Chemistry: electrical and wave energy – Processes and products – Processes of treating materials by wave energy
Patent
1992-04-24
1993-11-23
Evans, Joseph E.
Chemistry: electrical and wave energy
Processes and products
Processes of treating materials by wave energy
570142, C07C 1733, C07C 2513, C07G 1300
Patent
active
052640949
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The present invention relates to a process for the preparation of optionally substituted fluorobenzenes by thermal decarbonylation of the corresponding benzaldehydes with the aid of transition metal catalysts. Many of the substituted fluorobenzenes are valuable intermediates in the preparation of compounds having herbicidal, fungicidal or insecticidal activity; they are likewise usable for the preparation of important pharmaceutical active substances.
Fluorobenzenes have hitherto been prepared from the corresponding substituted anilines by diazotization and subsequent replacement of the diazo group by fluorine. Thus the preparation of fluorobenzene by diazotization of aniline hydrochloride, conversion of the resulting benzenediazonium chloride into the tetrafluoroborate and subsequent heating has long been known (G. Balz and G. Schiemann, Ber. 60 (1927) 1188; D. T. Flood, Org. Synth. Coll. Vol II (1943) 295). In addition, the preparation of fluorobenzene by diazotization of aniline in anhydrous hydrogen fluoride at 0.degree. C. with subsequent decomposition of the resulting benzenediazonium fluoride at 20.degree. C. is also described (Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd ed. Vol. 10, p. 908). 1,3-Difluorobenzene could be obtained analogously in a 31% yield, relative to m-phenylenediamine as starting compound, by heating benzene-1,3-bis-diazonium tetrafluoroborate (G. Schiemann and R. Pillarsky, Ber. 62 (1929) 3035-3043, especially 3029). The diazotization of 3-fluoroaniline in anhydrous hydrogen fluoride in the presence of either ammonium fluoride or tertiary amines or dimethyl sulfoxide produced in each case 1,3-difluorobenzene in a yield of 46 to 73% (U.S. Pat. No. 4,075,252 and U.S. Pat. No. 4,096,196).
The decarbonylation of aromatic aldehydes with replacement of the formyl group by hydrogen is a reaction which is described many times in the literature. The reaction is catalyzed inter alia by transition metal such as chromium, manganese, nickel, copper or zinc, but in particular by metals of the platinum group. For reasons of cost, these metals are generally precipitated on inert support materials. However, the use of soluble noble metal complexes is also described, whereby it is possible to carry out the reaction in homogeneous solution (Houben-Weyl, Methoden der Organischen Chemie [Methods in Organic Chemistry], Volume V/2b (1981) 332-336, Georg Thieme Verlag, Stuttgart).
The decarbonylation of fluorine-substituted benzaldehydes has only hitherto been achieved, however, in an economically untenably low yield. Thus, 1,3-difluorobenzene was obtained by heating 2,6-difluorobenzaldehyde in 50% strength aqueous potassium hydroxide solution in a yield of about 70% (G. Lock, Ber. 69 (1936) 2253).
Using the Balz-Schiemann process, unsubstituted or substituted fluorobenzenes can frequently only be obtained in unsatisfactory yield. It is, moreover, generally associated with the processing disadvantage that large amounts of salt are produced. The object was therefore to develop a process by which fluorobenzenes, which can be still further substituted, can be obtained in high yields from the corresponding fluorobenzaldehydes.
It has now been found that fluorobenzenes can be obtained in high yields by thermal decarbonylation from fluorine-substituted benzaldehydes if the resulting fluorobenzenes are immediately removed from the reaction zone. Since the fluorobenzenes have a lower boiling point than the fluorobenzaldehydes, this takes place in a particularly simple manner, by employing conditions under which the desired fluorobenzene is volatile and can be removed.
The invention thus relates to a process for the preparation of fluorobenzenes having at least one hydrogen atom as ring substituent and, optionally, further substituents, which, independently of each other, can be chlorine, bromine, nitro, hydroxyl, C.sub.1 -C.sub.3 -alkoxy or C.sub.1 -C.sub.3 -alkyl, the number of nitro groups being not more than 2 and the number of hydroxyl groups and alkoxy groups being not more than 3 i
REFERENCES:
patent: 4847442 (1989-07-01), Nalewajek et al.
patent: 4937395 (1990-06-01), Litterer et al.
patent: 4968852 (1990-11-01), Kawai et al.
Collection Czechoslov. Chem. Commun. 37, 3042-3051 (1972).
Leupold Ernst I.
Litterer Heinz
Sistig Frank P.
Evans Joseph E.
Hoechst Aktiengesellschaft
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