Carbon-halogen bonds catalytic reduction

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

In this part of the chapter, the discussion is focused on the direct cathodic reduction of halogenated organic compounds, although the last section will address the increasingly active area of catalytic reductions of carbon-halogen bonds. 

XII. Indirect or Catalytic Reduction of the Carbon-Halogen Bond... 

XII. INDIRECT OR CATALYTIC REDUCTION OF THE CARBON-HALOGEN BOND... 

Chromium contributes to organic transformations as reagents. Some examples include chromic acid Qones reagent), chromium acetate for the reduction of carbon-halogen bonds and conversion of ketoximes to ketones, chromyl acetate, chromium perchlorate, chromium sulfate (reduction of allq nes to alkenes), and Jacob s reagent (Cr02Cl2). The catalytic... 

The addition of an organopalladium intermediate to the double bond with subsequent cleavage of palladium hydride and restoration of unsaturation is the basis of another important process involving organopalladium compounds, the Heck reaction. In a catalytic version, the organopalladium intermediate is formed by oxidative addition to carbon-halogen or other carbon-heteroatom bonds, and the hydridopalladium complex is believed to be recycled by base-promoted reductive elimination (Scheme 5.7). 

Certain types of functionality can be removed and replaced by hydrogen under catalytic hydrogenation conditions. This is called hydrogenolysis. For example, aromatic halogen substituents are usually removed by reduction over metal catalysts. Aliphatic halides are less reactive but hydrogenolysis is promoted by base. Carbon oxygen bonds at benzyl and allyl positions are cleaved by catalytic hydrogenation. ... 

There is a much wider variety of indirect replacement approaches. In most cases these approaches introduce the labels at structurally predefined positions generated by (formally) oxidative processes, which are then followed in a second step by reductive operations. Examples of such approaches preferentially designed for the introduction of tritium include halogenation followed by tritiodehalogenation, the introduction of carbon-carbon multiple bonds followed by catalytic tritiation, the oxidation of carbon-heteroatom bonds followed by reduction using tritide reagents, etc. These approaches, which do not alter the skeleton of the target in the process, are discussed in Chapter 4, Sections 1-3 and Chapter 10, Sections 10.1.1.2. ... 

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