Halogen bonds, reductive cleavage

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... 

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. 

N- lomoallylic enamides 239 derived from pymvate were subjected to free radical cyclization in presence of R3SnH and AIBN. The radical species generated from the C-halogen bond homolytic cleavage underwent a 5-endo-cyclization followed either by a 5-exo- or 6-fV/z/o-cyclization and reduction of the final radical species (Scheme 63)... 

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... 

Reduction of carboxylic acids and esters, aldehydes, and nitriles, and the hydro-boration of alkenes with diborane in non-ethereal solvents is highly effective (Table 11.8), but reduction of nitro groups or cleavage of arena-halogen bonds does not occur [1]. However, in spite of the potential advantages, very little use appears to have been made of the procedure. 

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. 

Several research groups ha ve been involved in the study of ET reactions from an electrochemically generated aromatic radical anion to alkyl halides in order to describe the dichotomy between ET and polar substitution (SN2). The mechanism for indirect reduction of alkyl halides by aromatic mediators has been described in several papers. For all aliphatic alkyl halides and most benzylic halides the cleavage of the carbon-halogen bond takes place concertedly with the... 

The reductions of halogenated organic compounds (RX) involve the cleavage of carbon-halogen bonds [62]. Depending on the solvent, supporting electrolyte, electrode material and potential, it is possible to electrogenerate either alkyl radicals (R ) or carbanions (R ), which then can lead to the fonnation of dimers (R-R), alkanes (RH) and olefins [R(-H)] ... 

Studying the electrochemical reduction of halogenated organic compounds has practical importance, especially related to organic syntheses [62], Moreover, the reductive cleavage of the C-X bond is applicable as a method to convert hazardous chlorinated compound for example, polychlorinated biphenyls (PCBs) to biphenyl by reducing in DMF at -2.8 V vs SCE [63],... 

Because only one stable electroactive product arises from the reduction of /7-chlorobenzonitrile, the interpretation of the cyclic voltammetric behavior is relatively straightforward. The fact that electron attachment to halogen-containing compounds frequently results in the cleavage of the carbon-halogen bond immediately suggests benzonitrile as a possible product. The similarity between the cyclic voltammetric behavior of benzonitrile and that of the p-chlorobenzo-nitrile reduction product supports this prediction (see Fig. 21.1). 

The silver ion assisted carbon-halogen bond cleavage and the unraveling of the cyclopropane ring by the cyclopropyl-allyl rearrangement was first noted in the formation of 2-bromocyclohexen-l-ol from dibromobicyclo[3.l.0]hexane under solvolytic conditions (equation 86).220 The silver ion assisted solvolysis of the dihalocyclopropane adduct (43), derived from a Birch reduction product, smoothly rearranges to the tropone (equation 87).221 A number of other synthetic applications222-226 have beien reported... 

The corrinoid-mediated reduction of polyhaloethenes has been the subject of a recent study, which reports reaction via homolytic C-halogen bond fission. The elimination of a further halogen radical affords haloalkynes, which lead to acetylene itself.56 The electron transfer-induced reductive cleavage of alkyl phenyl ethers with lithium naphthalenide has been re-examined in a study which showed that it is possible to reverse regioselectivity of the cleavage (i.e. ArOR to ArH or ArOH) by introduction of a positive charge adjacent to the alkyl ether bond.57 A radical intermediate has been detected by ESR spectroscopy in the reduction of imines to amines with formic acid58 which infers reacts takes place via Lukasiewicz s mechanism.59... 

---