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

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.