Biferrocene-based diphosphine ligands: Synthesis and application of walphos analogues in asymmetric hydrogenations

Article abstract of DOI:10.1021/om3012147

A total of four biferrocene-based Walphos-type ligands have been synthesized, structurally characterized, and tested in the rhodium-, ruthenium- and iridium-catalyzed hydrogenation of alkenes and ketones. Negishi coupling conditions allowed the biferrocene backbone of these diphosphine ligands to be built up diastereoselectively from the two nonidentical and nonracemic ferrocene fragments (R)-1-(N,N-dimethylamino)ethylferrocene and (S_Fc)-2- bromoiodoferrocene. The molecular structures of (S_Fc)-2- bromoiodoferrocene, the coupling product, two ligands, and the two complexes ([PdCl₂(L)] and [RuCl(p-cymene)(L)]PF₆) were determined by X-ray diffraction. The structural features of complexes and the catalysis results obtained with the newly synthesized biferrocene-based ligands were compared with those of the corresponding Walphos ligands.

Full text of DOI:10.1021/om3012147

Article
pubs.acs.org/Organometallics
Biferrocene-Based Diphosphine Ligands: Synthesis and Application
of Walphos Analogues in Asymmetric Hydrogenations
Afrooz Zirakzadeh,† Manuela A. Groß,† Yaping Wang,‡ Kurt Mereiter,§ Felix Spindler,∥
and Walter Weissensteiner*,†
†Institute of Organic Chemistry, University of Vienna, Wahringer Straße 38, A-1090 Wien, Austria
̈
‡College of Pharmaceutical Sciences, Capital Medical University, Beijing, No. 10 Xitoutiao, You An Men Beijing 100069, People’s
Republic of China
§Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164SC, A-1060 Vienna,
Austria
∥Solvias AG, Synthese & Katalyse, Romerpark 2, CH-4303 Kaiseraugst, Switzerland
̈
S
* Supporting Information
ABSTRACT: A total of four biferrocene-based Walphos-type ligands have been
synthesized, structurally characterized, and tested in the rhodium-, ruthenium- and
iridium-catalyzed hydrogenation of alkenes and ketones. Negishi coupling conditions
allowed the biferrocene backbone of these diphosphine ligands to be built up
diastereoselectively from the two nonidentical and nonracemic ferrocene fragments
(R)-1-(N,N-dimethylamino)ethylferrocene and (SFc)-2-bromoiodoferrocene. The molecular structures of (SFc)-2-bromoiodo-
ferrocene, the coupling product, two ligands, and the two complexes ([PdCl2(L)] and [RuCl(p-cymene)(L)]PF6) were
determined by X-ray diffraction. The structural features of complexes and the catalysis results obtained with the newly
synthesized biferrocene-based ligands were compared with those of the corresponding Walphos ligands.
Chart 1
INTRODUCTION
■
B i a r y l - ba s ed d i ph o s ph i ne s s u c h a s 2 , 2 ′-bis-
(diphenylphosphino)-1,1′-binaphthyl (BINAP) belong to the
most widely and most successfully used catalyst ligands in
asymmetric catalysis.1 Since Noyori’s pioneering work on the
synthesis and application of BINAP,2 this field has seen
enormous development. Numerous investigations have been
carried out with the aim of making structural variations and
identifying further applications of BINAP. For this purpose the
binaphthyl unit has been replaced by other biaryl backbones,
the phosphine substituents have been varied, and spacers
between the biaryl backbone and phosphine units have been
introduced. From these attempts a variety of BINAP analogues
have evolved and many of these have been applied successfully
in asymmetric catalysis.3
Ito later investigated ligands that contain a biferrocene rather
than a biaryl backbone, and in 1991 he reported the C2-
symmetric BIFEP ligand4 and the TRAP ligand5 family (Chart
1). As an extension of this concept, P-chiral6 and unsymmetri-
cally substituted BIFEP ligands7 were investigated. Interest-
ingly, and in contrast to BINAP, the use of BIFEP ligands for
asymmetric hydrogenations resulted in only moderate product
ee’s. This fact was attributed to the conformational flexibility of
the biferrocene backbone, which is able to rotate around the
biferrocene axis even on coordination to transition metals.7b In
contrast to BIFEP, TRAP-type ligands performed excellently
when used in asymmetric hydrogenations. This behavior was
noted with surprise, as TRAP ligands preferentially adopt a
trans rather than a cis geometry when coordinated to transition
metals.5c,8
A few years later, ligands having a ferrocenyl−aryl backbone
were investigated. In 2002 Lotz and Knochel reported on
ferrocenyl−aryl-based BINAP/BIFEP analogues (Chart 1, Lotz
ligands) but, similarly to BIFEP, only moderate to good
product ee’s were obtained in asymmetric hydrogenations.9
Replacement of the biferrocene backbone of TRAP ligands by a
ferrocenyl−aryl unit (Chart 1, Schuecker ligands) also gave
trans-coordinating ligands, which in the hydrogenations of
Received: December 18, 2012
Published: February 11, 2013
© 2013 American Chemical Society
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Scheme 1
alkenes gave acceptable to excellent resultsalbeit with a
rather limited scope of substrates.10
Enantiopure (SFc)-2 could be obtained by applying Kagan’s
sulfoxide methodology.20 It is well-known not only that p-
tolylsulfinyl units direct diastereoselectively to ortho positions
but also that they can be exchanged with other electrophiles,
even when positioned next to a tributylstannyl unit. Therefore,
(R,SFc)-2-(p-tolylsulfinyl)-tributylstannylferrocene ((R,SFc)-4)20
was treated with t-BuLi at −78 °C and added to a solution of
tetrabromoethane (Scheme 2).
Like many other ferrocene-based diphosphine ligand
systems,11 the Walphos ligand family had originally been
developed with asymmetric hydrogenations in mind.12 On
complexation with transition metals these ligands form eight-
membered chelate rings and the ligand is coordinated cis to the
metal. Applications in rhodium- and ruthenium-catalyzed
asymmetric hydrogenation of olefins and ketones resulted in
product enantioselectivities of up to 95% and 97%,
respectively.12 Since one Walphos-type ligand proved to be
highly suitable for the asymmetric hydrogenation of a 2-
isopropylcinnamic acid derivative,12b an intermediate in the
synthesis of the renin inhibitor Aliskiren, these ligands were
commercialized and are now available on a technical scale. In
addition to hydrogenations,13 including cluster-based hydro-
genations,14 Walphos-type ligands were successfully applied in a
number of other enantioselective catalytic reactions such as
cycloadditions,15 coupling reactions,16 copper hydride mediated
reactions,17 and others.18 While originally developed as a
potential alternative to BINAP, the Walphos ligand family is
now considered a privileged class of ligands in its own right and
searches for novel applications are still continuing. On the basis
of the success of these ligands, we questioned whether Walphos
analogues with a biferrocene instead of a ferrocenyl−aryl
backbone could be synthesized and were curious as to how they
would perform in asymmetric catalysis. It is clear that such an
approach would not only result in a novel class of catalyst
ligands but would also bridge the structural gap between
BIFEP- and TRAP-type ligands.
Scheme 2
This resulted in a mixture of the desired product (SFc)-5 and
tributylstannylferrocene in a ratio of 87:13. The mixture could
not be fully separated by chromatography. Treatment of this
21
mixture with iodine in CH2Cl2 gave, after chromatography
with heptane as the eluent, bromoiodoferrocene (SFc)-2 in 60%
overall yield (based on 4). The molecular structure of (SFc)-2
could be determined by X-ray diffraction and this confirmed
not only its structural integrity but also its S absolute
configuration (Figure 1, top left; for crystallographic data see
the Supporting Information).
We report here the synthesis of four biferrocene-based
Walphos analogues and their application in the rhodium-,
ruthenium-, and iridium-catalyzed hydrogenation of alkenes
and ketones. A comparison of the results obtained with the
newly synthesized ligands and those of the corresponding
Walphos ligands is also discussed. This comparison concerns
the structural features of ligands, two of their palladium and
ruthenium complexes, and the hydrogenation results.
The Negishi coupling22 of (R)-1 and (SFc)-2 worked best
when the catalyst system [Pd2(dba)3]·CHCl3/tris(2-furyl)-
phosphine (tfp) was used. The desired diastereomer
(R,SFc,RFc)-3, which has a C2-symmetric biferrocene-2,2″-diyl
backbone, was obtained in 78% yield. A small amount of
(R,SFc,RFc)-3 was further reacted with BH3·THF to give the
amine−borane complex (R,SFc,RFc)-3·BH3. The molecular
structure of this compound was determined by X-ray diffraction
(Figure 1, top center; for crystallographic data see the
Supporting Information).
RESULTS AND DISCUSSION
■
Synthesis of Ligands and Complexes. In analogy to our
previously reported Walphos synthesis,12 a Negishi coupling of
(R)-1-(N,N-dimethylamino)ethylferrocene ((R)-1) with 2-
bromoiodoferrocene was attempted (Scheme 1). This approach
should ensure the high modularity of the original synthesis
route and should allow the phosphine units to be varied easily
and independently. It is clear, however, that a coupling reaction
between (R)-1 and racemic 2-bromoiodoferrocene would lead
to two coupling products, (R,SFc,RFc)-3 and its diastereomer
(R,SFc,SFc)-3. Therefore, in order to synthesize either of these
derivatives diastereoselectively, enantiomerically pure (RFc)- or
(SFc)-2 was needed.19
Diastereomer (R,SFc,RFc)-3 was further reacted with n-BuLi
and subsequently quenched with chlorodiphenylphosphine or
bis(3,5-dimethyl-4-methyoxyphenyl)chlorophosphine (Scheme
3). In both cases the bromide could be exchanged selectively
and the phosphines (R,SFc,RFc)-6 and (R,SFc,RFc)-7 were
obtained. Oxidation of these two aminophosphines with
H2O2 resulted in the formation of phosphine oxides
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Figure 1. Molecular structures of (SFc)-2, (R,SFc,RFc)-3·BH3, (R,SFc,RFc)-16, (R,SFc,RFc)-17, [PdCl2(15)] (molecule 1), and [RuCl(p-
−
cymene)(15)]PF6 showing 50% displacement ellipsoids. Hydrogen atoms and PF6 are omitted for clarity.
phenyl]phosphine,19 and the resulting phosphine−phosphine
Scheme 3
oxide 13a was isolated in its pure form in 66% yield.
In the last step, the reduction of phosphine oxides 10−13
with polymethylhydrosiloxane/titanium isopropoxide23 gave
the desired ligands (R,SFc,RFc)-14−17. The molecular struc-
tures of ligands 16 and 17 were determined by X-ray diffraction
and are both shown in Figure 1 (16, top right; 17, bottom left;
for crystallographic data see the Supporting Information). It
should be mentioned that the substitution patterns of
(R,SFc,RFc)-14−17 correspond to those of Walphos ligands
SL-W003-1 (14), SL-W002-1 (15), SL-W001-1 (16), and SL-
W005-1 (17) (Chart 2).
Since we had previously determined the molecular structures
of the palladium dichloride complex [PdCl2(L)]12a,24 and the
25
ruthenium p-cymene complex [RuCl(p-cymene)(L)]PF6 of
Walphos ligand SL-W002-1, the corresponding complexes of
the ligand (R,SFc,RFc)-15 were synthesized. The palladium
complex [PdCl2(15)] was obtained by reacting 15 with
[PdCl2(CH3CN)2], whereas reaction of the same ligand with
the ruthenium dimer [RuCl2(p-cymene)]2 and subsequent
treatment of the reaction mixture with ammonium hexafluor-
ophosphate gave the cationic ruthenium complex [RuCl(p-
cymene)(15)]PF6. The molecular structures of both complexes
of 15 were determined by X-ray diffraction and are depicted in
Figure 1 ([PdCl2(15)], bottom center; [RuCl(p-cymene)-
(15)]PF6, bottom right; for crystallographic data see the
Supporting Information).
Structural Features of Ligands and Complexes. The
overall structural features of ligands 16 and 17 and of the
precursor 3 (Figure 1) are highly comparable. In all three cases
the biferrocene backbone adopts a helical arrangement with the
mean planes defined by carbons C1−C5 (Cp) and C11−C15
(Cp″), which are rotated with respect to each other by
(R,SFc,RFc)-8 and (R,SFc,RFc)-9. Phosphine oxide (R,SFc,RFc)-8
could also be obtained directly from 3 when diphenylphos-
phinyl chloride was used as the electrophile.
In two further steps the dimethylamino group of 8 and 9 was
replaced by a dicyclohexyl- or diarylphosphino unit. The
reaction of phosphine oxide 8 in acetic acid at 70 °C with
dicyclohexylphosphine yielded, after chromatography, the bis
phosphine oxide 10b as the main product. Reaction of 8 with
diphenyl- or bis[3,5-bis(trifluoromethyl)phenyl]phosphine
mainly gave the phosphine−phosphine oxides 11a and 12a
(Scheme 4) together with small amounts of bisphosphine
oxides 11b and 12b.12a These mixtures were used in the last
step without further separation. In a way similar to that for 8,
compound 9 was reacted with bis[3,5-bis(trifluoromethyl)-
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Scheme 4
preoriented for metal coordination. Complexation of ligand 15
to a PdCl2 or to a RuCl(p-cymene) unit changes the ligand
conformation consistently. The biferrocene backbone of both
complexes again adopts a helical arrangement but with a much
smaller dihedral angle between Cp and Cp″ than in the free
ligand (83.5(1) and 82.3(1)° for the two independent
molecules of the palladium complex and 76.9(1)° for the Ru
complex). The orientation of the diphenylphosphino units is
also adjusted to achieve the complexation of both phosphorus
atoms to the metal. This rearrangement places the phenyl rings
bound to P2 in comparable positions, as can be seen from the
torsion angles C11−C12−P2−C35 and C11−C12−P2−C41
(Pd complex, 62.3(2)/67.6(2) and 177.8(2)/177.8(2)° for the
two independent molecules of this crystal structure; Ru
complex, 59.7(2) and 167.5(2)°).
Chart 2
124.3(1)° (16), 126.0(1)° (17) and 123.0(1)° (3). The
diarylphosphino units of 16 and 17 are reasonably well
As mentioned above, biferrocene ligand 15 can be considered
an analogue of Walphos ligand SL-W002-1 (Chart 2), in which
Figure 2. Superpositions of the molecular structures of [PdCl2(15)] and [PdCl2(SL-W002-1)] (left) and of [RuCl(p-cymene)(15)]PF6 with
[RuCl(p-cymene)(SL-W002-1)]PF6 (right; only the cations of the ruthenium complexes are shown). Ligand 15 is shown in green and SL-W002-1 in
blue. In both cases the Cp carbons C1−C5 and Fe1 are superimposed.
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Chart 3. Substrates Used for Catalyst Screening
the Walphos backbone phenyl ring is replaced by a second
ferrocene unit. It was therefore of interest to see how this
change in ligand structure would be reflected in the molecular
structures of the corresponding complexes [PdCl2(L)] and
[RuCl(p-cymene)(L)]PF6 (L = SL-W002-1, 15). For this
purpose geometrical parameters of the corresponding com-
plexes were analyzed (for representative superpositions see
Figure 2, and for a comparison of geometrical parameters see
the Supporting Information, Table S9).
slipped π−π interaction between the Cp ring C1−C5 and the
phenyl ring C35−C40 (with reference to the atom designation
in [PdCl2(15)]), which is generally present in PdCl2−Walphos
complexes, is absent in [PdCl2(15)].
A very similar picture is seen when the molecular structures
of the ruthenium complexes [RuCl(p-cymene)(15)]PF6 and
[RuCl(p-cymene)(SL-W002-1)]PF6 are superimposed. Re-
placement of the Walphos ferrocenyl−aryl backbone by a
biferrocene unit causes structural changes almost identical with
those found for the corresponding palladium complexes. The
phosphorus atom P2 and the whole ruthenium cymene unit are
moved away from the side chain substituted ferrocene, and this
is again reflected in a change of the tilt angle of Cp planes
against the P1−Ru−P2 plane (tilt angle 137.7(1)° for the
biferrocene complex and 118° for the Walphos complex). The
diphenylphosphino phenyl rings are also rearranged, and this
change is most pronounced for phenyl ring C35−C40.
In summary, both the PdCl2 and the RuCl(cymene) units
coordinate in a fashion very similar to that for Walphos ligand
SL-W002-1 and to biferrocene ligand 15, but it is clear that a
change from the Walphos ligand SL-W002-1 to its biferrocene
analog (R,SFc,RFc)-15 results mainly in a conformational change
of the phosphine aryl rings, and this leads to a significantly
different structural surrounding of the coordinated Pd and Ru
metals.
Asymmetric Hydrogenations. All newly synthesized
Walphos analogues 14−17 of R,SFc,RFc absolute configuration
were tested as ligands in the asymmetric hydrogenation of
alkenes and a few ketones (Chart 3). All catalysts were formed
in situ by reacting the ligands with an appropriate Rh, Ru, or Ir
source, and depending on the substrate, standard test reaction
conditions were used. A first set of reactions was carried out on
a customized Symyx high-throughput screening (HTS) system
with the substrates MAC, MAA, DMI, MCA, AllylOH, (Z)-
EAAC, DHNAA, and ketone MPG (Chart 3). These reactions
were carried out with ligands 14−17 in combination with the
metal precursors [RhCl(NBD)]2, [Rh(NBD)2]BF4, [Rh-
(COD)2]O3SCF3, [Ru(COD)(OOCCF3)2], [RuCl2(p-cym-
ene)]2, and [Ir(COD)2]BARF (BARF = B[3,5-
(CF3)2C6H3]4). For these reactions MeOH or THF was used
as the solvent.
Superposition of the molecular structures of complexes
[PdCl2(15)] and [PdCl2(SL-W002-1)] (Figure 2, left) shows
that the exchange of the backbone aryl ring of Walphos for a
ferrocene unit leads to barely any change in the position of the
ethyl side chain carbons and of the side chain bound
phosphorus atom (P1; for atom numbering see Figure 1) but
it does lead to a significant difference in the position of the
phosphorus atom attached to the backbone ferrocene unit
(P2). This change follows directly from the geometrical
differences of the six-membered aryl unit and the five-
membered ferrocene Cp″ ring to which P2 is bound. In
comparison to the Walphos complex [PdCl2(SL-W002-1)], P2
in the complex [PdCl2(15)] is shifted away from the ferrocene
Cp″ ring (P2 is moved away from carbon C1 by about 0.3 Å in
the direction of the biferrocene axis). A further significant
change is seen in the conformation of the phenyl rings of the
two diphenylphosphino units. In particular, one phenyl ring
attached to P2 (C35−C40) is rotated in such a way that its
interaction with Cp‴ (C16−C20) is minimized. Release of this
steric interaction is furthered by an additional out-of-plane
deformation that moves P2 a distance of 0.1 Å above Cp″
(C11−C15). This is in contrast to the Walphos complex, in
which the corresponding phosphorus atom is also moved out of
plane by 0.2 Å but in the opposite direction (below the
backbone phenyl ring).
Overall, replacement of the Walphos backbone phenyl ring
by a ferrocene unit causes the palladium unit to be positioned
higher above and further away from the ferrocene Cp ring. This
situation is consistent with a tilt of the square-planar metal unit
with Cp (tilt angle of planes Cp/P1−Pd−P2: 78.4(1) and
80.5(1)° in [PdCl2(15)] versus 59° in [PdCl2(SL-W002-1)]).
In addition, the arrangement of the phenyl rings surrounding
the palladium center is changed significantly and the orienta-
tional freedom of the phenyl rings at P2 is curtailed. The
The best results were obtained with rhodium catalysts, and
these are summarized in Table 1. The ruthenium and iridium
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Table 1. Selected High-Throughput Screening Results Obtained with Ligands (R,SFc,RFc)-14−17 in the Rhodium-Catalyzed
a
Hydrogenation of Double Bonds
entry
substrate
metal precursor
ligand
solvent
conversn, %
ee, %
1
MAC
[Rh(NBD)Cl]2
[Rh(NBD)Cl]2
[Rh(NBD)2]BF4
[Rh(NBD)Cl]2
[Rh(NBD)2]BF4
[Rh(NBD)Cl]2
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(COD)2]O3SCF3
[Rh(NBD)2]BF4
[Rh(NBD)Cl]2
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)Cl]2
[Rh(NBD)Cl]2
[Rh(NBD)Cl]2
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)Cl]2
[Rh(NBD)Cl]2
[Rh(COD)2]O3SCF3
[Rh(COD)2]O3SCF3
[Rh(NBD)Cl]2
[Rh(NBD)2]BF4
[Rh(NBD)Cl]2
[Rh(NBD)Cl]2
[Rh(NBD)Cl]2
[Rh(NBD)Cl]2
[Rh(NBD)Cl]2
[Rh(NBD)2]BF4
[Rh(COD)2]O3SCF3
[Rh(NBD)Cl]2
[Rh(NBD)Cl]2
[Rh(NBD)Cl]2
14
15
16
16
17
14
15
16
16
17
14
15
16
17
14
15
16
17
14
15
16
17
14
15
16
17
14
15
16
17
14
15
16
17
THF
>99
>99
>99
18
47
83
91
81
82
59
51
89
81
76
31
26
77
54
47
94
46
61
73
40
80
84
50
40
42
49
19
24
28
22
51
46
50
44
2
MAC
THF
3
MAC
MeOH
THF
4
MAC
5
MAC
MeOH
THF
>99
>99
>99
>99
>99
>99
99
6
MAA
7
MAA
MeOH
MeOH
THF
8
MAA
9
MAA
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
MAA
MeOH
THF
DMI
DMI
MeOH
MeOH
THF
99
DMI
>99
98
DMI
MCA
THF
>99
80
MCA
THF
MCA
MeOH
MeOH
THF
>99
>99
>99
>99
>99
>99
>99
>99
60
MCA
AllylOH
AllylOH
AllylOH
AllylOH
EAAC
EAAC
EAAC
EAAC
DHNAA
DHNAA
DHNAA
DHNAA
MPG
THF
THF
THF
THF
MeOH
THF
THF
86
THF
>99
81
THF
THF
16
MeOH
THF
>99
99
MPG
THF
99
MPG
THF
>99
99
MPG
THF
a
Reaction conditions: substrate, 41.67 μmol; catalyst, 1.67 μmol; catalyst loading S/C, 25; Vtotal, 500 μL.
precursors tested gave only poor to moderate results (see the
Supporting Information, Table S1). Conversions and product
enantiomeric excesses of ≥80% could be reached in the
rhodium-catalyzed hydrogenations for methyl acetamidoacry-
late (MAA), α-substituted cinnamic acid derivatives such as
methyl (Z)-α-acetamidocinnamate (MAC) and (E)-2-methyl-
cinnamic acid (MCA), and (E)-2-methyl-3-phenyl-prop-2-enol
(AllylOH). The best results for the hydrogenation of substrates
MAC, MAA, and DMI were obtained with ligands 15 and 16 in
combination with the precursor [Rh(NBD)2]BF4 in MeOH
(Table 1, entries 2−4, 8, 9, and 13) and for MCA with
[RhCl(NBD)]2 in THF (Table 1, entry 16). In the case of
AllylOH the combination of ligands 16 or 17 together with
precursor [Rh(COD)2]O3SCF3 in THF performed the best
(Table 1, entries 21 and 22).
For some of the substrates (MAC, DMI, MCA) the results
obtained with ligands 15 and 16 compare reasonably well with
data reported for the corresponding Walphos ligands SL-W002
and SL-W001.
As a result, it seemed interesting to investigate the
performance of these ligands in more detail and to extend
the scope of substrates to (E)-2-phenylcinnamic acid (PCA)
and to the commercially interesting cinnamic acid derivatives
IPCA-D12b,13g,26 and EOCA-D13k,27 (Chart 3), as well as to the
ketones acetylacetone (ACA) and ethyl 3-oxo-pentanoate
(EOP). Hydrogenations were carried out with the substrates
MAC, MAA, DMI, MAC, PCA, IPCA-D, EOCA-D, ACA, and
EOP (Chart 3), with biferrocene ligands (R,SFc,RFc)-15 and
(R,SFc,RFc)-16, with Walphos ligands SL-W001-1 (with R,RFc
configuration) and SL-W002-2 (with the opposite S,SFc
configuration), and with precursors [Rh(NBD)2]BF4 in
MeOH or [RhCl(NBD)]2 in THF. The results of these
hydrogenations are given in Table 2. The majority of reactions
were carried out in single experiments in either a glass or a steel
autoclave. For biferrocene ligands 15 and 16 and substrates
MAC, MAA, DMI, and, in part, MCA not all the high-
throughput screening results were repeated (Table 2, entries
1−8), but in these cases at least one reaction for each substrate
was run in a single experiment.
For example, the hydrogenation of MAA was carried out with
ligand 15, precursor [Rh(NBD)2]BF4 in MeOH, and an S/C
ratio of 100. The product was obtained with 100% conversion
and 50% ee, and this compares very well with the HTS result
(100% conversion and 51% ee; Table 2, entry 3).
Comparison of the data shows that substrates MAC, MAA,
and DMI can be hydrogenated to products with good to
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Table 2. Comparison of Hydrogenation Data Obtained with Use of Walphos and Biferrocene Ligands
T,
°C
p,
atm
Walphos
ligand
BiFc
ligand
entry
substrate
precursor
solvent
S/C conversn, % ee, %
S/C conversn, % ee, %
a
1
MAC
MAC
MAA
MAA
DMI
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[RhCl(NBD)]2
[RhCl(NBD)]2
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[RhCl(NBD)]2
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[RhCl(NBD)]2
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
THF
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
80
80
80
80
1
1
W002-2
W001-1
W002-2
W001-1
W002-2
W001-1
W002-2
W001-1
W002-2
W002-2
W002-2
W001-1
W002-2
W001-1
W002-2
W002-2
W001-1
W002-2
W002-2
W001-1
W002-2
W001-1
100
100
100
100
100
100
100
100
100
25
100
100
100
100
100
100
99
88
63
96
97
87
52
62
83
15
16
15
16
15
16
15
16
15
15
15
16
15
16
15
15
16
15
15
16
15
16
25
25
100
100
100
100
99
21
91
51
89
26
77
40
46
a
a
a
a
a
a
a
2
3
1
25
4
1
25
5
1
25
6
DMI
1
25
100
100
100
19
7
MCA
MCA
MCA
MCA
PCA
20
20
20
20
50
50
50
50
50
3.5
3.5
20
80
80
80
80
25
8
100
11
25
9
n.d.
n.d.
89
100
25
76
80
29
44
34
54
87
19
44
16
80
85
33
75
10
11
12
13
14
15
16
17
18
19
20
21
22
THF
14
63
MeOH
MeOH
MeOH
MeOH
THF
100
100
100
100
25
77
100
100
25
100
100
100
100
21
PCA
100
100
100
7
82
74
94
IPCA-D
IPCA-D
IPCA-D
25
n.d.
25
b
EOCA-D
EOCA-D
EOCA-D
MeOH
MeOH
THF
25
100
80
80
87
86
85
96
76
93
25
100
100
100
100
100
94
b
b
40
25
25
100
100
100
100
100
25
c
ACA
[RuI2(p-cymene)]2 MeOH
[RuI2(p-cymene)]2 MeOH
[RuI2(p-cymene)]2 EtOH
500
500
100
100
500
500
100
100
c
ACA
c
EOP
c
EOP
[RuI2(p-cymene)]2 EtOH
100
a
b
c
HTS results. Additive NaOMe. Additive HCl.
excellent ee values (77−97%; Table 2, entries 1−6) on using
either the Walphos ligands or their biferrocene analogues.
However, analogous ligands do not perform equally well. For
example, the hydrogenation of MAC with the Walphos ligand
SL-W002 led to the product with 88% ee, while that with the
analogous ligand 15 gave rise to only 21% ee. The Walphos
ligand SL-W001 gave 63% ee, while its biferrocene analogue 16
gave the product with 91% ee (Table 2, entries 1 and 2).
On the basis of the HTS results, for the hydrogenation of
MCA the catalyst system 15/[RhCl(NBD)]2/THF seemed to
be an interesting alternative to the original SL-W001/
[Rh(NBD)2]BF4/MeOH system (Table 1, entry 16). However,
a number of single experiments showed that, in order to
achieve useful conversions, not only high pressure but also a
high catalyst load is required. In an effort to identify the
appropriate reaction conditions, a number of test runs were
carried out. A conversion of 15% was obtained with 1% of
catalyst and a hydrogen pressure of 5 bar, but an increase in the
pressure to 50 bar only led to a marginal improvement in yield
to 17%. Only when the catalyst load was raised to 4% did the
conversion increase significantly. At a pressure of 20 bar 63%
(Table 2, entry 10) and at 50 bar 90% conversion was reached.
Interestingly, when ligand 15 was replaced by Walphos SL-
W002 and the catalyst system SL-W002/[RhCl(NBD)]2/THF
was applied, conversion became very low even when 4% of
catalyst was used (Table 2, entries 9 and 10). On the basis of
the high catalyst load required, it is clear that the 15/
[RhCl(NBD)]2/THF system must be considered of little
practical use, despite the reasonably high product ee obtained
(80%; Table 2, entry 10).
biferrocene ligands (PCA, SL-W002/15, 89/29% ee; IPCA-D,
SL-W001/16, 94/54% ee; EOCA-D, SL-W001/16, 87/44% ee;
Table 2, entries 11, 14, and 17).
In a way similar to that for MCA, the substrate IPCA-D gave
the 15/[RhCl(NBD)]2/THF catalyst system product with a
reasonably high ee value (87%)albeit again with very low
conversion (Table 2, entry 15).
Diketone ACA and β-keto ester EOP were hydrogenated at
80 bar and 80 °C in MeOH (ACA) or EtOH (EOP) with
[RuI2(p-cymene)]2 as the metal precursor. The best results
were obtained with Walphos ligand SL-W001 (ACA 96% ee
and EOP 93% ee, quantitative conversion), but with these
substrates the biferrocene analogue 16 also performed well
(ACA 85% ee and EOP 75% ee, quantitative conversion; Table
2, entries 21 and 22).
In all reactions the absolute configurations of products were
determined, and these are given in Table 3. It is important to
note that in the hydrogenation reactions we used Walphos
ligands with different absolute configurations. Ligand SL-W001-
1 had an R,RFc configuration, and ligand SL-W002-2 had an
S,SFc configuration. For ease of comparison the results given in
Table 3 are normalized. Results are given for ligands with the R
configuration at the ferrocene side chain carbon (Walphos
ligands with R,RFc and biferrocene ligands with R,SFc,RFc
configuration).
From the results in Table 3 it is immediately apparent that
the product absolute configurations obtained with analogous
Walphos28 and biferrocene ligands are not identical. For
example, with the substrate MAC and ligands SL-W001-1 and
SL-W002-1 (both with the R,RFc configuration) the product
with the S configuration was formed. However, with
biferrocenes (R,SFc,RFc)-15 and (R,SFc,RFc)-16 products of S
and R configurations, respectively, were obtained (Table 3,
entries 1 and 2). It is also clear that for all alkenes tested the use
of Walphos ligands SL-W001 and SL-W002 led to products of
Like MCA, substrates PCA, IPCA-D, and EOCA-D represent
2-substituted cinnamic acid derivatives with the carboxyl group
in a position trans to the aryl ring at position 3. In all of these
cases the [Rh(NBD)2]BF4/MeOH system performed much
better with Walphos ligands than with their analogous
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Organometallics
Article
Table 3. Product Absolute Configurations Given for
Walphos Ligands of R,RFc and Biferrocene Ligands of
R,SFc,RFc Absolute Configuration
Chart 4
Walphos
ligand
abs
BiFc
abs
entry substrate
precursor
confign ligand confign
1
MAC
MAC
MAA
MAA
DMI
DMI
MCA
MCA
MCA
MCA
PCA
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
[RhCl(NBD)]2
[RhCl(NBD)]2
[Rh(NBD)2]BF4
[Rh(NBD)2]BF4
W002
W001
W002
W001
W002
W001
W002
W001
W002
W002
W002
W001
W002
W001
W002
W002
W001
W002
W002
S
S
S
S
R
R
S
S
15
16
15
16
15
16
15
16
15
15
15
16
15
16
15
15
16
15
15
S
2
R
S
3
4
R
R
S
5
6
7
R
S
8
9
R
R
R
R
R
R
S
10
11
12
13
14
15
16
17
18
19
R
R
R
R
PCA
and cinnamic acid derivatives as well as for two ketones. A
comparative study was also carried out with two biferrocene-
based ligands (15 and 16) and their analogous Walphos ligands
(SL-W002 and SL-W001).
IPCA-D [Rh(NBD)2]BF4
IPCA-D [Rh(NBD)2]BF4
IPCA-D [RhCl(NBD)]2
EOCA-D [Rh(NBD)2]BF4
EOCA-D [Rh(NBD)2]BF4
EOCA-D [RhCl(NBD)]2
S
S
A comparison of the structural features of the complexes
[PdCl2(15)] and [RuCl(p-cymene)(15)]PF6 with those of
[PdCl2(SL-W002)] and [RuCl(p-cymene)(SL-W002)]PF6
showed that the replacement of the Walphos backbone aryl
ring by a ferrocene unit results mainly in a change in the
position of the diphenylphosphino unit directly attached to the
biferrocene backbone. This leads to a conformational change of
the phosphine aryl rings and thus to a significantly different
structural surrounding of the coordinated Pd and Ru metals.
The Walphos ligands and their analogous biferrocene ligands
showed significantly different performances in the hydro-
genation reactions. These reactions gave not only different
product ee values but, even more significantly, for a number of
substrates products with opposite absolute configurations were
obtained. Overall, only in some cases did the biferrocene-based
ligands perform as well or nearly as well as their Walphos
analogues.
Interestingly, in the rhodium-catalyzed hydrogenations with
Walphos ligands SL-W001 and SL-W002 it was found that, in
order to account for the observed product absolute
configurations, all alkene substrates used must coordinate in
the enantioselective step to the metal in a uniform way. This
structural correlation does not hold for the corresponding
biferrocene ligands 15 and 16.
S
S
S
S
ACA
ACA
EOP
EOP
[RuI2(p-
cymene)]2
R,R
S,S
20
21
22
[RuI2(p-
cymene)]2
W001
W002
W001
S,S
S
16
15
16
S,S
S
[RuI2(p-
cymene)]2
[RuI2(p-
cymene)]2
S
S
identical configuration (MAC, MAA, MCA, EOCA-D, S; DMI,
PCA, IPCA-D, R), but this is not the case for the biferrocene
ligands 15 and 16.
A closer analysis of the product absolute configurations
obtained with Walphos ligands showed another surprising
feature. It is evident that in order to obtain a product of S
configuration, in the enantiomer-determining step of the
hydrogenation reaction the substrate must be coordinated to
the metal through its Si side. A structural analysis showed that
in order to account for the observed product absolute
configurations, all alkene substrates used must coordinate to
the metal in a uniform way. In the enantiomer-determining step
rhodium has to coordinate to the side of the alkene that has the
carboxyl (or alkoxycarbonyl) group (COOR1), with the α (R2)
and the β substituent (R3) arranged in a counterclockwise
manner (Chart 4). This order does not depend on whether or
not the substituent R2 is able to coordinate to the metal. Such a
compelling relationship was not observed for the hydro-
genation of ketones or for any reactions in which the
biferrocene ligands were used.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving full experimental
descriptions and spectroscopic data for all newly synthesized
ligands and complexes as well as detailed hydrogenation and
transfer hydrogenation data, crystallographic and geometric
data for precursors (SFc)-2 and (R,SFc,RFc)-3·BH3, ligands
(R,SFc,RFc)-16 and (R,SFc,RFc)-17, and complexes [PdCl2(15)]
and [RuCl(p-cymene)(15)]PF6. This material is available free
of charge via the Internet at http://pubs.acs.org.
CONCLUSION
■
Four biferrocene analogues of Walphos ligands were prepared
by replacing the Walphos ferrocenyl−aryl backbone with a
biferrocene unit. The biferrocene backbone was built up by a
Negishi coupling of two nonracemic ferrocene fragments. For
this purpose a synthesis had to be developed for (S)-2-
bromoiodoferrocene. The newly synthesized ligands were
tested in both a high-throughput screening system and single
experiments in the hydrogenation of alkenes and ketones.
Moderate to high product ee values were obtained for acrylic
AUTHOR INFORMATION
Corresponding Author
*E-mail: walter.weissensteiner@univie.ac.at.
■
Notes
The authors declare no competing financial interest.
1082
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Organometallics
Article
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ACKNOWLEDGMENTS
■
We thank the Austrian Science Foundation FWF (project
P23376-N19), the Beijing Nova Program (2009 B 37), and the
E d u c a t i o n a l C o u n c i l F o u n d a t i o n o f B e i j i n g
(KM201010025012, PHR201008395, and PHR201007114) as
well as Solvias AG for their strong support of this work.
UMICORE is kindly acknowledged for a generous gift of metal
complexes.
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(28) In our first report12b some product absolute configurations were
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