Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Patent
1998-09-14
2000-02-29
Huang, Evelyn Mei
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
546144, 546173, 546329, C07D21338, C07D21342, C07D21361
Patent
active
060311061
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to N-phenylhydroxylamine compounds, a process for preparing these compounds and a process for converting these compounds into pyridyl-4-fluoraniline compounds.
2-Pyridyl-4-fluoraniline compounds are described in WO-A-95/02580. They correspond to the formula I ##STR1## and are suitable as herbicides for defoliation and desiccation. In addition, they are utilizable as intermediates for preparing phenylpyridines, which are likewise described in WO-A-95/02580. These compounds are synthesized by reaction of a halopyridine of the formula V with a boronic acid of the formula VI in the presence of a transition metal catalyst according to the following reaction scheme: ##STR2##
The compounds of the formula I can then be obtained by nitration of a compound of the formula VII to give a compound of the formula VIII and subsequent reduction according to the following reaction scheme: ##STR3##
The boronic acid VI is prepared by reaction of the corresponding aryllithium or arylmagnesium compounds with trialkyl borates and subsequent hydrolysis.
This synthesis route, however, is disadvantageous for the following reasons: are used. These are generally not easily accessible, but have to be prepared via a separate, in some cases multi-stage synthesis. Moreover, the halogen-containing compounds are expensive. the synthesis halogen-containing and in particular fluorine-containing waste substances are formed, whose disposal can be problematic. nucleophilically substituted. This demands that in all synthesis stages those reaction components and reaction conditions must be avoided which could lead to a nucleophilic substitution. In addition, reduction conditions are to be avoided, because in this case a defluorination can occur. conditions, highly corrosive hydrofluoric acid is formed, so that the reactions have to be carried out in expensive special apparatuses. mentioned has to be carried out at low temperatures, because otherwise the elimination of metal fluoride with aryne formation appears to interfere or is even the main reaction.
One possibility for preparing 4-fluoroaniline compounds is the introduction of the fluorine atom into the aromatic nucleus by the HF variant of the Bamberger reaction. This variant was until now used especially for the preparation of simple compounds, such as p-fluoroaniline, see Titov et al., Zh. Obshch. Khim 23, 346 (1953), U.S. Pat. No. 4,391,991, WO-A-91/17138 and U.S. Pat. No. 5,166,401. Moreover, Patrick et al. reported in J. Org. Chem. 39, (1974) pages 1759 to 1761 on investigations into substituted N-phenylhydroxy-lamines. In no example were their yields over 61%. It is pointed out here that the range of application of the Bamberger reaction mentioned is generally restricted by the poor availability of the N-phenylhydroxylamine compounds needed. The reason for this is that presently there are no reduction methods which can be generally employed for converting nitroaromatics to the N-phenylhy-droxylamines in high yield. In their reductions with zinc dust/ammonium chloride, ammonium sulfide or sodium borohydride, Patrick et al. obtained the desired N-phenylhydroxylamines in at best moderate yields.
In order to get around the disadvantage of the poor accessibility of the N-phenylhydroxylamines, a number of reactions have been described in which the reduction of the nitro compounds was performed in the presence of hydrogen fluoride in order to combine both stages, namely reduction and Bamberger rearrangement, in one stage (U.S. Pat. No. 2,884,458; U.S. Pat. No. 3,580,951; U.S. Pat. No. 3,558,707; DE-A-19 45 625 and GB-A-2,241,952). From this, however, mixtures of the desired p-fluoroaniline and the nonfluorinated aniline which is formed by direct reduction of the nitro compound generally result, see, for example, J. Fluorine Chem. 74 (1995) 251-254. The separation of these chemically very similar compounds, however, is very laborious and associated with high yield losses, see, for example DE-A-1,945,625.
One method for preparing N-phenylhydroxylamines is the long-
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Gotz Norbert
Harreus Albrecht
Isak Heinz
Rack Michael
Schafer Peter
BASF - Aktiengesellschaft
Huang Evelyn Mei
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