CA2103519A1

CA2103519A1 - Process for the preparation of aminophenyl acetates

Info

Publication number: CA2103519A1
Application number: CA002103519A
Authority: CA
Inventors: Matthias Beller; Heinz Strutz
Original assignee: Individual
Current assignee: Hoechst AG
Priority date: 1992-08-08
Filing date: 1993-08-06
Publication date: 1994-02-09
Also published as: EP0582929A1; KR940003918A; JPH06157438A

Abstract

Abstract of the disclosure:

Process for preparing an aminophenyl acetate or amino-phenyl diacetate The present invention relates to a process for preparing an aminophenyl acetate or aminophenyl diacetate by reacting a nitrophenyl acetate or diacetate in the presence of a hydrogenation catalyst with hydrogen at 0 to 60°C and an H2 pressure of at least 4 MPa in the presence of a solvent or a solvent mixture.

Description

2la3~j~s HOECHST ARTIENGESELLSCHAFT HOE 92/F 243 Dr. Bi/we Process for the preparation of aminophenyl acetates The present invention relates to an improved process, compared with the prior ~rt, for preparing ~minophenyl acetates by reaction of the corresponding nitrophenyl acetates with hydrogen in the presence of a hydrogen~tion catalyst.

Owing to their functional groups, aminophenyl acetates in general and 4-aminophenyl acetate in particular are suit-able starting materials for a multiplicity of differentchemical compounds. They serve, for example, B~ versatile precursors for the Heck reaction of diazonium salts prepared in situ as can be seen from R. gikukawa et al., J. Org. Chem. 1981, 46, 4885. Furthermore, aminophenyl acetates can be used as precursors for the preparation of a large number of fine chemicals, crop protection agents and pharmaceuticals.

The processes for preparing aminophenyl acetate deriva-tives previously described in the literature are at least to some extent fairly complicated and can therefore be carried out in industry only with very great difficulty and/or yield the desired valuable products only in low yields. Thus, according to Hazlet and Dornfeld, J. Am.
Chem. Soc. 1944, 66, 1781, 4-aminophenyl acetate can be p-repared by reduction of 4-nitrophenyl acetate with iron and a mineral acid. However, the yield iB only 9 - 13%~

A different route is described by L. Gslatis, Chem. Ber.
1926, 59, 848. p-Aminophenol i~ reacted with b~nzaldehyde to give the corre~ponding ~enzylidene compound, which is then subjected to O-acetylation and hydrolyzed in cold mineral acid. The yield exclusive of the last step is no more than about 35~, but no yield i~ reported for any of the preceding steps.

2t Q3~9 A more recent synthesis ~tarts with 4-aminophenol which i6 selectively reacted with acetic anhydride in the presence of cobalt(II) chloride to give the corresponding O-acetyl compound (S. Ahmad, J. Iqual, J. Chem. Soc.
Chem. Comm. 1987, 114). The yisld is 83~. However, this method ha~ disadvantages. On the one hand it i~ neces~ary to work with vigorous exclusion of water and, on the other, the reaction must be ~arried out under an inert gas atmosphere.

A further synthesis starting with 4-nitrophenyl acetate is disclo6ed in Sstoh et al., Chem. Pharm. Bull. 1981, 29, 1443. In this synthesis, 4-nitrophenyl acetate is reacted with excess tin(II) chloride and sodium boro-hydride in the presence of an aliphatic alcohol as the solvent. Although this method is suitable for a laboratory method, it i6 not suitable for an industrial proce6s owing to the large amount of tin(II) chloride required and owing to the use of expensive ~odium boro-hydride.

As is apparent from R. Feldstein et al., J. Org. Chem.
1961, 26, 1656, the reaction of 4-nitrophenyl acetate with hydrogen in the pre~ence of platinum oxide as the catalyst does not produce p-aminophenyl acetate but results in the formation of the corre~ponding p-aceta-midophenol, presumably by migration of the acetylradical, in a yield of not less than 77%.

The reaction of p-nitrophenylacyl esters with hydrogen in the presence of PtO2 as the cataly~t in ethanol described by J.E. Wynn et al., J. Pharm. Sci. 1932, 7, 772 must ~e seen as being related thereto. If p-nitrophenyl acstate and PtO2 as the catalyst and ab~oluts ethanol a8 the solvent are used, thi~ reaction leads to a yield of only 43% of p-aminophenyl acetate. Hydrogenation of sub6titu-ted nitrophenyl acetates is not described.

The general difficulties of synthesizing aminophenyl 21~3`~19 acetates and derivatives thereof may be explained by the instability of the phenyl ester group which is possibly due to the presence of an amino group. Obviously, in solution, the aminophenyl acetates gradually are con-verted into the thermodynamically more stable acetamido-phenols by migration of the acetyl group. EP-0,435,263 A1 has shown that traces of acid~ or ba~es catalyze this process.

As documented by the above obser~ations, there i8 a need for a process which avoid~ the difficulties de~cribed above, can moreover al~o be used on an $ndustrial scale, can be easily put into practice and u~es auxiliaries which are readily available in industry. Furthermore, the process should provide the desired product in high yield and, at the same time, prevent the formation of by-products as far as possible.

This object i6 achieved by a process for preparing an aminophenyl acetate or aminophenyl diacetate. It com-prises reactin~ a nitrophenyl acetate or a nitrophenyl diacetate in the pre~ence of a hydrogenation catalyst with hydrogen at O - 60~C and an H2 pre~sure of at least 4 MPa in the presence of a solvent or a solvent mixture.

In view of the results obtained by R. Feldstein et al., J. Org. Chem. 1961, 26, 1656 and J.E. Wynn et al., J.
Pharm. Sci. 1982, 7, 772 by means of catalytic hydrogena-tion, it was surprising that the proces~ according to the invention produces aminophenyl acetates by catalytic hydrogenation of the corresponding nitrophenyl acetate~
in high yields.

Suitable nitrophenyl acetates are not only nitrophenyl acetates or nitrophenyl diacetates which are unsubstitu-ted on the aromatic ring but also nitrophenyl acetates or nitrophenyl diacetates which are mono- or polysubstituted on the aromatic ring.

2~03~9 The nitrophenyl acetate which is usually used is a nitrophenyl acetate which is un~ubstituted or mono- or polysubstituted on the aromatic ring by an alkyl group having 1 to 12, in psrticular 1 to 4, carbon atoms, by a halogen atom, by a hydroxyl group, by an alkyl ether group having 1 to 4 carbon ~toms in the alkyl radical, by an aryl ether group, by a nitrile group, by an RS-, R3Si-or R-CO- group, in which R i8 an alkyl rad~cal h~ving 1 to 4 carbon atoms or an aryl ether group, by a sulfo group, by a phosphono group, by a sulfonyl group, by a carboxyl group and/or carboxylate salt group.

Expediently, the nitrophenyl acetate used is a nitro-phenyl acetate which is unsubstituted or mono- or poly-substituted on the aromatic ring by an alkyl group having 1 to 12, in particular 1 to 4, csrbon atoms, by a halogen atom, by a hydroxyl group, by an alkyl ether group having 1 to 4 carbon atoms in the alkyl radical, by an ~ryl ether group, by a nitrile group, by an R-CO- group, in which R i~ an alkyl radical havin~ 1 to 4 carbon atoms, or by a carboxyl group, in particular by an alkyl group having 1 to 12, preferably 1 to 4, carbon atom~, by a halogen atom, by an alkyl ether group having 1 to 4 carbon atoms in the alkyl radical, by a nitrile group or an R-CO- group, in which R is an alkyl radical having 1 to 4 carbon atom~, preferably by an alkyl group having 1 to 12, in particular having 1 to 4, carbon atoms, or by an alkyl ether group having 1 to 4 carbon atoms in the alkyl radical.

A suitable material to be u~ed is o-nitrophenyl acetate, m-nitrophenyl acetate or p-nitrophenyl acetate, each of which is unsubstituted or mono- or polysubstituted on the aromatic ring, in particular o-nitrophenyl scetste or p-nitrophenyl acetate, preferably p-nitrophenyl acetate.

Readily suitable nitrophenyl acetates are o-nitrophenyl ~5 acetate, m-nitrophenyl acetate, p-nitrophenyl ~cetate or 2-methyl-4-nitrophenyl acetate, in particular ` 2~Q3~3 p-nitrophenyl acetate or 2-methyl-4-nitrophenyl acetate.

Customary hydrogenation catalysts can be used for carry-ing out the reaction. As a rule, the hydrogenation catalysts contain copper, manganese, cobalt, nickel, S rhodium, platinum and/or palladium, and, if desired, suitable activators and promoters.

The suitability of cobalt-, nickel-, platinum- and/or palladium-containing hydrogenation c~talysts which, if desired, contain fiuitable activators and promoters is quite good.
Particularly suitable are nickel- and/or palladium-containing catalysts which, if desired, contain suitable activators and promoters.

The process according to the invention can be carried out either by means of a support-free cataly~t or with the use of a supported catalyst.

If a support-free c~talyst i8 used, Raney cobalt or Raney nickel, in particular Raney nickel, are reco ~ended. The supported catalysts contain customarily used ~upport materials. Suitable support material~ are Al2O3, clay, pumice, SiO2, ~ilica gel, silica, kieselguhr and/or activated carbon, in particular Al203, clay, kieselguhr and/or activated carbon, preferably A1203~ kieselguhr and/or activated carbon.

The process according to the invention ~8 particularly easy to carry out using ~upported catalyst~. Supported cataly~t~ containing noble metAls and supported catalysts containin~ cobalt and nickel are particularly suitable.
Platinum on activated carbon and/or palladium on acti-vated carbon, in particular palladium on activated carbonhave proven to be suitable supported catalysts containinq noble metal. However, platinum on Al2O3 and/or palladium on Al2O3, in particular palladium on Al2O3, can also be used. Cobalt- andtor nickel-, in particular nickel-2103~19 containing supported catalysts containing, for example, clay, SiO2, silica gel, gilica and/or kieselguhr, in particular ~ilica gel, ~ilica and/or kieselguhr, prefer-ably kieselguhr, as the support material are also parti-cularly suitable.

The proces6 can be carried out either batchwise or continuously. If ~atchwise operation is employed, it is recommended to suspend the catalyst in comminuted form and to carry out the reaction by means of the ~uspended catalyst. If continuous operstion is employed, it iE
recommended to use the cataly~t, for example, in bulk form and arranged as 8 solid bed. The catalyst arranged as a solid bed is usually pre~ent in an upright tube.
Depending on the manner of addition of the material to be hydrogenated, two methods may be distin~ui~hed, the trickle-phase and the liquid-phase method. In the trickle-phase method, the material to be hydrogenated is added to the catalyst arranged as a solid bed from abo~e, while the hydrogen i8 passed either co-currently from top to bottom or counter-currently from bottom to top. In the liquid-phase method, the material to be hydrogenated i8 passed through the catalyst-containing solid bed from bottom to top. In this method, hydrogen is usually passed through co-currently.

The process according to the invention i8 carried out at a temperature of from 0 to 60C. The reaction temperature to be u~ed al80 depends to a certain extent on the reac-tivity of the nitrophenyl acetate or nitrophenyl di-acetate. Relatively reactive nitrophenyl ncetates or nitrophenyl diacetates permit a reaction at relatively low temperatures, whereas comparatively inert nitrophenyl acetates or nitrophenyl diacetates require corres-pondingly higher temperatures.
Apart from the reactivity of the material used the activity of the hydrogenation catalyst also affects the reaction temperature to be used to a certain extent.
Active hydrogenation catalysts allow the reaction to be 210~19 carried out at comparatively lower temperatur~s, while less active hydrogenation cataly~ts make it nece~sary to carry out the reaction at higher temperatures.

In many ca~e~, it has pro~en 6uitab1e to carry out the reaction at 10 to 50, in particular 15 to 45, pref~rably 20 to 40C. Successful implementation of the process according to the invention requires that a ~pecific H2 pressure be maintained. The H2 pre~sure ~hould be at least 4 MPa.

Selection of the H2 pressure depends to a certain extent on the reaction temperature selected, on the reactivity of the material used, and on the activity of the hydro-genation catalyst. Thus, a comparatively high reaction temperature and/or a comparativeiy reactive nitrophenyl acetate or nitrophenyl diacetate and/or a comparatively active hydrogenation catalyst allows the rQaCtion to be carried out at relatively low pressures. In contrast, if a relatively low temperature and/or a relatively inert nitrophenyl acetate or nitrophenyl diacetate and/or a hydrogenation catalyst having low activity are used, it i8 recommended to carry out the reaction at relatively ele~ated pres~ures.

In most cases, it is sufficient to c~rry out the reaction at an H2 pre6sure of 4 to 100, in particular 5 to 50, preferably 5 to 30, MPa.

The nitrophenyl acetate or nitrophenyl diacetate i8 dissolved in a 601vent or colvent mixture, and this solution is u~ed for the catalytic reaction with hydro-gen. The solvent used is a straight-chain and/or branched, monohydric andtor polyhydric aliphatic alcohol, in particular a branched monohydric alcohol, an aliphatic ketone, a non-cyclic and/or cyclic aliphatic ether, an alkyl ester of an aliphatic carboxylic acid, an alkyl-benzene andtor an aliphatic N,N-dialkylcarboxamide.

2103~19 It has proved suitable to use, as the solvent, an ali-phatic alcohol having 3 to 8 carbon atoms, in particular isopropanol and~or sec.-butanol and/or tert.-butanol, an aliphatic ketone having 3 to 8 carbon atoms, in parti-cular acetone and/or methyl ethyl ketone, a non-cyclic aliphatic ether having 2 to 4 carbon atom~ per alkyl group, in particular diethyl ether and/or di-n-butyl ether, a cyclic aliphatic ether having 4 to 5 carbon atoms in the ring, in particular tetrahydrofuran and/or dioxane, an aliphatic alkyl carboxylate having 1 to 4 carbon atoms, in particular methylformate or e~hylformate and/or methyl acetate or ethyl acetate, an alkylbenzene, such as toluene, xylene and/or isopropylbenzene, in particular toluene, and/or an aliphatic N,N-dialkyl-carboxamide having 1 to 4 carbon atom~, in particulardimethylformamide, dimethylacetamide and/or N-methyl-pyrrolidine. Methanol and/or ethanol are not suitable solvents if they are used as the only solvent, i.e.
without the addition of another solvent. Methanol and/or ethanol po6sibly favor the formation of acetamidophenols which takes place by migration of the acetyl radical.

The solvent mixture u~ed is a ~olvent m$xture comprising a protie polar ~olvent and an aprotic solvent. The solvent mixture usually u~ed is a protic polar solvent, in particular a straight-chain or branched aliphatic alcohol having 1 to 4 carbon atoms, and an aprotic solvent, in particular tetrahydrofuran and/or dioxPne.
The use of a ~olvent mixture comprising isopropnnol and tetrahydrofuran gives a particularly good result.

The solvent mixture usually used i8 composed of 5 to g5, in particular 10 to 90, preferably 20 to 80, parts by weight of a protic polar solvent and 95 ~o 5, ~n parti-cular 90 to 10, preferably 80 to 20, parts by weight of an aprotic polar solvent.
In many cases, a solvent mixture comprising 30 to 70 parts by weight of a protic polar solvent and 70 to 30 parts by weight of an aprotic polar solvent has proven - 21~3~9 g suitable.

The Examples which follow illustrate the invention without limiting it.

Experimental Section Example 1 In an autoclsve (volume 250 ml), 8.0 g of p-nitrophenyl acetate (4-nitrophenyl acetate) are dissolved in a mixture of 60 ml of i~opropanol and 30 ml of dioxane, and 1.6 g of Raney nickel of medium activity are added.
Hydrogen is then in~ected up to a pressure of 10 MPa, and the solution of p-nitrophenyl scetate i5 reacted at room temperature for 4 hours with stirring. The H2 pressure is kept constant by addition of H2. The hydrogenation catalyst i8 then removed by filtration, and the resulting organic solution is concentrated in vacuo at room temperature to give 6.9 g of a 95~ pure p-aminophenyl acetate t4-aminophenyl acetate) in the form of sand-colored crystals (which corre~ponds to a yield of 98%).
All of the p-nitrophenyl acetate ig converted. Recrystal-lization in toluene/petroleum ethex g$ves 6.07 g ofp-aminophenyl acetate in a purity of ~ 99~ (which corre~-ponds to a yield of 91%).
Conversion (p-nitrophenyl acetate)s 100%
p-Aminophenyl acetate selectivitys 98%
Yield after recrystallization: 91%

Comparative Example 1 Example 1 is repeate~, carrying out the reaction at an H2 pressure of 1 MPa.
Reaction time: 24 hours Conversion (p-nitrophenyl acetate): 58%
p-Aminophenyl acetate selectivity: 84%

Comparative Ex2mple la Example 1 is repeated, except that 90 ml of ethanol 21Q3~
, (instead of dioxane and isopropanol) are used as the solvent.
Conversion (p-nitrophenyl acetate): 100~
p-Aminophenyl acetate selectivity: 68% (25% of p-acet-S amidophenol) Example 2 Example 1 is repeated, except that 90 ml of isopropanol (instead of dioxane and isopropanol) are used a~ the ~olvent.
Conversion (p-nitrophenyl acetate): 100%
p-Aminophenyl acetate selectivity: 85% (13% of p-acet-amidophenol) Comparative Example 2 Example 2 is repeated, except that 90 ml of isopropanol (instead of dioxane and isopropanol) are used as the solvent and the reaction is carried out at an H2 pres~ure of 1 MPa.
Conversion (p-nitrophenyl acetate): 45%
p-Aminophenyl acetate selectivity: 67% (4 by-products) Example 3 Example 1 is repeated, except that 1.91 g of a supported palladium catalyst (5~ of Pd on activated carbon~
(instead of Raney nickel) are used.
Conversion (p-nitrophenyl acetate): 100%
p-Aminophenyl acetate ~electivity: 96%

Comparative Example 3 Example 3 i~ repeated, except that 100 ml of toluene (instead of dioxane and isopropanol) are used as the solvent and the reaction is carried out at an H2 pressure of 1 MPa.
Reaction time: 1 hour Conversion (p-nitrophenyl acetate): 0%

- 21~3~19 Example 4 Example 1 i8 repeated, except that ~0 ml of toluene (instead of dioxane snd i~opropanol) are u~ed as the solvent.
Conversion (p-nitrophenyl acetate): 95%
p-Aminophenyl acetate ~electivity: 844 (3 by-products) Example S

Example 1 i8 repeated, except that 60 ml of methanol and 30 ml of tetrahydrofuran (instead of dioxane and i~opro-panol) are used as ~he solvent.
Conversion (p-nitrophenyl acetate): 100~
p-Aminophenyl acetate selectivity: 96% (4% of p-acet-amidophenol) Example 6 Example 1 i5 repeated, except that 50 ml of dioxane and 40 ml of isopropanol (instead of 30 ml of dioxane and 60 ml of isopropanol) ~re u~ed.
Conversion (p-nitrophenyl acetate): 100%
p-Aminophenyl acetate selectivity: 96% (4% unknown by-product) Example 7 Example 1 is repeated, except that 1.0 g of a supported nickel catalyst containing 52 to 55% by wsight of Ni and 25 to 30~ by weight of kieselguhr as the support (commer-cial product from Hoech~t AG; designation Ni 55/5 TS) (instead of Raney nickel) is used. Hydrogen ab~orption:
1 hour; reaction time a total of 4 hours Conversîon (p-nitrophenyl acetate): 100%
p-Aminophenyl acetate selectivity: 86%

2103~19 Example 8 Example 1 is repeated, except that 2-methyl-4-nitrophenyl acetate (instead of p-nitrophenyl acetate) is used a~ the starting material.
Conversion (2-methyl-4-nitrophenyl acetate): 100%
2-Methyl-4-aminophenyl acetate ~electivity: 97~

Claims

1. A process for preparing an aminophenyl acetate or aminophenyl diacetate, which comprises reacting a nitrophenyl acetate or a nitrophenyl diacetate in the presence of a hydrogenation catalyst with hydrogen at 0 to 60°C and an H2 pressure of at least 4 MPa in the presence of a solvent or a solvent mixture.

2. The process as claimed in claim 1, wherein the nitrophenyl acetate used is a nitrophenyl acetate which is unsubstituted or mono- or polysubstituted on the aromatic ring by an alkyl group having 1 to 12, in particular 1 to 4, carbon atoms, a halogen atom, a hydroxyl group, an alkyl ether group having 1 to 4 carbon atoms in the alkyl radical, an aryl ether group, a nitrile group, an RS-, R3Si- or R-CO-group, in which R is an alkyl radical having 1 to 4 carbon atoms or an aryl ether group, by a sulfo group, a phosphono group, a sulfonyl group, a carboxyl group and/or a carboxylate salt group.

3. The process as claimed in claim 1 or 2, wherein the nitrophenyl acetate used is a nitrophenyl acetate which is unsubstituted or mono- or polysubstituted on the aromatic ring by an alkyl group having 1 to 12, in particular 1 to 4, carbon atoms, a halogen atom, a hydroxyl group, an alkyl ether group having 1 to 4 carbon atoms in the alkyl radical, an aryl ether group, a nitrile group, an R-CO- group, in which R is an alkyl radical having 1 to 4 carbon atoms, or a carboxyl group, in particular by an alkyl group having 1 to 12, preferably 1 to 4, carbon atoms, a halogen atom, an alkyl other group having 1 to 4 carbon atoms in the alkyl radical, a nitrile group or an R-CO- group, in which R is an alkyl radical having 1 to 4 carbon atoms, preferably by an alkyl group having 1 to 12, in particular 1 to

4, carbon atoms or an alkyl ether group having 1 to 4 carbon atoms in the alkyl radical.

4. The process as claimed in one or more of claims 1 to 3, wherein an o-nitrophenyl acetate, m-nitrophenyl acetate and/or p-nitrophenyl acetate, each of which is unsubstituted or mono- or polysubstituted on the aromatic ring, in particular o-nitrophenyl acetate and/or p-nitrophenyl acetate, preferably p-nitro-phenyl acetate is used.

5. The process as claimed in one or more of claims 1 to 4, wherein the nitrophenyl acetate used is o-nitro-phenyl acetate, m-nitrophenyl acetate, p-nitrophenyl acetate and/or 2-methyl-4-nitrophenyl acetate, in particular p-nitrophenyl acetate and/or 2-methyl-4-nitrophenyl acetate.

6. The process as claimed in one or more of claims 1 to 5, wherein the hydrogenation catalyst contains copper, manganese, cobalt, nickel, rhodium, platinum and/or palladium, in particular cobalt, nickel, platinum and/or palladium, preferably nickel and/or palladium and, if desired, suitable activators and promoters.

7. The process as claimed in one or more of claims 1 to 6, wherein the hydrogenation catalyst used is a support-free catalyst or a supported catalyst.

8. The process as claimed in one or more of claims 1 to 7, wherein the support-free catalyst used is Raney cobalt or Raney nickel, in particular Raney nickel.

9. The process as claimed in one or more of claims 1 to 7, wherein the supported catalyst contains Al2O3, clay, pumice, SiO2, silica gel, silica, kieselguhr and/or activated carbon, in particular Al2O3, clay, kieselguhr and/or activated carbon, preferably Al2O3, kieselguhr and/or activated carbon.

10. The process as claimed in one or more of claims 1 to 9, wherein the reaction is carried out at 10 to 50, in particular 15 to 45, preferably 20 to 40°C.

11. The process as claimed in one or more of claims 1 to 10, wherein the reaction is carried out at an H2 pressure of 4 to 100, in particular 5 to 50, prefer-ably 5 to 30, MPa.

12. The process as claimed in one or more of claims 1 to 11, wherein the solvent used is a straight-chain and/or branched, monohydric and/or polyhydric aliphatic alcohol, in particular a branched mono-hydric, aliphatic alcohol, an aliphatic ketone, a non-cyclic and/or cyclic aliphatic ether, an ali-phatic alkyl carboxylate, an alkylbenzene and/or an aliphatic N,N-dialkylcarboxamide.

13. The process as claimed in one or more of claims 1 to 12, wherein the solvent used is an aliphatic alcohol having 3 to 8 carbon atoms, in particular isopro-panol and/or sec.-butanol and/or tert.-butanol, an aliphatic ketone having 3 to 8 carbon atoms, in particular acetone and/or methyl ethyl ketone, a non-cyclic aliphatic ether having 2 to 4 carbon atoms per alkyl group, in particular diethyl ether and/or di-n-butyl ether, a cyclic aliphatic ether having 4 to 5 carbon atoms in the ring, in parti-cular tetrahydrofuran and/or dioxane, an aliphatic alkylcarboxylate having 1 to 4 carbon atoms, in particular methylformate or ethylformate and/or methyl acetate or ethyl acetate, an alkylbenzene, such as toluene, xylene and/or isopropylbenzene, in particular toluene, and/or an aliphatic N,N-dialkyl-carboxamide having 1 to 4 carbon atoms, in parti-cular dimethylformamide, dimethylacetamide and/or N-methylpyrrolidine.

14. The process as claimed in one or more of claims 1 to 13, wherein the solvent mixture used is composed of a protic polar solvent, in particular a straight-chain or branched aliphatic alcohol having 1 to 4 carbon atoms, and an aprotic polar solvent, in particular tetrahydrofuran and/or dioxane.

15. The process as claimed in one or more of claims 1 to 14, wherein the solvent mixture used is composed of 5 to 95, in particular 10 to 90, preferably 20 to 80, parts by weight of a protic polar solvent and 95 to 5, in particular 90 to 10, preferably 80 to 20, parts by weight of an aprotic polar solvent.