Hydrocyanation

Hydrocyanation is, most fundamentally, the process whereby H⁺ and ^–CN ions are added to a molecular substrate. Usually the substrate is an alkene and the product is a nitrile. When ^–CN is a ligand in a transition metal complex, its basicity makes it difficult to dislodge, so, in this respect, hydrocyanation is remarkable. Since cyanide is both a good σ–donor and π–acceptor its presence accelerates the rate of substitution of ligands trans from itself, the trans effect.¹ A key step in hydrocyanation is the oxidative addition of hydrogen cyanide to low–valent metal complexes. In hydrocyanation of unsaturated carbonyls addition over the alkene competes with addition over the carbonyl group.

Industrially, hydrocyanation is commonly performed on alkenes catalyzed by nickel complexes of phosphite (P(OR)₃) ligands. A general reaction is shown:

The reaction proceeds via the oxidative addition of HCN to Ni(0) to give a hydridonickel(II) cyanide complex, abbreviated Ni(H)(CN)L₂. Subsequent binding of the alkene gives the intermediate Ni(H)(CN)L(alkene), which then undergoes migratory insertion to give an alkylnickel(II) cyanide Ni(R)(CN)L₂. The cycle is completed by the reductive elimination of the nitrile.

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