Palladium-catalyzed oxidative cyclization of aniline-tethered alkylidenecyclopropanes with O2: a facile protocol to selectively synthesize 2- and 3-vinylindoles

Article abstract of DOI:10.1039/c6cc08731k

A novel palladium-catalyzed oxidative cyclization of aniline-tethered alkylidenecyclopropanes using molecular oxygen as the terminal oxidant through β-carbon elimination of aminopalladation intermediates is disclosed. The reaction opens up an effective way to obtain a series of 2- and 3-vinylindoles which are important synthetic intermediates in many natural indole derivatives.

Full text of DOI:10.1039/c6cc08731k

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Palladium-catalyzed oxidative cyclization of
aniline-tethered alkylidenecyclopropanes with O2:
a facile protocol to selectively synthesize 2- and
3-vinylindoles†
Cite this:DOI: 10.1039/c6cc08731k
Received 31st October 2016,
Accepted 25th November 2016
Bo Cao,a Marwan Simaan,b Ilan Marek,b Yin Wei*c and Min Shi*ac
DOI: 10.1039/c6cc08731k
www.rsc.org/chemcomm
A novel palladium-catalyzed oxidative cyclization of aniline-tethered
alkylidenecyclopropanes using molecular oxygen as the terminal
oxidant through b-carbon elimination of aminopalladation inter-
mediates is disclosed. The reaction opens up an effective way to
obtain a series of 2- and 3-vinylindoles which are important synthetic
intermediates in many natural indole derivatives.
Indole derivatives are important structural motifs in many natural
products and pharmaceuticals which exhibit a wide range of
promising biological activities.1 Moreover, 2- and 3-vinylindoles
are key intermediates for the synthesis of biologically interesting
polycyclic products.2 During the past few decades, great efforts
have been devoted to the development of efficient and practical
strategies for the synthesis of these synthetic intermediates.3–7
Although vinylindoles could be accessed through a variety of
reactions such as Wittig,3 cross-coupling,4 SN20-type,5 elimination6
Scheme 1 Pd-catalyzed aza-Wacker-type cycloaddition for the synthesis
of indole.
or others,7 current synthetic methodologies lack substrate diversity the indole structures: (a) using terminal alkenes followed by a
as functional indoles such as formylindole or haloindole or related b-hydride elimination (Scheme 1, eqn (a))8,9 and (b) using aryl
species, possessing a leaving group, require an advanced prepara- substituted alkenes followed by an 1,2-aryl migratory reaction of the
tion. Therefore, the development of novel strategies leading to a aminopalladated intermediate (Scheme 1, eqn (b)).10 Inspired by
broad range of 2- and 3-substituted vinyl-indoles is still an on-going these methodologies, and with the hope to construct at the same
challenge that one needs to address.
time the indole framework and the vinyl group, we hypothesized
Palladium-catalyzed aza-Wacker-type cyclization has been proved that the aminopalladated intermediate may undergo a b-carbon
to be an effective approach for the preparation of indoles.8 In this and a b-hydride eliminations continuously to form the required
context, two distinct strategies have been explored to establish vinylindole in a single-pot operation bypassing the abovemen-
tioned two-step pathway. Based on this hypothesis and our ongoing
efforts to explore novel synthetic routes to indole-fused scaffolds,11
we decided to prepare ortho-aminoaryl-tethered alkylidenecyclo-
propanes from the important building blocks alkylidenecyclo-
propanes (ACPs),12 with aniline.13 Fortunately, we were delighted to
observe that these substrates could undergo the above proposed
cyclization to selectively give the desired 2- or 3-vinylindoles as
described below (Scheme 1, eqn (c)).
We initially investigated the reaction of 1a, as a model substrate,
by employing Pd(OAc)2 as a palladium source and K2CO3 as an
additive in toluene at 100 1C under an atmosphere of O2 (balloon).
Although in low yield (28%), we were pleased to observe that
the reaction proceeded as expected to give 2-vinylindole 2a
a Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
School of Chemistry & Molecular Engineering, East China University of Science
and Technology, Meilong Road No. 130, Shanghai 200237, China
b The Mallat Family Laboratory of Organic Chemistry, Schulich Faculty of
Chemistry and Lise Meitner-Minerva Center for Computational Quantum
Chemistry, Technion-Israel Institute of Technology, Technion City, Haifa 32000,
Israel
c State Key Laboratory of Organometallic Chemistry, University of Chinese Academy
of Sciences, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032, China. E-mail: weiyin@mail.sioc.ac.cn,
mshi@mail.sioc.ac.cn
† Electronic supplementary information (ESI) available: Experimental procedures and
characterization data of new compounds. CCDC 1491285. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c6cc08731k
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Table 1 Optimization of conditions for the Pd-catalyzed oxidative cycli-
zation of 1a
Additive
(equiv.)
Yieldb
Solvent Oxidant (%)
Entrya Catalyst
1
2
3
4
Pd(OAc)2
K2CO3 (2.0) Toluene O2
K2CO3 (2.0) Toluene O2
K2CO3 (2.0) Toluene O2
K2CO3 (2.0) Toluene O2
K2CO3 (2.0) Toluene O2
KaCO3 (2.0) Toluene BQ
K2CO3 (2.0) Toluene CuCl2 13
K2CO3 (2.0) Toluene AgOAc 13
KHCO3 (1.0) Toluene O2
KHCO3 (1.0) Toluene O2
28 (24)c
8
0
12
39c
19
Pd2(dba)3ꢀCHCl3
PdCl2
Pd(dppb)Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
PdIPrCl2
Pd[P(4-MeOC6H4)3]2Cl2 KHCO3 (1.0) Toluene O2
Pd[P(4-FC6H4)3]2Cl2 KHCO3 (1.0) Toluene O2
Pd[P(4-CF3C6H4)3]2Cl2 KHCO3 (1.0) Toluene O2
Pd[P(C6F5)3]2Cl2 KHCO3 (1.0) Toluene O2
Pd[P(4-CF3C6H4)3]2Cl2 KHCO3 (1.0) Toluene Air
5
6d
7e
8f
9
55
38
52
10
11
12
13
14
15g
64
74 (63)c
8
50
a
Reactions were performed with 1a (0.2 mmol), additive and 10 mol% of
catalyst in solvent (2.0 mL) under O2 (1 atm; balloon) at 100 1C in 12 h.
Scheme 2 Substrate scope for the synthesis of 2-vinylindoles 2.a
b
1
Yields were determined by H NMR using 1,3,5-trimethoxybenzene as
c
d
an internal standard. Yield of the isolated products. Using 2.0 equiv.
e
f
BQ under Ar. Using 2.0 equiv. CuCl2 under Ar. Using 2.0 equiv. AgOAc
under Ar. The reaction was performed in air.
g
of this reaction, we prepared functionalized substrates with a
heteroaromatic group (furan 1n and thiophene 1o) or with an
(Table 1, entry 1). We then further screened different Pd-catalysts, alkyl chain (methyl group 1p and n-butyl group 1q). The experi-
such as Pd2(dba)3ꢀCHCl3, PdCl2, Pd(dppb)Cl2, Pd(PPh3)2Cl2 mental results showed that 1o and 1q were suitable substrates for
(Table 1, entries 2–5), and various typical aza-Wacker oxidants this reaction under the current reaction conditions, giving the
including BQ, CuCl2 and AgOAc (Table 1, entries 6–8), but none corresponding products 2o and 2q in 40% and 45% yields,
of these conditions could offer a yield better than 39% (Table 1, respectively. However, the yields of 2n and 2p were low, probably
entry 5). Additional screening of the nature of solvents and due to the formation of byproducts (for detailed information,
additives (for further details, see the ESI†) revealed that the see the ESI†). Other N-sulfonyl protecting groups such as
presence of 1.0 equiv. of KHCO3 in toluene provided the best 4-bromobenzene sulfonyl (Bs) 1r, 4-nitrobenzene sulfonyl (Ns)
result (55%, Table 1, entry 9). Next, the nature of ligands on 1s and the N-carbonyl protecting group (Ac) 1t were compatible
palladium was tested, such as N-heterocyclic carbene and under these reaction conditions. To our delight, when using the
phosphine ligands with either electron-donating or electron- substrate 1u without the protecting group on the nitrogen
withdrawing substituents, and we were pleased to observe the atom, the reaction still proceeded smoothly giving 2u in 31%
formation of 2a in 74% yield when the complex Pd[P(4-CF3C6H4)3]2Cl2 yield. We also got the product 2u through detosylation of 2a
was used in the presence of KHCO3 (1.0 equiv.) in toluene under in 81% yield.
molecular oxygen at 100 1C within 12 h (Table 1, entry 13, for further
details, see the ESI†).
However, when using the substrate 3a, which has no sub-
stituent on the alkylidene moiety, the reaction proceeded to
Under the optimized conditions, we next surveyed the substrate furnish the 3-vinylindole 4a in 48% yield (Table 2, entry 5)
scope of this reaction and the results are shown in Scheme 2. We rather than the 2-vinylindole. Taking into account the lessons
first examined the effect of the position of substituents on the from the previous transformation (1 into 2), we reinvestigated
benzene ring. As for substrates 1b–1e, the reactions proceeded this reaction in detail. After screening the ligands of the
smoothly to furnish the corresponding 2-vinylindoles 2b–2e in catalysts and the additives, we found that when 10 mol% of
yields ranging from 56% to 65% regardless of the positions of the Pd[P(4-MeOC6H4)3]2Cl2 and 1.0 equiv. of KHCO3 were used in
substituents (ortho-, meta-, or para-position on the benzene ring). toluene under O2 at 100 1C, the yield of 4a could reach 60%
Then, the electronic effect at the para-position of the benzene (Table 2, entry 2).
ring was examined. Substrates with both electron-donating and
With these optimized reaction conditions in hand, the substrate
electron-withdrawing groups could afford the desired products scope for 3 was explored and the results are shown in Scheme 3.
2f–2k in yields ranging from 33% to 70%. The structure of 2f has Substrates 3 having both electron-donating and electron-withdrawing
been confirmed by X-ray diffraction analysis.14 When substrates 1l substituents (R1) at different positions of the aromatic ring were
(R1 = Ph and R2 = 5-Cl) and 1m (R1 = 2-ClC6H4 and R2 = 5-Cl) were tolerated, providing the 3-vinylindole derivatives (4b–4g) in moderate
tested, the corresponding products 2l and 2m were obtained in yields ranging from 39% to 58%. In addition, when methyl sulfonyl
59% and 60% yields, respectively. To further investigate the scope and 2,4-dimethoxybenzene sulfonyl were used as the N-protecting
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Table 2 Optimization of conditions for the Pd-catalyzed oxidative cycli-
zation of 3a
Additive
(1.0 equiv.) Solvent Oxidant (%)
Yieldb
Entrya Catalyst
1
2
3
4
5
6
7
8
Pd(PPh3)2Cl2
Pd[P(4-MeOC6H4)3]2Cl2 KHCO3
PdIPrCl2
Pd[P(4-FC6H4)3]2Cl2
Pd[P(4-CF3C6H4)3]2Cl2 KHCO3
Pd[P(C6F5)3]2Cl2 KHCO3
Pd[P(4-MeOC6H4)3]2Cl2 NaHCO3
Pd[P(4-MeOC6H4)3]2Cl2 Na3PO4
Pd[P(4-MeOC6H4)3]2Cl2 NaOAc
Pd[P(4-MeOC6H4)3]2Cl2 KHCO3
KHCO3
Toluene O2
Toluene O2
Toluene O2
Toluene O2
Toluene O2
Toluene O2
Toluene O2
Toluene O2
Toluene O2
Toluene O2
41
60 (58)c
24
KHCO3
KHCO3
56
48
17
39
50
8
45
9
10d
Scheme 5 Proposed mechanism.
a
Reactions were performed with 1a (0.2 mmol), additive and 10 mol% of
catalyst in solvent (2.0 mL) under O2 (1 atm; balloon) at 100 1C in 12 h.
b
1
Yields were determined by H NMR using 1,3,5-trimethoxybenzene as
c
d
an internal standard. Yield of the isolated products. Using 2.0 equiv.
KHCO3.
moiety was therefore transferred to the 2-position of the
3-vinylindole, shedding some light on the reaction mechanism.
On the basis of previous reports8,9 and our control experi-
ments, a plausible mechanism15 for this reaction is outlined in
Scheme 5. First, the electrophilic Pd(II) species coordinated
with the substrate 1 or 3 activates the double bond of the
ACPs to form complexes A or D. If R a H, A could undergo a
5-endo-aminopalladation to form B. Then following a b-carbon
elimination, the Pd-alkyl intermediate C is obtained. Finally,
the intermediate C undergoes a b-hydride elimination to form
the product 2. The reduced Pd catalyst was then oxidized
directly by molecular oxygen to regenerate the Pd(II) catalyst.
However, if R = H, complex D would undergo a 4-exo-
aminopalladation to yield E. Following a b-aryl elimination,
the Pd-aryl intermediate F would be formed and this process
transfers the alkylidene cyclopropane to the aniline nitrogen
and places Pd on the ortho-position of the aniline. Next, F
undergoes a 5-endo-carbonpalladation to generate G. Subsequently,
G could undergo a b-carbon elimination and a b-hydride elimina-
tion to form the product 4a. Another possible mechanism for the
generation of 4a is outlined in the ESI.†
In conclusion, a facile and versatile palladium-catalyzed
oxidative cyclization of aniline-tethered alkylidenecyclopropanes
with molecular oxygen could selectively provide either 2- or
3-vinylindoles in moderate to good yields. Different reaction
mechanisms have also been proposed on the basis of control
experiments. Further investigations on expanding the scope of
this reaction toward a variety of novel and potentially useful
polycyclic indole derivatives as well as the application of this
protocol to natural product synthesis are in progress.
Scheme 3 Substrate scope for the synthesis of 3-vinylindoles 4.a
groups, the reactions took place efficiently to afford the corresponding
3-vinylindoles 4h and 4i in 55% and 48% yields, respectively.
To further investigate the reaction mechanism, two control
experiments were performed and the results are shown in
Scheme 4. 2-Vinylindole 5 could not be transformed into
3-vinylindole 6 under our standard reaction conditions, suggesting
that 5 was not an intermediate in this reaction. Subsequently, a
deuterium-labeling experiment was conducted to obtain more
information on the mechanism. The treatment of [D]-7 with
Pd[P(4-MeOC6H4)3]2Cl2 produced [D]-8 in 60% yield with 66%
D content. The deuterium atom at the double bond of the ACP
This work was supported by the Joint NSFC-ISF Research
Program, jointly funded by the National Natural Science
Foundation of China and the Israel Science Foundation. We
are also grateful for the financial support from the National
Basic Research Program of China ((973)-2015CB856603) and
the National Natural Science Foundation of China (20472096,
21372241, 21361140350, 20672127, 21421091, 21372250, 21121062,
21302203, 20732008 and 21572052).
Scheme 4 Control experiments to investigate the mechanism.
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14 The crystal data of 2f have been deposited in CCDC with number
1491285.
15 We appreciate the referee’s important suggestion for the generation
of product 4 during the peer-review process.
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