Cooperative bimetallic asymmetric catalysis: Comparison of a planar chiral ruthenocene bis-palladacycle to the corresponding ferrocene

Article abstract of DOI:10.1021/cs500393x

Cooperative asymmetric catalysts often offer advantages in terms of activity, stereoselectivity, and generality as compared to more traditional single point activation catalysts. In cooperative bimetallic catalysis, the intermetallic distance is a crucial parameter for the outcome of a reaction and an optimal synergy of both metal centers. We have recently developed a number of catalytic asymmetric reactions, which are efficiently catalyzed by a planar chiral ferrocene based bispalladacycle and for which the cooperativity of two Pd centers has already been demonstrated. To get more insight into the role of the Pd/Pd distance in such metallocene bismetallacycles, in the present study a corresponding ruthenocene based Pd₂-complex has been prepared by the first direct diastereoselective biscyclopalladation of a chiral ruthenocene ligand. In addition, the first highly diastereoselective direct monocyclopalladation of a homochiral ruthenocene is reported. The effect of the increased Cp/Cp distance within the ruthenocene bispalladacycle has been examined in four catalytic asymmetric applications: the aza-Claisen rearrangement of Z-configured allylic N-aryltrifluoroacetimidates, the direct 1,4-addition of α-cyanoacetates to enones, a tandem azlactone formation/1,4-addition to enones and a tandem reaction to form quaternary α-aminosuccinimides by in situ azlactone formation, 1,4-addition to a nitroolefin, and a Nef-type nitro-to-carbonyl transformation as key steps. For each reaction studied, it was found that with some substrates the ferrocene based catalyst is superior, whereas for other substrates the ruthenocene backbone is more favorable. The ruthenocene based bispalladacycle can thus be considered to be a useful and complementary alternative for cooperative bimetallic catalysis.

Full text of DOI:10.1021/cs500393x

Research Article
pubs.acs.org/acscatalysis
Cooperative Bimetallic Asymmetric Catalysis: Comparison of a Planar
Chiral Ruthenocene Bis-Palladacycle to the Corresponding Ferrocene
Tina Hellmuth, Stefan Rieckhoff, Marcel Weiss, Konstantin Dorst, Wolfgang Frey, and Rene Peters*
́
Institut fur Organische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
̈
̈
S
* Supporting Information
ABSTRACT: Cooperative asymmetric catalysts often offer
advantages in terms of activity, stereoselectivity, and generality
as compared to more traditional single point activation
catalysts. In cooperative bimetallic catalysis, the intermetallic
distance is a crucial parameter for the outcome of a reaction
and an optimal synergy of both metal centers. We have
recently developed a number of catalytic asymmetric reactions,
which are efficiently catalyzed by a planar chiral ferrocene
based bispalladacycle and for which the cooperativity of two
Pd centers has already been demonstrated. To get more
insight into the role of the Pd/Pd distance in such metallocene
bismetallacycles, in the present study a corresponding
ruthenocene based Pd2-complex has been prepared by the first direct diastereoselective biscyclopalladation of a chiral
ruthenocene ligand. In addition, the first highly diastereoselective direct monocyclopalladation of a homochiral ruthenocene is
reported. The effect of the increased Cp/Cp distance within the ruthenocene bispalladacycle has been examined in four catalytic
asymmetric applications: the aza-Claisen rearrangement of Z-configured allylic N-aryltrifluoroacetimidates, the direct 1,4-addition
of α-cyanoacetates to enones, a tandem azlactone formation/1,4-addition to enones and a tandem reaction to form quaternary α-
aminosuccinimides by in situ azlactone formation, 1,4-addition to a nitroolefin, and a Nef-type nitro-to-carbonyl transformation
as key steps. For each reaction studied, it was found that with some substrates the ferrocene based catalyst is superior, whereas for
other substrates the ruthenocene backbone is more favorable. The ruthenocene based bispalladacycle can thus be considered to
be a useful and complementary alternative for cooperative bimetallic catalysis.
KEYWORDS: 1,4-addition, cyclopalladation, imidazoline, rearrangement, ruthenocene
INTRODUCTION
■
Inspired by the intriguing mode of action of dinuclear
metalloenzymes and their impressive catalytic efficiency,1 the
synergistic cooperation of two metal centers is currently also
intensively investigated for artificial catalyst systems, as it
frequently offers substantial advantages in terms of catalytic
activity and stereoselectivity.2 In the context of cooperative
asymmetric catalysis, our group has recently developed a planar
chiral ferrocene bisimidazoline bispalladacycle (FBIP, Figure
1)3,4 as a useful bimetallic catalyst, which enables various
different catalytic asymmetric reaction types.5−7
Figure 1. [FBIP-Cl]2 and [RuBIP-Cl]2.
A parameter of key importance for bimetallic catalysis is the
intermetallic distance of both catalytically relevant metal
centers.2 In this context, a major conceptual advantage of a
ferrocene (and other metallocene) backbone(s) seems to be a
partial rotational freedom around the Cp−Fe (or Cp−M) axes,
which allows the catalyst to adopt an appropriate distance of
both cooperating metal centers to simultaneously activate two
reacting substrates (or functional groups) via a bimetallic
pathway.8 The ferrocene based bismetallacycles can thus readily
open and close like a pair of scissors employing only a few
degrees of rotational freedom.
Due to the significance of the intermetallic distance in
bimetallic catalysis,2 we were interested in the effect of an
increased distance of both Cp-ligands in the metallocene core
on the catalytic performance in enantioselective bis-Pd
catalyzed reactions. Since it is known that the distance between
both Cp rings is significantly larger in the parent ruthenocene
Received: March 25, 2014
Revised: April 23, 2014
© XXXX American Chemical Society
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(distance of both Cp-rings ca. 3.68 Å)9 as compared to the
parent ferrocene (ca. 3.32 Å),10 we have investigated the
formation of the analogous ruthenocene bisimidazoline
bispalladacycle (RuBIP) and now report its synthesis and
structure as well as the catalytic performance in several
reactions, in which FBIP has previously been investigated
allowing for a comparison. In addition, we describe the
synthesis of a related ruthenocene imidazoline monopallada-
cycle which represents the first highly diastereoselective
monocyclopalladation of a ruthenocene. This monopalladacycle
was used for control reactions.
and NaOAc in a mixture of degassed tert-butanol/1,2-
dichloroethane at 80 °C furnished the C2-symmetric bispallada-
cycle in good yield and as a single diastereomer.
The constitution and the (all-Sp)-configuration19 of this
compound were confirmed by X-ray crystal structure analysis
(Figure 2).20 The structure closely resembles the one found for
the corresponding ferrocene derivative [FBIP-Cl]2.3a,b In the
dimeric structure of [RuBIP-Cl]2, two chloride bridged
palladium square planes are arranged in a nearly coplanar
fashion, both in the upper and lower Cp planes. While chloride
bridged ferrocenyl monoimidazoline palladacycle complexes
and other halide bridged palladacycles usually form geometric
cis/trans isomers around the PdII-centers,12 in the case of
[RuBIP-Cl]2 (and also [FBIP-Cl]2),3a,b the macrocyclic
structure enforces a coordination sphere, in which the all-
(Sp)-configured complexes form exclusively all-trans-configured
isomers, meaning that both imidazoline N-donors in the same
(PdCl)2-plane are always placed trans to each other. Like
expected, the most significant difference between the crystal
structure of [FBIP-Cl]2 and [RuBIP-Cl]2 is the distance of the
centroids of both Cp ligands in the same metallocene fragment
with values of 3.281−3.293 Å and 3.568−3.595 Å, respectively.
In contrast, the other bond lengths and also the bond angles
and conformations are very similar for both complexes.21 For
example, in the case of [RuBIP-Cl]2 the Pd−C/C′−Pd twist
angles are between 33.1(8)° and 35.9(8)° (resulting Pd/Pd
distances of 3.982(2) Å and 3.998(2) Å), while for [FBIP-Cl]2
they are between 34.6(3)° and 35.8(3)° (resulting Pd/Pd
distances of 3.7468(7) Å and 3.7685(7) Å).
RESULTS AND DISCUSSION
■
Synthesis and Structural Analysis of the Dimeric
Bispalladacycle [RuBIP-Cl]2. Compared to the cyclopallada-
tion of ferrocenes,11,12 the cyclopalladation of ruthenocene
derivatives has only been scarcely investigated. Most of the few
known methods provide racemic13 or achiral14 palladacycles.
To our knowledge the attempts toward direct enantioselec-
tive15 or diastereoselective16 cyclopalladations of ruthenocenes
providing planar chiral Pd(II)-complexes always resulted in
poor to moderate stereoselectivity thus far. Highly enantioen-
riched ruthenocene palladacycles have only very recently been
reported by Kundig et al., who employed indenyl derived
̈
ruthenocene substrates already containing the element of
planar chirality prior to the cyclopalladation step, thus elegantly
avoiding a stereoselectivity issue upon the cyclometalation
event.17
No examples for direct biscyclopalladations of ruthenocenes
have been reported so far. For the development of a
diastereoselective version of this reaction type, we have
prepared the ruthenocene bisimidazoline ligand RuBI (Scheme
1). The short ligand synthesis works in close analogy to the one
Synthesis and Structural Analysis of Ruthenocene
Imidazoline Monopalladacycle [RuIP-Cl]2. The corre-
sponding chloride bridged ruthenocene imidazoline pallada-
cycle dimer [RuIP-Cl]2 was formed by the first highly
diastereoselective direct monocyclopalladation of a chiral
ruthenocene ligand (Scheme 2). The synthesis started from
ruthenocene and could be performed in close analogy to the
corresponding ferrocene imidazoline palladacycle [FIP-
Cl]2.29g,k Monolithiation of ruthenocene22 and subsequent
trapping with CO2 provided ruthenocene carboxylic acid 2,
which was transformed into the primary amide 3 via acid
chloride formation (Scheme 2). The amide function was then
activated by [Et3O]BF4 for the subsequent formation of an
imidazoline, which was N-protected by tosylation. The direct
cyclopalladation using Na2[PdCl4] and NaOAc in MeOH
provided the palladacycle in high yield and with high
diastereoselectivity with regard to the element of planar
chirality. Since the chloride bridged palladacycles form
geometric cis/trans isomers around the PdII-centers resulting
Scheme 1. Diastereoselective Synthesis of the Chloride
Bridged Bispalladacycle [RuBIP-Cl]2
1
in relatively complex H NMR spectra, the diastereoselectivity
was estimated after formation of the monomeric acac complex
RuIP-acac, which is obtained in quantitative yield by treatment
of [RuIP-Cl]2 with Na(acac) in MeOH/benzene (Scheme 3).
The diastereomeric ratio of 20:1 is somewhat higher than for
the corresponding ferrocene (dr = 18:1).29k
The expected (Sp)-configuration of this compound could be
confirmed by X-ray crystal structure analysis of the chloride
bridged dimer, since the trans-isomer selectively crystallized
from a solution in CH2Cl2 (Figure 3).23 The structure is nearly
C2-symmetric in the solid state. In contrast to [RuBIP-Cl]2 the
four-membered (PdCl)2 ring is puckered with a dihedral angle
Pd−Cl−Pd−Cl of around 27°. Both ruthenocene moieties are
placed on the concave face of the four-membered ring, which is
also in analogy to structures of halide bridged ferrocene
of the corresponding ferrocene ligand.3a,b Dilithiation of
ruthenocene18 and trapping of the resulting biscarbanion with
N,N-dimethylthiocarbamoyl chloride provided bisthioamide 1.
The latter was activated by triethyloxonium tetrafluoroborate
for the bisimidazoline formation by treatment with the chiral
diamine (R,R)-1,2-diamino-1,2-diphenylethane ((R,R)-
DADPE). Both imidazoline rings were subsequently N-
tosylated to give the RuBI ligand. The direct biscyclopalladation
also proceeded efficiently under conditions optimized for the
synthesis of FBIP.3c Thus, treatment of RuBI with Na2[PdCl4]
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Figure 2. X-ray single crystal structure analysis of [RuBIP-Cl]2 (color code: C (gray); N (blue); O (red); S (yellow); Cl (green); Ru (turquoise); Pd
(magenta)). Hydrogen atoms and chloroform (7 per unit cell) are omitted for clarity in the ORTEP plot (ellipsoids at 50% probability level). Two
different views are shown. Selected bond lengths (Å) and angles (deg): C−Pd, 1.914(16)−2.00(2); N−Pd, 1.977(17)−2.075(16); Clcis‑to‑C−Pd,
2.309(5)−2.329(5); Cltrans‑to‑C−Pd, 2.428(5)−2.443(5); C−Pd−N, 78.7(8)−83.1(7); C−Pd−Clcis‑to‑C, 93.1(6)−95.8(6); N−Pd−Cltrans‑to‑C
93.3(4)−95.5(4); Cl−Pd−Cl, 90.48(17)−90.97(16); N−Pd−Clcis‑to‑C, 173.0(5)−175.0(4); C−Pd−Cltrans‑to‑C, 172.9(6)−176.2(6).
,
Scheme 2. Diastereoselective Synthesis of the Chloride
Bridged Palladacycle [RuIP-Cl]2
Figure 3. X-ray single crystal structure analysis of [RuIP-Cl]2 (color
code: C (gray); N (blue); O (red); S (yellow); Cl (green); Ru
(turquoise); Pd (magenta)). Hydrogen atoms and dichloromethane (1
per unit cell) are omitted for clarity in the ORTEP plot (ellipsoids at
50% probability level). Selected bond lengths (Å) and angles (deg):
C−Pd, 1.959(6)−1.967(5); N−Pd, 2.025(5)−2.042(5); Clcis‑to‑C-Pd,
2.3328(14)−2.3348(14); Cltrans‑to‑C-Pd, 2.4506(13); C−Pd−N,
80.4(2)−80.6(2); C−Pd−Clcis‑to‑C, 93.35(18)−94.41(17); N−Pd−
Cltrans‑to‑C, 96.01(13)−97.22(13); Cl−Pd−Cl, 88.87(5)−88.92(5);
Scheme 3. Formation of the Monomeric RuIP-acac for the dr
Determination
N−Pd−Clcis‑to‑C, 173.59(13)−173.64(13); C−Pd−Cltrans‑to‑C
174.87(16)−176.02(17).
,
distance would require different Pd−C/C′−Pd′ twist angles
and thus different reactive conformations for both catalyst
systems, which should result in different reactivities and
stereoselectivities. To allow for a first comparison, we have
selected several catalytic asymmetric reactions,26 for which we
have previously already demonstrated that a bimetallic catalyst
is either essential for sufficient reactivity or high enantiose-
lectivity or for the formation of a certain stereoisomer.
One of these applications is the catalytic asymmetric [3,3]-
rearrangement of (Z)-configured allylic N-aryltrifluoroacetimi-
dates, which is known as Overman or aza-Claisen rearrange-
ment.27 The bis-Pd catalyst FBIP3a,b and the corresponding
heterodinuclear Pd−Pt catalyst28 are the most active
enantioselective catalysts known so far for the rearrangement
of ZCC-configured allylic N-aryltrifluoroacetimidates. The
allylic imidate rearrangement catalyzed by chiral palladacycles
palladacycles.24 For the distance of the centroids of both Cp
ligands in one metallocene fragment, values between 3.627 and
3.641 Å have been determined.25
Comparison of RuBIP to FBIP in Asymmetric Catalysis.
As already mentioned above, the metal/metal distance is a
crucial parameter for cooperative effects in bimetallic catalysis.2
Since for identical Pd−C/C′−Pd′ twist angles the intermetallic
distance is larger in RuBIP as compared to FBIP, we were
interested in the effect of the increased Cp/Cp-distances in
bimetallic asymmetric catalysis. Albeit by rotation around the
Fe−Cp or Ru−Cp axes the Pd/Pd distances can adopt the
same values over a relatively broad range, an identical Pd/Pd-
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is known to proceed stereospecifically, i.e., ECC- and ZCC
-
(entries 1 and 2), but still provided the product with a very high
enantiomeric excess. In contrast, at 55 °C employing only 0.05
mol % of the precatalysts, RuBIP was more active than FBIP
(entries 3 and 4).
Very similar results were also obtained for substrate 4b
carrying a phenethyl residue RZ, either at room temperature
with a higher activity of FBIP (entries 5 and 6) or at 55 °C with
RuBIP being more active, in particular at a precatalyst loading
of only 0.1 mol % (87% vs 43% yield, entries 9 and 10). The
opposite behavior was noticed for the branched substrate 4c
(RZ = iBu, entries 11−14) with a slightly higher activity of
RuBIP at room temperature compared to FBIP, whereas at 55
°C working with low catalyst loadings, the FBIP system was
found to be more active.
configured substrates in general provide different enantiomers
in excess.29 As Z-olefins are often more readily available in
geometrically pure form (avoiding the need for a tedious
separation of isomers), the use of the ZCC-configured
substrates is synthetically attractive. On the other hand though,
the ZCC-substrates are much less reactive than the
corresponding ECC-substrates in the allylic imidate rearrange-
ments. This is due to an unfavorable axial orientation of the Z-
olefin substituent in the accepted halfchair-like transition state
of the rate determining C−N bond formation and the ensuing
cyclic σ-alkyl-Pd intermediate.30 With the above-mentioned
bimetallic complexes, a catalytic activity has been found that is
at least 1−2 orders of magnitude higher than with the most
active mono-Pd complexes using this difficult substrate class.29
This points to a synergistic action of both metal centers, which
has been explained by an initial bimetallic precoordination of
the substrate.28,27d
The conversion was monitored in the case of the
1
rearrangement of substrate 4a at 55 °C via H NMR (Figure
4). This comparison shows a higher activity of the RuBIP
Table 1 shows a comparison of the performance of the
3a,b
[FBIP-Cl]2 and [RuBIP-Cl]2 activated by silver tosylate for
chloride/tosylate ligand exchange (to facilitate the substrate
coordination) under identical reaction conditions previously
optimized for [FBIP-Cl]2.3a,b With substrate 4a carrying an nPr
residue as RZ, the ruthenocene based catalyst showed at room
temperature a slightly lower reactivity than the FBIP complex
Table 1. Comparison of RuBIP and FBIP in the Catalytic
Asymmetric Rearrangement of (Z)-Configured Allylic
Imidates 4
Figure 4. Comparison of the reactivity using the bis-Pd precatalysts
[RuBIP-Cl]2 (blue curve) and [FBIP-Cl]2 (red curve)activated by
AgOTsfor the rearrangement of substrates 4a by monitoring via 1H
NMR (conditions of Table 1, entries 1 and 2).
system in particular at higher conversions. Using 0.05 mol % of
precatalyst, after 24 h, 88% of the rearrangement products are
formed with RuBIP, whereas with FBIP the yield is just 65%
after that time (69% after 72 h). This might either indicate a
higher catalyst stability for the ruthenocene based bispallada-
cycle or a lower level of product inhibition.
Control experiments were performed with the ruthenocene
monopalladacycle under the conditions of Table 1, entry 3, but
using 0.1 mol % of [RuIP-Cl]2 to allow for the same Pd loading
as with 0.05 mol % of [RuBIP-Cl]2. After the catalyst activation
by 0.2 mol % of AgOTs, the product was formed in just 31%
yield with an ee of 68%. As expected, the monopalladacycle is
thus significantly less active and enantioselective in comparison
to the bispalladacycles supporting a cooperation of both metal
centers in RuBIP.
precatalyst
T
yield
ee
a
b
entry
M
4
a
RZ
loading: X mol % [°C] (%)
(%)
1
Ru
Fe
Ru
Fe
Ru
Fe
Ru
Fe
Ru
Fe
Ru
Fe
Ru
Fe
88
96
98
96
97
98
96
94
95
95
87
98
98
96
98
nPr
1.0
0.05
1.0
0.2
0.1
1.0
20
55
20
55
55
20
55
23b
3
96
87
68
88
99
98
90
87
43
90
87
71
86
a
b
b
b
c
nPr
Ph(CH2)2
Ph(CH2)2
Ph(CH2)2
iBu
428
5
63b
7
83b
9
1028
11
123b
13
143b
The above results demonstrate that both FBIP and RuBIP
are excellent catalysts for this reaction type, allowing for
excellent levels of enantioselectivity. It somewhat depends on
the substrate and the conditions, which of both catalysts show
the better catalytic performance.
0.2
0.1
c
iBu
a
b
Yield of isolated product. Determined by HPLC after hydrolysis of
the amide.
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The second application that has been studied is the direct
1,4-addition31 of α-cyanoacetates 6 to enones, which generates
all-C-substituted quaternary stereocenters32 in densely func-
tionalized products. The latter are synthetically attractive chiral
building blocks, e.g., toward enantioenriched α- and β-amino
acid derivatives.33 Our previous studies have shown that mono-
and bispalladacycles can be employed for stereodivergent access
to the addition products: while the use of vinylketones as
Michael-acceptors resulted in the preference of different
enantiomers using either FBIP or a pentaphenylferrocene
monopalladacycle catalyst, the use of cyclic ketones as Michael-
acceptors resulted in epimeric products possessing vicinal
quaternary and tertiary stereocenters.6b While (Sp)-configured
FBIP generated the (R,R)-configured products as the major
diastereomers with high enantioselectivity, the (Sp)-configured
pentaphenylferrocene monopalladacycle preferentially provided
highly enantioenriched (R,S)-diastereomers in excess. Kinetic
studies strongly support a bimetallic reaction pathway with the
dinuclear Pd-complex, in which both substrates are simulta-
neously activated by the two Pd-centers within one catalyst
molecule, culminating in an extraordinarily fast C−C bond
formation elementary step.6 Table 2 shows results obtained
instead of 0.5 mol % of [RuBIP-Cl]2, the product was formed in
a moderate yield of 55% and with only poor diastereoselectivity
(dr = 1.2:1) and low enantioselectivity (45% ee for the major
diastereomer).
The use of RuBIP for the significantly more reactive
methylvinylketone (MVK, 9) as Michael acceptor in combina-
tion with α-cyanoacetate 6a resulted in a nearly quantitative
yield (99%) and high enantioselectivity (ee = 92%, Scheme 4)
Scheme 4. Comparison of RuBIP and FBIP in the Direct 1,4-
Addition of α-Cyanoacetate 6a to MVK
with a precatalyst loading of 0.2 mol %. A comparison with
FBIP under identical reaction conditions (>99% yield, ee =
90%)6a reveals that the enantioselectivity is slightly better for
RuBIP in that case. A control experiment using 0.4 mol % of
[RuIP-Cl]2 provided the 1,4-adduct in 43% yield and as a
racemate, again supporting the cooperativity of both Pd centers
in RuBIP.
Table 2. Comparison of RuBIP and FBIP in the Direct 1,4-
Addition of α-Cyanoacetates to Cyclic Enones
The third application is the 1,4-addition of in situ generated
azlactones34 to enones, which is an attractive step-economic
reaction providing access to masked and functionalized highly
enantioenriched quaternary α-amino acid derivatives of bio-
logical interest.7a−c The azlactones are produced in the reaction
mixture from racemic N-benzoylated α-amino acids and acetic
anhydride, which is used as a cosolvent in combination with
acetic acid as a solvent. Our previous investigations have shown
that [FBIP-Cl]2 activated by silver triflate is capable of forming
the tandem reaction product in good yields and with high
enantioselectivity, if NaOAc is used as a Brønsted base
cocatalyst.7a,c In contrast, pentaphenylferrocene monopallada-
cycles showed poor activity in this reaction type in control
experiments supporting a bimetallic mechanism for FBIP, in
which both Pd-centers cooperate.7a,c,35 This synergy has been
explained by simultaneous activation of the enone electrophile
and the azlactone pronucleophile by different Pd centers.7c
Table 3 shows a comparison of results obtained with [FBIP-
yield
a
ee (%)
b
c
entry
M
Y
Z
n
8
(%)
dr
(R,R)-8
1
Ru
Fe
Ru
Fe
Ru
Fe
Ru
Fe
Ru
Fe
Ru
Fe
Ru
Fe
82
99
84
99
90
75
99
99
90
99
81
85
90
97
5.2:1
8.0:1
4.5:1
4.6:1
7.3:1
11.5:1
3.3:1
7.3:1
5.9:1
8.0:1
8.1:1
6.7:1
3.5:1
4.9:1
81
94
95
90
97
94
87
99
91
93
94
95
80
87
0.5
H
1
a
26b
3
0.5
0.5
H
0
1
1
b
c
d
e
f
46b
5
4-Me
4-Cl
66b
7
0.25
0.5
86b
9
3-OMe
3-Me
106b
11
126b
13
146b
0.5
7a,c
Cl]2 and [RuBIP-Cl]2.
0.25
3-Cl
g
Our previous studies with FBIP had shown that the use of
the racemic N-benzoylated alanine (R1 = Me) is relatively
difficult in terms of enantioselectivity.7a,c A comparison with
RuBIP shows that FBIP is clearly superior for this intricate
example (entries 1 and 2). For the other investigated substrates
the differences are much less pronounced. In the other
examples (entries 3−10) FBIP was found to be always
somewhat superior regarding the enantioselectivity, but the
latter is still high with RuBIP. However, in terms of reactivity
RuBIP was found to be superior for four out of six examples
(entries 1, 5, 7, 11).36
A related reaction has been investigated as a fourth
application, in which nitroolefins 14 are reacted with the
racemic N-benzoylated amino acids 11 and acetic anhydride in
the presence of manganese(II)acetate and acetic acid (Table
4).7e Using the bimetallic RuBIP and FBIP7e systems,
a
b
Yield of isolated product. dr = (R,R + S,S):(S,R + R,S), determined
c
by HPLC. Determined by HPLC.
with RuBIP and FBIP6b for comparison. In general, it can be
stated that both the RuBIP and the FBIP system usually
provide good activity and enantioselectivity. In terms of
diastereoselectivity FBIP is usually superior with only one
exception found (entries 11 and 12). In terms of
enantioselectivity it depends on the substrate which of both
systems is more efficient. A cooperativity of both Pd-centers
like in FBIP is also likely in RuBIP. This is supported by a
control experiment with the mono-Pd catalyst RuIP, which is
significantly less efficient in comparison to RuBIP. Under the
conditions of Table 2/entry 1, but using 1 mol % of [RuIP-Cl]2
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Table 3. Comparison of RuBIP and FBIP in the Domino Azlactone Formation/Michael Addition
a
b
c
entry
precatalyst
13
R1
R2
R3
yield (%)
dr
ee (%)
1
[RuBIP-Cl]2
[FBIP-Cl]2
[RuBIP-Cl]2
[FBIP-Cl]2
[RuBIP-Cl]2
[FBIP-Cl]2
[RuBIP-Cl]2
[FBIP-Cl]2
[RuBIP-Cl]2
[FBIP-Cl]2
[RuBIP-Cl]2
97
95
81
89
45
41
98
85
83
87
93
88
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
>98:2
46
76
97
98
74
81
94
98
83
90
92
96
a
Me
Ph
Me
27c
3
b
c
d
e
f
nPr
Bn
Ph
Ph
Me
Me
Me
Ph
47c
5
67c
7
nPr
nPr
nPr
4−Cl-C6H4
Ph
87c
9
107c
11
127c
2-furyl
Me
[FBIP-Cl]2
a
b
c
1
Yield of isolated product. Determined by H NMR of the isolated product. Determined by HPLC.
Table 4. Comparison of RuBIP and FBIP in the Asymmetric Synthesis of α-Aminosuccinimides 15 by Tandem Azlactone
Formation/Michael Addition/Nef Type Reaction
a
b
c
entry
precatalyst
15
R1
R2
yield (%)
dr
ee (%)
58
82
1
[RuBIP-Cl]2
[FBIP-Cl]2
83
95
49
91
58
46
21
>50:1
>50:1
9:1
a
Me
Ph
27e
3
d
[RuBIP-Cl]2
[FBIP-Cl]2
81 and 87
b
c
nPr
nPr
Ph
47e
5
>50:1
>50:1
>50:1
>50:1
>50:1
93
94
95
57
93
[RuBIP-Cl]2
[FBIP-Cl]2
iPr
67e
7
[RuBIP-Cl]2
d
(CH2)2CO2Me
3-MeOC6H4
87e
[FBIP-Cl]2
67
a
b
c
d
1
Yield of isolated product. Determined by H NMR of the isolated product. Determined by HPLC. For the minor diastereomer.
biologically interesting α-aminosuccinimides are generated. Our
previous investigations suggest a bimetallic reaction pathway
also in this case, because related ferrocene monopalladacycles
provided either no product at all (pentaphenylferrocene core)
under the standard conditions or led to significantly lower
enantioselectivity (ferrocene core).7e
A comparison between the RuBIP and FBIP catalyst in this
tandem reaction,37 which involves an in situ azlactone
formation, a 1,4-addition of the generated azlactone to the
nitroolefin, and a Nef-type-reaction as key steps,38 reveals that
the FBIP catalyst is superior in three out of four cases
investigated in terms of activity and enantioselectivity. Only
with a branched aliphatic nitroolefin, which provided the lowest
reactivity in our previous study using the FBIP catalyst, was the
RuBIP catalyst somewhat superior in terms of reactivity, and a
comparably high enantioselectivity was attained (entries 5 and
6). For this latter example, a control experiment was performed
under identical reaction conditions but with RuIP (10 mol %
[RuIP-Cl]2, activated by 20 mol % AgOTf) providing the
product with lower enantioselectivity (ee = 83%).39
CONCLUSION
■
In conclusion, we have reported the first highly diastereose-
lective direct mono- and biscyclopalladations of chiral
enantiopure ruthenocene derivatives. With N-tosylated imida-
zolines as ortho-directing groups the cyclopalladation con-
ditions previously optimized for the corresponding ferrocene
ligands could be employed and resultedin contrast to earlier
investigations on diastereoselective cyclopalladations of chiral
ruthenocenesin nearly diastereomerically pure palladacycles.
The bispalladacycle [RuBIP-Cl]2 was studied (after activation
by a chloride ligand exchange) as a bimetallic catalyst in a
number of different synthetic applications, in which the
intramolecular cooperation of two Pd centers is likely to play
a crucial role for the reaction outcome. In RuBIP, the Cp/Cp
distance is increased by ca. 10% as compared to FBIP, thus
resulting in larger Pd/Pd distances for identical Pd−C/C′−Pd′
twist angles. Since the intermetallic distance constitutes an
important parameter in bimetallic catalysis, the results obtained
with RuBIP in catalysis were directly compared to results
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acetonitrile (1 mL/5 mg silver salt) was stirred for 6 h at room
temperature under dinitrogen. The mixture was then filtered through a
pad of Celite, and the solvent was removed under reduced pressure. A
stock solution of the activated catalyst was subsequently prepared in
HOAc/Ac2O (7/3).
To the corresponding N-benzoyl amino acid (1.0 equiv), the
corresponding enone (2.0 equiv) was added under a dinitrogen
atmosphere. To this mixture, stem solutions of NaOAc (0.1 equiv) and
the above prepared activated catalyst (starting from 2 mol % [RuBIP-
Cl]2) in HOAc/Ac2O (7/3, total amount of solvent: 0.325 mL for 0.27
mmol substrate) were successively added. The mixture was heated to
30 °C while shaking at 450 rpm. After 23 h the reaction mixture was
cooled to room temperature and was subjected to column
chromatography (silica, pentane/Et2O, 4:1).
General Procedure for the Catalytic Asymmetric Synthesis
of α-Alkyl-α-Amino Succinimides 15 (GP4). [RuBIP-Cl]2 and
silver triflate (4 equiv per dimeric precatalyst molecule) were dissolved
in acetonitrile (1 mL per 5 mg [RuBIP-Cl]2). The solution was placed
in a supersonic bath at room temperature for 30 min. The mixture was
subsequently filtered through Celite, and free acetonitrile was removed
under reduced pressure. A stock solution in acetic acid (3.7 mg
[RuBIP-Cl]2 in 100 μL HOAc) was subsequently prepared.
The corresponding N-benzoyl amino acid (11, 1.2 equiv., 0.30
mmol), the corresponding nitroolefin (14, 1.0 equiv., 0.25 mmol), and
manganese acetate (5.0 equiv., 1.25 mmol, 216.3 mg) were charged
into a vial. A vacuum was then applied, and the vial was subsequently
flushed with nitrogen (3 times repeated). n-Hexane (0.25 mL) and the
activated catalyst (prepared from 0.05 equiv., 12.5 μmol, 31.6 mg
[RuBIP-Cl]2 as described above) as a stock solution in acetic acid
(0.85 mL) were added. After acetic anhydride (225 μL) was added, the
mixture was directly warmed to 50 °C. After 25 h at this temperature,
the mixture was cooled to room temperature, and chloroform (ca. 12
mL) was added. The resulting mixture was washed with water (ca. 50
mL) twice. The aqueous phases were extracted with chloroform (ca.
10 mL). The combined organic phases were then dried over Na2SO4.
After filtration and concentration, the crude material was used for silica
gel chromatography (PE:EE 1.5:1) to isolate the targeted compound.
previously reported for the corresponding ferrocene [FBIP-
Cl]2. From all four reaction types studied, it is obvious that the
catalytic outcome is influenced by the metallocene Cp/Cp
distance. For each reaction studied, it was found that with some
of the investigated substrates the ferrocene based catalyst is
superior, whereas for other substrates the ruthenocene
backbone is more favorable, even though the catalytic
applications have only been optimized for the FBIP system,
but not for RuBIP. Based on the above investigations, we
consider RuBIP to be a useful and complementary alternative
to FBIP.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were performed in oven-
dried glassware under a positive pressure of nitrogen. THF,
acetonitrile, and dichloromethane were dried under N2 over molecular
sieves in a solvent purification system. The solvents chloroform,
methanol, and ethyl acetate were used as purchased from commercial
suppliers. Solvents were usually removed at 30−40 °C by rotary
evaporation at 600−10 mbar pressure, and nonvolatile compounds
were dried in vacuo at 0.1 mbar. Yields refer to isolated, pure
compounds and are calculated in mol % of the used starting material.
NMR spectra were recorded at 21 °C operating at 300 or 500 MHz
(1H), 125 MHz (13C), and 235 MHz (19F). Chemical shifts are
referred to in terms of parts per million, and J-coupling constants are
given in hertz. Abbreviations for multiplicities are as follows: s
(singulet), d (doublet), t (triplet), m (multiplet), and b (broad signal).
IR spectra were recorded on an ATR unit, and the signals are given by
wavenumbers (cm−1). Optical rotation was measured at the sodium D
line in a cell with 100 mm path length. Melting points were measured
in open glass capillaries and are uncorrected. Mass spectra were
measured on an ESI spectrometer. Single crystal X-ray analysis was
performed by Dr. Wolfgang Frey (Universitat Stuttgart).
̈
General Procedure for the Enantioselective Aza-Claisen
Rearrangement (GP1). Silver p-toluenesulfonate (4.00 equiv) was
dissolved in MeCN (0.1 mL/mg), and the solvent was subsequently
removed by a stream of dinitrogen. A solution of [RuBIP-Cl]2 (1.00
equiv) in CH2Cl2 (0.1 mL/mg) was then added under a dinitrogen
atmosphere. The mixture was stirred overnight at room temperature
and subsequently filtered through Celite/CaH2. The filter cake was
extracted with CH2Cl2 until the organic solution was colorless. The
solvent was removed by a steady stream of dinitrogen and finally by
high vacuum. A stock solution of the activated catalyst was prepared by
dissolving the solid in dry CHCl3 (20 mmol/L). The required amount
of this solution was added to the substrate (prepared according to
Overman et al.29a) under an air atmosphere. The reaction tube was
sealed, and the reaction mixture was stirred for the indicated time at
the indicated temperature. Afterward, the reaction mixture was
suspended in petroleum ether/ethyl acetate (10/1) and subsequently
purified by filtration over silica gel.
ASSOCIATED CONTENT
■
S
* Supporting Information
Characterization data and CIF files. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rene.peters@oc.uni-stuttgart.de.
Notes
The authors declare no competing financial interest.
General Procedure for the Catalytic Asymmetric Michael
Addition of α-Cyanoacetates to Cyclic Enones (GP2). Silver
heptafluorobutyrate (4.0 equiv) was dissolved in acetonitrile (0.1 M),
and the solvent was then removed under reduced pressure. A solution
of the precatalyst [RuBIP-Cl]2 in CH2Cl2 (1 mL/5 mg silver salt) was
added, and the suspension was stirred for 1 h at room temperature.
Afterward, the suspension was filtrated over Celite, and the solvent was
removed by a steady flow of dinitrogen followed by high vacuum. A
stock solution was prepared in diglyme. To the corresponding α-aryl-
α-cyanoacetate (1 equiv) in diglyme were added acetic acid as a stem
solution in diglyme (0.2 equiv), the indicated amount of the activated
catalyst as a stem solution in diglyme (total amount of solvent: 0.34
mL for 0.18 mmol substrate), and finally the corresponding enone (2.0
equiv). The reaction mixture was stirred for 24 h at 35 °C. Afterward
n-pentane was added to precipitate the catalyst, and the mixture was
subjected to column chromatography (silica, pentane/EtOAc, 4:1).
General Procedure for the Catalytic Asymmetric Tandem
Azlactone Formation/Michael Addition (GP3). A solution of
[RuBIP-Cl]2 and silver triflate (4.0 equiv per [RuBIP-Cl]2) in
ACKNOWLEDGMENTS
■
This work was financially supported by the Deutsche
Forschungsgemeinschaft (DFG, PE 818/4-1) and the Land-
esgraduiertenforderung Baden-Wurttemberg (Ph.D. fellowship
̈
̈
to M.W.).
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New York, 2001. (b) Reviews about the Nef-reaction: Ballini, R.;
Petrini, M. Tetrahedron 2004, 60, 1017−1047. (c) Pinnick, H. W. Org.
React. 1990, 38, 655−792.
(39) Product yield under these conditions is 77%.
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