Low temperature process for preparing methyl esters

Details Low temperature process for preparing methyl esters Low temperature process for preparing methyl esters

2002-04-26

2003-02-04

Killos, Paul J. (Department: 1625)

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

Organic compounds

Carboxylic acid esters

C560S008000, C560S009000, C560S147000, C560S155000, C560S205000, C558S260000

Reexamination Certificate

active

06515167

ABSTRACT:

FIELD OF THE INVENTION
The present invention provides a process for preparing a methyl ester by reacting a carboxylic acid or salt thereof with dimethyl carbonate in the presence of a catalyst selected from 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and 1,4-diazabicyclo[2.2.2]octane (DABCO).
BACKGROUND OF THE INVENTION
Methylation of alcohols, amines, carboxylic acids and activated methylenes is an important process in chemistry. However, due to the environmental and human impact of using toxic and unsafe methylating reagents, such as methyl iodide or dimethyl sulfate, the investigation of safer, generally applicable alternatives continues. As an alternative to these toxic methylating agents, dimethyl carbonate has attracted considerable attention for the methylation of phenols, anilines and activated methylenes. Dimethyl carbonate is non-toxic and generates CO
2
and methanol as by-products during methylations. Dimethyl carbonate is also a volatile liquid with a boiling point of 90° C. Hence, the unreacted dimethyl carbonate can be easily recovered by distillation from the reaction mixture and reused. Furthermore, dimethyl carbonate has been shown to be quite selective in monomethylation of primary aromatic amines and C-methylation of arylacetonitriles and arylacetoesters.
However, the use of dimethyl carbonate as a methylating regent requires high temperatures and pressures, generally 140° C.-180° C. Therefore, autoclaves or the use of asymmetrical carbonates with a higher boiling point than dimethyl carbonate have to be employed. These restrictions lower the popularity of using dimethyl carbonate as a methylating reagent.
U.S. Pat. No. 4,513,146 describes a method for producing esters from highly-hindered carboxylic acids and carbonates. The method involves reacting the highly-hindered carboxylic acid with a carbonate with or without a catalyst at a temperature of 175° C. according to the examples. U.S. Pat. No. 4,513,146 states that exemplary catalysts are nitrogen-containing heterocyclic catalysts such as pyridine, 4-(dimethylamino)pyridine, imidazole, 2,6-lutidine and 2,4,6-collidine.
Therefore, it would be advantageous from a production and safety standpoint to develop a low temperature process which utilizes dimethyl carbonate as a reactant in the production of esters.
SUMMARY OF THE INVENTION
The invention provides a low temperature process for preparing a methyl ester having formula (III)
said process comprising reacting a carboxylic acid or salt thereof having formula (I)
with dimethyl carbonate having formula (II)
in the presence of a catalyst selected from the group consisting of 1,8-diazabicyclo[5.4.0]undec-7-ene; 1,4-diazabicyclo[2.2.2]octane; and combinations thereof, wherein said process is conducted at a temperature of about 10° C. to less than 120° C.;
R
1
is selected from the group consisting of an alkyl, aryl, alkoxy, alkenyl, cycloalkyl, benzocycloalkyl, cycloalkylalkyl, aralkyl, heterocyclic, heteroaralkyl, alkoxyalkyl, carboxyalkyl, alkylcarbonyl, alkoxycarbonyl, alkoxycarbonylalkyl and haloalkyl; and M is selected from the group consisting of hydrogen, a monovalent metal and a monovalent fractional part of a polyvalent metal.
According to another aspect, the invention provides a compound having formula (III)
wherein said compound is prepared by a process comprising reacting a carboxylic acid or salt thereof having formula (I)
with dimethyl carbonate having formula (II)
in the presence of a catalyst selected from the group consisting of 1,8-diazabicyclo[5.4.0]undec-7-ene; 1,4-diazabicyclo[2.2.2]octane; and combinations thereof, wherein said process is conducted at a temperature of about 10° C. to less than 120° C.;
R
1
is selected from the group consisting of an alkyl, aryl, alkoxy, alkenyl, cycloalkyl, benzocycloalkyl, cycloalkylalkyl, aralkyl, heterocyclic, heteroaralkyl, alkoxyalkyl, carboxyalkyl, alkylcarbonyl, alkoxycarbonyl, alkoxycarbonylalkyl and haloalkyl; and M is selected from the group consisting of hydrogen, a monovalent metal and a monovalent fractional part of a polyvalent metal.
The process of the invention is especially advantageous for preparing methyl esters since the process: (1) is conducted at a low temperature, less than 120° C.; (2) utilizes an environmentally friendly methylating reagent, dimethylcarbonate; (3) produces a high yield of the methyl ester, generally 97-98% conversion; and (4) does not require a high-pressure (autoclave) reactor.
DESCRIPTION OF THE INVENTION
The process of the invention is used to prepare a methyl ester having formula (III)
In formula (III), R
1
is selected from the group consisting of an alkyl, aryl, alkoxy, alkenyl, cycloalkyl, benzocycloalkyl, cycloalkylalkyl, aralkyl, heterocyclic, heteroaralkyl, alkoxyalkyl, carboxyalkyl, alkylcarbonyl, alkoxycarbonyl, alkoxycarbonylalkyl and haloalkyl. The reaction is conducted in the presence of a catalyst which is selected from 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,4-diazabicyclo[2.2.2]octane (DABCO). A combination of catalysts may also be used.
The process of the invention involves reacting a carboxylic acid or salt thereof having formula (I)
with dimethyl carbonate having formula (II)
In formula (I), R
1
is selected from the group consisting of an alkyl, aryl, alkoxy, alkenyl, cycloalkyl, benzocycloalkyl, cycloalkylalkyl, aralkyl, heterocyclic, heteroaralkyl, alkoxyalkyl, carboxyalkyl, alkylcarbonyl, alkoxycarbonyl, alkoxycarbonylalkyl and haloalkyl, and M is selected from the group consisting of hydrogen, a monovalent metal and a monovalent fractional part of a polyvalent metal. The metal is preferably selected from sodium, potassium, magnesium or calcium.
As used herein, “alkyl” means straight chain or branched alkyl, which may be, for example, C
1
-C
10
-alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, straight or branched pentyl, straight or branched hexyl, straight or branched heptyl, straight or branched nonyl or straight or branched decyl. Preferably alkyl is C
1
-C
4
-alkyl. “Aryl” means C
6
-C
14
-aryl, preferably C
6
-C
10
-aryl, and may be, for example, substituted by at least one group selected from mercapto, dialkylamino, nitro, alkoxy, halogen, keto, cyano or a combination. Preferably aryl is phenyl.
“Alkoxy” means straight chain or branched alkoxy and may be, for example, C
1
-C
10
-alkoxy, such as methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, isobutoxy, sec-butoxy, tert-butoxy or straight or branched pentoxy, hexyloxy, heptyloxy, octyloxy, nonyloxy or decyloxy. Preferably alkoxy is C
1
-C
4
-alkoxy.
“Alkenyl” means straight chain or branched alkenyl, which may be, for example, C
2
-C
10
alkenyl, such as vinyl, 1-propenyl, 2-propenyl, 1-butenyl, isobutenyl, or straight or branched pentenyl, hexenyl, heptenyl, octenyl, nonenyl or decenyl. Preferred alkenyl is C
2
-C
4
-alkenyl.
“Cycloalkyl” means C
3
-C
10
-cycloalkyl having 3- to 8-ring carbon atoms and may be, for example, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl or cycloheptyl, any of which can be substituted by one, two or more C
1
-C
4
-alkyl groups, particularly methyl groups. Preferably, cycloalkyl is C
3
-C
6
-cycloalkyl.
“Benzocycloalkyl” means cycloalkyl (e.g., one of the C
3
-C
10
-cycloalkyl groups mentioned hereinbefore), attached at two adjacent carbon atoms to a benzene ring. Preferably, benzocycloalkyl is benzo-C
5
-C
6
-cycloalkyl, especially benzocyclohexyl (tetrahydronaphthyl).
“Cycloalkylalkyl” means C
3
-C
10
-cycloalkyl-C
1
-C
10
-alkyl, where the C
3
-C
10
-cycloalkyl group has 3- to 8-ring carbon atoms and may be, for example, one of the C
1
-C
10
-alkyl groups mentioned hereinbefore, particularly one of the C
1
-C
4
-alkyl groups, substituted by one of the C
3
-C
10
-cycloalkyl groups mentioned hereinbefore. Preferably cycloalkylalkyl is C
3
-C
6
-cycloalkyl-C
1
-C
4
-alkyl.
“Aralkyl” means straight chain or branched C
6
-C
10
-aryl-C
1
-C
10
-alkyl and may be, for example, one of the C
1
-C
10
-alk

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