Pd(II)-Catalyzed Ci-H Activation of Styrylindoles: Short, Efficient, and Regioselective Synthesis of Functionalized Carbazoles

Article abstract of DOI:10.1002/chem.201503657

A novel Pd^II-catalyzed approach for the direct synthesis of highly functionalized carbazoles from unprotected styrylindoles has been developed. The reaction features a variety of olefin substrates, which are readily switchable by subtle tuning

Full text of DOI:10.1002/chem.201503657

DOI: 10.1002/chem.201503657
Communication
&
CÀH Functionalization
Pd(II)-Catalyzed CÀH Activation of Styrylindoles: Short, Efficient,
and Regioselective Synthesis of Functionalized Carbazoles
Rakesh K. Saunthwal,[a] Monika Patel,[a] Sonu Kumar,[a] Abhinandan K. Danodia,[a] and
Akhilesh K. Verma*[a, b]
Abstract: A novel PdII-catalyzed approach for the direct
synthesis of highly functionalized carbazoles from unpro-
tected styrylindoles has been developed. The reaction fea-
tures a variety of olefin substrates, which are readily
switchable by subtle tuning of the reaction conditions. In-
vestigations of the mechanism suggest that the CÀH acti-
vation proceeds via enamine formation.
In the past two decades, transition-metal-catalyzed CÀC bond
Figure 1. Selected examples of carbazole alkaloids.
formations[1] by CÀH activation have gained considerable at-
tention, and have become one of the most attractive alterna-
tives for the conversion of unfunctionalized substrates into
a variety of heterocycles, natural products, pharmaceuticals,
and functional materials. Regioselective CÀH functionalization
of heterocyclic substrates by electrophilic palladation has been
noted in recent years.[2] After the discovery of the Fujiwara–
Moritani reaction,[3] several groups[4] have explored the regiose-
lective migratory insertion for electron-rich olefins by using
heteroatom-based directing groups.
The carbazole framework is present in numerous natural
products, which are of immense biological importance.[5,6]
Many of these exhibit significant photophysical and physiologi-
cal activities.[7] Ellipticine, antiostatin A4, hyellazole, clausena-
line, murrayanine, and carbazomycine B are notable examples
Scheme 1. Heteroatom-directed CÀH activation.
of carbazole alkaloids (Figure 1). Because of their significant im-
portance, a number of methods are available in the literature
for the synthesis of carbazoles.[8–10] However to the best of our
knowledge, CÀH activation of (E)-3-styryl-1H-indoles with al-
kenes has not been reported and remains challenging. Recent-
ly, Itami[11a] and Yu[11b] have explored the Diels–Alder[12] reaction
for the synthesis of carbazole from N-methylindole using the
tri- and bimetallic systems of Pd-Cu-Ag and Pd-Cu, respectively.
Arguably, alkenylation of substituted benzenes, containing a di-
recting group (DG), with alkenes is one of the most frequently
applied methods to generate olefins through Pd-catalyzed di-
rected CÀH activation (Scheme 1i).[13] An elegant regioselective
functionalization of indoles using a Pd catalyst has already
been reported by Gaunt and Carretero (Scheme 1ii).[14]
In continuation of our ongoing research work on transition-
metal-catalyzed synthesis of heterocycles,[15] we envisioned
that functionalized carbazoles could be obtained directly from
styrylindoles, through successive regioselective CÀH activation
by electrophilic palladation using a variety of olefins (Sche-
me 1iii).
To identify the optimal conditions for the reaction, a variety
of reported Pd catalysts, along with various combinations of
organic solvents, were examined in the reaction of (E)-methyl
3-(1H-indol-3-yl)acrylate (1a) with styrene (2a) (Table 1). Using
Gaunt conditions[14a] of Pd(OAc)2 (10 mol%) with oxidant
(1.8 equiv) in DMF/DMSO (9:1) at 708C for 18 h, the desired
product 3a was obtained in 10% yield (Table 1, entry 1). In-
creasing the temperature from 70 to 1008C provided 3a only
in 20% yield (Table 1, entry 2). When Pd(OAc)2 (10 mol%) was
[a] R. K. Saunthwal, M. Patel, S. Kumar, A. K. Danodia, Prof. Dr. A. K. Verma
Department of Chemistry
University of Delhi, Delhi 110007 (India)
E-mail: averma@acbr.du.ac.in
[b] Prof. Dr. A. K. Verma
School of Physical Sciences (SPS)
Institution Jawaharlal Nehru University, Delhi 110067 (India)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503657.
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Table 1. Optimization of the reaction conditions.[a]
Table 2. CÀH Activation of styrylindoles with styrenes.[a]
Entry
Catalyst
Solvent
Time [h] Yield [%][b] 3a
1[c,d]
2[c]
3[c]
4[c]
5[c]
6[c]
7[c]
8
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
DMF/DMSO (9:1)
DMF/DMSO (9:1)
DMF/DMSO (7:1)
DMF/DMSO (5:1)
DMF/DMSO (5:1)
DMF/DMSO (5:1)
DMF/DMSO (5:1)
18
18
18
18
36
36
40
10
20
25
30
50
60
68
70
70
25
00
30
trace
[PdCl2(PPh3)2] DMF/DMSO (5:1) 40
9[e]
10
11
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
DMF/DMSO (5:1)
DMF
NMP
DMA
DMSO
40
40
40
40
40
[a] Reactions were performed using 1 (0.5 mmol), styrene (2; 1.0 mmol),
[PdCl2(PPh3)2] (10 mol%), and Cu(OAc)2 (1.5 equiv) in DMF/DMSO (5:1;
2.0 mL) at 1008C for 40 h; isolated yields are given.
12
13
[a] Reactions were performed using 3-acrolyl indole (1a; 0.5 mmol), 2a
(1.0 mmol), catalyst (10 mol%), and Cu(OAc)2 (1.5 equiv) in solvent
(2.0 mL) at 1008C. [b] Isolated yield. [c] Cu(OAc)2 (1.8 equiv). [d] At 708C.
[e] 15 mol% of catalyst used.
Table 3. CÀH activation of styrene-substituted indoles with acrylates.[a]
used in DMF/DMSO (7:1), 3a was obtained in 25% yield
(entry 3). It is interesting to note that the use of DMF/DMSO in
a 5:1 ratio provided 3a in an improved yield (entry 4). On run-
ning the reaction for 36 h, the desired product 3a was ob-
tained in 50% yield (entry 5). The use of [PdCl2(PPh3)2] provid-
ed 3a in an improved yield (entry 6). Increase in the reaction
time from 36 to 40 h further improved the yield of 3a (entry 7).
A slight decrease in the oxidant loading from 1.8 to 1.5 equiv,
led to the formation of 3a in 70% yield (entry 8). No significant
change in the yield of the product 3a was observed on using
15 mol% of the Pd catalyst (entry 9). The role of DMSO as an
oxidant has also been identified.[16] Inferior results were ob-
tained when DMF, NMP, DMA, and DMSO were used on their
own as a solvent (entries 10–13).
[a] Reactions were performed using 1 (0.5 mmol), acrylates (4; 1.0 mmol),
[PdCl2(PPh3)2] (10 mol%), and Cu(OAc)2 (1.5 equiv) in DMF/DMSO (5:1;
2.0 mL) at 1008C for 40 h; isolated yields are given.
With the optimized conditions in hand, the generality and
scope of the reaction was examined (Table 2). A variety of in-
doloacrylates (1a–d), bearing electron-neutral and -deficient
substituents, reacted with styrenes (2a and 2b) to provide
functionalized carbazoles (3a–f) in 62–70% yield with excellent
functional-group tolerance. The reaction of (E)-methyl 3-(1H-
indol-3-yl) acrylate (1a) with styrene (2a) and tert-butyl styrene
(2b) provided the desired products 3a and 3b in 70 and 65%
yields, respectively. When the ethyl group was used as R2, the
reaction was well implemented to form the intriguing cyclized
product 3c in 70% yield. The reaction of butylindoloacrylate
(1c) with electron-rich alkenes, 2a and 2b, provided the prod-
ucts 3d and 3–e in good yields (68 and 64%, respectively);
however, the reaction of the electron-deficient substrate 5-bro-
moindole (1d) with 2b provided the desired product 3f only in
moderate yield (Table 2).
indole (1e) with electron-deficient acrylates (4a and 4b) provid-
ed the desired products 5aand5b in 57 and 58% yields, re-
spectively. When bromo and methoxy groups were used as R1,
the reaction was well implemented to form the intriguing cy-
clized products 5c–f in moderate to good yields (Table 3).
We further extended the scope of the reaction by using styr-
ylindoles (1) with a variety of alkenes (4) under optimized con-
ditions for 10–12 h (Table 4). The reactions of butylstyrylindoles
(1c and 1h) with ethylacrylate (4a) and methylacrylate (4c) pro-
vided the carbazoles 6a–c in good yields with excellent regio-
selectivities. It is interesting to note that substrate 1i, bearing
a strong electron-withdrawing -NO2 group, provided the de-
sired product 6d in 60% yield. The reaction of styrylindoles 1a-
and -1b with methyl vinyl ketone (4d) fruitfully provided the
corresponding carbazoles 6e-and 6f in 75 and 72% yields, re-
spectively
Encouraged by the above results, the reactions for the CÀH
activation of styrene-substituted indoles with acrylate was next
performed (Table 3). The reaction of electron-rich styryl-1H-
In order to support the proposed mechanistic pathway, vari-
ous preliminary experiments were performed (Scheme 2). For
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reaction conditions (Scheme 3). Astonishingly, the carbazole 8a
was obtained in 72% yield; however, the carbazole 9 (with N-
methyl) was not observed. The formation of 8a invokes the dif-
ferent N-deprotonating/protonating abilities of 1b and 1k.
These results suggest that the presence of free NH-group of
the indole is crucial for the reaction.
Table 4. CÀH Activation of acrylate-substituted indoles with electron-de-
ficient alkenes.[a]
To further provide the support for our mechanism, we per-
formed additional experiments using deuterated reagents
(Scheme 4). The isotopic labeling study of 1b with deuterated
styrene (10) provided the isotopic carbazole 11 in 40% yield.
This control experiment suggests that the CÀH activation is oc-
curring regioselectively first onto the alkene.
Based on the evidences from the control experiments, two
plausible pathways are proposed in Scheme 5.[17] The mecha-
nism is initiated by the flow of electrons in 1. Based on an en-
amine-like motif i, this flow of electrons would be consistent
with an attack of the carbon adjacent to R2 and palladium. The
loss of an NH proton would lead to the formation of a palladi-
[a] Reactions were performed using 1 (0.5 mmol), acrylates (4; 1.0 mmol),
[PdCl2(PPh3)2] (10 mol%), and Cu(OAc)2 (1.0 equiv) in DMF/DMSO (5:1;
2.0 mL) at 1008C for 12 h; isolated yields are given. [b] Time = 10 h.
the validation of a possible reac-
tion intermediate, we performed
gram-scale experiments using
styrylindoles 1b and 1j with al-
kenes 4a and 4c. Thus, carba-
Scheme 3. Study of protected and deprotected styrylindoles.
zoles 8a and 8b were obtained
in 65 and 70% yields, respective-
ly, along with the intermediates
um-allyl system iia (Path A). The regioselective olefin
insertion at the a-position of R2 will generate the
species iiia. The newly formed PdÀC bond would
next attack the imine (C=N) to establish the tricyclic
core skeleton iva. The intermediate 7 would be
formed through the b-hydride elimination followed
by N-protonation and CÀH deprotonation on the CÀ
H bond next to R2. The intermediate 7 would under-
go the same CÀPd bond-forming process as men-
tioned above with a proton loss of the NÀH bond
and nucleophilic attack of the terminal carbon carry-
ing R3 to palladium to form skeleton via, which will
then undergo intramolecular carbopalladation of the
C=N bond (viia) leading to the formation of carba-
zoles through b-hydride elimination (viiia) followed
by a CÀH deprotonation/N-protonation sequence.
The palladium(II)–hydrido complex reduces into Pd0,
which is then oxidized by Cu(OAc)2 to PdII (Path A).
Alternatively, the regioselective palladation can also
take place on styrylindole (1) at the a-position of R2
(Path B). The intermediate 7 is produced following
Scheme 2. Control experiments.
7a and 7b in 3 and 4% yields, respectively (Scheme 2i). The
treatment of intermediate 7b with [PdCl2(PPh3)2] (5 mol%) and
Cu(OAc)2 (1.0 equiv), provided the carbazole 8b in 80% yield
(Scheme 2ii). However, under thermal conditions and in the ab-
sence of a Pd catalyst, the carbazole was not observed. These
observations clearly support the formation of carbazoles
through successive CÀH activation steps.
We next carried out the reaction of 1b (free NH indole) and
1k (N-methyl indole) with ethyl acrylate using the optimized
Scheme 4. Isotopic labeling studies.
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(1.0 mmol) were added under an
inert atmosphere. The resulting re-
action mixture was heated at
1008C for 10–40 h and progress of
the reaction was monitored by
TLC. After complete consumption
of vinylindole, the reaction was
brought to room temperature.
Note: an additional amount of ac-
rylate was added, if required, for
complete conversion. The reaction
mixture was diluted with ethyl ace-
tate (10 mL) and water (15 mL),
and then filtered through a plug
of celite. The layers were separat-
ed, and the organic layer was
washed with aqueous saturated
brine solution and dried over
Na2SO4. The organic layer was con-
centrated under reduced pressure
and the resulting crude material
was purified by column chroma-
tography on silica gel (hexane/eth-
ylacetate = 90:10). The structure
and purity of known starting mate-
rials were confirmed by compari-
son of their physical and spectral
data (1H NMR, 13C NMR, and
HRMS).
Scheme 5. Plausible mechanism.
the oxidative Heck cycle through regioselective palladation (i),
olefin insertion (iib), and syn b-hydride elimination of Pd from
iiib. Further the catalytic cycle is completed by C2-palladation
(ivb), followed by the oxidative intramolecular cyclization (vb)
and b-hydride elimination (vib), which leads to the carbazole
formation (Path B). To support the mechanism, the key inter-
mediate 7 was isolated and confirmed by NMR and HRMS
spectra. In Path A, the steps rely on NÀH deprotonation, which
cannot be accomplished with N-CH3 system. This was indeed
found experimentally as shown in Scheme 3, which favors the
proposed mechanism via enamine formation.
Acknowledgements
This work was supported by DST and UPE-II, JNU. R.K.S., M.P.,
and S.K. are thankful to UGC, DST and CSIR for fellowship
grants. We gratefully acknowledge USIC, University of Delhi,
and AIRF, JNU for providing the instrumentation facilities.
Keywords: carbazoles
·
CÀH activation
·
palladium
·
regioselectivity · styrylindoles
In conclusion, a novel PdII-catalyzed approach for the direct
synthesis of highly functionalized carbazoles from unprotected
styrylindoles has been developed. The three pairs of substrate
scopes, namely acrylate-substituted styrylindoles with styrene,
styrene indoles with acrylates, and acrylate-substituted styrylin-
doles with arylate/ketone were established for the synthesis of
the functionalized carbazoles. The results of the control experi-
ments, deuterium labeling and isolation of the key intermedi-
ate suggests that the mechanism of CÀH activation proceeds
via enamine formation (Path A over Path B).
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Received: September 11, 2015
Published online on November 3, 2015
Chem. Eur. J. 2015, 21, 18601 – 18605
www.chemeurj.org
18605
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim