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Concept
Hydrocyanation
Summary
In organic chemistry, hydrocyanation is a process for conversion of alkenes to nitriles. The reaction involves the addition of hydrogen cyanide and requires a catalyst. This conversion is conducted on an industrial scale for the production of precursors to nylon.
Industrially, hydrocyanation is commonly performed on alkenes catalyzed by nickel complexes of phosphite () ligands. A general reaction is shown:
RCH=CH2 + HCN -> RCH2-CH2-CN
The reaction involves the addition of and cyanide () to the substrate. Usually the substrate is an alkene and the product is a nitrile.
The reaction proceeds via the oxidative addition of HCN to a low-valent metal complex to give a hydrido cyanide complex. Subsequent binding of the alkene gives the intermediate , which then undergoes migratory insertion to give an alkylmetal cyanide. The cycle is completed by the reductive elimination of the nitrile.
Lewis acids, such as triphenylboron (), induce reductive elimination of the nitrile product, increasing rates.
In the case of nickel-based systems, catalyst deactivation involves formation of dicyanonickel(II) species, which are unreactive toward alkenes. The dicyanide arises via two pathways (L = phosphite):
Ni(H)(CN)L2 + HCN -> Ni(CN)2L2 + H2
Ni(R)(CN)L2 + HCN -> Ni(CN)2L2 + RH
Most alkenes are prochiral, meaning in this context that their hydrocyanation generates chiral nitriles. Conventional hydrocyanation catalysts, e.g. , catalyse the formation of racemic mixtures. When however the supporting ligands are chiral, the hydrocyanation can be highly enantioselective. For asymmetric hydrocyanation, popular chiral ligands are chelating aryl diphosphite complexes.
The most important industrial application is the nickel-catalyzed synthesis of adiponitrile () synthesis from buta-1,3-diene (). Adiponitrile is a precursor to hexamethylenediamine (), which is used for the production of certain kinds of Nylon.
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