Method for producing compounds containing fluorine, in particula

Details Method for producing compounds containing fluorine, in particula Method for producing compounds containing fluorine, in particula

2000-02-28

2000-12-26

Tsang, Cecilia

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

Organic compounds

Nitriles

568437, C07C25500, C07C 4500

Patent

active

061662429

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

Process for preparing fluorine-containing compounds, in particular fluorobenzaldehydes and fluorobenzonitriles
The present invention relates to a process for preparing fluorine-containing compounds, preferably fluorine-containing aromatic compounds, in particular fluorobenzaldehydes and fluorobenzonitriles, in high purity.
In particular, the invention relates to a process which is improved compared to the prior art and in which fluorination is achieved with high selectivity and in high purity by means of a halogen-fluorine exchange reaction (halex process).
Fluorine-containing compounds are employed, inter alia, in liquid crystal mixtures (EP-A-0 602 596).
The replacement of hydrogen bound to an aromatic ring by fluorine is also very important for the synthesis of bioactive substances or for the preparation of precursors of such compounds.
Furthermore, it is generally known that fluorine has strong and often unexpected effects on the biological activity of chemical compounds. The replacement of hydrogen by fluorine in a biologically active molecule often leads to an analogous compound having increased or modified biological action.
Apart from direct fluorination, the preparation of fluorine compounds by replacement of a halogen (Cl, Br) by fluorine (known as the "halex process") is an extremely valuable reaction which is of great industrial importance.
In the case of aromatic compounds, in particular activated aromatic compounds, the halogen-fluorine exchange proceeds in the form of a nucleophilic substitution.
Carrying out this reaction requires comparatively high temperatures, frequently in the range from 200 to 300.degree. C., as a result of which sometimes considerable amounts of decomposition products are formed. In general, a solvent cannot be dispensed with, so that the space-time yields are significantly lower compared to solvent-free processes.
The halex reaction is frequently accompanied by further secondary reactions which form significant amounts of by-products, especially reductively dehalogenated aromatics, whose removal from the product is extremely difficult and very expensive because of a similarity of boiling points. Owing to these secondary reactions, the applications of the halex reaction are relatively restricted.
The use of phase transfer catalysts belongs to the prior art designed to circumvent some of the abovementioned problems. However, other problems such as poor stirrability of the reaction suspension in the case of solvent-free processes remain.
D1 teaches the use of quaternary ammonium or alkylphosphonium salts as phase transfer catalysts. According to D2, pyridinium salts are used as phase transfer catalysts, and D3 uses crown ethers or tetraphenylphosphonium salts as phase transfer catalysts. Some of these phase transfer catalysts have comparatively low activity and are only moderately stable at the temperatures required for carrying out the reaction.
D4 proposes carrying out the chlorine-fluorine exchange in the presence of polymerization inhibitors such as dinitrobenzene. However, the use of dinitrobenzene unfortunately has some serious disadvantages. As has recently become known, dinitrobenzene reacts with KF with replacement of NO.sub.2 and formation of nitrite anions which leads to the formation of phenol derivatives and consumption of additional amounts of fluorination reagents.
According to D5, the chlorine-fluorine exchange reaction is carried out in nitrobenzene as solvent, so that the space-time yield is considerably lower compared to solvent-free processes. In addition, the separation of the products from nitrobenzene can be difficult, particularly in the case of compounds having similar boiling points.
D6 discloses a procedure in which the dehalogenation of monochlorinated benzaldehydes during the halex reaction is reduced. 4-Chloro-benzaldehyde is reacted with sulfolane, KF and tetraphenylphosphonium bromide in the presence of nitrobenzene or nitronaphthalene. Although this reduces the rate of formation of the by-product benzaldehyde, the amount for

REFERENCES:
patent: 5466859 (1995-11-01), Schach et al.
Mochizuki Hiromitsu, Ihara Chem. Ind. Co. LTD, Pat. Abs. of Japan 96: No 8 of JP 08 092148 (1996).
Ihara Chem. Ind. Co. LTD, Derwent Abstract of JP 5194303, XP-002085380 (1993).
Shin-Akita Kasei KK, Derwent Abstract of JP63 039824, XP-002085381 (1998).

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