Carbon-halide bond activation reaction

The bond dissociation energy of fluoromethane is 115 kcal mol , which is much higher than the other halides (C-Cl, C-Br and C-1, respectively 84, 72 and 58 kcal mol ) [6], Due to its strength, the carbon-fluorine (C-F) bond is one of the most challenging bonds to activate [7], A variety of C-F bond activation reactions have been carried out with different organometallic complexes [8], Among them, nickel [9] and ruthenium complexes have proven to proceed selectively under mild conditions [10],... 

Benzimidazoles bearing a halogen in the activated 2-position show a remarkable reactivity in palladium catalyzed carbon-nitrogen bond forming reactions. TV-protected 2-chlorobenzimidazoles reacted smoothly with a series of amines (6.81.). The activity of the aryl halide, besides the ready coupling of the chloro derivative, is also emphasized by the low catalyst loading used.112... 

In 2009, a one-step protocol for the synthesis of xanthones via Pd-catalyzed coupling between 1,2-dibromoarenes and salicylaldehydes was developed. The success of the reaction relies heavily on the careful selection of a proper palladium catalyst, solvent and base moderate yields of the desired products were formed (Scheme 3.2). In this communication, a reaction mechanism was proposed by the authors. The Pd(0) catalyst first underwent oxidative insertion into one of the carbon-halide bonds to generate the aryl-Pd(ii) intermediate, which reacted with the phenolate to displace the halide, then underwent C-H activation of the C-H bond of the aldehyde to form intermediate A. After abstraction of the hydrogen atom by the base and reductive elimination to exclude the Pd(0) catalyst, the biaryl ketone intermediate was... 

On the other hand, NHC-Ni complexes have been used as suitable starting point to study various mechanisms ie. oxidative addition of organic halides, bond activation, carbon-carbon couplings or the Heck reaction, see Chapter 2). In recent years, a renewed interest has been dedicated... 

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

The activated nickel powder is easily prepared by stirring a 1 2.3 mixture of NiL and lithium metal under argon with a catalytic amount of naphthalene (1(7 mole % based on nickel halide) at room temperature for 12 h in DME. The resulting black slurry slowly settles after stirring is stopped and the solvent can be removed via cannula if desired. Washing with fresh DME will remove the naphthalene as well as most of the lithium salts. For most of the nickel chemistry described below, these substances did not affect the reactions and hence they were not removed. The activated nickel slurries were found to undergo oxidative addition with a wide variety of aryl, vinyl, and many alkyl carbon halogen bonds. 

The first step in the cycle, analogous to the cross-coupling reactions, is the oxidative addition of an aryl (vinyl) halide or sulfonate onto the low oxidation state metal, usually palladium(O). The second step is the coordination of the olefin followed by its insertion into the palladium-carbon bond (carbopalladation). In most cases palladium is preferentially attached to the sterically less hindered end of the carbon-carbon double bond. The product is released from the palladium in a / -hydrogen elimination and the active form of the catalyst is regenerated by the loss of HX in a reductive elimination step. To facilitate the process an equivalent amount of base is usually added to the reaction mixture. 

Catalysis. Catalytic properties of the activated carbon surface are useful in both inoiganic and oiganic synthesis. For example, the fumigant sulfuryl fluoride is made by reaction of sulfur dioxide with hydrogen fluoride and fluorine over activated carbon (114). Activated carbon also catalyzes the addition of halogens across a carbon—carbon double bond in the production of a variety of oiganic halides (85) and is used in the production of phosgene... 

Elimination reactions can also occur when a carbon halogen bond does not completely ionize, but merely becomes polarized. As with the El reactions, E2 mechanisms occur when the attacking group displays its basic characteristics rather than its nucleophilic property. The activated complex for this mechanism contains both the alkyl halide and the alkoxide ion. 

Processes (b) and (c) are limited by diffusion and heat removal. The activation energies of these processes are low. Process (a) involves common chemical reactions and is improbable at low temperatures ( 80 K). Indeed, as already mentioned, only the most active organic halides with weakened carbon-halogen bonds react with magnesium immediately in the course of condensation. Therefore, only the aggregation and stabilization processes are actually important. Let us consider them in the light of quantum-chemical calculations. 

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