A Neutral, Chiral, Bis(imidazolidine)-Derived NCN-Type Palladium Pincer Complex with Catalytic Activity (original) (raw)

The function of pincer ligands is typically to chelate transition metals through the action of three adjacent coplanar coordination sites, resulting in complexes that are both especially stable and rigid. [1] Since the first reports by Shaw, [2] van Koten [3] and Noltes [3] in the 1970s, a variety of pincer complexes have been designed and synthesized with possible applications in catalysis and materials science. The stable, so-called XCX-type palladium pincer complexes, incorporating a strong carbon-metal s bond and two additional chelating moieties (X), are particularly useful in promoting catalysis. One example of the successful application of these compounds is the use of the PCP-type palladium catalyst to promote the Heck and/or Suzuki-Miyaura coupling reactions. [4] Owing to the unique properties of metal pincer complexes, recent work has focused on their use in asymmetric catalysis for the synthesis of optically active compounds. For example, a series of chiral oxazoline-derived NCN-type pincer ligands (Phebox) [5] have been reported for the metal complexes as an analogue of the well-known coordination complex of pybox-metal catalyst. [6] The use of such chiral metal pincer complexes in highly enantioselective catalysis has, however, proved challenging. The meridional orientation of the pincer ligands in the coordination complex creates an unconstrained reaction sphere, leading to difficulties in the control of stereoselective reactions. We previously developed a chiral N,N,N-terdentate PyBidine ligand (where PyBidine is an abbreviation of bis(imidazolidine)pyridine) [7] as an analogue of the pybox ligand. Xray crystallographic analysis of a single crystal of the PyBidine-CuA C H T U N G T R E N N U N G (OTf) 2 complex showed that the two imidazolidine rings are oriented perpendicular to the equatorial terdentate coordination plane. The imidazolidine rings therefore act as "chiral fences" to shield the first and third quadrants of the (S,S