence of at least one iodide source and that the process is carried out at 10 to 215 °C.

CO2 can be used indirectly under mild conditions when carbonate or bicarbonate salts are used as the carbonyl source. Carbonates or bicarbonates are easily accessible by reaction of CO2 in alkaline aqueous solution.

It has been reported that O-alkyl carbamates can be obtained from aliphatic and aromatic amines with alkyl halide and cesium carbonate in presence of tetra-butyl ammonium iodide (WO 2002/034698 A2). However, the use of cesium and tetra-butyl ammonium iodide is unfavorable on a large scale for cost reasons, and the synthesis is preferably performed in dimethylformamide, which is unfavorable for toxicological reasons. Another synthesis route uses tetraethylammonium bicarbonate as carbonyl source. Using amines and alkyl halides in presence of tetraethyl ammonium bicarbonate under mild conditions results in high yields up to 98% for linear (J. Org. Chem. 1998, 63, 1337-1338) and cyclic carbamates (Synthesis 2010, 6, 943-946). For linear carbamates alkylating agents based on bromide, iodide, and tosylate have been employed in a large excess, typically 3 to 5 fold over amine, which makes this processes particularly unattractive in view of the high price of these alkylating agents.

A general reaction sequence starting from an organic amine, a bicarbonate and an organic chloride is:

R-NH ₂ + Y ⁺ HC0 ₃ + Cl-R'→ R-NH-C(=0)-0-R' + Y ⁺ CT + H ₂ 0 with R and R' being organic radicals and Y ⁺ being an organic ammonium cation.

Anionic exchange resins are known in the art. They can also be modified to serve as a source for hydrogen carbonate (bicarbonate) ions. US 2,999,821 relates to a method for removing chloride and silicate anions from a styrene-divinylbenzene quaternary ammonium anion-exchange resin which has at least one of said anions on the resin's ion-exchange sites, said method comprising treating the anion-exchange resin containing any chloride and silicate anions with an alkali metal bicarbonate, whereby said anions on the resin are replaced by bicarbonate ions and the resin is then capable of being used so as ultimately to take on more of said anions.

WO 93/11071 Al discloses a method of producing an alkali metal carbonate comprising passing a solution or suspension of the chloride and the bicarbonate of the alkali metal through a solid ion exchange resin which in the aqueous environment is chloride retaining and has a basicity greater than that of the bicarbonate ion, and recovering an aqueous solution or suspension of the alkali metal carbonate from the resin.

With respect to the synthesis of organic liquids, the publication Bull. Mater. Sci. 2013, 36, 1121- 1125 describes that salts were obtained by reacting tetraethylammonium cation [N ⁺ 2222] with inorganic anions like BF ^, NO 3, NO ^_ 2, SCN ^" , BrO 3, IO 3, PF ^" e and HCO 3 using ion exchange methods. These ionic liquids (ILs) were characterized using thermal methods, infrared spectroscopy and densitometry.

The present invention has the object of providing a process for producing O-alkyl carbamates that can also be conducted on an industrial scale with a recycling method for the quaternary ammonium salt.

According to the invention this object is achieved by a method for the production of carbamate compounds, the method comprising the steps of:

A) reacting an organic primary amine with an organic halogen compound in the presence of a quaternary organic ammonium carbonate and/or bicarbonate, thereby obtaining a reaction mixture comprising a carbamate compound and a quaternary organic ammonium salt; B) separating the quaternary organic ammonium salt from the reaction mixture obtained after step A); wherein the method further comprises the steps of:

C) contacting the quaternary organic ammonium salt obtained after step B) with a carbonate and/or bicarbonate anion-exchange resin, thereby obtaining a quaternary organic ammonium carbonate and/ or bicarbonate;

D) repeating step A) at least once, wherein the quaternary organic ammonium carbonate and/or bicarbonate employed in this next step A) is at least partially sourced from the quaternary organic ammonium carbonate and/or bicarbonate obtained from the preceding step C).

In step A) of the method according to the reaction an organic amine is brought to react with an organic halogen compound and a carbonate and/or bicarbonate as the carbonyl source. As a result, a carbamate is formed and the halogen of the organic halogen compound used as a starting material forms a halogenide salt with the organic ammonium compound. The quaternary organic ammonium salt in the reaction mixture may, of course, also comprise unreacted carbonate/bicarbonate salts if these have been employed in a stoichiometric excess during the reaction. The carbamate synthesis may be carried out in a solvent. Suitable solvents are for example DMF, DMSO, 1 ,2-dimethoxy ethane or N-methyl-2-pyrrolidone. In the cases wherein the syntheses are carried out under heat treatment, it is preferred to work under reflux. Moreover, the syntheses can be carried out under normal pressure (1 atm), reduced pressure or increased pressure.

Organic amines suitable as starting materials include open-chain aliphatic primary amines, carbocyclic aliphatic primary amines and aromatic primary amines. The alkyl halides which are suitable in the inventive process preferably comprise alkyl groups with at least 3 carbon atoms, preferably with at least 4 carbon atoms.

Step B) is a separation step where ammonium salt is separated from the reaction mixture. The obtained carbamate compound may be then purified and further processed, if desired. Examples for suitable separation techniques include distilling the reaction mixture, liquid/liquid extraction with an aqueous phase and an organic phase or solid/liquid extraction with a solvent in which the salts are insoluble.

The present invention has recognized that in a work-up step the quaternary organic ammonium salt may be converted back to a carbonyl source by treating it with an ion-exchange resin which is a source of carbonate and/or bicarbonate ions. Then, with freshly provided carbonate/bicarbonate counterions, the compound may be employed again as a starting material for carbamate synthesis. Surprisingly there is little loss of carbamate yield even if the ammonium compound is re -used and subsequently regenerated several times.

In contrast to these findings the inventors have found that a regeneration of the ammonium salt using hydroxides and carbon dioxide according to the following reaction sequence (exemplified by tetraethylammonium compounds and potassium hydroxide) leads to a pronounced decrease in chemical yield of the carbamates after several synthesis cycles:

Et ₄ NCl + KOH→ EttNOH + KC1

Et ₄ NOH + C0 ₂ → Et ₄ NHC0 ₃

Anion exchange resins comprise cationic groups bound to a matrix polymer and exchangeable anions. In principle, strongly basic and weakly basic resins may be used. Examples for strongly basic resins are those containing quaternary ammonium groups such as poly(acrylamido-N- propyltrimethylammonium or poly[(3-(methacryloylamino)-propyl] trimethylammonium type resins. Weakly basic resins typically feature primary, secondary, and/or ternary amino groups, e.g. polyethylene amine. The carbonate and/or bicarbonate anion-exchange resins may be prepared by eluting a column filled with commercially available chloride-type anion exchange resins with water until no chloride ions can be detected in the eluted liquid and then by eluting an aqueous carbonate and/or bicarbonate solution through the column.

The regeneration of the carbonate/bicarbonate salt in the column is preferably performed in aqueous solution. It is preferred that after this step the water solvent is removed to yield the organic ammonium carbonate/bicarbonate as a dry solid.

In step D) of the method according to the invention at least some of the regenerated carbonate/bicarbonate is re-introduced into a subsequent synthesis cycle. This may constitute the entire amount that is used in the synthesis step or carbonates/bicarbonates from other sources may be added to augment the amount of carbonyl sources in the reaction. The amount of regenerated carbonate and/or bicarbonate salt may be > 70 mol-%, preferably > 80 mol-%, more preferred > 90 mol-%) and most preferred > 95 mol-%> of the total amount of quaternary organic ammonium carbonate and/or bicarbonate employed in the subsequent step A).

As step D) calls for at least one repetition of step A), the minimal sequence of steps in the method according to the invention is A), B), C), D), A). Further repetitions are also contemplated, such as A), B), C), D), A), B), C), D), A) or, in more general terms, [A), B), C), D)] _n A) with n being an integer from 1 to 20, preferably 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

Regarding the chemical yield for the carbamate synthesis in step A), preferred embodiments of the method according to the invention provide that the chemical yield when step A) is performed for the first time does not decrease by more than 10%, preferably does not decrease by more than 5%, over the next two, three or four times step A) is performed. Further aspects of the present invention will be described in greater detail below. They can be combined freely unless the context clearly indicates otherwise.

In one embodiment of the method according to the invention the organic amine is an organic polyamine compound. Preferred are primary diamines, triamines and tetraamines.

In another embodiment of the method according to the invention the organic amine is selected from the group of 1,5-diaminopentane, 1 ,6-diaminohexane, 4,4'-methylenebis(cyclohexylamine), 5- amino-l,3,3-trimethylcyclohexane-methylamine, 1 ,4-diaminobenzene and/or 2,4-diaminotoluene.

In another embodiment of the method according to the invention the organic halogen compound is an organochlorine compound.

In another embodiment of the method according to the invention the organic halogen compound is selected from the group of n-, iso- and/or t-butyl chloride.

In another embodiment of the method according to the invention the ammonium cation of the quaternary organic ammonium carbonate and/or bicarbonate is selected from the group of tetraalkylammonium, a cation according to general formula (I) and/or a cation according to general formula (II):

(I) (Π) wherein:

X and Y independently represent a C2-C20 segment which may be substituted or unsubstituted, linear or branched, undisrupted or disrupted in the carbon chain by heteroatoms selected from the group of oxygen, sulfur or nitrogen and/or carbocyclic rings; R ¹ and R ² are independently selected from the group of aliphatic, cycloaliphatic, aromatic or araliphatic C1-C20 substituents, which can be saturated or unsaturated, linear or branched, optionally substituted and/or can contain heteroatoms in the carbon chain selected from the group of oxygen, sulfur or nitrogen.

The use of these compounds for the modification of isocyanates is described in the patent applications WO 2015/124504 Al and EP 15181246.8.

Preferred tetraalkylammonium cations are tetramethylammonium, tetraethylammonium, tetrapropylammonium and/or tetrabutylammonium.

With respect to general formulas (I) and (II), compounds of this structural type may be synthesized by reacting secondary amines with the appropriate mono- or dihalogen substituted compounds. If the secondary amine is a heterocyclic compound such as oxazolidine, isooxazolidine, oxazinane, morpholine or oxazepane the heteroatom containing compounds may be obtained.

By appropriately choosing the alkylating agent structural variations may also be introduced into the ring segments X or Y. An example is the reaction of bis(2-halogenethyl)ethers with the above- mentioned secondary cyclic amines. Examples of such syntheses are given in US 2007/0049750 Al, in particular in paragraphs [0015] to [0039] of this publication.

Specific compounds satisfying formulas (I) and (II) are:

In another embodiment of the method according to the invention the separation step B) comprises extracting the reaction mixture with an unpolar solvent and filtering off the insolubles. Alkane solvents, ethers, chloroform and dichloromethane are particularly suitable. In another embodiment of the method according to the invention the carbonate and/or bicarbonate anion-exchange resin comprises quaternary ammonium groups bound to the resin.

In another embodiment of the method according to the invention after step C) a further step C) is performed:

C) contacting the anion-exchange resin used in step C) with a carbonate and/or bicarbonate solution.

As step D) calls for at least one repetition of step A), the minimal sequence of steps in the method according to this embodiment is A), B), C), C), D), A). Further repetitions are also contemplated, such as A), B), C), C), D), A), B), C), C), D), A) or, in more general terms, [A), B), C), C), D)]„ A) with n being an integer from 1 to 20, preferably 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10. In another embodiment of the method according to the invention the method is conducted as a continuous process.

Examples

The present invention will be further described with reference to the following examples without wishing to be limited by them.

Synthesis of tetraethylammonium bicarbonate | ^" Et4N] [HCC l ^:

Tetraethylammonium chloride (kept overnight under high vacuum at room temperature) was dissolved in methanol (ca. 2 mL/mmol) and KOH (1 equiv.) was added. A white precipitate was formed. The reaction was allowed to stir for 2h at room temperature and then the precipitate was filtered off. CO2 was bubbled into the solution for 1 h and the solvent was evaporated under reduced pressure. The resulting white solid was kept overnight under high vacuum.

Synthesis of 5-azoniaspiro| ^" 4.4 ^" |nonane chloride:

0.02 mol of KOH in 15 mL water were kept under stirring at 100 °C for 10 min, then 0.02 mol of pyrrolidine were slowly added to the mixture at the same temperature and, after 5 min, 0.02 mol of 1,4-dichlorobutane were slowly added. The solution was kept at 100 °C for 18 h, then 1 night at room temperature. The solvent was eliminated under reduced pressure, obtaining a solid, which was dissolved in CHCI3 and filtrated. The chloroform phase was concentrated under vacuum, obtaining 2.84 g of pure 5-azoniaspiro[4.4]nonane chloride (88%). Example 1 (comparative example):

In a 100 mL round bottom flask, tetraethylammonium bicarbonate (3.0 mmol) was dissolved in acetonitrile. A solution of hexamethylene diamine (1.0 mmol) in acetonitrile (3-4 mL) was added to the flask and the reaction was allowed to stir for 1 h at room temperature.

Butyl chloride (5.0 mmol) was added and the mixture was stirred at 80 °C for 3 h. The solvent was evaporated in vacuo and the resulting residue was extracted 4 times with diethyl ether. The collected organic fractions were concentrated to dryness to afford the pure dibutyl carbamate ester.

Recycling of tetraethylammonium bicarbonate by precipitation:

The solid residue after the ethereal extractions in the synthesis of the carbamate esters as described above was dissolved in 30 mL MeOH and filtered off, then the filtrate was dried under reduced pressure and kept overnight under high vacuum. Treatment as described under the section Synthesis of tetraethylammonium bicarbonate above with dissolution of the residue in methanol, reacting with 1 equiv. KOH and bubbling CO2 gas through the reaction mixture allowed the regeneration of tetra-ethyl ammonium bicarbonate, which was used in a new synthesis cycle.

The resulting yields of the isolated dibutyl carbamate ester after several synthesis cycles were: First synthesis cycle: 91%

Second synthesis cycle: 75%

Third synthesis cycle: 76%

Fourth synthesis cycle: 64%

Fifth synthesis cycle: 66%

The amount of diamine used in the subsequent cycles was calculated based on the amount of recycled bicarbonate salt, i.e.: first synthesis: 1.00 mmol; second synthesis: 0.95 mmol; third synthesis: 0.83 mmol; fourth synthesis: 0.91 mmol; fifth synthesis: 0.83 mmol.

Example 2 (according to the invention): In a 50 mL round bottom flask, the quaternary ammonium salt - tetraethylammonium bicarbonate or 5-azoniaspiro[4.4]nonane bicarbonate - (1.5 mmol) was dissolved in 12 mL acetonitrile. A solution of the diamine (0.5 mmol) in acetonitrile (1 mL) was added to the flask and the reaction was allowed to stir for 1 h at room temperature.

Butyl chloride (2.0 mmol) was added and the mixture was stirred at 80 °C for 3 h. The solvent was evaporated in vacuo and the resulting residue was extracted 4 times with diethyl ether (3 x 40 mL respectively). In the case of 5-azoniaspiro[4.4]nonane salts, the fourth extraction lasted 2 h. The collected organic fractions were concentrated to dryness to afford the pure dibutyl carbamate ester. Diethyl ether collected from the ethereal phase was reused in the following extraction.

The solid residue after the ethereal extractions was dissolved in 2 mL water and treated by ion- exchange resins as described below.

Recycling of ammonium bicarbonates by ion-exchange synthesis:

The ionic exchange chromatography column to generate the ammonium bicarbonates was a normal glass column for flash chromatography (internal diameter: 1 cm) filled with the resin Amberlite® IRA-400 chloride form (strongly basic gel-type resin, quaternary ammonium functionality) from Sigma- Aldrich (height: 18 cm) for 1.5 mmol of salt. Initially the column was packed using water as solvent, then it was:

- washed with H2O until chloride absence (checked with AgNC );

- slowly washed with NaHCOs 1 M (20 mL);

- washed with H2O until NaHCCb absence (checked with HC1). Before every recycling experiment, the same packed column was washed with 1 M NaHCC and then water. A solution of the solid residue after the ethereal extractions of the carbamate synthesis step comprising tetraethylammonium chloride or 5-azoniaspiro[4.4]nonane chloride (1.5 mmol, calculated as chloride) in 2 mL water was slowly passed through a column containing the bicarbonate exchange resin; then the resin was washed with water till the collection of 50 mL of solution. The solvent was evaporated under reduced pressure. The resulting white solid was kept overnight under high vacuum.

It should be noted that no fresh bicarbonate was added in the second or subsequent synthesis cycles.

The results of the syntheses are summarized in the table below.

Diamine Bicarbonate Biscarbamate yield after n

synthesis cycles (using recycled bicarbonate in step «≥2)

1 ,6-Hexamethylenediamine Tetraethylammonium n =\ : 92%

bicarbonate n =2: 94%

n =3: 91%

n =4: 98%

n =5: 94%

1 ,6-Hexamethylenediamine 5 - Azoniaspiro [4.4] nonane n =\ : 86%

bicarbonate n =2: 90%

n =3 88%

n =4: 97%

n =5: 93%

Methylenedicyclohexyl-4,4 ' - Tetraethylammonium n =\ : 94%

diamine bicarbonate n =2: 96%

Methylenedicyclohexyl-4,4 ' - 5 - Azoniaspiro [4.4] nonane n =\ : 88%

diamine bicarbonate

Isophoronediamine Tetraethylammonium n =\ : 87%

bicarbonate n =2: 98%

Isophoronediamine 5 - Azoniaspiro [4.4] nonane n =\ : 83%

bicarbonate

w-Xylylenediamine Tetraethylammonium n =\ : 87%

bicarbonate n =2: 98%

w-Xylylenediamine 5 - Azoniaspiro [4.4] nonane n =\ : 54%

bicarbonate

Reactions using /j-phenylenediamine/tetraethylammonium bicarbonate, 2,4-toluylenediamine/ tetraethylammonium bicarbonate and 2,4-toluylenediamine/5-azoniaspiro[4.4]nonane bicarbonate as starting materials did not yield a carbamate product in any appreciable amount after the first cycle and were not followed through with subsequent synthesis steps.