Carbon-Halogen Cleavage

The photochemistry of 1-deoxy 1-halogenosugars has been developed in recent years to create new bonds at the anomeric carbon such as  

If both a and [3-halogenosugars 8a and 8 c are reduced to the same anomeric mixture with the predominant deuterio compound 10, an easier cleavage of the axial anomeric carbon-bromine bond is observed with shorter times of photoreactions. 

In comparison, the stereoselectivity of the radical reduction slightly decreases if the anomeric center is substituted by an electron withdrawing substituent (CN) using AIBN as radical initiator [17 b]. The corresponding radical is supposed to be more planar than pyramidal to explain the lower degree of stereoselectivity for the reduction process of 8 b into 9. 

The preceding photoreduction of 1-halosugars with tin hydride under photolytic conditions can be carried out in the presence of electron-poor alkenes to form a 

Bu3SnO hv slow Bu3SnH AIBN Bu3SnD hv fast 

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Carbon-halogen bond cleavage in aryl halides is believed to involve initial excitation into an upper singlet state that is stable with respect to cleavage, followed by intersystem crossing to an upper triplet state that is either dissociative itself or can cross to an upper dissociative (a, a ) triplet [65-67]. The two-color approach has been used to demonstrate directly that excitation to an upper triplet state results in carbon-halogen cleavage. For example, in 2-bromonaphthalene (81) and 9-bromo-phenanthrene (82) [68], promotion of Tj to an upper triplet by dye laser excitation... 

Catalytic reactions of organohalides using palladium, nickel, or rhodium catalysts are well investigated. Interestingly, however, in the case of ruthenium-catalyzed reactions, only a few examples involving carbon-halogen cleavage have been reported. 

The part played by the more stable fragments resulting from carbon-halogen cleavage is again uncertain, but Heine (7) has indicated a reduced tendency for these radicals to add to unsaturated systems. Dimeric products have been isolated in the case of photolyses carried out in benzene, e.g. (7). 

Allylic Systems. Allylic Halides. Allylic halides also undergo homolytic carbon-halogen cleavage by pentacyanocobaltate(II) to form equimolar quantities of halo- and allylcobalt complexes (21, 22, 23). It is assumed that this reaction involves generation of the allylic radical (Reaction 19), which then reacts with pentacyanocobaltate(II) (Reaction 20). 

Step (1) Alkyl halide dissociates by heterolytic cleavage of carbon-halogen bond (Ionization step)... 

The alkyl halide m this case 2 bromo 2 methylbutane ionizes to a carbocation and a halide anion by a heterolytic cleavage of the carbon-halogen bond Like the dissoci ation of an aUcyloxonmm ion to a carbocation this step is rate determining Because the rate determining step is ummolecular—it involves only the alkyl halide and not the base—It is a type of El mechanism... 

The value of alkyl halides as starting materials for the preparation of a variety of organic functional groups has been stressed many times In our earlier discussions we noted that aryl halides are normally much less reactive than alkyl halides m reactions that involve carbon-halogen bond cleavage In the present chapter you will see that aryl halides can exhibit their own patterns of chemical reactivity and that these reac tions are novel useful and mechanistically interesting... 

The strength of their carbon-halogen bonds causes aryl halides to react very slowly in reactions in which carbon-halogen bond cleavage is rate determining as m nude ophilic substitution for example Later m this chapter we will see examples of such reactions that do take place at reasonable rates but proceed by mechanisms distinctly dif ferent from the classical S l and 8 2 pathways... 

Acid, hydrogen haUde, or halogen cleavage of tetraorganotins is not used except on a laboratory scale because they are wasteful of tin—carbon bonds and uneconomical on a commercial scale. 

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... 

Heterolytic cleavage of the tin-carbon bond is reviewed in references (94-96). Cleavage by electrophiles (e.g, HgXj or halogen) is dominated by electrophilic attack at carbon, and cleavage by nucleophiles principally involves nucleophilic attack at tin. Much of the interest in these processes centers on the intermediate mechanisms that may exist between these extremes, in which electrophilic attack is accompanied by some nucleophilic assistance, and vice versa. Allylic, al-lenic, and propargylic compoimds show a special reactivity by a special (Se2 or SE2y) mechanism. 

Electrogenerated nickel(I)251 and cobalt(I)252 complexes of Salen (Salen = bis(salicylidene)ethane-1,2-diamine) have displayed good catalytic properties in the cleavage of carbon-halogen bonds in a variety of organic compounds. Recent research in this field has been reviewed.253... 

Burk et al. showed the enantioselective hydrogenation of a broad range of N-acylhydrazones 146 to occur readily with [Et-DuPhos Rh(COD)]OTf [14]. The reaction was found to be extremely chemoselective, with little or no reduction of alkenes, alkynes, ketones, aldehydes, esters, nitriles, imines, carbon-halogen, or nitro groups occurring. Excellent enantioselectivities were achieved (88-97% ee) at reasonable rates (TOF up to 500 h ) under very mild conditions (4 bar H2, 20°C). The products from these reactions could be easily converted into chiral amines or a-amino acids by cleavage of the N-N bond with samarium diiodide. 

Elimination of an alkoxy group or of a halogen in the case of zirconacyclopentenes has been investigated in combination with the (3, (3-carbon—carbon bond-cleavage reaction. As shown in Eq. 2.73, an OR group or a halogen at a (3-position is eliminated and trapped by the zirconium metal center [53],... 

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... 

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]. 

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. 

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]. 

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