Catalytic dehalogenation

Hydroxyphthalazin-l(2//)-one is obtained in a smooth reaction between phthalic anhydride and hydrazine hydrate and this is again the starting compound for many 1-substituted and/or 1,4-disubstituted phthalazines. The transformations of 1,4-dichloro-phthalazine, which is prepared in the usual manner, follow a similar pattern as shown for pyridazines in Scheme 110. On the other hand, phthalonitrile is the preferential starting compound for amino- and hydrazino-phthalazines. The most satisfactory synthesis of phthalazine is the reaction between a,a,a, a -tetrachloro-o-xylene and hydrazine sulfate in sulfuric acid (67FRP1438827), alt iough catalytic dehalogenation of 1-chloro- or 1,4-dichloro-phthalazine or oxidation of 1-hydrazinophthalazine also provides the parent compound in moderate yield. 

It is interesting to remark that classical hydrogenation is a method used to prepare deuterium-labeled compounds by aromatic dehalogenation, but the usual reaction conditions suffer from several limitations. A microwave-enhanced catalytic dehalogenation procedure for rapid and specific deuterium labeling of N-4-picolyl-4-halogenobenzamide 68, by use of deuterated formate, [41] was recently reported (Scheme 8.25). 

Catalytic Dehalogenation with Various Reducing Agents I 517... 

The applications of polyoxometalates in catalytic dehalogenation of halocar-bons have been succinctly reviewed by Hill and coworkers [188]. This reaction involves the photocatalytic transformation of organic halides coupled with the oxidation of sacrificial organic reductants (secondary alcohols or tertiary amides) (Eq. (9)) [189, 190] ... 

In contrast to [L Pd], oxidative addition of aryl halides on [L Ni] often proceed by single electron transfer mechanism [2, 197]. SN2 and SNAr types of oxidative addition as a step of the catalytic dehalogenation have also been proposed in the literature for low-valent Ti [40, 114], Zr(II) [115], Ru(II) [20, 74, 81, 98],... 

Catalytic dehalogenation cycles with a binuclear reductive elimination step have not yet been reported, but many examples are known with a single metal. The RH product eliminates easily from d8 metals (Ni(II), Pd(II)) or d6 metals (Ru(II), Rh(III)) (Eq. (25)) [196]. This reaction is believed to go by a three-center transition state [193, 194, 212] ... 

Streptomyces aureofaciens, then oxytetracycline was derived from Streptomyces rimosus and then tetracycline was obtained by catalytic dehalogenation of chlortetracycline. They are classified as in table 9.2.1. 

Catalytic dehalogenation (mostly by H2/Pd) and desulfurization (by Raney-Ni) are important tools for structural analysis. This way chlorine can be removed from positions 2, 5, and 7 [64CPB204 66JCS(C)2031]. Zinc was used for dechlorination at C-7 (59YZ903). The 5,7-dichloro-TPs (61CPB801) and 6,7-dichloro-TPs (59CPB903) in the presence of H2/Pd first lose the more reactive chlorine from C-7 the remaining 6-chloro compound can be dechlorinated only by Raney-Ni. 

Bases are added frequently as promoters in catalytic dehalogenations to neutralize the liberated halogen acid that may inhibit the action of catalyst.196 Denton et al. studied the effects of added potassium acetate to the rates of hydrogenolysis of various halogen compounds with a Pd-C as catalyst in methanol (Table 13.8).196... 

Pyridazines with hydroxyl groups (generally in the oxo form) at different positions in the ring can be prepared by several methods as outlined in Section III. Syntheses from open-chain compounds are particularly suitable for obtaining 3(2/I)-pyridazinones and their 6-hydroxy analogs. If more complex pyridazinones are available, such as halogenated pyridazinones, catalytic dehalogenation is one of the preferred methods. In this manner 3(2 I)-pyridazinone ... 

Methoxypyridazine when treated with hydrogen peroxide in acetic acid yielded 3-methoxypyridazine 1-oxide, obtainable also by catalytic dehalogenation of 3-methoxy-6-chloropyridazine... 

BGBI I 1993, 1720, EMA 8053-6-20, last changes on 25.2.2003 Y. Sakata, T. Bhaskar, M. A. Uddin, A. Muto and T. Matsui, Development of a catalytic, dehalogenation (Cl, Br) process for municipal waste plastic-derived oil, J. Mater. Cycles Waste Manag., 5, 113-124 (2003). 

Among many catalysts, Pd/C is less susceptible to the poisoning effect of halide ions hence, this catalyst is preferred by most investigators. Platinum and rhodium are not effective for catalytic dehalogenations. Schemes 1 and 2 illustrate representative Pd/C catalyzed hydrogenolyses of an alkyl iodide and an alkyl bromide. ... 

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