Dihydrophenanthridine: A new and easily regenerable NAD(P)H model for biomimetic asymmetric hydrogenation

Article abstract of DOI:10.1021/ja211684v

A new and easily regenerable NAD(P)H model 9,10-dihydrophenanthridine (DHPD) has been designed for biomimetic asymmetric hydrogenation of imines and aromatic compounds. This reaction features the use of hydrogen gas as terminal reductant for the regenerat

Full text of DOI:10.1021/ja211684v

Article
pubs.acs.org/JACS
Dihydrophenanthridine: A New and Easily Regenerable NAD(P)H
Model for Biomimetic Asymmetric Hydrogenation
Qing-An Chen,† Kai Gao,† Ying Duan,† Zhi-Shi Ye,† Lei Shi,† Yan Yang,† and Yong-Gui Zhou*,†,‡
†State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
S
* Supporting Information
ABSTRACT: A new and easily regenerable NAD(P)H model 9,10-
dihydrophenanthridine (DHPD) has been designed for biomimetic
asymmetric hydrogenation of imines and aromatic compounds. This
reaction features the use of hydrogen gas as terminal reductant for the
regeneration of the DHPD under the mild condition. Therefore, the
substrate scope is not limited in benzoxazinones; the biomimetic
asymmetric hydrogenation of benzoxazines, quinoxalines, and quinolines
also gives excellent activities and enantioselectivities. Meanwhile, an
unexpected reversal of enantioselectivity was observed between the
reactions promoted by the different NAD(P)H models, which is ascribed to the different hydride transfer pathway.
efficient method for in situ regeneration of HEH from
Hantzsch pyridine under hydrogen gas in biomimetic
asymmetric hydrogenation (Scheme 1).7a Although excellent
INTRODUCTION
■
As a couple of the most important coenzymes found in living
cells, reduced nicotinamide adenine dinucleotide (NADH) and
nicotinamide adenine dinucleotide phosphate (NADPH) play
great roles in reduction−oxidation (redox) metabolism.1
Therefore, NAD(P)H mimics have become one of the most
significant fields in biomimetic chemistry over the past few
decades (Figure 1). Despite that much progress has been
Scheme 1. Biomimetic Asymmetric Hydrogenation of
Benzoxazinones Using Catalytic Amount of Hantzsch Esters
enantioselectivities were obtained, the regeneration condition
of HEH was harsh, and the substrate scope was limited to
benzoxazinones which underwent no background reaction.
Developing a milder biomimetic asymmetric hydrogenation is
of great interest in the field of NAD(P)H mimics and good for
extending the substrate generality. Based on our previous work
on asymmetric hydrogenation,8 we envisioned that looking for
a new and easily regenerable NAD(P)H model is probably a
good choice.
To the best of our knowledge, the dihydropyridine amido
group is the key structure in NAD(P)H models and plays an
important part in the hydride transfer process. Therefore, most
of the currently successful NAD(P)H models, such as HEH3
and 1-benzyl-1,4-dihydronicotinamide (BNAH),9 contain a
dihydropyridine skeleton. Based on the design of NAD(P)H
models, the search of NAD(P)H models that can be used in the
Figure 1. (A) NAD(P)H models-mediated biomimetic reduction
(Cat. I, regeneration catalyst; Cat. II, reduction catalyst; HEH,
Hantzsch esters; DHPD, 9,10-dihydrophenanthridine).
achieved, most of the current research focuses on the hydride
transfer ability and selectivity in redox reactions rather than the
renewable capability of NAD(P)H models.2
As one of the simplest NAD(P)H models, Hantzsch esters
(HEH)3 have been widely and successfully used as reductant in
the enantioselective transfer hydrogenation of unsaturated
bonds (CC, CN, and CO) using organocatalysts4,5
and metal catalysts (Figure 1).6 Recently, we reported an
Received: December 15, 2011
Published: January 3, 2012
© 2012 American Chemical Society
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biomimetic asymmetric hydrogenation process takes into
account four requirements: (a) easy preparation of the
NAD(P)H model from commercially available materials; (b)
easy hydride transfer from the NAD(P)H model to the
prochiral substrate; (c) easy regeneration of the NAD(P)H
model from the oxidized form under hydrogen gas; (d) easy
control of the reaction enantioselectivity. Herein, we developed
a new and easily regenerable NAD(P)H model 9,10-
dihydrophenanthridine (DHPD) for biomimetic asymmetric
hydrogenation of imines and aromatic compounds (Scheme 2).
[Ru(p-cymene)I2]2 exhibited similar catalytic efficiency in the
reduction of phenanthridine 1 in different solvents under 200
psi of H2 (entries 1−6). No decrease of conversion was
observed with the change of the pressure of H2, even under
atmospheric pressure using CH2Cl2 as the solvent (entries 7−
9).
Biomimetic Asymmetric Hydrogenation of Benzox-
azinones. Subsequently, the feasibility of the in situ
regeneration of DHPD for the asymmetric hydrogenation of
benzoxazinone 3a was explored, and the results are presented in
Table 2.10 The evaluation of chiral Brønsted acids demon-
strated that (S)-5d was the best catalyst for the hydride transfer
and delivered 4a in 94% ee (entries 1−4).11,12 No improvement
of the conversion was achieved through the investigation of
solvent effect (entries 5−8). With the increasing of the pressure
of hydrogen gas or the catalyst loading, an improvement of
reaction conversion was observed (entries 9−10). Prolonging
the reaction time delivered the dihydrobenzoxazinone 4a with
excellent conversion (entry 11).
Scheme 2. Biomimetic Asymmetric Hydrogenation
Promoted by Dihydrophenanthridine
With the optimized conditions, the generality of the
biomimetic asymmetric hydrogenation of benzoxazinones 3
using a catalytic amount of phenanthridine 1 (10 mol %) was
investigated (Table 3). In general, moderate to excellent yields
(77−96%) and excellent enantioselectivities (87−97% ee) were
achieved in this hydrogenation system for various benzox-
azinones 3 regardless of the electronic properties of
substituents (entries 1 and 3−12). Interestingly, an unexpected
reversal of enantioselectivity was observed between the
different NAD(P)H models (DHPD vs HEH)-promoted
biomimetic asymmetric hydrogenation (entry 1 vs 2).7a,10a
Enantioreversal Phenomenon in Biomimetic Asym-
metric Hydrogenation. The origin of enantioreversal in
biomimetic asymmetric hydrogenation of benzoxazinones 3 can
be explained by the stereochemical model as illustrated in
Figure 2.13 For the biomimetic asymmetric hydrogenation
promoted by DHPD, the substrate 3a and DHPD interact with
chiral phosphoric acid (S)-5 through two hydrogen bonds (eq
1).14 These two hydrogen bonds and the effect of steric
hindrance build up a “three-point contact model” that
determines the stereoselectivity in this 1,2-hydride transfer
process. In the biomimetic asymmetric hydrogenation
promoted by HEH,7a 3a/(S)−5/HEH form another three-
point contact model leading to Re-face reduction based on
Goodman and Himo’s calculation (eq 2).15 The different steric
Owing to the fact that only mild condition was required for
regeneration of DHPD, the substrate scope is not just limited in
benzoxazinones; the biomimetic asymmetric hydrogenation of
benzoxazines, quinoxalines, and quinolines also gives excellent
activities and enantioselectivities.
RESULTS AND DISCUSSION
■
Regeneration of NAD(P)H Model-DHPD from Phenan-
thridine. Its easy preparation and dehydroaromatization make
9,10-dihydrophenanthridine (DHPD) a promising candidate to
act as a new and renewable NAD(P)H model in biomimetic
asymmetric hydrogenation systems. The 1,2-hydride transfer
pathway occurring in DHPD-mediated reduction is different
from the common 1,4-hydride transfer of the current NAD(P)
H model. This feature perhaps is good for controlling the
reaction enantioselectivity owing to the decrease in steric
distance between the substrate and the catalyst. Based on our
research on biomimetic asymmetric hydrogenation,7 the Ru(II)
complex was chosen as the catalyst for the regeneration of
DHPD 2 from phenanthridine 1 under hydrogen gas (Table 1).
a
Table 1. Generation of DHPD 2 from Phenanthridine 1
b
entry
solvent
H2 (psi)
conv. (%)
1
2
3
4
5
6
7
8
9
toluene
THF
200
200
200
200
200
200
500
50
71
89
81
86
80
83
83
84
83
dioxane
EtOAc
CHCl3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
15
a
b
1 (0.02 mmol), [Ru(p-cymene)I2]2 (5 mol %), H2, solvent (2 mL), 16 h. Determined by 1H NMR.
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a
Table 2. In Situ Regeneration of DHPD 2 for the Biomimetic Hydrogenation of Benzoxazinone 3a
b
c
entry
solvent
5
conv. (%)
ee (%)
1
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
THF
(S)-5a
(S)-5b
(S)-5c
(S)-5d
(S)-5d
(S)-5d
(S)-5d
(S)-5d
(S)-5d
(S)-5d
(S)-5d
17
16
17
19
12
<5
12
9
70
90
74
94
90
−
95
93
94
94
94
2
3
4
5
6
7
8
9
toluene
EtOAc
CH2Cl2
CH2Cl2
CH2Cl2
d
36
52
>95
de
,
10
11
def
, ,
a
b
3a (0.20 mmol), 1 (10 mol %), [Ru(p-cymene)I2]2 (0.5 mol %), (S)-5 (1 mol %), solvent (2 mL), H2 (50 psi), 16 h. Determined by 1H NMR.
c
d
e
f
Determined by HPLC. H2 (500 psi). (S)-5d (2 mol %). 48 h.
a
Table 3. Biomimetic Asymmetric Hydrogenation of Benzoxazinones 3 Using Phenanthridine 1
b
c
entry
1
R in 3
Ar in 3
yield (%)
ee (%)
H
H
Ph
Ph
96 (4a)
93 (4a)
94 (4b)
96 (4c)
94 (4d)
89 (4e)
82 (4f)
82 (4g)
82 (4h)
78 (4i)
87 (4j)
77 (4k)
90 (4l)
94 (R)
98 (S)
94 (R)
93 (R)
90 (R)
96 (R)
97 (R)
95 (R)
94 (R)
95 (S)
89 (R)
87 (R)
93 (R)
d
2
3
4
H
4-MeOC6H4
4-MeC6H4
3,4-Me2C6H3
4-ClC6H4
4-BrC6H4
4-FC6H4
3-FC6H4
2-thienyl
Ph
H
5
H
6
H
7
H
8
H
9
H
10
11
12
13
H
6-Cl
6-Me
7-Me
Ph
Ph
a
b
3 (0.20 mmol), 1 (10 mol %), [Ru(p-cymene)I2]2 (0.5 mol %), (S)-5d (2 mol %), H2 (500 psi), CH2Cl2 (2 mL), 48 h, RT. Isolated yields.
c
d
Determined by HPLC. The data were quoted from ref 7a: 3 (0.20 mmol), Hantzsch ethyl ester (10 mol %), [Ru(p-cymene)I2]2 (1.25 mol %), (S)-
5a (2 mol %), H2 (1000 psi), THF/CH2Cl2 1/3 (2 mL), 48 h, 50 °C.
demand between 1,2- and 1,4-hydride transfer pathway is
responsible for the reversal of enantioselectivity which we had
observed in the asymmetric disproportionation of dihydroqui-
noxalines.7b
enantioselectivity was also observed in the reduction of
benzoxazines 6 promoted by different hydride transfer reagents
(DHPD vs HEH, entry 1 vs 3). Notably, the obtained chiral
products dihydrobenzoxazines 7 are efficient catalysts for the
enantioselective transfer hydrogenation of α,β-unsaturated
aldehydes with Hantzsch esters as hydrogen source.16c
Biomimetic Asymmetric Hydrogenation of Aromatic
Compounds Quinoxalines. The biomimetic asymmetric
hydrogenation promoted by DHPD could also be successfully
applied to the reduction of aromatic compounds,17 such as
quinoxalines18,19 and quinolines.20,21 The optically pure 1,2,3,4-
tetrahydroquinoxaline derivatives are of great synthetic
potential in the preparation of pharmaceuticals and agro-
chemicals. Therefore, various transitional metal catalysts
Biomimetic Asymmetric Hydrogenation of Benzox-
azines. Encouraged by the successful hydrogenation of
benzoxazinones 3, we then examined the hydrogenation of
benzoxazines 6. The pressure of H2 could be further reduced in
the biomimetic asymmetric reduction of benzoxazines 6 using a
catalytic amount of phenanthridine 1 (10 mol %) with full
conversion and up to 92% ee (Table 4, entries 1 and 4−11).16
Even under 1 atm of H2, full conversion was observed in the
reduction of benzoxazine 6a with 2 mol % of [Ru(p-
cymene)I2]2 in 72 h (entry 2). As expected, a reversal of
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genation of the two double bonds (CN) in the moiety of
quinoxalines 8 with high yields (Table 5). Generally, good to
excellent enantioselectivities (85−95%) were obtained in this
biomimetic asymmetric hydrogenation system regardless of the
electronic property of the substituent on quinoxalines 8 (entries
1 and 3−12). As predicted, an enantioreversal phenomenon
occurred once again (entry 1 vs 2).
Biomimetic Asymmetric Hydrogenation of Aromatic
Compounds Quinolines. 1n 2003, our group developed the
first example of iridium-catalyzed highly enantioselective
hydrogenation of quinolines providing a simple access to chiral
1,2,3,4-tetrahydroquinolines, which are of great synthetic
importance in the preparation of natural products and
pharmaceuticals.8a−f Subsequently, much important progress
has been achieved in the asymmetric hydrogenation of
quinolines using numerous transition-metal catalysts.20 Mean-
while, the organocatalytic asymmetric transfer hydrogenation of
quinolines using Hantzsch esters as hydrogen source has been
well developed by Rueping and Du.21
Due to the fact that two different unsaturated double bonds
(CC and CN) are reduced in quinolines, the biomimetic
asymmetric hydrogenation of quinolines 10 will be more
difficult than the hydrogenation of quinoxalines considering the
catalyst system’s selectivities. In order to achieve high
conversions, the catalyst loading, hydrogen gas pressure, and
reaction temperature have to be increased (Table 6). To our
delight, excellent enantioselectivities (86−93%) could also be
obtained in the biomimetic asymmetric hydrogenation of
quinolines 10 using a catalytic amount of phenanthridine 1
(entries 1−8).
Figure 2. Origin of enantioreversal in the biomimetic asymmetric
hydrogenation resulting from different hydride transfer pathways.
involving rhodium, iridium, and ruthenium complexes have
been developed for the enantioselective hydrogenation of
quinoxalines since 1987.18 Recently, Rueping and co-workers
reported highly enantioselective Brønsted acid-catalyzed trans-
fer hydrogenation of quinoxalines with HEH as hydrogen
source.19 Owing to the fact that there are two unsaturated
bonds (CN) reduced in quinoxalines, at least 2 equiv of
Hantzsch esters are required in order to achieve full conversion.
Besides this, separating the chiral products from the pyridine
derivatives (more than 2 equiv) generated by dehydrogenation
of Hantzsch ester is difficult. These two disadvantages will be
overcome by the biomimetic asymmetric hydrogenation of
quinoxalines 8 promoted by a catalytic amount of phenan-
thridine 1.
Proposed Catalytic Cycle for Biomimetic Asymmetric
Hydrogenation. A proposed mechanism for the biomimetic
asymmetric hydrogenation of imines is illustrated in Scheme 3.
Just like the enzymatic reactions where NAD(P)H is involved,
this biomimetic asymmetric hydrogenation comprises two
cascade redox cycles promoted by metal/Brønsted acid relay
catalysis.22 Owing to the easy regeneration of DHPD 2 from
phenanthridine 1, a milder reaction condition was required
The optimized condition was obtained through an
exploration of appropriate Brønsted acid catalysts and the
examination of reaction parameters, such as solvent, hydrogen
gas pressure, catalyst loading, and temperature. Only 10 mol %
of phenanthridine 1 was required to promote the hydro-
a
Table 4. Biomimetic Asymmetric Hydrogenation of Benzoxazines 6 Using Phenanthridine 1 under Mild Conditions
b
c
entry
1
R in 6
yield (%)
ee (%)
Ph
99 (7a)
99 (7a)
95 (7a)
98 (7b)
99 (7c)
98 (7d)
98 (7e)
98 (7f)
99 (7g)
98 (7h)
99 (7i)
92 (R)
90 (R)
97 (S)
86 (R)
88 (R)
91 (R)
88 (R)
88 (R)
92 (R)
88 (R)
87 (R)
d
2
Ph
e
3
Ph
4
5
4-MeC6H4
4-PhC6H4
4-BrC6H4
4-ClC6H4
4-FC6H4
3-BrC6H4
3,4-Cl2C6H3
2-naphthyl
6
7
8
9
10
11
a
b
6 (0.20 mmol), 1 (10 mol %), [Ru(p-cymene)I2]2 (1 mol %), (S)-5d (1 mol %), H2 (50 psi), CH2Cl2 (2 mL), 32 h, RT. Isolated yields.
c
d
e
Determined by HPLC. H2 (1 atm), [Ru(p-cymene)I2]2 (2 mol %), (S)-5d (1 mol %), 72 h, RT. The data were quoted from ref 10a. (R)-5a was
used in Rueping’s work for the formation of (R)-7a. For a simple discussion, (S)-5a was employed in this table. Reaction conditions: 6a, Hantzsch
ethyl ester (1.25 equiv) and (S)-5a (0.1 mol %), CH2Cl2, 12 h.
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a
Table 5. Biomimetic Asymmetric Hydrogenation of Quinoxalines 8 Using Phenanthridine 1
b
c
entry
1
Ar in 8
yield (%)
ee (%)
Ph
99 (9a)
98 (9a)
96 (9b)
98 (9c)
98 (9d)
98 (9e)
98 (9f)
93 (9g)
99 (9h)
96 (9i)
99 (9j)
96 (9k)
90 (R)
90 (S)
91 (R)
85 (R)
91 (R)
91 (R)
92 (R)
93 (R)
90 (R)
92 (R)
93 (R)
95 (R)
d
2
Ph
3
4
4-MeC6H4
4-MeOC6H4
4-FC6H4
4-ClC6H4
4-BrC6H4
4-CF3C6H4
3-FC6H4
3-ClC6H4
3-BrC6H4
2-naphthyl
5
6
7
8
9
10
11
12
a
b
8 (0.20 mmol), 1 (10 mol %), [Ru(p-cymene)I2]2 (0.5 mol %), (S)-5e (1 mol %), H2 (100 psi), benzene (2 mL), 48 h, RT. Isolated yields.
c
d
Determined by HPLC. The data were quoted from ref 19. (R)-5f was used in Rueping’s work for the formation of (R)-9a. For a simple discussion,
(S)-5f was employed in this table. Reaction conditions: 8, Hantzsch ethyl ester (2.4 equiv) and (S)-5f (10 mol %), 35 °C, CHCl3 (0.5 M), 24 h.
a
Table 6. Biomimetic Asymmetric Hydrogenation of Quinolines 10 Using Phenanthridine 1
b
c
entry
Ar in 10
yield (%)
ee (%)
1
2
3
4
5
6
7
8
Ph
95 (11a)
96 (11b)
98 (11c)
96 (11d)
94 (11e)
90 (11f)
91 (S)
90 (S)
86 (S)
90 (S)
91 (S)
93 (S)
92 (S)
4-MeC6H4
4-MeOC6H4
4-FC6H4
4-ClC6H4
4-BrC6H4
4-CF3C6H4
2-naphthyl
d
64 (11g)
92 (11h)
91 (S)
a
b
10 (0.20 mmol), 1 (10 mol %), [Ru(p-cymene)I2]2 (0.5 mol %), (S)-5e (4 mol %), H2 (400 psi), benzene (2 mL), 40 °C, 48 h, RT. Isolated
c
d
1
yields. Determined by HPLC. Conversion determined by H NMR.
Scheme 3. Proposed Mechanism for Biomimetic
Hydrogenation of Imines Using Phenanthridine 1 (CPA =
Chiral Phosphoric Acid)
CONCLUSIONS
■
In summary, we have successfully developed a new and easily
regenerable NAD(P)H model DHPD for biomimetic asym-
metric hydrogenation of benzoxazinones, benzoxazines, qui-
noxalines, and quinolines. The easy regeneration of DHPD 2
from phenanthridine 1 suggests that this biomimetic cascade
reduction could be performed under mild conditions. There-
fore, the substrate scope is not limited in benzoxazinones; the
biomimetic asymmetric hydrogenation of benzoxazines,
quinoxalines, and quinolines also gave excellent activities and
enantioselectivities. Meanwhile, an unexpected reversal of
enantioselectivity was observed between the reactions pro-
moted by the different NAD(P)H models. This work also
features the use of hydrogen gas as the terminal reductant for
the regeneration of the NAD(P)H model and combining
transition-metal catalyst and organocatalyst for two biomimetic
cascade redox cycles. Further investigations on the application
of this method are currently ongoing in our lab.
compared with the biomimetic asymmetric hydrogenation
using a catalytic amount of HEH.7a The excellent enantiose-
lectivities achieved in this biomimetic asymmetric transfer
hydrogenation are attributed to the fact that the reaction rate of
this principal reaction k2 is faster than that of the undesired side
reaction k3 which gives racemic products.
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10, 2035. For selected work on chiral thiourea-catalyzed asymmetric
transfer hydrogenation with Hantzsch esters, see: (g) Martin, N. J. A.;
Ozores, L.; List, B. J. Am. Chem. Soc. 2007, 129, 8976. (h) Martin, N. J.
A.; Cheng, X.; List, B. J. Am. Chem. Soc. 2008, 130, 13862.
(i) Schneider, J. F.; Lauber, M. B.; Muhr, V.; Kratzer, D.; Paradies,
J. Org. Biomol. Chem. 2011, 9, 4323.
ASSOCIATED CONTENT
■
S
* Supporting Information
Complete experimental procedures and characterization data
for the prepared compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
(6) For selected work on metal-catalyzed asymmetric transfer
hydrogenation with Hantzsch esters, see: (a) Zehani, S.; Gelbard, G.
J. Chem. Soc., Chem. Commun. 1985, 1162. (b) Yang, J. W.; List, B. Org.
Lett. 2006, 8, 5653. (c) Wang, D. W.; Zeng, W.; Zhou, Y. G.
Tetrahedron: Asymmetry 2007, 18, 1103.
AUTHOR INFORMATION
■
Corresponding Author
ygzhou@dicp.ac.cn
(7) (a) Chen, Q. A.; Chen, M. W.; Yu, C. B.; Shi, L.; Wang, D. S.;
Yang, Y.; Zhou, Y. G. J. Am. Chem. Soc. 2011, 133, 16432. (b) Chen,
Q. A.; Wang, D. S.; Zhou, Y. G.; Duan, Y.; Fan, H. J.; Yang, Y.; Zhang,
Z. J. Am. Chem. Soc. 2011, 133, 6126.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21032003 and 21125208), National
Basic Research Program of China (2010CB833300), and
Dalian Institute of Chemical Physics (K2010F1). We also
thank Prof. Xumu Zhang of Rutgers University and Prof. Wen-
Jing Xiao of Central China Normal University for very helpful
discussions.
(8) For selected work from our group on asymmetric hydrogenation
and transfer hydrogenation, see quinolines: (a) Wang, W. B.; Lu, S.
M.; Yang, P. Y.; Han, X. W.; Zhou, Y. G. J. Am. Chem. Soc. 2003, 125,
10536. (b) Lu, S. M.; Han, X. W.; Zhou, Y. G. Adv. Synth. Catal. 2004,
346, 909. (c) Lu, S. M.; Wang, Y. Q.; Han, X. W.; Zhou, Y. G. Angew.
Chem., Int. Ed. 2006, 45, 2260. (d) Wang, D. W.; Wang, X. B.; Wang,
D. S.; Lu, S. M.; Zhou, Y. G.; Li, Y. X. J. Org. Chem. 2009, 74, 2780.
(e) Wang, D. W.; Wang, D. S.; Chen, Q. A.; Zhou, Y. G. Chem.Eur.
J. 2010, 16, 1133. Pyridines: (f) Wang, X. B.; Zeng, W.; Zhou, Y. G.
Tetrahedron Lett. 2008, 49, 4922. Indoles: (g) Wang, D. S.; Chen, Q.
A.; Li, W.; Yu, C. B.; Zhou, Y. G.; Zhang, X. M. J. Am. Chem. Soc. 2010,
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G.; Yu, C. B.; Fan, H. J.; Duan, Y. J. Am. Chem. Soc. 2011, 133, 8866.
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J. B.; Fish, R. H. Angew. Chem., Int. Ed. 1999, 38, 1429. (b) Konno, H.;
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(e) Matsubara, Y.; Koga, K.; Kobayashi, A.; Konno, H.; Sakamoto, K.;
Morimoto, T.; Ishitani, O. J. Am. Chem. Soc. 2010, 132, 10547. For
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