Dearomatization of Indoles via Palladium-Catalyzed Allylic C-H Activation

Article abstract of DOI:10.1021/acs.orglett.5b03053

The first Pd-catalyzed allylic dearomatization of substituted indoles triggered by C-H bond activation is reported. The presence of a catalytic amount of 2,5-DMBQ is proven to be a key factor for the high yield. This one-pot tandem allylic C-H activation/dearomatization sequence provides a straightforward access to 3,3-disubstituted indolines.

Full text of DOI:10.1021/acs.orglett.5b03053

Letter
pubs.acs.org/OrgLett
Dearomatization of Indoles via Palladium-Catalyzed Allylic C−H
Activation
Heng Zhang,† Rong-Bin Hu,† Na Liu,† Shi-Xia Li,† and Shang-Dong Yang*,†,‡
†State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China
‡State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Lanzhou 730000, P. R.
China
S
* Supporting Information
ABSTRACT: The first Pd-catalyzed allylic dearomatization of
substituted indoles triggered by C−H bond activation is reported.
The presence of a catalytic amount of 2,5-DMBQ is proven to be
a key factor for the high yield. This one-pot tandem allylic C−H
activation/dearomatization sequence provides a straightforward
access to 3,3-disubstituted indolines.
Scheme 1. Strategies for Dearomatization of Indoles
he dearomatization process has been widely recognized as a
Tpowerful strategy for the construction of cyclic or
heterocyclic frameworks from simple starting materials.1,2
Therefore, a great deal of effort has been devoted to
dearomatization reactions, and various methods have been
established in alkylative and arylative dearomatizations.3 Among
them, transition-metal-catalyzed dearomatization of indoles
derivatives attracts much more attention for its extensive
application in the synthesis of different indole alkaloids. In
general, the classical and efficient dearomatization reactions of
indoles have been made with electrophiles containing a leaving
group.4 However, from a step- and atom-economic point of view,
the dearomatization reactions triggered by a simple C−H bond
activation5 would be a more straightforward and advanced
alternative. Very recently, Luan’s group and You’s group
independently reported the dearomatization of naphthols via
transition-metal-catalyzed C−H functionalization.6a−c The
Tanaka group successfully developed a gold-catalyzed dearoma-
tization of 1-aminonaphthalene derivatives with an intra-
molecular C−H functionalization.6d Despite this progress,
transition-metal-catalyzed direct dearomatization via C−H
bond functionalization remains rare and sluggish.
It is well-known that transition-metal-catalyzed allylic C−H
activation is potentially a more efficient and complementary
process to the conventional Tsuji−Trost-type reactions.7
Though allylic C−H bond activation studies have made
significant advancements recently,8−11 application of this
strategy to the dearomatization of indoles has not yet been
described. Previous strategies for the allylic dearomatization of
indoles mainly employed allylic halides or esters (Scheme 1, path
A).12 As a result of our endeavors, a new method for the
dearomatization of indoles triggered by a Pd(II)-catalyzed allylic
C−H activation was developed (Scheme 1, path B). Compared
with existing allylic dearomatization of indoles, our reaction
combines allylic C−H activation with dearomatization of indoles
and provides a rapid and concise route to the synthesis of
polysubstituted indolines.
At the outset of this study, we chose 2,3-dimethylindole (1a)
and allylbenzene (2a) as model substrates to optimize suitable
reaction conditions. We screened a number of conditions
including palladium sources, oxidants, solvents, and additives
(Table 1). After consulting previous reports of allylic C−H
activations, we selected Pd(PPh3)2Cl2 (5 mol %) as the catalyst,
Ag2CO3 (1.1 equiv) as the oxidant as well as the base, and
benzoquinone (BQ) (1.1 equiv) as an additive. A series of
solvents were then evaluated at 80 °C in the reaction of allylic C−
H activation and dearomatization of indoles. Gratifyingly, we
found that Pd(PPh3)2Cl2 could catalyze the reaction and that
dichloromethane (DCM) is the best solvent (entries 1−7); the
desired product was obtained in 23% yield. Encouraged by this
result, we further optimized the reaction conditions. Sub-
sequently, control experiments indicated that Ag2CO3 and BQ
were essential to the reaction (entries 8 and 9). A screening of
different palladium catalysts showed that Pd(PPh3)2Cl2 is still the
best choice in this transformation (entries 10−14). It is well-
known that BQ is pivotal in many Pd-catalyzed allylic
functionalization reactions and often serves as the oxidant and
ligand.13 Therefore, various BQ derivatives such as additives 2−8
on an equivalent scale were examined. When substituted electron
benzoquinones such as 2-methylbenzoquinone, 2,6-dimethyl-
benzoquinone, and 2,5- dimethylbenzoquinone (2,5-DMBQ),
Received: October 20, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03053
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
with 0.44 mmol of allylbenzene at 80 °C under an argon
atmosphere (entry 22).
Table 1. Reaction Conditions Screening
Having discovered optimized conditions for the allylic
dearomatization reaction of 2,3-disubstituted indole, we wished
to demonstrate the generality of this straightforward trans-
formation. We first investigated the compatibility of the olefin
partners (Scheme 2). Indeed, electron-rich allylbenzene such as
a,b
Scheme 2. Substrate Scope of Alkenes
a
The reaction was carried out with Pd(PPh3)2Cl2 (5 mol %), 2,5-
DMBQ (20 mol %), Ag2CO3 (1.1 equiv), 1a (0.20 mmol), and 2a−m
b
(0.44 mmol) in DCM (1.5 mL) at 80 °C for 16 h under argon. Yield
of isolated product.
m-methoxyallylbenzene, p-methoxyallylbenzene, and 3,5-dime-
thoxyallylbenzene afforded the desired products in 84%, 75%,
and 60% yields, respectively (3b−d). 3-Methylallylbenzene
furnished the desired product with 88% yield (3e). Unfortu-
nately, the electron-poor allylbenzenes such as 4-Cl- and 4-CF3-
substituted allylbenzene delivered the desired products in 57%
and 33% yield, respectively (3f,g). The allylpentafluorobenzene
only gave trace amounts of the desired product (3h). Meanwhile,
possibly due to a steric effect, 2-methylallylbenzene gave the
corresponding product in 60% yield (3i), and 2-naphthylpro-
pene gave the corresponding product in 48% yield (3j). Notably,
heteroaryl-substituted propenes such as 2-allylthiophene and 5-
allylbenzo[d][1,3]dioxole also produced the desired products in
good yields (3k, 84%; 3l, 79%). Moreover, the 4-acetylallylben-
zene afforded the desired product in 74% yield (3m).
Nonactivated alkenes such as 1-decene or 1-octene are not
compatible in this reaction.
Next, various substituted indoles were examined (Scheme 3).
Both electron-donating group (5-OMe) and electron-with-
drawing groups (5-Cl, and 5,7-di-Cl) on the indoles were well-
tolerated and gave the corresponding products in moderate to
good yields (4b−d). Moreover, carbon-cycle-fused indoles were
also proven to be suitable for this transformation. For example,
the 1,2,3,4-tetrahydrocarbazole and indolo(2,3-B)cycloheptene
afforded dearomatizated products in 88% and 67% yields (4e and
4f). 3-Monosubstituted indoles were then studied. 3-methyl-
indole delivered the desired product 4g in 56% yield. Tryptophol
a
The reactions were carried out by using 1a (0.20 mmol), 2a (0.44
mmol), palladium (5 mol %), ligand (10 mol %), oxidant (1.1 equiv),
and additive (1.1 equiv) in solvent (1.5 mL) at 80 °C for 16 h under
b
c
d
argon. Isolated yields. Additive (20 mol %). Additive (10 mol %).
e
Additive 2 = p-toluquinone; additive 3 = 2,6-dimethyl-1,4-quinone;
additive 4 = p-xyloquinon; additive 5 = duroquinone; additive 6 = 2,6-
di-tert-butyl-p-benzoquinone; additive 7 = 2,6-dimethoxy-1,4-benzqui-
none; additive 8 = 2,6-dichloro-1,4-benzquinone.
were examined, the yield of the desired product improved
(entries 15−17). The 2,5-dimethylbenzoquinone (2,5-DMBQ)
gave an excellent yield of 93% (entry 17). Owing to the electronic
and steric effects, duroquinone, 2,6-di-tert-butyl-p-benzoqui-
none, and 2,6-dimethoxyquinone only provided lower yields
(entries 18−20). Moreover, electron-poor 2,6-dichlorobenzo-
quinone also was unfavored for this reaction (entry 21). Next,
decreasing the amount of 2,5-dimethylbenzoquinone (2,5-
DMBQ) to 20 mol % left the yield of product completely
unchanged (entry 22). A further decrease of the amount of 2,5-
dimethylbenzoquinone (2,5-DMBQ) is not beneficial to the
reaction (entry 23). Finally, we also investigated other oxidants,
but no improvements were obtained (entries 24−28). Thus, the
optimal reaction conditions were obtained by using Pd-
(PPh3)2Cl2 (5 mol %) as the catalyst, 2,5-dimethylbenzoquinone
(2,5-DMBQ) (20 mol %) as additive, 1.1 equiv Ag2CO3 as the
base as well as the oxidant in 1.5 mL of DCM for 0.2 mmol of 1a
B
DOI: 10.1021/acs.orglett.5b03053
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Letter
a,b
L2PdIICl2 and 2,5-dimethylbenzene-1,4-diol to complete the
catalytic cycle. Meanwhile, 2,5-dimethylbenzene-1,4-diol was
oxidized by Ag2CO3 to regenerate the 2,5-DMBQ. In this
reaction, we also could not exclude that 2,5-DMBQ performed as
the ligand and prompted the reductive elimination to proceed
with a swing.
Scheme 3. Substrate Scope of Indoles
In conclusion, we have developed the first general example of
direct palladium-catalyzed C3-allylic dearomatization of sub-
stituted indoles through C−H activation/dearomatization
tandem processes. This Pd-catalyzed transformation is effective
for indoles and allylbenzenes containing sterically and electroni-
cally diverse substituents and affords the C3-phenylpropenyl
indolenine products in good yield.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03053.
Experimental details, characterization data, and copies of
NMR spectra of new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
a
The reaction was carried out with Pd(PPh3)2Cl2 (5 mol %), 2,5-
*E-mail: yangshd@lzu.edu.cn.
Notes
DMBQ (20 mol %), Ag2CO3 (1.1 equiv), 1b−i (0.20 mmol), and 2a
b
(0.44 mmol) in DCM (1.5 mL) at 80 °C for 16 h under argon. Yield
of isolated product.
The authors declare no competing financial interest.
gave the corresponding furanoindoline product in 44% yield
(4h). When N-tosyltryptamine was used, the corresponding
product was observed in lower yield (4i), and the allylic
amination product was obtained as byproduct concomitantly.
Moreover, indolylcarboxyamide gave the double allylic sub-
stituent product at the C-3 and N-1 positions in 20% yield (4j),
and meanwhile, the C−H bond amination product was also
produced.
ACKNOWLEDGMENTS
We are grateful for the NSFC (Nos. 21272100 and 21472076)
for financial support.
■
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D
DOI: 10.1021/acs.orglett.5b03053
Org. Lett. XXXX, XXX, XXX−XXX