Reductive Removal of Functional Groups

The reductive removal of halogen can be accomplished with lithium or sodium. Tetrahydrofuran containing /-butanol is a useful reaction medium. Good results have also been achieved with polyhalogenated compounds by using sodium in ethanol. 

An important synthetic application of this reaction is in dehalogenation of dichloro- and dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce deuterium at a specific site. The mechanism of the reaction involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

Phosphate groups can also be removed by dissolving-metal reduction. Reductive removal of vinyl phosphate groups is one method for conversion of a carbonyl compound to an alkene.224 (See Section 5.7.2 for other methods.) The required vinyl phosphate esters are obtained by phosphorylation of the enolate with diethyl phospho-rochloridate or /V A /V -tetramethyldiamidophosphorochloridate.225 

Carbonyl Groups, and Other Functional Groups 

Ketones can also be reduced to alkenes via enol triflates. The use of Pd(OAc)2 and triphenylphosphine as the catalyst and tertiary amines as the hydrogen donors is effective.226 

Reductive removal of oxygen from aromatic rings can also be achieved by reductive cleavage of aryl diethyl phosphate esters. 

There are also examples where phosphate esters of saturated alcohols are reductively deoxygenated. Mechanistic studies of the cleavage of aryl dialkyl phosphates have 

CHAPTER 5 REDUCTION OF CARBONYL AND OTHER FUNCTIONAL GROUPS 

The mechanism of the reaction presumably involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

---

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... 

---