Reduction carbon-halogen bonds

The dynamics of carbon-halogen bond reductive cleavage in alkyl halides was studied by MP3 ab initio calculations, using pseudopotentials for the halogens and semidiffuse functions for the heavy atoms [104], The effect of solvent was treated by means of the ellipsoidal cavity dielectric continuum model. Both a concerted (i.e., a one-step) and a stepwise mechanism (in which an anion radical is formed at first) were... 

Bertran, J., Gallardo, I., Moreno, M. and Saveant, J. M. Dissociative electron transfer. Ab initio study of the carbon-halogen bond reductive cleavage in methyl and perfluoromethyl halides. Role of the solvent, JAm.Chem.Soc., 114 (1992), 9576-9583... 

As solvents, alcohols and ethers are the most frequently used, but other types, especially dipolar aprotic solvents, can be used in more demanding dehalogenations. Alcohols are used in simple cases where there is no risk of a carbon halogen bond reduction. Among them,... 

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. 

An irreversible reaction of the intermediate of a redox reaction will greatly facilitate redox catalysis by thermodynamic control. A good example is the reduction of the carbon halogen bond where the irreversible reaction is the cleavage of the carbon halogen bond associated, or concerted, with the first electron transfer -pEe... 

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). ... 

The question 26-27> whether there is a preferred surface orientation for facile reduction of the carbon-halogen bond may also be answered by reference to the electrochemical behavior of 16-18. This question was investigated in two ways. The first method involved determination of the amount of iso-topically labelled chlorine remaining in 20 isolated from the electrochemical reduction of 16. This proportion was found to be 7 1 % under a variety of experimental conditions 25-29). This means that reduction of the exo chlorine... 

There is on the other hand a great deal of evidence showing that the electrochemical reduction of 1,2-dihalides to olefins can occur via a concerted pathway, i.e., via a transition state (39) in which both carbon-halogen bonds are partially broken and the carbon-carbon double bond is partially formed. An important, indeed critical, point of evidence supporting the conclusion that reduction is concerted lies in the remarkable ease with which vicinal dihalides are reduced. For example, the half-wave potentials of ethyl bromide and 1,2-dibromoethane are -2.08 V and -1.52 V (vs. s.c.e.), respectively 15 >46) those of ethyl iodide and /J-chloroethyl iodide are -1.6 V and -0.9 V, respectively 47). These very large differences must reflect the lower energy of delocalized transition state 39 relative to the transition state for reduction of an alkyl monohalide. 

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. 

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 ease of reduction of a carbon-halogen bond decreases in the order J > Br > Cl > F. The carbon-fluorine bond is the most difficult to reduce due to its having the highest electronegativity. 

Generally, the two-electron reduction of organic halides produces carbanion species. In fact, cathodic reduction of organic halides under certain conditions gives the product derived from the corresponding carbanion intermediates. Silicon is known to stabilize the carbanion at the a position by dn-pn interaction. Therefore, we can expect that silicon promotes the electron transfer from carbon-halogen bonds and the formation of the carbanion at the a position. 

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. 

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]. 

Studies of aliphatic a-haloaldehydes in water-dioxane have shown that the electrochemical behavior of these compounds is affected by the equilibrium between the hydrated and unhydrated forms of the aldehyde [92]. Each carbon-halogen bond is broken in a two-electron, one-proton process, and the resulting nonhalogenated aldehyde undergoes reduction at a more negative potential [93]. 

For the replacement of halogen by hydrogen - hydrogenolysis of the carbon-halogen bond - many reduction methods are available. Since breaking of carbon-halogen bonds is involved, the ease of hydrogenolysis decreases in... 

---