Diastereoselectivity in chiral ruthenium complexes of chiraphos monoxide ligands: Epimerization, quasiracemates, and Lewis acid catalysis

Heterobidentate chelating ligands comprising P,O-donor sets produce chiral metal centers when bound to arene-ruthenium complexes. The cymene complex [ CyRu (eta(2)-chelate-P,O) Cl](+) cations are precursors to 16-electron dica tionic strong Lewis acids which have potential use in asymmetric catalysis. Sixteen-electron complexes of this type, however, also provide a pathway w ith a low energy barrier to epimerization of the metal center in intermedia tes. With the chiral ligand (R/R)-chiraphos monoxide, the metal center in [ CyRu(eta(2)(R,R)-Ph2PCHMeCHMe Ph2PO-P,O)Cl](+) epimerizes to minimize steri c interactions in the two diastereomers yielding a 8.2:1 dr in solution. Ne vertheless, upon crystallization a new equilibrium is obtained in the solid and the crystals obtained have a 1:1 dr. These crystals are quasiracemates , which contain equal amounts of the (R-RU) and (S-RU)-Cy-Ru-P-O-Cl stereoc enters within the same crystal. The doubly charged Lewis acid, [CyRu(eta(2) -(R,R)-Ph2PCHMeCHMe Ph2PO-P,O) (solvate)](2+) derived from the chloro compl ex by chloride abstraction with AgSbF6 gave modest ee's (30%) in the Diels- Alder reaction of methacrolein with cyclopentadiene. (C) 2000 Wiley-Liss, I nc.