Ruthenium Complexes of Phosphino-Substituted Ferrocenyloxazolines in the Asymmetric Hydrogenation and Transfer Hydrogenation of Ketones: A Comparison

Abstract

Three novel routes have been developed for the synthesis of ferrocenylbased phosphino-oxazolines in which the phosphino unit is attached to a ferrocenylmethyl or a ferrocenylethyl side chain. In two of the routes the phosphino-substituted ethyl side chain was built up diastereoselectively. Ruthenium complexes of the type [RuCl 2 PPh 3 (L)] of 12 bidentate phosphine-oxazoline ligands were synthesized, characterized, and tested in the transfer hydrogenation of acetophenone. For the best performing complexes a total of 12 additional ketones were screened in transfer hydrogenations and hydrogenations under transfer hydrogenation conditions. Two catalyst precursors in particular delivered products with an enantiomeric excess of up to 98% in transfer hydrogenations and 99% ee in hydrogenations. The transfer hydrogenation results obtained with all novel ligands were compared to those of two well-established FOXAP ligands. Furthermore, a qualitative comparison with the hydrogenation data was carried out. In both cases surprising similarities in product enantiomeric excess and product absolute configuration were found. Attempts were made to rationalize some of the observed features by considering a transition-state model. The molecular structures of one synthesis intermediate, two catalyst precursors, and two corresponding acetonitrile complexes were studied by X-ray diffraction.

Enantioselective Transfer Hydrogenation of Ketones Catalyzed by a Manganese Complex Containing an Unsymmetrical Chiral PNP′ Tridentate Ligand

“…[a] Reaction conditions: Acetophenone (1 mmol), i PrOH (5 mL), 25 °C. [b] 10 bar=1.0 MPa [c] Determined by GC on a β‐Dex 110 (30 m) column …”

Synthesis and Characterization of Half‐Sandwich Ruthenium Complexes Containing Aromatic Sulfonamides Bearing Pyr­id­inyl Rings: Catalysts for Transfer Hydrogenation of Acetophenone Derivatives

“…[36,49] The stronger the base, the higher the general conversion rankings: KOH Ͼ NaOH Ͼ KOtBu Ͼ K 2 CO 3 . NaOH and KOH are known to yield better conversions than K 2 CO 3 and KOtBu in TH reactions.…”

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Keywords: Transfer hydrogenation / Sulfonamides / Ruthenium / N ligands benzenesulfonamides 1-6 were successfully synthesized by the reaction of 8-aminoquinoline and various benzenesulfonyl chlorides. [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52] Herein, a series of half-sandwich Ru II complexes containing sulfonamide ligands were synthesized and characterized by various spectroscopic techniques. The synthesized compounds were characterized by NMR and FTIR spectroscopy and elemental analysis, and compounds 8 and 9 were further ana-

[a]

3224 lyzed by X-ray diffraction.

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Ferrocene-derived P,N ligands: synthesis and application in enantioselective catalysis

“…Enantioselectivities of > 95% ee were usually obtained using i-PrOH as a hydrogen donor [78][79][80][81][82].…”