Transition metals dehalogenation reactions

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

Martin and associates [124, 125] have studied the dehalogenation of CHC13 in boiling methanol by Schiff-base complexes of some transition metals in the presence of TMEDA. The kinetics of chloride ion formation has been measured without characterizing the organic products. Nahar and Mukhedkar [126, 127] found that the reactivities of related Schiff-base complexes in the above reaction decreased in the order Pd>Pt>Ni>Cu>Zn. 

It is well known that the comparatively inert Si-H bonds of triorganosilane R3SiH (R = alkyl, aryl) can be activated by transition metal complexes. Chatgililaloglu et al. have used Et3SiH in palladium-catalyzed dehalogenation reactions, which occur with the involvement of free radicals.245... 

Transition metal-catalyzed cross-coupling reactions of halogenopyrazines is an efficient synthetic method for alkyl-, alkenyl-, and alkynylpyrazines (Section 6.03.5.4.2). Palladium and nickel complexes are particularly effective as catalysts in this reaction. The Wittig reaction of halogenomethyl-pyrazines likewise leads to the formation of alkenylpyrazines (Section 6.03.8.1). Dehalogenation of halogeno pyrazines is a very practical synthetic method for alkyl- or aryl-substituted pyrazines. For example, phenylpyrazine has been prepared by catalytic hydrogenolysis of the 2-chloro-3-phenyl compound in the presence of triethylamine. This product is also obtained by decarboxylation of... 

The chapter The SN -Amination of Aromatic Compounds , authored by Anna Gulevskaya and Alexander Pozharsky (Rostov-on-Don University, Russia), presents a comprehensive review on the direct Sn amination of electron-deficient heteroaromatic compounds. Recent advances in this area and many new aspects of the Sn amination are discussed, including new types of reagents, metal-free catalysts, solvents and the hydride ion acceptors. The review shows that the amination is rather promising synthetic alternative to both classic and transition metal-catalyzed amino-dehalogenation reactions. 

Using the above procedure, arene complexes of many of the block transition metals may be prepared, and substituted benzenes such as toluene, mesitylene (Mes) and hexamethylbenzene may be used instead of benzene. Generally, mesitylene and hexametl lbenzene metal complexes are more readily prepared than the unsubstituted benzene analogues. When halogenated aromatics are used, they undergo dehalogenation reactions affording hydrocarbon Ti-arene-metd complexes [25, 26]. 

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] ... 

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