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Analysis of Autoinduction, Inhibition, and Autoinhibition in a Rh-Catalyzed C–C Cleavage: Mechanism of Decyanative Aryl Silylation

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journal contribution
posted on 14.08.2018, 00:00 by Eric C. Keske, Thomas H. West, Guy C. Lloyd-Jones
The mechanism the Chatani–Tobisu rhodium-catalyzed decyanative silylation of aryl nitriles by hexamethyldisilane (Me3Si-SiMe3) has been investigated by in situ NMR spectroscopy. The production of Ar-SiMe3 evolves in three distinct phases: slow catalyst induction is followed by a period of rapidly accelerating turnover and then, after approximately three catalyst turnovers, the onset of progressive inhibition. The processes giving rise to these phenomena have been elucidated by isotopic labeling (13C/15N) and kinetic analysis, and it is shown that, in addition to facilitating catalyst turnover to generate Ar-SiMe3, the reactants serve other roles. Me3Si-SiMe3 functions as a slow exogenous precatalyst activator and as a moderately powerful catalyst inhibitor. In contrast, ArCN acts as a precatalyst inhibitor. Moreover, the coproduct from the reaction (trimethylsilyl cyanide, Me3SiCN) acts as a powerful endogenous precatalyst activator and catalyst inhibitor, together giving rise to sigmoidal temporal concentration profiles for [Ar-SiMe3]. Kinetic studies of the reaction during the phase of progressive inhibition suggest that, for a given initial catalyst concentration, the maximum rate of turnover is achieved when the concentrations of [Me3Si-SiMe3] and [ArCN] partition the Rh equally between two major resting states, one on-cycle, the other off-cycle. The off-cycle resting-state was identified as a Rh­(III) complex: (Me3Si)3Rh­(CN-SiMe3)3, as confirmed by independent synthesis and isotopic labeling (13C/15N); the on-cycle resting state has been tentatively assigned as (Me3Si-NC)3RhAr. Overall, the results indicate that the catalytic process can, in principle, be made much more efficient by engendering a pathway or process for Me3SiCN sequestration.