Palladium-catalyzed difunctionalization of internal alkynes via highly regioselective 6- endo cyclization and alkenylation of enynoates: Synthesis of multisubstituted pyrones

Article abstract of DOI:10.1021/acs.orglett.5b00272

An efficient Pd(II)-catalyzed difunctionalization reaction of internal alkynes is reported. In this reaction, various enynoates and electron-deficient alkenes were used for the syntheses of useful highly substituted pyrone derivatives via a 6-endo cycliza

Full text of DOI:10.1021/acs.orglett.5b00272

Letter
pubs.acs.org/OrgLett
Palladium-Catalyzed Difunctionalization of Internal Alkynes via
Highly Regioselective 6-Endo Cyclization and Alkenylation of
Enynoates: Synthesis of Multisubstituted Pyrones
Pan-Pan Tian,† Sai-Hu Cai,† Qiu-Ju Liang, Xing-Yi Zhou,† Yun-He Xu,*,† and Teck-Peng Loh*,†,‡
†Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry, University of Science and
Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, China
‡Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
S
* Supporting Information
ABSTRACT: An efficient Pd(II)-catalyzed difunctionalization
reaction of internal alkynes is reported. In this reaction, various
enynoates and electron-deficient alkenes were used for the
syntheses of useful highly substituted pyrone derivatives via a
6-endo cyclization and subsequent alkenylation pathway. The
corresponding products could be obtained in moderate to
good yields under very mild reaction conditions. The high
regioselectivity and wide compatibility with different functional
groups found in the desired products demonstrate this method to be a general and useful tool for the synthesis of highly
substituted pyrone derivatives.
functional pyrones deserve to be developed. Recently, methods
developed using different electrophiles such as I2, ICl, NIS,
PhSeCl mediated cyclization reactions of enynoates have been
reported by Larock,4 Rossi,5 Burton6 et al. as useful supplements
for the synthesis of heteroatom substituted pyrone compounds.
To achieve the multicarbon-substituted pyrone fragments, these
obtained products need to be further functionalized in
subsequent steps.7 Simultaneously, transition-metal-catalyzed
cyclization reactions of enynes to prepare the pyrone derivatives
have been studied which would be more efficient and convenient
methods for the synthesis of this type of products.8 However, the
high control of high 5-exo or 6-endo regioselectivity of the
products and suppression of protonation side reaction are
difficult.9 To the best of our knowledge, few examples of
palladium-catalyzed difunctionalization of conjugated enynes via
6-endo cyclization have been achieved.10 Inspired by the fruitful
research results of the application of vinylpalladium species
generated in situ in coupling reactions to synthesize complex
molecules,11−13 as well as our continuous interest in developing
cross-coupling reactions between alkenes,14 we embarked on the
study of a palladium-catalyzed difunctionalization reaction of
internal alkynes15 via highly selective 6-endo cyclization and
alkenylation using easily available enynoates16 and electron-
deficient alkenes (Scheme 1).
yrone structural subunits are widely observed in many
Pnatural products and pharmaceutical molecules (Scheme
1).1 Pyrones are also useful building blocks for the synthesis of a
variety of heterocyclic compounds.2 Consequently, the con-
struction of this class of compounds has attracted much
attention. Traditional approaches and organometallic methods
to access this class of compounds have been well studied.3
However, limited substrate scope and/or harsh reaction
conditions restrict the wide application of these methods.
Therefore, more efficient and general methods to access
Scheme 1. Palladium-Catalyzed Cyclization of Enynoates for
Synthesis of Highly Substituted Pyrones
Initially, the optimization of reaction conditions was carried
out using Z-enynoate 1a and methyl acrylate 2a; the summarized
results are shown in Scheme 2. After numerous attempts, it was
Received: January 28, 2015
Published: March 19, 2015
© 2015 American Chemical Society
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DOI: 10.1021/acs.orglett.5b00272
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Organic Letters
Letter
Scheme 2. Optimization of Reaction Conditions for the
Cyclization and Alkenylation of Enynoate 1a with Methyl
Acrylate 2a
Scheme 4. Pd(II)-Catalyzed 6-Endo Cyclization and
a b
,
Alkenylation of 1 with Methyl Acrylate
ab
,
a
Reaction conditions: The mixture of 1a (0.3 mmol), 2a (0.6 mmol),
oxidant in indicated solvent (1.0 mL) was stirred for corresponding
b
c
hours. Isolated yields. 5 mol % Pd(PhCN)2Cl2 was used as catalyst.
Scheme 3. Different Enynoates were Tested for Palladium-
Catalyzed Cyclization and Alkenylation Reactions
found that the system of the Pd(II) catalyst with an oxidant could
favor the product’s formation. The use of dimethyl sulfoxide
(DMSO) as solvent in the reaction could greatly improve the
product’s yield as compared to other tested solvents. Moreover,
the combined oxidant systems such as BQ with oxygen, silver salt
with oxygen, or tert-butylhydroperoxide in air could afford the
desired product in reasonable yield, while the use of only oxygen,
copper(II) acetate with oxygen, or hydrogen peroxide in air were
found to be completely inefficient. Next, various Pd(II) catalysts
were screened; except for Pd(PPh3)2Cl2, other Pd(II) species
could all afford the desired product in acceptable yields.
Especially, a high yield of product could be obtained when
Pd(PhCN)2Cl2 was applied in this reaction. Further detailed
studies using Pd(PhCN)2Cl2 as the catalyst showed that
elevating the temperature to 50 °C would shorten the reaction
time significantly with a slightly decreased product yield.
Furthermore, it was observed that replacing the oxidant system
with BQ and an oxygen atmosphere would favor the product’s
formation, while a high loading of BQ or low loading of the
a
Reaction conditions: The reactions were carried out under the
standard reaction conditions: 1 (0.3 mmol), 2a (0.6 mmol, 2 equiv),
Pd(PhCN)2Cl2 (0.03 mmol, 0.1 equiv), BQ (0.5 equiv), and O2 (1
atm) in 1.0 mL of DMSO were stirred at 32 °C for corresponding
b
c
hours shown in parentheses. Isolated yields. The reactions were
carried out at 60 °C.
palladium catalyst would diminish the yield of product 3a
(Scheme 2, entries 20 and 21 respectively).
Furthermore, enynes with different ester groups were
investigated to react with methyl acrylate respectively under
the optimized reaction conditions. It was noticed that the ethyl-
group-substituted compound 1a is the best choice in terms of
yield and reaction duration (Scheme 3).
Therefore, the generality of the cascade reactions between
ethyl-group-substituted enynoates and methyl acrylate was
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Organic Letters
Letter
duration. Finally, excellent yields could also be obtained for the
substrates with a β-substituent (Scheme 4, 3r and 3s).
Next, various electron-deficient alkenes were tested as the
coupling partners (Scheme 5). The corresponding products
could be smoothly obtained in high yields when different
acrylates (Scheme 4, 3a, 4a−4d) as well as the acrylamide
(Scheme 5, 4f) were used in the reaction. However, only low
yields of the products 4e and 4h were found when acrolein and
styrene were used as the coupling partners, respectively.
A plausible reaction pathway for the Pd(II)-catalyzed 6-endo
cyclization and alkenylation reactions of enynoates was proposed
as shown in Scheme 6.17 First, a 6-endo cyclization of the
enynoate will occur which was activated by the alkyne Pd(II) ion.
Therefore, an active vinylpalladium species B will be generated to
react with methyl acrylate and afford the intermediate C via
migratory insertion. The desired product 3a could be formed
after reductive elimination and followed by the reoxidation of
Pd(0) species into the next catalytic cycle by benzoquinone
(BQ).
Scheme 5. Pd(II)-Catalyzed Cyclization and Alkenylation of
1a with Different Electron-Deficient Alkenes 2
ab
,
In conclusion, Pd(II)-catalyzed highly regioselective 6-endo
cyclization and alkenylation reactions have been developed.
Various multifunctionalized pyrones could be obtained in good
yields under mild reaction conditions in a one-pot manner.
Further studies on the application of this method for the
synthesis of natural products are in progress.
a
Reaction conditions: The reactions were conducted under the
standard reaction conditions: 1a (0.3 mmol), 2 (0.6 mmol, 2 equiv),
Pd(PhCN)2Cl2 (0.03 mmol, 0.1 equiv), BQ (0.5 equiv) in 1.0 mL of
DMSO under an oxygen atmosphere (1 atm) were stirred at 32 °C for
ASSOCIATED CONTENT
b
■
24 h. Isolated yields.
S
* Supporting Information
Experimental details and spectroscopic data for all products. This
material is available free of charge via the Internet at http://pubs.
acs.org.
Scheme 6. Proposed Possible Mechanism for the Pd(II)-
Catalyzed 6-Endo Cyclization and Olefination Reactions of
Enynoates
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: xyh0709@ustc.edu.cn.
*E-mail: teckpeng@ntu.edu.sg.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the funding support of the National
Natural Science Foundation of China (21372210), the State Key
Program of National Natural Science Foundation of China
(21432009), Natural Science Foundation of Anhui Province
(1308085QB37), and the National Science Foundation for
Postdoctoral Scientists of China (2012M521231). The authors
also thank Nanyang Technology University for financial support
of this research. The authors are grateful to Mr. Koh Peng Fei
Jackson (Nanyang Technological University, Singapore) for his
careful proofreading of the final manuscript.
evaluated (Scheme 4). The substrates with electron-donating or
-withdrawing group on the phenyl ring all could afford the
desired products in moderate to excellent yields. However, the
presence of a methyl substituent at the ortho-position of the
phenyl ring will prolong the reaction time greatly due to possible
steric hindrance. Additionally, the cyclohexenyl as well as the
aliphatic group substituted enynoates could also be employed in
the cascade reactions. The tolerance of various functional groups
such as halide, ether, and ester in products allows them to be
further functionalized in subsequent steps. When thioenyl- or
indole-substitued enynoate was used to react with methyl
acrylate, the desired products 3p and 3q could be generated in
62% and 72% yields respectively by prolonging the reaction
REFERENCES
■
(1) (a) Chen, K. K.; Kovarikova, A. J. J. Pharm. Sci. 1967, 56, 1535.
(b) Venkataraman, H.; Cha, J. K. Tetrahedron Lett. 1987, 28, 2455.
(c) Abraham, W. R.; Arfmann, H. Phytochemistry 1988, 27, 3310.
(d) Posner, G. H.; Nelson, T. D.; Kinter, C. M.; Johnson, N. J. Org.
Chem. 1992, 57, 4083. (e) Vara Prasad, J. V. N.; Para, K. S.; Lunney, E.
A.; Ortwine, D. F.; Dunbar, J. B., Jr.; Ferguson, D.; Tummino, P. J.;
Hupe, D.; Tait, B. D.; Domagala, J. M.; Humblet, C.; Bhat, T. N.; Liu, B.;
Guerin, D. M. A.; Baldwin, E. T.; Erickson, J. W.; Sawyer, T. K. J. Am.
Chem. Soc. 1994, 116, 6989. (f) Schlingmann, G.; Milne, L.; Carter, G. T.
Tetrahedron 1998, 54, 13013. (g) Yokoe, H.; Mitsuhashi, C.; Matsuoka,
1638
DOI: 10.1021/acs.orglett.5b00272
Org. Lett. 2015, 17, 1636−1639

Organic Letters
Letter
Y.; Yoshimura, T.; Yoshida, M.; Shishido, K. J. Am. Chem. Soc. 2011, 133,
8854.
(2) (a) Kozytska, M. V.; Dudley, G. B. Tetrahedron Lett. 2008, 49, 2899.
(b) Larsson, R.; Sterner, O.; Johansson, M. Org. Lett. 2009, 11, 657.
(c) Nelson, H. M.; Gordon, J. R.; Virgil, S. C.; Stoltz, B. M. Angew.
Chem., Int. Ed. 2013, 52, 6699.
(3) (a) Komiyama, T.; Takaguchi, Y.; Tsuboi, S. Tetrahedron Lett.
2004, 45, 6299. (b) Sheibani, H.; Islami, M. R.; Khabazzadeh, H.; Saidi,
K. Tetrahedron 2004, 60, 5931. (c) Ma, S.; Yu, S.; Yin, S. J. Org. Chem.
2003, 68, 8996. (d) Biagetti, M.; Bellina, F.; Carpita, A.; Rossi, R.
Tetrahedron Lett. 2003, 44, 607. (e) Anastasia, L.; Xu, C.; Negishi, E.
Tetrahedron Lett. 2002, 43, 5673. (f) Fringuelli, F.; Piermatt, O.; Pizzo,
F. Heterocycles 1999, 50, 611. (g) Tidwell, T. T.; Sammtleben, F.;
Christl, M. J. Chem. Soc., Perkin Trans. 1 1998, 2031. (h) Cerezo, S.;
H.; Jin, W.; Sun, C.; Chen, J.; Du, W.; He, S.; Yu, Z. Angew. Chem., Int.
Ed. 2010, 49, 5792. (h) Hu, P.; Huang, S.; Xu, J.; Shi, Z.-J.; Su, W. Angew.
Chem., Int. Ed. 2011, 50, 9926. (i) Yeung, C. S.; Dong, V. M. Chem. Rev.
2011, 111, 1215. (j) Besset, T.; Kuhl, N.; Paturean, F. W.; Glorious, F.
Chem.Eur. J. 2011, 17, 7167. (k) Li, C.; Zhang, Y.; Li, P.; Wang, L. J.
Org. Chem. 2011, 76, 4692. (l) Bai, Y.; Zeng, J.; Cai, S.; Liu, X.-W. Org.
Lett. 2011, 13, 4394. (m) Bi, L.; Georg, G. I. Org. Lett. 2011, 13, 5413.
(n) Yu, Y.-Y.; Niphakis, M. J.; Georg, G. I. Org. Lett. 2011, 13, 5932.
(o) Shi, Z.; Zhang, C.; Tang, C.; Jiao, N. Chem. Soc. Rev. 2011, 41, 3381.
(p) Liwosz, T. W.; Chemler, S. R. J. Am. Chem. Soc. 2012, 134, 2020.
(q) Yuan, F.-Q.; Han, F.-S. Org. Lett. 2012, 14, 1218. (r) Zhang, Y.; Cui,
Z.; Li, Z.; Liu, Z.-Q. Org. Lett. 2012, 14, 1838. (s) Dong, J.; Huan g, Y.;
Qiu, X.; Cheng, Y.; Hao, J.; Wan, D.; Li, W.; Liu, X.; You, J. Chem.Eur.
J. 2012, 18, 6158. (t) Wencel-Delord, J.; Nimphius, C.; Patureau, F. W.;
Glorius, F. Chem.Asian. J. 2012, 7, 1208. (u) Giant, N.; Giuaizeau, I.
Org. Lett. 2012, 14, 3304. (v) Yu, W.; Chen, J.; Gao, K.; Liu, Z.; Zhang, Y.
Org. Lett. 2014, 16, 4870 and ref 12.
(13) Examples of vinylpalladium species generated via nucleophilic
addition of alkynes: (a) Ma, S.; Lu, X. J. Org. Chem. 1993, 58, 1245.
(b) Zhang, Q.; Lu, X. J. Am. Chem. Soc. 2000, 122, 7604. (c) Lei, A.; Lu,
X. Org. Lett. 2000, 2, 2699. (d) Zhao, L.; Lu, X. Org. Lett. 2002, 4, 3903.
(e) Song, J.; Shen, Q.; Xu, F.; Lu, X. Org. Lett. 2007, 9, 2947. (f) Shen, K.;
Han, X.; Lu, X. Org. Lett. 2012, 14, 1756. (g) Liu, H.; EI-Salfiti, M.; Chai,
D. I.; Auffret, J.; Lautens, M. Org. Lett. 2012, 14, 3648. (h) Keilitz, J.;
Newman, S. G.; Lautens, M. Org. Lett. 2013, 15, 1148. (i) Chinchilla, R.;
Moreno-Manas, M.; Pleixats, R. Tetrahedron Lett. 1998, 54, 7813.
̃
(i) Dubuffet, T.; Cimetiere, B.; Lavielle, G. Synth. Commun. 1997, 27,
1123. (j) Liebeskind, L. S.; Wang, J. Tetrahedron 1993, 49, 5461.
(k) West, F. G.; Hartke-Karger, C.; Koch, D. J.; Kuehn, C. E.; Arif, A. M.
J. Org. Chem. 1993, 58, 6795. (l) Tsuda, T.; Morikawa, S.; Saegusa, T. J.
Chem. Soc., Chem. Commun. 1989, 9. (m) Dieter, R. K.; Fishpaugh, J. R. J.
Org. Chem. 1988, 53, 2031. (n) Henry, W.; Hughes, R. P. J. Am. Chem.
Soc. 1986, 108, 7862. (o) Pavlik, J. W.; Patten, A. D.; Bolin, D. R.;
Bradford, K. C.; Clennan, E. L. J. Org. Chem. 1984, 49, 4523. (p) Ishibe,
N.; Sunami, M.; Odani, M. J. Am. Chem. Soc. 1973, 95, 463. (q) Ishibe,
N.; Yutaka, S. J. Org. Chem. 1978, 43, 2138. (r) Hayasi, Y.; Nozaki, H.
Tetrahedron 1971, 27, 3085.
́
Najera, C. Chem. Rev. 2014, 114, 1783 and references cited therein.
(14) (a) Zhou, H.; Chung, W.-J.; Xu, Y.-H.; Loh, T.-P. Chem. Commun.
2009, 45, 3472. (b) Zhou, H.; Xu, Y.-H.; Chang, W.-J.; Loh, T.-P. Angew.
Chem., Int. Ed. 2009, 48, 5355. (c) Xu, Y.-H.; Chok, Y.-K.; Loh, T.-P.
Chem. Sci. 2011, 2, 1822. (d) Pankajakshan, S.; Xu, Y.-H.; Cheng, J.-K.;
Low, M.-T.; Loh, T. P. Angew. Chem., Int. Ed. 2012, 51, 5701. (e) Feng,
C.; Loh, T.-P. Chem. Sci. 2012, 3, 3458. (e) Zhang, X.; Wang, M.; Xu, Y.-
H.; Loh, T.-P. Org. Lett. 2013, 15, 5531. (g) Xu, Y.-H.; He, T.; Zhang,
Q.-C.; Loh, T.-P. Chem. Commun. 2014, 50, 2784.
(15) Examples: (a) Ma, S.; Lu, X. J. Org. Chem. 1991, 56, 5120.
(b) Zhou, C.; Emich, D. E.; Larock, R. C. Org. Lett. 2003, 5, 1579.
(c) Patel, S. J.; Jamison, T. F. Angew. Chem., Int. Ed. 2003, 42, 1364.
(d) Nakao, Y.; Oda, S.; Hayama, T. J. Am. Chem. Soc. 2004, 126, 13904.
(e) Zhou, C.; Larock, R. C. J. Org. Chem. 2006, 71, 3184. (f) Nakao, Y.;
(4) (a) Yao, T.; Larock, R. C. Tetrahdron Lett. 2002, 43, 7401. (b) Yao,
T.; Larock, R. C. J. Org. Chem. 2003, 68, 5936.
(5) (a) Bellina, F.; Biagetti, M.; Carpita, A.; Rossi, R. Tetrahedron 2001,
57, 2857. (b) Biagetti, M.; Bellina, F.; Carpita, A.; Viel, S.; Mannina, L.;
Rossi, R. Eur. J. Org. Chem. 2002, 1063.
(6) Wang, Y.; J. Burton, D. J. Org. Chem. 2006, 71, 3859.
(7) Frebault, F.; Oliveira, M. T.; Wostefeld, E.; Maulide, N. J. Org.
́
̈
Chem. 2010, 75, 762 and ref 5.
(8) (a) For a Ag(I)-catalyzed cyclization of enyne carboxylic acids,
please see: Yoshikawa, T.; Shindo, M. Org. Lett. 2009, 11, 5378. (b) For
Cu-catalyzed cyclization and bromination examples, see: Liang, Y.; Xie,
Y.-X.; Li, J.-H. Synthesis 2007, 400. (c) Please see the example of a Pd-
catalyzed 6-endo cyclization and arylation reaction: Rossi, R.; Bellina, F.;
Biagetti, M.; Catanese, A.; Mannina, L. Tetrahedron Lett. 2000, 41, 5281.
(d) Two examples of a Pd-catalyzed tandem annulation of enyne with
methyl vinyl ketone: Wang, H.; Han, X.; Lu, X. Tetrahedron 2013, 69,
8626.
(9) (a) Arai, S.; Nishida, A. Synlett 2012, 23, 2880 and references cited
therein. (b) Xia, Y.; Qu, S.; Xiao, Q.; Wang, Z.-X.; Qu, P.; Chen, L.; Liu,
Z.; Tian, L.; Huang, Z.; Zhang, Y.; Wang, J. J. Am. Chem. Soc. 2013, 135,
13502. (c) Hu, F.; Xia, Y.; Ma, C.; Zhang, Y.; Wang, J. Org. Lett. 2014, 16,
4082. Also please see ref 5a.
́
Yada, A.; Hayama, T. J. Am. Chem. Soc. 2010, 132, 10024. (g) Suarez, L.
L.; Greaney, M. F. Chem. Commun. 2011, 7992. (h) Minami, Y.;
Yoshiyasu, H.; Nakao, Y.; Hiyama, T. Angew. Chem., Int. Ed. 2013, 52,
883. (i) Walkinshaw, A. J.; Xu, W.; Suero, M. G.; Gaunt, M. J. J. Am.
Chem. Soc. 2013, 135, 12532.
(16) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 16, 4467. (b) Andrus, M. B.; Lepore, S. D.; Turner, T. M. J. Am.
Chem. Soc. 1997, 119, 12159. (c) Paterson, I.; Paquet, T. Org. Lett. 2010,
12, 2158.
(17) (a) Huang, Q.; Larock, R. C. J. Org. Chem. 2003, 68, 980.
(b) Zhao, P.; Chen, D.; Song, G.; Han, K.; Li, X. J. Org. Chem. 2012, 77,
1579.
(10) Negish, E.; Kotora, M. Tetrahedron 1997, 53, 6707. Also please
see refs 8c and 8d.
(11) (a) Examples of vinylpalladium species generated from alkenyl
halides: (a) Heck, R. F.; Nolley, J. P. J. Org. Chem. 1972, 37, 2320.
(b) Mizoroki, T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jpn. 1971, 44, 581.
(c) de Meijere, A.; Meyer, F. E. Angew. Chem., Int. Ed. Engl. 1994, 33,
2379. (d) Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009.
(e) Dounay, A. B.; Overman, L. E. Chem. Rev. 2003, 103, 2945.
(f) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005,
44, 4442. Examples of vinylpalladium species generated from alkenyl
organometallic compounds: (g) Negish, E.; King, A. O.; Okukado, N. J.
Org. Chem. 1977, 42, 1821. (h) Miyaura, N.; Yamada, K.; Suzuki, A.
Tetrahedron Lett. 1979, 20, 3437. (i) Hallberg, A.; Westerlund, C. Chem.
Lett. 1982, 1993. (j) Still, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508.
(12) Examples of vinylpalladium species generated via transition-metal
catalyzed alkenyl C−H bond palladation: (a) Hatamoto, Y.; Sakaguchi,
S.; Ishii, Y. Org. Lett. 2004, 6, 4623. (b) Giri, R.; Yu, J.-Q. J. Am. Chem.
Soc. 2008, 130, 14082. (c) Ge, H.; Niphakis, X.; Georg, M. J. G. I. J. Am.
Chem. Soc. 2008, 130, 3708. (d) Han, X.; Lu. Org. Lett. 2009, 11, 2381.
(e) Mochida, S.; Hirano, K.; Satoh; Miura, M. J. Org. Chem. 2009, 74,
6295. (f) Ho, C.-Y.; He, L. Angew. Chem., Int. Ed. 2010, 49, 9182. (g) Yu,
1639
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