Reductive removal of functionality

It is sometimes necessary to reduce a functionalized site to a CH or CH group. Starting from the acid level of oxidation, this is usually done in two stages. For example, an ester may be reduced to an alcohol with LiAlH, the alcohol converted to a tosylate, and then reduced again to a CH group. Ketones and aldehydes can be reduced completely via the tosylhydrazone using catecholborane followed by decomposition of the hydroboration intermediate as in Equation 6.71 [116]. 

Benzylic, secondary, and tertiary alcohols can be reduced to the corresponding alkanes in the presence of esters, halogens, nitro groups, or even primary alcohols via an indium trichloride catalyzed reduction using chlorodiphenylsilane as the hydride source (Eq. 6.72) [117], 

The reduction of alkyl hahdes has been important in many syntheses. Sodium cyanoborohydride in HMPA will reduce alkyl iodides, bromides, and tosylates selectively in the presence of ester, amide, nitro, chloro, cyano, alkene, epoxide, and aldehyde groups [118]. Tri-n-butyltin hydride will replace chloro, bromo, or iodo groups with hydrogen via a free radical chain reaction initiated by thermal decomposition of AIBN [119]. Other functionality such as ketones, esters, amides, ethers, and alcohols survive unchanged. The less toxic tris(trimethylsilyl) silane can be used similarly [120]. 

---

The reactions are radical chain processes (Scheme 3) and, therefore, the initial silyl radicals are generated by some initiation. The most popular thermal initiator is azobisisobutyronitrile (AIBN), with a half-life of 1 h at 81 °C. Other azocompounds are used from time to time depending on the reaction conditions. EtsB in the presence of very small amounts of oxygen is an excellent initiator for lower temperature reactions (down to —78°C). The procedures and examples for reductive removal of functional groups by (TMSlsSiH are numerous and have recently been summarized in the book Organosilanes in Radical Chemistry. ... 

Scheme 5.12. Reductive Removal of Functional Groups from a-Substituted Carbonyl... 

The synthetic potential of reductions by formate has been extended considerably by the use of ammonium formate with transition metal catalysts like palladium and rhodium. This forms a safe alternative to use of hydrogen. In this fashion it is possible to reduce hydrazones to hydrazines, azides and nitro groups to amines, to dehalogenate chloro-substituted aromatics, and to carry out various reductive removals of functional groups. For example, phenol triflates are selectively deoxygenated to the aromatic derivatives using triethylammonium formate as reductant and a palladium catalyst. - These recent af li-cations have been reviewed. 

Many new methods for the preparation of alkanes by reductive removal of functional groups have been reported during the year. Barton and his co-workers have presented a new radical decarboxylation for the conversion of carboxylic acids into hydrocarbons. Following esterification with /ran5-9-hydroxy-10-phenylthio-(or -10-chloro-)9,10-dihydrophenanthrene, a primary, secondary, or tertiary carboxylic acid is smoothly reduced under neutral conditions by tri-n-butylstannane and a radical initiator (e.g. Scheme 1). Formation of phenanthrene... 

Reductive removal of functional groups from a-substituted carbonyl compounds... 

---