Reduction of Carbon-Halogen Bonds

The sonochemical reduction of the carbon-halogen bond to a C-H bond was not extensively used. One example is the reduction of a polychlorocarbon to the 

A similar reaction applied to a i fc-dibromide with formation of an olefin was used in sugar chemistry.  

However, the domain of choice for these reductive processes is undoubtedly the preparation of organometallic reagents. After the initial work reported by Renaud, a second paper appeared in 1959, on the preparation of phenylsodium from ultrasonically activated sodium and chlorobenzene. These initial reports can be seen as the initial steps towards modem organometallic sonochemistry. 

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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. 

With the exception of a few special cases of mixed halogen compounds, the ease of reduction of carbon-halogen bonds follows the expected order, I>Br>CI>F, and becomes more facile as the number of halogen atoms on a given carbon atom increases. Accessible halfwave potentials range from-2.23 V (see) (C—Cl) to -0.3 V (see) (Br2C—Br)15. The isolated C—F bond cannot be reduced electrochemically below the solvent cut-off limit. However, when the C—F functionality is adjacent to an aromatic or carbonyl moiety, or is geminal, reduction often does occur. 

Reduction of carbon-halogen bond by metal yields carbanion. Reaction of alkyl halide with Mg in the presence of anhydrous ether as solvent generates Grignard reagent. The Grignard reagent behaves like a carbanion. Alkyllithiums are also obtained from alkyl halides and behave as carbanions. 

The overall reaction results in the reduction of carbon-halogen bonds to carbon- hydrogen bonds. 

Another important homolytic reaction of tin hydrides is the reduction of carbon-halogen bonds the reaction is promoted by initiators and retarded by radical traps (Scheme 13). Reactivity decreases in the sequence X = I > Br > Cl and BusSnH > Bu2SnH2 Ph3SiiH > BuSnH3. Other groups X in RX that can be reduced by tin hydrides include -OC(S)R, SR, SePh, TePh, NC, and NO2. The intermediate radical, R in Scheme 13, can be trapped by additional substances, e.g. alkenes, or may undergo... 

C. Reduction of carbon-halogen bonds at chemically modified electrodes References... 

Later in this volume. Chapter 18 presents a discussion of the electrochemical behavior of heterocyclic species. However, it is appropriate here to offer a short summary of the reduction of carbon-halogen bonds found in several classes of heterocyclic compounds. Many halogenated heterocyclic substances display behavior similar to that of aryl halides the carbon-halogen bond is reductively cleaved in a process involving two electrons and a proton to give a hydrogenated product. 

C. Reduction of Carbon-Halogen Bonds at Chemically Modified Electrodes... 

These reagents are interesting because aldehydes and ketones are affected in acidic media only, which permits the reduction of carbon-halogen bonds (Section 2.1) without affecting carbonyl groups, esters, or nitriles. 

Chromium contributes to organic transformations as reagents. Some examples include chromic acid Qones reagent), chromium acetate for the reduction of carbon-halogen bonds and conversion of ketoximes to ketones, chromyl acetate, chromium perchlorate, chromium sulfate (reduction of allq nes to alkenes), and Jacob s reagent (Cr02Cl2). The catalytic... 

Reductions of carbon-halogen bonds are initiated similarly. In this case, several tin hydrides (di- and tributyl, triphenyl) can be used with practically equal effectiveness. lododecane gives the hydrocarbon in 88% yield after sonication for 2 h with triphenyltin hydride in THF at - 64°C. An equivalent yield is obtained using the tributyl analogue in toluene at 6°C. With substrates containing an unsaturation in a suitable position, cyclization occurs (Eq. 9). Interception by oxygen of the intermediate radical can be stopped at the hydroperoxide stage. 4... 

Palladium-catalyzed reduction of carbon-halogen bonds can also be achieved by the formate anion in a biphasic system in the presence of cyclodextrins, which form a hydrophilic host-guest complex with hydrophobic substrates fitting into their cavities, and thus transfer them into the aqueous phase [86] ... 

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