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Mechanism for Activation of the C–CN Bond of Nitriles by a Cationic CpRhIII–Silyl Complex: A Systematic DFT Study

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journal contribution
posted on 23.06.2014 by Song-Lin Zhang, Lu Huang, Wen-Feng Bie
A theoretical study of reaction mechanisms for C–CN bond activation of nitriles by a RhIII–silyl complex is reported. Various mechanisms including direct oxidative addition, insertion of the cyanide CN triple bond into the RhIII–silyl bond followed by β-carbon elimination, insertion of the cyanide CN triple bond into the RhIII–silyl bond followed by α-carbon elimination (deisocyanide), radical mechanisms, and other possible alternatives have been evaluated. Our results provide strong evidence for the sequential mechanism of cyano insertion/α-C elimination (deisocyanide). The cyanide insertion step should be the rate-limiting step, while the deisocyanide step is facile. The intermediate from cyanide insertion, i.e., the Rh­(III) η2-iminoacyl complex, has been identified and is in good agreement with the experimentally characterized X-ray crystal structure. The oxidative addition and cyanide insertion/β-C elimination mechanisms are kinetically inhibited due to extremely high activation barriers. Radical mechanisms are also kinetically unfavorable due to the electrophilic nature of the cationic Rh­(III) complex. These findings distinguish the cationic RhIII–silyl complex from the electron-rich Ni(0) systems frequently exploited for the activation of cyanide C–CN bonds, where an oxidative addition mechanism should be operative. Furthermore, the rate-limiting step of cyano insertion into the Rh–silyl bond has been examined for various nitriles. The reactivity trend for these nitriles is also in good agreement with experimental observations, which show significant steric effects but small electronic effects for the RCN R group. The origin of the favorable insertion to give a Rh­(III) η2-iminoacyl complex versus the formation of a Rh­(III) η1-imino complex has been elucidated by using natural charge population analyses. It is attributed to the presence of two pairs of favorable stabilizing Rh···C and Si···N interactions in the transition state TS1 for cyano insertion in the insertion/deisocyanide mechanism. However, this effect is replaced by detrimental repulsive Rh···N and Si···C interactions in the isomeric transition state TS1′ with the reverse orientation of Rh–Si versus CN bonds.