Influence of the Counterion on the Activation of Nickel(σ-Aryl) Precatalysts

Abstract

Complexes of the type L_nNi(σ-aryl)Cl are known to be competitive precatalysts for various transformations, avoiding the use of expensive and sensitive Ni(0)-precursors, such as Ni(cod)₂. The in situ activation requires a transmetalation step with a nucleophile, yielding the active Ni(0) catalyst after reductive elimination. Steric hindrance is usually implemented in the σ-aryl group (e.g., o-tolyl or 1-naphthyl) to enhance kinetic stability. Simultaneously, this steric hindrance can render the activation process slow, thus increasing the reaction time and possibly reducing the amount of active catalyst. To circumvent this issue, we envisaged substitution of the anionic chloride ligand of the precatalyst with more labile ligands that would facilitate transmetalation. In this work, a series of (Xantphos)Ni(o-tolyl)X complexes was successfully synthesized, and the effect of the counterion X on the reaction profile was investigated using C–S cross-coupling as the model reaction. (Xantphos)Ni(o-tolyl)OTf was identified as the most efficient precatalyst, probably due to the weak coordinating ability of the triflate anion that facilitated the activation step. Finally, this concept was also studied in Suzuki–Miyaura coupling and Buchwald–Hartwig amination reactions using (dppf)Ni(o-tolyl)X precatalysts.