Chiral and Nonchiral [OsX2(diphosphane)(diamine)] (X: Cl, OCH2CF3) Complexes for Fast Hydrogenation of Carbonyl Compounds

Abstract

The osmium complexes trans-[OsCl₂(dppf)(diamine)] (dppf: 1,1′-bis(diphenylphosphino)ferrocene; diamine: ethylenediamine in 3, propylenediamine in 4) were prepared by the reaction of [OsCl₂(PPh₃)₃] (1) with the ferrocenyl diphosphane, dppf and the corresponding diamine in dichloromethane. The reaction of derivative 3 with NaOCH₂CF₃ in toluene afforded the alkoxide cis-[Os(OCH₂CF₃)₂(dppf)(ethylenediamine)] (5). The novel precursor [Os₂Cl₄(P(m-tolyl)₃)₅] (2) allows the synthesis of the chiral complexes trans-[OsCl₂(diphosphane)(1,2-diamine)] (6–9; diphosphane: (R)-[6,6′-dimethoxy(1,1′-biphenyl)-2,2′-diyl]bis[1,1-bis(3,5-dimethylphenyl)phosphane] (xylMeObiphep) or (R)-(1,1′-binaphthalene)-2,2′-diylbis[1,1-bis(3,5-dimethylphenyl)phosphane] (xylbinap); diamine=(R,R)-1,2-diphenylethylenediamine (dpen) or (R,R)-1,2-diaminocyclohexane (dach)), obtained by the treatment of 2 with the diphosphane and the 1,2-diamine in toluene at reflux temperature. Compounds 3–5 in ethanol and in the presence of NaOEt catalyze the reduction of methyl aryl, dialkyl, and diaryl ketones and aldehydes with H₂ at low pressure (5 atm), with substrate/catalyst (S/C) ratios of 10 000–200 000 and achieving turnover frequencies (TOFs) of up to 3.0×10⁵ h⁻¹ at 70 °C. By employment of the chiral compounds 6–9, different ketones, including alkyl aryl, bulky tert-butyl, and cyclic ketones, have successfully been hydrogenated with enantioselectivities up to 99 % and with S/C ratios of 5000–100 000 and TOFs of up to 4.1×10⁴ h⁻¹ at 60 °C.