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Diazene, also called diimine or diimide, is a compound having the formula HN=NH. Azo compounds can be considered to be formally derived from diazene by substituting the hydrogen atoms with organyl groups. Because each nitrogen atom bears one hydrogen and one lone pair and double bonds cannot rotate, diazene exists in distinct cis and trans geometric isomers.
cis-Diazene is also a useful reagent in organic synthesis, as it can be used to hydrogenate alkenes, alkynes and polyenes, with selective delivery of hydrogen from one face of the substrate. This gives the same stereoselectivty as metal-catalysed syn addition of H2. The advantages of using Diazene, however, are that there is no need for high pressures or potentially explosive hydrogen gas (the only gas released is relatively inert nitrogen) and no need for expensive metal catalysts.
Mechanism:
Diazene can be efficiently generated in-situ by elimination of sulfonohydrazides using a suitable base. For example 2,4,6-Triisopropylbenzenesulfonohydrazide can eliminate to give diazene using sodium hydrogen carbonate, a very mild base. This generates both the cis and trans forms of diazene and the isomerisation is slow. However, the trans - cis equilibrium is driven to the right due to the rapid consumption of the cis isomer by reaction with the substrate.
References
- Wiberg, E.; Holleman, A. F. (2001). "1.2.7: Diimine, N2H2", Inorganic Chemistry. Elsevier, 628.
- Miller, Charles E. Hydrogenation with diimide. Journal of Chemical Education (1965), 42(5), 254-9. CODEN: JCEDA8 ISSN:0021-9584.[1]
- Trost et al Journal of the American Chemical Society 127 11 2005 3667 DOI:10.1021/ja042435i[2]
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