An Insoluble Copper(II) Acetylacetonate-Chiral Bipyridine Complex that Catalyzes Asymmetric Silyl Conjugate Addition in Water

Article abstract of DOI:10.1021/jacs.5b11418

Acicular purplish crystals were obtained from Cu(acac)₂ and a chiral bipyridine ligand. Although the crystals were not soluble, they nevertheless catalyzed asymmetric silyl conjugate addition of lipophilic substrates in water. Indeed, the reactions proceeded efficiently only in water; they did not proceed well either in organic solvents or in mixed water/organic solvents in which the catalyst/substrates were soluble. This is in pronounced contrast to conventional organic reactions wherein the catalyst/substrates tend to be in solution. Several advantages of the chiral Cu(II) catalysis in water over previously reported catalyst systems have been demonstrated. Water is expected to play a prominent role in constructing and stabilizing sterically confined transition states and accelerating subsequent protonation to achieve high yields and enantioselectivities.

Full text of DOI:10.1021/jacs.5b11418

Communication
pubs.acs.org/JACS
An Insoluble Copper(II) Acetylacetonate−Chiral Bipyridine Complex
that Catalyzes Asymmetric Silyl Conjugate Addition in Water
Taku Kitanosono,§ Lei Zhu,§ Chang Liu, Pengyu Xu, and Shu Kobayashi*
̅
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
S
* Supporting Information
C−Si bond to be made available in chemical synthesis and in
materials and medicinal chemistry.3 Given that α-chiral silanes
ABSTRACT: Acicular purplish crystals were obtained
from Cu(acac)2 and a chiral bipyridine ligand. Although
the crystals were not soluble, they nevertheless catalyzed
are deemed to be versatile stereodefined placeholders for C−C
bonds4,5 as well as C−O bonds,6 enantioselective installation of
asymmetric silyl conjugate addition of lipophilic substrates
in water. Indeed, the reactions proceeded efficiently only
in water; they did not proceed well either in organic
solvents or in mixed water/organic solvents in which the
catalyst/substrates were soluble. This is in pronounced
contrast to conventional organic reactions wherein the
catalyst/substrates tend to be in solution. Several
advantages of the chiral Cu(II) catalysis in water over
previously reported catalyst systems have been demon-
strated. Water is expected to play a prominent role in
constructing and stabilizing sterically confined transition
states and accelerating subsequent protonation to achieve
high yields and enantioselectivities.
silicon groups has developed into an very promising area of
research. Among the available methods to furnish organosilicon
compounds in an enantioselective fashion, conjugate addition of
silyl metal reagents is currently the most reliable. The history of
this reaction commenced with classic protocols involving
stoichiometric amounts of silylcuprates,7 followed by activation
of silyl reagents by catalytic amounts of metals such as Hayashi’s
1,4-disilylation protocol with palladium catalysts.8 However, the
limited scope of this approach stimulated the development of
more efficient catalysts for asymmetric silyl transfers. Although
Hartmann and Oestreich,9 Hoveyda,10 and Proctor11 developed
Rh(I)-phosphine, Cu(I)-N-heterocyclic carbene (NHC), and
metal-free NHC catalysts for activation of Suginome’s interele-
ment silylboron reagent [dimethylphenylsilyl pinacolatoboro-
nate PhMe2SiB(pin)] toward asymmetric conjugate addition,12
these approaches tended to have limited coverage of acceptors
and required an extensive array of ligands.
xploring chemical reactions in water is of paramount
Eimportance in multidisciplinary research, including life
science. Since water is abundant, inexpensive, nonflammable,
nontoxic, and an environmentally benign medium, today’s
increasing awareness of environmental damage has prompted
the use of water as an ideal solvent alternative to organic solvents.
Moreover, the use of water as solvent sometimes leads to novel
modes of transformations and can produce unusual rate
acceleration and unusual stereoinduction. For instance, non-
immobilized chiral heterogeneous catalysts in water have elicited
much interest, and such systems have been shown to have a
number of impressive characteristics.1 Composed of insoluble
metal salts or zero-valent metals with chiral modifiers, these
catalyst systems are generally advantageous in terms of simplicity,
durability, reusability, prevention of metal contamination into
products, and suitability for integration into, for example, tandem
reactions. Moreover, they sometimes exhibit superior perform-
ance in water over analogous homogeneous systems even in
reactions between water-insoluble substrates.1 In spite of the
unique and unpredictable nature of such catalysts, there remain
only a few examples of nonimmobilized, chiral heterogeneous
catalysts in water.
Herein, we describe our efforts to highlight the use of water-
insoluble chiral Cu(II) catalysts in asymmetric silyl conjugate
addition of PhMe2SiB(pin) to α,β-unsaturated acceptors.13 The
absolute durability and ready availability of Cu(II) underscore
the value of the Cu(II)-based catalyst over an array of traditional
Cu(I)-based catalysts.14
We first prepared chiral Cu(II) complexes with fixed
coordination geometry from Cu(II) salts and chiral 2,2′-
bipyridine L1.15 After examining several conditions, it was
found that acicular purplish crystals were obtained from
Cu(acac)2 and L1. The crystals were fully characterized by
spectrometric methods (see the Supporting Information (SI))
and were used in the reaction of chalcone 1a with PhMe2SiB-
(pin) 2 in water, providing the desired product with high
selectivity (Table S1, entry 1). The heterogeneity of the Cu(II)
complex was confirmed by inductively coupled plasma (ICP)
analysis (see SI). The use of insoluble Cu(OH)2 also afforded
good results (entry 2). In contrast, use of a possible
conformational mixture of complexes that was prepared in situ
in water led to a reduction in both the yield and selectivity
(entries 3 and 4). The use of highly soluble Cu(II) salts also
resulted in poor results (entries 5−7), whereas Cu(II) halides
gave moderate enantioselectivities (entries 8 and 9). Although all
components involved in the reaction under the conditions
The physical and electronic nature of silicon differs in a
number of respects from that of carbon, and this can result in
increased robustness under harsh conditions, increased lip-
ophilicity, and significantly altered polarization and pharmaco-
kinetic properties of organosilicon compounds compared with
their carbon analogues.2 The incorporation of a silicon atom at a
specific site in a substrate allows the attractive properties of the
Received: November 1, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b11418
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Journal of the American Chemical Society
Communication
detailed in entry 1 are immiscible with water, the reaction did not
proceed at all without any solvent even in the presence of 1 equiv
of water (entry 10). The reaction in anhydrous solvents did not
furnish any product including a β-silyl boron enolate (entry 11).
The reaction run in diethyl ether in the presence of an equimolar
amount of methanol, which worked well in boron conjugate
addition,14 provided reduced yield and enantioselectivity (entry
12). The reaction run in alcohol alone also afforded poor results
(entries 13 and 14). Finally, it was established that the desired
catalysis could be exerted only in water.
lactones (1s and 1t) reacted with 2 in a highly enantiomeric
manner. Of the two possible isomers, the 1,4-addition products
were obtained exclusively in high yields and with high
enantioselectivities when acyclic α,β,γ,δ-unsaturated dienones
3v and 3w were applied as substrates. The produced allylsilanes
can be employed in Hosomi−Sakurai reactions16 and other
transformations such as 1,4-phenyl migration from the silyl
group to a carbocation at the δ-position upon treatment with
HBF417 and intramolecular allylation.2b It was therefore apparent
that a wide range reactions could be catalyzed by the Cu(II)-
based catalyst with multifarious substrates.
The scope of the reaction was surveyed under the optimized
reaction conditions (Table 1). Chalcone derivatives bearing
The catalyst system was suitable for use with other electron-
deficient α,β-unsaturated compounds (Table 2). α,β-Unsatu-
Table 1. Scope of the Reaction with α,β-Unsaturated
Carbonyl Compounds
Table 2. Scope of the Reaction with Electron-Deficient
Alkenes
a
b
entry
EWG
R
yield (%)
ee (%)
1
2
3
4
5
CN
Ph
1x
78
80
70
83
90
61
63
58
94
82
90
87
90
82
78
81
87
CN
4-MeOC6H4
4-FC6H4
2-Furyl
Me
1y
CN
1z
CN
1aa
1bb
1cc
1dd
1ee
1ff
CN
c
6
NO2
NO2
NO2
NO2
Ph
7
8
9
4-MeC6H4
4-FC6H4
3-Furyl
67
a
b
c
Isolated yield. Determined by HPLC analysis. Performed for 48 h
with 2 (1.5 equiv).
rated nitriles (1x−bb) could be used under the optimal reaction
conditions, and the reaction was not inhibited by other functional
groups that were capable of coordinating to the Cu(II) core
(1aa). It was found that the catalyst was also reactive toward a
nonconjugated α,β-unsaturated nitrile (1bb). The first successful
examples involving the application of β-nitrostyrenes (1cc−ff) in
asymmetric silyl conjugate addition are notable because this
provides a synthetic route to useful β-aminosilanes and the
corresponding β-silyl diazonium intermediates,18 which are not
available even through analogous boron conjugate addition.
When cyclic α,β,γ,δ-unsaturated dienones were employed as
acceptors, δ-silylated products were obtained exclusively, as
expected (Scheme 1). In boron conjugate addition, the use of
heterogeneous Cu(OH)2-based catalysts tends to lead to the 1,6-
addition pathway being followed.1b,d To gain insights into the
mechanism of the 1,6-addition pathway, the progress of the
reaction was investigated in detail (Table S2). At the beginning of
the reaction, kinetically protonated δ-silylated product 4hh,
bearing a β,γ-alkene moiety, was obtained exclusively with almost
a
b
c
Isolated yield. Determined by HPLC analysis. Gram-scale synthesis
d
e
to give the product (1.13 g). Performed for 48 h. At 10 mol %
catalyst loading. Triton X-100 (25 mg) was added.
f
either electron-donating or -withdrawing groups at any position
of the benzene ring reacted smoothly with 2 to afford the desired
products in high yields with high enantioselectivities (1a−h).
The catalyst was suitable for use in a gram-scale reaction without
any loss of enantioselectivity. Not only acyclic aliphatic α,β-
unsaturated ketones (1i−l), but also cyclic ketones (1m and 1n)
were also applicable without significant erosion of enantiose-
lectivity. The silyl group could be transferred to a crowded
position to generate a quaternary carbon center bearing a C−Si
bond (3o). Similarly, acyclic α,β-unsaturated esters (1p−r) and
Scheme 1. Silyl Conjugate Addition to Cyclic Dienones
B
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Journal of the American Chemical Society
Communication
complete Z-preference. After stirring for 3 h, the reaction
furnished γ-protonated isomer 3hh predominantly. Eventually,
no 4hh remained, and a significant amount of 3hh was obtained
after 24 h. Hoveyda et al. reported that by using their NHC-
Cu(I) catalyst system, 4hh was obtained that contained 15−20%
3hh as a result of adventitious isomerization.10b,d The Cu(II)-
based system ultimately reached thermodynamic convergence
with the formation of 3hh. Considering that an analogous δ-
borylated enolate intermediate underwent protonation at the γ-
position directly,1b,d the preferential protonation of a δ-silylated
enolate intermediate at the α-position may be interpreted as a
result of a hyperconjugative interaction. It is reasonable to
suppose that the reaction environment at the interface between
the heterogeneous Cu(II) catalyst and water is responsible for
the propagation of a stepwise process that includes protonation
and subsequent isomerization.
bond. The electron-withdrawing nature of the Ph ring on the
silicon atom would also facilitate polarization across the Si−B
bond and subsequent cleavage. The putative silylcopper(II)
species would undergo nucleophilic silyl transfer to α,β-
unsaturated acceptors. Unlike the well-documented, isolable
silylcopper(I) species,22 only a few Cu(II)−carbon or silicon
structures are known.23 Although multidisciplinary research is
required to comprehend the electronic nature of such species, the
d9 configuration of the copper may lead to p-character of the
bond24 with silicon, which is anticipated to play a crucial role in
stereoinduction. The “soft” nature of Cu(I) has been shown to
lead to high activity for couplings with sp2 carbon centers, as
represented by conjugate additions through d−π interactions;
however, this entails the use of anhydrous solvents and an inert
atmosphere. In contrast, Cu(II) tends to coordinate to the lone
pair of electrons on the carbonyl oxygen rather than to the π-
electrons of a double bond because of an intrinsic instability of
the d9−π interaction caused by its unpaired electron. This leads
to a new mechanism involving an alternative sense of
enantioselection via an O-enolate intermediate. The correspond-
ing Cu(II) enolate would readily release the β-silylated carbonyl
compounds. Aqueous environments would tend to stabilize the
geometry of the chiral silylcopper(II) intermediate and lead to
instantaneous protonation of the O-enolate intermediate to
deliver the corresponding β-silylated product.
Given that a CuBr2-based complex was shown by crystallo-
graphic analysis to have a square pyramidal structure,19 it was
assumed that, in Cu(acac)2-L1 crystals, Cu(II) is coordinated by
L1 in a tridentate fashion through N,N,O-type coordination.
UV−vis absorption spectroscopy indicated that the conforma-
tion of an isolated Cu(acac)2-L1 crystal in aqueous tetrahy-
drofuran (THF) was close to that of the X-ray crystal structure.20
The absorption is characterized by a strong major absorption at
2
980 nm that may be assigned to the transition of dxz or dyz → dz ,
suggesting a trigonal bipyramidal geometry (see SI).1c,21 The
catalyst behavior in the solution state was then investigated using
aqueous THF solutions (Figure 1). A sharp increase in
Catalyst reusability was evaluated in the reaction of cyclo-
pentenone 1m with 2. After the first run, the reaction tube was
centrifuged at 3000 rpm to separate the reaction mixture into
aqueous, organic, and solid phases (Figure 2). No product,
Figure 2. Separated phases after centrifugation of the reaction mixture.
copper, or chiral ligand was detected in the aqueous phase,
whereas the product 3m and a trace amount of the chiral ligand
were found in the organic phase (see SI). The product 3m was
isolated in 85% yield with 87% ee. The collected solid was then
used in a second run, resulting in a comparable result with little
erosion of the selectivity (80% yield, 82% ee).
Figure 1. Homogeneous versus heterogeneous catalysis.
In summary, we have isolated crystals of a chiral heterogeneous
Cu(II) catalyst prepared from Cu(acac)2 and L1, which,
although not soluble in water, nevertheless catalyzes asymmetric
silyl conjugate addition of lipophilic substrates in this medium.
The catalytic asymmetric reactions proceed in high yields and
with high enantioselectivities only in water. The reactions either
did not proceed at all or gave low yields with low
enantioselectivities in many organic solvents and even in mixed
water/organic solvents in which the catalyst and substrates
dissolved completely. This result is in striking contrast to many
conventional organic reactions in which solubility is required. As
a synthetic methodology, chiral Cu(II) catalysis in water has
several advantages over previously reported catalyst systems,
such as wide substrate scope and high enantioselectivity. The
approach is also in line with the concepts of green sustainable
chemistry because it leads to a reduction in the amount of organic
solvents used and is amenable to catalyst recovery and recycling.
The antagonistic combination of water-insoluble chiral hetero-
geneous catalyst coupled with the use of water as a solvent may
enantioselection correlated with a decrease in solubility of the
catalyst, whereas the yields of 3a were maintained. When the
amount of water was equal to or less than the amount of THF,
the catalyst solution became transparent. Given that leaching of
Cu was not detected in the absence of THF, the catalyst should
be entirely heterogeneous in water. It is therefore asserted that a
water-insoluble heterogeneous Cu(II) catalyst coupled with a
large excess of water is required for highly stereoselective
execution of asymmetric silyl conjugate addition.
The postulated catalytic cycle commences with sp3-hybrid-
ization of the boron atom in 2 caused by association with the
uncoordinated OH group of L1 (Scheme S1). The chiral Cu(II)
complex would react with sterically more congested 2 to deliver a
chiral silylcopper(II) complex. The latter may involve a
nonbonding π−π interaction between the slightly distorted
bipyridine and the phenyl group on silicon. It was surmised that
the selective transformation of the silicon group might be
rationalized by the energetically favored formation of a B−O
C
DOI: 10.1021/jacs.5b11418
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Journal of the American Chemical Society
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ASSOCIATED CONTENT
■
S
* Supporting Information
(10) (a) Wu, H.; Garcia, J. M.; Radomkit, S.; Zhugralin, A. R.;
Hoveyda, A. H. J. Am. Chem. Soc. 2015, 137, 10585. (b) Lee, K.-s.; Wu,
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b11418.
Experimental details and characterization of the products
(PDF)
AUTHOR INFORMATION
■
(12) (a) Oestreich, M.; Hartmann, E.; Mewald, M. Chem. Rev. 2013,
113, 402. (b) Suginome, M.; Matsuda, T.; Nakamura, H.; Ito, Y.
Tetrahedron 1999, 55, 8787.
Corresponding Author
*shu_kobayashi@chem.s.u-tokyo.ac.jp
Author Contributions
(13) Currently, there are a limited number of reports on Cu(II)-
induced activation of PhMe2SiB(pin), and all transformations lead to
racemic mixtures, see: (a) Calderone, J. A.; Santos, W. L. Angew. Chem.,
Int. Ed. 2014, 53, 4154. (b) Xuan, Q.-Q.; Zhong, N.-J.; Ren, C.-L.; Wang,
D.; Chen, Y.-J.; Li, C.-J. J. Org. Chem. 2013, 78, 11076. (c) Calderone, J.
A.; Santos, W. L. Org. Lett. 2012, 14, 2090.
§These authors contributed equally.
Notes
The authors declare no competing financial interest.
(14) Zhu, L.; Kitanosono, T.; Xu, P.; Kobayashi, S. Chem. Commun.
2015, 51, 11685.
ACKNOWLEDGMENTS
■
This work was partially supported by a Grant-in-Aid for Science
Research from the Japan Society for the Promotion of Science
(JSPS), Global COE Program, The University of Tokyo, MEXT,
Japan, the Japan Science and Technology Agency (JST). P.X.
thanks the JSPS for a Research Fellowship for Young Scientists.
(15) Analogous Cu(II) complexes were reported, see: (a) For
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