Diastereoselectivity in chiral ruthenium complexes of bidentate bisphosphine monoxide ligands: Controlling epimerization in aldehyde complexes and 16-electron intermediates

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(eta(2)-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(eta(2)-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(eta(2)- Ph2PCHR)Ph2PO-P,O)Cl](+) is the preferred diastereomeric pair. In the parent, [CyRu(eta(2)-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 [(R-Ru)-CyRu(eta(2)-dppmO-P,O)(R-C-ligand)](2+) and [(S-Ru)-CyRu(eta(2)-dppmO-P,O)(R-C-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 (R-C)-ligand by either [(R-Ru,R-C)CyRu(eta(2)-Ph2PCHPr)Ph2PO-P,O)](2+) or [(S-Ru,S-C)-CyRu(eta(2)-Ph2PCHPr)Ph2PO-P,O)](2+). An X-ray structure of rac-[(R-Ru*,R-C)-CyRu(eta(2)-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.