Planar Chiral Ferrocendiyl and Ruthenocendiyl Bisimidazoline Bispalladacycles Featuring Pyridin‐2‐olates and Ketophenolates as Potentially Hemilabile Ligands in Asymmetric 1,4‐Additions

Abstract

AbstractPlanar chiral bispalladacycles based on ferrocene have previously been shown to be excellent catalysts for asymmetric 1,4‐additions of α‐cyanoacetates to enones. For a bimetallic reaction pathway, it was found that product decomplexation is probably rate‐determining as a result of a bimetallic two‐point binding. We hypothesized that the use of hemilabile chelating ligands might accelerate this step to improve the catalytic activity. Here we report the use of pyridin‐2‐olates (pyridonates) as potentially hemilabile 1,3‐N,O‐chelating ligands in this catalytic application. In this article, we describe the first coordination of pyridonate ligands to planar chiral, metallocene based palladacycles. Four of the resulting ferrocene and ruthenocene bis‐palladium complexes were characterized by X‐ray single crystal structure analysis revealing a μ2‐(O,N) coordination mode. We suggest that an observed lower catalytic efficiency of the new complexes has its origin in this particular coordination mode. For that reason, a related trifluoromethylketophenolate ligand was installed for which κ2‐(O,N) coordination was expected and also experimentally found. Indeed, in that case catalytic activity was improved compared to the benchmark system.