Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2002-09-27
2004-06-15
Lambkin, Deborah C. (Department: 1626)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C549S064000
Reexamination Certificate
active
06750354
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for the alkylation of dihydroxythiophenedicarboxylic esters or their alkali metal or alkaline earth metal salts in the presence of organic onium salts.
2. Brief Description of the Prior Art
3,4-Dialkoxythiophenes and 3,4-alkylenedioxythiophenes are starting compounds for the preparation of electrically conductive polymers which may be transparent in thin layers and have recently been finding a wide range of uses. The polymers are used, for example, as electrodes, sensors, for producing capacitors or electroluminescent displays and other electro-optic components, for producing photovoltaic devices, as electrochromic layers, as auxiliaries for the production of metal coatings, as thin films for dissipating static charges, in gel electrolytes or in ion-exchange membranes. The properties of these polymers can be varied within wide limits by means of the length and the substitution pattern of the alkoxy or alkylene group. The polymers are generally prepared from the corresponding monomers by chemical or electrochemical methods. A particularly important 3,4-alkylenedioxythiophene is 3,4-ethylenedioxythiophene.
3,4-Dialkoxythiophenes and 3,4-alkylenedioxythiophenes are frequently prepared in a multistage synthesis from the corresponding 3,4-dialkoxythiophene-2,5-dicarboxylic acids and 3,4-alkylenedioxythiophene-2,5-dicarboxylic acids by decarboxylation. These acids are in turn prepared from the corresponding esters. 3,4-Dialkoxythiophene-2,5-dicarboxylic esters and 3,4-alkylenedioxythiophene-2,5-dicarboxylic esters can be prepared by alkylation of 3,4-dihydroxythiophene-2,5-dicarboxylic esters. While 3,4-dihydroxythiophene-2,5-dicarboxylic esters and their alkali metal and alkaline earth metal salts have for a long time been able to be obtained conveniently in good yield (cf. for example, O. Hinsberg, Ber. Dt. Chem. Ges. 43, 1910, 901-906 and 45, 1912, 2413-2418), their alkylation is in many cases difficult and able to be carried out only in moderate yields.
Merz et al., J. Prakt. Chem. 338, 1996, 672-674, describe the alkylation of 3,4-dihydroxythiophene-2,5-dicarboxylic esters which is carried out using dimethyl sulphate in toluene in the presence of the cation solvator [18]crown-6, giving a yield of free dicarboxylic acid of 73% of theory. However, a disadvantage of this process is the long reaction time of 48 hours. Furthermore, the starting material is used in the form of its dipotassium salt which has to be prepared separately from the diol. The alkylation using 1,2-dichloroethane as alkylating agent, which is said to lead to 3,4-ethylenedioxythiophene-2,5-dicarboxylic ester which is preferably used as starting material for preparing 3,4-ethylenedioxythiophene, was not able to be reproduced in the laboratory under the conditions stated.
M. Coffey et al., Synthetic Communications 26 (11), 1996, 2205-2212, describe the alkylation of 3,4-dihydroxythiophene-2,5-dicarboxylic esters in the form of the free diol by means of 1,2-dibromoethane in the presence of potassium carbonate at 150° C. in dimethylformamide. A disadvantage is the low yield of 52% of theory. In addition, the preparation of the free diol from the alkali metal salt initially obtained is an additional process step which has an adverse effect on the economics of the synthesis. A variant of this method in which the reaction temperature is 90° C. has been described by Sankaran and Reynolds, polym. Mater. SCI 72, 1995, 319-320.
There is therefore a need for a process for the alkylation of 3,4-dihydroxythiophene-2,5-dicarboxylic esters or their alkali metal or alkaline earth metal salts which is simple to carry out and gives good yields of the corresponding 3,4-dialkoxythiophene-2,5-dicarboxylic esters and 3,4-alkylenedioxythiophene-2,5-dicarboxylic esters at comparatively short reaction times.
SUMMARY OF THE INVENTION
We have now surprisingly found a process for the alkylation of compounds of the formula (I)
where
R
1
and R
2
are identical and are each hydrogen or are identical or different and are each an alkali metal or an alkaline earth metal and
R
3
and R
4
are identical or different and are each a straight-chain or branched alkyl radical having from 1 to 10 carbon atoms,
comprising reacting said compounds with alkylating agents in a polar diluent, wherein the reaction is carried out in the presence of quaternary onium salts of the formula (II)
where
A is nitrogen or phosphorus,
Y
−
is an anion and
R
5
to R
8
are identical or different and are each an alkyl radical having from 1 to 20 carbon atoms, an aryl radical having from 6 to 15 carbon atoms or an aralkyl radical having from 7 to 20 carbon atoms.
The process of the invention allows the alkylation of compounds of the formula (I) under mild conditions at low temperatures and short reaction times while giving excellent yields.
DETAILED DESCRIPTION OF THE INVENTION
In the process of the invention, preference is given to using compounds of the formula (I) in which R
1
and R
2
are identical or different and are each an alkali metal or an alkaline earth metal, for example lithium, sodium, potassium, rubidium, magnesium, calcium or strontium. They are obtained from the corresponding free 3,4-dihydroxythiophene-2,5-dicarboxylic esters by addition of alkali metal alkoxide or carbonate or alkaline earth metal alkoxide or carbonate or are obtained directly in the preparation of 3,4-dihydroxythiophene-2,5-dicarboxylic esters by reaction of thiodiacetic esters with oxalic esters in the presence of alkali metal alkoxide (“Hinsberg condensation”). Particular preference is given to using compounds of the formula (I) in which R
1
and R
2
are identical or different and are each an alkali metal, for example lithium, sodium, potassium or rubidium.
If compounds of the formula (I) in which R
1
and R
2
are different are used, then R
1
is particularly preferably sodium and R2 is particularly preferably potassium.
Very particular preference is given to using compounds of the formula (I) in which R
1
and R
2
are identical and are each lithium, sodium, potassium or rubidium, among which sodium and potassium are preferred.
The process of the invention is preferably carried out using compounds of the formula (I) in which R
3
and R
4
are identical or different and are each a straight-chain or branched alkyl radical having from 1 to 8 carbon atoms, particularly preferably a straight-chain or branched alkyl radical having from 1 to 6 carbon atoms, for example methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, n-pentyl, branched pentyls, n-hexyl, branched hexyls, 2-methylbutyl or 2-ethylbutyl.
If compounds of the formula (I) in which R
3
and R
4
are identical are used in the process of the invention, then R
3
and R
4
are each preferably methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, 2-methylbutyl or 2-ethylbutyl, particularly preferably methyl, ethyl, n-propyl or isopropyl, with the compounds mentioned being particularly preferably used in the form of their disodium or dipotassium salts (R
1
and R
2
are identical and are each sodium or potassium), very particularly preferably in the form of their disodium salt (R
1
and R
2
are identical and are each sodium). Very particular preference is given to using dimethyl 3,4-dihydroxythiophene-2,5-dicarboxylate in the form of its disodium salt.
In a preferred embodiment of the process of the invention, use is made of mixtures of compounds of the formula (I) as are obtained, for example, in the preparation of 3,4-dihydroxythiophene-2,5-dicarboxylic esters by reacting dibutyl thiodiacetate with diethyl oxalate in the presence of sodium methoxide in methanol (“Hinsberg condensation”). These are preferably mixtures in which R
3
and R
4
are identical or different and are each methyl, ethyl or butyl. Particular preference is given to using mixtures comprising dimethyl 3,4-dihydroxythiophene-2,5-dicarboxylate, methyl ethyl 3,4-dihydroxythiophene-2,5-dicarboxylate and methyl butyl 3,4-dihydrox
Bremen Josef
Klausener Alexander
Rauchschwalbe Günter
Schenkel Ralf-Ingo
Winkler Adolf
Akorli Godfried R.
Bayer Aktiengesellschaft
Eyl Diderico van
Lambkin Deborah C.
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