Wolff-Kishner reduction nitrogen elimination

A slighdy more vigorous method, known as the Wolff-Kishner reduction, is driven by the elimination of nitrogen gas from a hydrazone. Hot concentrated sodium hydroxide solution deprotonates the hydrazone, which can then eliminate an alkyl anion—a reaction you would usually be wary of writing, but which is made possible by the thermodynamic stability of N2. 

These types are particularly involved whose one of the leaving groups is part of the chain. Thus, in the abnormal Wolff-Kishner reduction 4,5-dihydro-lH-pyrazoles are formed (e.g. 311), whose pyrolysis leads to [2,3,(4)I,(2)3(4)]-eliminations of nitrogen with-cyclopropane formation. In this way 311 forms 312 (75%)163). The three assistant figures are necessary in order to fix which chain-bonds migrate and where the second leaving group is located. Even the eliminations of carbenes (formally)... 

Cinnolines may be formed by the cyclization of 2-hydrazone or 2-hydrazonophosphorane derivatives of 3-(2-fluoro- or chlorophenyl)-3-oxopropionates (Scheme 87), which are prepared from the oxopropionates via reaction with /i-toluenesulfonyl azide and treatment of the diazo derivatives with phosphines. Simple hydrazones undergo cyclization by displacement of fluoride under thermal or basic conditions, and reaction of chloro compounds is successful under basic conditions, though Wolff-Kishner reduction may compete in the absence of a methyl group on the terminal nitrogen. The diazo compounds cyclize, via tri- -butylphosphazines, more readily than do the hydrazones and base catalysis is not required it may be that cyclization proceeds via a phosphonium salt, with subsequent hydration and elimination of tri- -butylphosphine oxide. The triphenylphosphazines do not cyclize, but are labile intermediates to the hydrazones <88Cpbi321>. 

A ketone can be reduced to a methylene group using the Wolff-Kishner reduction, which involves heating the ketone with hydrazine and base. The mechanism for the Wolff-Kishner reduction involves initial formation of a hydrazone followed by a tautomeriza-tion and elimination of nitrogen. The Wolff-Kishner reduction is complementary to the Clemmensen reduction (Section 15.9A), which involves acid, and to the reduction of dithioacetals (Section 16.7D), which involves catalytic hydrogenation. 

Mechanism of Nitrogen Elimination in the Wolff-Kishner Reduction... 

---