Diastereoselectivity in Chiral Ruthenium Complexes of Bidentate Bisphosphine Monoxide Ligands: Controlling Epimerization in Aldehyde Complexes and 16-Electron Intermediates
journal contributionposted on 10.07.1999, 00:00 by J. W. Faller, Ben P. Patel, Mauricio A. Albrizzio, Michael Curtis
Heterobidentate and hemilabile ligands involving P,O-donor chelates produce chiral metal centers when bound to arene−ruthenium complexes. This chirality in cymene complexes produces diastereotopic methyl groups in the isopropyl ligand which serve as a detector of the chirality at the metal. [CyRu(η2-chelate-P,O)Cl]+ cations are precursors to strong 16-electron dicationic Lewis acids which have potential use in asymmetric catalysis. Sixteen-electron complexes of this type, however, provide a pathway with a low barrier to racemization or epimerization of the metal center in intermediates, such as [CyRu(η2-chelate-P,O)(PhCHO)]2+. Substitution of the central carbon in diphenylphosphinomethane monoxide (dppmO) forces the ligand to adopt a configuration with the substituent in an endo position, thus forcing the 16-electron intermediate to return diastereoselectively to its original configuration and prevents epimerization. Thus, an X-ray structure shows that (R*Ru,R*C)-[CyRu(η2- Ph2PCHR)Ph2PO-P,O)Cl]+ is the preferred diastereomeric pair. In the parent, [CyRu(η2-dppmO-P,O)(ligand)]2+, racemization occurs at the metal center, since there is nothing driving the preferential formation of either the R or S ruthenium center. When the ligands are chiral, however, the metal center epimerizes to minimize steric interactions in the two diastereomers. The equilibrium between [(RRu)-CyRu(η2-dppmO-P,O)(RC-ligand)]2+ and [(SRu)-CyRu(η2-dppmO-P,O)(RC-ligand)]2+ reflects a 37% de for (1R)−(−)-myrtenal. Since a substituent on the central carbon prevents epimerization at the metal center, this diastereoselectivity is reflected in a preference for binding of (RC)-ligand by either [(RRu,RC)-CyRu(η2-Ph2PCHPr)Ph2PO-P,O)]2+ or [(SRu,SC)-CyRu(η2-Ph2PCHPr)Ph2PO-P,O)]2+. An X-ray structure of rac-[(RRu*,RC*)-CyRu(η2-Ph2PCHPr)Ph2PO-P,O)(PhCHO]2+ shows that the aldehyde assumes an orientation that would suggest one stereoface of the aldehyde may be more susceptible to attack.