Reductive Deoxygenation of Carbonyl Groups to Methylene

Zinc and hydrochloric acid form a classical reagent combination for conversion of carbonyl groups to methylene groups, a reaction known as the Clemmensen reduction 12 The corresponding alcohols are not reduced under the conditions of the 

The Wolff-Kishner reaction175 is the reduction of carbonyl groups to methylene groups by base-catalyzed decomposition of the hydrazone of the carbonyl compound. It is thought that alkyldiimides are formed and then collapse with loss of nitrogen.276 

The reduction of tosylhydrazones by LiAlH4 or NaBH4 also converts carbonyl groups to methylene.277 It is believed that a diimide is involved, as in the Wolff-Kishner reaction. 

R2C=NNHS02Ar NaS 4 R2CHN=LHi3 S02Ar — R2CHN=NH — R2CH2 

Reduction of tosylhydrazones of a, (3-unsaturated ketones by NaBH3CN gives alkenes with the double bond located between the former carbonyl carbon and the a-carbon.280 This reaction is believed to proceed by an initial conjugate reduction, followed by decomposition of the resulting vinylhydrazine to a vinyldiimide. 

---