Polymer-incarcerated chiral Rh/Ag nanoparticles for asymmetric 1,4-addition reactions of arylboronic acids to enones: Remarkable effects of bimetallic structure on activity and metal leaching

Article abstract of DOI:10.1021/ja307913e

Robust and highly active bimetallic Rh nanoparticle (NP) catalysts, PI/CB Rh/Ag, have been developed and applied to the asymmetric 1,4-addition of arylboronic acids to enones without leaching of the metals. We found that the structures of the bimetallic Rh/Ag catalysts and chiral ligands strongly affect their catalytic activity and the amount of metal leaching. PI/CB Rh/Ag could be recycled several times by simple operations while keeping high yields and excellent enantioselectivities. To show the versatility of the PI/CB Rh/Ag catalyst, a one-pot, oxidation-asymmetric 1,4-addition reaction of an allyl alcohol and an arylboronic acid was demonstrated by combining the PI/CB Rh/Ag catalyst with PI/CB Au as an aerobic oxidation catalyst.

Full text of DOI:10.1021/ja307913e

Communication
pubs.acs.org/JACS
Polymer-Incarcerated Chiral Rh/Ag Nanoparticles for Asymmetric
1,4-Addition Reactions of Arylboronic Acids to Enones: Remarkable
Effects of Bimetallic Structure on Activity and Metal Leaching
Tomohiro Yasukawa, Hiroyuki Miyamura, and Shu Kobayashi*
̅
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
S
* Supporting Information
investigated for several C−C bond-forming reactions by using
ABSTRACT: Robust and highly active bimetallic Rh
nanoparticle (NP) catalysts, PI/CB Rh/Ag, have been
developed and applied to the asymmetric 1,4-addition of
arylboronic acids to enones without leaching of the metals.
We found that the structures of the bimetallic Rh/Ag
catalysts and chiral ligands strongly affect their catalytic
activity and the amount of metal leaching. PI/CB Rh/Ag
could be recycled several times by simple operations while
keeping high yields and excellent enantioselectivities. To
show the versatility of the PI/CB Rh/Ag catalyst, a one-
pot, oxidation-asymmetric 1,4-addition reaction of an allyl
alcohol and an arylboronic acid was demonstrated by
combining the PI/CB Rh/Ag catalyst with PI/CB Au as an
aerobic oxidation catalyst.
immobilized phosphine ligands; however, these systems require
complicated preparation of monomeric ligands and individual
fabrication of heterogeneous polymers. In addition, these
catalysts themselves are not robust because of the instability of
the immobilized ligands, such as intolerance to oxidative
conditions. In contrast, the latter strategy easily allows the
optimization of ligand structures by external addition of ligands
to immobilized metal catalysts. However, control of metal
leaching is more difficult with this strategy because strong
coordination of the ligands to cationic Rh may allow the metal to
leach out from the support.7a,g To the best of our knowledge,
there has been no report of an asymmetric version of the 1,4-
addition reaction of boron compounds with α,β-unsaturated
carbonyl compounds in heterogeneous systems using the latter
strategy. Therefore, to develop immobilized chiral Rh catalysts
for asymmetric C−C bond-forming reactions with both high
efficiency and robustness, a new strategy is required.
eterogeneous metal nanoparticle (NP) catalysis has been
H
of great interest in both academia and industry and has
expanded rapidly because of these catalysts’ unique reactivity and
selectivity, stability, recyclability, avoidance of metal contami-
nation of products, and availability for reaction integration such
as in continuous flow systems and tandem reactions.1 Also, their
activities and selectivities can be controlled by the formation of
multimetallic NPs.2 Several recent reports have shown that
homogeneous transition-metal-catalyzed reactions affording
complex organic molecules can be conducted by using metal
NP catalytic systems, with the advantages mentioned above.3
However, there have been few reports of metal NP catalysts that
can provide excellent enantioselectivity in asymmetric C−C
bond-forming reactions.3c,e,f,h,i Moreover, only single-metal NPs
are generally used in those reactions, and the effects of a second
dopant metal to form bimetallic NPs have scarcely been
discussed.
Our group has already developed polystyrene (PS)-based
polymer-incarcerated metal (PI-M) NP catalysts and reported
that they show high catalytic activity and robustness, such as
recyclability and restoration of the activity of deactivated catalysts
that had been used repetitively for many kinds of reactions
without metal leaching.1c,8 With this PI method, multimetallic
NP catalysts were readily prepared, and some bimetallic catalysts
showed higher catalytic activities and unique selectivities
compared with the monometallic NP catalysts.1e,9 This work
led us to hypothesize that the development of truly robust, chiral
Rh NP catalysts10 could be achieved using the PI method.11
Herein we report the development of novel, robust, chiral Rh and
bimetallic NP catalysts by the PI method and their use in
asymmetric 1,4-additions of boronic acids to enones.
We first chose Rh(PPh3)3Cl as the Rh precursor for the
preparation of PI/CB Rh catalyst 7 containing Rh NPs through
in situ reduction of cationic Rh by NaBH4 in the presence of a PS-
based copolymer, followed by thermal cross-linking (Scheme 1).
With the copolymer and carbon black (CB) as a second support
to expand the surface area, PI/CB catalysts were prepared using a
reported method.12 2-Cyclohexenone 1a and phenylboronic acid
2a were chosen as model substrates, and the reaction was
conducted with the external addition of BINAP 4 as a chiral
ligand (substrates and ligands are shown in Chart 1). In a 1:2
toluene/water cosolvent system, the desired product 3aa was
The Rh-catalyzed asymmetric 1,4-addition of boron com-
pounds to α,β-unsaturated carbonyl compounds was developed
by Hayashi and Miyaura and is one of the most useful tools for
constructing asymmetric C−C bonds.4 Although both academic
research and large-scale (up to 20 kg) processes involving this
reaction have been widely reported,5 recyclable and robust chiral
heterogeneous Rh catalysts that can work with high catalytic
turnover and long lifetimes without metal leaching are required
because Rh is one of the most expensive precious metals and Rh
contamination in products is problematic.5c Reported strategies
for immobilization of Rh catalysts have used two approaches:
immobilization of ligands on supports6 or immobilization of
cationic Rh on supports.7 The former strategy has already been
Received: August 8, 2012
Published: September 24, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
a
Scheme 1. Preparation of Rh NP catalysts
Table 1. Optimization of the Reaction Conditions
Chart 1. Substrates and Ligands
a
Reaction conditions: either 1a (0.2 mmol), 2a (0.3 mmol), toluene
(0.25 mL), and water (0.5 mL) or 1b (0.3 mmol), 2a (0.45 mmol),
toluene (0.375 mL), and water (0.75 mL) at 100 °C for t h.
b
c
Determined by GC analysis. Determined by HPLC analysis. “−” =
d
not determined. ND = below the detection limit of the ICP
equipment (<0.003−0.005 ppm).13 The solvent was heated to 100 °C
e
f
g
obtained in high yield with high enantiomeric excess (ee) at 100
°C, but >10% leaching of Rh was observed by inductively
coupled plasma (ICP) analysis (Table 1, entry 1). As our control
experiments showed that no leaching of Rh occurred in the
presence of only 4 or only the substrates under the same
conditions,13 we hypothesized that the leaching might result
from the formation of an intermediate involving interactions with
both the ligand and the substrates.
before use. Rh/M = 1:1. ICP analysis confirmed that there was no
h i j
leaching of Ag. Rh/Ag = 1:2. Rh/Ag = 1:3. 2a (0.6 mmol), 15 (1.5
mol % as Rh), and 6 (2 mol %) were used.
and amount of Rh leaching were improved by increasing the
proportion of Ag (entries 15 and 16), and further optimization
gave 3ba in 91% yield and 92% ee with no leaching of the metals
(entry 17). Furthermore, a filtrate transfer experiment3b,13
confirmed that the filtrate after the reaction did not catalyze
the 1,4-addition, indicating that the reaction is not catalyzed by
homogeneous leached species. It is noteworthy that the
notorious metal leaching was suppressed by the use of bimetallic
NP catalysts.
To prevent the leaching, chiral ligands with various structures
were examined to see whether the size or strength of the
interaction affected the degree of leaching. Interestingly, we
chose chiral dienes as ligands4b,g,14 and found that the leaching
was dramatically decreased when chiral diene 5 was used, in spite
of the lower yield and ee (Table 1, entry 2). On the other hand,
when chiral diene 6 was used, excellent ee was obtained and ∼5%
Rh leaching was observed (entry 3). Preheating the solvents to
100 °C improved the yield (entry 4), probably because side
reactions occurring at T < 100 °C were suppressed. Finally, the
use of PI/CB Rh catalyst 8 prepared from [Rh(OAc)2]2
completely suppressed the leaching (entry 5), while the
combination of 8 and 4 could not suppress the leaching (entry
6). The second reduction in the catalyst preparation (Scheme
1)13 provided better reproducibility of the catalytic activity and
metal leaching. We also applied this system to the acyclic
substrate 1b. Disappointingly, the product 3ba was obtained in
poor yield in the presence of 8 (entry 8). Because our previous
work showed that the use of bimetallic NP catalysts enhanced the
catalytic activity,9 we then screened bimetallic NP catalysts
prepared by the same procedure as for 8 with simultaneous
reduction of the Rh salt and another metal salt. Initially, catalysts
prepared with a 1:1 Rh/M ratio (M = Ag, Co, Pd, Ru, Au) and a
commercially available 5% Rh on carbon (Rh/C) catalyst were
examined (entries 9−14), and PI/CB Rh/Ag catalyst 9 showed
the highest catalytic activity with 2.3% Rh leaching (entry 9). The
high catalytic activity of 9 for cyclic substrate 1a was also
confirmed, and no leaching of either Rh or Ag was observed
(entry 7) and the loading of ligand 6 was successfully reduced to
0.0375 mol % without loss of catalytic activity and ee.13 The yield
With the optimized bimetallic catalysts 9 and 15, the substrate
scope of the asymmetric 1,4-addition was surveyed (Table 2).
Arylboronic acids with either electron-donating or electron-
withdrawing groups were employed for the reaction with 1a
under the optimized conditions, and in all cases the desired 1,4-
addition products were obtained in high yields with excellent ee
(entries 2−4, 7, 8, and 10−12). No leaching of Rh occurred in
any cases. Arylboronic acids with substituent groups at the ortho,
meta, and para positions also provided the desired products in
high yields with excellent ee without leaching of Rh (entries 3, 4,
and 10−12). o-Methyl-substituted phenylboronic acid 2b and 1-
napthylboronic acid 2h were found to be less reactive, but slight
increases in the amounts of arylboronic acid, catalyst, and ligand
led to high yields of the desired products with excellent ee
(entries 2 and 9). In the reaction of 2e, Rh/Ag catalyst 15 gave a
high yield with excellent ee and no leaching of Rh (entry 6), while
9 showed a small amount of metal leaching (entry 5). The five-
membered-ring enone 1c also afforded the 1,4-addition product
in high yield with good ee under the same conditions (entry 14).
For acyclic enones 1d−f, the optimized conditions for 1b (entry
13) were used, and in all cases the desired products were
obtained in good to high yields with excellent ee and no leaching
of Rh (entries 15−17). In the reaction of 1f, the yield was
improved by using Rh catalyst 8 (entry 18). Overall, a wide
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Journal of the American Chemical Society
Communication
a
Table 2. Scope of the Asymmetric 1,4-Addition
a
Reaction conditions: 100 °C, enone (0.3 mmol), toluene (0.375 mL),
water (0.75 mL); all solvents were heated to 100 °C before use.
b
Calculated from product isolated from 70% of the reaction mixture.
c
d
Determined by HPLC analysis. “−” = not determined. ND = below
the detection limit of the ICP equipment (<0.003−0.005 ppm).13
e
f
Determined by GC analysis. The catalyst (1.5 mol % as Rh) and 6 (2
g
Figure 1. (a, b) STEM images of (a) 8 and (b) 9. (c) EDS mappings of 9.
(d) EDS line analyses of 9.
mol %) were used. The catalyst (1.5 mol % as Rh) and 5 (2 mol %)
were used. The catalyst was further treated in mesitylene at 150 °C
h
for 5 h before use.
small NPs (∼3 nm) were formed, they were assembled on one
area and not dispersed widely over the support (Figure 1a). In
contrast, the NPs in Rh/Ag catalyst 9 were dispersed over the
support (Figure 1b), and EDS mapping showed that bimetallic
alloy NPs were formed (Figure 1c). EDS line analyses of those
alloy NPs (Figure 1d) showed that the Rh/Ag ratio in the NPs
was random (1:2−2:1). Thus, the effects of the bimetallic
structure might be explained by the size effect10a and the
existence of metal−metal interactions. At this stage, we believe
that the reaction proceeds on the surface of the bimetallic NPs, as
there were no significant morphological changes in the catalyst
after use and after heating. We did not determine the oxidation
states of the metals on the surface of the NPs at this stage.
To show the general versatility and applicability of the PI/CB
Rh/Ag catalyst, a one-pot, oxidation-asymmetric 1,4-addition
reaction of an allylic alcohol and an arylboronic acid was
performed (Scheme 3). The reaction was started from allylic
a
Scheme 2. Reuse of the Catalyst
a
The recovered catalyst was heated at 170 °C for 4 h between the 10th
and 11th uses. In all cases, 3aa was obtained with 98% ee and no metal
leaching.
substrate scope with high yields and excellent ee was found, and
in all cases, no metal leaching occurred.
The viability of recycling 9 was examined in the asymmetric
1,4-addition reaction of 1a with 2a (Scheme 2). The catalysts
were easily recovered by simple filtration and reused for the next
cycle with additional 6 after washing with tetrahydrofuran and
drying. For eight cycles, no significant loss of activity was
observed; however, the yield suddenly dropped to 67% for the
ninth use. Similar low activity was observed for the 10th use when
no special treatment was performed before use. Interestingly,
when the catalyst collected after the 10th use was heated at 170
°C for 4 h, the high activity was successfully restored and
maintained for a further four uses, probably because the heating
eliminated impurities such as small amounts of solvents,
products, or side products that were adsorbed on the surface of
the NPs. No significant structural changes before use, after the
10th use, or after heating were observed using scanning
transmission electron microscopy (STEM).13 In all cycles, the
product was obtained in excellent yield and ee with no leaching of
Rh. This demonstrates the robustness of the PI/CB Rh/Ag
catalyst in asymmetric catalysis.
Scheme 3. One-Pot Reaction Using Allylic Alcohol 16
alcohol 16 at 60 °C under an O2 atmosphere in the presence of
K2CO3 and a PI/CB Au catalyst that was shown to be an
excellent catalyst for aerobic oxidation of alcohols.12,15 After the
oxidation step was complete, 2a, 15, and 6 were added to the
same pot, and the reaction mixture was stirred at 100 °C; this
gave the desired product 3ga in high yield with excellent ee. Such
one-pot processes are very useful because workup purification
steps are reduced and a relatively unstable intermediate can be
generated in situ. Moreover, this one-pot reaction shows the
To understand why Ag affects both the catalytic activity and
the Rh leaching, STEM and energy-dispersive X-ray spectrom-
etry (EDS) analyses were performed. In Rh catalyst 8, although
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Journal of the American Chemical Society
Communication
Becker, C. F.; Elliott, S. J.; Grinstaff, M. W.; Porco, J. A. J. Am. Chem. Soc
2010, 132, 7514. (e) Mori, K.; Kondo, Y.; Yamashita, H. Phys. Chem.
Chem. Phys. 2009, 11, 8949. (f) Sawai, K.; Tatumi, R.; Nakahodo, T.;
Fujihara, H. Angew. Chem., Int. Ed. 2008, 47, 6917. (g) Abad, A.; Corma,
A.; Garcia, H. Top. Catal. 2007, 44, 237. (h) Park, K. H.; Chung, Y. K.
Adv. Synth. Catal. 2005, 347, 854. (i) Akiyama, R.; Kobayashi, S. Angew.
Chem., Int. Ed. 2001, 40, 3469.
(4) (a) Tian, P.; Dong, H.-Q.; Lin, G.-Q. ACS Catal. 2012, 2, 95.
(b) Feng, C.-G.; Xu, M.-H.; Lin, G.-Q. Synlett 2011, 1345. (c) Hargrave,
J. D.; Allen, J. C.; Frost, C. G. Chem.Asian J. 2010, 5, 386.
(d) Edwards, H. J.; Hargrave, J. D.; Penrose, S. D.; Frost, C. G. Chem.
Soc. Rev. 2010, 39, 2093. (e) Youn, S. W. Eur. J. Org. Chem. 2009, 2597.
(f) Miyaura, N. Bull. Chem. Soc. Jpn. 2008, 81, 1535. (g) Defieber, C.;
robustness of 15, as its catalytic activity was maintained in the
presence of the Au catalyst and K2CO3.
In summary, we have developed robust and active bimetallic
NP catalysts, PI/CB Rh/Ag, and used them in chiral metal-NP-
catalyzed asymmetric 1,4-additions of arylboronic acids to
enones without leaching of the metals. Wide substrate scope,
high yields, high ee, and no metal leaching were attained in the
chiral metal-NP-catalyzed asymmetric C−C bond-forming
reactions. We also found that the structures of the bimetallic
Rh/Ag NPs and chiral ligands strongly affected the catalytic
activity and the amount of metal leaching. STEM analysis and
EDS mapping showed that Ag formed alloy NPs with Rh,
preventing aggregation of the Rh NPs to enhance the catalytic
activity and control the metal leaching. This is a remarkable
example of the positive effects of a second dopant metal in metal-
NP-catalyzed systems. In addition, the PI/CB Rh/Ag catalyst
could be recycled using simple operations while keeping high
yields and high ee for several cycles, and the deactivated catalyst
after repetitive use could be restored simply by heating. Finally,
to show the versatility of the PI/CB Rh/Ag catalysts, we
developed a one-pot, oxidation-asymmetric 1,4-addition reaction
of an allyl alcohol and an arylboronic acid by combining the PI/
CB Rh/Ag catalyst with a PI/CB Au catalyst as an aerobic
oxidation catalyst. Thus, the developed catalytic system is not
only useful as a synthetic methodology but also should provide
precious scientific knowledge and new avenues for heteroge-
neous, homogeneous, and metal-NP catalysis.
Grutzmacher, H.; Carreira, E. M. Angew. Chem., Int. Ed. 2008, 47, 4482.
̈
(h) Paquin, J.-F.; Defieber, C.; Stephenson, C. R. J.; Carreira, E. M. J.
Am. Chem. Soc 2005, 127, 10850. (i) Guo, H. C.; Ma, J. A. Angew. Chem.,
Int. Ed. 2006, 45, 354. (j) Hayashi, T.; Yamasaki, K. Chem. Rev. 2003,
103, 2829. (k) Takaya, Y.; Ogasawara, M.; Hayashi, T.; Sakai, M.;
Miyaura, N. J. Am. Chem. Soc. 1998, 120, 5579.
(5) (a) Howell, G. P. Org. Process Res. Dev. 2012, 16, 1258. (b) Magano,
J.; Dunetz, J. R. Chem. Rev. 2011, 111, 2177. (c) Brock, S.; Hose, D.;
Moseley, J.; Parker, A.; Patel, I.; Williams, A. Org. Process Res. Dev. 2008,
12, 496.
(6) (a) Makhubela, B. C. E.; Jardine, A.; Smith, G. S. Green Chem. 2012,
14, 338. (b) Jana, R.; Tunge, J. A. J. Org. Chem. 2011, 76, 8376. (c) Jana,
R.; Tunge, J. A. Org. Lett. 2009, 11, 971. (d) Laska, U.; Frost, C. G.;
Plucinski, P. K.; Price, G. J. Catal. Lett. 2007, 122, 68. (e) Otomaru, Y.;
Senda, T.; Hayashi, T. Org. Lett. 2004, 6, 3357. (f) Uozumi, Y.;
Nakazono, M. Adv. Synth. Catal. 2002, 344, 274. (g) Guerreiro, P.;
Ratovelomanana-Vidal, V.; Genet
2001, 42, 3423.
̂
, J.-P.; Dellis, P. Tetrahedron Lett.
ASSOCIATED CONTENT
* Supporting Information
Reaction procedures, STEM images, and spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
(7) (a) Motokura, K.; Hashimoto, N.; Hara, T.; Mitsudome, T.;
Mizugaki, T.; Jitsukawa, K.; Kaneda, K. Green Chem. 2011, 13, 2416.
(b) Neatu, F.; Besnea, M.; Komvokis, V. G.; Genet, J. P.; Michelet, V.;
̂
■
S
Triantafyllidis, K. S.; Parvuescu, V. I. Catal. Today 2008, 139, 161.
(c) Laska, U.; Frost, C. G.; Plucinski, P. K.; Price, G. J. Catal. Lett. 2008,
122, 68. (d) Kantam, M. L.; Subrahmanyam, V. B.; Kumar, K. B. S.;
Venkanna, G. T.; Sreedhar, B. Helv. Chim. Acta 2008, 91, 1947.
(e) Handa, P.; Witula, T.; Reis, P.; Holmberg, K. ARKIVOC 2008,
No. vi, 107. (f) Handa, P.; Holmberg, K.; Sauthier, M.; Castanet, Y.;
Mortreux, A. Microporous Mesoporous Mater. 2008, 116, 424. (g) Fujita,
N.; Motokura, K.; Mori, K.; Mizugaki, T.; Ebitani, K.; Jitsukawa, K.;
Kaneda, K. Tetrahedron Lett. 2006, 47, 5083.
AUTHOR INFORMATION
Corresponding Author
shu_kobayashi@chem.s.u-tokyo.ac.jp
■
Notes
The authors declare no competing financial interest.
(8) Akiyama, R.; Kobayashi, S. Chem. Rev. 2009, 109, 594.
ACKNOWLEDGMENTS
■
(9) (a) Kaizuka, K.; Miyamura, H.; Kobayashi, S. J. Am. Chem. Soc
2010, 132, 15096. (b) Miyamura, H.; Matsubara, R.; Kobayashi, S.
Chem. Commun. 2008, 2031.
(10) (a) Yuan, Y.; Yan, N.; Dyson, P. J. ACS Catal. 2012, 2, 1057.
́
(b) Guyonnet-Bile, E.; Denicourt-Nowicki, A.; Sassine, R.; Beaunier, P.;
This work was partially supported by a Grant-in-Aid for Scientific
Research from the Global COE Program of JSPS, the University
of Tokyo, MEXT, and NEDO. We thank Mr. Noriaki Kuramitsu
(The University of Tokyo) for STEM and EDS analyses.
Launay, F.; Roucoux, A. ChemSusChem 2010, 3, 1276. (c) Han, D.; Li,
X.; Zhang, H.; Liu, Z.; Li, J.; Li, C. J. Catal. 2006, 243, 318. (d) Bile, E. G.;
Cortelazzo-Polisini, E.; Denicourt-Nowicki, A.; Sassine, R.; Launay, F.;
Roucoux, A. ChemSusChem 2012, 5, 91. (e) Crabtree, R. H. Chem. Rev.
2012, 112, 1536. (f) Heitbaum, M.; Glorius, F.; Escher, I. Angew. Chem.,
Int. Ed. 2006, 45, 4732.
(11) Before developing the PI method, we exploited micro-
encapsulated (MC) catalysts. MC Pd with an NP structure was
successfully used in asymmetric allylic substitution.3i,8
(12) Lucchesi, C.; Inasaki, T.; Miyamura, H.; Matsubara, R.;
Kobayashi, S. Adv. Synth. Catal. 2008, 350, 1996.
REFERENCES
■
(1) (a) Metal Nanoclusters in Catalysis and Materials Science: The Issue of
Size Control; Corain, B., Schmid, G., Toshima, N., Eds.; Elsevier:
Amsterdam, 2007. (b) Feldheim, D. L.; Foss, C. A. Metal Nanoparticles:
Synthesis, Characterization, and Applications; CRC Press: Boca Raton,
FL, 2001. (c) Kobayashi, S.; Miyamura, H. Chem. Rec. 2010, 10, 271.
(d) Yan, N.; Xiao, C.; Kou, Y. Coord. Chem. Rev. 2010, 254, 1179.
(e) Yoo, W.-J.; Miyamura, H.; Kobayashi, S. J. Am. Chem. Soc 2011, 133,
3095. (f) Wang, N.; Matsumoto, T.; Ueno, M.; Miyamura, H.;
Kobayashi, S. Angew. Chem., Int. Ed. 2009, 48, 4744.
(2) (a) Munoz-Flores, B. M.; Kharisov, B. I.; Jimenez-Perez, V. M.;
Martinez, P. E.; Lopez, S. T. Ind. Eng. Chem. Res. 2011, 50, 7705.
(b) Ferrando, R.; Jellinek, J.; Johnston, R. L. Chem. Rev. 2008, 108, 845.
(c) Liu, P.; Nørskov, J. K. Phys. Chem. Chem. Phys. 2001, 3, 3814.
(d) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 22, 1179.
(13) See the Supporting Information for details.
(14) (a) Okamoto, K.; Hayashi, T.; Rawal, V. H. Chem. Commun. 2009,
4815. (b) Okamoto, K.; Hayashi, T.; Rawal, V. H. Org. Lett. 2008, 10,
4387.
(15) Miyamura, H.; Matsubara, R.; Miyazaki, Y.; Kobayashi, S. Angew.
Chem., Int. Ed. 2007, 46, 4151.
(3) (a) Cong, H.; Porco, J. A. ACS Catal. 2012, 2, 65. (b) Witham, C.
A.; Huang, W.; Tsung, C.-K.; Kuhn, J. N.; Somorjai, G. A.; Toste, F. D.
Nat. Chem. 2010, 2, 36. (c) Ranganath, K. V. S.; Kloesges, J.; Schafer, A.
̈
H.; Glorius, F. Angew. Chem., Int. Ed. 2010, 49, 7786. (d) Cong, H.;
16966
dx.doi.org/10.1021/ja307913e | J. Am. Chem. Soc. 2012, 134, 16963−16966