Synthesis, Characterization, and Application in Asymmetric Hydrogenation Reactions of Chiral Ruthenium(II) Diphosphine Complexes

Four new complexes of the type [Ru(CF3CO2)2(X)2(PP)] 7 (X = methanol or ethanol, PP = chiral diphosphine) have been synthesized from the reaction of PP with [Ru2(CF3CO2)4(H2O)(COD)2] and characterized by both NMR spectroscopy and single-crystal X-ray diffraction studies. A common feature of these complexes is a pseudooctahedral geometry for ruthenium, an unusual monodentate ligation mode for the mutually trans trifluoroacetate groups, along with two solvent molecules coordinated in mutual cis positions (MeOH for 7a and 7b, EtOH for 7c). 7a (PP = (2S,4S)-BDPP) belongs to the orthorhombic P212121 space group, Z = 4, a = 9.896(1) Å, b = 19.099(3) Å, and c = 19.689(2) Å. 7b (PP = (4S,5S)-DIOP) also crystallizes in the orthorhombic P212121 space group, Z = 4, a = 12.679(1) Å, b = 16.744(2) Å, and c = 18.944(2) Å. 7c (PP = (R)-1-[(S)-2-(diphenylphosphino)ferrocenyl]ethyldicyclohexylphosphine, abbreviated as PPFCy2) belongs to the monoclinic P21 space group, Z = 4, a = 18.153(2) Å, b = 12.364(2) Å, c = 20.431(1) Å, and β = 97.40(1)°. The air-stable [RuCl(p-cymene)(PP)]PF6 9 were synthesized from the reaction of PP with [RuCl2(p-cymene)]2 in a refluxing CH2Cl2/EtOH mixture, followed by metathesis with KPF6 in MeOH, in 72−89% yields (PP = (S)-(R) or (R)-(S)-2-(diphenylphosphino or arsino)ferrocenyl]ethylbisphosphine derivative). 9a (PP = (S)-(R)-PPFCy2 crystallizes in the monoclinic space group P21, Z = 2, a = 11.965(2) Å, b = 14.852(3) Å, c = 13.999(2) Å, and β = 111.50(2)°. Complexes 7 were probed for their catalytic behavior in the hydrogenation of methyl acetamidocinnamate 10, acetamidocinnamic acid 11, and dimethyl itaconate 12. Using a typical substrate to catalyst ratio of 50−100:1 and p(H2) = 50 bar, modest enantiomeric excesses (ee) of up to 75% were achieved. The highest ee's were attained in CH2Cl2 or a 5:4 THF/CH2Cl2 mixture, whereas higher activities were observed when methanol was employed as solvent. The addition of NEt3 had a beneficial effect on the ee of the reactions involving 11, whereas with 10 a detrimental effect was observed. The reactions involving 12 gave poor enantioselectivity.