Diastereoselectivity an 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(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.
Faller
J W
JW
Department of Chemistry, Yale University, New Haven, Connecticut, USA. jack.faller@yale.edu
Liu
X
X
Parr
J
J
D000097:Acetonitriles / Q000737:chemistry; D000143:Acids / Q000737:chemistry*; D000447:Aldehydes / Q000737:chemistry; D002384:Catalysis; D000081007:Cymenes; D004583:Electrons; D008024:Ligands; D008670:Metals; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D039821:Monoterpenes / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry*; D017975:Ruthenium Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D013237:Stereoisomerism; D014867:Water / Q000737:chemistry
D000097:Acetonitriles; D000143:Acids; D000447:Aldehydes; D000081007:Cymenes; D008024:Ligands; D008670:Metals; D039821:Monoterpenes; D017975:Ruthenium Compounds; D014867:Water; C051707:1-methyl-3-isopropylbenzene; D010100:Oxygen; C032159:acetonitrile