Process for preparing fluorine-containing compounds

Details Process for preparing fluorine-containing compounds Process for preparing fluorine-containing compounds

1999-07-07

2001-02-06

Richter, Johann (Department: 1621)

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

Organic compounds

Halogen containing

C570S124000, C570S127000, C544S217000, C544S218000, C544S224000, C544S334000, C544S409000, C546S345000, C546S346000

Reexamination Certificate

active

06184425

ABSTRACT:

The present invention relates to a process which is an improvement over the prior art for preparing fluorine-containing compounds by means of a halogen-fluorine exchange reaction.
Fluorine-containing compounds are employed, inter alia, in liquid-crystalline mixtures (EP 0 602 596).
The halogen-fluorine exchange reaction is also known by the name halex reaction. It is an extensively employed method of introducing fluorine substituents into a compound containing fluorine-exchangeable halogen.
In the case of aromatic compounds, especially activated aromatic compounds, the halogen-fluorine exchange proceeds in the manner of a nucleophilic substitution. The implementation of this reaction requires relatively high reaction temperatures, which are often between 200 and 300° C., leading to the formation of in some cases considerable amounts of decomposition products. In general it is impossible to operate without a solvent, so that the space-time yields are much lower than those of solvent-free processes. As an alternative to this it is possible to use conventional phase transfer catalysts, by means of which some of the abovementioned disadvantages can be reduced.
Other problems, such as poor stirrability of the reaction suspension in the case of solvent-free processes, continue to exist. Phase transfer catalysts used to date have been quaternary alkylammonium or alkylphosphonium salts (U.S. Pat. No. 4,287,374), pyridinium salts (WO 87/04194), crown ethers or tetraphenylphosphonium salts (J. H. Clark et al., Tetrahedron Letters 28 [1987], pages 111 to 114). Some of these phase transfer catalysts have comparatively low levels of activity and are only moderately stable at the temperatures required for the implementation of the reaction.
In view of these restrictions and disadvantages there is a great need for a process which avoids the disadvantages inherent in the known processes, especially high reaction temperatures and long reaction times, and which, moreover, provides the desired fluorine-containing compounds in yields ranging from good to very good at lower reaction temperatures and in shorter reaction times.
This object is achieved by a process for preparing fluorine-containing compounds by reacting a compound which contains fluorine-exchangeable halogen with a fluoride or a mixture of fluorides with the formula I
MeF  (I),
in which Me is an alkaline earth metal ion, NH
4
+
-ion or alkali metal ion, in the presence or absence of a solvent at a temperature from 40 to 260° C., which comprises carrying out the reaction in the presence of a compound or a mixture of compounds of the formula (II)
in which A
1
, A
2
, A
3
, A
4
, A
5
, A
6
, A
7
, A
8
independently of one another are identical or different and are a straight-chain or branched alkyl or alkenyl having 1 to 12 carbon atoms, cycloalkyl having 4 to 8 carbon atoms, an aryl having 6 to 12 carbon atoms, or an aralkyl having 7 to 12 carbon atoms, or A
1
A
2
, A
3
A
4
, A
5
A
6
, and A
7
A
8
independently of one another are identical or different and are connected to one another directly or by way of O or N—A
9
to form a ring having 3 to 7 ring members, A
9
is an alkyl having 1 to 4 carbon atoms and B
−
is a monovalent acid radical or the equivalent of a polyvalent acid radical.
It should be regarded as surprising that the use of the compounds of the formula (II) as catalyst leads to a strong acceleration of the reaction, thereby making it possible to implement the halogen-fluorine exchange reaction (halex reaction) under considerably milder conditions, in particular at lower temperatures and/or with shorter reaction times. At the same time it is also possible by this means to suppress or substantially avoid the formation of unwanted byproducts.
The novel process has other advantages as well. For instance, the compounds of the formula (II) are not toxic, or at most only slightly toxic, and in addition possess a resistance to thermal stress which is superior to that of the prior art catalysts that are frequently employed. If a compound of the formula (II) in which the radicals A
1
to A
8
are each a methyl group is used and is subjected over 10 hours to a temperature of not less than 230° C., then only 0.2% of carbonaceous decomposition products are subsequently found.
The term fluorine-exchangeable halogen refers to chlorine, bromine or iodine, especially chlorine or bromine, preferably chlorine, which can be exchanged for fluoride in the context of a nucleophilic substitution.
A further advantage of the novel process is that a large number of compounds can be employed as starting material.
Thus it is possible to employ, as the compound containing fluorine-exchangeable halogen, an aromatic compound whose ring system has from 0 to 3 nitrogen atoms and carries a chlorine or bromine substituent, in particular a chlorine substituent, which can be exchanged for fluorine which compound may have at least one further substituent which promotes nucleophilic substitution of the aromatic compound.
With no claim to completeness, suitable starting compounds for the novel process are aromatic compounds of the benzene, naphthalene, pyridine, anthracene, phenanthrene, pyrimidine and pyrazine type and of the type of benzofused ring systems of pyridine (quinoline, isoquinoline, acridine or acridone type), of pyrimidine, pyrazine and piperazine (benzodiazines of the cinnoline, phthalazine, quinazoline, quinazoline, phenazine or phenoxazine type) and derivatives thereof which may have at least one further substituent which promotes the nucleophilic substitution of the aromatic compound. This further substituent which promotes the nucleophilic substitution of the aromatic compound usually leads to an activation of the aromatic compound, which is thereby more readily amenable to a halogen-fluorine exchange reaction.
The further substituent which promotes the nucleophilic substitution of the aromatic compound comprises I and M substituents, which reduce the electron density or, respectively, the nucleophilicity of the aromatic compound and thereby make electrophilic substitution more difficult. However, the aromatic compound is thereby activated with respect to a nucleophilic substitution. The activating effect of these substituents is particularly great when they stand ortho or para to the halogen that is to be exchanged for fluorine, this halogen being, in particular, chlorine or bromine, preferably chlorine.
With no claim to completeness mention may be made, as further substituents which promote the nucleophilic substitution and thus the halogen-fluorine exchange reaction, especially the chlorine-fluorine exchange reaction, of F, Cl, Br, I, NO
2
, NO, CF
3
, CN, CHO, COF, COCl, SO
2
F, SO
2
Cl, OCF
3
, SCF
3
, SOCF
3
, SO
2
CF
3
, COOR, CONRR′, SO
2
R, COR, OR or a radical —CO—O—CO—, —CO—NR—CO—, which links two ortho positions, especially F, Cl, NO
2
, CF
3
, CN, CHO, COCl, SO
2
Cl, COOR, SO
2
CF
3
, CONRR′, SO
2
R, or COR, preferably F, Cl, NO
2
, CF
3
, CN, CHO or COCl, where R and R′ each independently of one another are identical or different and are H, a straight-chain or branched alkyl having 1 to 6, especially 1 to 4 carbon atoms, an aryl having 6 to 12 carbon atoms or aralkyl having 7 to 12 carbon atoms and where the alkyls and aralkyls are unsubstituted or substituted from one to three times by halogen, especially by fluorine or chlorine.
It is possible to employ an aromatic compound which possesses on the ring system a fluorine-exchangeable chlorine or bromine substituent, especially a chlorine substituent, and which has at least one further substituent from the series consisting of F, Cl, Br, I, NO
2
, CF
3
, CN, CHO, COF, COCl, SO
2
F, SO
2
Cl, OCF
3
, SCF
3
, SOCF
3
, SO
2
CF
3
, COOR, CONRR′, SO
2
R, COR and OR or a radical —CO—O—CO—, —CO—NR—CO—, which links two ortho positions, where R and R′ each independently of one another are identical or different and are H, a straight-chain or branched alkyl having 1 to 6 carbon atoms, an aryl having 6 to 12 carbon atoms or ar

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