Photoredox Catalysis Induced Bisindolylation of Ethers/Alcohols via Sequential C-H and C-O Bond Cleavage

Article abstract of DOI:10.1021/acs.orglett.7b03073

A visible-light-engaged 2-fold site-selective alkylation of indole derivatives with aliphatic ethers or alcohols has been accomplished for easy access to symmetric 3,3′-bisindolylmethane derivatives. The experimental data suggest a sequential photoredox catalysis induced radical addition and proton-mediated Friedel-Crafts alkylation mechanism.

Full text of DOI:10.1021/acs.orglett.7b03073

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
Photoredox Catalysis Induced Bisindolylation of Ethers/Alcohols via
Sequential C−H and C−O Bond Cleavage
Lu Ye,† Sai-Hu Cai,†,‡ Ding-Xing Wang,† Yi-Qiu Wang,† Lin-Jie Lai,† Chao Feng,*,†
and Teck-Peng Loh*,‡,§
†Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for
Advanced Materials, Nanjing Tech University, Nanjing 211816, P. R. China
‡Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China
§Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
S
* Supporting Information
ABSTRACT: A visible-light-engaged 2-fold site-selective
alkylation of indole derivatives with aliphatic ethers or alcohols
has been accomplished for easy access to symmetric 3,3′-
bisindolylmethane derivatives. The experimental data suggest a
sequential photoredox catalysis induced radical addition and
proton-mediated Friedel−Crafts alkylation mechanism.
isindolylmethanes (BIMs) which consist of two indole
Bmotifs bridged by a single carbon at the 3 and 3′ positions
are known as an important class of indole alkaloids1 and
frequently occur in marine2 and terrestrial3 natural sources.
These BIM alkaloids possess diverse biological properties,4 such
as antifungal,5 antimicrobial,6 antiinflammatory,7 antibacter-
ial,2b,8 antihyperlipidemic,9 and anticancer10 activity and also
are found to serve as potential hypolipidemic and antiobesity
agents.11 Moreover, oxidized forms of BIMs have been utilized
as dyes as well as colorimetric chemosensors.12 Owing to their
prevalence in natural resources and versatile biological activity,
there has been significant interest in the synthesis of BIMs.
Traditionally, Lewis acids or protic acids were used as catalysts
to promote the electrophilic substitution reaction of indoles
with various carbonyl compounds13 or ethers (Scheme 1a).14 In
particular, a variety of organo,15 transition-metal,16 ionic
liquid,17 nanomaterial,18 and solid acidic catalysts19 have also
been employed. Despite these achievements, the development
of a more sustainable and mild protocol for the preparation of
BIMs by circumventing the inherent drawbacks remains a
compelling subject. Avoiding the necessity of high catalyst
loading, expensive or highly toxic catalysts, harsh reaction
conditions, and tedious workup procedures are desirable goals.
In recent years, photocatalysis chemistry has witnessed a
significant renaissance, which could enable otherwise unattain-
able organic transformations to occur and allow very mild
reaction conditions, thus attracting increasing attention from
synthetic organic chemists.20 In conjunction with our
continuing efforts in the realm of visible-light photocatalysis,21
we would report our recent work in visible-light-induced
synthesis of symmetric 3,3′-bisindolylmethanes. The reaction
proceeds through the successive alkylation of indole derivatives
with ethers/alcohols mediated by an arenediazonium salt as a
single-electron-transfer reagent (Scheme 1b).22
Initially, the reaction of N,N-dimethyl-1H-indole-1-carbox-
amide 1a and tetrahydrofuran (THF) was examined (Table 1).
We were pleased to find that the desired bis-indolylmethane
3aa was obtained in 81% yield by using Ru(bpy)3(PF6)2 as
photocatalyst and DCE as solvent, while DCM and CH3CN led
to 72% and 69% isolated yield, respectively (Table 1, entries 1−
3). No desired product was observed by conducting the
reaction in DMF or under air (Table 1, entries 4 and 5). In
addition, several photocatalysts were examined, which proved
to be less effective for such a transformation (Table 1, entries
6−8). Control experiments showed that the visible-light-
induced activation did not proceed, and the starting material
was recovered completely without either light irradiation or the
presence of photocatalyst (Table 1, entries 9 and 10).
Moreover, arenediazonium salt was demonstrated to be a
Scheme 1. One-Pot Synthesis of 3,3′-Bisindolylmethanes
Received: October 1, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03073
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
a,b
Table 1. Optimization of Reaction Conditions
Scheme 2. Reaction Scope
b
entry
photocatalyst
solvent
yield (%)
1
2
3
4
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ir(ppy)2(dtbbpy)PF6
[Ru(bpy)3]Cl2·H2O
fac-Ir(ppy)3
DCE
DCM
MeCN
DMF
DCE
DCE
DCE
DCE
DCE
DCE
DCE
81
72
69
0
c
5
0
6
7
8
75
79
55
0
d
9
Ru(bpy)3(PF6)2
10
0
e
11
Ru(bpy)3(PF6)2
0
a
Unless otherwise noted, the reactions were carried out at room
temperature under Ar atmosphere using 1a (1 equiv, 0.1 mmol), THF
(15 equiv, 1.5 mmol), Ru(bpy)3(PF6)2 (2 mol %, 0.002 mmol), and 2-
methoxybenzenediazonium tetrafluoroborate (1.2 equiv, 0.12 mmol)
b
c
d
in solvent (1 mL) for 12 h. Isolated yield. Air conditions. The
e
reaction was run in the dark. Without 2-methoxybenzenediazonium
tetrafluoroborate.
necessary stoichiometric oxidant to furnish the desired product
(Table 1, entry 11).
With the optimized conditions in hand, the substrate scopes
with respect to indoles and ethers/alcohols were surveyed, and
the results are shown in Scheme 2. It was found that diversely
decorated N-protected indoles 1 were suitable substrates for
this transformation, while N-H indoles afforded no products.
Generally, both electron-donating and electron-withdrawing
groups containing substrates worked well in this reaction. For
example, substrates with methyl and methoxyl groups were all
applicable and provided the desired products in moderate to
excellent yields (62%−90%, 3aa−da). N-Protected indoles
bearing synthetically important substituents, such as CN, NO2,
and COOMe, were competent in this reaction, affording the
corresponding products in moderate to excellent yields (53%−
94%, 3ea−na). It should be noted that halogen substituents
remain intact after the reaction, which provides the opportunity
for further derivatizations of the products. In addition, the
substitution position was found to show little impact on the
reaction efficiency. The influence of the N-protecting group of
indoles to this reaction was also probed. For example, replacing
the N,N-dimethylcarbamoyl group with N,N-diethylcarbamoyl,
N,N-diphenylcarbamoyl, and pyrrolidine-1-carbonyl groups led
to somewhat attenuated efficiency (3oa−qa). N-Acetyl and
-benzoyl indoles worked smoothly in this transformation and
furnished the desired products in good yields (3ra, 3sa).
Notably, the N-protecting group of indoles could also be
changed to aryl groups, such as 2-pyrimidyl and phenyl, albeit
with diminished reaction efficiency (3ta, 3ua). When
tetrahydropyran (THP) was employed instead of THF, the
homologous product 3ab was isolated in 44% yield. In addition,
common acyclic ethers were also investigated in the reaction.
Interestingly, dimethoxyethane gave two products, 3ac and
3ac′, through the cleavage of two kinds of C−O bonds. Diethyl
ether afforded the desired product 3ad in good yield.
Furthermore, more challenging substrate alcohols were
a
Unless otherwise noted, the reactions were carried out at room
temperature under Ar atmosphere using 1 (1 equiv, 0.1 mmol), THF
(15 equiv, 1.5 mmol), Ru(bpy)3(PF6)2 (2 mol %, 0.002 mmol), 2-
methoxybenzenediazonium tetrafluoroborate (1.2 equiv, 0.12 mmol)
b
c
d
in DCE (1 mL) for 12 h. Isolated yield. 2.5 mmol scale. Products
e
obtained from dimethoxyethane. Obtained from diethyl ether.
f
Obtained from ethanol.
examined instead of ethers. To our delight, ethanol gave the
product 3ad successfully although in moderate yield. Then
benzyl alcohol and 4-methyl benzyl alcohol were subjected to
reaction and delivered the desired products in 81% and 82%
yield (3ae and 3af). The increased yields could probably be
attributed to the activation of aryl group to benzylic C−H
bond.
To shed light on the reaction mechanism, diverse controls
reactions were performed. First, given aldehydes have been
reported to react with aromatic compounds to yield diaryl-
methane derivatives through Friedel−Crafts reaction,23 5-
hydroxypentaldehyde 4 which is able to be formed from the
oxidation of THP24 was examined in the reaction. As shown in
Scheme 3a, the aldehyde 4 was found to be an unproductive
reaction partner, which seems to rule out its participation in
this transformation. Second, an experiment was carried out with
B
DOI: 10.1021/acs.orglett.7b03073
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Letter
alkylation strategy, is present. The reaction proceeds through
photoinduced radical alkylation of indole followed by a
Friedel−Crafts alkylation of the second indole substrate. The
use of environmentally benign photoredox catalysis circum-
vents the employment of traditional strong oxidants or acids.
Moreover, the mild conditions and the wide functional group
tolerance of this method underline its potential for the
synthesis of more complicated molecules.
Scheme 3. Mechanistic Study
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03073.
Experimental procedures, characterization data, and
NMR spectra of new and previously reported com-
pounds (PDF)
radical-trapping reagent to gain additional insight into the
reaction mechanism. The reaction was inhibited in the presence
of TEMPO, which reveals that a radical mechanism may
operate in this reaction. Third, the monoindolylated THF 5 was
prepared and subjected to the standard reaction conditions. In
this reaction, the bisindolylated product 3ta was obtained in
90% yield, which tentatively suggests the intermediacy of 5 in
the present process (Scheme 3c). Finally, two types of kinetic
isotope effect (KIE) experiments were carried out under the
standard conditions, from which the KIE values were measured
to be 3.3 and 1.7, respectively (see the Supporting Information
for details). These results clearly demonstrate that C−H/D
abstraction from the THF occurs during the rate-determining
step of the reaction. The effect of photo irradiation on the
reaction was further studied under “on/off” light conditions
(see the Supporting Information for details). A dramatically
decreased reaction rate was observed by turning off the light,
which strongly suggests dependence of the present reaction on
photoirradiation.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: iamcfeng@njtech.edu.cn.
*E-mail: teckpeng@ntu.edu.sg.
ORCID
Chao Feng: 0000-0003-4494-6845
Teck-Peng Loh: 0000-0002-2936-337X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the “1000-Youth Talents Plan”, a Start-up Grant
(39837110) from Nanjing Tech University, and financial
support from a SICAM Fellowship by Jiangsu National
Synergetic Innovation Center for Advanced Materials.
Based on our observations, we propose a plausible
mechanism for bisindolylation process with ethers (Scheme
4). The activated photocatalyst PC* formed by irradiation
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