Reductive Deoxygenation of Carbonyl Groups

The Wolf-Kishner reaction is the reduction of carbonyl groups to methylene groups by base-catalyzed decomposition of the hydrazone of the carbonyl compound. Alkyldiimides are believed to be formed and then collapse with loss of nitrogen. 

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

Reduction of tosylhydrazones of a,j5-unsaturated ketones by NaBH3CN gives alkenes with double bond located between the former carbonyl carbon and the a carbon. This reaction is believed to proceed by an initial conjugate reduction, followed by decomposi- 

CHAPTER 5 REDUCTION OF CARBONYL AND OTHER FUNCTIONAL GROUPS 

Catecholborane or sodium borohydride in acetic acid can also be used as reducing reagents in this reaction. 

Several methods are available for reductive removal of carbonyl groups from organic compounds. Reduction to methylene groups or conversion to alkenes can be achieved. 

Zinc and hydrochloric acid is a classical reagent combination for conversion of carbonyl groups to methylene groups. The reaction is known as the Clemmensen reduction The corresponding alcohols are not reduced under the conditions of the reaction, so they are evidently not intermediates. The mechanism is not known in detail, but it may involve carbon-zinc bonds at the metal surface. 

The reaction is commonly carried out in hot concentrated hydrochloric acid with ethanol as a co-solvent. These conditions preclude the presence of acid-sensitive or hydrolyzable functional groups. The Clemmensen reaction works best for aryl ketones and is less reliable with unconjugated ketones. A modification in which the reaction is run in ether saturated with dry hydrogen chloride gave good results in the reaction of steroidal ketones.  

The reaction is commonly carried out in hot concentrated hydrochloric acid with ethanol as a cosolvent. These conditions preclude the presence of acid-sensitive or hydrolyzable functional groups. This methodology is usually quite efficient for 

The Wolff-Kishner reaction involves the base-catalyzed decomposition of 

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Reductive Carbon-Carbon Bond Formation Reductive Deoxygenation of Carbonyl Groups General References... 

Reduction to Alkanes. Carbonyl groups can be reductively deoxygenated to methylene functions if both of the two steps represented by Eqs. 1 and 2 proceed to completion. With aldehydes, this process leads to the transformation of the CHO group into a CH3 group. 

Creative transformation of primary taxanes to pachtaxel was achieved by the group of B. Ganem at Cornell University. The authors had long experience with reductive deoxygenation of various carbonyl functionalities by the zirconium complex Cp2ZrHCl (31, Scheme 14.14). 

Wolff—Kishner reduction Deoxygenation of an aldehyde or ketone converting a carbonyl group to... 

The Tafel rearrangement only occurs in acid medium. Simultaneous reduction of both carbonyl groups leads to interaction and formation of a cyclopropane. Acid catalysed cyclopropane ring opening follows to yield an a-diketone 28 which undergoes the electrochemical Clemmensen reduction step to the hydrocarbon. Side products include the two monoketones derived by partial deoxygenation of the a-diketone and the secondary alcohols from reduction of these raonoketones. Separate experiments show that the a-diketone 28 can be reduced to the hydrocarbon. 

This reductive coupling involves two steps. The coupling is induced by single electron transfer to the carbonyl groups from alkali metal, followed by deoxygenation of the 1,2-diol with low-valent titanium to yield the alkene. 

Deoxygenation. This substance reduces epoxides to alkenes in 85-95% yield, generally with retention of configuration, particularly in reduction of trans-tpox-ides. This reduction proceeds more readily than that of carbonyl or ester groups, but a,(3-enals are coupled reductively by 1 at 25° to diallylic ethers. 

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