Domino Carbopalladation/C-H Functionalization Sequence: An Expedient Synthesis of Bis-Heteroaryls through Transient Alkyl/Vinyl-Palladium Species Capture

Article abstract of DOI:10.1002/chem.201503708

A microwave-assisted highly efficient intermolecular domino carbopalladation/C-H functionalization sequence has been developed to access bis-heteroaryl frameworks in a single operation. The reaction involves carbopalladation of the halogenated acrylamides

Full text of DOI:10.1002/chem.201503708

DOI: 10.1002/chem.201503708
Communication
&
Synthetic Methods
Domino Carbopalladation/CÀH Functionalization Sequence: An
Expedient Synthesis of Bis-Heteroaryls through Transient Alkyl/
Vinyl–Palladium Species Capture
Upendra K. Sharma,*[a] Nandini Sharma,[a] Yogesh Kumar,[a, b] Brajendra K. Singh,[b] and
Erik V. Van der Eycken*[a]
In this regard, considerable progress has been achieved to-
wards capturing of these s-alkyl/vinyl–palladium(II) intermedi-
ates by anionic, neutral, as well as organometallic species
(Scheme 1).[5] However, these intermediates are rarely inter-
Abstract: A microwave-assisted highly efficient inter-
molecular domino carbopalladation/CÀH functionalization
sequence has been developed to access bis-heteroaryl
frameworks in a single operation. The reaction involves
carbopalladation of the halogenated acrylamides or phe-
nylpropiolamides by the Pd(0) catalysis, followed by the
direct (hetero)arylation to give products with good to ex-
cellent yields. The synthetic utility of this method was also
extended towards the application of the Ugi-adduct as
the starting material.
The rapid generation of complex (bis)heterocyclic frameworks
has always been challenging and has received considerable at-
tention due to several inherent benefits.[1] Recent advances in
CÀH functionalization[2] of arenes and heteroarenes have
begun to alter the way organic chemists execute synthesis of
such complex structures. One such interesting approach is the
amalgamation of transition-metal-catalyzed CÀH bond func-
tionalization with another CÀC bond formation step. By trig-
gering such domino reactions with well-defined functionalities
in the structure of the starting material, these reactions in-
crease the efficiency and modularity of the synthetic protocol,
allowing the rapid generation of molecular diversity in a
minimum number of steps.[3] It has already been established
that the carbopalladation of the halogenated acrylamides or
phenylpropiolamides can generate the transient s-alkyl/vinyl–
palladium(II) intermediates, which are living species, and the
reaction is only terminated by exchanging the PdÀX bond
with an anion or nucleophile followed by reductive
elimination.[4]
Scheme 1. Cross-coupling reactions by vinyl- or alkylpalladium species.
cepted by the intermolecular CÀH functionalization.[5b] In the
course of our ongoing research on domino reactions[6] and the
direct CÀH functionalization[7] of heteroarenes, we wondered if
such transient vinyl- or alkylpalladium(II) species could activate
the sp2 CÀH bond of azoles in a domino process. The heteroar-
yl azole skeleton[8] is a key structural unit in a number of bio-
logically active compounds and finds profound applications in
medicinal chemistry as well as material sciences.[9] In recent
years, considerable progress has been achieved in the direct
arylation, alkenylation, and alkynylation of azoles using aryl
halides or pseudohalides as coupling partners.[10] However,
similar strategies for the construction of heteroaryl–heteroaryl
scaffolds, such as the oxindole–azole framework, remain far
less explored. Moreover, 3,3-oxindoles and 3-alkylidene-
oxindoles are known versatile compounds in terms of their
biological activity and synthetic applicability (Figure 1).[11]
[a] Dr. U. K. Sharma, Dr. N. Sharma,+ Y. Kumar,+ Prof.Dr. E. V. Van der Eycken
Laboratory for Organic & Microwave-Assisted Chemistry (LOMAC)
Department of Chemistry, University of Leuven (KU Leuven)
Celestijnenlaan 200F, 3001 Leuven (Belgium)
E-mail: usharma81@gmail.com
erik.vandereycken@chem.kuleuven.be
Homepage: http://chem.kuleuven.be/en/research/mds/lomac
[b] Y. Kumar,+ Dr. B. K. Singh
Bioorganic Lab, Department of Chemistry
University of Delhi, Delhi 110007 (India)
[+] Both authors contributed equally to this work.
To develop this domino protocol, the conditions must be
carefully selected to avoid the direct heteroarylation as an
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503708.
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the product were obtained, while no reaction was observed
with Et3N (entries 8–12). Similarly, no improvement in the yield
was observed upon changing the reaction time (entries 13 and
14). Follow-up reactions revealed that running the reaction in
acetonitrile at 110 8C provided the best yield (entries 15–18).
Shifting from the conventional heating to microwave irradia-
tion, considerably lowered the reaction time[12] from 12 h to
30 min without compromising the yield (entries 18–20). More-
over, decreasing the catalytic loading of [Pd(PPh3)4] to 5 mol%
still allowed the domino reaction to occur smoothly, providing
the desired product 3a in 97% isolated yield (entry 21). How-
ever, a further decrease of the catalyst to 3 mol% resulted in
a reduced yield (entry 22).
Figure 1. Important molecules containing the 3-vinylidene or 3,3-disubsti-
tuted oxindole framework.
unwanted side reaction. We started our investigations for the
synthesis of indolinone–azole framework by attempting the in-
termolecular cyclization using 1a, which was easily prepared
from 2-iodoaniline,[5d] and 2-phenyl-1,3,4-oxadiazole (2a) as
model substrates in toluene for 12 h at 1008C (Table 1).
[Pd(PPh3)4] proved to be a superior catalyst to Pd(OAc)2 for the
reaction (entries 1–5). The product yield was significantly im-
proved (89%; entry 6) when the temperature was increased to
110 8C; however, further rising the temperature resulted in a de-
creased yield (entry 7). In the presence of other inorganic
bases, such as tBuOLi, K2CO3, Na2CO3, and K3PO4, low yields of
With the optimized reaction conditions in hand (Table 1,
entry 21), we evaluated the scope of this methodology
(Table 2). Initially, the one-pot reaction was applied to a series
of 2-iodoanilines, all giving the corresponding products (3a–
3e) in good to excellent yields. Acrylamide (3 f), derived from
the atropic acid, gave a relatively low yield, which could be at-
tributed to the steric factors. Next, the influence of the azole
subunit on the reaction outcome was examined. Different
1,3,4-oxadiazoles with electron-donating- or -withdrawing
Table 2. Scope of domino carbopalladation/CÀH functionalization
sequence by an alkyl–Pd species.[a]
Table 1. Optimization of reaction conditions.[a]
Entry Catalyst
Ligand
PPh3
Base
Solvent
T [8C] Yield [%][b]
1
Pd(OAc)2
Cs2CO3 toluene 100
19
2
3
Pd(OAc)2
Pd(OAc)2
XantPhos Cs2CO3 toluene 100
DPEPhos Cs2CO3 toluene 100
40
33
4
5
6
7
8
9
10
11
12
13[c]
14[d]
15
16
17
18
19[e]
20[f]
Pd(OAc)2
DBPF
Cs2CO3 toluene 100
Cs2CO3 toluene 100
Cs2CO3 toluene 110
Cs2CO3 toluene 120
LiOtBu toluene 110
24
51
89
61
17
22
21
24
nd
67
80
87
74
39
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
K2CO3
toluene 110
Na2CO3 toluene 110
K3PO4
Et3N
toluene 110
toluene 110
Cs2CO3 toluene 110
Cs2CO3 toluene 110
Cs2CO3 o-xylene 110
Cs2CO3 DCE
110
110
110
110
110
110
110
Cs2CO3 DMSO
Cs2CO3 MeCN
Cs2CO3 MeCN
Cs2CO3 MeCN
Cs2CO3 MeCN
Cs2CO3 MeCN
100 (97)
100 (98)
100 (97)
100 (97)
51
21[f,g] [Pd(PPh3)4]
22[f,h] [Pd(PPh3)4]
[a] All reactions were performed with 1a (0.2 mmol), 2a (0.22 mmol), cat-
alyst (10 mol%), ligand (20 mol%), and base (0.4 mmol) in solvent (1 mL)
at the indicated temperature for 12 h under N2 atmosphere. [b] Yield
determined by 1H NMR using 3,4,6-trimethoxy benzaldehyde as internal
standard; isolated yields in parenthesis. [c] Reaction run for 10 h. [d] Reac-
tion run for 16 h. [e] Microwave irradiation for 1 h at 100 W maximum
power. [f] Microwave irradiation for 30 min. [g] Catalyst (5 mol%).
[h] Catalyst (3 mol%).
[a] Reaction conditions: 1 (0.2 mmol), 2 (1.1 equiv), [Pd(PPh3)4] (5 mol%),
and Cs2CO3 (2 equiv) in MeCN (1.5 mL) under N2 atmosphere at 1108C for
30 min under microwave irradiation at 100 W maximum power.
[b] N-(2-bromophenyl)-N-methylmethacrylamide used as substrate.
[c] Conventional reaction for 12 h.
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groups on the aryl ring were well tolerated (3g–3k). Replace-
ment of the phenyl ring of the oxadiazole with a benzyl group
(3l) or 3-pyridyl ring (3m) led to the desired products in satis-
factory yields. Also, the alkyl (3n) or benzyl substitutions (3o
and 3p) on the nitrogen atom were well tolerated. However,
the electron-withdrawing tosyl group (3q) provided only 12%
yield, while the unsubstituted indole was incompatible under
these conditions (3r). To further demonstrate the catalytic effi-
cacy, we expanded the scope of this protocol to other azole
derivatives, resulting in good to moderate yields of the prod-
ucts (3s–3w). Also, the acrylamide, with a bromo group in-
stead of iodo, worked equally well under these conditions
(3x). It is worth noting that our efforts for the ring expansion
by employing the higher homologues of acrylamide 1 failed
(3y and 3z) and heteroarylated products, resulted by the com-
petitive direct arylation reaction, were observed instead. This
could be attributed to the more difficult formation of seven- or
eight-membered palladacycle intermediates.
of the conventional heating proved better than the microwave
irradiation for this transformation. After careful examination, it
was ascertained that the best conditions for this domino reac-
tion were Pd(OAc)2 (10 mol%), PPh3 (20 mol%), and Cs2CO3
(2 equiv) in toluene at 1008C for 24 h under N2 (see Table S1 in
the Supporting information) that provided the desired product
9a in 81% yield as a single isomer. The E-configuration of the
obtained isomer was deduced from NOESY experiments (for
9b and 9j, see the Supporting Information). The observed
double-bond geometry is due to the regioselective syn inser-
tion of the arylpalladium species to the triple bond of the pro-
piolamide. The optimized reaction conditions were applied to
evaluate the scope of the method (Table 3). To our satisfaction,
Table 3. Scope for domino carbopalladation/CÀH functionalization
sequence by a vinyl–Pd species.[a]
To enhance the practical utility of our methodology, com-
pounds 4a–c were synthesized by the Ugi four-component
reaction (Ugi-4CR)[13] and subjected to the optimized reaction
conditions, which led to the facile synthesis of indolinones
5a–c in good yields as diastereomeric mixtures (Scheme 2).
Scheme 2. Expansion of the reaction scope to post-Ugi tandem-domino
carbopalladation/CÀH functionalization.
[a] Reaction conditions: all reactions were performed with 8 (0.2 mmol), 2
(1.1 equiv), Pd(OAc)2 (10 mol%), PPh3 (20 mol%), and Cs2CO3 (2 equiv) in
toluene (1.5 mL) under N2 atmosphere at 1008C for 24 h.
Further, the above protocol was successfully extended to
a domino reaction starting from 1-(8-iodo-3,4-dihydroquinolin-
1-yl)-2-methylprop-2-en-1-one 6a (derived from 8-iodo-1,2,3,4-
tetrahydroquinoline) that delivered compound 7a in 78% yield
(Scheme 3).
a wide range of substituents with different electronic proper-
ties on the aryl ring of 1,3,4-oxadiazoles (9a–9 f), provided the
corresponding products in moderate to good yields. Also,
benzyl substitution (9g–9i) on the nitrogen was well tolerated;
however, the electron-withdrawing tosyl group (9m) was in-
compatible under these conditions. Moderate to good yields
were obtained with 3-pyridyl-substituted oxadiazoles (9j and
9k) and 2-phenyl-1,3,4-thiadiazole (9l). A highly substituted
compound derived from the Ugi-4CR underwent facile reaction
to provide 9n in moderate yield.
Next, we targeted the synthesis of a 3-ethylidene indoli-
none–azole framework using N-(2-iodophenyl)-N-methyl-3-phe-
nylpropiolamide (8a) as the substrate. However, employment
A mechanism for the synthesis of the indolinone–azole
framework was proposed involving the formation of a s-alkyl-
palladium intermediate (Scheme 4). Initially, the oxidative addi-
tion of acrylamide 1a by the ligated Pd(0) species, generates
the Pd(II) complex A. This is followed by the intramolecular
Scheme 3. Expansion of the reaction scope to a tetrahydroquinoline
derivative.
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stirred under nitrogen at 1008C for 24 h. The reaction mixture was
cooled to ambient temperature, diluted with ethyl acetate (5 mL)
and passed through a small bed of celite. The filtrate was concen-
trated under vacuum to provide the crude product, which was pu-
rified by column chromatography on silica gel using heptane/
1
EtOAc as eluent. The products were further identified by H NMR,
13C NMR and HRMS, and were all in good agreement with the
assigned structures.
Acknowledgements
The authors wish to thank the FWO Fund for Scientific Re-
search-Flanders (Belgium) and the Research Fund of the Uni-
versity of Leuven (KU Leuven) for financial support. U.K.S. and
N.S. are grateful to the University of Leuven for postdoctoral
fellowships. Y.K. is thankful to the Erasmus Mundus Experts 2
program for providing a visiting doctoral fellowship.
Scheme 4. Proposed mechanistic pathway for the indolinone–azole
framework.
Keywords: CÀH activation · domino reactions · heterocycles ·
microwave chemistry · palladium
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Method A: Synthesis of 3,3-substituted indolinone–azole
framework
In an oven-dried microwave vial (10 mL) equipped with a magnetic
stirring bar was added acrylamide 1 (0.2 mmol, 1 equiv), azole 2
(1.1 equiv), Cs2CO3 (0.4 mmol), [Pd(PPh3)4] (5 mol%), and MeCN
(1.5 mL). The vial was degassed, backfilled with nitrogen and
heated under microwave irradiation (100 W) for 30 min at 110 8C.
After completion of the reaction as monitored by TLC analysis, the
mixture was filtered, concentrated, and the resulting crude product
was purified by column chromatography on silica gel using hep-
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1H NMR, 13C NMR, and HRMS and were all in good agreement with
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Method B: Synthesis of 3-ethylidene indolinone–azole
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Received: September 15, 2015
Published online on December 2, 2015
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