Functional group removal, halogen

Two different sets of experimental conditions have been used. Buu-Hoi et al. and Hansen have employed the method introduced by Papa et using Raney nickel alloy directly for the desulfurization in an alkaline medium. Under these conditions most functional groups are removed and this method is most convenient for the preparation of aliphatic acids. The other method uses Raney nickel catalysts of different reactivity in various solvents such as aqueous ammonia, alcohol, ether, or acetone. The solvent and activity of the catalyst can have an appreciable influence on yields and types of compounds formed, but have not yet been investigated in detail. In acetic anhydride, for instance, desulfurization of thiophenes does not occur and these reaction conditions have been employed for reductive acetylation of nitrothiophenes. Even under the mildest conditions, all double bonds are hydrogenated and all halogens removed. Nitro and oxime groups are reduced to amines. 

Since enolates also add via a ligand attack process, the regioselectivity that they exhibit is quite comparable to soft caibon nucleophiles. Alkyl or aiyl substituents at the allyl termini direct attack to the less substituted terminus (equations 228-232) functional groups such as COaMe and halogen at one allyl terminus direct attack to the remote terminus (equations 233 and 234). Remote functionalities such as —OR also direct addition to the allyl terminus more removed from the substituent (equations 23S and 236). 

Radical substitution reactions can also be used to remove functional groups from molecules. A useful reagent for this (and, as you will see, for other radical reactions too) is tributyltin hydride, Bu3SnH. The Sn-H bond is weak and B SnH will react with alkyl halides to replace the halogen atom with H, producing BL SnHal as a by-product. 

In this section, only oxidations that result in the removal of halogen substituents will be discussed. Oxidations that do not affect halogens are discussed in sections describing oxidations of individual functional groups. The exception is the oxidative cleavage of haloalkenes, which leads to carboxylic acids and which is mentioned in the section Alkenes and Cy-cloalkenes. 

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