Visible-light-induced cascade dearomatization cyclization between alkynes and indole-derived bromides: A facile strategy to synthesize spiroindolenines

Article abstract of DOI:10.1039/d0cc05672c

A visible-light-initiated intermolecular dearomatization cyclization cascade reaction between alkynes and indole-derived bromides has been explored. This transformation exhibits a wide substrate scope and significant functional group tolerance, providing an efficient way to access a variety of spiroindolenines under mild conditions. This journal is

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DOI: 10.1039/D0CC05672C
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Visible-light-induced cascade dearomatization cyclization
between alkynes and indole-derived bromides: a facile strategy to
synthesize spiroindolenines
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaoshuang Gao,a Yao Yuan,a Xiaomin Xie,a Zhaoguo Zhang*a,b
DOI: 10.1039/x0xx00000x
A visible-light-initiated intermolecular dearomatization cyclization subsequently demonstrated by Xu and co-workers (Scheme
cascade reaction between alkynes and indole-derived bromides has 1c).6c
been explored. This transformation exhibits a wide substrates
scope and significant functional groups tolerance, providing an
efficient way to access a variety of spiroindolenines under mild
conditions.
(a) EDA complex-enabled intramolecular alkene trifluoromethylation and dearomatization of indoles
F3C
X
X
Umemoto's reagent
R2
R1
R1
R2
visible light
N
N
H
Synthesizing spiroindolenines is vital importance in organic
and medicinal chemistry since spiroindolenines scaffolds are
widely embedded in pharmaceuticals and natural products.1
Therefore, it is not surprising that many efforts have been
devoted to construct such useful skeletons.2 However, these
protocols often suffer from harsh reaction conditions or a limited
substrate scope. Hence, the development of practical and green
methods for the straightforward construction of these
spiroindolenine frameworks is still an urgent and attractive task.
Nowadays, the visible-light-induced catalysis cascade
dearomatization cyclization strategy has emerged as a uniquely
powerful and straight forward tool for the assembly of novel and
complex cyclic molecular architectures from simple precursors
under mild reaction conditions.3-5 However, only a handful of
examples on the syntheses of spiroindolenines have been
reported via visible-light photoredox catalysis. In 2018, You and
(b) Photoredox-Catalysed C-S bond formation throught Spirocyclization of Indolylynones
O
O
SPh
R
PhSH
Potocatalyst
R
visible light
N
N
H
(c) Visible-Light-Promoted Selenylative Spirocyclization of Indolylynones
O
O
SeAr
(ArSe)2
R2
R3
R1
R3
visible light
R2
R1
N
N
H
Scheme 1 Photoredox-based synthesis of spiroindolenines
Furthermore, another particularly intriguing transformation
is the photocatalytic cascade radical functionalization of aryl
alkynes followed by intermolecular cyclization reaction which
can easily construct many important heterocycles.7,8 In 2016,
Xiao group has demonstrated a photo-driven intermolecular
formal (4+2) cycloaddition for the efficient synthesis of
carbazole using alkynes and indole-derived bromides as the
substrates (Scheme 2a).8b
Inspired by these works and our continuous efforts devoted
to visible-light-induced dearomatization reactions,9 we would
like to present a visible-light-mediated intermolecular cascade
dearomatization cyclization reaction between alkynes and
indole-derived bromides, with this strategy, several series of
spiroindolenines frameworks could be obtained under facile
conditions (Scheme 2b).
coworkers
discovered
a
novel
cascade
alkene
trifluoromethylation and dearomatization of indole derivatives,
and mechanistic studies suggested an electron donor-acceptor
(EDA) complex formed between indole derivatives and
Umemoto’s reagent (Scheme 1a).6a Later, Unsworth’s group
reported a radical spirocyclization of indolyl-tethered ynones
leading to sulfur-containing spiroindolenines under the air
condition (Scheme 1b).6b A similar visible-light-promoted
selenylative spirocyclization of indolyl-tethered ynones was
a. Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs, School of
Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800
Dongchuan Road, Shanghai 200240, China.
b. State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, 345 Lingling Road, Shanghai 200032, China.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI:10.1039/x0xx00000x
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(a) Photo-driven intermolecular formal (4+2) cycloaddition using alkynes and indole-derived bromides
reaction was performed in the absence of a base, which indicated
DOI: 10.1039/D0CC05672C
that the base played a key role for a successful outcome in this
transformation. (Table 1, entry 2). Encouraged by this result, the
reaction conditions were further optimized. Firstly, we screened
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COOEt
HO
R3
O
R2
Br
Photocatalyst
R2
R3
+
O
Visible light
R1
R1
N
O
N
other
photocatalysts
such
as
Ru(bpy)3Cl2•6H2O,
(b) This work:visible-Light-Induced Dearomatization Cyclization of indole-derived bromides and alkynes
Ir(ppy)2(dtbpy)PF6 and Eosin Y, which gave inferior results
(Table 1, entries 3–5). To further improve the reaction
efficiency, many inorganic and organic bases were subsequently
screened, but none of the other bases gave a higher yield than
Na2CO3 (Table 1, entries 6–12). Considering that the identity of
solvents sometimes play a key role in photoredox catalysis, we
further screened many commonly solvents (Table 1, entries 13–
23). To our delight, when DCE replaced CH3CN as the solvent,
the yield of the desired product 3aa increased to 82%. Control
experiments suggested that photocatalyst and visible light
irradiation are indispensable to this transformation (Table 1,
entries 24 and 25).
R5
O
R4
O
O
Br
R2
Photocatalyst
Visible light
R5
O
R4
+
R1
R1
R2
R3
HN
N
R3
Scheme 2 Photoredox reaction based alkynes and indole-derived bromides.
Table 1 Optimization of the reaction conditionsa
O
O
O
O
Br
O
O
O
Photocatalyst
O
+
Base, Solvent
7W blue light
HN
2a
N
3aa
1a
Scheme 3 Substrates scope of alkynes.a,b
entry
1
2
3
4
5
6
7
8
photocatalyst
fac-Ir(ppy)3
fac-Ir(ppy)3
base
Na2CO3
-
Na2CO3
Na2CO3
Na2CO3
Li2CO3
K2CO3
NaHCO3 CH3CN
Na2HPO4 CH3CN
K2HPO4
Et3N
solvent
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
yieldb
74
0
O
O
O
O
O
Br
O
O
fac-Ir(ppy)3
O
+
Ir(ppy)2(dtbpy)PF6
Ru(bpy)3Cl2∙6H2O
Eosin Y
8
0
0
8
73
65
4
65
60
R1
R1
Na2CO3, DCE
7W blue LED
N
HN
2a
1
3
1
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
1
3aa
3ba
3ca
, R =H,
78% yield
3fa
, R =Cl,
1
67% yield
42% yieldc
48% yield
86% yield
O
O
1
O
, R =Me, 61% yield
3ga
3ha
, R =Br,
1
c
1
, R =OMe, 76% yield
, R =CF3,
R1 3da
1
t
O
1
, R = Bu, 83% yield
1
3ia
3ja
, R =CN,
1
3ea
, R =F,
67% yield
, R =COOMe, 88% yield
9
10
11
N
CH3CN
CH3CN
O
O
R1=Me, 25% yield
O
3ka
,
O
O
O
3la
,
R1=OMe, 69% yield
1
2,6-
12
fac-Ir(ppy)3
CH3CN
51
O
3ma
, R =F,
R1=Cl,
R1=Br,
73% yield
58% yield
78% yieldc
O
Lutidine
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
3na
,
R1
13
14
15
16
17
18
19
20
21
22
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
DMF
DMA
DMSO
Toluene
MeOH
DCM
DCE
CHCl3
THF
EA
72
64
57
52
0
3oa
,
N
N
3pa
, 61% yield
O
O
O
O
O
OMe
O
O
O
O
N
S
O
O
O
76
OMe
N
82(78)c
74
N
N
3qa
, 66% yield
3sa
3ra
, 53% yield
, 34% yield
O
O
63
54
O
O
O
O
Ph
O
O
O
H
Ph
O
O
O
O
1,4-
dioxane
DCE
23
fac-Ir(ppy)3
Na2CO3
72
N
H
N
H
N
3ua
3ta
,0% yield
3va
, 82% yield
, 0% yield
24
none
fac-Ir(ppy)3
Na2CO3
Na2CO3
0
0
25d
DCE
a
a Reaction conditions: 1a (30.6 mg, 0.3 mmol), 2a (229.3 mg, 0.6
mmol), base (0.6 mmol), catalyst (0.006 mmol), solvent (3 mL), rt,
24 h, under N2 atmosphere. b Determined by 1H NMR analysis with
benzyl ether as an internal standard. c The value in parentheses was
isolated yield. d In the dark.
Reaction conditions: 1 (0.3 mmol), 2a (229.3 mg, 0.6 mmol),
Na2CO3 (63.6 mg, 0.6 mmol), fac-Ir(ppy)3 (3.9 mg, 0.006mmol), DCE
(3 mL), irradiation with a 7W blue LED light, rt, 24 h. b Isolated yields.
c Irradiation with a 7 W blue LED for 48 h.
With the optimized cyclization reaction conditions in hand,
the substrate scope of this reaction was investigated (Scheme 3).
Firstly, different substituent groups on the aryl alkynes were
examined. Gratifyingly, both electron-donating group (e.g., Me,
Initially,
we
investigated
this
reaction
using
phenylacetylene 1a and indole-derived bromide 2a as the starting
materials with Na2CO3 as the base and fac-Ir(ppy)3 (2 mol%) as
the catalyst. To our delight, the reaction proceeded smoothly
after 24 h of irradiation with a 7 W blue LED in CH3CN at room
temperature, affording the desired spiroindolenines 3aa in 74 %
yield (Table 1, entry 1). However, no 3aa was detected when the
t
OMe, Bu) and electron-withdrawing group (e.g., CF3, CN,
COOMe) substituents at the para position of aryl alkynes
proceeded efficiently to afford the spiroindolenines 3aa-3ja in
moderate to excellent yields. Notably, synthetically attractive
2 | J. Name., 2012, 00, 1-3
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groups (F, Cl, Br) in substrates were also tolerated in this system, delivering the desired product 3ai in 84% yield. Finally,
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reaction providing the products 3ea-3ga in moderate to good other bromocarbonyl indoles, such as ketDoOeIs:t1e0r.s1023j9/oDr0dCiCk0e5to67n2eCs
yields. Subsequently, for a meta-substituent aryl alkynes also 2k, were also effective in this transformation affording the
underwent cyclization smoothly to give spiroindolenines 3ka- spiroindolenines 3aj and 3ak in 58% and 45% yield, respectively.
3oa. Nevertheless, the meta-Me-substituent aryl alkynes was
When we scaled the reaction of 1a in a 4.5 mmol with 2a,
obtained corresponding product 3ka in only 25% yield. Other to our delight, the reaction proceeded smoothly delivering
substituents such as ortho-Me and 3,5-di-OMe on aryl alkynes product 3aa in 74% yield, which highlights practicality of this
could also delivered the desired products 3pa and 3qa in method (Scheme 5a). In addtion, several transformations based
satisfactory yields. In addition, heteroaryl thiophene and on spiroindolenine 3aa were carried out. Spirocycle 4 could be
pyridine-based acetylenes were well tolerated with this mild obtained in 79% yield as a single diastereoisomer through the
system affording the spiroindolenines 3ra and 3sa in 34% and reduction of the imine moiety of 3aa in the presence of
53% yield, respectively. Remarkably, the aryl alkyne with a NaBH3CN and AcOH (Scheme 5b). Moreover, the product 3aa
natural product estrone steroid skeleton was also amenable to could be readily converted to spirocycle diol compound 5 in 54%
this reaction under the standard conditions giving the yield using LiAlH4 as reducing agent (Scheme 5c).
corresponding product 3ta in 82% yield, which may be applied
Scheme 5. Gram-scale reaction and transformations of product 3aa
in the late-stage functionalization of a bioactive molecule.
Unfortunately, for the substrate internal alkynes diphenyl
acetylene 1u or aliphatic alkyne 1-octyne 1v failed to give the
target compound, and starting material 1u and 1v was recovered.
O
O
O
O
Br
O
O
O
fac-Ir(ppy)3
O
+
Na2CO3, DCE
7w blue LED, 24h
a)
HN
2a
N
1a
, 4.5mmol, 0.46g
Scheme 4 Substrates scope of indole-derived bromide.a,b
3aa
, 1.35g, 74% yield
R5
R4
O
O
EtOOC
EtOOC
EtOOC
EtOOC
Br
O
R2
R5
O
fac-Ir(ppy)3
R4
NaBH3CN(1.2 equiv)
AcOH(1.2 equiv)
+
Na2CO3, DCE
7W blue LED
b)
R2
R3
HN
MeOH, rt, 40min
R3
O
N
N
H
N
1a
2
3
3aa
4
, 79% yield
OH
O
EtOOC
EtOOC
O
O
O
O
O
O
O
O
O
LiAlH4 (6 equiv)
O
HO
c)
O
O
Cl
O
F
THF, 0C to rt, 5h
O
MeO
N
N
H
N
N
N
3aa
N
3ae
, 75% yield
3ad
, 61% yield
5
, 52% yield
3ac
, 26% yield
3ab
, 57% yield
To gain additional mechanistic insights, 3 equiv. of TEMPO
O
O
O
O
O
O
relative to 1a was added to the reaction system, no desired
product 3aa was observed, indicating that a radical process is
probably involved in this reaction. The Stern-Volmer analysis
revealed that the photoluminescence of fac-Ir(ppy)3 was
quenched by indole-derived bromides 2a in DCE at room
temperature (see SI ). Meanwhile, we also ruled out the radical-
chain propagation mechanism based on the light on/off
experiments (see SI).
O
O
O
O
O
O
O
O
O
Br
O
Ph
N
N
N
H
N
3'ag
3af
, 54% yield
,67% yield
3ag
,0% yield
3ah
, 48% yield
O
O
O
O
O
O
O
O
O
N
N
N
, 45% yield
3ai
, 84% yield
3ak
3aj
, 58% yield, dr=1:1
a
On the basis of experimental observations and previous
literature reports,9 a plausible mechanism was proposed for this
transformation (Scheme 6). Initially, Ir(III) photocatalyst was
excited to generate the excited species Ir(III)* under blue light
irradiation, which underwent single electron transfer (SET)
process with indole-derived bromide 2a generate radical A and
Ir(IV) metal complex. Subsequently, radical A underwent a rapid
addition with phenylacetylene 1a afforded radical intermediate
B, followed by intramolecular cyclization lead to radical
intermediate C, which was further oxidized through SET process
Reaction conditions: 1a (30.6 mg, 0.3 mmol), 2 (0.6 mmol),
Na2CO3 (63.6 mg, 0.6 mmol), fac-Ir(ppy)3 (3.9 mg, 0.006 mmol),
b
DCE (3 mL), irradiation with a 7W blue LED light, rt, 24 h.
Isolated yield.
To further expand the substrate scope of this transformation,
we then examined the substituents on the indole-derived
bromides (Scheme 4). A range of C5-substituted on indoles also
showed good applicability for this reaction affording their
corresponding dearomatized products in moderate yields (3ab-
3af). However, somewhat lower yield was obtained with the 5-
OMe-indole derived bromide 2c. When indoles C2 position
bearing phenyl group, we only got 2g direct cyclization
byproduct 3ag in 54% yield. In addition, when indoles C2
position bearing hydrogen moiety was also achieved desired
spiroindolenines 3ah in 48% yield. Moreover, dimethyl
malonate indole-derived bromide 2i was also applicable to this
to give key carbocation
D
and regenerated Ir(III)
photocatalyst.5d-5f Finally, deprotonation of intermediate D under
basic condition gave the desired product 3aa.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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and F. Glorius, Chem. Soc. Rev., 2018, 47, 7190-7202; (h) C.-S.
Wang, P. H. Dixneuf and J.-F. Soulé, Chem. Rev., 20V1ie8w, 1A1rti8cl,e7O5n3li2ne-
Scheme 6 Postulated cyclization reaction mechanism.
DOI: 10.1039/D0CC05672C
OO
7585; (i) Y. Zhao and W. Xia, Chem. Soc. Rev., 2018, 47, 2591-
2608; (j) H. Wang, X. Gao, Z. Lv, T. Abdelilah and A. Lei, Chem.
Rev., 2019, 119, 6769-6787.
4 For selected reviews of visible-light-induced dearomatizatio
reaction, see: (a) A. A. Festa, L. G. Voskressensky and E. V. Van
der Eycken, Chem. Soc. Rev., 2019, 48, 4401-4423; (b) M.
Okumura and D. Sarlah, Eur. J. Org. Chem., 2020, 2020, 1259-
1273; W.-C. Yang, M.-M. Zhang and J.-G. Feng, Adv. Synth.
Catal., 2020, doi: 10.1002/adsc.202000636.
O
N
O
O
O
Br
O
O
[Ir(III)]*
- H+
base
3aa
[Ir(III)]
HN
2a
Oxidative
quenching cycle
OO
O
HN
O
O
[Ir(IV)]
D
5 For selected papers of visible-light-mediated dearomatization
reaction, see: (a) Z. Gu, H. Zhang, P. Xu, Y. Cheng and C. Zhu,
Adv. Synth. Catal., 2015, 357, 3057-3063; (b) Y.-Z. Cheng, K.
Zhou, M. Zhu, L.-A.-C. Li, X. Zhang and S.-L. You, Chem. Eur. J.,
2018, 24, 12519-12523; (c) N. Hu, H. Jung, Y. Zheng, J. Lee, L.
Zhang, Z. Ullah, X. Xie, K. Harms, M.-H. Baik and E. Meggers,
Angew. Chem. Int. Ed., 2018, 57, 6242-6246; (d) Q. Wang, Y. Qu,
Q. Xia, H. Song, H. Song, Y. Liu and Q. Wang, Adv. Synth. Catal.,
2018, 360, 2879-2884; (e) Q. Wang, Y. Qu, Q. Xia, H. Song, H.
Song, Y. Liu and Q. Wang, Chem. Eur. J., 2018, 24, 11283-11287;
(f) Q. Wang, Y. Qu, Y. Liu, H. Song and Q. Wang, Adv. Synth.
Catal., 2019, 361, 4739-4747; (g) Y. Han, Y. Jin, M. Jiang, H. Yang
and H. Fu, Org. Lett., 2019, 21, 1799-1803; (h) Y.-Z. Cheng, Q.-R.
Zhao, X. Zhang and S.-L. You, Angew. Chem. Int. Ed., 2019, 58,
18069-18074; (i) M. Zhu, C. Zheng, X. Zhang and S.-L. You, J. Am.
Chem. Soc., 2019, 141, 2636-2644; (j) C.-L. Dong, X. Ding, L.-Q.
Huang, Y.-H. He and Z. Guan, Org. Lett., 2020, 22, 1076-1080.
6 (a) M. Zhu, K. Zhou, X. Zhang and S.-L. You, Org. Lett., 2018, 20,
4379-4383; (b) H. E. Ho, A. Pagano, J. A. Rossi-Ashton, J. R.
Donald, R. G. Epton, J. C. Churchill, M. J. James, P. O'Brien, R. J.
K. Taylor and W. P. Unsworth, Chem. Sci. , 2020, 11, 1353-1360;
(c) X.-J. Zhou, H.-Y. Liu, Z.-Y. Mo, X.-L. Ma, Y.-Y. Chen, H.-T.
Tang, Y.-M. Pan and Y.-L. Xu, Chem. Asian J., 2020, 15, 1536-
1539.
7 For selected reviews of alkyne reaction, see: (a) R. Chinchilla and
C. Nájera, Chem. Rev., 2014, 114, 1783-1826; (b) R. Salvio, M.
Moliterno and M. Bella, Asian J. Org. Chem., 2014, 3, 340-351; (c)
V. P. Boyarskiy, D. S. Ryabukhin, N. A. Bokach and A. V.
Vasilyev, Chem. Rev., 2016, 116, 5894-5986.
8 For selected papers of visible-light-mediated alkyne reaction, see:
(a) H. Jiang, Y. Cheng, Y. Zhang and S. Yu, Org. Lett., 2013, 15,
4884-4887; (b) Z.-G. Yuan, Q. Wang, A. Zheng, K. Zhang, L.-Q.
Lu, Z. Tang and W.-J. Xiao, Chem. Commun., 2016, 52, 5128-5131;
(c) H. Zhou, X. Deng, Z. Ma, A. Zhang, Q. Qin, R. X. Tan and S.
Yu, Org. Biomol. Chem. , 2016, 14, 6065-6070; (d) A. Shao, X.
Luo, C.-W. Chiang, M. Gao and A. Lei, Chem. Eur. J., 2017, 23,
17874-17878.
O
O
OO
OO
O
O
O
O
O
HN
HN
HN
1a
C
A
B
In summary, we have disclosed an operationally convenient
visible-light photocatalytic intermolecular dearomatization
cyclization cascade reaction between indole-derived bromides
and alkynes, which cyclization process features a broad substrate
scope and high reaction efficiency. This protocol also presents a
mild and efficient way to furnish a variety of spiroindolenines.
Conflicts of interest
There are no conflicts to declare.
Notes and references
1 For selected reviews of spiroindolenines, see: (a) Q. Ding, X. Zhou
and R. Fan, Org. Biomol. Chem. , 2014, 12, 4807-4815; (b) S. P.
Roche, J.-J. Youte Tendoung and B. Tréguier, Tetrahedron, 2015,
71, 3549-3591; (c) M. J. James, P. O'Brien, R. J. K. Taylor and W.
P. Unsworth, Chem. Eur. J., 2016, 22, 2856-2881; (d) J. Bariwal,
L. G. Voskressensky and E. V. Van der Eycken, Chem. Soc. Rev.,
2018, 47, 3831-3848; (e) C. Zheng and S.-L. You, Acc. Chem. Res.,
2020, 53, 974-987.
2 For selected examples synthesize of spiroindolenines, see: (a) K.-J.
Wu, L.-X. Dai and S.-L. You, Org. Lett., 2012, 14, 3772-3775; (b)
C.-X. Zhuo, Y. Zhou, Q. Cheng, L. Huang and S.-L. You, Angew.
Chem. Int. Ed., 2015, 54, 14146-14149; (c) P.-F. Wang, C. Chen,
H. Chen, L.-S. Han, L. Liu, H. Sun, X. Wen and Q.-L. Xu, Adv.
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