Well-defined NHC-Pd complex-mediated intermolecular direct annulations for synthesis of functionalized indoles (NHC = N-hetero-cyclic carbene)

Article abstract of DOI:10.1002/aoc.1793

As alternatives to the common tertiary phosphine/Pd systems, well-defined N-heterocyclic carbene-Pd complexes have been proven to be highly efficient precatalysts for intermolecular direct annalution of o-haloanilines and ketones at lower catalyst loadings. A highly efficient and practical protocol for synthesis of functionalized indoles was developed using (IPr)Pd(acac)Cl as catalyst. Both o-bromoanilines and o-chloroanilines gave rise to efficient coupling under the reaction conditions. Related to acyclic ones, cyclic ketones coupled more effectively with o-haloanilines. With [Pd(IPr)₂] as catalyst, the base-sensitive groups including OH and CO₂H groups could be tolerated. Copyright

Full text of DOI:10.1002/aoc.1793

Full Paper
Received: 7 January 2011
Revised: 26 February 2011
Accepted: 5 March 2011
Published online in Wiley Online Library: 5 May 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1793
Well-defined NHC–Pd complex-mediated
intermolecular direct annulations for synthesis
of functionalized indoles (NHC = N-hetero-
cyclic carbene)
Zhong Jina∗, Su-Xian Guoa, Ling-Ling Qiua, Gui-Ping Wua
and Jian-Xin Fanga,b∗
As alternatives to the common tertiary phosphine/Pd systems, well-defined N-heterocyclic carbene–Pd complexes have been
proven to be highly efficient precatalysts for intermolecular direct annalution of o-haloanilines and ketones at lower catalyst
loadings. A highly efficient and practical protocol for synthesis of functionalized indoles was developed using (IPr)Pd(acac)Cl
as catalyst. Both o-bromoanilines and o-chloroanilines gave rise to efficient coupling under the reaction conditions. Related to
acyclic ones, cyclic ketones coupled more effectively with o-haloanilines. With [Pd(IPr)2] as catalyst, the base-sensitive groups
c
including OH and CO2H groups could be tolerated. Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: indole synthesis; N-heterocyclic carbene; annulation; palladium
Introduction
perature (up to 140 ◦C) were required for efficient conversion.
In addition, the extreme air-sensitivity and degradable property
of the phosphine ligand at high temperatures,[8] together with
phosphine ligand and ligand decomposition byproduct removal
difficulties, seriously hampered application of the procedure in the
fine chemical and pharmaceutical industries.
Over the past decade, transition metal-mediated homogeneous
catalysis using N-heterocyclic carbenes (NHCs) as supporting
ligands has witnessed a vast development.[1] Although tertiary
phosphines are still widely popular as ancillary ligands, the use
of NHCs as alternative ligands in transition metal-catalyzed cross-
coupling reactions is still steadily increasing.[2] The shielding steric
substitutents in the heterocyclic backbone, high thermal stability
and strong σ-donor character result in very high stability of the
metal–NHC bond. Particularly in the field of Pd-catalyzed cross-
coupling reactions, well-defined NHC-containing Pd(II) complexes
have proven to be a superior alternative for catalytic systems of
tertiary phosphines.[3]
Activepharmaceuticalingredients(APIs)andnaturallyoccurring
alkaloids,[4] which exhibit significant, diverse biological activities,
are often found to share an indole unit as the central core in
their skeletons. For this reason, indole synthesis, including both
the construction of indole nucleus and the functionalization of
the indole ring, has attracted remarkable attention from synthetic
chemistry community.[5] Notwithstanding, many strategies suffer
from lack of availability of the starting materials, harsh reaction
conditions, multi-step reactions, low yields and resulting trou-
blesome separation and purification. In this sense, Pd-catalyzed
intermolecular direct annulation from the commercially avail-
able o-haloanilines and simple ketones appears to be a highly
efficient and practical strategy for the construction of an in-
dole nucleus (Scheme 1).[6] The tertiary phosphine/Pd complex
[Pd(Pt-Bu3)2] has recently proven to be a superior catalyst for
this transformation.[7] The procedure is applicable for both o-
chloroanilines and o-chloroaminopyridines. Unfortunately, high
catalyst loading (10 mol% of substrate) and elevated reaction tem-
Very recently, it has been shown that well-defined NHC–Pd
complex (NHC)Pd(allyl)Cl (NHC = SIPr) was able to catalyze the
coupling of 3-bromo-2-aminopyridine and simple ketones to form
therelatedazoindolederivatives.[9] However, highcatalystloading
(up to 20 mol%) was needed for this transformation. Given the
stringent US Food and Drug Administration (FDA) requirements
on the amount of Pd and P present in pharmaceuticals, combined
withthecost, productpurity, toxicityandenvironmentalconcerns,
reducing the amount of palladium to as little as possible in
the catalytic process, particularly on an industrial scale, is also
important.[10] In view of the robust catalytic performance of
various well-defined NHC–Pd complexes, we are very interested
in achieving this transformation at lower catalyst loadings by
utilizing well-defined NHC–Pd complexes as an alternative to the
common tertiary phosphine–Pd system. We herein report our
investigation of catalytic performance of well-defined NHC–Pd
∗
Correspondenceto:Zhong JinandJian-XinFang,InstituteofElemento-Organic
Chemistry, Nankai University, Tianjin 300071, China.
E-mail: zjin@nankai.edu.cn; fjx@nankai.edu.cn
a
Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071,
China
b
StateKeyLaboratoryofElemento-OrganicChemistry,NankaiUniversity,Tianjin
300071, China
c
Appl. Organometal. Chem. 2011, 25, 502–507
Copyright ꢀ 2011 John Wiley & Sons, Ltd.

Well-defined NHC–Pd complex-mediated intermolecular direct annulations
R2
O
X
[Pd]
R1
R
R1
R
conditions
N
NH2
R2
H
Scheme 1. Pd-catalyzed intermolecular direct annulations reaction.
complexes in intermolecular direct annulation for the synthesis of
functionalized indoles. In addition, the scope and limitations of
substrates and reaction mechanism are also discussed.
o-bromoaniline into the desired 2-phenylindole to a certain extent
with decreased catalyst loadings (5 mol%; entries 1–7, Table 1).
Amongst them, the (IPr)Pd(acac)Cl, 6, was found to give the best
result (entry 6, Table 1).
For optimizing reaction conditions, various bases were investi-
gated. In addition to KOtBu, NaOtBu also proved to be effective,
but to a lesser extent (entry 8, Table 1). In contrast, with Cs2CO3
as base, only 22% indole was obtained (entry 9, Table 1). Other
bases, including inorganic bases LiOH, KF and KOAc, and organic
base DBU, were completely ineffective. A survey of reaction sol-
vent and temperature revealed that toluene (100 ◦C) was optimal,
while other solvents, such as tetrahydrofuran (THF) (60 ◦C), 1,2-
dimethoxyethane (DME, 80 ◦C) and 1,4-dioxane (100 ◦C), caused
slightly lower yields. Meanwhile, elevated reaction temperature
(140 ◦C in xylene) and longer reaction time (24 h) did not give an
obviouslyhigheryield(entries13and14, Table 1). Additionally, itis
noteworthy that, with further decreased catalyst loading (2 mol%),
parallel yield could be also obtained, although a longer reaction
time (36–48 h) was required for complete conversion (entry 15,
Table 1).
Results and Discussion
Screening of Well-defined NHC–Pd Complexes for Direct
Annulations Reactions
In recent years, structurally diverse well-defined NHC–Pd com-
plexes have been developed for different synthetic utilizations.[11]
These complexes show prominent catalytic activities in the broad
cross-coupling reactions, including the Suzuki–Miyaura, Negishi,
Kumada–Tamao–Corriu and Buchwald–Hartwig reactions, and
α-arylation of ketones. They not only consist of an accurate ratio
of the Pd/ligand that avoids the use of excess costly ligand, but
generally also exhibit enhanced reactivity associated with easy
access to a highly active low-coordinate [Pd0L] species.[12]
Firstly, we examined the catalytic potential of the commercially
available NHC–Pd complexes (NHC)Pd(allyl)Cl, 1,[13] and PEPPSI,
2,[14] in direct annulation reactions. In early experiments we used
acetophenone and o-bromoaniline as the coupling substrates
and established that annulation reactions were performed in the
presence of 5 mol% NHC–Pd complex with KOtBu as base in
toluene. We found that both compounds 1 and 2 could efficiently
catalyze the transformation at 100 ◦C to give the corresponding
2-phenylindole. It has been demonstrated recently[15,16] that, in
addition to the dominant NHC ligand, other ancillary ligands
surrounding the Pd metal center also play a crucial role in
their catalytic performance. As a result, a variety of NHC–Pd
complexes stabilized by different anionic ligands, including
cinnamyl, 3,[17] and Cp, 4,[18] OAc, 5,[19] acac, 6[20] and PPh3, 7,[21]
were also prepared and their catalytic potential evaluated in direct
annulation reactions of o-haloanilines and ketones (Scheme 2). We
observed that, of the NHC–Pd complexes that we have examined
thus far, all were able to catalyze conversion of acetophenone and
Annulation Reaction of Ketones with o-Haloanilines
With optimized catalytic conditions in hand, we thus tested
(NHC)Pd(acac)Cl, 6, as a precatalyst in an intermolecular direct
annulation reaction between a wide array of acyclic and cyclic
ketones and o-bromoanilines at 5 mol% catalyst loadings. As
shown in Table 2, both cyclic and acyclic ketones reacted well
with o-bromoanilines under the present catalytic conditions.
Among the ketones investigated, cyclic ones gave rise to more
efficient annulation than the corresponding acyclic ones (entries
5, 11 and 16, Table 2). With regard to cyclic ketones, coupling
reactions of six-membered cyclic ketones were more effective
than those of five-membered analogs. For the aromatic ketones
examined, both electron-donating and electron-withdrawing
substituents in the aryl group had the same reactivity. For
instance, the annulation reaction of p-F-acetophenone with
4-methyl-2-bromoaniline gave the desired indole in an isolated
yield of 77% (entry 10, Table 2). Furthermore, it is remarkable
that the steric effect on o-bromoaniline was not obvious
and the sterically hindered 3-methyl-2-bromoaniline showed
equally high reactivity to 2-bromoaniline (entries 14–16,
Table 2).
Unfortunately, owing to the base-sensitivity of the groups OH
and CO2H, using KOBut as a base in an attempt to achieve annu-
lation between either o-bromoanilines or ketones incorporating
such groups was unsuccessful and a complicated mixture was
obtained. In the present catalytic system, the use of a strong base
such as KOBut has two results: (1) activation of these well-defined
NHC–Pd precatalysts to produce active [NHC–Pd0] species in a
catalytic cycle;[3] and (2) deprotonation of the enolizable ketone.
Previous research by Nazare has demonstrated that mild bases
such as K3PO4 are also effective for deprotonation of ketones.[7]
Therefore, if an active NHC–Pd0 catalyst, such as [Pd0(NHC)2],
IPr
Pd
IPr
Cl Pd Cl
N
IPr
Pd
IPr
Pd
O
O
Cl
Cl
O
O
O
H
H
R
Cl
1 R = H
3 R = Ph
5
2
4
IPr
IPr
O
N
N
Pd
O
Cl Pd Cl
P
Cl
Ph
Ph
Ph
6
7
IPr
Scheme 2. The structures of well-defined N-heterocyclic carbene–Pd
complexes.
c
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Z. Jin et al.
Table 1. Catalytic activity of well-defined NHC–Pd complexes in the annulation reactionsa
NHC-Pd complex
(5 mol %)
Br
O
Ph
Ph
N
H
base, solvent/T
NH2
Entry
NHC–Pd complex
Base
Solvent/temperature (◦C)
Yield (%)b
1
2
1
2
3
4
5
6
7
6
6
6
6
6
6
6
6
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
NaOtBu
Cs2CO3
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
Toluene/100
Toluene/100
Toluene/100
Toluene/100
Toluene/100
Toluene/100
Toluene/100
Toluene/100
Toluene/100
THF/60
52
50
45
47
43
68
44
62
22
47
52
59
67
66c
62d
3
4
5
6
7
8
9
10
11
12
13
14
15
DME/80
1,4-Dioxane/100
Xylene/140
Toluene/100
Toluene/100
a Reaction conditions: o-bromoaniline, 1.0 mmol; acetophenone, 3.0 mmol; NHC–Pd catalyst, 5 mol%; base, 4.0 mmol; MgSO4, 1.0 mmol; solvent,
3 ml; reaction time t, 10 h unless indicated.
b Isolated yields based on the average of two runs.
c Reaction time, 24 h. 14
d 2 mol% 6 was used; reaction time, 36 h.
Mechanism
Theoretically, construction of indole nuclei by annulation between
N
N
N
N
N
N
N
N
o-haloanilines and ketones may be achieved via two possible
pathways: (1) an α-arylation reaction[24] of ketones followed by
intramolecular dehydration cyclization process (path a); and (2) an
enamine formation followed by an intramolecular Heck coupling
reaction (pathb; Scheme 5).[6] The previous study has showed that
alone,potentiallyreactiveClsubstituentdoesnotcarryoutrelative
α-arylation of ketone under those reaction conditions, which
supports an enamine-Heck mechanism (path b).[7] In the present
study, we observed further that isolated enamine, prepared by
condensation of o-bromoanile with acetophenone mediated by
a TiCl4/TEA system,[25] was able to be cyclized to afford the 2-
phenylindole in an overall yield of 72% by utilizing the present
catalytic protocol (Scheme 6).
Pd
Pd
8a
8b
Scheme 3. The structures of [Pd0(NHC)2] complexes.
8a[22] and 8b[23] (Scheme 3), was employed, a strong base should
be avoidable so as to broaden the scope of substrates. In fact, we
have demonstrated that using mild K3PO4 as a base with 5 mol%
NHC–Pd complex 8b as the catalyst, o-bromoaniline could be
efficiently coupled with pyruvic acid to give 2-indolecarboxylic
acid in 89% yield (Scheme 4).
Conclusion
Owing to the increased availability and low cost of
o-chloroanilinesrelatedtoo-bromoanilines,usingo-chloroanilines
as the starting substrates remains a very attractive target in the
industrial applicable process. Therefore, we further investigated
the annulation reactions of o-chloroanilines with a variety of ke-
tones utilizing (NHC)Pd(acac)Cl, 6, as the catalyst. As expected,
using the present catalytic protocol, o-chloroanilines showed the
same reactivity and allowed for efficient annulation to produce the
functionalized indoles (Table 2). Like that in o-bromoanilines, in
general, cyclic ketones and aliphatic ketones gave rise to the cor-
responding indoles in higher yields than aromatic ketones (entries
5, 7 and 16, Table 2). Notably, heteroaromatic ketone exhibited
prominent reactivity under the present catalytic system (entries
18 and 19, Table 2).
In conclusion, a variety of well-defined NHC–Pd complexes have
been evaluated for their catalytic activity in direct annulations of
o-haloanilines with ketones. We have demonstrated that, instead
of common tertiary phosphine ligand, well-defined NHC–Pd
complexes, such as (IPr)Pd(acac)Cl, 6, can also be utilized as
highly efficient and practical catalysts for intermolecular direct
annulation reaction between o-haloanilines and ketones at lower
catalyst loadings. Both o-bromoanilines and o-chloroanilines are
applicable substrates under the present system. Cyclic ketones
give rise to more effective coupling than acyclic ones. Moreover,
using[Pd(IPr)2], 8b, ascatalyst, thebase-sensitivegroupsincluding
OH and CO2H group can be tolerated in this transformation.
Relative to the common tertiary phosphine–Pd catalytic system,
c
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Appl. Organometal. Chem. 2011, 25, 502–507

Well-defined NHC–Pd complex-mediated intermolecular direct annulations
Table 2. Annulation reaction of ketones with o-haloanilinesa,b
R2
O
X
NH2
t
6
(5 mol %), KOBu
R1
R
R
R1
toluene, 100 oC, 10 h
N
H
R2
F
OMe
N
H
N
N
H
N
N
H
H
H
5a X = Br, 98 %
5b X = Cl, 92 %
1a X = Br, 68 %
1b X = Cl, 66 %
4a X = Br, 65 %
4b X = Cl, 58 %
2a X = Br, 67 %
2b X = Cl, 62 %
3a X = Br, 62 %
3b X = Cl, 68 %
F
N
H
N
H
N
N
H
N
H
H
6a X = Br, 69 %
6b X = Cl, 65 %
7a X = Br, 81 %
7b X = Cl, 80 %
8 X = Br, 70 %
9 X = Br, 72 %
10 X = Br, 77 %
F
N
H
N
H
N
H
N
H
N
H
15a X = Br, 73 %
15b X = Cl, 70 %
14a X = Br, 61 %
14b X = Cl, 55 %
11 X = Br, 92 %
12 X = Br, 69 %
13 X = Br, 57 %
OMe
N
N
N
H
N
H
N
H
N
H
16a X = Br, 87 %
16b X = Cl, 80 %
17 X = Cl, 72 %
18 X = Cl, 86 %
19 X = Cl, 85 %
a Reaction conditions: o-haloaniline, 1 mmol; ketone, 3 mmol; (NHC)Pd(acac)Cl, 6, 5 mol%; KOtBu, 4 mmol; MgSO4, 1 mmol; toluene, 3 ml; reaction
temperature, T, 100 ◦C oil bath; reaction time, t, 10 h; reaction time not optimized.
b Isolated yield based on the average of two runs.
Br
O
8b (5 mol %), K3PO4
CO2H
dioxane,100 °C,10 h
CO2H
N
NH2
89%
H
Scheme 4. Synthesis of 2-indole-carboxylic acid.
the strategy for indole synthesis described here represents a
practically and industrially applicable alternative.
with uncorrected thermometers. Flash column chromatography
was performed on silica gel 60 (230–400 mesh). Well-defined
NHC–Pd complexes (NHC)Pd(allyl)Cl, 1,[13b] and PEPPSI, 2,[14d]
are commercially available and could also be prepared easily
according to the literature method. (NHC)Pd(cinnamyl)Cl, 3,[17a]
CpPd(NHC)Cl, 4,[18] Pd(OAc)2(NHC)(H2O), 5,[19] (NHC)Pd(acac)Cl,
6[20b] and PdCl2(IPr)(PPh3), 7,[21] were prepared following the
literature procedures.
Experimental Section
General Comments
All reactions were carried out under an Ar2 atmosphere unless
indicated otherwise. Toluene, THF, DME, 1,4-dioxane and xy-
lene were dried and distilled over Ph2CO/Na prior to use. All
ketones, o-bromoanilines and o-chloroanilines were used as re-
ceived from commercial sources. Potassium tert-butoxide and
sodium tert-butoxide was stored under argon in a desiccator. 1H-
and 13C-NMR spectra were recorded on a Bruker 400 MHz spec-
trometer at ambient temperature in CDCl3 (Cambridge Isotope
Laboratories, Inc.). Melting points of compounds were recorded
General Procedure for Annulation Reaction of o-Haloanilines
with Ketones
In air, potassium tert-butoxide (4 mmol, 450 mg), (NHC)Pd(acac)Cl,
6 (5 mol%, 32 mg) and anhydrous MgSO4 (1 mmol, 120 mg) were
weighed into a 10 ml screw-cap threaded vial that was sealed
with a septum and evacuated and backfilled with argon three
times. The o-bromoaniline or o-chloroaniline (1 mmol), ketone
c
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Z. Jin et al.
bromoaniline or 3-methyl-2-chloroaniline and acetophenone in
73 and 70% yield, respectively. Colorless solid. M.p. 99–100 ◦C, 1H
NMR δ (ppm): 8.24 (br, 1H, NH), 7.63–7.60 (m, 2H, ArH), 7.23 (d, 1H,
J = 8.4 Hz, ArH), 7.15–7.08 (m, 3H, ArH), 6.92 (d, 1H, J = 7.0 Hz,
ArH), 6.76 (s, 1H, ArH), 2.57 (s, 3H, CH3). 13C NMR δ (ppm): 163.6,
161.1, 136.5, 136.4, 130.2, 129.2, 128.9, 128.8, 126.8, 126.7, 122.6,
120.5, 116.2, 115.9, 108.5, 98.5, 18.8. ESI-MS [M+ + H]: 226. Anal.
calcd C15H12FN: C, 79.98; H, 5.37; N, 6.22. Found: C, 79.95; H, 5.40;
N, 6.21.
Similarly, 4-methyl-2-(4ꢁ-methoxy)phenyl-1H-indole (entry 17,
Table 2) was obtained from 3-methyl-2-chloroaniline and 4-
methoxyacetophenone in 72% yield. Colorless solid. M.p.
154–155 ◦C, 1H NMR δ (ppm): 8.20 (br, 1H, NH), 7.56 (d, 2H,
J = 8.5 Hz, ArH), 7.21 (d, 1H, J = 8.3 Hz, ArH), 7.09–7.05 (t, 1H,
ArH), 6.95 (d, 2H, J = 8.5 Hz, ArH), 6.90 (d, 1H, J = 7.1 Hz, ArH), 6.71
(s, 1H, ArH), 3.82 (s, 3H, CH3), 2.57 (s, 3H, CH3). 13C NMR δ (ppm):
159.3, 137.4, 136.3, 129.9, 129.3, 126.5, 125.3, 122.1, 120.3, 114.5,
108.4, 97.4, 55.4, 18.8. ESI-MS [M+ + H]: 238. Anal. calcd C16H15NO:
C, 80.98; H, 6.37; N, 5.90. Found: C, 80.90; H, 6.39; N, 5.89.
Ph
HX
O
NH2
α-arylation
path a
H2O
Ph
X
O
Ph
N
H
NH2
HX
Heck coupling
X
N
H
Ph
H2O
path b
Scheme 5. Possible pathways for annulation to indoles.
4-Methyl-2-(pyridin-4-yl)-1H-indole (entry 19, Table 2) was ob-
tained from 3-methyl-2-chloroaniline and 4-acetylpyridine in 85%
◦
1
yield. Colorless solid. M.p. 210 C, H NMR δ (ppm): 8.78 (br, 1H,
NH), 8.65 (d, 2H, J = 4.8 Hz, ArH), 7.55 (d, 2H, J = 4.9 Hz, ArH), 7.26
(d, 1H, J = 7.1 Hz, ArH), 7.18–7.14 (t, 1H, ArH), 7.07 (s, 1H, ArH),
6.95 (d, 1H, J = 7.0 Hz, ArH), 2.59 (s, 3H, CH3). 13C NMR δ (ppm):
150.4, 139.5, 137.1, 133.9, 131.0, 128.8, 123.9, 120.8, 119.0, 108.8,
101.5, 18.7. ESI-MS [M+ + H]: 209. Anal. Calcd. C14H12N2: C, 80.74;
H, 5.81; N, 13.45. Found: C, 80.88; H, 5.80; N, 13.49.
Br
6 (5 mol %), KOBut
Ph
toluene,100 °C,12 h
N
H
N
Ph
72%
H
Scheme 6. Catalytic synthesis of 2-phenyl indole.
(3 mmol, 3 equiv.) and toluene (3 ml) were added in turn via
a syringe. The mixture was heated in an oil bath of 100 ◦C for
12 h. After cooling to room temperature the reaction mixture was
filtered with celite, water (10 ml) was added to the filtrate and
the mixture was abstracted with ethyl acetate (10 ml × 3). The
combined organic phases were dried over anhydrous MgSO4 and
filtered. After removal of the solvents under reduced pressure, the
residue was purified either by flash chromatography on silica or
by recrystallization from n-hexane.
Acknowledgment
Financial support from the National Natural Science Foundation
of China (no. 20502012) is gratefully acknowledged.
Supporting information
Supporting information may be found in the online version of this
article.
According to the above-mentioned general procedure, the
following indole derivatives were obtained and their m.p.,
1H- and 13C-NMR were identical to those described in the
literature. 2-Phenyl-1H-indole (entry 1, Table 2, in 68 and 66%
yield),[26] 2-(4ꢁ-methyl)phenyl-1H-indole (entry 2, Table 2, in
67 and 62% yield),[27] 2-(4ꢁ-fluoro)phenyl-1H-indole (entry 3,
Table 2, in 62 and 68% yield),[27] 2-(4ꢁ-methoxy)phenyl-1H-indole
(entry 4, Table 2, in 65 and 58% yield),[28] 2,3,4,9-tetrahydro-
1H-carbazole (entry 5, Table 2, in 98 and 92% yield),[29] 6,11-
dihydro-5H-benzo[a]carbazole (entry 6, Table 2, in 69 and 65%
yield),[30] 2-ethyl-3-methyl-1H-indole (entry 7, Table 2, in 81 and
80% yield),[31] 5-methyl-2-phenyl-1H-indole (entry 8, Table 2, in
70% yield),[27] 5-methyl-2-(4ꢁ-methyl)phenyl-1H-indole (entry 9,
Table 2, in 72% yield),[32] 5-methyl-2-(4ꢁ-fluoro)phenyl-1H-indole
(entry 10, Table 2, in 77% yield),[33] 6-methyl-2,3,4,9-tetrahydro-
1H-carbazole (entry 11, Table 2, in 92% yield),[31] 8-methyl-6,11-
dihydro-5H-benzo[a]carbazole (entry 12, Table 2, in 69% yield),[34]
8-methyl-5,10-dihydroindeno[1,2-b]indole (entry 13, Table 2, in
57% yield),[35] 4-methyl-2-phenyl-1H-indole (entry 14, Table 2, in
61 and 55% yield),[36] 5-methyl-2,3,4,9-tetrahydro-1H-carbazole
(entry 16, Table 2, in 87 and 80% yield),[37] 2-(pyridin-4-yl)-1H-
indole (entry 18, Table 2, in 86% yield).[38]
References
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