Tridentate P,N,N-ligand promoted copper-catalyzed [3 + 2] cycloaddition of propargylic esters with β-enamino esters: Synthesis of highly functionalized pyrroles

Article abstract of DOI:10.1039/c5ra19304d

A copper-catalyzed [3 + 2] cycloaddition of propargylic esters with β-enamino esters under mild reaction conditions for the construction of highly functionalized pyrroles has been developed. By employment of a newly developed tridentate P,N,N-ligand, a va

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Tridentate P,N,N-ligand promoted copper-catalyzed
[3+2] cycloaddition of propargylic esters with β-
enamino esters: synthesis of highly functionalized
pyrroles
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Qing Li,ab Chuan-Jin Hou,*ab Yun-Ze Hui,c Yan-Jun Liu,a Rui-Feng Yanga and
Xiang-Ping Hu*b
DOI: 10.1039/x0xx00000x
www.rsc.org/
A copperꢀcatalyzed [3+2] cycloadditon of propargylic esters construction of pyrroles from propargylic esters with βꢀenamino
with βꢀenamino esters under mild reaction conditions for esters, generating pyrroles with different substitution mode to those
the construction of highly functionalized pyrroles has been with Pdꢀcatalyst due to the different reaction pathway between the
developed. By employment of a newly developed tridentate Cuꢀ and Pdꢀcatalyzed reaction. As a result, herein we report an
P,N,Nꢀligand, a variety of fully substituted pyrroles were efficient and alternative synthesis of highly functionalized
achieved in good to high yields.
pyrroles by a Cuꢀcatalyzed [3+2] cycloadditon of propargylic
esters with βꢀenamino esters with a new benzylamineꢀderived
P,N,Nꢀligand.
Pyrroles are one of the most important heterocyclic compounds,
which present in many natural products1 and synthetic
intermediates,2 along with a broad range of applications in
medicinal chemistry,3 bioorganic chemistry,4 polymer
chemistry5 and material science.6 Therefore, great efforts have
been devoted to develop efficient methods for the synthesis of
pyrroles.7 The present synthetic methods generally depend on
the classical condensation such as Knorr,8 PaalꢀKnorr,9 and
Hantzsch reactions.10 Recently, some more efficient approaches
based on the transitionꢀmetal catalyzed cyclizations7a,f and the
multicomponent reactions7d,g have also been developed. Despite
these impressive achievements, the regioselective synthesis of
highly functionalized pyrroles with structural diversity, in
particular fully substituted pyrroles, remains a challenging and
desirable task.11
In the past decades, catalytic propargylic transformation has
made significant progress.12 In particular, the cycloadditions of
propargylic alcohol derivatives with bisꢀnucleophiles for the
construction of cyclic frameworks have been disclosed.
Recently, Yoshida13 reported a Pdꢀcatalyzed cyclization of
propargylic carbonates with βꢀenamino esters for the synthesis
of fully substituted pyrroles (Scheme 1). Considering structural
diversity of pyrrole derivatives, further exploration of new
strategies for the synthesis of fully substituted pyrroles with different
regioselectivities remains in high demand. Based on the recent
advances on the copperꢀcatalyzed propargylic transformation
for the synthesis of heterocyclic compounds,14 we envisioned that
this methodology should be also a suitable approach for the
Scheme 1. Cu-catalyzed [3+2] cycloaddition of propargylic esters with β-enamino
esters for the synthesis of pyrroles.
At the outset of our investigation, the cycloaddition of 1ꢀ
phenylpropꢀ2ꢀynꢀ1ꢀyl acetate
(1a) with (Z)ꢀmethyl 3ꢀ(4ꢀ
methylphenylsulfonamido)ꢀ3ꢀphenylacrylate (2a) was chosen
as the tested reaction to examine the effect of several
commercially available ligands which have proved to be
efficient in the Cuꢀcatalyzed reactions. To our disappointment,
some diphosphine ligands such as DPPF (L1), DPPP (L2) and
BINAP (L3), as well as bidentate nitrogen ligands such as biPy
(
L4) and 1,10ꢀPhenanthroline (L5), all gave low to moderate
yield of the corresponding [3+2] cycloaddition product 3aa
(Scheme 2). Recently, we have developed a series of chiral
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DOI: 10.1039/C5RA19304D
P,N,Nꢀligands which showed excellent diastereoꢀ and benzylamine intermediate, which was further lithiated and
enantioselectivities in the Cuꢀcatalyzed asymmetric propargylic orthoꢀphosphinated with Ph2PCl to give 2ꢀ(diphenylphosphino)
substitution,15 decarboxylative propargylic substitution16 and benzylamine
5 in 56% overall yield. Condensation of 5 with 2ꢀ
[3+3] cycloaddition reactions.17 In particular, we have reported pyridinecarboxaldehyde in the presence of MgSO4 gave the
an example of highly enantioselective synthesis of optically desired P,N,Nꢀligand L6 in 89% yield.
active 2,3ꢀdihydrofurans by Cuꢀcatalyzed [3+2] cycloadditon of
With this newly developed ligand L6 in hand, we then
propargylic esters with βꢀketoesters with chiral P,N,Nꢀligand.18 investigated its efficiency in the Cuꢀcatalyzed [3+2]
The distinct structure of tridentate chelates generating from the cycloaddition (Table 1). As expected, good result of 84% yield
complexion of P,N,Nꢀligands to copper should be responsible was obtained with ligand L6 (entry 1). Next investigation of Cu
to high reactivity and enantioselectivity observed in the copperꢀ salts modified with L6 showed that except for CuI, all of the Cu
catalyzed propargylic transformation.16a,17 Due to excellent salts, including Cu(OTf)2 and Cu(OAc)2, displayed good
performance of P,N,Nꢀligands in the Cuꢀcatalyzed propargylic performance (entries 2ꢀ4). Among them, Cu(OTf)2 proved to be
transformation, we envisioned that this tridentate P,N,Nꢀligand the best choice, which gave the corresponding product in 87%
type should be also efficient to the Cuꢀcatalyzed [3+2] yield (entry 3). The addition of a base proved to be crucial to
cycloadditon of propargylic esters
the synthesis of pyrroles. Considering the high cost and the its absence (entry 5). Et3N provided better result than
1
with βꢀenamino esters
2
for this reaction since none of the desired product was observed in
ꢀPr2NEt
i
difficult synthesis of chiral P,N,Nꢀligand, a structurally similar and DBU (entries 3, 6 and 7). Solvent also highly affected the
and nonchiral P,N,Nꢀligand L6 was then designed for this reaction. Of the solvents that we tested, MeOH was the only
reaction.
suitable one, and few reactions occurred in CH2Cl2, toluene and
Et2O (entries 3, 8ꢀ10). Thus, the optimal reaction condition was
identified as follow: Cu(OTf)2 (5 mol %), ligand L6 (5.5 mol
%), Et3N (1.2 eq.) in MeOH at room temperature for 12h.
Table 1 Optimization of the reaction conditions a
Entry
[Cu]
Base
Et3N
Et3N
Et3N
Et3N
None
Solvent
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
CH2Cl2
Toluene
Et2O
Yield (%)b
1
2
3
4
5
6
7
8
9
10
Cu(CH3CN)4BF4
Cu(OAc)2·H2O
Cu(OTf)2
CuI
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
84
70
87
47
NR
78
69
24
ꢀ c
iꢀPr2NEt
DBU
Et3N
Et3N
Et3N
ꢀ c
Scheme 2. Ligands evaluated for the Cu-catalyzed [3+2] cycloaddition.
a
The reaction was carried out with 1a (0.36 mmol), 2a (0.3 mmol), [Cu]
(0.015 mmol, 5 mol %), L6 (0.0165 mmol, 5.5 mol %) and base (0.36 mmol)
in 3 mL of solvent at room temperature for 12 h. b Yield of isolated product.
c Trace reaction detected by TLC.
SiMe3
NH
n-BuLi, Ph2PCl
2 n-BuLi, Me3SiCl
N
H
Et2O, -35℃ to rt
Et2O, -35℃ to rt
4
Having established the optimized conditions, we first examined
the scope and limitation of the reaction with respect to propargylic
esters 1. As shown in Table 2, a wide range of propargylic esters
reacted with βꢀenamino esters 2a to give the corresponding fully
substituted pyrroles in high yields. It appeared that the position of
the substituent on the phenyl ring had important effect on the
reaction. Thus, substrates with an orthoꢀchloro group on the phenyl
ring gave lower yield in comparison with its metaꢀ or paraꢀchloro
substituted analogues (entries 2ꢀ4). Both electronꢀwithdrawing (F,
Cl, Br) and ꢀdonating substituents (OMe, Me) at the paraꢀposition of
the phenyl ring were tolerated in this transformation (entries 4ꢀ8). In
addition, 2ꢀnaphthyl substrate 1i proved to be suitable for this
NH2
picolinaldehyde
MgSO4, EtOH, reflux
N
PPh2
N
PPh2
5
L6
Scheme 3. Procedure for the synthesis of P,N,N-ligand L6.
The new P,N,Nꢀligand L6 was prepared from readily
available and cheap benzylamine via a concise procedure as
outlined in Scheme 3. In the first step, benzylamine was
treated with ꢀBuLi and TMSCl to give a silyꢀprotected
4
4
n
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reaction (entry 9). Meanwhile a heteroaromatic propargylic ester 1j
was also efficient, giving the cycloaddition product in 80% yield of
(entry 10). Furthermore, the vinyl substrate 1k also worked smoothly exocyclic double bond at the 2ꢀposition. The structure of
to give the desired product in 88% yield (entry 11). For the aliphatic compound
was confirmed by Xꢀray analysis.21 Isomerization
substrate, the pentafluorobenzoate 1l butꢀ3ꢀynꢀ2ꢀyl of 2,3ꢀdihydropyrrole gave the corresponding pyrrole 3aa
E. Then, intramolecular nucleophilic attack of
N
at the Cβ atom
F
would at last afford the 2,3ꢀdihydropyrrole
I
bearing an
I
'
(
I
.
pentafluorobenzoate) should be used instead of the Isotopic labeling experiments using CD3OD as solvent (Scheme
corresponding acetate in order to achieve acceptable yield (entry 5) also confirmed the above conclusion.
1
2).
Table 3 Subtrate scope of ßꢀenamino esters 2 a
Table 2 Subtrate scope of propargylic esters 1 a
Entry 2 (R2, R3)
Product
3aa
3ba
3ca
3da
3ea
Yield b (%)
2a (R2 = C6H5, R3 = Me)
87
80
85
83
86
88
74
80
82
88
85
NR
Entry
1
2
3
4
5
6
7
8
1 (R1)
1a (C6H5)
Product
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
Yield b (%)
1
2
3
4
5
6
7
8
2b (R2 = 4ꢀMeC6H4, R3 = Me)
2c (R2 = 4ꢀFC6H4, R3 = Me)
2d (R2 = 4ꢀBrC6H4, R3 = Me)
2e (R2 = 4ꢀClC6H4, R3 = Me)
2f (R2 = 3ꢀClC6H4, R3 = Me)
2g (R2 = 2ꢀClC6H4, R3 = Me)
2h (R2 = 6ꢀOMeꢀ2ꢀnaphthyl, R3 = Me)
2i (R2 = Me, R3 = Me)
87
72
82
88
90
86
85
83
82
80
88
70
1b (2ꢀClC6H4)
1c (3ꢀClC6H4)
1d (4ꢀClC6H4)
1e (4ꢀFC6H4)
1f (4ꢀBrC6H4)
1g (4ꢀOMeC6H4)
1h (4ꢀMeC6H4)
1i (2ꢀnaphthyl)
1j (2ꢀthienyl)
1k (C6H5CH=CH)
1l' (Me)
3fa
3ga
3ha
3ia
3ja
3ka
9
10
11
12c
2j (R2 = C6H5, R3 = Et)
9
2k (R2 = Me, R3 = Et)
10
11
12 c
3aj
3ak
3al
2l (R2 = C6H5, R3 = Me)
a The reaction was carried out with 1a (0.36 mmol), 2 (0.3 mmol), Cu(OTf)2
(0.015 mmol, 5 mol %), L6 (0.0165 mmol, 5.5 mol %) and Et3N (0.36 mmol)
in 3 mL of MeOH at room temperature for 12 h. b Yield of isolated product.
c Substrate 2l ((Z)ꢀmethyl 3ꢀaminoꢀ3ꢀphenylacrylate ) without tosyl group
was used.
a The reaction was carried out with 1 (0.36 mmol), 2a (0.3 mmol), Cu(OTf)2
(0.015 mmol, 5 mol %), L6 (0.0165 mmol, 5.5 mol %), and Et3N (0.36 mmol)
in 3 mL of MeOH at room temperature for 12 h. b Yield of isolated product.
c The pentafluorobenzoate was used instead of the corresponding acetate and
the reaction was conducted at reflux temperature for 24 hours.
The scope of βꢀenamino esters
2 was subsequently examined
after the investigating of propargylic esters, and the results are
summarized in Table 3. Good performance was observed for
aryl βꢀenamino esters with both electronꢀdonating and ꢀ
withdrawing groups at the para or meta position (entries 2ꢀ6).
The substrate 2g with 2ꢀCl substituent resulted in the decreased
yield presumably due to the steric hindrance (entry 7). 6ꢀOMeꢀ
2ꢀnaphthyl substrate 2h also worked well, giving the
cycloaddition product in 80% yield (entry 8). Pleasingly, the
aliphatic substituted βꢀenamino ester 2i was also suitable for
this reaction (entry 9). Meanwhile, the ethyl ester substrates
turned out to be tolerated in this transformation (entries 10 and
11). However, no desired product was obtained when the
substrate 2l ((Z)ꢀmethyl 3ꢀaminoꢀ3ꢀphenylacrylate) without
tosyl group was employed in the reaction (entry 12), probably
due to the weaker acidity of hydrogen at the nitrogen atom.
We proposed the plausible mechanism for the formation of
3aa as shown in Scheme 4. In the first step, the Cu complex
Scheme 5. Isotopic labeling experiments using CD3OD as solvent.
Conclusions
In conclusion,
a new nonchiral P,N,Nꢀligand has been
synthesized from benzylamine via a concise procedure and
successfully applied to the Cuꢀcatalyzed [3+2] cycloadditon of
propargylic esters with βꢀenamino esters. Under the optimized
condition, a wide range of substitution patterns at the
propargylic esters and βꢀenamino esters were well tolerated to
the reaction, providing the corresponding fully substituted
pyrroles in good to high yields. The present work provides an
efficient and alternative access to the highly functionalized
pyrroles.
formed
Deprotonation with Et3N gave Cuꢀacetylide complex
an acetyl group from formed Cuꢀallenylidene complex
where the Cuꢀacetylide complex bearing a cationic γꢀcarbon
exists as a resonance structure.20 Nucleophilic attack of 2a at
the Cγ atom of gave the corresponding Cuꢀacetylide complex
a
π
ꢀcomplex
A
with propargylic acetate 1a 19
. Loss of
.
B
B
C,
D
C
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Scheme 4. Proposed mechanism
4
5
A. Fürstner, Angew. Chem., Int. Ed., 2003, 42, 3582.
Acknowledgements
P. S. Sharma, A. PietrzykꢀLe, F. D’Souza and W. Kutner, Anal.
Bioanal. Chem., 2012
, 402, 3177.
The authors are grateful for the financial support from the
National Natural Science Foundation of China (21403022) and
Natural Science Foundation of Liaoning Province of China
(2015020194).
6
7
(a) S. J. Higgins, Chem. Soc. Rev., 1997, 26, 247; (b) P. Novák, K.
Müller, K. S. V. Santhanam and O. Hass, Chem. Rev., 1997, 97
,
207.
For reviews, see: (a) I. Nakamura and Y. Yamamoto, Chem. Rev.,
2004, 104, 2127; (b) C. Schmuck and D. Rupprecht, Synthesis 2007,
3095; (c) N. Ono, Heterocycles, 2008, 75, 243; (d) V. Estévez, M.
Villacampa and J. C. Menéndez, Chem. Soc. Rev., 2010, 39, 4402;
Notes and references
a School of Light Industry and Chemical Engineering, Dalian Polytechnic
University, Dalian 116034, China. Email: houcj@dlpu.edu.cn
b Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian 116023, China. Email:xiangping@dicp.ac.cn
(e) W. JiaꢀJie, Y. Zhu and Z.ꢀP. Zhan, Asian J. Org. Chem., 2012, 1,
108; (f) A. V. Gulevich, A. S. Dudnik, N. Chernyak and V.
Gevorgyan, Chem. Rev., 2013, 113, 3084; (g) P. K. Maji, R. U.
Islam and S. K. Bera, Heterocycles, 2014, 89, 869.
c Faculty of Engineering and Built Environment, University of Newcastle,
8
9
L. Knorr, Ber. Dtsch. Chem. Ges., 1884, 17, 1635.
Callaghan, NSW 2308, Australia.
C. Pall, Ber. Dtsch. Chem. Ges., 1885, 18, 367.
†
Electronic Supplementary Information (ESI) available: Experimental
10 A. Hantzsch, Ber. Dtsch. Chem. Ges., 1890, 23, 1474.
details, characterization data and copies of NMR spectra for all products,
11 For selected examples for the synthesis of highly functionalized
pyrroles, see: (a) O. A. Attanasi, G. Favi, F. Mantellini, G.
Moscatelli and S. Santeusanio, J. Org. Chem., 2011, 76, 2860; (b)
A. Palmieri, S. Gabrielli, C. Cimarelli and R. Ballini, Green Chem.,
2011, 13, 3333; (c) Z.ꢀH. Guan, L. Li, Z.ꢀH. Ren and M.ꢀN. Zhao,
Green Chem., 2011, 13, 1664; (d) D. Dhara, K. S. Gayen, S.
Khamarui, P. Pandit, S. Ghosh and D. K. Maiti, J. Org. Chem.,
2012, 77, 10441; (e) R. Suresh, S. Muthusubramanian, M. Nagaraja
and G. Manickam, Tetrahedron Lett., 2013, 54, 1779; (f) Y. Lian, T.
Huber, K. D. Hesp, R. G. Bergman and J. A. Ellman, Angew. Chem.,
Int. Ed., 2013, 52, 629; (g) D. R. Magar, Y.ꢀJ. Ke and K. Chen,
and Crystallographic Information Framework (CIF) file of compound
(CCDC 1407815). See DOI: 10.1039/c000000x/
I
1
2
3
H. Fan, J. Peng, M. T. Hamann and J.ꢀF. Hu, Chem. Rev., 2008, 108
264.
,
B. A. Trofimov, L. N. Sobenina, A. P. Demenev and A. I.
Mikhaleva, Chem. Rev., 2004, 104, 2481.
A. Aiello, M. D’Esposito, E. Fattorusso, M. Menna, W. E. G. Müller,
S. PerovićꢀOttstadt and H. C. Schröder, Bioorg. Med. Chem., 2006,
14, 17.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 5 of 5
Journal Name
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View Article Online
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DOI: 10.1039/C5RA19304D
Asian J. Org. Chem., 2013, 2, 330; (h) N. Bhunia and B. Das,
Synthesis, 2013, 45, 1045; (i) S. Karamthulla, S. Pal, M. N. Khan
and L. H. Choudhury, Synlett, 2014, 25, 1926; (j) Y. Li, J. Shi, Z.
Wu, X. Wang, X. Wu, J. Gu, H. Bu and H. Ma, Tetrahedron 2014,
70, 2472; (k) H. Mehrabi, M. AnaryꢀAbbasinejad and F.
Mirhashemi, Tetrahedron Lett., 2014, 55, 4310; (l) K Lida, T. Miura,
J. Ando, S. Saito, Org. Lett. 2013, 15, 1436; (m) D. Srimani, Y.
BenꢀDavid, D. Milstein, Angew. Chem., Int. Ed., 2013, 52, 4012; (n)
S. Michlik, R. Kempe, Nature Chem., 2013,
J. Obenauf, M. Friedrich, S.ꢀM. Hühne, W. Mader, G. Motz, R.
Kempe, Catal. Sci. Technol. 2014, , 4188.
5, 149; (o) D. Forberg,
4
12 For reviews in catalytic propargylic transformation, see: (a) N. Ljungdahl
and N. Kann, Angew. Chem., Int. Ed., 2009, 48, 642; (b) R. J. Detz, H.
Hiemstra and J. H. van Maarseveen, Eur. J. Org. Chem., 2009, 6263; (c)
Y. Miyake, S. Uemura and Y. Nishibayashi, ChemCatChem, 2009, 1, 342;
(d) C.ꢀH. Ding and X.ꢀL. Hou, Chem. Rev., 2011, 111, 1914; (e) Y.
Nishibayashi, Synthesis, 2012, 44, 489; (f) E. B. Bauer, Synthesis, 2012,
44, 1131; (g) D.ꢀY. Zhang and X.ꢀP. Hu, Tetrahedron Lett., 2015, 56,
283; (h) X.ꢀH. Hu, Z.ꢀT. Liu, L. Shao and X.ꢀP. Hu, Synthesis, 2015, 47,
913.
13 M.Yoshida and C. Sugimura, Tetrahedron Lett., 2013, 54, 2082.
14 (a) Z.ꢀP. Zhan, S.ꢀP. Wang, X.ꢀB. Cai, H.ꢀJ. Liu, J.ꢀL. Yu and Y.ꢀY.
Cui, Adv. Synth. Catal., 2007, 349, 2097; (b) Y.ꢀM. Pan, S.ꢀY. Zhao,
W.ꢀH. Ji and Z.ꢀP. Zhan, J. Comb. Chem., 2009, 11, 103; (c) G.
Hattori, Y. Miyake and Y. Nishibayashi, ChemCatChem, 2010, 2,
155; (d) M. Lin, Q.ꢀZ. Chen, Y. Zhu, X.ꢀL. Chen, J.ꢀJ. Cai, Y.ꢀM. Pan
and Z.ꢀP. Zhan, Synlett, 2011, 1179; (e) W. Shao, H. Li, C. Liu, C.ꢀJ.
Liu and S.ꢀL. You, Angew. Chem., Int. Ed., 2015, 54, 7684ꢀ7687; (f)
D.ꢀY. Zhang, L. Shao, J. Xu and X.ꢀP. Hu, ACS Catal., 2015, 5, 5026.
15 (a) C. Zhang, Y.ꢀH. Wang, X.ꢀH. Hu, Z. Zheng, J. Xu and X.ꢀP. Hu,
Adv. Synth. Catal., 2012, 354, 2854; (b) F.ꢀZ. Han, F.ꢀL. Zhu, Y.ꢀH.
Wang, Y. Zou, X.ꢀH. Hu, S. Chen and X.ꢀP. Hu, Org. Lett., 2014,
16, 588; (c) D.ꢀY. Zhang, F.ꢀL. Zhu, Y.ꢀH. Wang, X.ꢀH. Hu, S.
Chen, C.ꢀJ. Hou and X.ꢀP. Hu, Chem. Commun., 2014, 50, 14459;
(d) F. Zhu and X.ꢀP. Hu, Chin. J. Catal., 2015, 36, 86.
16 (a) F.ꢀL. Zhu, Y. Zou, D.ꢀY. Zhang, Y.ꢀH. Wang, X.ꢀH. Hu, S. Chen,
J. Xu and X.ꢀP. Hu, Angew. Chem., Int. Ed., 2014, 53, 1410; (b) Y.
Zou, F.ꢀL. Zhu, Z.ꢀC. Duan, Y.ꢀH. Wang, D.ꢀY. Zhang, Z. Cao, Z.
Zheng and X.ꢀP. Hu, Tetrahedron Lett., 2014, 55, 2033; (c) F.ꢀL.
Zhu, Y.ꢀH. Wang, D.ꢀY. Zhang, X.ꢀH. Hu, S. Chen, C.ꢀJ. Hou, J. Xu
and X.ꢀP. Hu, Adv. Synth. Catal., 2014, 356, 3231.
17 C. Zhang, X.ꢀH. Hu, Y.ꢀH. Wang, Z. Zheng, J. Xu and X.ꢀP. Hu, J.
Am. Chem. Soc., 2012, 134, 9585.
18 F.ꢀL. Zhu, Y.ꢀH. Wang, D.ꢀY. Zhang, J. Xu and X.ꢀP. Hu, Angew.
Chem., Int. Ed., 2014, 53, 10223.
19 F. Himo, T. Lovell, R. Hilgraf, V. V. Rostovtsev, L. Noodleman, K. B.
Sharpless and V. V. Fokin, J. Am. Chem. Soc., 2005, 127, 210.
20 Y. Imada, M. Yuasa, I. Nakamura and S.ꢀI. Murahashi, J. Org. Chem.,
1994, 59, 2282.
21 CCDC 1407815.
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