Nickel-Catalyzed Domino Heck Cyclization/Suzuki Coupling for the Synthesis of 3,3-Disubstituted Oxindoles

Article abstract of DOI:10.1021/acs.orglett.7b03713

The first nickel-catalyzed domino Heck cyclization/Suzuki coupling reaction for the synthesis of 3,3-disubstituted oxindoles bearing quaternary all-carbon centers is reported. A wide range of electrophiles, such as aryl iodides, bromides, triflates, and c

Full text of DOI:10.1021/acs.orglett.7b03713

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Nickel-Catalyzed Domino Heck Cyclization/Suzuki Coupling for the
Synthesis of 3,3-Disubstituted Oxindoles
Yuxiu Li, Kuai Wang, Yuanyuan Ping, Yifan Wang, and Wangqing Kong*
The Institute for Advanced Studies, Wuhan University, 299 Bayi Road, Wuchang District, Wuhan, Hubei 430072, P. R. China
S
* Supporting Information
ABSTRACT: The first nickel-catalyzed domino Heck cycliza-
tion/Suzuki coupling reaction for the synthesis of 3,3-
disubstituted oxindoles bearing quaternary all-carbon centers
is reported. A wide range of electrophiles, such as aryl iodides,
bromides, triflates, and chlorides, are all compatible with the
reaction conditions. Moreover, cheap aryl esters, which
undergo catalytic C−O bond cleavage, could also be employed
as electrophiles. The approach shows good yields and broad scope, complementing a more practical and sustainable alternative to
the conventional palladium-based analogues.
he intramolecular Heck cyclization has been intensely
Herein, we report our investigations of the first example of
Tinvestigated for decades and has been utilized for the nickel-catalyzed cyclizative cross coupling of aryl halides and
pivalates with boronic acids for the synthesis of 3,3-
disubstituted oxindoles through a domino Csp2−X cleavage/
Heck cyclization/Suzuki coupling process (Scheme 1, eq 2).
Our study commenced with the reaction of 2-(N-
methylmethacrylamido)phenyl trifluoromethanesulfonate 1a
and phenylboronic acid 2a (for the optimization of reaction
conditions, see Table S1). In the presence of Ni(cod)2 (10 mol
%), L1 (20 mol %), and K2CO3 (2 equiv), the desired oxindole
3aa could be isolated in 84% yield in toluene at 100 °C. When
an air-stable NiCl2dppp (10 mol %) complex and PPh3 (20 mol
%) were employed in place of Ni(cod)2 and the ligand, 74%
isolated yield of 3aa could also be obtained (Scheme 2).
synthesis of a range of natural products and pharmaceutically
active compounds bearing quaternary carbon centers.1
Although palladium-based catalysts are commonly used in
this reaction,2,3 considerable efforts have been made to replace
this highly expensive and scarce metal with nickel, which is a
low-cost, nontoxic, abundant, and sustainable element.4 In
addition, nickel is a facile oxidative addition on account of its
smaller size, allowing for the use of much less reactive
electrophiles, such as aryl chlorides, carbamates, esters, ethers,
and so on.5 Nickel catalyzed Heck reaction was known about as
early as the 1980s;6 however, Heck reactions with Ni are still
relatively rare and challenging.7,8
Recently, nickel-catalyzed intramolecular Heck cyclization of
aryl halides for construction of quaternary carbon centers has
been reported (Scheme 1, eq 1).9 In connection with our
interest in transition-metal-catalyzed domino processes,10 we
envisaged that the in situ generated σ-alkyl-Ni(II) intermediate
could directly undergo Suzuki coupling with boronic acids
without an external stoichiometric amount of reducing agent.11
None have yet achieved the generality, economy, and efficiency
of this domino process with catalytic amount of nickel.12
Scheme 2. Optimized Conditions for Domino Heck
Cyclization/Suzuki Coupling of Aryl Triflate
Scheme 1. Nickel-Catalyzed Domino Heck Cyclization
Reaction
With the optimized conditions in hand (Scheme 2), reaction
scope with regard to the structure of boronic acids was first
explored. Both electron-rich and electron-poor p-aryl boronic
acids could react with 1a to give the target products 3aa−ag
with good yields (Scheme 3). Notably, functional groups (Cl,
OMe) were found to be compatible with this transformation,
providing an opportunity to further functionalize these
obtained products. The steric hindrance did not affect the
Received: November 29, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03713
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Scope for Boronic Acids
Scheme 4. Scope for Aryl Triflates
a
b
2a (3 equiv). The reaction was run using NiCl2dppp (10 mol %)
and PPh3 (20 mol %).
a
NiCl2dppp (10 mol %), PPh3 (20 mol %), and 2c (3 equiv) at 90 °C.
b
NiCl2dppp (10 mol %), PPh3 (20 mol %), and 2g (5 equiv) at 90 °C.
The effects of electrophiles were investigated subsequently
(Scheme 5). The aryl triflate, chloride (4), bromide (5), and
c
d
NiCl2dppp (10 mol %) and PPh3 (20 mol %). Reaction performed
at 120 °C.
Scheme 5. Effects of the Electrophiles
efficiency, and all meta- and ortho-substituted aryl boronic acids
(3ah−aj) showed comparable reactivities. The fluoro (3ad and
3ai), trifluoromethyl (3ae), trifluoromethoxyl (3af), and ketal
(3ak) substituents were all tolerated. Both 2- and 1-
naphthylboronic acids could be employed to provide 3al and
3am in 96% and 91% yield, respectively. Moreover, substrate
scope toward (hetero)aryl boronic acids was also screened. As
shown by the formation of 3an−at, thiophene, benzofuran,
benzothiophene, dibenzofuran, dibenzothiophene, and carba-
zole heterocycles were transformed into the desired products
successfully with satisfied yields. These results demonstrate the
potential utility of this methodology for the synthesis of
medicinally relevant compounds are feasible.13 Excitingly, the
trans-β-styreneboronic acid 2u could also be efficiently
transformed into the desired product 3au in 60% yield at 120
°C.
Subsequently, the scope of the current method with respect
to aryl triflates was evaluated and found to proceed smoothly in
all the cases (Scheme 4). The domino reaction between N-
benzyl triflate 1b and boronic acid 2a proceeded efficiently to
provide 3ba in 91% yield. As the N-benzyl is easily removed, it
constitutes a route to N-H oxindoles. The influence of the Cα
substituents (R2) of the acrylamide double bond on the
reaction outcome was tested. Isopropyl, benzyl, and n-hexyl
substituents were readily transformed into the desired
oxindoles 3ca−ea in good yields. The phenyl at Cα position
of the double bond was also found to be effectively leading to
the desired oxindole 3fa in 66% yield. Finally, the influence of
substitution pattern in the aniline part was evaluated. Para-,
meta-, or ortho-substituted aryl triflates were well tolerated,
affording the corresponding oxindoles 3ga−ja in 79−99%
yields. More importantly, the reaction was successfully applied
to the synthesis of 5,6-benzoxindole 3ka, 6,7-benzoxindole 3la,
and 4,5-benzoxindole 3ma.
iodide (6) are competent electrophiles to afford the desired
product 3aa in 60−84% yields, thus extending the scope
compared to Desrosiers’ report.9b
Encouraged by these results, we decided to further explore
the scope of aryl electrophiles. In the past decade, C−O
electrophiles have received considerable attention in the cross-
coupling reactions due to their lack of toxicity, stability and
readily availability compared with commonly used aryl halides.
Among the C−O electrophiles, aryl sulfonates have been widely
employed due to their high reactivities. Nevertheless, relatively
high price and producing of stoichiometric amounts of sulfur-
containing waste have limited the application in chemical
industry. Use aryl esters as coupling partners could be a
synthetically ideal route that is cheaper and more environ-
mentally friendly. However, employing aryl esters as electro-
philes via catalytic C−O bond-cleavage is still a formidable
challenge due to the requirement of higher activation energy to
cleavage the inert Csp2−O bond.4,5 Despite the progress made,
the overwhelming majority of C−O bond activation reactions
are still restricted to the direct cross-coupling for single C−C
bond or C-heteroatom bond formation.4,5,8,14 Indeed, the
development of catalytic domino reaction initiated by C−O
bond activation, involving multibond formation in one pot, still
remains an elusive task.
2-(N-methylmethacrylamido)phenyl pivalates 7a and 2a
were chosen as model substrates to evaluate the feasibility of
reaction design (Table 1). Using previously optimized
conditions (Scheme 2), however, failed to furnish any of the
desired product (Table 1, entries 1 and 2). The choice of a
B
DOI: 10.1021/acs.orglett.7b03713
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of the Reaction Conditions
Scheme 6. Scope for Domino Csp2−O Bond Cleavage/Heck
Cyclization/Suzuki Coupling Reaction
entry
catalyst
ligand
base (equiv)
yield (%) of 3aa
1
NiCl2dppp
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
NiCl2
PPh3
L1
K2CO3 (2)
0
0
2
K2CO3 (2)
3
L1
t-BuOLi (4)
t-BuONa (4)
t-BuOK (4)
t-BuONa (4)
t-BuONa (4)
t-BuONa (4)
t-BuONa (4)
t-BuONa (4)
t-BuONa (4)
t-BuONa (4)
t-BuONa (4)
t-BuONa (4)
t-BuONa (4)
t-BuONa (4)
t-BuONa (2.5)
t-BuONa (2.5)
0
4
L1
15
0
5
L1
6
PCy3
L2
11
trace
11
13
25
0
7
8
L3
a
9
L4
2a (3 equiv).
10
11
12
13
14
15
16
17
18
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
we decided to screen a series of chiral ligands in an attempt to
render the reaction asymmetric (for details, see Table S2).
Unfortunately, all attempts with aryl triflate 1a afforded racemic
product. When aryl chloride 4 was treated with phosphor-
amidite ligand, oxindole 3aa was obtained in 17% yield with
64:36 er. Though the result is far from satisfactory, it might
suggest a Ni(0)/Ni(II) or Ni(I)/Ni(III) catalytic cycle instead
of a radical-induced pathway.
In summary, an efficient strategy for the first nickel-catalyzed
domino Heck cyclization/Suzuki coupling for the construction
of 3,3-disubstituted oxindoles bearing quaternary carbon
centers has been presented. The cheap and traditionally
unreactive aryl esters via catalytic Csp2−O bond cleavage
could also be employed in this domino process. This protocol is
distinguished by its wide substrate scope and diverse set of
substitution patterns, representing a more practical and
sustainable alternative to the conventional palladium-catalyzed
Heck cyclization reaction. Further investigations on the
asymmetric version and detailed mechanistic studies of this
transformation are currently underway in our laboratory.
NiBr2
0
Ni(acac)2
Ni(PPh3)2Cl2
Ni(PPh3)4
0
0
41
49
56
70
b
Ni(PPh3)4
b
Ni(PPh3)4
c
Ni(PPh3)4
a
Reaction conditions: 7a (0.1 mmol), 2a (0.25 mmol), [Ni] (10 mol
%), ligand (20 mol %), toluene (2.0 mL), sealed tube, 100 °C, 16 h.
Ni(PPh3)4 (15 mol %) and PPh3 (30 mol %). Ni(PPh3)4 (20 mol %)
b
c
and PPh3 (40 mol %).
proper base was crucial to this reaction. t-BuONa (4 equiv)
provided promising results (Table 1, entry 4), and no product
took place with t-BuOLi or t-BuOK. Subsequently, a survey of
the ligands was performed (Table 1, entries 6−10). To our
delight, the desired product 3aa could be obtained in 25% yield
when PPh3 was employed (Table 1, entry 10). Various nickel
complexes were tested (Table 1, entries 11−15), and
Ni(PPh3)4 was found to be the most effective, providing 3aa
in 41% yield (Table 1, entry 15). Further screening the reaction
conditions (Table 1, entries 16−18) allowed us to define the
following optimized conditions: Ni(PPh3)4 (20 mol %)/PPh3
(40 mol %) as catalyst ,and t-BuONa (2 equiv) as a base in
toluene at 100 °C. Under these conditions, oxindole 3aa was
isolated in 70% yield (Table 1, entry 18).
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.7b03713.
With the optimized conditions in hand (Table 1, entry 18),
the substrate scope and generality of our Ni-catalyzed domino
C−O bond activation/Heck cyclization/Suzuki coupling
reaction was examined (Scheme 6). Once again, various
substituted boronic acids and aryl pivalates all underwent the
domino process without event, and the desired oxindoles 3
were obtained in moderate to good yields. These results are
noteworthy, taking into account the higher activation energy
required for Csp2−OPiv cleavage in the electron-rich o-
aminoaryl pivalates 7. Previous reports on Ni-catalyzed cross-
couplings of electron-rich phenyl pivalates usually resulted in a
reduced yield.7j,14 Aryl esters are unreactive with palladium
catalysis, which clearly manifests the superiority of the current
synthetic method.
Experimental procedures, characterization data, and
spectra data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wqkong@whu.edu.cn.
ORCID
Wangqing Kong: 0000-0003-3260-0097
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the “1000-Youth Talents Plan” and the
National Natural Science Foundation of China (No. 21702149)
■
Since Ni-catalyzed enantioselective intramolecular (reduc-
tive) Heck cyclization reactions were reported very recently,9c,d
C
DOI: 10.1021/acs.orglett.7b03713
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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facilities.
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