Preparation of enol ethers

Details Preparation of enol ethers Preparation of enol ethers

2001-06-21

2003-01-07

Keys, Rosalynd (Department: 1621)

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

Organic compounds

Oxygen containing

C568S689000

Reexamination Certificate

active

06504064

ABSTRACT:

The present invention relates to a process for preparing enol ethers of the formula (I)
where R
1
to R
3
are each, independently of one another, hydrogen or a carbon-containing organic radical and R
4
is an unsubstituted or substituted alkyl radical, by reacting alcohols of the formula (II)
R
4
—OH  (II),
with alkynes of the formula (IIIa), alkadienes of the formula (IIIb)
R
1
—C≡C—CHR
2
R
3
  (IIIa),
R
1
—CH═C═CR
2
R
3
  (IIIb)
or mixtures thereof, where R
1
to R
4
are as defined above.
Enol ethers are important intermediates in the synthesis of many chemical products, in particular pharmaceuticals, agrochemicals and cosmetics. Furthermore, enol ethers are important monomeric building blocks in the preparation of polymers.
Enol ethers are generally prepared by alkenylation of alcohols in the presence of basic catalysts. The ethenylation of alcohols is usually carried out under superatmospheric pressure, e.g. 1.6 MPa in the case of methyl vinyl ether, and high temperatures in the range from 120 to 180° C. (cf. Ullmann's Encyclopedia of Industrial Chemistry, 6
th
edition, 1999 Electronic Release, Chapter “VINYL ETHERS”).
B. A. Trofimov, Russian Journal of Organic Chemistry, Vol. 31, 1995, pages 1233 to 1252, describes the reaction of alcohols with ethyne under atmospheric pressure and at from 20 to 100° C. in the presence of a superbasic catalyst system comprising potassium hydroxide in dimethyl sulfoxide (DMSO).
If alkenylations are carried out using propyne or higher molecular weight alkynes, the reaction conditions are generally more drastic. DD 263 890 A3 describes the preparation of alkoxypropenes from propyne and alcohols at from 190 to 240° C. and a pressure of from 41 to 62 atm in the presence of potassium hydroxide in the liquid phase. Disadvantages are the relatively severe reaction conditions in respect of temperature and pressure.
High temperature and high pressure represent a safety risk when handling alkynes and alkenes. Thus, according to L. Bretherick et al., “Bretherick's Handbook of Reactive Chemical Hazards”, 5
th
edition, Butterworth-Heinemann, Oxford 1995, pages 5410 to 5411, decomposition of propyne in the gas phase at 20° C. can occur at as low as 0.35 MPa abs and decomposition of 1,2-propadiene in the gas phase at 25° C. can occur at as low as 0.2 MPa abs.
DD 267 629 A3 and DD 265 289 A3 describe the alkenylation of methanol and ethanol using propene over heterogeneous zinc-containing catalysts in the gas phase at from atmospheric pressure to 10 atm and at from 250 to 300° C. A disadvantage is the very high temperature required in this process.
EP 0 887 331 A1 and EP 0 887 332 A1 disclose the preparation of enol ethers by reaction of alcohols with alkynes in the gas phase in the presence of an X-ray-amorphous zinc or cadmium silicate. The preparation of 2-methoxypropene at 170° C. and a pressure of 1.2 bar abs is described. In this process, too, a high temperature is necessary for the reaction.
Owing to the disadvantages of the alkenylation processes mentioned, 2-methoxypropene, for example, is generally obtained by dissociation of the corresponding ketal in the gas phase over various heterogeneous catalysts. Appropriate methods are described, for example, in WO 98/58894, EP 0 887 330 A1 and EP 0 776 879 A1. Disadvantages of this synthetic route are the unfavorable starting material basis, namely the corresponding ketones, and the multistep nature of the process chain, which comprises ketalization of the ketones and subsequent dissociation of the ketals.
It is an object of the present invention to find a process for preparing enol ethers of the propene group and higher molecular weight alkene groups, which no longer has the abovementioned disadvantages, is based on economical, readily available raw materials and, in particular, makes possible a high yield of enol ether in only one synthesis step under mild reaction conditions. We have found that this object is achieved by a process for preparing enol ethers of the formula (I)
where R
1
to R
3
are each, independently of one another, hydrogen or a carbon-containing organic radical and R
4
is an unsubstituted or substituted alkyl radical, by reacting alcohols of the formula (II)
R
4
—OH  (II),
with alkynes of the formula (IIIa), alkadienes of the formula (IIIb)
R
1
—C≡C—CHR
2
R
3
  (IIIa),
R
1
—CH═C═CR
2
R
3
  (IIIb)
or mixtures thereof, where R
1
to R
4
are as defined above, wherein the reaction is carried out in the presence of an alkali metal alkoxide and a polar, aprotic solvent.
Alkali metal alkoxides which can be used in the process of the present invention are in principle all aliphatic, araliphatic, saturated, noncyclic or cyclic alkoxides of lithium, sodium, potassium, rubidium or cesium. Examples which may be mentioned are alkali metal methoxides, alkali metal ethoxides, alkali metal 1-propoxides (alkali metal propoxides), alkali metal 2-propoxides (alkali metal isopropoxides), alkali metal 1-butoxides (alkali metal butoxides), alkali metal 2-butoxides (alkali metal sec-butoxides), alkali metal 2-methyl-1-propoxides (alkali metal isobutoxides), alkali metal 1,1-dimethyl-1-ethoxides (alkali metal tert-butoxides), alkali metal 1-pentoxides, alkali metal 2-pentoxides, alkali metal 3-pentoxides, alkali metal 2-methyl-1-butoxides, alkali metal 3-methyl-1-butoxides (alkali metal isoamylates), alkali metal 3-methyl-2-butoxides, alkali metal 2,2-dimethyl-1-propoxides, alkali metal 2-methyl-2-butoxides (alkali metal tert-amylates), alkali metal cyclopentoxides, alkali metal cyclohexoxides, alkali metal benzylates, alkali metal 2-phenyl-1-ethoxides, alkali metal 2-phenyl-2-propoxides or mixtures thereof, where alkali metal may be lithium, sodium, potassium, rubidium or cesium.
Preference is given to using the tertiary alkali metal alkoxides, for example sodium tert-butoxide, potassium tert-butoxide, cesium tert-butoxide, sodium tert-amylate, potassium tert-amylate, cesium tert-amylate or mixtures thereof. Particular preference is given to using potassium tert-butoxide.
The alkali metal alkoxides can also be used in combination with alkali metal hydroxides, for example sodium hydroxide, potassium hydroxide or cesium hydroxide, and/or alkali metal halides, preferably fluorides such as sodium fluoride, potassium fluoride or cesium fluoride. If the alkali metal alkoxides are used in combination with further alkali metal compounds, the combination with cesium fluoride is preferred.
In the process of the present invention, the alkali metal alkoxide is generally used in an amount of from 1 to 50 mol %, based on the alcohol (II) used. Preference is given to an amount of from 5 to 30 mol %, particularly preferably from 10 to 20 mol %.
The alkali metal alkoxides used according to the present invention are mostly commercially available, but can, if necessary, be prepared by known methods. Suitable methods are, for example, reaction of the corresponding alcohols with (i) elemental alkali metals, (ii) with alkali metal hydroxides and removal of the water of reaction formed and (iii) with other alkoxides and removal of the other alcohols formed.
Furthermore, the presence of a polar, aprotic solvent is essential in the process of the present invention. Suitable polar, aprotic solvents are liquid under the reaction conditions and preferably also at room temperature. Examples which may be mentioned are tetrahydrofuran, 1,4-dioxane, N-methylpyrrolidone, N-methylpiperidone, dimethyl sulfoxide, glycol ethers (e.g. 1,2-dimethoxyethane or bis(2-methoxyethyl) ether), dimethylformamide, dimethylformanilide or mixtures thereof. Particular preference is given to using N-methylpyrrolidone.
The polar, aprotic solvent can also be used in admixture with further aprotic solvents, for example saturated aliphatic or aromatic compounds. Examples which may be mentioned are alkanes such as hexanes, heptanes, octanes, nonanes, decanes or petroleum spirit, and aromatic compounds such as toluene or xylenes. The polar, aprotic solv

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