Process for the preparation of substantially fluorinated alkyl b

Details Process for the preparation of substantially fluorinated alkyl b Process for the preparation of substantially fluorinated alkyl b

1993-02-16

1994-08-09

Siegel, Alan

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

Organic compounds

Halogen containing

570163, C07C 1720, C07C 1908

Patent

active

053368185

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a process for the preparation of substantially fluorinated alkyl bromides, in particular of perfluoroalkyl bromides, starting from substantially fluorinated alkyl iodides, in particular from perfluoroalkyl iodides.
Substantially fluorinated alkyl bromides, in particular perfluoroalkyl bromides are used, for example, as intermediates for the preparation of polymer liquids, resins and elastomers, as X-ray contrast medium, for the preparation of pharmaceutical preparations and in aqueous emulsion as a blood substitute. A perfluoroalkyl bromide which is preferred as blood substitute is perfluorooctyl bromide.
A series of processes are already known for the preparation of perfluoroalkyl bromides. Thus, for example, according to Japanese Patent JP 60 184 033 (C.A. Vol. 104 (1986), 88106p) perfluoroalkyl iodides are reacted in the presence of free radical initiators with elemental bromine to give perfluoroalkyl bromide. Hazeldine (J. Chem. Soc. 1953, 3761-3768) describes on page 3763 and 3766 the reaction of perfluoroalkyl iodides with elemental bromine and with irradiation using UV light. Both methods have considerable problems associated with them in terms of materials and safety precautions, due to the use of elemental bromine, the release of elemental iodine, interhalogen compounds and hydrogen fluoride.
Examples of further preparation processes for perfluoroalkyl bromides are (R.sub.F =perfluoroalkyl): reaction of bromine with compounds R.sub.F --SF.sub.5 at 500.degree. C. in the presence of nickel (U.S. Pat. No. 3,456,024); reaction of bromine with compounds R.sub.F --SO.sub.2 Na in the presence of KI/I.sub.2 (C.A., Vol. 107 (1987), 236043); reaction of bromine with salts of perfluorinated carboxylic acids (U.S. Pat. No. 2,678,953), in particular with R.sub.F COOAg (U.S. Pat. No. 2,678,953) and Hauptschein et. al., J. Am. Chem. Soc. 74 (1952), 1347ff); reaction of bromine with compounds R.sub.F H with simultaneous irradia-tion using UV light (J. Chem. Soc. 1953, 3761). In all these processes, the use of elemental bromine leads to significant problems in terms of materials and safety precautions. Moreover, the starting compounds are difficult to obtain or have to be prepared from the corresponding perfluoroalkyl iodides via an additional process step. This is also true of the preparation of perfluoroalkyl bromides by reaction of R.sub.F SO.sub.3 Cl with HBr gas in the presence of a catalyst at 125.degree. C. (EP-A1-0,298,870).
According to Fainberg et. al. JACS 79. 4172 (1957), perfluoroallyl bromide can be prepared by reaction of perfluoroallyl iodide with lithium bromide in acetone. Applying this transhalogenation to other perfluoroalkyl iodides is obvious but unsuccessful, since in normal perfluoroalkyl iodides there is no activation of iodine by an allyl group. As can be seen from Comparative Example 1 which follows, the reaction conditions described by Fainberg et. al. cannot be applied successfully to perfluorooctyl iodide. Comparative Example 2 which follows shows that the attempt of accelerating the reaction by phase transfer catalysis does not lead to a satisfactory result either.
The result of the comparative examples is as expected, since it is known that fluorine atoms considerably reduce the reactivity of alkyl halides in nucleophilic substitution reactions (cf., for example, Houben-Weyl, Methoden der organischen Chemie (Methods of Organic Chemistry), 4th Edition, Vol. 5/4, p. 685, 688).
The previously known processes for preparing substantially fluorinated alkyl bromides, in particular perfluoroalkyl bromides, are not satisfactory for the abovementioned reasons. Accordingly, the object of the present invention is to provide a technically simple process for preparing substantially fluorinated alkyl bromides, in particular perfluoroalkyl bromides, starting from the easily accessible substantially fluorinated alkyl iodides, in particular perfluoroalkyl iodides.
This object is achieved by a process for preparing substantially fluorinated alkyl bromides, starting fro

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