Activation of Carbon-Halogen Bonds

Abstraction of one of the metal-bound hydrides from complex 5a provides the cationic iridium(lll) complex 28, which is an efficient precatalyst for alkyl halide reduction in the presence of EtsSiH (Equation 12.11) [31]. 

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The complex [HB 3,5-(Cp3)2pz 3]Ag(THP) (7) has been employed in the silver-catalyzed activation of carbon-halogen bonds through a carbene insertion process that occurs at room temperature, resulting in formation of a new sp -sp carbon carbon bond. ... 

More recently, refinements and new approaches for controlling radical polymerization have been described (155). Two of the most studied methods feature either stable counter-radicals, eg, nitroxyl-mediated pol5unerization (NMP), or reversible activation of carbon-halogen bonds by transition-metal species in a... 

RoUin Y, Troupel M, Perichon J, Fauvatque JF (1981) Electroreduction of nickel(II) salts in tetrahy-drofuran-hexamethylphosphoramide mixtures. Application to electrochemical activation of carbon-halogen bonds. J Chem Res (S) 322-323... 

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. 

Transition-metal catalyzed metathesis of carbon-halogen bonds with Si-Si bonds provides useful access to organosilicon compounds. Most of the reaction may involve initial oxidative addition of the carbon-halogen bond onto the transition-metal followed by activation of the Si-Si bond to give (organosilyl)(orga-no)palladium(II) complex, which undergoes reductive elimination of the carbon-silicon bond. 

The only type of halogenating enzymes known until 1997 were peroxidases and perhydrolases which catalyze the formation of carbon halogen bonds using halide ions, hydrogen peroxide and an organic substrate activated for electrophilic attack. 

The reasons why complexes 8 and 9 are active in ATRP are presently nnclear. These complexes possess indeed two 18-electron rathenium centres and, as snch, shonld be nnable to activate the carbon-halogen bond of the initiator or of the growing polymer chain end. On the other hand, the fact that an indnction period was fonnd for the ATRP of MMA indicates that the mtheninm-vinylidene complexes have to be activated prior to the ATRP process. There are in principle several plansible explanations for the formation of a coordinatively unsatnrated 16-electron rathenium species from the rathenium-vinylidene complexes 8 or 9 either the splitting of the bimetallic scaffold into two different unsaturated rathenium intermediates (Path A, Scheme 5), the opening of a p.-chloro bridge (Path B), or the release of the vinylidene ligand (Path C, Scheme 5). 

On the other hand, the strength of carbon-halogen bonds is lower than carbon-hydrogen bonds, hence recently the halogen-terminated surface is preferred as an activated platform for further modification. 

Isse AA, Gennaro A, Lin CY, Hodgson JL, Coote ML, Guhashvili T. Mechanism of carbon-halogen bond reductive cleavage in activated alkyl halide initiators relevant to living radical polymerization theoretical and experimental study. J Am Chem Soc 2011 133 6254-6264. 

The most noteworthy limitation to this approach is the complexity and multi-functionality that exist in these waste streams. For example, a bleached kraft pulp mill is estimated to produce greater than 400 compounds including tannins, resins, and lignin degradation products (5). However, as only a relatively small number of chemical transformations offer benefits to an end-user, i.e. polymerisation, selective oxidation or reduction, C-H bond activation, and carbon-halogen bond cleavage, the problem is simplified to some extent. 

Ketene Insertions. Ketenes insert into strongly polarized or polarizable single bonds, such as reactive carbon—halogen bonds, giving acid hahdes (7) and into active acid haUdes giving haUdes of p-ketoacids (8) (46). Phosgene [77-44-5] (47) and thiophosgene [463-71-8] (48) also react with ketenes. 

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). 

Vinylic halides are virtually unreactive and a high selectivity is to be found in the preferential cleavage of aliphatic carbon-halogen bonds of haloalkanoic amides and esters, and of nitro- and cyanoaryl derivatives. Activated haloarenes, e.g. 1-chloro-2,4-dinitrobenzene, however, give a complex mixture of products [7]. 

Mann and Barnes [45] have discussed the mechanism of reduction of substituted and optically active 1-bromo-and 1-iodocyclopropanes, and Hazard and coworkers [46] have investigated the reduction of l-bromo-l-carboxy-2,2-diphenyl-cyclopropane. At mercury cathodes, electrolyses of 1-bromo- and 1-iodonorbornane proceed via two-electron cleavage of the carbon-halogen bond to give mainly nor-bomane, plus a small amount of bis(l-norbomyl)mercury [47]. 

Apart from the carbon-halogen bond, the carbon-oxygen one is rather active toward the reductive cleavage due to its polarity, so different types of compounds bearing a carbon-oxygen bond are able to undergo this reaction. 

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