Tandem C-C/C-N Formation via Palladium-Catalyzed C-H Activation/Styrenation and Sequential Annulation of O-Methylketoxime with Styrenes

Article abstract of DOI:10.1021/acs.orglett.9b00702

A novel route for tandem C-C/C-N formation via palladium-catalyzed C-H activation/styrenation and annulation of O-methylketoxime with styrenes to synthesize benzothienopyridines and benzofuropyridines has been developed. Furthermore, the intermolecular alkenylation of the ketoxime with acrylates produces 3-alkenyl O-methylketoximes in good to excellent yields. The method features mild reaction conditions and good functional group tolerance, providing a direct approach for the preparation of fused heterocycles.

Full text of DOI:10.1021/acs.orglett.9b00702

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Tandem C−C/C−N Formation via Palladium-Catalyzed C−H
Activation/Styrenation and Sequential Annulation of
O‑Methylketoxime with Styrenes
Xiaopan Fu,† Jinyue Yang,† Kezuan Deng,† Lingyan Shao,† Chengcai Xia,*,‡ and Yafei Ji*,†
†School of Pharmacy, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China
‡Pharmacy College, Shandong First Medical University & Shandong Academy of Medical Sciences, 619 Changcheng Road, Taian
271016, P. R. China
S
* Supporting Information
ABSTRACT: A novel route for tandem C−C/C−N formation via palladium-catalyzed C−H activation/styrenation and
annulation of O-methylketoxime with styrenes to synthesize benzothienopyridines and benzofuropyridines has been developed.
Furthermore, the intermolecular alkenylation of the ketoxime with acrylates produces 3-alkenyl O-methylketoximes in good to
excellent yields. The method features mild reaction conditions and good functional group tolerance, providing a direct approach
for the preparation of fused heterocycles.
ver the past decades, the directing group protocol of
Otransition-metal-catalyzed C−H bond functionalization
Scheme 1. O-Methyloxime-Directed Oxidative Heck
Reaction
has become a powerful method for C−C and C−X bond
formation reactions.1 The value of this strategy achieves high
regio- and stereoselectivity in terms of atom- and step-
economic nature. As one of the most widely used protocols,
the oxidative functionalizations of alkenes have attracted more
attention in recent years.2 Ortho alkenylation with the
assistance of various directing groups such as carboxyl,3
amide,4 ester,5 pyridine,6 and others7 has been extensively
reported.
Oxime ethers possess an excellent directing ability for C−H
bond activation.8 Previous works have demonstrated palla-
dium-catalyzed oxime-ether-directed ortho C(sp2)−H func-
tionalization,9 such as arylation,9a,c acylation,9b acyloxyla-
tion,9d,h alkoxylation,9h and hydroxylation.9g Aside from the
above functionalized reactions, the alkenylation assisted by
oxime ether has not been well explored. In 2011, Ellman and
co-workers9e reported the oxidative coupling of oxime ethers
with unactivated alkenes using a cationic Rh (III) catalyst.
Sun’s group9f demonstrated the ortho olefination of arylalde-
hyde oximes with activated olefins through a Pd(II) catalyst.
More recently, Jeganmohan’s group9i described the ruthenium-
catalyzed oxidant-free ortho alkenylation of aromatic amides,
ketoximes, and anilides with alkenes (Scheme 1). Despite
Received: February 24, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00702
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
tremendous progress, these reactions were limited in the
intermolecular alkenylation. Therefore, the functionalization of
oxime ethers, which can not only act as the directing group but
also be transformed into value-added molecules with
olefins,10−12 would be of great significance.
Herein, we report the palladium-catalyzed benzothieno- and
benzofuro-O-methyloxime-directed dual functionalization with
substituted styrenes, generating benzothienopyridines and
benzofuropyridines. In addition, the intermolecular alkenyla-
tion with acrylates produces 3-alkenyl products in good to
excellent yields. Benzothieno- and benzofuropyridines are
important fused heterocyclic compounds, playing an indis-
pensable role in diverse pharmaceuticals, natural products,
druglike scaffolds, and organic materials.13 Therefore, it is
highly desirable to develop a strategy using oxime ethers as the
directing group for the synthesis of benzothieno- and
benzofuropyridines through a tandem approach.14
to the fact that acidic medium was favorable for the reaction.9f
Among the oxidants tested, AgTFA was ideal (entries 5−8,
Table 1). Meanwhile, we also used other Pd catalysts,
including Pd(TFA)2, Pd(PPh3)2Cl2, Pd(CH3CN)2Cl2, and
PdCl2, revealing that Pd(OAc)2 was the best choice (entries
9−12, Table 1). Although amino acid was reported to be able
to promote C−H activations,15,16 it was not applicable to our
reaction system (entries 13−16, Table1). In contrast, the
simple organic base12a,b could improve the yield, and pyridine
was the best base for the reaction conversion (entries 17−20,
Table1). Considering the influence of the reaction temper-
ature, we observed that the yield was slightly decreased to 43%
under 90 °C (entry 17). When the amount of pyridine was
increased from 40% to 50%, the yield was not improved at all
(entry 21). In addition, the yield was slightly decreased under
N2 protection.
With the established conditions in hand, the scope of
substituted styrenes was then explored, and the representative
products are shown in Table 2. A variety of substituted
styrenes were subjected to the protocol ,favorably delivering
the desired products (3a−s). Of note, a remarkably broad
variety of styrenes with different groups including OCH3, Me,
i-Pr, F, Cl, Br, CF3, NO2, and naphthyl were transformed. The
styrene containing the more electron-withdrawing nitro group
at the para position was less reactive, which is possibly due to
the lower catalytic overturn (3h). Notably, electron-with-
drawing groups of styrenes provided better yields than those
bearing electron-donating groups (3i, 3j, 3l, 3m−o). Besides,
1-vinylnaphthalene and 2-vinylnaphthalene were eligible
substrates affording the moderate yields (3q, 3r). In addition,
trisubstituted partners could be also applied to the trans-
formation to deliver the desired product in moderate yields
(3s). Meanwhile, benzofuropyridines were also obtained under
this reaction system. The reactions of the electron-deficient
styrenes with (E)-1-(benzofuran-2-yl) ethan-1-one O-methyl
oxime led to the benzofuropyridine products (3t−w).
Unfortunately, we did not obtain benzofuropyridines with
electron-donating styrenes. In addition, the 1-(1H-indol-2-
yl)ethan-1-one O-methyl oxime was not reacted with styrene
(3x). The protectd 1-(1H-indol-2-yl)ethan-1-one gave a
moderate yield of 40% (3y). 3-Acetylbenzthiophene O-methyl
oxime could afford a better yield of 49% (3z). Other
unactivated alkenes such as n-heptene and cyclohexene were
not suitable for the reaction (3A, 3B).
Then the scope of acrylates as the coupling partners with 1a
was explored, and the 3-alkenylbenzothieno-O-methyloximes
instead of cycloaddition products were produced. Therein, a
series of acrylates was subjected to the protocol to give the
corresponding products in 61−84% yields (5a−f) when we
changed the additive from pyridine to the amino acid and
reduced the temperature to 90 °C. Meanwhile, it was found
that when (E)-1-(benzofuran-2-yl) ethan-1-one O-methyl
oxime was used as the reactant, the yield was significantly
improved up to 92%. However, only a trace amount of product
was delivered while using butyl acrylate, due to steric
hindrance (see the SI).
As an initial experiment, we performed the reaction of the
model substrate 1a and 4-methylstyrene 2a in the presence of
Pd(OAc)2 (10 mol %) as the catalyst, AgTFA (1.5 equiv) as
the oxidant, and N-Ac-Val-OH as the additive in AcOH at 110
°C (Table1). Surprisingly, the alkenylation−cyclization prod-
uct 3a was isolated in 35% yield, and no alkenylation product
was observed (Table 1, entry 1). Inspired by the results, we
screened different solvents and found that AcOH was a
suitable solvent (entries 1−4, Table 1), which is possibly due
a
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst
Pd(OAc)2
oxidant
additive
yield (%)
1
2
3
4
5
6
7
8
AgTFA
AgTFA
AgTFA
AgTFA
AgOAc
Ag2CO3
Ag3PO4
Ag2O
AgTFA
AgTFA
AgTFA
AgTFA
AgTFA
AgTFA
AgTFA
AgTFA
AgTFA
AgTFA
AgTFA
AgTFA
AgTFA
Ac-Val-OH
Ac-Val-OH
Ac-Val-OH
Ac-Val-OH
Ac-Val-OH
Ac-Val-OH
Ac-Val-OH
Ac-Val-OH
Ac-Val-OH
Ac-Val-OH
Ac-Val-OH
Ac-Val-OH
Glycine
35
c
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(TFA)2
Pd(PPh3)2Cl2
Pd(CH3CN)2Cl2
PdCl2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
25
10
d
e
0
23
15
9
3
9
31
29
26
21
35
32
22
10
11
12
13
14
15
16
17
18
19
20
21
Ac-Gly-OH
PivOH
<10
f
pyridine
NaOAc
t-BuOK
Et3N
53 (43 )
44
42
22
In addition, the intermolecular competition experi-
ments4b,d,f−h were carried out to probe the electronic effect.
When the mixture of electronically biased styrene 2d and 2b
(1:1) was treated with 1a, the yield of 3d was about 8.3 times
more than that of 3b under the standard reaction conditions
(Scheme 2), showing electron-deficient alkenes to be
preferentially functionalized, which can be attributed to an
g
h
i
pyridine
69 (68, , 68 )
a
Reaction conditions: 1a (0.3 mmol), 2a (0.6 mmol), catalyst (10
mol %), oxidant (1.5 equiv), additive (20 mol %), AcOH (2 mL), 110
b
c
d
°C, 20 h. Isolated yield of 3a. DCE (2 mL). DMSO (2 mL).
e
f
g
h
Dioxane (2 mL). 90 °C. Pyridine (40 mol %). Pyridine (50 mol
i
%). N2
B
DOI: 10.1021/acs.orglett.9b00702
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
electron-donating group substituted on the stryenes being less
active toward formation of the cyclopalladated intermediate.9f
In addition, the reaction on gram scale has also been
completed in good yield (see the SI).
Table 2. Substrate Scope of the Styrenes
To obtain further mechanistic insight, we performed some
control experiments with isotopically labeled solvents. First,
when 2-acetylbenzothieno oximes (1a) was exposed to the
standard conditions in AcOD, deuterium incorporation was
observed at the 3-position and the methyl of 2-acetylbenzo-
thieno analogue 1a (Scheme 3A). In contrast, when the same
Scheme 3. Deuterium Incorporation Studies
reaction was performed with the styrene 2d, nearly 90% of the
deuterium incorporation at the methyl of 3d signals a
deprotonation step. No other changes of deuterium incorpo-
ration at the methyl group as well as the reduction to 15% at
the 3-position in unreacted 1a are observed (Scheme 3B).
These results suggest that the first step of the reaction is a
reversible C−H activation at the 3-position of 1a,10g,h and the
deprotonation at the methyl of 3d is favorable after the first
irreversible step of the reaction.
Based on the mechanistic experiments described above and
relevant literature reports,9f,12 a plausible mechanism for this
reaction is depicted in Scheme 4. First, the coordination of the
nitrogen atom in compound 1a with Pd (II) species triggers
reversible cyclopalladation to form a five-membered cyclo-
palladated (II) intermediate A through a concerted metal-
ation−deprotonation pathway9f,18 or an agostic intermediate,17
Scheme 4. Proposed Mechanism
a
Reaction conditions: 1a (0.3 mmol), 2 (0.6 mmol), Pd(OAc)2 (10
mol %), AgTFA (1.5 equiv), pyridine (40 mol %), AcOH (2 mL), 110
b
°C, 20 h. Isolated yield of 3.
Scheme 2. Competition Experiment
C
DOI: 10.1021/acs.orglett.9b00702
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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In summary, we have developed a novel method for tandem
C−C/C−N formation via palladium-catalyzed C−H activa-
tion/styrenation and cyclization of ketoxime with styrenes to
synthesize benzothienopyridines and benzofuropyridines, and
the intermolecular alkenylation of the ketoxime with acrylates
forms 3-alkenyl O-methylketoximes in good to excellent yields.
This method is anticipated to construct structurally diversified
benzothienopyridines and benzofuropyridines for the screening
of potential pharmaceuticals in the future.
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.9b00702.
Experimental procedures and spectral data (PDF)
Accession Codes
CCDC 1887152 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.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: jyf@ecust.edu.cn.
*E-mail: xiachc@163.com.
ORCID
Chengcai Xia: 0000-0002-1943-5720
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We gratefully thank the National Natural Science Foundation
of China (Project Nos. 21676088 and 21476074) for financial
support.
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DOI: 10.1021/acs.orglett.9b00702
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