Hydrodecyanation reactions

Grignard reagents tBuMgBr or Bu2Mg were also used as the hydride source for nickel-catalysed hydrodecyanation reactions of aryl and allyl 

Figiire 14.7 Plausible mechanism of the nickel/Lewis acid-catalysed carbocyanation of alkynes. 

In the presence of a nickel catalyst electron-deficient allynes are prone to homo- and/or crosscyclotrimerisation reactions. Hence, it does not seem easy to carry out allynylcyanation of allynes using a allynyl cyanide. However, in the presence of a Lewis acid (BPhs) the trimerisation reaction can be successfully suppressed and nickel-catalysed allynylcyanation of alkynes achieved. In addition, bully silyl substituted alkynyl cyanides exhibit better reactivity in the allynylcyanation reactions than other allynyl cyanides the former require lower catalyst loadings and give higher product yields.  

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Alternatively, iron- or rhodium-catalyzed hydrodecyanation reaction is proposed to be initiated by the C-CN activation of aromatic and aliphatic nitriles with silylmetal species to give iminoacylmetal intermediates and then sUylisonitrile metal complexes, which produce decyanated products and silyl cyanides upon the reaction with hydrosUanes to reproduce the catalyticaUy active silylmetal intermediates (Scheme 7). Irradiation is required to generate coordinatively unsaturated iron complexes (Scheme 8) [45, 46]. A similar reaction mechanism also operates... 

Oxidative addition of C-CN bonds to nickel(0) can be followed by transmetalation with various main-group organometaUic reagents, and subsequent reductive elimination can result in the functionalization of C-CN bonds of nitriles (Scheme 5). As the simplest case, C-CN bonds can be transformed to C-H bonds via transmetalation with metal hydrides. Indeed, nickel-catalyzed hydrodecyanation of various aromatic and aliphatic nitriles proceeds with tetramethyldisUoxane as a hydride donor (Scheme 6) [44]. While a wide range of nitriles can be decyanated by this protocol, a relatively high amount of catalyst is required in this process, presumably because of the formation of catalyticaUy inactive (PCy3)2Ni(CN)2. The use of AlMe3 as a Lewis acid is effective in some cases to promote the C-CN bond activation. Under these reaction conditions, the relative reactivity order of different aryl electrophiles is estimated Ar-SMe>Ar-CN>Ar-OAr>Ar-OMe. 

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