Catalytic Asymmetric [4 + 1] Annulation of Sulfur Ylides with Copper-Allenylidene Intermediates

Article abstract of DOI:10.1021/jacs.6b04414

The first copper-catalyzed asymmetric decarboxylative [4 + 1] cycloaddition of propargylic carbamates and sulfur ylides was successfully developed. This strategy led to a series of chiral indolines with synthetically flexible alkyne groups in good yields and with high enantio- and diastereoselectivities (up to 99% yield, 98% ee, and >95:5 dr). A possible mechanism and stereoinduction mode with copper-allenylidenes were proposed as the possible dipolar intermediate.

Full text of DOI:10.1021/jacs.6b04414

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Communication
Catalytic Asymmetric [4+1] Annulation of Sulfur
Ylides with Copper-Allenylidene Intermediates
Qiang Wang, Tian-Ren Li, Liang-Qiu Lu, Kai Zhang, Wen-Jing Xiao, and Miao-Miao Li
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b04414 • Publication Date (Web): 29 Jun 2016
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Catalytic Asymmetric [4+1] Annulation of Sulfur Ylides with
Copper-Allenylidene Intermediates
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Qiang Wang,† Tian-Ren Li,† Liang-Qiu Lu,*,† Miao-Miao Li,† Kai Zhang,† Wen-Jing Xiao*,†,‡
†College of Chemistry, Central China Normal University, 152 Luoyu Road, Wuhan, Hubei 430079, China
‡State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, Gansu 730000, China
Supporting Information Placeholder
ABSTRACT: The first copper-catalyzed asymmetric decar-
boxylative [4+1] cycloaddition of propargylic carbamates and
sulfur ylides has been successfully developed. This strategy
led to a series of chiral indolines with synthetically flexible
alkyne groups in good yields and with high enantio- and
diastereoselectivities (up to 99% yield, 98% ee and >95:5 dr).
A possible mechanism and stereo-induction mode with cop-
per allenylidenes were proposed as the possible dipolar in-
termediate.
Transition-metal-catalyzed cycloaddition reactions have
been the focus of extensive study because of their fundamen-
tal importance in organic, medicinal and materials chemis-
try.1 Many reactions proceed via metal-associated dipolar
intermediates, which involve two independent reaction cen-
ters: one acts as an electrophile, and the other acts as a nu-
Figure 1. Cycloaddition reactions via metal-associated dipo-
cleophile. For example, various nucleophile-containing π-
lar intermediates.
allyl-Pd complexes2 (Figure 1a, type-I) and metallo-
enolcarbenes1c,3 (type-II: M = Rh and Au) have been widely
various carbo- and heterocyclic systems beyond three-
membered rings.8,9 In this work, we disclose the first example
applied in transition-metal-catalyzed cycloadditions. To ex-
of catalytic asymmetric formal [4+1] cycloaddition of sulfur
pand this cycloaddition chemistry, we applied asymmetric
ylides with copper-allenylidene dipolar intermediates (Figure
catalysis by an earth-abundant metals to achieve the first
1c). Using this protocol, we have produced a vast range of
example of formal [4+1] cycloaddition of copper-allenylidene
chiral indolines10 with synthetically flexible alkyne groups in
dipolar intermediates with high reaction yields and enantio-
high reaction efficiencies and selectivities, which is a com-
and diastereoselectivities (Figure 1c).
plement to previous achievements.9c,d Notably, this study
The metal-allenylidene species is a promising synthetic
represents one of the limited reports on the transition-metal-
intermediate for organic chemists; it enables the integration
catalyzed asymmetric cycloadditions of sulfur ylides.11
of a synthetically flexible alkyne functional group.4 Over the
past decade, Ru- or Cu-catalyzed asymmetric transfor-
mations of terminal propargylic alcohols and their deriva-
tives have been extensively developed, particularly transfor-
mations involving asymmetric processes with excellent enan-
tiocontrols.5,6 However, the cycloaddition reaction with met-
al-allenylidene dipolar intermediates has remained underde-
veloped. The only such transformation which produced cy-
cloaddition products in racemic form was disclosed in 2013
(Figure 1b).7 In that work, an Ru-catalyzed [3+2] cycloaddi-
tion of ethynyl cyclopropanes with aldehydes/aldimines was
elegantly designed and well implemented using stoichio-
metric Lewis acids, which efficiently produced 2-
ethynyltetrahydrofurans/pyrrolidines. Over the past few
years, we have devoted our efforts to developing new meth-
odologies using sulfur ylides, and we efficiently constructed
Initially, we performed the cycloaddition reaction of
ethynyl benzoxazinanone 1a and benzoyl sulfur ylide 2a at
room temperature (rt) in the presence of i-Pr2NEt, Cu(OTf)2
and chiral ligand R-BINAP (L1) in MeOH (Table 1, entry 1).
To our delight, the reaction did indeed occur and produced
the desired indoline product 3aa in trans configuration in
good yield, albeit with low enantioselectivity (entry 1: 88%
yield and 8% ee). Encouraged by this result, we evaluated
chiral ligands widely used in Cu-catalyzed asymmetric pro-
pargylic alkylation of propargyl esters for the present cy-
cloaddition reaction (entries 2-7). Accordingly, the commer-
cially available phenyl-substituted Pybox ligand L4 stood out
as the superior choice, producing chiral indoline 3aa in 66%
yield and 50% ee (entry 4). The investigation of the solvent
effect revealed that THF provided the best reaction efficiency
despite similar enantiocontrol (entry 8: 97% yield, 53% ee;
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Table 1. Selected Condition Optimizationa Table 2. Scope of Sulfonium Saltsa
Page 2 of 5
1
2
3
4
5
6
7
8
entry ligand
time
yield (%)b
ee (%)c
entry
4: R’
4a: C6H5
3
yield (%)b
ee (%)c
9
1
2
L1
L2
L3
L4
L5
L6
L7
L4
L4
L4
L4
30 min
30 min
30 min
30 min
30 min
30 min
30 min
40 min
40 min
16 h
88
36
8
36
50
50
44
-6
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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28
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1
2
3aa
3ab
3ac
3ad
3ae
3af
94
98
95
96
4b: 4-NO2-C6H4
4c: 4-CN-C6H4
4d: 4-F-C6H4
4e: 4-Cl-C6H4
4f: 4-Br-C6H4
4g: 4-Me-C6H4
4h: 3-Br-C6H4
4i: 2-F-C6H4
3
53
3
99
99(99) d
94
94(92)d
4
66
4
5
5
56
92
98
6
32
6
7e
8
9
10
93
96
7
35
-38
53
88
91
3ag
3ah
3ai
96
90
8d
9d,e
10e,d,f
11d,f,g
97
97
94
99
99
90
99
95(94)h
4j: 2,4-F2C6H4
3aj
97
93
24 h
95
aReaction conditions: 1a (0.1 mmol), 2a (0.2 mmol),
Cu(OTf)2 (10 mol%), L (11 mol%) and i-Pr2NEt (1.2 eq.) in
11 4k: 2-thienyl
12f 4l: 2-benzofuryl
13f 4m: methyl
3ak
3al
95
84
90
94
MeOH at rt. bDetermined by H NMR of the reaction
1
3am
3an
3ao
3ap
95
91
mixture containing 1,3,5-trimethoxybenzene as an inter-
nal standard. cDetermined by chiral HPLC analysis.
14
15
16
4n: cyclopropyl
4o: cyclohexyl
4p: i-Bu
99
92
dUsing THF as the solvent. Using sulfur ylide 2b. 0 C.
e
f
o
90
95
gUsing sulfonium salt 4a (0.2 mmol) and i-Pr2NEt (3.2
95
93
h
aUnless otherwise noted, the reactions were performed at 0.2
eq.). Isolated yields in parentheses. THF: tetrahydrofu-
ran.
b
c
mmol scale as entry 11 in Table 1. Isolated yield. Determined
by a chiral HPLC analysis. dA gram-scale reaction was per-
formed with 1.0 g of 1a and 2.3 g of 4d in 28 h and 1.26g of 3ad
was obtained (some results are given in parentheses.). eCorre-
f
sponding sulfur ylide was used. Tetrafluoroborate sulfonium
salt was used.
with high efficiency and selectivity (3aa−3ag: 92-99% yields,
90-98% ee and >95:5 dr). Precursors with various substituent
positions on the sulfonium salts, such as 3-bromo (4h), 2-
fluoro (4i) and 2,4-difluoro (4j), tolerated this cycloaddition
and were converted into the corresponding products with
good results (3ah−3aj: 97-99% yields, 90-94% ee and >95:5
dr). In addition, the identical transformation with heteroaryl-
substituted sulfonium salts 4k and 4l also proceeded notably
well and produced 3ak and 3al in 95% and 90% yields with
84% and 94% ee, respectively (entries 10 and 11). Significant-
ly, the success of this transformation was further extended to
aliphatic sulfonium salts (entries 13-16). For example, sub-
strates with methyl (4m), cyclopropyl (4n), cyclohexyl (4o)
and i-butyl (4p) reacted well with ethynyl benzoxazinanone
1a in the chiral copper catalyst system and produced chiral
indoline products 3am-3ap in 90-99% yield, 91-95% ee and >
95:5 dr.
see Table S2 in the supporting information for more details).
To further improve the result, other sulfur ylides were tested
(Table S3). As a result, sulfur ylide 2b, in which one methyl
group was replaced with a phenyl group, was converted into
the same product 3aa in 99% yield and 88% ee (entry 9).
Decreasing the reaction temperature gave a slightly im-
proved enantioselectivity with 99% yield at a prolonged reac-
tion time (entry 10). Delightfully, when applying a simplified
operation using easily available sulfonium salt 4a and excess
of i-Pr2NEt to in situ generate sulfur ylide 2b, the enantiose-
lectivity increased to 95% ee with 94% isolated yield.
With the optimal conditions in hand, we examined the
scope of sulfonium salts for this cycloaddition reaction. As
summarized in Table 2, excellent levels of yield, diastereo-
and enantioselectivity were obtained using sulfonium salts
with various substituents on the benzene ring (entries 1-10).
The substrates with electron-withdrawing groups (e.g., NO2,
CN), and those with fluoro, chloro, bromo and methyl at the
4-position were transformed into chiral indoline products
We next explored the cycloaddition reaction of sulfonium
salt 4a with various ethynyl benzoxazinanones (Table 3). The
use of substrates with a bromo (1b), methyl (1c) or methoxyl
(1d) group at the 6-position gave the corresponding products
in high yields and with great enantioselectivities (3ba-3da:
95-99% yields, 81-91% ee and >95:5 dr). Introducing a chlo-
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Table 3. Scope of Ethynyl Benzoxazinanonesa
substituted chiral indoline 10 in 65% yield without significant
loss in enantiopurity (Scheme 1b, 10).13 Treatment of 10 with
magnesium powder afforded the N-free 2-indole-substituted
indoline 11 with high yield (Scheme 1b, 11).
1
2
3
4
5
Scheme 1. Synthetic Transformation
6
7
8
9
entry
1: R1
1a: H
3
yield (%)b
ee (%)c
1
2
3aa
3ba
3ca
3da
3ea
3fa
94
97
99
95
99
96
93
82
95
90
91
10
11
12
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1b: 6-Br
1c: 6-Me
1d: 6-MeO
1e: 7-Cl
1f: 7-CF3
1g: 5-F
3
4d
81
5
95
94
80
88
6
7
3ga
8
1h: 8-F
3ha
aUnless otherwise noted, the reactions were performed at
0.2 mmol scale as entry 11 in Table 1. Isolated yield. cDeter-
b
mined by chiral HPLC analysis. dSulfur ylide 2b was used.
A nonlinear relationship between the enantiopurity of
product 3aa and ligand L4 was clearly observed in the cop-
per-catalyzed asymmetric cycloaddition of 1a with 4a (Figure
S1). This result indicates that a dinuclear complex of copper
salts and chiral ligand may function as an active catalytic
species to promote this transformation according to previous
works.14 A plausible mechanism is proposed in Scheme 2.
First, the copper complex likely activates the alkyne part of
substrate 1a by forming a π-complex A, which generates the
copper-acetylide species B upon deprotonation with i-
Pr2NEt. Then, a copper-allenylidene intermediate C, which is
stabilized by its resonance form C’, is generated through a
CO2 extrusion process. Subsequently, the selective capture of
sulfur ylide 2b by intermediate C forms the transient species
D, which converts into copper-containing cycloadduct E via
an intramolecular SN2 reaction. Finally, the chiral indoline is
produced through a proton transfer process, and the dinu-
clear copper catalyst is simultaneously regenerated.
rine atom (1e) to the 7-position of ethynyl benzoxazinanone
yields the corresponding product 3ea in an excellent stereo-
control (entry 5: 99% yield, 95% ee and >95:5 dr). Similarly,
addition of a trifluoro group to the 7-position of the ethynyl
benzoxazinanone was compatible with the present catalyst
system, converting into desired product 3fa in an excellent
reaction efficiency and selectivity (entry 6: 96% yield, 94% ee
and >95:5 dr). The substrates with a fluoro atom at the 5- and
8-positions were tested under the optimal conditions. Fluo-
ro-incorporated chiral indolines 3ga and 3ha were obtained
in good yields and with high enantiocontrol (entry 7: 93%
yield, 80% ee and >95:5 dr; entry 8: 80% yield, 88% ee and
>95:5 dr). Relatively low enantiomeric excess of chiral indo-
line 3ga was probably attributed to the steric effects of the F-
substituent at 5 position. Moreover, we have successfully
used this Cu-catalyzed asymmetric cycloaddition to prepare
chiral pyrrolidines. For example, the reactions of ethynyl
carbamate 5 with sulfonium salt 4a and 4f could afford the
corresponding pyrrolidine 6 and 7 were produced in high
enantio- and diastereoselectivity, respectively (eq 1).
Scheme 2. Proposed Mechanism
Synthetic transformations were performed to demon-
strate the utility of this method. For example, a copper-
catalyzed 1,3-dipolar cycloaddition of 3aa with TsN3 pro-
duced 1,2,3-triazole-substituted chiral indoline 8 in 99% yield
with retained enantiopurity (Scheme 1a). Although the active
sulfur ylides were not suitable for this cycloaddition,12 the
deoxygenation operation of the products with triethyl silane
and boron trifluoride (e.g., 3aa) produced the indoline with
an alkyl group at the 2-position in good results (Scheme 1b,
9: 70% yield, 95% ee and >95:5 dr). A Pd/Cu-catalyzed se-
quence reaction can easily convert 9 into a 2-indole-
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(c) Ohmatsu, K.; Imagawa, N.; Ooi, T. Nat. Chem. 2014, 6, 47. (d) Xu,
C.-F.; Zheng, B.-H.; Suo, J.-J.; Ding, C.-H.; Hou, X.-L. Angew. Chem.
Int. Ed. 2015, 54, 1604.
The absolute configuration of the indoline products was
unambiguously determined to be (S,S) on the basis of the X-
ray crystallographic analysis of 3af (Figure S2).13 The stere-
ocontrol that led to this isomer might be rationalized with
Maarseveen's model of cooperative catalysis (Figure 2b),14
which was established according to crystallographic results
(Figure 2a).14b,15 The propargylation step possibly favors the
re-face attack of the copper-allenylidene complex by sulfur
ylides, where the sulfur ylide reacts with its re-face.
1
2
3
4
5
6
7
8
(3) For select examples, see: (a) Guzmn, P. E.; Lian, Y.; Davies, H.
M. L. Angew. Chem. Int. Ed. 2014, 53, 13083. (b) Cheng, Q.-Q.;
Yedoyan, J.; Arman, H.; Doyle, M. P. J. Am. Chem. Soc. 2016, 138, 44,
and references therein.
(4) For reviews, see: (a) Ljungdahl, N.; Kann, N. Angew. Chem. Int.
Ed. 2009, 48, 642. (b) Detz, R. J.; Hiemstra, H.; van Maarseveen, J. H.
Eur. J. Org. Chem. 2009, 2009, 6263. (c) Miyake, Y.; Uemura, S.;
Nishibayashi, Y. ChemCatChem 2009, 1, 342. (d) Ding, C.-H.; Hou,
X.-L. Chem. Rev. 2011, 111, 1914. (e) Nishibayashi, Y. Synthesis 2012, 44,
489. (f) Bauer, E. Synthesis 2012, 44, 1131. (g) Hu, X.-H.; Liu, Z.-T.;
Shao, H.; Hu, X.-P.; Synthesis 2015, 47, 913. (h) Zhang, D.-Y.; Hu, X.-P.
Tetrahedron Lett. 2015, 56, 283.
(5) For selected examples of Ru catalysis, see: (a) Inada, Y.; Nishi-
bayashi, Y.; Uemura, S. Angew. Chem. Int. Ed. 2005, 44, 7715. (b)
Ikeda, M.; Miyake, Y.; Nishibayashi, Y. Angew. Chem. Int. Ed. 2010,
49, 7289. (c) Senda, Y.; Nakajima, K.; Nishibayashi, Y. Angew. Chem.
Int. Ed. 2015, 54, 4060.
9
a)
b)
10
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O
O
N
Cu
N
N
R
Cu
R
Cu
(X-)2
R
R
Cu
N
N
N
O
O
Ph
H
Re-favored
O
Me
Two Copper:
Cooperative
Catalysis
S
results of Gamasa and
Nishibayashi
(X = OTf or PF6)
Ph
NH
Ts
Maarseveen' model
(6) For selected examples of Cu catalysis, see: (a) Detz, R. J.; Del-
ville, M. M. E.; Hiemstra, H.; van Maarseveen, J. H. Angew. Chem. Int.
Ed. 2008, 47, 3777. (b) Hattori, G.; Matsuzawa, H.; Miyake, Y.; Nishi-
bayashi, Y. Angew. Chem. Int. Ed. 2008, 47, 3781. (c) Fang, P.; Hou,
X.-L. Org. Lett. 2009, 11, 4612. (d) Yoshida, A.; Ikeda, M.; Hattori, G.;
Miyake, Y.; Nishibayashi. Y. Org. Lett. 2011, 13, 292. (e) Zhang, C.; Hu,
X.-H.; Wang, Y.-H.; Zheng, Z.; Xu, J.; Hu, X.-P. J. Am. Chem. Soc. 2012,
134, 9585. (f) Zhu, F.-L.; Zou, Y.; Zhang, D.-Y.; Wang, Y.-H.; Hu, X.-H.;
Chen, S.; Xu, J.; Hu, X.-P. Angew. Chem. Int. Ed. 2014, 53, 1410. (g)
Zhu, F.-L.; Wang, Y.-H.; Zhang, D.-Y.; Xu, J.; Hu, X.-P. Angew. Chem.
Int. Ed. 2014, 53, 10223. (h) Shao, W.; Li, H.; Liu, C.; Liu, C. J.; You, S.
L. Angew. Chem. Int. Ed. 2015, 54, 7684.
Figure 2. Possible asymmetric induction mode.
In conclusion, we developed a copper-catalyzed asym-
metric formal [4+1] cycloaddition for the first time through
trapping copper-allenylidene dipolar intermediates by sulfur
ylides. Thus, a new approach to chiral indoline products and
related cycloadducts was explored in high reaction yields and
stereoselectivities (up to 99% yield, 98% ee and >95:5 dr).
Mechanistic studies suggest that this reaction is a sequence
process that involves decarboxylative propargylation/SN2
reactions promoted by dinuclear copper complexes. Further
studies with this type of metal-associated dipolar intermedi-
ates are currently in progress.
(7) Miyake, Y.; Endo, S.; Moriyama, T.; Sakata, K.; Nishibayashi, Y.
Angew. Chem. Int. Ed. 2013, 52, 1758.
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5841. (b) Sun, X.-L.; Tang, Y. Acc. Chem. Res. 2008, 41, 937. (c) Li, G.-
C.; Wang, L.-Y.; Huang, Y. Chin. J. Org. Chem. 2013, 33, 1900.
(9) For recent work on sulfur ylides from our group, see: (a) Lu,
L.-Q.; Chen, J.-R.; Xiao, W.-J. Acc. Chem. Res. 2012, 45, 1278. (b) Yang,
Q.-Q.; Xiao, C.; Lu, L.-Q.; An, J.; Tan, F.; Li, B.-J.; Xiao, W.-J. Angew.
Chem. Int. Ed. 2012, 51, 9137. (c) Li, T.-R.; Tan, F.; Lu, L.-Q.; Wei, Y.;
Wang, Y.-N.; Liu, Y.-Y.; Yang, Q.-Q.; Chen, J.-R.; Shi, D.-Q.; Xiao,
W.-J. Nat. Commun. 2014, 5. 5500. (d) Wang, Q.; Qi, X.; Lu, L.-Q.; Li,
T.-R.; Yuan, Z.-G.; Zhang, K.; Li, B.-J.; Lan, Y.; Xiao, W.-J. Angew.
Chem. Int. Ed. 2016, 55, 2840.
(10) For select examples on the biological significance and synthe-
sis of chiral indolines, see: (a) Cui, H.-L.; Feng, X.; Peng, J.; Lei, J.;
Jiang, K.; Chen, Y.-C. Angew. Chem., Int. Ed. 2011, 50, 10661. (b) Duan,
Y.; Li, L.; Chen, M.-W.; Yu, C.-B.; Fan, H.-J.; Zhou, Y.-G. J. Am. Chem.
Soc. 2014, 136, 7688. (c) Romano, C.; Jia, M.; Monari, M.; Manoni, E.;
Bandini, M. Angew. Chem. Int. Ed. 2014, 53, 13854.
(11) (a) Chen, J.-R.; Dong, W.-R.; Candy, M.; Pan, F.-F.; Jörres, M.;
Bolm, C. J. Am. Chem. Soc. 2012, 134, 6924. (b) Huang, X.; Klimczyk,
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10365, and ref. 9c.
(12) Phenyl-substituted sulfur ylide and Corey ylide were tested,
but only the fast decomposition of substrate 1a was observed.
(13) CCDC 1471938 and CCDC 1450138 contain the crystallographic
data of 10 and 3af. These data can be obtained free of charge from
ASSOCIATED CONTENT
Supporting Information
Experimental procedures; spectral data. This material is
available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
luliangqiu@mail.ccnu.edu.cn; wxiao@mail.ccnu.edu.cn
Notes
*The authors declare no competing financial interests.
ACKNOWLEDGMENT
We are grateful to the National Natural Science Foundation
of China (No. 21232003, 21472057 and 21572074) and other
financial supports (No. 201422, CCNU15A02007 and
2015CFA033) for support of this research.
REFERENCES
The
Cambridge
Crystallographic
Data
Centre
via
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(2) For recent examples, see: (a) Trost, B. M.; Morris, P. J.; Sprague,
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(15) (a) Díez, J.; Gamasa, M. P.; Panera, M. Inorg. Chem. 2006, 45,
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