Faller J W, Liu X, Parr J
Department of Chemistry, Yale University, New Haven, Connecticut, USA.
Chirality. 2000 Jun;12(5-6):325-37. doi: 10.1002/(SICI)1520-636X(2000)12:5/6<325::AID-CHIR5>3.0.CO;2-6.
Heterobidentate chelating ligands comprising P,O-donor sets produce chiral metal centers when bound to arene-ruthenium complexes. The cymene complex [CyRu(eta2-chelate-P,O)Cl]+ cations are precursors to 16-electron dicationic strong Lewis acids which have potential use in asymmetric catalysis. Sixteen-electron complexes of this type, however, also provide a pathway with a low energy barrier to epimerization of the metal center in intermediates. With the chiral ligand (R,R)-chiraphos monoxide, the metal center in [CyRu(eta2)-(R,R)-Ph2PCHMeCHMe Ph2PO-P,O)Cl]+ epimerizes to minimize steric interactions in the two diastereomers yielding a 8. 2:1 dr in solution. Nevertheless, 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 (RRu) and (SRu)-CyRu-P-O-Cl stereocenters within the same crystal. The doubly charged Lewis acid, [CyRu(eta2-(R, R)-Ph2PCHMeCHMe Ph2PO-P,O)(solvate)]2+ derived from the chloro complex by chloride abstraction with AgSbF6 gave modest ee's (30%) in the Diels-Alder reaction of methacrolein with cyclopentadiene.
