Carbon-halogen bonds oxidation

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). ... 

Initially we tried the standard approach, reduction of NiL, NiB, or NiC with 2.0 equivalents of potassium in refluxing THF. Finely divided black nickel powders were obtained however, they showed rather limited reactivity toward oxidative addition with carbon-halogen bonds. Similar results were found for palladium and platinum. 

Klabunde has reported limited reactivity toward oxidative addition reactions of carbon halogen bonds with nickel slurries prepared by the metal vaporization technique(65). 

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. 

Cobalt represents an interesting contrast to the many activated metal powders generated by reduction of metal salts. As will be seen, the cobalt powders are highly reactive with regard to several different types of reactions. However, in contrast to the vast majority of metals studied to date, it shows limited reactivity toward oxidative addition with carbon halogen bonds. 

In marked contrast to the majority of activated metals prepared by the reduction process, cobalt showed limited reactivity toward oxidative addition with carbon halogen bonds. Iodopentafluorobenzene reacted with 2 to give the solvated oxidative addition products CoL and Co(C,F5)2 or Co(C F )L The compound CoiOJF 2PEt, was isolated in 54% yield by addition of triethylphosphine to tne solvated materials. This compound was also prepared in comparable yield from 1 by a similar procedure. This compound had previously been prepared by the reaction of cobalt atom vapor with C6F5I(81). 

The iron slurries show exceptional reactivity toward oxidative addition reactions with carbon halogen bonds. In fact, the reaction with C.FcI is so exothermic that the slurry has to be cooled to 0 °C before the addition of C F L The reaction of iron with C F Br is also quite exothermic, hence, even for this addition, the iron slurry is cooled to about 0 ° C. The organoiron compound formed in the above reactions, solvated Fe(C.F )2, reacts with CO at room temperature and ambient pressure to yiela Fe(C,F3)2(CO)2(DMEL. 

Other dimer dications have been prepared from dichalcogenoethers by treatment with two equivalents of the one-electron oxidant NO (Fig. 42), but these dimer dications are not sufficiently stable to be isolated and characterized by elemental and spectroscopic analyses, typically undergoing carbon-halogen bond cleavage. 

Electrochemical oxidation of alkyl bromides and iodides leads to loss of a nonbonding electron from the halogen substituent, followed by cleavage of the carbon-halogen bond to form a carbonium ion and a halogen atom. The products isolated are formed by further reactions of the carbonium ion while two of the halogen at-... 

The interaction of metal atoms with carbon-halogen bonds is characterized by oxidative cleavage of that linkage and is discussed more fully in Section IV. An exception to this generalization appears to be perfluorobut-2-ene which reacts with transition metal atoms in a manner similar to C2H4 and norbornene (54, 55) ... 

Since carbon-halogen bond cleavage is not a favorable process, and since both carbon and oxygen atoms have an even number of electrons, the oxidation proceeds either via free radicals or via carbenes and triplets. As a result, the oxidation mechanisms are considerably simpler than with hydrocarbons. Section III of this review treats free radical oxidation, whereas Section IV treats oxidation by processes not involving free radicals. In this discussion a free radical is considered to be a species with an odd number of electrons carbenes and triplets are called biradicals and are not included in this category. 

In a general sense, the Reformatsky reaction can be taken as subsuming all enolate formations by oxidative addition of a metal or a low-valent metal salt into a carbon-halogen bond activated by a vicinal carbonyl group, followed by reaction of the enolates thus formed with an appropriate electrophile (Scheme 14.1).1-3 The insertion of metallic zinc into a-haloesters is the historically first and still most widely used form of this process,4 to which this chapter is confined. It is the mode of enolate formation that distinguishes the Reformatsky reaction from other fields of metal enolate chemistry. 

The indirect oxidation and reduction of the carbon-halogen bond has been an area of major activity. Mediated electrochemical reactions, in which the primary electron transfer... 

The atmospheric fate of a halocarbon molecule depends upon whether or not it contains a hydrogen atom. Hydrohalomethanes are oxidized by a series of reactions with radicals prominant in the troposphere, predominantly hydroxyl OH. Fully halogenated methanes are unreactive towards these radicals and consequently are transported up through the troposphere into the stratosphere, where their oxidation is initiated by UV photolysis of a carbon-halogen bond. 

Davila et al. [68] reported the transformation of the pesticides bromoxynil, dichlorophen, and pentachlorophenol by the versatile peroxidase from B. adusta. For the three transformed pesticides by versatile peroxidase, an oxidative dehalo-genation was observed, a very important process since the halogenated pesticides are considered more persistent and toxic than the organophosphorus pesticides, because of the carbon-halogen bond. Enzymatic transformation of dichlorophen compound... 

The reaction scheme is easy enough to draw, but what is the mechanism Overall it involves an insertion of magnesium into the new carbon-halogen bond. There is also a change in oxidation state of the magnesium, from Mg(0) to Mg(II). The reaction is therefore known as an oxidative insertion or oxidative addition, and is a general process for many metals such as Mg, Li (which we meet shortly), Cu, and Zn. 

Scheme 1 depicts some of the outcomes possible (5) when a metal atom reacts with an organic molecule, which might be a monomer or a substituent on a polymer. Some of the more common reactions are generalized. Oxidative addition is relevant to insertion of a metal atom into a carbon-halogen bond, such as might be found in polyvinyl chloride or the monomer, allyl chloride, or... 

A long-standing success in transition metal catalysis is the carbonylation reaction [66], in particular the synthesis of acetic acid [67]. Formally this is the insertion of CO into another bond, in particular into a carbon-halogen bond. After the oxidative addition to the transition metal (the breaking of the carbon-halogen bond), a reaction with a CO ligand takes place. This reaction is often called an insertion. Mechanistic studies have, however, shown that the actual reaction... 

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