Enantioselective hydrovinylation via asymmetric counteranion-directed ruthenium catalysis

Article abstract of DOI:10.1039/c1cc12499d

The first Ru-catalyzed enantioselective hydrovinylation has been realized by using an asymmetric counteranion-directed catalysis strategy. Styrene derivatives react with ethylene in excellent yields and promising enantioselectivity using this approach. Th

Full text of DOI:10.1039/c1cc12499d

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COMMUNICATION
Enantioselective hydrovinylation via asymmetric counteranion-directed
ruthenium catalysisw
Gaoxi Jiang and Benjamin List*
Received 28th April 2011, Accepted 22nd July 2011
DOI: 10.1039/c1cc12499d
The first Ru-catalyzed enantioselective hydrovinylation has been
realized by using an asymmetric counteranion-directed catalysis
strategy. Styrene derivatives react with ethylene in excellent
yields and promising enantioselectivity using this approach.
quite to our disappointment, these species proved to be either
unreactive or did not furnish enantioselectivity in any of the
hydrovinylations we have investigated. Two examples shall
illustrate this (Scheme 1): subjecting olefin 1a under an atmosphere
of ethylene to a catalyst system comprised of dimeric allyl
nickel bromide, (2-(benzyloxy)phenyl)diphenyl-phosphine,
and silver phosphate Ag-1, did not lead to the formation of
even traces of the expected hydrovinylation product. In
contrast, when we utilized chiral disulfonimide Ag-2, which
we have recently introduced in the context of Lewis acid
organocatalysis,12 product 2a was formed in quantitative yield
but with no enantioselectivity. Similar results were obtained
with many different Ni-salt/chiral anion/achiral phosphine-
combinations.13
The hydrovinylation of olefins with ethylene is a highly
attractive and perfectly atom economic carbon–carbon-bond
forming reaction based on feedstock chemicals (eq (1))1.
ð1Þ
Typical catalysts are based on nickel2–5 but cobalt,6 palladium,7
and ruthenium complexes8,9 have also been described.
Asymmetric hydrovinylation catalysis goes back to pioneering
studies by Wilke et al., who in 1972 used [(allyl)NiCl]2/
(ꢀ)-dimenthylmethylphosphane/Et3Al2Cl3 to catalyze the
reaction of 1,3-cyclooctadien with ethylene to 3-vinylcyclooctene
in up to 85 : 15 er.2 The reaction has been significantly
advanced in more recent years by studies of Rajanbabu
et al.,3 Leitner et al.,4 and Zhou et al.,5 who have provided
state of the art enantioselective Ni-catalysts. Herein, we report
the first asymmetric hydrovinylation, which is catalyzed by a
ruthenium complex. Our approach capitalizes from an
asymmetric counteranion directed catalysis strategy and gives
promising enantioselectivies in the hydrovinylation of styrene
derivatives.
We next switched our attention to Ru-catalysts. Despite the
discovery of ruthenium salt catalysis of hydrovinylation reactions
already in 1965,8 there has not been a single example of an
asymmetric variant.9 Our interest in Ru-based catalysts was
spurred by the previous observation of strong counteranion
effects in the hydrovinylation, suggesting the possibility of an
asymmetric version by incorporating a chiral non-racemic
counteranion.9 In 2001, Yi and co-workers reported a pentavalent
ruthenium hydride complex, RuHCl(CO)(PCy3)2 (Ru-1), that
promotes the hydrovinylation of olefins with ethylene.9a In
subsequent years, first Rajanbabu and co-workers and later
Connell and co-workers developed cationic variants by combining
Ru-1 with silver salts (Scheme 2).3f,9e Inspired by these
observations and as a part of our continuing interest in
The generally accepted mechanism of the Ni-catalyzed
hydronvinylation of styrene suggests involvement of a cationic
nickel hydride with a weakly coordinating anion.3 Insertion of
styrene into the Ni–H-bond following coordination of ethylene,
its insertion into the Ni–C-bond, and b-hydride elimination,
then leads to the product. All steps in this mechanism,
including the stereo-determining one, formally involve
cationic intermediates, which to us implies the opportunity
for the utilization of enantiopure counteranions in an asymmetric
counteranion-directed catalysis (ACDC) approach.10,11
In our initial studies we have therefore prepared dozens of
different Ni-complexes with chiral counteranions. However,
Max-Planck-Institut fur Kohlenforschung, Kaiser-Wilhelm-Platz 1,
¨
45470 Mulheim an der Ruhr, Germany,
¨
E-mail: list@mpi-muelheim.mpg.de; Fax: (+49) 208-306-2999
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc12499d
Scheme 1 Attempts towards an enantioselective hydrovinylation via
asymmetric counteranion-directed Ni-catalysis.
c
10022 Chem. Commun., 2011, 47, 10022–10024
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Table 1 Enantioselectivity for the asymmetric hydrovinylation of 1a
with ethylene to 2a catalyzed by diverse Ru/chiral Ag salts
Scheme 2 An ACDC strategy for the design of an asymmetric
Ru-catalyzed hydrovinylation reaction.
asymmetric-counteranion-directed-transition-metal catalysis,11
we hypothesized that replacing the chloride anion at the
Ru-complex with an enantiopure counteranion to furnish a
chiral catalyst might allow for the development of enantio-
selective Ru-hydrovinylation catalysis. The synthesis of such a
chiral Ru-complex was anticipated to be achievable by treating
Ru-1 with a silver salt that bears a chiral counteranion.
Indeed, initial feasibility studies provided encouragement
for our reaction design. Activating complex Ru-1 (2.0 mol%)
with silver phosphate Ag-1 (2.2 mol%) gave a catalyst for the
hydrovinylation of 1a at room temperature that smoothly
yielded the desired product 2a in excellent yield and with an
er of 73 : 27 (eq (2)). The formation of the isomerized product
was not observed.
Entrya Ru-cat. Ag salt Conv. (%)b Selectivity(%)b erc
1
2
3
4
5
6
7
8
Ru-1
Ag-1
Ag-2
Ag-3
Ag-4
Ag-5
Ag-6
Ag-1
Ag-1
>99
n.r.
>99
97
95
97
>99
—
>99
98
96
97
>99
>99
73 : 27
—
75 : 25
74 : 26
74 : 26
74 : 26
70 : 30
77 : 23
Ru-2
Ru-3
>99
>99
a
Reaction conditions: 1a (0.2 mmol), ethylene (1 atm), [Ru] (2.0 mol%),
b
[Ag] (2.2 mol%), benzene (1 mL), 22 1C, 12 h. Determined by
1H-NMR analysis of the crude reaction mixture. Determined by
HPLC.
c
Ni-catalysis (Scheme 1). The best result was obtained with ion-
pair catalyst based on Ru-3 and Ag-1, which gave an er of
77 : 23 and >99% NMR yield (entry 8). Further optimization,
which included lowering the temperature, the examination of
other solvents, and the study of isocyanide and N-heterocyclic
carbenes to replace the CO or phosphine-ligand did not lead to
further improvements of the enantioselectivity.
ð2Þ
We next investigated the effect of different phosphine
ligands on our Ru-catalyst. During the extensive screening
of a variety of phosphine ligands to prepare analogs of Ru-1,
we found that most of the frequently used phosphine ligands,
with the notable exception of triisopropylphosphine and
di-(1-adamantyl)-n-butylphosphine, are either not suitable
for the preparation or the corresponding ruthenium complexes
or their complexes are unreactive (for details, see Electronic
Supplementary Information (ESI)w). Analogs Ru-214a and
Ru-3 can be easily obtained by the treatment of RuCl3ꢁ
nH2O or [RuCl2(COD)]2 with triisopropylphosphine and
di-(1-adamantyl)-n-butylphosphine under the reported conditions,
respectively.14 In the meantime, different chiral phosphates as
silver salts, bearing substituents at different positions of the
3,30-aryl groups, including our recently developed spirocyclic
phosphate Ag-6,15 were investigated for the asymmetric
hydrovinylation of 1a with ethylene to 2a at room temperature
in benzene for 12 h. Table 1 shows the details of the screening
results. Remarkably, the disulfonimide anion introduced
by Ag-2 fails entirely to produce catalytically active
ruthenium complexes, a stark difference to the corresponding
Finally, we conducted a small exploration of the substrate
scope of our asymmetric hydrovinylation using the best
catalyst system (Ru-3/Ag-1). As illustrated in Table 2, the
asymmetric hydrovinylation of styrene derivatives 1b–f gave
Table 2 Symmetric hydrovinylation of styrene derivativesa
Entry
Product (RQ)
Conv. (%)c
Selectivity (%)c
erd
1
2
3
2b (H)
2c (Me)
2d (OMe)
2e (Br)
2f (CO2Me)
>99
>99
>99
>99
96
>99
>99
>99
>99
96
68 : 32
67 : 33
67 : 33
72 : 28
68 : 32
4
5b
a
Reaction conditions: 1 (0.2 mmol), ethylene (1 atm), Ru-3 (2.0 mol%),
b
c
Ag-1 (2.2 mol%), benzene (1 mL), 8 1C, 12 h. At r.t. Determined by
1H-NMR analysis of the crude reaction mixture. Determined by chiral
GC or HPLC.
d
c
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generally quantitative conversion to the desired products 2b–f
in high selectivity but moderate enantioselectivity.
7 For examples with Pd-catalysts, see: (a) N. J. Hovestad,
E. B. Eggeling, H. J. Heidbuchel, J. T. B. H. Jastrzebski,
¨
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In summary, the first ruthenium-catalyzed asymmetric
hydrovinylation has been discovered.16 The reaction is based
on the utilization of our ACDC-strategy. It is interesting to
speculate on the role of the anion and the nature and stereo-
chemistry (chiral at Ru) of our actual catalyst complex. Future
structural and mechanistic studies are expected to shed light
on this fascinating aspect of our reaction. Furthermore,
improvements of our catalyst system and additional applications
of chiral anions in Ru-catalysis are anticipated.
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We thank S. Muller, H. v. Thienen, and S. Liao for
¨
providing chiral acids. This work was supported by the
Max-Planck-Society.
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16 After the submission of this manuscript, Zhou and co-workers
published attempts to use a ruthenium catalyst containing a chiral
phosphine ligand in an asymmetric hydrovinylation. However, in
the best case, the product was obtained with 17% yield and with an
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c
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