Chiral Bis(oxazolinyl)phenyl Ru^II Catalysts for Highly Enantioselective Cyclopropanation

Abstract

Transition-metal complexes with monoanionic meridional ligands have been widely utilized in stoichiometric and catalytic reactions, such as CÀH, CÀC, and CÀO bond activation and alkane dehydrogenation, because of their unique reactivity.[1] Novel PCP and NCN pincer ligands have recently made it possible to design new pincer-type metal complexes using a combination of two-electron donors and an anionic center. In this context, introduction of a chiral moiety as part of the ligand framework to create transition-metal catalysts for asymmetric reactions has been investigated. Recently, several highly reactive Ru and Os complexes with chiral CNN-type ligands [2] and Fe and Co complexes with chiral bis(pyridylimino)isoindole ligands [3] were reported as active chiral catalysts.During our investigation of transition-metal catalysts with chiral meridional NCN ligands, we reported the bis(oxazolinyl)phenyl (phebox) ligand as one of a larger family of ligands including bis(oxazolinyl)pyridine (pybox).[4] Although the meridional geometry of the phebox ligand is similar to that of the pybox ligand, the metal complex with the phebox ligand possesses a metal-carbon s bond that may induce different reactivities. We anticipated that the simple change from a benzene substituent to a pyridine ring as the ligand backbone would lead to a change in the catalytic performance of the complex. Indeed, phebox-Rh catalysts were found to catalyze 1,4-conjugated reductions of a,b-unsaturated carbonyls and related reductive aldol coupling reactions with high selectivity.[4] As part of our effort to develop Ru catalysts, we recently synthesized new phebox-Ru dinuclear complexes 1 and 2, which have been successfully applied as catalysts for enantioselective hydrogenation and transfer hydrogenation of aromatic ketones.[5]

Designing ‘Totem’ C2‐Symmetrical Iron Porphyrin Catalysts for Stereoselective Cyclopropanations

“…[83,84] All the other starting materials were commercial products used as received. Analytical data of cyclopropanes 3-10, [65,69,85,86] 12, [85] 16, [87] and 17 [88] were in accordance with reported data (see Supporting Information).…”

Phosphatation of Zeolite H‐ZSM‐5: A Combined Microscopy and Spectroscopy Study

“…The dimeric complex was in turn treated with sodium acetate or sodium acetylacetonate at room temperature to destroy the dimeric structure and furnish (Phebox‐ i‐P r)Ru(CO) (acetate)/(acac), 41 and 42 , respectively in good yield (Scheme ) 54. Additionally, treatment of the Phebox ligand 39a with the same ruthenium precursor under similar conditions with replacement of zinc metal by magnesium affords the monomeric pincer complex [RuCOCl(Phebox)(H 2 O)], 43 54c…”

“…Intramolecular cyclopropanation with high yield and ee value was also observed. It is believed that replacement of the aqua coordinated to the metal with diazo compounds triggers the catalytic cycle 54c…”

Asymmetric Cyclopropanation Reactions

“…111 Conversely, bis(oxazolinyl)phenyl ruthenium complex (Ru-phebox; 0.5 mol%) was efficient for the cyclopropanation reactions of various styrene derivatives with α-diazoacetate (Scheme 37). 112 The reaction proceeded in high yields of 85-92% with 98-99% ee and 4.5:1 to 24:1 dr for the trans-(1R,2R)-cyclopropane carboxylates.…”