“Chiral-at-Metal” Octahedral Ruthenium(II) Complexes with Achiral Ligands:  A New Type of Enantioselective Catalyst

Abstract

cis-[Ru(dmp)(2)(CH(3)CN)(2)][PF(6)](2) (dmp = 2,9-dimethyl-1,10-phenanthroline), complex 1[PF(6)](2), exists in two enantiomeric forms, Delta and Lambda. During treatment with the chiral anion tris[tetrachlorobenzene-1,2-bis(olato)]phosphate(V), also named Trisphat, in dichloromethane it has been possible to selectively precipitate each enantiomer, associated with Trisphat in the form of the heterochiral pair. This enantiomerically pure compound has been characterized in solution by UV-visible, CD, ESI-MS, and NMR spectroscopy and by X-ray crystallography in the solid state. Trisphat was also used as an NMR chiral shift reagent to determine the enantiomeric excess of the complex preparations. The "chiral-at-metal" ruthenium complex has been evaluated as a catalyst for the oxidation of sulfides to sulfoxides by hydrogen peroxide. The reactions displayed a low but significant level of enantioselectivity (18% ee in the case of 4-bromophenyl methyl sulfide). Our results thus provide the first demonstration that the chiral information carried by a stereogenic metal center can be catalytically transferred to molecules during stereoselective oxidation.

Crystallization‐Induced Asymmetric Transformation of Chiral‐at‐metal Ruthenium(II) Complexes Bearing Achiral Ligands

“…In both structures, as was observed for 2-(PF 6 ) 2 , the cis N-Ru-N angles between the two nitrogen atoms of two different dmp ligands and between the nitrogen atom of one dmp ligand and the amine nitrogen atom located in front of the methyl substituents are increased by about 108 and 88, respectively (that is, N(2)-Ru-N(3) and N(2)-Ru-N(5) for 3-(PF 6 ) 2 ; Table 1). The steric hindrance generated by the methyl groups is probably also at the origin of the curvatures observed for every phenanthroline, with angles of 14.5±17.18 between the two planes of the pyridinyl moiet- [9] To evaluate its photochemical stability, the isolated opti- These results, namely, the selective crystallization of the heterochiral pair and the light-dependent racemization, led us to use the following strategy for converting the racemic 1-(PF 6 ) 2 complex into a unique optically active diastereomer. [nBu 3 NH][L-Trisphat] (2.1 equiv) was added to a dichloromethane solution of the complex 1-(PF 6 ) 2 and the resulting suspension was then irradiated as described above.…”

“…However, the preparation of chiral-at-metal complexes with only achiral ligands required a subsequent step to substitute the optically active ligands with achiral ones and this has to be achieved without any racemization. [8] Considering these problems and with the aim of preparing optically active chiral-at-metal ruthenium(ii) bis(diimine) complexes bearing only achiral ligands, [9] we investigated a new strategy allowing the efficient preparation of such complexes from racemic mixtures. This is an original and rare reaction in coordination chemistry, namely, an asymmetric transformation induced by light and selective crystallization in the presence of the chiral Trisphat anion.…”

“…6 ] 2 (1-(PF 6 ) 2 , dmp= 2,9-dimethyl-1,10-phenanthroline) was prepared according to a described procedure. [9] Three new complexes, namely, rac-…”

Asymmetrische Katalyse über die Ligandensphäre von oktaedrischen Komplexen mit chiralem Metallzentrum

“…[25,26] [28] Beispielsweise wurde aus der Hydrierung von (E)-1-Hexyl-2-nitrostyrol mit dem Ditert-butyl-Derivat des Hantzsch-Esters in Toluol bei Raumtemperatur bei nur 0.1 Mol-% von L-7 das Produkt (R)-2-Phenyl-1-nitrooctan in 89 % Ausbeute mit 94 % ee erhalten (Schema 5 a). Da der entwickelte Iridium-Komplex inert gegenüber Substitution ist, muss die beobachtete Katalyse vollständig durch die Ligandensphäre vermittelt werden.…”

Metal-templated chiral Brønsted base organocatalysis

“…We recently introduced octahedral chiral-at-metal [22][23][24][25][26][27][28][29][30][31][32][33][34][35] iridium(III) complexes as inert templates for the design of lowloading asymmetric hydrogen bonding catalysts 36,37 . Octahedral stereocenter [38][39][40] permit the straightforward generation of compounds with high shape and stereochemical complexity and should therefore be highly suitable as structural templates for multifunctional catalysts.…”