EP1210310A1

EP1210310A1 - Method for preparing acid chlorides and benzene aldehydes

Info

Publication number: EP1210310A1
Application number: EP00960819A
Authority: EP
Inventors: Christophe Ruppin; Françoise Chosson
Original assignee: Tessenderlo Chemie NV SA
Current assignee: Tessenderlo Chemie NV SA
Priority date: 1999-09-10
Filing date: 2000-09-07
Publication date: 2002-06-05
Also published as: AU7299700A; JP2003509391A; KR20020041428A; BR0013925A; FR2798380B1; FR2798380A1; WO2001019766A1

Abstract

The invention concerns a semi-continuous method for preparing acid chlorides and benzene aldehydes which consists in: producing in a reactor a starter comprising for the major part a dichloromethylated benzene compound; simultaneously and continuously introducing simultaneously in the reactor, a carboxylic acid or its anhydride, a dichloromethylated benzene compound (2) and at least a Friedel-Crafts catalyst; and in causing them to react, under stirring, under a pressure P1 not higher than atmospheric pressure and at a temperature ranging between 60 DEG C and 180 DEG C; continuously extracting chloride acid (3) from the reaction medium and optionally formed HCI, by distillation under pressure P1 and at reaction temperature, then in ceasing to introduce reagents when the working volume of the reactor containing for the major part benzene aldehyde is reached; and in performing total or partial fractionated distillation of the reaction medium, consisting for the major part of benzene aldehyde, under pressure P2 < P1 and at a temperature not higher than 180 DEG C.

Description

PROCESS FOR THE PREPARATION OF ACID CHLORIDES AND BENZENIC ALDEHYDES

The invention relates to a process for preparing chlorides of benzene acids and aldehydes. Acid chlorides are important synthesis intermediates in the chemical industry. They are widely used for the synthesis of various pharmaceutical products (antiviral, anti-inflammatory agents) or phytosanitary products (herbicides, insecticides). They are also used for the synthesis of peroxides which are in particular used as initiators for radical polymerizations.

The main routes of access to acid chlorides include processes which react conventional reagents such as phosgene, sulfonyl chloride, phosphorus tri- or pentachlorides and thionyl chloride.

However, all of these processes represent complex technologies, given the reagents involved and the need to carry out costly treatments of the products obtained and the effluents, which are unacceptable for industrial production.

Thus, for example in the process described by Butlerow (Justus Liebiegs Ann. Chem., P. 373, 1,874) which consists in reacting pivalic acid on phosphorus pentachloride according to the reaction:

(CH ₃ ) ₃ C COOH + PCI ₅ (CH ₃ ) ₃ C COCI + POCI ₃ + HCI

the POCI ₃ and the pivaloyl chloride obtained have temperatures so close (1 04-1 06 ° C) that it is almost impossible to separate them.

It has been proposed to use phosphorus trichloride which reacts with a carboxylic acid according to the reaction:

PCI ₃ + RCOOH ^ RCOCI + H ₃ PO ₃ + HCI

Although high yields are generally obtained, it is very difficult to remove the last traces of phosphorous acid, which are prohibitive for the use of acid chlorides in certain syntheses. One of the processes for the synthesis of acid chlorides most mentioned in the literature is that using thionyl chloride according to the reaction:

RCOOH + SOCI ₂ ^ RCOCI + SO ₂ + HCI

In general, the reaction is carried out in the presence of a molar excess of 20% to 50% of SOCI ₂ .

According to these conditions, molar yields of distilled acid chlorides are obtained, for example around 90%. The addition of catalysts such as DMF, pyridine or N-methylacetamide makes it possible to increase the reaction kinetics and improve the selectivity (lowering the percentage of secondary products such as anhydride). However, the use of SOCI ₂ has the disadvantage of leading to acid chlorides which may contain sulfur. In addition, in the event that a catalyst is used, its recycling is difficult.

The acid chlorides can also be obtained from phosgene according to the reaction:

RCOOH + COCI ₂ ^ RCOCI + HCI + CO ₂

However, these methods have the drawback of using phosgene which is a highly toxic reagent and difficult to handle. In addition, in order to have acceptable yields, greater than 90%, it is necessary to use nitrogen catalysts such as tertiary amines, imidazoles, imides, iactams.

The chlorides of aliphatic and aromatic acids can be obtained simultaneously by reaction of an aromatic compound containing at least one residue -CCI ₃ linked to an aromatic carbon with an aliphatic carboxylic acid in the presence of catalysts such as H ₂ SO ₄ (US 4 , 1 63,753) H ₃ PO ₄ , HCIO ₄ (US 1, 965,556) FeCI ₃ or ZnCI ₂ (FR 2,686,601). Benzene aldehydes and in particular benzaldehyde are used in perfumery and as intermediates for the synthesis of various chemicals (dyes, antibiotics). The access routes to benzene aldehydes, in particular benzaldehyde, include processes which constitute the hydrolysis of the corresponding dichloromethylated benzene compounds.

Thus, benzaldehyde is obtained industrially by hydrolysis, acid or alkaline, of benzylidene chloride according to the reaction:

C ₆ H ₅ CHCI ₂ + H ₂ O ^ C ₆ H ₅ CHO + 2HCI

The acid hydrolysis of benzylidene chloride, catalyzed by metal salts, leads to high yields of benzaldehyde (> 90%).

The acids used are in particular H ₃ PO ₄ , mentioned in the patent application (FR 21 6 374), HCI (DE 2 756 61 2) and HCO ₂ H (EP 41 672). Sulfuric acid has also been used, but has the drawback of leading to discharges which consist of dilute solutions of H ₂ SO ₄ .

However, H ₂ SO ₄ has been used for the hydrolysis of certain substituted benzylidene chlorides such as 2,4- and 2,6-dichloro-1 - (dichloromethyl) benzenes.

Benzaldehyde can also be obtained by partial oxidation of toluene. Thus, in international application WO 95.20560, a process is described which consists in partially oxidizing toluene in the liquid phase at temperatures between 10 ° C. and 200 ° C. under pressures ranging from 2 to 50 bars in the presence of catalysts based on Co and Mn.

The simultaneous industrial production of acid chlorides and benzene aldehydes is little described in the literature.

Only a few tests at the laboratory stage have been carried out. Thus in patent DE 1 1 494 (1 879), it is mentioned that a mole of

C ₆ H ₅ CHCI ₂ can react with one mole of acetic acid in the presence of

ZnCI ₂ , SbCI ₃ or CuCI ₂ to lead to benzaldehyde, acetyl chloride and hydrogen chloride depending on the reaction. C ₆ H ₅ CHCI ₂ + CH ₃ CO ₂ H ^ C ₆ H ₅ CHO + CH ₃ COCI + HCI

In US Patent 3,691, 21 7 describes, Example 1 6, the preparation of benzaldehyde and propionyl chloride by reacting, with stirring, at 100 ° C for 1 h 30, 0.5 mole of chloride benzylidene and 0.5 mole of propionic acid in the presence of 0.005 mole of SnCI ₄ .

No performance is mentioned.

The major drawback of this process is to implement a

5 volatile catalyst such as SnCI ₄ (Eb = 1 14, 1 ° C), which is likely to be entrained with the acid chlorides during their distillation. This is unacceptable for the purity of said acid chlorides and for a continuous or semi-continuous industrial process.

The Applicant has carried out batch tests to reproduce the methods of the prior art described above. It obtained conversions and low yields and found the presence of significant quantities of secondary products, which in particular contain cinnamic acid derivatives in the case of the reaction with acetic acid (or its anhydride) and, in a manner general, along with the other 15 acids, aromatic diesters such as benzylidene dicarboxylates

Without the Applicant being bound by any explanation on the formation of these diesters, it believes that they can be obtained from the products formed according to the scheme:

20

C _fi H ₅ CHO + RCOCI ^ C ₆ H ₅ CHCI-OC- (O) R

which can react on the acid present according to:

25 C ₆ H ₅ CHCIOC (O) R + RCOOH> C ₆ H ₅ CH [OC (O) R] ₂ + HCI

We have now found a process for the preparation of acid chloride (3) and benzene aldehyde (4) by reaction of a carboxylic acid (1) or its anhydride (1 a) with a benzene compound

Dichloromethylated (2), in the presence of at least one catalyst of the Friedel and Crafts type, according to reactions (I) or (II):

π RCOOH + _n HCI (I)

where R represents an alkyl radical, linear or branched, having a number of carbon atoms ranging from 1 to 1 2 and, preferably, ranging from 1 to 8, optionally substituted by 1 or more halogen atoms such as Cl, Br, F; an alkenyl radical having a number of carbon atoms ranging from 2 to 8, optionally substituted by 1 or more halogen atoms such as Cl, F; a phenyl radical optionally substituted by one or more atoms of Cl, Br or F, by a linear or branched alkyl residue, having a number of carbon atoms ranging from 1 to 4, said residue possibly containing 1 or more halogen atoms such as Cl, Br, F; (R ¹ ) ™ denotes that the benzene nucleus may have one or more substituents, identical or different, chosen from the group consisting of a Cl atom, a Br atom, an F atom, an alkyl radical, linear or branched, having a number of carbon atoms ranging from 1 to 4, -CF ₃ and -CH ₂ CI; m = O or an integer ranging from 1 to 3; n = 1 or 2; said process being characterized in that it consists: a) in producing in a reactor a base stock comprising mainly the dichloromethylated benzene compound (2), b) to be carried out, simultaneously and continuously: b ¹ ) on the one hand, the simultaneous introduction into the reactor of the carboxylic acid (1) or its anhydride (1 a), of the dichloromethylated benzene compound (2) and of at least one catalyst of the Friedel and Crafts type, and of reacting them, with stirring, under a pressure P1 less than or at most equal to atmospheric pressure and, preferably, under a pressure ranging from 100 mbar to 500 mbar and at a temperature ranging from 60 ° C to 1 80 ° C and, preferably, ranging from 80 ° C to 150 ° C; b ² ) on the other hand, the continuous extraction of the acid chloride (3) from the reaction medium and optionally from the HCl formed, by distillation under a pressure P1 and at the reaction temperature, then c) to be stopped the introduction of the reagents as far as the useful volume of the reactor containing mainly the benzene aldehyde is reached; and d) undertaking fractional distillation, total or partial, of the reaction medium, consisting mainly of benzene aldehyde, under a pressure P2 <P1 and at a temperature at most equal to 180 ° C. The total or partial fractional distillation of the reaction medium can be carried out separately in a fractional distillation installation. In this event, the contents of the reactor where the reaction was carried out is transferred, in whole or in part, to said distillation installation. In the case of a partial transfer, the non-transferred part serves as the base for a subsequent operation.

According to the present invention, the base stock mainly comprises dichloromethylated benzene compound (2) as well as a defined proportion of catalyst. It can also comprise, in small amounts, acid chloride and / or benzene aldehyde originating from a previous operation, in particular intermediate sections of fractional distillation of the reaction medium.

According to the present invention, the base stock represents at most 30% by volume of the reactor used for the implementation of the process.

According to the present invention, use will be made of the dichloromethylated benzene compound (2) and the carboxylic acid (1) or its anhydride (1 a) in a molar ratio (2) / (1) or (2) / (1 a) ranging from 1 / n to 1.2 / n and preferably ranging from 1 / n to 1.05 / n. The term “Friedel and Crafts type catalyst” is used to denote a Lewis acid or a Bronsted acid.

By way of illustration of Lewis acid which can be used according to the invention, mention will be made of FeCl ₃ , ZnCI ₂ , SbCI ₅ , CoCI ₂ , BiCl ₃ , ZrCI ₄ . Preferably, ZnCI ₂ or FeCI _{3 will be used} . By way of illustration of Brônsted acid which can be used according to the present invention, mention will be made of sulfuric acid, phosphoric acid.

These Friedel and Crafts type catalysts can be introduced into the reaction zone as such or in the form of aqueous solutions or in solution in one of the reactants. According to the present invention, a molar amount of pure Lewis acid of between 0.01% and 2% and, preferably, between 0.05% and 0.5% relative to the amount of the dichloromethylated compound will be used ( 2) implementation.

According to the present invention, use will be made of a molar amount of pure Brnsted acid of between 0.1% and 5% and preferably between 0.5% and 2%, relative to the amount of the dichloromethylated compound (2 ) implemented. The present invention applies very particularly well to carboxylic acids (1) or to their anhydrides (1 a) having the carbon in α of CO (carbonyl) as substituted as possible. By way of illustration of such acids, mention will be made of isobutyric, pivalic, 2-chloropropionic, trifluoroacetic, trichloroacetic acids. The invention also applies well to benzoic acid or its substituted derivatives.

By way of illustration of dichloromethylated benzene compounds (2) which can be used according to the present invention, there may be mentioned:

- (dichloromethyl) benzene or benzylidene chloride, - 2-chloro, 3-chloro and 4-chloro-1 - (dichloromethyl) benzenes,

- 4-fluoro-1 - (dichloromethyl) benzene,

- 4-trifluoromethyl-1 - (dichloromethyl) benzene,

- 1, 2- and 1, 4-bis (dichloromethyl) benzenes.

The process of the invention is particularly applicable to the preparation of acid chlorides having the most substituted CO carbon at α and benzaldehyde from benzylidene chloride and the preferred carboxylic acids (or their anhydrides) mentioned above.

According to the present invention, in the case where a carboxylic acid (1) is used, the hydrogen chloride formed can be washed against the current with the dichloromethylated benzene compound (2) in order to recover any acid chlorides entrained . The HCI recovered is advantageously absorbed in water in a slaughter column to lead to aqueous solutions of very pure commercial HCI. The organic washing solution can be reused for synthesis.

The process has the advantage of leading to acid chlorides and to benzenic aldehydes of high purity and of constant quality with high yields and selectivities.

This process also has the advantage of generating little or no secondary products, no gaseous effluents in the case where an acid anhydride is used as reagent or else generates HCI which can be recovered in the form of commercial solutions. when using a carboxylic acid.

In addition, this process also has the advantage of not using volatile catalysts which would be entrained practically continuously during the extraction of the products, which would generate pollution of the products and loss of activity during the reaction stage.

The following examples illustrate the invention. EXAMPLE 1 (not in accordance with the invention):

The reaction is carried out in batch in a glass reactor of

500 ml surmounted by a water cooler and equipped with a stirring anchor, a temperature probe and an inerting system. The refrigerant outlet is connected to a dry ice trap (to trap any volatile organics) followed by a water washer to absorb the HCl.

After having purged the entire assembly with nitrogen, the following are successively introduced into the reactor at room temperature:

1 mol of pivalic acid (1 03 g at approximately 99% purity) 1 mol of benzylidene chloride (1 62.6 g at approximately 99% purity) and 2 mmol of anhydrous FeCl ₃ (1 65 mg)

The mixture, with stirring, is gradually heated to 105 ° C. over 30 min and then kept at this temperature for 2 h 30 (until the evolution of HCl is stopped).

The HCI absorbed continuously during the reaction corresponds to 80% of the theoretical amount.

GPC analysis of the reaction crude then reveals a conversion of benzylidene chloride of 66% and a conversion of pivalic acid of 60% for a crude molar yield of pivaloyl chloride (relative to the initial pivalic acid) of only 33% and a gross molar yield of benzaldehyde (relative to the initial benzylidene chloride) of only 47%.

The poor selectivity for pivaloyl chloride (55%) corresponds to the formation of benzylidene pivalate C ₆ H ₅ CH [OC (O) C (CH ₃ ) ₂ ] ₂ identified by CPG / S coupling.

EXAMPLE 2 (according to the invention):

The reaction is carried out semi-continuously in a 500 ml glass flask surmounted by an Oldershaw type distillation column with 10 theoretical plates. The column head equipped with a reflux timer is connected to two dual circulation refrigerants to condense and recover the organics in a first recipe. The refrigerant outlet is connected to a line of vents including a dry ice trap (second recipe) followed by a water washer to absorb the HCl. After having purged the entire assembly with nitrogen, the following are introduced into the reactor:

0.68 mol of benzylidene chloride (1 1 2 g at 98% purity) and 1 80 mg of anhydrous ZnCI ₂ (approximately 1, 3 mmol). This base stock is brought to a temperature of 150 ° C., with stirring and at atmospheric pressure. While maintaining the temperature at 150 ° C., 258 g (2.5 mol) of pivalic acid, 300 g (1.82 mol) of benzylidene chloride and 490 mg (3.6 mmol) are then introduced continuously. of ZnCI ₂ in 6 h 30. In order to facilitate the implementation, the reagents have been previously mixed to obtain a stock solution (maintained at a temperature below 25 ° C.) which can then be introduced into the reactor using a single metering pump at a flow rate of 80 ml / h.

The start of the release of HCI is observed from the start of the introduction of the reagents and this release continues at a flow rate of between 0.3 and 0.4 mol / h.

The pivaloyl chloride begins to condense at the head of the column after approximately 1 hour 30 minutes of introduction, it is then withdrawn continuously with an average flow rate of the order of 25 g / h. At the end of the introduction of the reagents, the total quantity of HCI absorbed corresponds to 102.5% of the theoretical quantity. The mass of pivaloyl chloride extracted is 1 34.6 g (of which 1 1 8, 1 g in the first recipe and 1 6.5 g in the second recipe) with a purity of 97.6%, which represents a yield of 43.6%. The reaction crude remaining in the flask is then distilled under reduced pressure (from 500 to 50 mbar) and one thus obtains:

- an additional fraction of 82.8 g of pivaloyl chloride with a purity of 99.1%,

- a 7.8 g recyclable intermediate cut containing 26.8% pivaloyl chloride and 66.7% benzaldehyde,

- a fraction of 248.5 g rich in benzaldehyde (88.5%) but containing 10% of pivalic anhydride,

- then 73 g of distillation residue containing 2.7% of benzaldehyde, 20.8% of pivalic anhydride and 0.7% of residual benzylidene chloride.

At the end of the operation, the conversions of benzylidene chloride and pivalic acid are respectively 99.8% and 97.7% with an overall yield of pivaloyl chloride of 71.6% and a yield of crude benzaldehyde of 85.6%. EXAMPLE 3 (according to the invention):

The reaction is carried out in an assembly similar to that of Example 2 but the synthesis step is carried out under reduced pressure and with a molar excess of benzylidene chloride of 5.6% relative to pivalic acid.

The vacuum is ensured by a membrane pump (all Teflon) connected to the line of vents at the outlet of the dry ice trap and the outlet of which is connected to the water washer to absorb the HCl.

After having purged the entire assembly with nitrogen, 1 1 2 g (0.68 mol) of benzylidene chloride and 1 92 mg (# 1.4 mmol) of anhydrous ZnCl ₂ are introduced into the reactor.

This base stock is brought to a temperature of 120 ° C., with stirring and under pressure of 180 mbar. While maintaining the temperature at 120 ° C., 206 g (2 mol) of pivalic acid, 236.4 g (1, 432 mol) of benzylidene chloride and 391 mg (2.9) are then introduced continuously by gravity. mmol) of ZnCI ₂ in 5 h, the rate of introduction of the mixture, adjusted using a micrometric valve, is 80 ml / hour. The start of the release of HCI is observed from the start of the introduction of the reagents and this release continues at an average flow rate of 0.4 mol / h.

The pivaloyl chloride begins to condense at the head of the column after approximately 30 min of introduction, it is then withdrawn continuously with an average flow rate of the order of 45 g / h.

At the end of the introduction of the reagents, the total amount of HCI absorbed corresponds to 97.5% of the theoretical amount. The mass of pivaloyl chloride extracted is 224.7 g (of which 1 61.8 g in the first recipe and

62.9 g in the second recipe) with a purity of 99.2%, which represents a yield of 92.4%.

The reaction crude remaining in the flask is then distilled under reduced pressure (at 150 mbar) and one thus obtains:

- a complementary fraction of 1 1 g of pivaloyl chloride with a purity of 98%, - a recyclable intermediate cut of 1 1, 6 g containing 27.8% of pivaloyl chloride and 64.9% of benzaldehyde,

- a fraction of 200.4 g of benzaldehyde with a purity of 98%,

- then 1 6, 1 g of distillation residues containing 38% of benzaldehyde and 14% of residual benzylidene chloride. At the end of the operation, the conversions of benzylidene chloride and pivalic acid are respectively 99, 1% and 99.9% with an overall yield of distilled pivaloyl chloride (purity greater than 99%) of 97% relative to the pivalic acid used and a yield of distilled benzaldehyde of 92.5%.

EXAMPLES 4-7:

The reaction is carried out according to a process analogous to that of Example 3 but by reaction of benzylidene chloride with isobutyric acid (example 4), isobutyric anhydride (examples 5 and 6) and 2-chloropropionic acid (example 7). The purity of the benzylidene chloride used is between 97% and 99%. In all cases, the catalyst used is ZnCl ₂ with a molar ratio of 0.2% relative to the benzylidene chloride, except in Example 4 where the molar ratio is only 0.1%.

Table 1 indicates the reaction conditions.

Table 2 indicates the respective conversions of the reagents, the yields of acid chloride extracted simultaneously with the supply of said reagents, the overall molar yields of acid chloride and the yields of benzaldehyde obtained after distillation under reduced pressure at the bottom. reaction.

TABLE N ° 1

TABLE N ° 2

Claims

1. Process for the preparation of acid chloride (3) and benzene aldehyde (4) by reaction of a carboxylic acid (1) or its anhydride (1 a) on a dichloromethylated benzene compound (2), in the presence at least one Friedel and Crafts type catalyst, according to reactions (I) or (II):

HCL n RCOOH ⁺ uT3 / ( ^CHCi2 ) _n * ^" " (1)

_n ((π)

where R represents a radical, linear or branched, having a number of carbon atoms ranging from 1 to 1 2 and, preferably, ranging from 2 to 8, optionally substituted by 1 or more halogen atoms such as Cl, Br , F; an alkenyl radical having a number of carbon atoms ranging from 2 to 8 optionally substituted by 1 or more halogen atoms such as Cl, F; a phenyl radical optionally substituted by one or more atoms of Cl, Br or F, by an alkyl residue, linear or branched, having a number of carbon atoms ranging from 1 to 4, said residue possibly containing 1 or more atoms of halogen such as Cl, Br, F;

(R ¹ ) _m denotes that the benzene nucleus may have one or more substituents, identical or different, chosen from the group consisting of a Cl atom, a Br atom, an F atom, an alkyl radical, linear or branched, having a number of carbon atoms ranging from 1 to 4, -CF ₃ and -CH ₂ CI; m = O or an integer ranging from 1 to 3; n ≈ 1 or 2; said process being characterized in that it consists: a) in producing in a reactor a base stock comprising mainly the dichloromethylated benzene compound (2), b) to be carried out, simultaneously and continuously: b ¹ ) on the one hand, the simultaneous introduction into the reactor of the carboxylic acid (1) or its anhydride (1 a), of the dichloromethylated benzene compound (2) and of at least one catalyst of the Friedel type and Crafts, and reacting them, with stirring, under a pressure P1 less than or at most equal to atmospheric pressure and at a temperature ranging from 60 ° C to 1 80 ° C; b ² ) on the other hand, the continuous extraction of the acid chloride (3) from the reaction medium and optionally from the HCl formed, by distillation under a pressure P1 and at the reaction temperature, then c) to be stopped the introduction of the reagents as far as the useful volume of the reactor containing mainly the benzene aldehyde is reached; and d) undertaking fractional distillation, total or partial, of the reaction medium, consisting mainly of benzene aldehyde, under a pressure P2 <P1 and at a temperature at most equal to 180 ° C.

2. Method according to claim 1, characterized in that one operates in step b) under a pressure P1 ranging from 100 mbar to 500 mbar and at a temperature ranging from 80 ° C to 1 50 ° C.

3. Method according to claim 1 or 2, characterized in that the base stock represents at most 30% by volume of the reactor.

4. Method according to any one of claims 1 to 3, characterized in that one uses the dichloromethylated benzene compound (2) and the carboxylic acid (1), or its anhydride (1 a) in a molar ratio ( 2) / {1) or (2) / (1 a) ranging from 1 / n to 1, 2 / n.

5. Method according to claim 4, characterized in that the molar ratio (2) / (1) or (2) / (1 a) ranges from 1 / n to 1.05 / n.

6. Method according to any one of claims 1 to 5, characterized in that the catalyst of the Friedel and Crafts type is a Lewis acid.

7. Method according to claim 6, characterized in that the Lewis acid is ZnCI ₂ or FeCI ₃ .

8. Method according to one of claims 6 or 7, characterized in that a molar amount of Lewis acid is used, between 0.01% and 2%, and preferably, between 0.05% and 0.5 %% relative to the amount of dichloromethylated compound used.

9. Method according to any one of claims 1 to 8, characterized in that the dichloromethylated compound (2) is benzylidene chloride.

10. Method according to any one of claims 1 to 8, characterized in that the carboxylic acid (1) or its anhydride (1 a) have the carbon in α of CO as substituted as possible.

11. Method according to claim 1 0, characterized in that the carboxylic acid (1) is pivalic acid, 2-chloropropionic acid, isobutyric acid, trifluoroacetic acid or trichloroacetic acid.

12. Method according to claim 1 0, characterized in that the anhydride (1 a) is isobutyric anhydride.