Ruthenium amino carboxylate complexes as asymmetric hydrogen transfer catalysts

Abstract

The synthesis and characterization of optically active amino carboxylate complexes of formula [(η(6)-arene)Ru(Aa)Cl] (arene = C(6)H(6), C(6)Me(6), Aa = amino carboxylate) as well as those of the related trimers [{(η(6)-arene)Ru(Aa)}(3)][BF(4)](3) are reported. Trimerization takes place with chiral self-recognition: only diastereomers equally configured at the metal, R(Ru)R(Ru)R(Ru) or S(Ru)S(Ru)S(Ru), are detected. The crystal structures of the complexes [(η(6)-C(6)H(6))Ru(Pip)Cl] and [{(η(6)-C(6)Me(6))Ru(Pro)}(3)][BF(4)](3) have been determined by X-ray diffraction methods. Both types of complexes catalyse the hydrogen transfer reaction from 2-propanol to ketones with moderate enantioselectivity (up to 68% ee). The enantiodifferentiation achieved can be accounted for by assuming that Noyori's bifunctional mechanism is operating.

Extending the range of pentasubstituted cyclopentadienyl compounds: The synthesis of a series of tetramethyl(alkyl or aryl)cyclopentadienes (Cp ∗ R ), their iridium complexes and their catalytic activity for asymmetric transfer hydrogenation

“…[22,23,[36][37][38][39][40] All complexes adopt the three-legged piano-stool (half-sandwich) configuration, as expected. As is the case with Cp ⁄ Ir amino acid complexes previously reported, the iridium atom becomes a pseudo-tetrahedral chiral center upon coordination of the amino acid, resulting in diastereomers that differ at configuration of the metal when using an enantiomerically pure amino acid.…”

“…Prior work has shown that the ring based systems are the most selective, at least in the case of aromatic substrates [22,23,44,45]. This is due to only one active hydride complex being formed under ATH conditions.…”

“…22 was identified based on the following information. (23) 0.1000 g (0.107 mmol) of [(g 5 -C 5 Me 4 C 6 H 10 )IrCl 2 ] 2 was combined with 0.0259 g (0.225 mmol) of L-proline and 0.019 g (0.225 mmol) of sodium hydrogen carbonate in a round bottom flask. Upon addition of 30 mL methanol and a magnetic stir bar the mixture slowly turned yellow over the course of 30 min.…”

“…asymmetric transfer hydrogenation (ATH), for which a share of the Nobel Prize was awarded to Ryoji Noyori in 2001 [21]. Noyori and co-workers have shown that half-sandwich compounds of ruthenium are excellent catalysts for ATH [22][23][24]. It has been postulated that interaction of ATH substrates with the methyl groups of C 6 Me 6 ruthenium compounds plays an important role in the enantioselectivity of the products [25].…”

Hydroxymethylpyridine containing half-sandwich complexes of Rh(iii), Ir(iii) or Ru(ii)

“…10 The main difference concerns the Ir-Cl bond length in (R)-3b, which is slightly shorter than those observed in related α-amino carboxylates of the formula [(η 5 -C 5 Me 5 )Ir-(Aa)Cl] (range 2.409(3)-2.421(2) Å). 10c This dissimilarity may be related to minor differences in the intramolecular hydrogen bonding net established in the solid (see below), although a direct relationship is not easy to establish.…”