Process for preparing esters

Details Process for preparing esters Process for preparing esters

2002-03-25

2003-02-25

Killos, Paul J. (Department: 1625)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acid esters

C560S103000, C560S174000

Reexamination Certificate

active

06525213

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a process for preparing carboxylic acid esters. Carboxylic acid esters can be used, for example, as solvents. They are also intermediates in chemical synthesis.
U.S. Pat. No. 5,405,991 discloses the preparation of esters of trifluoroacetic acid and of chlorodifluoroacetic acid from the acid chlorides and the corresponding alcohol in the presence of “onium” salts of the carboxylic acid corresponding to the carboxylic acid chloride used.
Published German patent application no. DE 197 32 031 discloses the preparation of esters of trifluoroacetic acid and of chlorodifluoroacetic acid in the presence of the corresponding “onium” salts with 2-phase formation for the purpose of simple separation of the products.
SUMMARY OF THE INVENTION
It is an object of the present invention to devise an improved process for the preparation of carboxylic acid esters from alcohols and carboxylic acids, carboxylic acid chlorides or carboxylic acid bromides.
These and other objects are achieved in accordance with the present invention by providing a process for preparing a carboxylic acid ester from an alcohol and a carboxylic acid, carboxylic acid chloride or carboxylic acid bromide, wherein esterification is effected while avoiding precipitation of “onium” salts, by operating in the presence of an onium salt adduct of the corresponding carboxylic acid and 1,5-diazabicyclo[4.3.0]-non-2-ene or 1,8-diazabicyclo[5.4.0]-undec-7-ene, or operating in the presence of the corresponding carboxylic acid and an onium salt of the corresponding carboxylic acid.
The process according to the invention for the preparation of carboxylic acid esters from alcohols and carboxylic acids, carboxylic acid chlorides or carboxylic acid bromides provides for the esterification to be performed, avoiding the precipitation of “onium” salts, by
operating in the presence of the adduct (“onium” salt) of the corresponding carboxylic acid and 1,5-diazabicyclo[4.3.0]-non-5-ene (“DBN”) or 1,8-diazabicyclo[5.4.0]-undec-7-ene (“DBU”), or
operating in the presence of the corresponding carboxylic acid and an “onium” salt of the corresponding carboxylic acid.
Preferably the starting materials in the preparation of carboxylic acid esters according to the invention are alcohols and carboxylic acid chlorides. The advantage of the process of the invention is that the precipitation of solids (primarily hydrohalide salts of the “onium” cations) is prevented. Precipitated solids are undesirable in the reaction and working-up of the reaction mixture. The term “alcohols” also includes thioalcohols. The terms “carboxylic acids, carboxylic acid chlorides or carboxylic acid bromides” also cover the corresponding thiocarboxylic acids or thiocarboxylic acid derivatives.
The process will be explained in further detail with reference to illustrative representative embodiments using carboxylic acid chlorides. Preferably carboxylic acid esters corresponding to the formula (I) or (II)
R
1
—C(O)—OR
2
  (I)
R
1
—C(O)—CH
2
—C(O)—OR
2
  (II)
are prepared, wherein R
1
is C1-C4 alkyl or C1-C4 alkyl substituted by at least 1 halogen atom, and R
2
is C1-C4 alkyl, C1-C4 alkyl substituted by at least 1 halogen atom, aryl or benzyl. R
1
may also be aryl, e.g. phenyl. Particularly preferably, R
1
is methyl, ethyl or methyl or ethyl substituted by at least 1 halogen atom.
Particularly preferably, C1-C4-alcohols and acid chlorides of halogenated acetic acids or halogenated acetylacetic acids are esterified. Very particularly preferably, corresponding alkyl esters of chloro-, dichloro-, trichloro-, trifluoro-, difluoro- or chlorodifluoroacetic acid are prepared, or corresponding alkyl esters of tri-, difluoro- or chlorodifluoroacetylacetic acid, in particular of trifluoroacetylacetic acid.
The invention can be performed in two variants. In the first variant, the “onium” salts of two special amines, DBN or DBU, are used. In this case, no additional acid is used. In the second variant, the reaction is performed in the presence of free carboxylic acid. In this variant, any nitrogen-based “onium” salts can be used, including DBN and DBU. The second variant will be explained in greater detail first.
According to one variant, the esterification is performed in the presence of an “onium” salt of the corresponding acid (preferably of a halogenated acetic acid or of the halogenated acetylacetic acid) and additionally of the corresponding (free) acid (preferably of a halogenated acetic acid or of (free) halogenated acetylacetic acid). Accordingly, both a corresponding “onium” salt and the free acid are present in the mixture. It is assumed that a portion of the corresponding halogenated acid can be replaced by another carboxylic acid or mineral acid. However, it is preferred if the acid chloride and the acid used to form the “onium” salt are the same. The molar ratio of “onium” salt to (free) carboxylic acid is advantageously between 1:0.2 and 1:3. Good results are for example also achieved with a ratio between 1:0.2 and 1:2. The concentration of “onium” salt is advantageously 5 to 20 mole % of the reactants (calculated without solvent). The presence of the free carboxylic acid can be brought about by adding it to the reaction mixture. It can also be produced in situ by adding water to the reaction mixture, because the acid chloride and water react to form the corresponding free carboxylic acid. The process can be performed continuously.
The term “onium” in this variant indicates cations having a positively-charged nitrogen, for example protonated aromatic nitrogen bases such as pyridinium or protonated alkyl-, dialkyl- or trialkylammonium cations, or ammonium compounds substituted by cycloalkyl, or cycloaliphatic nitrogen bases such as piperidinium or quaternary ammonium cations.
Especially suitable carboxylic acid salts are “Onium” salts in which “onium” identifies a cation of nitrogen having the formula R′R″R′″R″″N+ wherein R′,R″,R′″ and R″″, independently of each other, represent hydrogen, alkyl with 1 to 20 carbon atoms, aryl or aralkyl. R′ and R″ or R′″ and R″″, or R′, R′ and R′″ or R′, R″, R′″ and R″″ may also, optionally with inclusion of the nitrogen atom, form saturated or unsaturated ring systems. “Aryl” here stands in particular for phenyl or for phenyl substituted by 1 or more C1-C2 alkyl groups. Particularly advantageous salts are those in which “onium” is ammonium, pyridinium or R
1
′R
2
′R
3
′R
4
′N
+
, wherein R
1
′, R
2
′, R
3
′ and R
4
′, independently of each other, represent hydrogen, alkyl with 1 to 15 carbon atoms, phenyl or benzyl. Examples of such cations include pyridinium, piperidinium, N-methylpiperidinium, anilinium, benzyltriethylammonium and triethylammonium.
Amines substituted by hydroxy groups, particularly cycloaliphatic amines, in particular hydroxy-substituted piperidines and N- C1-C4 alkylpiperidines, can also be used. Suitable examples include piperidines substituted at the C4 atom such as 4-hydroxypiperidine, N-methyl-4-hydroxypiperidine, N-ethyl-4-hydroxypiperidine and N-propyl-4-hydroxypiperidine.
Cations of amines which are disclosed in German Offenlegungsschrift no. DE 101 04 663.4, which does not constitute a prior publication, can also be used. These are “onium” cations based on a mono-or bicyclic compound with at least two nitrogen atoms, wherein at least one nitrogen atom is incorporated in the ring system. Thus “onium” cations based on monocyclic compounds may be used. These are then saturated or unsaturated 5-member ring, 6-member ring or 7-member ring compounds. At least one nitrogen atom is incorporated in the ring. A further nitrogen atom may also be incorporated in the ring system. Alternatively or additionally, the ring may be substituted by one or more amino groups. Dialkylamino groups in whi

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