Structure-selectivity relationship in the chemoselective hydrogenation of unsaturated nitriles

Several unsaturated nitriles of various structures (cinnamonitrile, cyclohex-1-enyl-acetonitrile, acrylonitrile, 3,3-dimethyl-acrylonitrile, geranylnitrile, and 2- and 3-pentenenitrile) with different substituents at the double bond were hydrogenated over Cr-doped Raney cobalt and nickel and over their undoped equivalents. The substitution and the position of the double bond relative to the nitrile group are crucial in determining the chemoselectivity for the unsaturated amine. The double bond is not hydrogenated when it is sterically hindered or if it is too far from the nitrile group (cyclohex-1-enyl-acetonitrile, double bond at C-6 in geranylnitrile). In conjugated systems, such as acrylonitrile or 2-pentenylnitrile, the activated double bond is hydrogenated before the nitrile. An additional methyl substituent at the double bond enhances the selectivity for unsaturated amines and, thus, 3,3-dimethyl-acrylonitrile and geranylnitrile were hydrogenated with selectivity up to 40%. The highest selectivities for unsaturated amines (up to 90%) were reached during the hydrogenation of nonconjugated systems, such as cyclohex-1-enyl-acetonitrile and 3-pentenylnitrile.