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

Planar chiral bispalladacycles based on ferrocene have previously been shown to be excellent catalysts for asymmetric 1,4-additions of alpha-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 mu 2-(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 kappa 2-(O,N) coordination was expected and also experimentally found. Indeed, in that case catalytic activity was improved compared to the benchmark system. The coordination of pyridonates and a ketophenolate to metallocene based bispalladacycles is reported. Using the phenolate ligand in a catalytic asymmetric 1,4-addition, higher activity than with the benchmark system from literature was found, while the pyridonate decreased the activity. The different mu 2- and kappa 2-coordination modes are discussed as reason. image