Reductions with Hydrazine
Hydrazine itself and derivatives of hydrazine can behave as reducing agents, in most cases by formation of the very stable N≡N, which formally releases two molecules of H2.
Wolff-Kishner Reduction
The Wolff-Kisher reduction is used to convert ketones to methylene groups, and aldehydes to methyl groups. It cannot be used to reduce the carbonyl groups of amides and esters. The reaction involves conversion of the ketone or aldehyde to a hydrazone, which undergoes base-catalyzed isomerizion to the azo compound A followed by fragmentation. The conditions are very harsh - strongly basic and high temperatures - which limits the application of the method to hydrocarbons or molecules with very thermally and base-stable protecting groups.
Genudin Model: Renoud-Grappin, M.; Vanucci, C.; Lhomet, G. J. Org. Chem. 1994, 59, 3902.
Reduction of Tosylhydrazones
A much milder method synthetically equivalent to the Wolff-Kishner is the reduction of tosylhydrazones with sodium borohydride or sodium cyanoborohydride. The mechanism is believed to go through the same intermediate azo compound A as the Wolff- Kishner reaction.
Cytisine: Stead, D.; O'Brien, P.; Sanderson, A. J. Org. Lett. 2005, 7, 4459-4462
Reduction with Diimide
The unstable molecule diimide (HN=NH) can be generated in a variety of ways. It reacts with alkynes and unhindered alkenes to transfer two hydrogens in a cyclic process, resulting in a strictly cis reduction. This is the mildest way of performing such reductions.
Epothilone B, D: White, J. D.; Carter, R. G.; Sundermann, K. F.; Wartmann, M.. J. Am. Chem. Soc. 2001, 123, 5407.
One generally useful application of diimide reduction is in the conversion of iodoalkynes to cis vinyl iodides. Most other reducting agents will cause competing or complete cleavage of the C-I bond. Brasilenyne: Denmark, S.; Yang, S. M. J. Am. Chem. Soc. 2004, 126, 12432. DOI
