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 (scheme 1).1

Inorganic Chemistry
Hydrocyanation is performed on alkenes and alkynes with copper, palladium, and most commonly, nickel catalysts.1 Industrial hydrocyanation utilizes phosphite (P(OR)3) complexes of nickel. Phosphites give excellent catalysts, whereas the related phosphine (PR3) ligands, which are more basic, are catalytically inactive.1 Chiral, chelating aryl diphosphite complexes are commonly employed in asymmetric hydrocyanation. An example of a nickel–phosphite catalyzed hydrocyanation of ethene is displayed in scheme 2.1

Lewis acids, such as B(C6H5)3, can increase hydrocyanation rates and allow for lower operating temperatures.2 Triphenylboron may derive this ability from sterically protecting the –CN as it is bound to nitrogen.1 Rates can also be amplified with electron–withdrawing groups (NO2, CF3, CN, C(=O)OR, C(=O)R) on the phosphite ligands, because they stabilize Ni(0).3 A major problem when using nickel catalysts for hydrocyanation is the production of Ni0(CN)x as a result of excess HCN.3 Bulky ligands impede the formation of these unreactive Ni0(CN)x complexes.4

Usage
Hydrocyanation is important due to the versatility of alkyl nitriles (RCN), which are important intermediates for the syntheses of amides, amines, carboxylic acids, and ester compounds.3,5 The most popular industrial usage of nickel-catalyzed hydrocyanation is for adiponitrile (NC–(CH2)4–CN) synthesis from 1,3–butadiene (CH2=CH–CH=CH2).1 Adiponitrile is a precursor to hexamethylenediamine (H2N–(CH2)6–NH2), which is used for the production of certain kinds of Nylon. The DuPont ADN process3,4 to give adiponitrile is shown in scheme 3.1

Naproxen, an anti-inflammatory drug, utilizes an asymmetric enantioselective hydrocyanation of vinylnaphthalene from a phosphinite(OPR2) ligand, L1 (figure 1).1,5 The enantioselectivity of this reaction is important because only the S enantiomer is medicinally desirable, whereas the R enantiomer produces harmful health effects.5 The synthesis of naproxen nitrile is featured in scheme 4.5 This reaction can produce the S enantiomer with > 90% selectivity. Upon recrystallization of the crude product, the optically pure nitrile can be attained.5

History
Hydrocyanation was first reported by Arthur and Pratt in 1954, when they homogeneously catalyzed the hydrocyanation of linear alkenes.6