Pd-Catalyzed Tandem Coupling Reaction of 2-gem-Dibromovinylanilines and N-Tosylhydrazones to Construct 2-(1-phenylvinyl)-indoles

Article abstract of DOI:10.1002/adsc.201900230

A novel palladium(0)-catalyzed intermolecular coupling reaction of 2-gem-dibromovinylanilines and N-tosylhydrazones was reported to construct 2-(1-phenylvinyl)-indoles efficiently. The indole bearing 1, 1-disubstituted alkenes were obtained in one step wi

Full text of DOI:10.1002/adsc.201900230

Title: Pd-Catalyzed Tandem Coupling Reaction of 2-gem-
Dibromovinylanilines and N-Tosylhydrazones to Construct 2-(1-
phenylvinyl)-indoles
Authors: Jian Song, Xiaochen Chi, Long Meng, Pingping Zhao,
fenggang Sun, Daopeng Zhang, Luyang Jiao, Qing Liu,
Yunhui Dong, and Hui Liu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900230
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10.1002/adsc.201900230
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Pd-Catalyzed Tandem Coupling Reaction of 2-gem-
Dibromovinylanilines and N-Tosylhydrazones to Construct 2-
(1-phenylvinyl)-indoles
Jian Song,a Xiaochen Chi,a Long Meng,a Pingping Zhao,b Fenggang Sun,a Daopeng
Zhang,a Luyang Jiao,a Qing Liu,a Yunhui Donga and Hui Liua,*
a
School of Chemistry and Chemical Engineering, Shandong University of Technology, 266 West Xincun Road, Zibo
255049, P. R. China.
E-mail: huiliu1030@sdut.edu.cn
College of chemical and environmental engineering, Shandong university of science and technology, 579
b
Qianwangang Road, Huangdao District, Qingdao, 266590, P.R.China
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
catalyzed regioselective C2 alkenylation of indoles
coupling reaction of 2-gem-dibromovinylanilines and N- through utilizing alkenyl bromides, directed by a N-2-
Abstract. A novel palladium(0)-catalyzed intermolecular
tosylhydrazones was reported to construct 2-(1-
phenylvinyl)-indoles efficiently. The indole bearing 1, 1-
disubstituted alkenes were obtained in one step with short
reaction time, in high yields and broad substrate scope. The
formed indole derivates could be easily transformed into
much more valuable molecules.
pyrimidinyl group under relatively mild conditions
(Scheme 1b).[5f] Schipper and other groups
successively achieved Co or Ru catalyzed
intermolecular C2-alkenylation of indoles by using
alkynes instead of alkenes, in which N,N-
dimethylcarbonyl or pyrimidyl group was employed
as a directing group (Scheme 1c).[6]
Keywords: Palladium; 2-gem-dibromovinylanilines;
Hydrazones; Alkenylindoles; Alkenylation
The indole scaffold is an integral feature of several
biologically active compounds, natural products and
pharmaceuticals.[1] In the past few decades,
tremendous advances have been made in the
development of functionalized indoles.[2] 2-
Alkenylindoles are important building blocks in
organic synthesis, which exist in many important
naturally alkaloids.[3] However, direct alkenylation of
indoles at the C2-position is a challenging subject due
to the reaction selectivity at C2/C3-position.[4] To
date, alkenylation of indoles at the C2-position is less
reported.[5] In 2005, Gaunt’s group developed a
palladium(II)-catalyzed alkenylation of indoles to
afford C2- or C3-alkenylated products by adjusting
the solvent and additives.[5a] However, the yields of
C2 alkenylation were quite low and substrates were
limited to a few examples (Scheme 1a). Ricci and co-
workers achieved regioselective C2 alkenylation of
indoles, directed by a N-2-pyridylmethyl group with
the assistance of Pd catalyst (Scheme 1b).[5b]
Simultaneously, Arrayás’s group reported
a
Scheme 1. Different Approaches to Alkenylindoles.
palladium(II)-catalyzed alkenylation of indoles at the
C2-position by using excess of alkenes with N-
pyridylsulfonyl as a directing group (Scheme 1b).[5c]
Recently, Punji and co-workers described a Ni-
Moreover, some examples without N-directing
groups were developed. In 2006, Nakao, Hiyama and
1
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10.1002/adsc.201900230
Advanced Synthesis & Catalysis
co-workers described a C2-addition to internal
insertion
reaction,
who
used
alkynes under mild nickel catalysis.[4a] In recent years, trimethylsilyldiazomethane as the carbene precursor
Yoshikai’s group[4b] demonstrated an iron-catalyzed
directly C2-alkenylation of indoles bearing formyl
group at the C3 position with alkynes. Miura’ et al.[4c]
investigated the oxidative coupling of indole-3-
carboxylic acids with alkenes to afford the
corresponding 2-alkenylindoles via directly C-H
alkenylation and decarboxylation catalyzed by
and benzyl halides as electrophiles. In 2007,
Barluenga and co-workers demonstrated the first
cross-coupling reaction between N-tosylhydrazones
and aryl halides, during which aryl migratory
insertion mechanism was also suggested.[10] From
then on, the N-tosylhydrazones as carbene precursor
were extensively investigated.[11]
palladium
(Scheme
1d).
Basically,
these
On the basis of pioneering study and our work[2j] in
developing new methodologies to functionalized
methodologies require the electron-withdrawing
groups at C3-position, such as CN, CO2R and CHO.
Besides the direct alkenylation of indoles at C2-
position, Lautens and co-workers demonstrated a
one-pot palladium-catalyzed tandem reaction to give
a broad scope of substituted 2-vinylic indoles from
gem-dibromovinylanilines and alkenyl boronic
acids[7a] or alkenes[7b] (Scheme 1e). However, most of
the processes provided 1, 2-disubstituted alkenes,
except that the reaction reported by Lautens and co-
workers produced 1,1-disubstituted alkenes via
Suzuki coupling. In 1980s, Akgün’s groups
indoles, we disclose
a
palladium(0)-catalyzed
alkenylation of indoles at the C2-position from 2-
gem-dibromovinylanilines and hydrazones, during
which indole ring bearing 1,1-disubstituted alkenes
were obtained in one step (Scheme 1f).
Initially, our study commenced from the reaction
of 2-gem-dibromovinylanilines 1a with N-
tosylhydrazones 2a catalyzed by PdCl2/PPh3 using
3.0 equivalent tBuOLi as base in dioxane under N2
atmosphere (Entry 1, Table 1). As expected, the
desired indole derivatives 3aa was obtained in 62%
yield (Entry 1). The yield could be increased to 72%
when Pd(OAc)2 was used, and it was elevated to 77%
by using Pd(dba)2 (Entries 2 and 3). However, the
yield was decreased to 20% by using PCy3 as ligand
(Entry 4). To our delighted, the yield was increased to
88% by employing PdCl2(PPh3)2 as catalyst (Entry 6).
Subsequently, the bases such as DMAP, tBuOK,
TEA, CsCO3, and CsF were screeninged, while no
better results were obtained than tBuOLi (Entries 7-
11). Furthermore, we examined several solvents, such
developed
a
process towards 1,1-disubstituted
alkenes at the C2-position of indole via the aldol
reaction of 2-lithioindoles, followed by an
elimination reaction of the resulting indole-carbinols
to form the 2-vinylindoles.[8] However, this reaction
required multiple steps to complete, which limited the
application.
Table 1. Optimization of reaction conditions.a
Table 2. Scope of N-Tosylhydrazones in the Construction
of tandem reaction.a
aReaction conditions: 1a (1.0 equiv., 0.3 mmol), 2a (2.0
equiv., 0.6 mmol), [Pd] (10 mol %), base (3.0 equiv., 0.9
mmol), solvent (4 mL), N2, 1 h, 110 oC. bIsolated Yields.
Diazo compounds have been frequently exploited
as carbene or metal carbene precursors. In 2001, Van
Vranken’s group[9] developed the first palladium-
aReaction conditions: 1a (0.3 mmol), 2 (0.6 mmol),
PdCl2(PPh3)2 (10 mol %), tBuOLi (0.9 mmol), dioxane (4
mL), N2, 1 h, 110 oC. bIsolated Yields.
catalyzed cross-coupling involving
a
carbene-
2
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10.1002/adsc.201900230
Advanced Synthesis & Catalysis
as PhMe, DMF, DME and THF. Unfortunately, no
positive results were obtained (Entries 12-15). When
(3ag-3aj). Furthermore, the tri-substituted alkenes
were synthesized from the corresponding N-
tosylhydrazones, which exhibited high Z-selectivity
determined by NOE (3ak-3am). In addition, we
investigated the scope of heteroaromatic hydrazones,
such as pyridyl and furyl hydrazones. Unfortunately,
the reaction of 2-gem-dibromovinylanilines with
pyridyl hydrazones could not be converted to the
desired product. For the reaction of furyl hydrazones,
we isolated one spot from the mixture, however, the
1H NMR showed it was a mixture which could not be
purified although we tried a lot of methods (see the
Supporting Information IX).
Subsequently, the substitutions on 2-gem-
dibromovinylaniline were then investigated in Table
3. Substrates bearing strong electron-withdrawing
groups as -CO2Me, provided the corresponding
products in 61% yield (3ba). The Br and F
substitutions could survive well, giving 3ca and 3da
in 94% and 80% yields respectively. Ns-protected 2-
gem-dibromovinylaniline was also suitable for this
transformation, providing the desired product in 86%
yield (3ea). The tri-substituted alkenes were obtained
in good yields with excellent Z-selectivity (3fa and
3ga). Furthermore, substrates with different aryl
groups, such as Ph, 4-Me-Ph, 4-OMe-Ph, 4-tBu-Ph,
4-F-Ph and 4-Cl-Ph, provided the desired products in
good yields (3ha-3ma). We examined different
nitrogen-protected 2-gem-dibromovinylanilines, such
as N-Ac and N-Ms. Although the substrates 1n and
1o could give the corresponding products, the yields
were relatively low (3na: 35%; 3oa: 27%). In
addition, we tested the reaction of 2-gem-
dibromovinylphenols with 2a, but no desired product
was detected (3pa).
o
the temperature was lowered down to 100 C and 90
oC, the yield was decreased to 70% and 68% sharply
(Entry 16-17). High temperature reduced the yield
slightly (Entry 18). In addition, we examined
different metal catalysts, but these catalysts were
unsuitable for this transformation (Entry 19-20).
Finally, PdCl2(PPh3)2 (10 mol %) and tBuOLi (3.0
o
equiv.) in anhydrous dioxane at 110 C under N2
atmosphere were chosen to the standard conditions
(Entry 6).
With the optimized conditions in hand, the
substrate scope of N-tosylhydrazones was
investigated with 2-gem-dibromovinylanilines, and
the results were illustrated in Table 2. The
substitutions on phenyl group of N-tosylhydrazones
exhibited quite good tolerance. Both electron-
donating groups (m-OMe, p-OMe and -OCH2O-) and
electron-withdrawing groups (NO2 and CN) could
give the corresponding products in moderate to good
yields (3ab-3af). Not only fluro and chloro
substituted N-tosylhydrazones performed smoothly,
but also the substrates bearing m- and p-Br
substitutions could survive well under the standard
conditions, which could be readily transformed to
more valuable molecules via cross-coupling reactions
Table 3. Scope of gem-dibromovinylaniline derivatives.a
To show the synthetic versatility of this
methodology, several derivatizations of the products
were performed, and the results were shown in
Scheme 2. The oxidative cleavage of double bond
using PCC in DCE provided the corresponding
ketone in 84% yield.[12] On the other hand, the
oxidation with m-chloroperbenzoic acid could afford
hydroxyaldehyde in 69% yield.[13] Furthermore, the
products bearing bromo group could be
functionalized via coupling reactions, such as Suzuki-
Miyaura Coupling[14] and Sonogashira reaction[15]
,
providing the coupled derivatives in considerable
yields (80% and 97%).
Scheme 2. Derivatization of the 2-(1-phenylvinyl)-indole
derivatives.
aReaction conditions: 1 (0.3 mmol), 2a (0.6 mmol),
PdCl2(PPh3)2 (10 mol %), tBuOLi (0.9 mmol), dioxane (4
mL), N2, 1 h, 110 oC. bIsolated Yields.
3
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10.1002/adsc.201900230
Advanced Synthesis & Catalysis
complex 9 delivers the desired product 2-(1-
phenylvinyl)-indole 3aa.
To prove mechanism for the tandem reaction, we
carried out the control experiments using 2-Br indole
and N-tosylhydrazones under the standard conditions
(Scheme 4), and the desired product was obtained in
96% yield, indicating this mechanism for the tandem
reaction is convincing.
Scheme 4. Control experiment.
Based on the pioneering work and the results of
our study, a plausible mechanism including two
catalytic cycles for this tandem coupling reaction is
proposed in Scheme 3.[7][11][16] In cycle A, oxidative
addition of palladium(0) to gem-dibromovinylaniline
In summary, we have demonstrated a novel
palladium-catalyzed
phenylvinyl)-indoles
construction
utilizing
of
2-(1-
2-gem-
Scheme 3. Possible mechanism for the tandem reaction.
dibromovinylanilines and hydrazones. The indoles
bearing 1, 1-disubstituted alkene are furnished in one
pot. This tandem reaction could be finished in 1 hour,
and produce the corresponding products in high
yields with a broad substrate scope. Notably, this
methodology shows excellent Z-selectivity for tri-
substituted alkenes. Meanwhile, the products are
easily to be transformed into much more valuable
indole derivates. Further investigation to the
application of this strategy is underway.
Experimental Section
The 2-gem-dibromovinylanilines 1a (135 mg, 0.3
mmol, 1.0 equiv.) and N-tosylhydrazones 2a (174 mg,
0.6 mmol, 2.0 equiv.), tBuOLi (75 mg, 0.9 mmol, 3.0
equiv.) and PdCl2(PPh3)2 (22 mg, 0.03 mmol, 10
mol %) were added bottle. Adding 4 ml of anhydrous
dioxane into mixture with stirred, and reaction was
heated with 110℃ for 1 h. Lastly, the extractive was
extracted with EA and washed with water to get
crude product. The crude product was purified by
column chromatography (PE:EA, 30:1) to give the
3aa (98.6 mg, 0.264 mmol, 88 %) as white solid.
Acknowledgements
1a provides the stabilized palladacycle 4. The
deprotonation of aniline affords palladium complex 5,
which then produces compound 6 via reductive
elimination. Compound 6 could be observed by TLC
and GC-MS, and it disappears in the end of the
reaction. If the transformation was interrupted before
finished, compound 6 could be isolated and then be
converted to 3aa under the standard conditions in
high yield. In cycle B, the oxidative addition of
compound 6 to palladium(0) generates palladium(II)
complex 7, which subsequently forms alkylpalladium
species 9 with carbene intermediate generated from
diazo compound 8. Finally, the β-H elimination of
We thank the Shandong Provincial Natural Science Foundation
(ZR2018MB008, ZR2018MB012), the National Natural Science
Foundation of China (21671121), the Shandong Provincal Key
Research and Development program of China (2017GGX20131).
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Bognar, S. Lee, M. Lautens, Org. Lett. 2017, 19, 5058-
5061.
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10.1002/adsc.201900230
Advanced Synthesis & Catalysis
UPDATE
Pd-Catalyzed Tandem Coupling Reaction of 2-
gem-Dibromovinylanilines and N-Tosylhydrazones
to Construct 2-(1-phenylvinyl)-indoles
Adv. Synth. Catal. Year, Volume, Page – Page
Jian Song,a Xiaochen Chi,a Long Meng,a Pingping
Zhao,b Fenggang Sun,a Daopeng Zhang,a Luyang
Jiao,a Qing Liu,a Yunhui Donga and Hui Liua,*
7
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