Mechanism, Raney nickel alkylation

Alkyl bromides and especially alkyl iodides are reduced faster than chlorides. Catalytic hydrogenation was accomplished in good yields using Raney nickel in the presence of potassium hydroxide [63] Procedure 5, p. 205). More frequently, bromides and iodides are reduced by hydrides [505] and complex hydrides in good to excellent yields [501, 504]. Most powerful are lithium triethylborohydride and lithium aluminum hydride [506]. Sodium borohydride reacts much more slowly. Since the complex hydrides are believed to react by an S 2 mechanism [505, 511], it is not surprising that secondary bromides and iodides react more slowly than the primary ones [506]. The reagent prepared from trimethoxylithium aluminum deuteride and cuprous iodide... 

In this context it is interesting to note that benzonitrile, Ph—C=N, trimerizes to a triazine on a Raney nickel surface. It was assumed that Jt-bonded nitriles were involved in the reaction mechanism.10 This reaction resembles the well-known template synthesis of phthalocyanine complexes from phthalodinitrile. Formation of linear polymers [—C(R)—N—] occurs on heating aryl or alkyl cyanides with metal halides.11... 

Reactions.— Nitriles can be reduced to aldehydes in aqueous media by photochemically generated hydrated electrons. The reduction of nitriles to amines by sodium borohydride is catalysed by Raney nickel. The transition metal promoted reductive decyanation of alkyl nitriles to homologous hydrocarbons with one less carbon atom is reported, and has been shown by van Tamelen to proceed by a different mechanism from that of a similar reduction by alkali metals in ammonia, where a process of stepwise two-electron transfer has been proposed. In the former reaction, where use is made of ferric acetylacetonate and sodium, the proton required for alkane production is derived exclusively from the acetylacetonate ligand, alkane being formed before a proton source is added this suggests a mechanism which involves initial co-ordination of the nitrile to iron, followed by reductive cleavage of the nitrile 1,2-carbon bond and proton transfer to this area, as pictured in Scheme 42. 

---