Arrested Catalysis: Controlling Kumada Coupling Activity via a Redox-Active N-Heterocyclic Carbene

Abstract

Optimized syntheses for 1,3-dimesitylnaphthoquinimidazolium chloride [1H][Cl] and the corresponding silver NHC complex [AgCl(1)] (2) were developed, enabling access to this versatile reagent in near-quantitative yield. Transmetalation from 2 to [NiCl2(PPh3)(2)], trans-[PdCl2(PhCN)(2)], or trans-[PtCl2(PhCN)(2)] afforded the Group 10 complexes trans-[MCl2(1)(2)] (3a-c, M = Ni, Pd, and Pt, respectively) in excellent overall yield (>95%) after three steps from commercially available starting materials. Electrochemical measurements indicated that the E-1/2 and Delta E-1/2 values for the quinone reduction couples were independent of the identity of the bridging transition metal in these complexes. Whereas attempts to isolate the reduced complexes were unsuccessful, UV/vis spectroelectrochemical analysis confirmed that electrochemical reduction of 3a-c in situ afforded optical difference spectra consistent with the formation of the expected reduced species. Complex 3a was found to catalyze the Kumada cross-coupling reaction between PhMgCl and a range of bromoarenes at room temperature. Addition of 2 equiv of cobaltocene (with respect to 3a) to the coupling reaction with bromotoluene caused a decrease in catalytic activity (from 4.7 x 10(-5) to 2.7 x 10(-6) s(-1)), which was attributed to the conversion of 3a to an arrested state. Subsequent introduction of ferrocenium tetrafluoroborate (2 equiv with respect to 3a) restored a significant degree of catalytic activity (k(obs) = 1.2 x 10(-5) s(-1)). Redox-switching experiments performed over different time scales revealed that the catalyst was stable in the reduced/inactive state and that extended durations in this state did not impede catalytic reactivation upon subsequent oxidation.