Through-Space 1,4-Palladium Migration and 1,2-Aryl Shift: Direct Access to Dibenzo[a,c]carbazoles through a Triple C-H Functionalization Cascade

Article abstract of DOI:10.1002/chem.201503474

A palladium-catalyzed expeditious synthesis of dibenzofused carbazoles from readily available 2-arylindoles and diaryliodonium salts is reported. Interestingly, after the electrophilic C3 palladation of indole, an unexpected "through-space" 1,4-palladium migration to the 2-aryl moiety, by remote C-H bond activation followed by C-H arylation with diaryliodonium salt, and an unprecedented 1,2-aryl shift take place. Finally, an intramolecular cross-dehydrogenative coupling (CDC) at the C2 position affords dibenzo[a,c]carbazoles in high yields. Remarkably, the present migratory annulation occurs through three C-H bond activation one C-C bond cleavage, and the simultaneous construction of three new C-C bonds in a single operation.

Full text of DOI:10.1002/chem.201503474

DOI: 10.1002/chem.201503474
Communication
&
Cascade Reactions
Through-Space 1,4-Palladium Migration and 1,2-Aryl Shift: Direct
Access to Dibenzo[a,c]carbazoles through a Triple CÀH
Functionalization Cascade
Samir Kumar Bhunia,[a, b] Arghya Polley,[a, b] Ramalingam Natarajan,[a] and Ranjan Jana*[a, b]
lence in natural products, pharmaceuticals, and functional ma-
Abstract: A palladium-catalyzed expeditious synthesis of
dibenzofused carbazoles from readily available 2-arylin-
doles and diaryliodonium salts is reported. Interestingly,
after the electrophilic C3 palladation of indole, an unex-
pected “through-space” 1,4-palladium migration to the 2-
aryl moiety, by remote CÀH bond activation followed by
CÀH arylation with diaryliodonium salt, and an unprece-
dented 1,2-aryl shift take place. Finally, an intramolecular
cross-dehydrogenative coupling (CDC) at the C2 position
affords dibenzo[a,c]carbazoles in high yields. Remarkably,
the present migratory annulation occurs through three CÀ
H bond activation one CÀC bond cleavage, and the simul-
taneous construction of three new CÀC bonds in a single
operation.
terials, such as polymer and organic light-emitting diodes
(PLED, OLED).[5] However, efficient protocols for the synthesis
of dibenzofused carbazoles are limited.[6] Palladium-catalyzed
multistep synthesis from heavily prefunctionalized indoles[6b]
and photocatalyzed annulation of 2,3-diaryl indoles have been
reported.[6c] Recently, one-shot annulation through multiple CÀ
H bond activation has proven a powerful strategy for the p-ex-
tension of hydrocarbons and heterocycles.[7] Hypothetically,
substituted indoles could also be converted to the correspond-
ing dibenzofused carbazoles through multiple CÀH arylations
in one stroke.
Owing to their distinct reactivity, hypervalent iodine(III) re-
agents have attracted considerable attention for the site-selec-
tive CÀH arylation of indoles.[8] Sanford and co-workers report-
ed their use for C2 arylations by PdII/PdIV catalytic cycle,[8c]
whereas Gaunt and co-workers reported C3-selective arylation
through the generation of a highly electrophilic CuIII-species in
situ.[8d] A CuI-catalyzed atom-efficient 1,3-diarylation of indole
by CÀC and CÀN bond formation was reported by Greaney
and co-workers.[8f] Synthesis of fused carbazoles through dual
CÀH functionalization of indole by using a tricyclic iodonium
species was also reported.[6a] However, this protocol is limited
to the formation of symmetrical carbazoles only. The annula-
tion of indoles with easily available linear diaryliodonium salts
to afford unsymmetrical carbazoles through multiple CÀH acti-
vation is not known. As a part of our current research focus on
cascade CÀH activation for molecular complexity, we recently
reported the oxidative annulation of arylureas with styrenes to
provide 2-arylindoles and indolines.[9] We hypothesized that
a combination of a palladium complex and diaryliodonium
salts may lead to the C3 arylation of 2-arylindoles followed by
cross-dehydrogenetive coupling (CDC) to access symmetrical,
as well as unsymmetrical, dibenzofused carbazoles directly. Sur-
prisingly, an unprecedented “through-space” 1,4-palladium mi-
gration and 1,2-aryl shift was observed in this triple CÀH acti-
vation cascade (Scheme 1).
Transition metal-catalyzed cascade reactions involving CÀH
bond activation has great potential to achieve rapid increases
in molecular complexity, avoiding rigorous prefunctionaliza-
tions.[1] In the CÀH activation cascade, the requisite structural
motif that is generated through the initial transformation trig-
gers subsequent CÀH activations in a single operation.[2] Al-
though an impressive array of CÀH functionalizations have
been achieved in the last decade, cascade reactions involving
remote CÀH activation have limited success. In this vein, nor-
bornene-mediated ortho CÀH functionalization and “through-
space” palladium migration through remote CÀH activation
have attracted much attention in the cascade reaction mani-
folds.[3] Mechanistically, they have been proposed to proceed
through a Pd0/PdII/PdIV catalytic cycle. The Larock group and
others have also demonstrated that “through-space” 1,4-Pd mi-
gration is a powerful tool for the construction of privileged
scaffolds and fused heterocycles.[4] Among the N-heterocycles,
fused carbazoles are of particular interest due to their preva-
[a] S. K. Bhunia, A. Polley, Dr. R. Natarajan, Dr. R. Jana
Organic and Medicinal Chemistry Division
CSIR-Indian Institute of Chemical Biology
4 Raja S. C. Mullick Road, Jadavpur, Kolkata 700032 (India)
Fax: (+91)33-2473-5197
Although a palladium-catalyzed 1,2-aryl shift during aza-
Wacker cyclization of 2-alkenylanilines was reported,[10] the 1,2-
aryl shift of the preformed indole is, to our knowledge, not
known. However, a 1,2-silyl migration on the preformed indole
was reported by the Hiyama group.[11] We report herein,
a mechanistically unique palladium-catalyzed C3 palladation,
1,4-palladium migration, CÀH arylation, 1,2-aryl shift, and intra-
molecular CDC cascade of 2-arylindoles to access dibenzo[a,c]-
carbazoles.
E-mail: rjana@iicb.res.in
[b] S. K. Bhunia, A. Polley, Dr. R. Jana
Academy of Scientific and Innovative Research-Kolkata
4 Raja S. C. Mullick Road, Jadavpur, Kolkata 700032 (India)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503474.
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proved the yield further and some amount of product decom-
position was observed. Interestingly, a decent yield of the de-
sired product was also obtained by using a Pd0 precatalyst
(Table 1, entry 12). However, reactions with iodobenzene in lieu
of diaryliodonium salts or without palladium did not furnish
any desired product. Remarkably, the present transformation is
not air or moisture sensitive, thus providing a user-friendly
method.
A number of substituted 2-phenylindoles underwent CÀH ar-
ylation/CDC sequences with diphenyliodonium salt to afford
dibenzo[a,c]carbazoles in high yields (2a–k, Table 2). However,
highly electron-rich 2-phenylindoles were prone to decomposi-
tion under the reaction conditions and provided moderate
yield of the desired products (2i, 2j, Table 2). Next we intend-
ed to expand the substrate scope to the formation of unsym-
metrical carbazoles. The reaction between 2-(p-anisyl)indole
and bis(p-bromophenyl)iodonium tetrafluoroborate afforded
the carbazole product 2l in 76% yield (Scheme 2, Table 2). Sur-
Scheme 1. Migratory annulation of 2-arylindoles to dibenzo[a,c]carbazoles.
Table 1. Optimization of the reaction conditions.[a,b]
Entry
[Pd]
Base
Solvent
Yield [%]
1
2
3
4
5[c]
6[d]
7
8
9
Pd(OAc)2
Pd(OAc)2
Pd(tfa)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
NaOAc AcOH
35
65
60
72
0
n.d.
80
85
75
35
80
70
NaOAc AcOH/TFE (1:1)
NaOAc AcOH/TFE (1:1)
K2HPO4 AcOH/TFE (1:1)
K2HPO4 AcOH/TFE (1:1)
K2HPO4 AcOH/TFE (1:1)
K2HPO4 AcOH/HFIP (1:1)
K2HPO4 AcOH/HFIP (3:1)
K2HPO4 AcOH/HFIP (1:3)
K2HPO4 HFIP
10
11
12
Scheme 2. Reaction between 2-(p-anisyl)indole and bis(p-bromophenyl)iodo-
nium tetrafluoroborate.
[Pd(CH3CN)4](BF4)2
[Pd2(dba)3]
K2HPO4 AcOH/HFIP (3:1)
K2HPO4 AcOH/HFIP (3:1)
[a] All reactions were carried out on 0.1 mmol scale, 0.05m. [b] Yields
refer to isolated products. [c] 2.0 equiv of K2S2O8 was used. [d] 2.0 equiv
of Cu(OAc)2 was used. tfa=trifluoroacetate; dba=dibenzylideneacetone;
TFE=trifluoroethanol; HFIP=hexafluoroisopropanol; n.d.=not deter-
mined.
prisingly, NMR spectroscopy and X-ray crystallography revealed
that the 2-aryl moiety had shifted to the C3 position of the
indole, presumably via an unprecedented 1,2-aryl shift. Most
surprisingly, the p-bromo group of the iodonium salt appeared
at the C7 position, rather than, as anticipated for a C3 aryla-
tion/CDC cascade, at the C3 position of the fused carbazole
(Scheme 2). Alternatively, 1,2-aryl shift from C2 to C3 of the
indole followed by C2 arylation and CDC cascade may lead to
the corresponding carbazole with the bromo group at C6 in-
stead of C7. An electrophilic C3 palladation of 2-arylindole, 1-4-
palladium migration to the 2-aryl moiety, CÀH arylation with
diaryliodonium salt followed by 1,2-aryl shift and CDC cascade
at the C2 position may explain the formation of 2l.
Encouraged by this unexpected result, we next examined
the generality of this migratory product formation. A systemat-
ic study revealed that several substituted indoles and substitut-
ed diaryliodonium salts afforded the migratory product consis-
tently and it depends mainly on the electronic nature of the
C2-aryl group of indoles. For example, electron-rich p-tolyl and
p-anisyl (2l–ac, Table 2) moieties promoted the 1,2-aryl shift.
However, electron-deficient 2-aryl moiety (p-chlorophenyl) of
To optimize the reaction conditions, a mixture of 2-phenylin-
dole (1a) diphenyliodonium tetrafluoroborate, and a catalytic
amount of palladium acetate was heated at 110 8C in acetic
acid. Gratifyingly, the desired dibenzo[a,c]carbazole 2a was ob-
tained in 35% yield (Table 1, entry 1). Subsequently, we found
that solvent and base had prominent roles in the reaction out-
come (for details, see the Supporting Information). After a rigor-
ous screening, a mixture of acetic acid and hexafluoroisopropa-
nol (AcOH/HFIP=3:1) as solvent and K2HPO4 as base were
found to be optimal for this transformation (Table 1, entry 8).
The non-coordinating tetrafluoroborate counterion on iodoni-
um salt was found to be beneficial, presumably due to the for-
mation of cationic palladium species for facile electrophilic pal-
ladation.[12] Complete conversion of the starting material was
achieved by using an excess of diphenyliodonium salt
(2.5 equiv). No external additives, oxidants, or ligands im-
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Table 2. Generality of the migratory dibenzo[a,c]carbazole formation.[a,b]
[a] All reactions were carried out on 0.2 mmol scale, 0.05m. [b] Yields refer to isolated products, averaged over at least two experiments. [c] 10 mol%
[Pd(CH3CN)4](BF4)2 was used. [d] For crystallographic data, see the Supporting Information. [e] Diaryliodonium triflates were used. [f] Migratory and non-mi-
gratory product, 2ad/2ad’=3:5.
indoles provided a mixture of migratory and non-migratory
products (2ad/2ad’=3:5; Table 2). Relatively electron-rich aryls
from diaryliodonium species (2-phenyl vs. p-methoxyphenyl)
also afforded mixtures of migratory and non-migratory prod-
ucts (2af/2af’=3:1; Table 2). These results indicate that the mi-
gration may occur through electrophilic aromatic substitution
(SEAr). Interestingly, halogen substituents such as bromo,
chloro, and fluoro, which are useful for further cross-couplings
to extend the p system, were compatible under the reaction
conditions.
unsubstituted indole, a 1,2-palladium migration from C3 to C2
was reported.[13] However, we report here for the first time
that, in the case of 2-arylindoles, a 1,4-palladium migration to
the ortho position of the 2-phenyl group occurs instead of the
1,2-migration. After the 1,4-palladium migration, cross-coupling
with the diaryliodonium salt through a PdII/PdIV catalytic
cycle[8c] may lead to the intermediate IV (R=OMe, Scheme 4).
To probe this, the intermediate IV (R=OMe) was synthesized
separately and subjected to the reaction conditions without
hypervalent iodine(III) reagent. A mixture of migratory (2af)
and non-migratory carbazole (2af’) was obtained in compara-
ble yield (Scheme 3b). The migratory product may form by
a 1,2-shift of the cross-coupled product IV (R=OMe, Scheme-
s 3and 4) followed by CDC.
To elucidate the reaction mechanism, several control experi-
ments were performed (Scheme 3). Owing to the substitution
patterns in the substrate and the product, we assumed that,
after the initial electrophilic C3 palladation, a 1,4-palladium mi-
gration to the pendant 2-aryl moiety followed by CÀH aryla-
tion occurs. To probe this, the deuterated substrate, [D5]1a
was synthesized and subjected to the reaction conditions with-
out hypervalent iodine(III) reagent. The exchange of ortho deu-
terium with proton (ca. 70%) was observed (Scheme 3a), indi-
cating a reversible “through-space” 1,4-palladium migration via
five-membered hydridopalladium(IV) metallacycle formation
and protodepalladation (Scheme 4).[4c] Notably, in the case of
To test the 1,2-migratory aptitude of the electronically neu-
tral 2-phenyl group of indole, [D5]1a was subjected to the op-
timized reaction conditions with diphenyliodonium salt
(Scheme 3c). A significant amount (60%) of 1,2-migratory
1
product was also formed, as determined by H NMR spectros-
copy, which may be a general tendency for symmetrical carba-
zoles (2a–k, Table 2). In addition, the reaction between 2-(p-
tolyl)indole with bis(m-chlorophenyl)iodonium afforded 2ag
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and 2ag’, with the chloro substituent at C6 and C3 in the mi-
gratory and non-migratory products, respectively (Scheme 3d).
This positional change of the substituents supports the 1,4-pal-
ladium migration/1,2-aryl shift/CDC cascade mechanism. It also
eliminates the alternative possibilities, such as direct C3 aryla-
tion/CDC or 1,2-aryl shift/C2 arylation/CDC sequences, since, in
such cases, the chloro group would be at C2 or C7 respectively.
When the reaction was stopped after 2 h, an inseparable mix-
ture of 2ah and 2ah’ was isolated and characterized by 1H
NMR spectroscopy and mass spectrometry (Scheme 3e). The
intermediate 2ah may form by electrophilic C3 palladation
and 1,4-palladium migration followed by CÀH arylation. More-
over, a trace amount of 2ah’ may form by double 1,4-palladi-
um migration and CÀH arylation sequences.[14] The reactions
with 3-phenylindole or N-acyl-2-phenylindole did not furnish
any product, indicating that electrophilic C3 palladation fol-
lowed by 1,4-Pd migration must precede the 1,2-aryl shift.
These control experiments all suggest that, after the forma-
tion of the intermediate IV, the reaction may proceed by two
distinct routes to generate two different products in a diver-
gent manner (Scheme 4). In Path A, the second electrophilic C3
palladation and CÀH insertion followed by reductive elimina-
tion can generate the 1,2-non-migratory product 2af’
(Scheme 4). Alternatively, after the second electrophilic C3 pal-
ladation, a concomitant 1,2-palladium and aryl migration may
generate the intermediate IX (Scheme 4). Finally, CÀH inser-
tion/reductive elimination sequences may provide the ob-
served migratory product 2af, as depicted in Path B. Possibly,
the Pd0 species, which is generated after reductive elimination,
is reoxidized to PdII by a portion of hypervalent iodine(III) re-
agent to complete a PdII/PdIV/Pd0 catalytic cycle.[15]
Having a library of dibenzofused carbazoles in hand, we per-
formed preliminary studies on their photochemical proper-
ties.[11] A representative set of seven compounds exhibited UV
absorption in the region l=250–350 nm (Figure 1,a), whereas
the fluorescence emission of these compounds was in the
range l=370–450 nm upon the excitation at 350 nm in etha-
nol (Figure 1, b). Detailed studies of mechanistic and photo-
Scheme 3. Control experiments.
Scheme 4. Proposed mechanism of migratory and non-migratory carbazole formation.
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Acknowledgements
R.J. thanks DST (no. SR/S2/RJN-97/2012) and CSIR (miND; no.
BSC 0115), and R.N. thanks DST (no. SR/S2/RJN-62/2012) of the
Government of India for financial support. S.K.B. and A.P. thank
UGC and CSIR, respectively, for their fellowships. We thank S.
Kundu, S. Pramanik, S. Chatterjee, and B. Achari for their contri-
butions.
Keywords: annulation · cascade reactions · CÀH activation ·
carbazoles · palladium
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Figure 1. Photochemical properties of representative carbazoles.
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route to dibenzofused carbazoles from 2-arylindoles and diary-
liodonium salts. The photochemical properties of the carba-
zoles have also been preliminarily studied. The highly reactive
hypervalent iodine(III) reagents accelerated the serendipitous
discovery of unanticipated “through-space” 1,4-palladium mi-
gration through remote CÀH activation and 1,2-aryl shift by
a triple CÀH activation cascade. Besides the synthetic aspects,
mechanistic interpretation of this present procedure allows for
the further development of novel synthetic procedures.
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Experimental Section
General experimental procedure for the Dibenzo[a,c]carbazoles:
In
a 15 mL pressure tube, to a mixture of 2-phenylindole
(0.2 mmol), Pd(OAc)2 (0.02 mmol), diphenyliodonium salt
(0.5 mmol) and K2HPO4 (0.4 mmol) was added acetic acid (3.0 mL)
and hexafluoroisopropanol (1.0 mL). This reaction mixture was
stirred for 22 h at 1108C. After completion (established by TLC), the
mixture was cooled to room temperature and poured into saturat-
ed NaHCO3 solution (30 mL). This mixture was stirred until the evo-
lution of CO2 had ceased. The mixture was then extracted with
ethyl acetate (50 mL). The organic layer was dried over anhydrous
Na2SO4 and the solvent was removed under reduced pressure. The
crude residue was purified by column chromatography using ethyl
acetate/hexane as eluent to afford the desired product 2a in 85%
yield.
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Received: August 31, 2015
Published online on October 6, 2015
Chem. Eur. J. 2015, 21, 16786 – 16791
www.chemeurj.org
16791
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