Catalysis Based on C?I???π Halogen Bonds: Electrophilic Activation of 2-Alkenylindoles by Cationic Halogen-Bond Donors for [4+2] Cycloadditions

Article abstract of DOI:10.1002/anie.201904689

Homo- and cross-[4+2] cycloadditions of 2-alkenylindoles, catalyzed by cationic halogen-bond donors, were developed. Under mild reaction conditions, 3-indolyl-substituted tetrahydrocarbazole derivatives were obtained in good to excellent yields. Experimental and quantum calculation studies revealed that the electrophilic activation of 2-alkenylindoles was achieved by C?I???π halogen bonds.

Full text of DOI:10.1002/anie.201904689

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Accepted Article
Title: Catalysis Based on C−I···π Halogen Bonds: Electrophilic
Activation of 2-Alkenylindoles by Cationic Halogen-Bond-Donors
for [4+2] Cycloadditions
Authors: Takayoshi Arai, Satoru Kuwano, Takumi Suzuki, Masahiro
Yamanaka, and Ryosuke Tsutsumi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201904689
Angew. Chem. 10.1002/ange.201904689
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10.1002/anie.201904689
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COMMUNICATION
Catalysis Based on C−I···π Halogen Bonds: Electrophilic
Activation of 2-Alkenylindoles by Cationic Halogen-Bond-Donors
for [4+2] Cycloadditions
Satoru Kuwano,[a] Takumi Suzuki,[a] Masahiro Yamanaka,[b] Ryosuke Tsutsumi[b] and Takayoshi Arai*[a]
Abstract: Cationic halogen bond-donor-catalyzed homo- and cross-
[4+2] cycloadditions of 2-alkenylindoles were developed. Under mild
reaction conditions, 3-indolyl-substituted tetrahydrocarbazole
derivatives were obtained in good to excellent yields. Experimental
and quantum calculation studies revealed that the electrophilic
activation of 2-alkenylindoles was achieved by C−I···π halogen bonds.
Non-covalent interactions, such as hydrogen bonding,
halogen bonding (XB), chalcogen bonding, and π interactions [π-
π stacking, XH-π (X = B, C, N, O), cation-π, anion-π, and lone
pair-π], are involved in many chemical transformations (Figure
1a).[1] These non-covalent interactions contribute to catalytic
processes by mimicking enzymatic catalysis, leading to various
small-molecule catalysts that utilize hydrogen bonds.[2] However,
catalysis based on non-covalent interactions other than hydrogen
bonding remains challenging.
Electrostatic non-covalent interactions between the σ-hole
of electron-deficient halogen atoms (e.g., C−X, X = I, Br, Cl; XB
donor) and a Lewis base (XB acceptor) can form XB complexes
(Figure 1b).[3] The highly directional and soft character of XB
complexes have been applied in crystal engineering,[4]
biomolecular systems,[5] and catalysis.[6,7,8] In 2008, Bolm et al.
reported XB donor-catalyzed reduction of quinolines by Hantzsch
esters.[7a] After this seminal work, other catalytic reactions were
successfully developed for the activation of lone-pair-possessing
heteroatoms (Y) in the substrates (e.g., imine derivatives,[7a,e,h,o,q,r]
carbonyl compounds,[7c,f,j,l,m,n,r] alkyl halides,[7d,g] thioamides,[7i]
iodonium ylides,[7k] and N-haloamides[7p]) (Figure 1b-left). In
contrast to activation of the lone pair (n-electron) of heteroatoms,
catalyst development of XB for the activation of π-electrons
Figure 1. Application of non-covalent π interactions in catalysis.
(C−X···π XB) has not been explored, although C−X···π XB
interactions have been reported in crystal engineering and
biomolecular systems (Figure 1b-right). For example, a recent
survey on high-quality structures of the PDB (Protein Data Bank)
by Zhu et al. showed that a variety of π electron systems (Try,
Phe, and Trp) were involved as XB acceptors.[9] In addition to
solid-phase studies, the thermodynamics of X···π XB in solution
[a]
Dr. S. Kuwano, T. Suzuki, Prof. Dr. T. Arai
phase was also examined.[10] Based on these findings, C−X···π
XB was expected to have catalysis applications.
Soft Molecular Activation Research Center (SMARC)
Chiba Iodine Resource Innovation Center (CIRIC)
Molecular Chirality Research Center (MCRC)
Synthetic Organic Chemistry, Department of Chemistry, Graduate
School of Science, Chiba University
1-33 Yayoi, Inage, Chiba 263-8522 (Japan)
E-mail: tarai@faculty.chiba-u.jp
Prof. Dr. M. Yamanaka, Dr. R. Tsutsumi
In 2014, Tamamura et al. reported formation of an
imidazolium salt and indole complex through cation-π interactions
(Figure 1c).[11,12] To develop a reaction using C−X···π XB
catalysis, 2-alkenylindole was chosen as the substrate because it
has an electron-rich π system to interact with XB donors. In 2009,
Xiao et al. reported Brønsted acid-catalyzed [4+2] cycloaddition
reaction of 2-vinylindoles.[13] The present report describes
electrophilic activation of 2-vinylindoles by a cationic halogen-
bond-donor for [4+2] cycloaddition reactions (Figure 1d).
[b]
Department of Chemistry, Rikkyo University
3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501 (Japan)
Supporting information for this article is given via a link at the end of
the document.
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Initially, various XB donor catalysts (10 mol%) were
screened for [4+2] cycloaddition of 2-vinylindole 2a (Table 1).
Compared to the neutral XB donor 1a, 2-iodoimidazolium salts 1b
and 1c smoothly catalyzed the reaction to give 3-indolyl-
tetrahydrocarbazole 3a in moderate yields (entries 1–3).[14]
Among the azolium salts tested, 2-iodoimidazolinium salt 1d
promoted the reaction most efficiently, resulting in 81% yield of
the product (entry 4). For the reaction using 1d, Lewis basic
solvents, such as THF, acetonitrile, and methanol, or a π-
donating solvent, such as toluene, were not effective for the
production of 3a (entries 5–8), presumably due to XB formation
with catalyst. Chloroform was the best solvent choice (entry 10,
Table 1). Notably, a catalyst loading of 1d could be reduced to 2.5
mol%, which produced 3a in 84% yield after 32 h (entry 11).[15]
Under optimized reaction conditions, substrate scope of the
homo-[4+2] cycloaddition reaction was examined (Table 2). For
N-benzyl-2-vinylindoles, introduction of electron-donating and
electron-withdrawing substituents at the 5-position successfully
gave dimerized products in moderate to high yields (entries 2–4).
The NH-free indoles, which have substituents at the 5-position,
were also suitable for the reaction, resulting in good yields of
products (entries 5–8). Bulky substrates, such as 4a–4c, were
tolerated, furnishing the corresponding products as single
diastereomers (entries 10–13).
Table 1. Optimization of reaction conditions.
Entry
1
Solvent
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MeCN
THF
Time
50
17
17
24
24
24
24
24
24
24
32
Yield [%][a]
1
2
1a
1b
1c
1d
1d
1d
1d
1d
1d
1d
1d
0
65
3
68
4
81
5
0
6
0
7
MeOH
toluene
(CH2Cl)2
CHCl3
CHCl3
0
8
0
42
9
10
11[b]
93
84 [84][c]
To expand the utility of this reaction, the cross-[4+2]
cycloaddition reaction was investigated using different 2-
alkenylindole components (Scheme 1). When the XB donor
catalyst 1d was applied to the cross reaction, 5a was obtained as
the main product, while dimerized compounds were not produced.
When previously examined trifluoroacetic acid[13] was used, yield
of the cross-product was decreased, and an uncyclized byproduct
6a was obtained. Results suggest that XB catalyst has the
potential to become a complemental tool for Brønsted acid
catalysis.
[a] Yield was determined by 1H NMR analysis using terephthalonitrile as an
internal standard. [b] Reaction was conducted with 2.5 mol % of 1d in CHCl3
(0.1 M). [c] Isolated yield.
Table 2. Substrate scope of [4+2] cycloaddition reaction of 2-vinylindoles.
Entry
1
2
R1
Bn
Bn
Bn
Bn
H
R2
H
R3
3
3a
3b
3c
3d
3e
3f
Yield [%]
84
2a
2b
2c
2d
2e
2f
H
2
5-Me
5-MeO
5-Br
H
H
96
3
H
81
4
H
43
Scheme 1. Difference between halogen bond and Brønsted acid catalysis.
5[a]
H
79
6[a]
H
5-Me
5-MeO
5-Br
7-Br
H
H
63
Various 2-vinylindoles and 2-styrylindoles were coupled
successfully using the XB donor catalyst 1d to give diverse
tetrahydrocarbazole derivatives in high yields with syn-
diastereoselective manners (Table 3). Interestingly, when the
reaction of entry 7 was examined using catalytic TfOH catalyst (5
mol %), significant amount of homo-dimerization of 2 was
generated (cross-coupling/homo-dimer = 1:0.51), which would
suggest the nonselective-activation by the Brnsted acid.
7[a]
2g
2h
2i
H
H
3g
3h
3i
69
8[a]
H
H
H
53
9[a]
H
45
10[a,b]
11[a,b]
12[a,b]
13[a,b]
4a
4b
4c
4d
H
Ph
3j
92
H
5-MeO
H
Ph
3k
3l
99
H
4-BrC6H4
4-MeOC6H4
52
H
H
3m
49
Several control experiments were performed for
investigating the reaction mechanism shown in Scheme 2. When
the reaction was conducted in CDCl3, no decomposition of 1d
[a] Reaction was conducted in CHCl3 (0.2 M). [b] Reaction was conducted
at 40 ˚C.
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occurred, as indicated by 1H NMR (Scheme 2a). The possibility of
a boost by Brønsted acid catalysis, generated by hydrolysis of
1d,[16] was eliminated by performing an experiment involving
addition of DIPEA (Scheme 2b). It should be noteworthy that the
homo-dimerization of 2a using HI (5 mol %) gave a complex
mixture.
in facilitating this reaction. To examine the interaction between 2-
iodoimidazolinium salt 1d and indoles (2a, 2j) in solution, 1H NMR
analysis in CDCl3 was conducted (Figure 2). Addition of 10 mol
eq. of 2-vinylindole 2a to catalyst 1d shielded the methyl proton
(Ha) peak in 0.024 ppm, and the addition of 2j, which does not
contain a 2-vinyl moiety, to 1d similarly shielded the methyl proton
(Ha) peak resulted in 0.028 ppm (Figure 2a). This NMR study
suggests that cation-π interactions through C−X···π XB occurred
between 1d and the π electrons of the indole moiety in 2a to give
2-iodoimidazolinium/2-vinylindole complex. A Job plot indicated
that the complex of 1d with 2j was 1:1. Therefore, the binding
constant for the 1:1 complexation [1d·2j] was Ka 0.22 M−1 by fitting
the data to the 1:1 binding model (Figure 2b).
Table 3. Substrate scope of cross-[4+2] cycloaddition reaction.
Entry
R1
Bn
H
R2
H
R3
H
R4
MeO
MeO
MeO
H
5
Yield [%]
80
dr
3:1
1
2
3
4
5
6
7
5a
5b
5c
5d
5e
5f
H
H
88
12:1
5:1
Bn
Bn
Bn
H
MeO
H
H
72
Bn
H
82
5:1
Figure 2. (a) 1H NMR spectral analyses for interaction of 1a with 2a or 2j (in
CDCl3 at room temperature). (b) 1H NMR titration of 1d (CDCl3, 298 K; Change
in chemical shift |Δ| (ppm) vs. [2j] (M)
H
H
82
3:1
H
H
H
86
12:1
>20:1
H
Cl
H
H
5g
88
Scheme 2. Control experiments.
The possibility of a radical pathway was also excluded by reaction
with the radical scavenger BHT (Scheme 2c). Reaction was
inhibited by addition of a chloride anion source (Scheme 2d).[17]
When the iodine atom of 1d was replaced with a hydrogen atom,
the chemical yield was decreased dramatically from 84% to 11%
(Scheme 2e). Because the catalyst safely survives under the
reaction conditions, the catalyst amount can be reduced to 1
mol % for giving 5g in 97 % yield. See the additional control
experiments in SI.
Figure 3. (a) DFT calculation of reaction mode between 2-vinylindole and N,N-
dimethyl 2-iodoimidazolinium salt. (b) Plausible transition states for [4+2]
cycloaddition.
In addition, DFT calculations was conducted to clarify the
halogen-bonding promoted catalysis (Figure 3).[18] For the
combination
of
2-vinylindole
with
N,N-dimethyl
2-
iodoimidazolinium salt, the mode A using halogen bonding
interaction is 2.3 kcal/mol more stable than the conventionally
well-studied π-π stacking mode B (Figure 3a). Because the
These results indicate that a cationic heteroaromatic ring or CH-
π interactions were not effective for activation of the 2-
alkenylindoles, and suggest that a C−X···π XB plays a crucial role
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In conclusion, cationic halogen bond donor-catalyzed
homo- and cross-[4+2] cycloaddition reactions of 2-alkenylindoles
were developed. Under mild reaction conditions, various 3-
indolyl-substituted tetrahydrocarbazole derivatives were obtained
in good to high yields. From experimental and quantum
calculation studies, electrophilic activation of 2-alkenylindoles
based on C−I···π halogen bond was realized. Further
investigations to determine details about the activation mode
using C−X···π halogen bonds and to discover new reactions
using this non-covalent interaction are underway.
Acknowledgements
This research is supported by JSPS KAKENHI Grant Number
JP19H02709 in Grant-in-Aid for Scientific Research (B),
JP16H01004 and JP18H04237 in Precisely Designed Catalysts
with Customized Scaffolding, JP18H04660 (Hybrid Catalysis),
19K15553 in Grant-in-Aid for Young Scientists, and the Futaba
Research Grant Program of the Futaba Foundation.
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Keywords: halogen bond • cycloaddition • catalysis • indole •
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DFT
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[18] M06-2X/LANL2DZdp for I and 6-31+G* for C,H,N. Computational details
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[19] Possible diastereomeric halogen bonding complex (mode A) and TSs
corresponding to the facial selectivity of 2-vinylindole were also
addressed. Chiral center of 1d has no impact on the gross structures and
energetics (See details in SI).
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Entry for the Table of Contents
COMMUNICATION
Satoru Kuwano, Takumi Suzuki,
Masahiro Yamanaka, Ryosuke
Tsutsumi, Takayoshi Arai*
Page No. – Page No.
Catalysis Based on C−I···π Halogen
Bonds: Electrophilic Activation of 2-
Alkenylindoles by Cationic Halogen-
Bond-Donors for [4+2]
Halogen bond-donor-catalyst meets with akenylindoles by making C−I···π
interaction to provide diverse 3-indolyl-substituted tetrahydrocarbazoles.
Cycloadditions
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