Hydrogenation selective poisoning effects

Primary amine formation is equally well promoted in alkaline medium, e.g., aqueous ethanolic NaOH solution, that selectively poisons the catalyst for hydrogenolysis reactions. However, saturated NH3/alcohol solutions best afford almost quantitative yields of primary amines from catalytic reduction of nitriles. Ammonia adds to imine 1 to give a 1,1-diamine, which is hydrogenolyzed to the primary amine. In the presence of NHj, finely divided Ni can be used, platinized finely divided Ni for the hydrogenation of hindered nitriles, and rhodium-on-alumina for sensitive compounds. Mild reduction of 3-indoleacetonitrile to tryptamine [equation (c)] is effected at RT over 5% rhodium-on-alumina in 10% ethanolic NH3 with little side reaction , and branched chain amino sugars are conveniently prepared using this selective hydrogenation [equation (d)] . 

As noted in Section 10.12, the partial hydrogenation of benzene to cyclohexene is an economically attractive first step in the synthesis of polyamides, because it is more easily separated from unreacted benzene than is cyclohexane. The reaction is however a difficult one to accomplish, because cyclohexene is normally much the more easily hydrogenated, so its appearance in any significant amount depends upon constructing a catalyst which is in effect selectively poisoned for the hydrogenation of the alkene but not for the more strongly adsorbed benzene. There have been many attempts to modify metals (mainly ruthenium ) to secure this result, in ways that are now briefly recounted. 

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