Organic compounds -- part of the class 532-570 series – Organic compounds – Nitriles
Patent
2000-01-11
2000-10-17
McKane, Joseph K.
Organic compounds -- part of the class 532-570 series
Organic compounds
Nitriles
560 8, 570185, 570196, C07C25500, C07C 6976, C07C 2200, C07C 1700
Patent
active
061334685
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for preparing substituted benzyl bromides of the formula I ##STR3## where at least one substituent R.sup.1-5 is an electron-attracting group such as fluorine, chlorine, bromine, C.sub.1 -C.sub.4 -alkoxycarbonyl, cyano or nitro, and the other substituents R.sup.1-5 are hydrogen or methyl, ##STR4## with a brominating agent at from 20 to 95.degree. C.
Side-chain bromination of alkylaromatic compounds has been known for a long time (Houben-Weyl, Volume 5/4, pages. 331 et seq. (1960)).
It is pointed out in this review that electron-attracting substituents, such as halogen atoms or nitro groups, make this reaction difficult. Compounds which can be substituted only with great difficulty, specifically nitrotoluenes, can often be induced to react only at temperatures above 100.degree. C. and under pressure, which involves considerable safety problems because of the low thermal stability of the compounds.
There is a description in more recent literature (EP-A 336 567) of the particularly difficult preparation of o-nitrobenzyl bromide by bromination of o-nitrotoluene with hydrogen bromide in the presence of hydrogen peroxide while irradiating with light, it having been possible to obtain selectivities of more than 90%. This process has been found to have the following disadvantages:
The bromine free radicals which are required are generated by irradiating with light which, on continuous operation, may result in coating of the lamps and thus considerable adverse effects.
Achieving the optimum depends on maintaining the narrow temperature range of 60-70.degree. C. Good selectivities are obtained only with relatively low conversions.
For optimal reaction conditions, the hydrogen peroxide/substrate molar ratio and the hydrogen peroxide/hydrogen bromide molar ratio can be varied only within relatively narrow limits.
It has now been found that benzyl bromides substituted by electron-attracting groups are obtained with very good selectivities when the bromination is carried out in the presence of an azo carbonitrile or of an azo carboxylic ester and in the presence of an oxidizing agent.
It is surprising that it is possible to generate bromine free radicals using organic initiators even in the presence of strong oxidizing agents. Azo carboxylic esters and azo carbonitriles are particularly stable in the presence of said oxidizing agents and are therefore predestined for use as initiators in the novel process. The novel process has a number of industrial and economic advantages which are briefly listed below and are explained in detail hereinafter: used reversed. The brominating agent is metered into a mixture of the substrate to be brominated and the oxidizing agent. The concentration of corrosive brominating agent, especially hydrogen bromide, in the reaction vessel can be kept very low in this way.
Solvents suitable for the novel process are those which are inert during the bromination, for example aromatic hydrocarbons such as benzene, tert-butylbenzene and tert-amylbenzene, halogenated hydrocarbons such as methylene chloride, chloroform and chlorobenzene, 1,2-dichloroethane, tetrachloromethane, dichlorobenzene or trichlorobenzene. It is also possible to use mixtures of said solvents.
Halogenated hydrocarbons such as methylene chloride, 1,2-dichloroethane, chloroform, tetrachloromethane, ortho- or para-dichlorobenzene, 1,2,4-trichlorobenzene and, in particular, chlorobenzene are preferred.
The ortho-nitrotoluenes II employed in the novel process can in most cases be purchased or are easily obtainable by processes described in the literature (eg. Organikum, Barth Verlagsgesellschaft (1993) 320 et seq.).
In contrast with the disclosure in EP-A 336 567, the procedure for the novel process is very flexible. It is possible to employ brominating agents such as elemental bromine, or bromine salts such as sodium bromide inter alia, and hydrogen bromide, preferably in the form of its aqueous solution, hydrobromic acid. Industrial azeotropic mixtures containing hydrobromic acid are partic
REFERENCES:
Patent Abstracts of Japan, vol. 014, No. 531 (C-0780), 1990.
Houben-Weyl, vol. 5/4, pp 334-336, 1960 with translation.
Gotz Norbert
Keil Michael
Muller Bernd
Wingert Horst
BASF - Aktiengesellschaft
McKane Joseph K.
Murray Joseph
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