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Synthesis, Characterization, and Application in Asymmetric Hydrogenation Reactions of Chiral Ruthenium(II) Diphosphine Complexes
journal contribution
posted on 1996-01-23, 00:00 authored by Nadia C. Zanetti, Felix Spindler, John Spencer, Antonio Togni, Grety RihsFour 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.
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Keywords
cis positionsCF 3 CO 2enantiomeric excessesTHFspace group P 2 1monodentate ligation modeBDPPNEt 3Ru 2DIOPP 2 1 2 1 2 1 space groupacetamidocinnamic acid 11PPFCy 27 b7 cKPF 650 barÅ.trans trifluoroacetate groupsCH 2 Cl 2catalyst ratioRuCl 2Asymmetric Hydrogenation ReactionsP 2 1 space groupmethyl acetamidocinnamate 10dimethyl itaconate 12pseudooctahedral geometryNMR spectroscopychiral diphosphine
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