Dehalogenation Raney nickel

Nitropyridazines are reduced catalytically either over platinum, Raney nickel or palladium-charcoal catalyst. When an N-oxide function is present, palladium-charcoal in neutral solution is used in order to obtain the corresponding amino N-oxide. On the other hand, when hydrogenation is carried out in aqueous or alcoholic hydrochloric acid and palladium-charcoal or Raney nickel are used for the reduction of the nitro group, deoxygenation of the N- oxide takes place simultaneously. Halonitropyridazines and their N- oxides are reduced, dehalogenated and deoxygenated to aminopyridazines or to aminopyridazine N- oxides under analogous conditions. 

Chlorine substituents may be replaced by hydrogen by reductive dehalogenation with, for example, Raney nickel or palladium-charcoal catalyst. The 1,7-phenanthroline skeleton is not reduced under these conditions.96,171,172 The bromine group in 4-bromo-2-trifluoromethyl-... 

Amidine derivatives are effective dehalogenation inhibitors for the chemoselective hydrogenation of aromatic halonitro compounds with Raney nickel catalysts. The best modifiers are unsubstituted or N-alkyl substituted formamidine acetates and dicyandiamide which are able to prevent dehalogenation even of very sensitive substrates. Our results indicate that the dehalogenation occurs after the nitro group has been completely reduced i.e. as a consecutive reaction from the halogenated aniline. A possible explanation for these observations is the competitive adsorption between haloaniline, nitro compound, reaction intermediates and/or modifier. The measurement of the catalyst potential can be used to determine the endpoint of the desired nitro reduction very accurately. 

Raney nickel is very probably the most commonly used nickel catalyst. It is also the most versatile of catalysts. In one or other of its several modifications it has been used for hydrogenations over a wide range of pressures varying from high to subatmospheric, for desulfuration, for dehalogenation, and for very many other reactions. 

Grigorovskii (113a), investigating the dehalogenation of chloroacri-dines over Raney nickel, found that halogen atoms in position other than 9 were unaffected. Chloroacridine (10 g.) and Raney nickel paste (10 g.) in 250 ml. of methanol were boiled with stirring for 4 hours. Upon treatment of the reaction mixture, a 66% yield of 9,9 -biacridine was isolated. When 3,9-dichloroacridine was treated in a similar manner, 3,3 -dichloro-... 

Little is known of the effect of nuclear substituents on the dehalogenation of aromatic halogen compounds. Schwab (114) debrominated several 4-bromoanthraquinones by means of hydrogen over Raney nickel. A p-hydroxyl group causes a much more rapid removal of bromine than a p-methoxy group. Methoxy groups in the 6 or 8 positions had very little inhibitory effect. 

Raney alloy has also been used (116) for the dehalogenation of 4-chloro-5-methylquinaldine and 4-chloro-7-methylquinaldine to form the 2,5- and 2,7-dimethylquinaldines. Zinc, in alkaline solution, in the presence of Raney nickel has been used to dehalogenate organic compounds (117). The halogen is then determined by a Volhard determination. 

Raney nickel was recommended as a convenient reagent for desulfurization of phenothiazine, and it has been used in many structural investigations. More recently, the method has been applied to various phenothiazines, - in its classical form, that is Raney nickel and hydrogen, as well as in some modified variants. Thus, working with a special Raney nickel, without hydrogen, 1,3,7,9-tetrachlorophenothiazine was desulfurized without dehalogenation the usual procedure leads, for example, to diphenylamine, when applied to 3,7-dichlorophenothiazine. ... 

Reductive dehalogenation of 2-chloro-3-(3-piperidinopropylamino)-6-methoxyquinoxa-line with hydrogen and Raney nickel proceeds in the presence of sodium hydroxide omission of the sodium, hydroxide m.akes dehalogenation inconveniently slow. " ... 

The selective reduction of nitroarenes containing benzyl-protected phenolic groups without concomitant hydrogenolysis has also been achieved using hydrazine hydrate and Raney nickel. This procedure avoids strongly acidic conditions. The reaction is selective and a variety of (V-benzyl- and chloro-sub-stituted nitroarenes are reduced to the corresponding anilines without dehalogenation or debenzylation. 

The removal of a halogen from an aromatic heterocycle takes place more readily than the hydrogenolysis of a phenyl halide of the same type (Eqn. 20.61). 45 The relative difference between the ease of dehalogenation of a phenyl and a heterocyclic halide is more pronounced over Raney nickel in basic medium than it is over palladium (Eqn. 20.62). 46... 

The usual basic, palladium catalyzed dehalogenation conditions are, however, preferred over Raney nickel for nonselective halide hydrogenolysis. Dehalogenation over palladium occurs in preference to ring and cyano group... 

Dehalogenation. With either Raney nickel or Raney cobalt catalyst it is possible to reduce aromatic trifluoromethyl groups to methyl groups. ... 

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