Assembly of Indole Cores through a Palladium-Catalyzed Metathesis of Ar-X σ-Bonds

Article abstract of DOI:10.1021/acs.orglett.0c03611

We describe the development of a new method for construction of highly substituted indole scaffolds through the strategic utilizing of the metathesis of Ar-X σ-bonds based on the dynamic nature of palladium-based oxidative addition/reductive elimination. A suitable and simple catalytic system has provided an appropriate platform for a productive ligand exchange and consecutive carbopalladation/C-H activation/amination of phosphine ligands with alkynes and aromatic/aliphatic amines for construction of structurally diverse indoles.

Full text of DOI:10.1021/acs.orglett.0c03611

pubs.acs.org/OrgLett
Letter
Assembly of Indole Cores through a Palladium-Catalyzed Metathesis
of Ar−X σ‑Bonds
Farnaz Jafarpour,* Mehran Ghasemi,† Hamed Navid,† Niloofar Safaie, Saideh Rajai-Daryasarei,
Azizollah Habibi, and Robert C. Ferrier Jr.
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ABSTRACT: We describe the development of a new method for
construction of highly substituted indole scaffolds through the strategic
utilizing of the metathesis of Ar-X σ-bonds based on the dynamic nature of
palladium-based oxidative addition/reductive elimination. A suitable and
simple catalytic system has provided an appropriate platform for a productive
ligand exchange and consecutive carbopalladation/C−H activation/amination
of phosphine ligands with alkynes and aromatic/aliphatic amines for
construction of structurally diverse indoles.
Scheme 1. Pd-Catalyzed Redistribution of Aryl Groups
hosphines play a crucial role in most transition-metal-
catalyzed cross-coupling reactions in stabilizing the active
P
metal center and fine-tuning the selectivity of the conversion.1
The initially formed aryl palladium species, however, have been
reported to frequently undergo an undesirable aryl/aryl
interchange with the aryl substituents of triarylphosphine
ligands via C−P bond scission, leading to undesired scrambled
side products and deactivation of the catalyst.2 Accordingly,
early studies were focused on shutting down this side trail.
However, by the accumulation of mechanistic data,3 new
avenues for developing various catalytic reactions through the
strategic utilization of the metathesis of Pd−C and P−C bonds
have been recently opened.4 The catalytic redistribution of aryl
groups between aryl electrophiles and aryl phosphines via
reductive elimination of the phosphine ligand and the aryl
group from the phosphine-ligated PdII−aryl complex to form
the phosphonium intermediate has been developed into a
useful synthetic transformation (Scheme 1A). In this regard, in
2017, Morandi et al. pioneered a Pd-catalyzed C−S or C−P
bond metathesis via catalytic redistribution of aryl groups
between aryl electrophiles and aryl sulfides and aryl phosphines
(Scheme 1B).5 Furthermore, the Morandi and Arndtsen
research groups developed in parallel a functional group
metathesis between iodoarenes and acid chlorides employing
metathesis-active sterically encumbered Xantphos ligand to
promote scrambling of the aryl groups on the aryl iodide and
the aroyl chloride through reversible oxidative addition and
carbonylation (Scheme 1C).6 Obviously, creating a suitable
catalytic system to provide an appropriate platform for a
productive ligand exchange would find great applications in
organic synthesis.
the benzoic ring built by the aryl section of simple
triarylphosphines via the metathesis of Ar−X σ-bonds (Scheme
1D). Aryl’s attachment to phosphorus can be activated via a
controlled exchange between P−Ar and Pd−Ar, and it can
Received: October 29, 2020
Inspired by recent advances in this area, we set out to
identify reaction conditions to construct indole scaffolds with
https://dx.doi.org/10.1021/acs.orglett.0c03611
© XXXX American Chemical Society
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participate in a cascade annulation reaction for the purpose of
the construction of more complex molecules, which is
unprecedented to the best of our knowledge and would be
very attractive. This transformation exploits palladium-
catalyzed reversible oxidative addition/reductive elimination
chemistry to offer the synthesis of a diverse array of highly
functionalized indoles in high yields and opens a new window
for the application of simple phosphines in the assembly of
indole cores. Because indole rings are ubiquitous in natural
products and pharmaceuticals7 and are nominated as the
fourth most dominant heteroaromatics,8 a high-yielding
general approach to their synthesis is highly desirable.9
The advancement of a controlled ligand exchange in a
palladium-catalyzed carbopalladation/annulation/amination
cascade for the construction of indole core, however, would
encounter challenges via some probable side reactions.
Classical Larock indole synthesis via the assembly of amines
and alkynes,9h Buchwald−Hartwig amination of iodoarenes for
the construction of diaryl amines,10 and the assembly of alkyne
and arenes for the construction of various carbocycles11 are
some competing paths in case. Suppressing the side oxidative
addition of iodoarene to palladium(0) and the alkyne
carbopalladation reaction is a further challenge.12
(entries 1−3). Screening solvents showed that although
solvents including ACN, chlorobenzene, THF, and toluene
were undesirable compared with DMF, using DMSO improved
the yield (entries 4−8). A screening of different bases revealed
that switching the base to NaHCO3 improved the yield (entry
12). No improved yield was obtained by varying the reactant
equivalents. (See Table S2 in the Supporting Information for
details.) The reaction was totally suppressed when Pd(OAc)2
was removed. A control experiment also confirmed the crucial
role of the iodoanisole as the additive. We probed if other
iodoarenes may be more suited to favor the aryl scrambling
process; however, no more improvements were achieved using
various aryl iodides (Table 1). Eventually, we found that
treating triphenylphosphine 1a with 2-methoxy aniline 2a (2
equiv), diphenylacetylene 3a (1 equiv), iodoanisole (0.5
equiv), NaHCO3 (2.0 equiv), and Pd(OAc)2 (10 mol %) in
DMSO (1 mL) led to the chemoselective assembly of indole
4a in 66% isolated yield.
With the optimized reaction conditions in hand, we next
investigated the substrate scope for the assembly of indole
cores employing triarylphosphines to construct the benzoic
ring (Scheme 2). The catalytic reaction tolerated a wide range
of functionalities, including electron-rich and -deficient aniline
derivatives. Anilines with meta- and para-alkyl and -alkoxy
groups participated well in this annulation reaction (4b, 4h,
4i). Interestingly, ortho-, meta-, and para-halo-substituted
anilines were well-tolerated in the current system, affording
the desired indoles 4d−g with suitable handles for further
functionalization reactions in yields ranging from 70 to 86%.
Fortunately, electron-withdrawing nitro and CF3 groups were
compatible to achieve the reaction, albeit in lower yields (4j
and 4k). Fascinatingly, the strategy could be well extended to
alkyl amines to afford N-alkyl indoles (4m−r), which are
prevalent in pharmaceuticals and agrochemicals, in moderate
to good yields. Next we turned our attention to the scope of
arylphosphines. Alkyl-, alkoxy-, and chloro-substituted aryl-
phosphines participated well in this phosphine metathesis
reaction, rendering the corresponding indoles 4s−w in yields
exceeding 64%. The structure of representative product 4s was
further confirmed by X-ray analysis (CCDC 2022004).
Triarylphosphines bearing electron-withdrawing fluoro- and
trifluoromethyl groups also offered the desired indole cores 4x
and 4y in 72 and 41% yields, respectively. Fortunately, methyl
groups at the meta-positions were well tolerated under the
optimized reaction condition, and the expected indole product
4z was obtained in moderate yield. Unfortunately, the reaction
was unsuccessful with ortho-methyl-substituted triarylphos-
phine. When tris(2-methoxyphenyl)phosphine was used,
however, a mixture of 4- and 6-methoxy-substituted indoles
was obtained in a combined 70% yield (entry 27, 4aa and
4aa′). The results showed that para-iodoanisole participated
more efficiently than the ortho-substituted arylphosphine in the
annulation reaction (o/p = 1/5).
At the outset of our investigations, triphenylphosphine (1a),
2-methoxy aniline (2a), and diphenylacetylene (3a) were
selected as the model substrates (Table 1). In general, it has
been reported that Ar/Ar exchange between P−Ar and Pd−Ar
usually becomes more realistic with electron-rich aryl groups,
especially in polar solvents.13 Accordingly, iodoanisole was
added for C−P bond stimulation via phosphonium salt
assembly. Among the palladium catalysts tested, Pd(OAc)2
afforded the annulation product in a better yield of 48%
a
Table 1. Optimization of Reaction Conditions
b
entry
Pd
PdCl2
base
solv
yield (%)
1
2
3
4
5
6
7
8
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
KOH
DMF
DMF
DMF
ACN
DMSO
PhCl
THF
toluene
DMSO
DMSO
DMSO
DMSO
26
48
35
33
57
28
trace
21
55
Pd(OAc)2
Pd(dba)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
Pd(OAc)2
9
10
11
12
49
trace
66
Finally, some substituted alkynes were examined, and the
desired 2,3-diaryl indoles 4ab and 4ac were collected in high
yields (entries 28 and 29). Remarkably, with the unsym-
metrical 1-phenyl-1-propyne alkyne, the desired products 4ad
and 4ae were smoothly delivered as single regioisomeric
products in 78 and 48% yields, respectively. The high
regioselectivity of the reaction may be attributed to the
carbopalladation sequence of the alkyne. In this order, the
relatively large aryl group of the arylpalladium species
approaches the internal alkyne from the less hindered end to
NaHCO3
a
Reaction conditions: phosphine 1a (0.1 mmol), aniline 2a (0.2
mmol), alkyne 3a (0.1 mmol), additive (0.05 mmol), Pd (10 mol %),
b
base (0.2 mmol), solvent (1 mL), at 120 °C for 24 h. Isolated yields.
B
https://dx.doi.org/10.1021/acs.orglett.0c03611
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a
Scheme 2. Substrate Scope for the Construction of Indoles
a
Reaction conditions: Phosphine (0.1 mmol), aniline 2a (0.2 mmol), alkyne 3a (0.1 mmol), iodoanisole (0.05 mmol), Pd(OAc)2 (10 mol %),
b
NaHCO3 (0.2 mmol), DMSO (1 mL), at 120 °C for 24 h. Phosphine (5.0 mmol) was used.
reduce the steric repulsion and replaces the larger group at the
C-2 position of the indole ring.14 Fortunately, when the
reaction was scaled up to the gram scale, 4d was collected in
53% isolated yield.
In the proposed mechanism (Scheme 3), an initial oxidative
addition of iodoarene to palladium(0) results in the aryl
palladium species I. The reductive elimination of phosphine
ligand and anisole from the palladium complex to form the
phosphonium intermediate followed by the oxidative addition
of another carbon−phosphor bond results in aryl group
scrambling between aryl groups of phosphine and palladium to
form a new palladium-aryl species II.15 Succeeding the
exchange process, the carbopalladation reaction of aryl
palladium II and alkyne16 followed by an intramolecular C−
H functionalization,17 forms a five-membered palladacycle
(IV). For the insertion of an amine group into the C,C-
palladacycle next, two paths are proposed. A transmetalation
between two Pd(II) species, introduced by Cardenas and
C
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Scheme 3. Proposed Reaction Mechanism
AUTHOR INFORMATION
■
Corresponding Author
Farnaz Jafarpour − Faculty of Chemistry and Faculty of
Chemistry, Kharazmi University, 15719-14911 Tehran,
Iran; School of Chemistry, College of Science and School of
Chemistry, College of Science, University of Tehran, 14155-
6455 Tehran, Iran; Department of Chemistry and
Department of Chemical Engineering and Materials Science,
Michigan State University, East Lansing, Michigan 48824,
United StatesSchool of Chemistry, College of Science and
School of Chemistry, College of Science, University of Tehran,
14155-6455 Tehran, IranFaculty of Chemistry and Faculty
of Chemistry, Kharazmi University, 15719-14911 Tehran,
IranDepartment of Chemical Engineering and Materials
Science and Department of Chemistry, Michigan State
University, East Lansing, Michigan 48824, United States;
orcid.org/0000-0001-5242-2748; Email: jafarpur@
khayam.ut.ac.ir
Authors
Mehran Ghasemi − Faculty of Chemistry and Faculty of
Chemistry, Kharazmi University, 15719-14911 Tehran,
Iran; School of Chemistry, College of Science and School of
Chemistry, College of Science, University of Tehran, 14155-
6455 Tehran, Iran; Department of Chemistry and
Echavarren18 and developed by Derat and Catellani19 (path
A), may generate a binuclear palladium species V, which upon
the reductive elimination of Pd(0) and the successive
intramolecular N−Pd bond formation results in Pd(II) species
VII. A direct trapping of palladacycle with alkyl or arylamines
is the next proposed path, which may result in the same
palladium−amine complex VII (path B). A final reductive
elimination reaction releases Pd(0) and makes the indole core
4.
Overall, although C−P bond scission has a profound impact
on the catalyst deactivation and side reactions, we have
uncovered its untapped potential in a cascade palladium-
catalyzed ortho- and ipso-functionalization of arylphosphines
for the construction of privileged indole scaffolds. The
metathesis of Ar−X σ-bonds via a reversible Ar−X oxidative
addition/reductive elimination on palladium by means of very
simple triaryl phosphines offers a platform to synthesize
structurally diverse indoles via the carbopalladation/C−H
activation/amination cascade. The approach is scalable and
obviates any requisite for large bite angle phosphine ligands to
fulfill the C−P bond scission and offers flexible substituents at
each point of the indole ring.
Department of Chemical Engineering and Materials Science,
Michigan State University, East Lansing, Michigan 48824,
United StatesSchool of Chemistry, College of Science and
School of Chemistry, College of Science, University of Tehran,
14155-6455 Tehran, IranFaculty of Chemistry and Faculty
of Chemistry, Kharazmi University, 15719-14911 Tehran,
IranDepartment of Chemical Engineering and Materials
Science and Department of Chemistry, Michigan State
University, East Lansing, Michigan 48824, United States
Hamed Navid − Faculty of Chemistry and Faculty of
Chemistry, Kharazmi University, 15719-14911 Tehran,
Iran; School of Chemistry, College of Science and School of
Chemistry, College of Science, University of Tehran, 14155-
6455 Tehran, Iran; Department of Chemistry and
Department of Chemical Engineering and Materials Science,
Michigan State University, East Lansing, Michigan 48824,
United StatesSchool of Chemistry, College of Science and
School of Chemistry, College of Science, University of Tehran,
14155-6455 Tehran, IranFaculty of Chemistry and Faculty
of Chemistry, Kharazmi University, 15719-14911 Tehran,
IranDepartment of Chemical Engineering and Materials
Science and Department of Chemistry, Michigan State
University, East Lansing, Michigan 48824, United States
Niloofar Safaie − Faculty of Chemistry and Faculty of
Chemistry, Kharazmi University, 15719-14911 Tehran,
Iran; School of Chemistry, College of Science and School of
Chemistry, College of Science, University of Tehran, 14155-
6455 Tehran, Iran; Department of Chemistry and
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03611.
Experimental procedures, optimization, compound
characterization data, X-ray data, and 1H/13C NMR
spectra (PDF)
Department of Chemical Engineering and Materials Science,
Michigan State University, East Lansing, Michigan 48824,
United StatesSchool of Chemistry, College of Science and
School of Chemistry, College of Science, University of Tehran,
14155-6455 Tehran, IranFaculty of Chemistry and Faculty
of Chemistry, Kharazmi University, 15719-14911 Tehran,
IranDepartment of Chemical Engineering and Materials
Science and Department of Chemistry, Michigan State
Accession Codes
CCDC 2022004 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.
D
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Saideh Rajai-Daryasarei − Faculty of Chemistry and Faculty
of Chemistry, Kharazmi University, 15719-14911 Tehran,
Iran; School of Chemistry, College of Science and School of
Chemistry, College of Science, University of Tehran, 14155-
6455 Tehran, Iran; Department of Chemistry and
Department of Chemical Engineering and Materials Science,
Michigan State University, East Lansing, Michigan 48824,
United StatesSchool of Chemistry, College of Science and
School of Chemistry, College of Science, University of Tehran,
14155-6455 Tehran, IranFaculty of Chemistry and Faculty
of Chemistry, Kharazmi University, 15719-14911 Tehran,
IranDepartment of Chemical Engineering and Materials
Science and Department of Chemistry, Michigan State
University, East Lansing, Michigan 48824, United States;
orcid.org/0000-0003-0418-2211
Azizollah Habibi − Faculty of Chemistry and Faculty of
Chemistry, Kharazmi University, 15719-14911 Tehran,
Iran; School of Chemistry, College of Science and School of
Chemistry, College of Science, University of Tehran, 14155-
6455 Tehran, Iran; Department of Chemistry and
Department of Chemical Engineering and Materials Science,
Michigan State University, East Lansing, Michigan 48824,
United StatesSchool of Chemistry, College of Science and
School of Chemistry, College of Science, University of Tehran,
14155-6455 Tehran, IranFaculty of Chemistry and Faculty
of Chemistry, Kharazmi University, 15719-14911 Tehran,
IranDepartment of Chemical Engineering and Materials
Science and Department of Chemistry, Michigan State
University, East Lansing, Michigan 48824, United States
Robert C. Ferrier Jr. − Faculty of Chemistry and Faculty of
Chemistry, Kharazmi University, 15719-14911 Tehran,
Iran; School of Chemistry, College of Science and School of
Chemistry, College of Science, University of Tehran, 14155-
6455 Tehran, Iran; Department of Chemistry and
Department of Chemical Engineering and Materials Science,
Michigan State University, East Lansing, Michigan 48824,
United StatesSchool of Chemistry, College of Science and
School of Chemistry, College of Science, University of Tehran,
14155-6455 Tehran, IranFaculty of Chemistry and Faculty
of Chemistry, Kharazmi University, 15719-14911 Tehran,
IranDepartment of Chemical Engineering and Materials
Science and Department of Chemistry, Michigan State
University, East Lansing, Michigan 48824, United States;
orcid.org/0000-0002-5123-7433
(10) Dorel, R.; Grugel, C. P.; Haydl, A. H. Angew. Chem., Int. Ed.
2019, 58, 17118.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03611
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Author Contributions
†M.G. and H.N. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support of University
of Tehran and Kharazmi University.
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