Adiponitrile by HCN Addition to Butadiene

Adiponitrile is an important intermediate for the manufacture of hexa-methylenediamine (Section 3.4), which, together with adipic acid, is used to produce nylon 6,6. Although adiponitrile is still largely produced from adipic acid, obtained by vapour phase oxidation of cyclohexane (Section 2.2), it is also manufactured from butadiene by DuPont on the basis of the process first patented in 1970 (Equations 24-26). Conversion is 99% with 90% selectivity to adiponitrile. 

Equation 24 describes the Ni(0)-catalyzed addition of HCN to butadiene, which leads to 3-pentenenitrile together with its allylic isomer 2-methyl-3-butenenitrile (1.5 1 molar ratio). The branched allylic isomer, however, progressively isomerizes to the linear 3-pentenenitrile. 

Double-bond isomerization of 3-pentenenitrile (Equation 25) gives an equilibrium mixture containing only ca.1% of 4-pentenenitrile with respect to the 3-isomer. This isomerization occurs much faster than that leading to the thermodynamically favoured 2-pentenenitrile. 4-Pentenenitrile is preferentially hydrocyanated to adiponitrile because it reacts faster than the 3-isomer for steric reasons (Equation 26). In addition to adiponitrile, the final reaction mixture contains 2-methylglutaronitrile (from hydrocyanation of 2-methyl-3-butenenitrile), ethylsuccinonitrile (from cyanation of 3-pentenenitrile before isomerization) and 2-pentenenitrile (from isomerization of 3-pentenenitrile, which does not undergo further hydrocyanation). 

Although not all the fine details of the DuPont process have been disclosed, enough is known to show the fascinating chemistry underlying it. For example nickel(O) is probably a better catalyst than palladium(O) (coordinated by the 

The use of tris(o-tolyl) phosphite in place of triphenylphosphine has been found to favour the reductive coupling of the metal-coordinated methylallyl (or of the cyanobutyl group in the second step) with the CN group, which liberates the metal in its original (0) oxidation state. 

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