Rhodium(III)-Catalyzed Oxidative Allylic C-H Indolylation via Nucleophilic Cyclization

Article abstract of DOI:10.1021/acs.orglett.9b01553

Reported herein is a mild synthesis of 3-allylindoles via Rh(III)-catalyzed allylic C-H activation of olefins and coupling with o-alkynylanilines. The reaction proceeded via initial nucleophilic cyclization of o-alkynylanilines followed by oxidative coupl

Full text of DOI:10.1021/acs.orglett.9b01553

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rhodium(III)-Catalyzed Oxidative Allylic C−H Indolylation via
Nucleophilic Cyclization
Jiaqiong Sun,† Kuan Wang,† Peiyuan Wang,† Guangfan Zheng,*,† and Xingwei Li*,†,‡
†Key Laboratory of Applied Surface and Colloid Chemistry of MOE & School of Chemistry and Chemical Engineering, Shaanxi
Normal University (SNNU), Xi’an 710062, China
‡School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang 453007, China
S
* Supporting Information
ABSTRACT: Reported herein is a mild synthesis of 3-
allylindoles via Rh(III)-catalyzed allylic C−H activation of
olefins and coupling with o-alkynylanilines. The reaction
proceeded via initial nucleophilic cyclization of o-alkynylanilines
followed by oxidative coupling with allylic C−H bonds via an
η3-allyl intermediate.
etal-catalyzed direct C(sp3)−H bond activation has
and Glorius5a,b have successfully developed Cp*Rh(III)-
catalyzed allylic C−H amination, etherification, and arylation
of terminal and internal alkenes (Scheme 1b). Rovis6b and
Glorius5c also independently realized intermolecular allylic C−
H amidation of terminal olefins. Despite these achievements,
development of novel allylic C−H functionalization systems is
still challenging, especially for C(allylic)−C(aryl) coupling. In
reported allylic C−H arylation systems, arenes and aryl
boronic acids are common arylating reagents,5a,b with the
key M−aryl species being generated via C−H activation and
transmetalation, respectively. It is well-known that cyclization
of alkynes bearing a proximal nucleophilic offers a versatile and
atom-economic strategy to access M−C(sp2) species.7−11
However, this strategy has not been integrated into activation
of C(allylic)−H bonds.
M
emerged as a convenient and powerful strategy to
deliver numerous important synthetic targets.1 Although
impressive achievements have been made in catalytic C-
(sp3)−H activation (Scheme 1a), a directing group is generally
Scheme 1. Catalytic C(sp3)−H Activation
Indoles are prominent heterocyclic structures in numerous
natural and synthetic bioactive products. Among the various
indole derivatives, 3-allylindoles are useful intermediates in
synthetic organic chemistry and promising candidates in drug
design.12 Given the numerous reports of catalytic nucleophilic
cyclization of o-alkynylanilines for indole synthesis,8e,f,9c−h,13 it
is possible that Cp*Rh(III) complexes may also catalyze the
cyclization of o-alkynylanilines toward the indolylation of
allylic olefins. Thus, our objective was to integrate allylic C−H
activation and cyclization of o-alkynylanilines via Cp*Rh(III)
catalysis under oxidative conditions (Scheme 1c). Despite the
design, the following challenges should be addressed: (1) The
reaction may stop at the cyclization−protonolysis stage to
deliver an unreactive C(3)-unfunctionalized indole. (2)
Intermolecular allylic C−H amination may occur to lead to
undesired selectivity.4a (3) Homocoupling of indoles or olefins
may occur to lower the reaction efficiency. We now report a
indispensable to ensure reactivity and selectivity of an
unactivated C(sp3)−H bond.2 Consequently, development of
a nondirected strategy to realize activation of a specific
C(sp3)−H bond is of great demand. Among the various classes
of C(sp3)−H bonds, allylic C−H bonds are intrinsically
reactive owing to their tendency to form thermodynamically
stable η3-allyl species.3−6 Thus, following the pioneering work
by White, Shi, and others, Pd-catalyzed regioselective allylic
functionalization of olefins evolved rapidly.3 Recently, Blakey4
Received: May 2, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01553
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a b
,
Table 1. Optimization of the Reaction Conditions
yield (%)
entry
catalyst (mol %)
oxidant
solvent
T (°C)
3aa
57
4a
1
2
3
4
[Cp*RhCl2]2 (4)/AgSbF6 (16)
[Cp*RhCl2]2 (4)/AgSbF6 (16)
[Cp*RhCl2]2 (4)/AgSbF6 (16)
[Cp*RhCl2]2 (4)/AgSbF6 (16)
[Cp*RhCl2]2 (4)/AgSbF6 (16)
[Cp*RhCl2]2 (4)/AgSbF6 (16)
[Cp*Rh(MeCN)3](SbF6)2 (5)
[Cp*Rh(MeCN)3](SbF6)2 (5)
[Cp*Rh(MeCN)3](SbF6)2 (5)
[Cp*Rh(MeCN)3](SbF6)2 (5)
[Cp*Rh(MeCN)3](SbF6)2 (5)
[Cp*Rh(MeCN)3](SbF6)2 (5)
[Cp*Rh(MeCN)3](SbF6)2 (5)
[Cp*Rh(MeCN)3](SbF6)2 (5)
[Cp*Rh(MeCN)3](SbF6)2 (5)
[Cp*Rh(MeCN)3](SbF6)2 (5)
[Cp*RhCl2]2 (4)/AgSbF6 (16)
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
AgOAc
Ag2CO3
AgF
AgOPiv
AgOAc
AgOAc
DCM
DCE
PhCF3
TFE
100
100
100
100
100
100
100
100
100
100
100
100
120
90
41
38
54
45
35
34
31
22
16
71
38
42
33
16
15
6
61
44
55
65
66
69
75
82
25
60
53
51
83
85
c
5
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
c d
,
6
7
8
9
c d
,
c
c
−
e
e
−
c
c
c
c
c
c
c
c
−e
−e
−e
−e
−e
−e
−e
−e
10
11
12
13
14
15
16
17
AgOAc
AgOAc
AgOAc
80
70
70
93 (92)
77
14
a
b
Reaction conditions: 1a (0.1 mmol), 2a (1.5 equiv), Rh catalyst, oxidant (2.2 equiv), solvent (2 mL) under N2 for 12 h. Yields were determined
by H NMR spectroscopy using 1,3,5-trimethylbenzene as an internal standard (isolated yield in parentheses). 2a (2.0 equiv). Under air.
Oxidant (3.0 equiv) was used.
c
d
1
e
a b
,
Rh(III)-catalyzed cyclization/oxidative allylation cascade for
synthesis of 3-allylindoles.
Scheme 2. Scope of Alkynes in Allylation
We commenced our investigation with optimization studies
of the coupling of o-alkynylaniline (1a) and trans-1,3-
diphenylpropene (2a) as model substrates in the presence of
a [Cp*RhCl2]2/AgSbF6 catalyst (100 °C). A coupling
occurred in the presence of Cu(OAc)2 oxidant to give the
desired product 3aa in 57% yield alongside with indole 4a as
the side product (Table 1, entry 1). Screening of solvents
revealed that DCE was the best choice, which afforded the
desired product in 61% yield (entries 2−4). The yield of 3aa
was improved to 65% when 2 equiv of 2a was used (entry 5).
The reaction efficiency was essentially unaffected when
conducted under air (entry 6). Switching the catalyst to
[Cp*Rh(MeCN)3](SbF6)2 (5 mol %) improved the yield to
69% (entry 7). The yield was further improved by increasing
the amount of Cu(OAc)2 (entry 8). Although other silver(I)
oxidants were less effective (entires 10−12), AgOAc turned
out to be ideal for this reaction (82%, entry 9). Gratifyingly, an
excellent yield of 3aa (92%) was isolated when the reaction
was performed at 70 °C (entries 13−16). Switching the
catalyst to [Cp*RhCl2]2/AgSbF6 gave lower yields due to
formation of 4a (entry 17).
With the optimized conditions in hand, we next explored the
scope and limitation of this cyclization/allylation reaction
coupling with trans-1,3-diphenylpropene 2a (Scheme 2). The
presence of methyl and methoxyl groups at the para position of
the phenyl ring afford 3ba and 3ca in 88% and 87% yield,
respectively. Alkynes with halogen and electron-withdrawing
groups delivered 3da−3ga in 46−77% yields. Biphenyl and 2-
naphthyl groups were also tolerated, affording 3ha and 3ia in
86% and 48% yields. For meta-substituted substrates 1j and 1k,
the reaction proceeded efficiently with the corresponding 3ja
a
Reaction conditions A: 1 (0.2 mmol), 2a (2.0 equiv), [Cp*Rh-
(MeCN)3](SbF6)2 (5 mol %), AgOAc (3.0 equiv), DCE (2 mL), 70
°C for 12 h. Isolated yield after column chromatography. 2a (3.0
equiv), under 100 °C.
b
c
and 3ka being isolated in 86% and 73% yields. We noted that
o-Me- and o-OMe-substituted substrates 1l and 1m led to a
formation of a mixture of diastereoisomers caused by hindered
rotation of the two aromatic rings. In addition, an alkyl-
terminated alkyne was also found to be compatible and
afforded product 3na in 42% yield. Moreover, good coupling
efficiency was observed for substrates bearing 5-and 6-alkyl,
halogen, or ester groups in the aniline ring, affording coupling
products (3oa−3sa) in 72−94% yields. In addition, product
3qa has been characterized by X-ray crystallography (CCDC
1890239). Next, we explored the effects of N-activating groups
of o-alkynylaniline. Our results revealed that sulfonamides with
B
DOI: 10.1021/acs.orglett.9b01553
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different functional groups furnished the corresponding
products 3ta−3wa in good to excellent yields (62−91%).
Unfortunately, TMS-substituted o-alkynylaniline, unprotected
o-alkynylaniline, and o-alkynylphenol failed to deliver any of
the desired coupling products (see the SI).
Scheme 4. Scale-up Reaction and Derivatization of 3aa
The scope of the internal and terminal olefins was next
explored under slightly modified reaction conditions (Scheme
3). Tolyl- and chlorophenyl-substituted diarylpropene 2b and
a b
,
Scheme 3. Substrate Scope of Olefins
Scheme 5. Mechanistic Studies
a
Reaction conditions B: 1a (0.2 mmol), 2 (3.0 equiv), [Cp*Rh-
(MeCN)3](SbF6)2 (5 mol %), AgOAc (3.0 equiv), DCE (2 mL), 100
°C for 12 h. Isolated yield after column chromatography.
b
2c coupled smoothly with o-alkynylaniline 1a, affording the 3-
allylation indoles in good yields (3ab, 3ac). Then the internal
olefins 7-tetradecene and 4-octene were examined, which both
delivered the corresponding 3-allylindoles as 1:1 regioisomers
(3ad, 3ae). Terminal olefins were also tolerated and give a
10:1 and 3:1 regioisomeric ratio favoring the linear product
(3af, 3ai). Substituted allylbenzenes were also tolerated and
gave the desired products 3ag and 3ah in 37% and 34% yields
with 8:1 and 7:1 l/b ratios, respectively. However, no product
was obtained when ester-, aldehyde- and acid-functionalized
olefins were used, and cyclic olefins were also found to be
incompatible (see the SI).
To demonstrate the practicability of this reaction, a 2 mmol
scale reaction was conducted and 3aa was isolated in 74% yield
(0.8 g). To further investigate the synthetic application of this
product, we next investigated deprotection of the sulfonyl
group (Scheme 4). Thus, treatment of 3aa with NaOH
afforded NH indole 5 in 72% yield.14
A series of experiments have been conducted to gain
mechanistic insight into this coupling system (Scheme 5).
First, we performed two independent couplings of allylbenzene
and (E)-prop-1-en-1-ylbenzene and o-alkynylaniline 1a, and
the product was obtained in nearly the same yield and
regioselectivity (Scheme 5a). This result indicates the common
intermediacy of a π-allyl species in these reactions.
Furthermore, indole 4a proved to be inactive for this
transformation (Scheme 5b). Therefore a Rh(III)-indolyl,
instead of a protonolyzed indole, is the active species. The C−
H activation step in the coupling of 1a and 2a was found to be
irreversible under the reaction conditions, as evidenced by
absence of deuterium incorporation in both 3-allylindoles
(3aa) and the recovered olefin (Scheme 5c). We also evaluated
the sequence of cyclization versus allylic C−H activation. By
increasing the amount of olefin 2a with a fixed amount of 1a
substrate, the ratio of 3aa/4a also increased (Scheme 5d). This
observation indicated that cyclization occurred prior to C−H
activation (vide infra). Our competition experiment also
showed that the C−H functionalization is kinetically favored
for a more electron-rich allylic C−H bond (Scheme 5e).
On the basis of the preliminary mechanistic studies and
previous reports,4,5 a plausible catalytic cycle for the reaction of
1a and 2a is proposed (Scheme 6). An active catalyst
Cp*RhX2 was generated via ligand exchange. Coordination
of the o-alkynylaniline 2a then gives a metal-bound alkyne
intermediate A. Subsequent nucleophilic cyclization gives a
Rh(III) indolyl species B. Off-cycle competitive protonolysis of
the Rh-indolyl bond accounts for the cyclization byproduct.
C
DOI: 10.1021/acs.orglett.9b01553
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Scheme 6. Plausible Mechanism
ACKNOWLEDGMENTS
■
Financial support from the NSFC (No. 21525208) and SNNU
is gratefully acknowledged.
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without prefunctionalization of the substrate. Further studies
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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.9b01553.
Experimental procedures, spectral data of new com-
pounds, and crystallographic data of 3qa (PDF)
Accession Codes
CCDC 1890239 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: zhenggf@snnu.edu.cn.
*E-mail: xwli@dicp.ac.cn, lixw@snnu.edu.cn.
ORCID
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D
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