Cascade π-Extended Decarboxylative Annulation Involving Cyclic Diaryliodonium Salts: Site-Selective Synthesis of Phenanthridines and Benzocarbazoles via a Traceless Directing Group Strategy

Article abstract of DOI:10.1021/acs.orglett.9b03775

A novel cascade π-extended decarboxylative annulation (PEDA) involved with cyclic diaryliodonium salts is described. Via fine-tuning of the reaction conditions, the Pd(II)-catalyzed site-selective N1/C2 or C2/C3 annulation of commercially available indole

Full text of DOI:10.1021/acs.orglett.9b03775

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cascade π‑Extended Decarboxylative Annulation Involving Cyclic
Diaryliodonium Salts: Site-Selective Synthesis of Phenanthridines
and Benzocarbazoles via a Traceless Directing Group Strategy
Zenghui Ye, Yong Li, Kai Xu, Na Chen, and Fengzhi Zhang*
College of Pharmaceutical Science, Zhejiang University of Technology, Hangzhou 310014, P. R. China
S
* Supporting Information
ABSTRACT: A novel cascade π-extended decarboxylative annula-
tion (PEDA) involved with cyclic diaryliodonium salts is described.
Via fine-tuning of the reaction conditions, the Pd(II)-catalyzed site-
selective N1/C2 or C2/C3 annulation of commercially available
indole-2-carboxylic acids can be achieved, affording valuable
phenanthridines or benzocarbazoles, respectively. The key strategy
is the carboxylic acid functionality being employed as both a traceless
directing group for the ortho C−N or C−C coupling and a reactive
group for the cascade π-extended decarboxylative annulation in a
highly step economical manner.
Scheme 1. Site-Selective Arylation of Indoles
he indole scaffold is one of the most intensively studied
T
heteroaromatics due to its prevalence in bioactive natural
products,1 drugs,2 and functional materials.3 Hence, consid-
erable attention has been focused on the direct and selective
functionalization of indole derivatives.4 However, how to
switch and achieve complete regioselectivity is still a
challenging task due to the presence of multiple C−H or
N−H bonds in the indole structure.5 Various elegant methods
have been developed to enable selective C/N−H functional-
ization on either the pyrrole-type ring or the benzenoid moiety
of indole.6 Among them, the regioselective construction of
aryl−indolyl bonds for the synthesis of arylindoles,7 the key
moieties of a variety of biologically active molecules, is of
fundamental importance in organic synthesis.8 As a readily
available electrophilic arylating reagent, the linear diary-
liodonium salts9,10 have been employed for the direct and
site-selective arylation of indoles (Scheme 1a). For example, in
2006, the Sanford group reported Pd-catalyzed direct and
selective C2−H arylation of indoles under mild conditions.11
In 2008, by changing the protecting groups on the nitrogen
atom, the Gaunt group disclosed a Cu(II)-catalyzed direct and
site-selective C2 or C3 arylation of indoles.12 In 2015, by
making use of both aryl groups from the linear diaryliodonium
salts, the Greaney group reported a Cu-catalyzed one-pot
tandem C3−H/N−H arylation affording various 1,3-diary-
lindoles.13 Later, the Shi group achieved the Cu-catalyzed
regioselective C−H arylation of indoles at positions C4−C6
(Scheme 1a).14 Compared to the mono-C−H functionaliza-
tion of indoles, the one-step double C−H arylative annulation
of indoles with cyclic diaryliodomium salts15,16 had never been
reported until the Wen group reported the double C−H
functionalization of indoles at positions C2 and C3 affording
the benzocarbazoles (Scheme 1b).17 However, the N1/C2
double annulation cannot be achieved with this protocol.
Annulative π-extension (APEX) has been frequently
employed for the construction of polycyclic arenes that have
important applications in material chemistry.18 Although the
number of steps for the synthesis of polycyclic arenes can be
reduced by using direct C−H activation in this approach, it is
usually very difficult to achieve complete site selectivity due to
the presence of multiple C−H bonds in the substrates.
Recently, by a π-extended decarboxylative annulation (PEDA)
strategy, we reported the synthesis of triphenylenes and
Received: October 24, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03775
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Letter
dibenzo[f,h]quinolines with benzoic acids and cyclic diary-
liodonium salts as the starting materials.19 As a continuation of
our research interest in novel arylation chemistry involved with
diaryliodonium salts,20 we envisioned that the carboxylic acid
functionality in readily available indole-2-carboxylic acids could
be employed as both a directing group for the palladium-
catalyzed ortho N−H or C−H arylation and a reactive group
for the tandem ipso-decarboxylative21 annulation for the
synthesis of valuable phenanthridines and benzocarbazoles
(Scheme 1c). The challenges for this novel one-pot cascade π-
extended decarboxylative annulation (PEDA) are how to avoid
side reactions such as protodecarboxylation, etc.,22 how to
achieve the N−H or C−H activation with complete
regioselectivity, and how to make the consecutive series of
reactions proceed smoothly to achieve the site-selective N1/
C2 or C2/C3 annulation.
phosphinite (Ph2POEt), respectively, were used as the ligands
(Table 1, entries 7 and 8, respectively). The yield was further
increased to 72% when the reaction was conducted in a more
diluted solution (Table 1, entry 9). To our delight, it was
found the yield can be improved significantly by increasing the
amount of iodonium salt 2a (Table 1, entries 10 and 11), and
finally, the reaction gave phenanthridine 3aa in 94% yield when
2.0 equiv of 2a was used (Table 1, entry 11). In the absence of
Ph2POEt, a mixture of 3a and 4a was obtained in 40% and
10% yields, respectively, with DMF as the solvent (Table 1,
entry 12), and the yield of carbazole 4a was improved to 84%
with HOAc as the solvent (Table 1, entry 13).
With optimum conditions in hand, we first examined the
substrate scope of this novel acid-directed double N1/C2
arylative annulation protocol with various commercially
available indole-2-carboxylic acid derivatives (Scheme 2).
We initiated the condition optimizations by reacting the
commercially available indole-2-carboxylic acids 1a with cyclic
diaryliodonium salt 2a (Table 1).
Scheme 2. Carboxylic Acids for N1/C2 Annulation
a
Table 1. Optimization of the Reaction Conditions
b
yield (%)
entry
1
2
2a (equiv)
ligand
solvent
3a
4a
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.6
2.0
2.0
2.0
−
−
−
−
−
−
DMF
DMF
DMA
DCE
DMSO
HOAc
DMF
DMF
DMF
DMF
DMF
DMF
HOAc
30
−
10
11
30
−
47
58
72
87
94
40
−
−
−
−
−
−
30
−
−
−
−
−
10
84
c
3
4
5
6
7
8
PPh3
Ph2POEt
Ph2POEt
Ph2POEt
Ph2POEt
−
d
9
d
10
d
11
d
12
d
13
−
First, the substrates without substitutents at position C3
were tested. Substrates with methyl or methoxyl substituents
gave the corresponding products in good to excellent yields
(3aa−3ad). Substrates displaying extra halogenated function-
alities such as F or Cl were tolerated under the reaction
conditions (3ae and 3af). Second, 3-aryl-indole-2-carboxylic
acid substrates were examined. All of the substrates with strong
electron-donating or -withdrawing substituents in the phenyl
ring afforded the corresponding cyclized products 3ag−3ai in
excellent yields. Both 3-thiophenyl and 3-furanyl indole-2-
carboxylic acids are effective, as well (3aj and 3ak). It was
found that the reaction with simple pyrrole-2-carboxylic acid as
the substrate also gave the pyrrolo[1,2-f]phenanthridine 3al in
98% yield.
To fully establish the scope of this one-pot N1/C2
annulation process, a range of substituted cyclic diary-
liodonium salts were prepared and subjected to the optimized
reaction protocol with indole-2-carboxylic acids and pyrrole-2-
carboxylic acids (Scheme 3).23 The symmetrical cyclic
diaryliodonium salts were examined first (3ba−3bh). Both
the indole-2-carboxylic acid and the pyrrole-2-carboxylic acid
reacted with the cyclic diaryliodonium salts bearing simple
a
Reaction conditions: 1a (0.3 mmol), Pd(OAc)2 (10 mol %), ligand
(20 mol %), base (2.2 equiv), solvent (1 mL), 12 h, 145 °C, air
b
c
d
atmosphere. Isolated yields. Without K2CO3 or Pd(OAc)2. With 2
mL of solvent.
After intensive screening of the catalytic systems (see the
Supporting Information), it was found that the N1/C2 cyclized
product indolo[1,2-f ]phenanthridine 3aa was obtained in 30%
yield without the detection of C2/C3 cyclized carbazole 4a
with Pd(OAc)2 (10 mol %) as the catalyst and K2CO3 (2.2
equiv) as the base in DMF at 145 °C for 12 h (Table 1, entry
1). There is no reaction in the absence of either base or Pd
catalyst (Table 1, entry 2). A base screening (see the
Supporting Information) and a solvent screening (Table 1,
entries 3−5) were then carried out; unfortunately, there was
no improvement in the yield of 3aa. Surprisingly, when HOAc
was used as the solvent, the site selectivity was switched
completely with C2/C3 cyclized carbazole 4a isolated as the
only product (Table 1, entry 6). A ligand screening was then
conducted. It was found the yields were improved to 47% and
58% when triphenylphosphine (PPh3) and ethyl diphenyl-
B
DOI: 10.1021/acs.orglett.9b03775
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Organic Letters
Letter
Scheme 3. Cyclic Diaryliodonium Salts for N1/C2
Annulation
Scheme 4. Substrates for C2/C3 Annulation
Scheme 5. Control Experiments
methyl substituents and gave the corresponding products in
moderate yields. The halogen (F and Cl) and strong electron-
withdrawing groups such as CF3 were tolerated under the
optimum conditions, affording the corresponding products in
moderate to good yields (3bd−3bh). The unsymmetrical
cyclic diaryliodonium salts were then tested (3bi−3bl).
Cyclized products 3bi and 3bj were isolated as single products,
and the structure of 3bi was further confirmed by single-crystal
X-ray diffraction analysis, which demonstrated that N-arylation
took place from the less hindered side of the salt and formed
the C−N bond first. For the unsymmetrical cyclic diary-
liodonium salt with an ester substituent on one of the phenyl
rings, the reactions afforded a mixture of products in moderate
yields, and the main product was formed from the nitrogen
attacking from the electron deficient ring of the salt.
The substrate scope for the cascade one-pot C2/C3
annulation was then examined (Scheme 4). The indole-2-
carboxylic acids without substituents on position N1 were
tested first (4a−4i). All gave the corresponding C2/C3
annulated products with complete site selectivity. However,
the substrates with halogen substituents gave the products in
poor yields (4c, 4d, 4h, and 4i). The indole-2-carboxylic acids
with a methyl or benzyl substituent at position N1 were
effective and gave the desired products 4j−4l successfully.
To further understand this cascade decarboxylative
annulative π-extension protocol, we carried out the following
control experiments (Scheme 5). First, the effect of the
carboxylic functional group was evaluated. It was found the
reaction did not take place if the carboxylic functional group
was masked as ester 5, which demonstrates the essential
directing effect of carboxylic acid (Scheme 5, eq 1). Only a
trace amount of 3aa was obtained if the carboxylic acid group
was removed from the substrate (Scheme 5, eq 2). The impact
of the substitutents on the pyrrole-type ring was then tested.
With N-blocked indole-2-carboxylic acids 7 with a substrate,
carbazole product 8 was obtained in 83% yield (Scheme 5, eq
3). If substrate 9 was used as the substrate with both N1 and
C3 positions blocked, proto-decarboxylated product 10 was
obtained in 60% yield (Scheme 5, eq 4). Finally, if cyclic
diaryliodonium salt 2a was replaced with 2-iodo-1,1′-biphenyl
11 and reacted with 7 and 1a separately, carbazole product 4j
and phenanthridine product 3aa were obtained in only 36%
and 10% yields, respectively (Scheme 5, eqs 5 and 6).
C
DOI: 10.1021/acs.orglett.9b03775
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Organic Letters
Letter
On the basis of the experiments described above, the
proposed reaction mechanisms are shown in Scheme 6. With
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.
Scheme 6. Proposed Mechanism
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zhangfengzhi@zjut.edu.cn.
ORCID
Fengzhi Zhang: 0000-0001-9542-6634
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the National Natural Science
Foundation of China (21871234) and the Zhejiang Provincial
NSFC for Distinguished Young Scholars (LR15H300001).
HOAc as the solvent, the catalytic cycle would start with a C−
H activation by Pd(II) species to give a five-membered
cyclopalladate(II) intermediate A. Because the N atom is more
electronegative than the C atom, a ring opening of cyclic
diaryliodonium salt 2a then occurs at the C position, which has
a lower barrier, and a Pd(IV) complex B would be generated,
which is the key step that determines the regioselectivity of this
reaction. A reductive elimination would give the C−C coupled
intermediate C. After sequential decarboxylation (D), oxidative
addition (E), and reductive elimination, carbazole product 4a
would form. When DMF is used as the solvent with Ph2POEt
as an additive, a Pd(0)/Pd(II) catalytic cycle instead of the
aforementioned Pd(II)/Pd(IV) pathway is proposed. The
catalytic cycle would start with the ring opening of cyclic
diaryliodonium salt 2a by in situ-generated Pd(0) to give
Pd(II) species I, which would be attacked by indole-2-
carboxylic acids 1a to give intermediate II. The following key
step, N−H activation, might generate a five-membered
cyclopalladate(II) species III that would undergo sequential
reductive elimination, oxidative addition, decarboxylation, and
reductive elimination to give phenanthridine product 3aa.
In summary, we have successfully developed a cascade π-
extended decarboxylative annulation (PEDA) strategy for the
construction of privileged phenanthridine and benzocarbazole
scaffolds from commercially available indole-2-carboxylic acids
and readily available cyclic diaryliodonium salts. This highly
step economical process involves the Pd(II)-catalyzed site-
selective ortho N1 or C2 arylation followed by tandem
intramolecular decarboxylative annulation. The key is the
successful development of a two-in-one traceless directing
group strategy by which the carboxylic acid functionality was
employed as both a directing group for the Pd-catalyzed N−
H/C−H arylation and a reactive group for the cascade
intramolecular decarboxylative annulation.
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Experimental procedures and spectroscopic character-
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Accession Codes
CCDC 1951128 contains the supplementary crystallographic
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