Selective Syntheses of Benzo[b]carbazoles and C3-Substituted Indoles via Tunable Catalytic Annulations of β-Alkynyl Ketones with Indoles

Article abstract of DOI:10.1002/adsc.202000491

Tunable catalytic annulation reactions of β alkynyl ketones with indoles have been developed, enabling multiple chemical bond-forming events to selectively access skeletally diverse indole-containing heterocycles with generally good yields. Silver-catalyzed intermolecular benzannulation reaction of β-alkynyl ketones with indoles afforded tetracyclic benzo[b]carbazoles whereas isochromen-1-yl-substituted indoles could be obtained using the same silver catalysis by lowering the reaction temperature (0 °C or rt). Interestingly, using Sc(OTf)₃ and AgOTf as a combined catalytic system led to the formation of C3-naphthylated indoles via intramolecular benzannulation reaction. (Figure presented.).

Full text of DOI:10.1002/adsc.202000491

Title: Selective Syntheses of Benzo[b]carbazoles and C3-Substituted
Indoles via Tunable Catalytic Annulations of β-Alkynyl Ketones
with Indoles
Authors: Bo Jiang, Dan Wang, Shi-Chao Wang, Wen-Juan Hao, and
Shu-Jiang Tu
This manuscript has been accepted after peer review and appears as an
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202000491
Link to VoR: https://doi.org/10.1002/adsc.202000491

10.1002/adsc.202000491
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Selective Syntheses of Benzo[b]carbazoles and C3-Substituted
Indoles via Tunable Catalytic Annulations of β-Alkynyl Ketones
with Indoles
Dan Wang,[a],1 Shi-Chao Wang,[a],1 Wen-Juan Hao,*,[a] Shu-Jiang Tu,[a] and Bo Jiang*,[a]
[a]
School of Chemistry and Material Science, Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional
Materials, Jiangsu Normal University, Xuzhou, 211116, P. R. China; E-mail: wjhao@jsnu.edu.cn (WJH);
jiangchem@jsnu.edu.cn (BJ); Tel./fax: +86 516 83500065; 1These authors contributed equally
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. Tunable catalytic annulation reactions of β
alkynyl ketones with indoles have been developed,
enabling multiple chemical bond-forming events to
selectively access skeletally diverse indole-containing
heterocycles with generally good yields. Silver-catalyzed
intermolecular benzannulation reaction of β-alkynyl
using the same silver catalysis by lowering the reaction
temperature (0 C or rt). Interestingly, using Sc(OTf)3 and
AgOTf as a combined catalytic system led to the formation
o
of
C3-naphthylated
indoles
via
intramolecular
benzannulation reaction.
Keywords: Benzannulation;
Benzo[b]carbazoles;
ketones
with
indoles
afforded
tetracyclic
Chemoselectivity; Isochromenes; oxo-Cyclization
benzo[b]carbazoles whereas isochromen-1-yl-substituted
indoles could be obtained
Introduction
Benzo[b]carbazoles constitute a class of important
polycyclic skeletons commonly present in a myriad
of naturally occurring alkaloids such as ellipticine B
and 9-methoxyellipticine as well as in biologically
active substances.[1] These molecules are, due to their
co-planar structures and chemiluminescent and
optoelectronic properties, widely applied in organic
semiconductor
and
luminescent
materials.[2]
Therefore, numerous efforts have been made to
establish effective methods for producing these
targets, with these methods often including
cycloaromatization of keteneimines (Scheme 1, path
a),[3] intramolecular Diels–Alder cycloaddition
reactions (Scheme 1, path b),[4] and benzannulation
reactions with indoles (Scheme 1, path c),[5]
naphthalenes,[6a,b] or carbazoles.[6c] However, most of
these methods suffer from the need to carry out
prefunctionalizations of starting materials,[3],[4] use of
expensive transition metal catalysts,[5],[6] harsh
reaction conditions[3-6] and uneconomical atomic
transformations.[3],[4],[6] Therefore, atom-economic
approaches are still needed to synthesize
benzo[b]carbazoles and their derivatives from simple
precursors and inexpensive catalysts.
Metal-catalyzed annulation reactions of β-alkynyl
ketones have been recognized as a versatile tool for
the assembly of bioactive cyclic scaffolds in a
convergent
Scheme 1. Profiles for the synthesis of benzo[b]carbazoles
manner.[7,8] Since the pioneering works reported by
Asao, Yamamoto and coworkers,[9] great advances in
the metal-catalyzed [4 + 2] cycloadditions of β-
1
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10.1002/adsc.202000491
Advanced Synthesis & Catalysis
alkynyl ketones with alkenes or alkynes have been
made, with the key advance here involving the in situ
generation of benzopyrylium intermediates as 1,4-
dipoles.[10] Recently, we developed silver-catalyzed
selective dimerizations and trimerizations of β-
alkynyl ketones to access functionalized spiro-
substituted isochromenes.[11] While carrying out this
project, we reasoned that under suitable silver
catalysis conditions, benzopyrylium intermediates
from β-alkynyl ketones could react with indoles,
enabling an intermolecular [4 + 2] cycloaddition and
generated isochromen-1-yl substituted indole 5a in
almost quantitative yield (98%, entry 10). Then,
Sc(OTf)3 as a Lewis acid was added into this reaction
system, and was found to clearly suppress the
formation of 3a but to favor the generation of 4a
(entry 11). A similar reaction tendency was observed
when elevating the reaction temperature. An increase
in the temperature to 80 oC provided an elevated yield
of 4a (37%) along with a reduced yield of 3a (40%,
entry 12). The reaction catalyzed by AgOTf/Sc(OTf)3
in toluene showed an even better yield of 4a (47%,
entry 13). To our satisfaction, this Ag/Sc-co-catalysis
only produced 4a in 60% yield when the reaction
temperature was elevated to 100 oC (entry 14).
Further increasing the temperature led to a lower
conversion into 4a (entry 15). Without AgOTf, the
reaction did not work and the starting materials were
recovered (entry 16).
oxygen
migration
process
to
give
benzo[b]carbazoles.[5g] In the current work, tunable
catalytic annulation reactions of β-alkynyl ketones 1
with indoles 2 could be well controlled by adjusting
reaction parameters including the catalyst, solvent
and reaction temperature, which allowed for the
selective syntheses of benzo[b]carbazoles 3, C3-
naphthylated indoles
4
and isochromen-1-yl
substituted indoles 5 with generally good yields. i)
Silver-catalyzed benzannulations between β-alkynyl
ketones 1 and indoles 2 in tetrahydrofuran (THF) at
40 oC accessed the benzo[b]carbazoles as major
o
products. ii) At 0 C or rt, the reactions proceeded in
a different direction to give isochromen-1-yl-
substituted indoles 5. iii) And, C3-naphthylated
indoles 4 were obtained upon carrying out synergistic
silver/scandium catalyses of these substrates in
o
toluene at 100 C (Scheme 1, path d). Herein, we
report these three types of catalytic annulation
reactions.
Results and Discussion
At the outset of our studies, the preformed β-alkynyl
ketone 1a and 5-methyl-1H-indole (2a) were chosen
as model substrates for the reaction in commercial
THF in the presence of silver trifluoroacetate
Scheme 2. Substrate scope for the synthesis of 3.
With these acceptable reaction conditions in hand, we
set out to investigate the generality of these
controlled catalytic annulation cascades for accessing
the three different functionalized indoles 3, 4 and 5
by examining β-alkynyl ketone and indole
components (Scheme 2, Scheme 3 and Scheme 4).
Under the optimized reaction conditions for forming
product 2a (Table 1, entry 2), 5-methyl-1H-indole
(2a) was first reacted with β-alkynyl ketones 1 having
either electronically poor, neutral, or rich groups
relative to the arylalkynyl moiety (R1), and gave
access to the corresponding benzo[b]carbazoles 3b-3g
in 40-89% yields (Scheme 2). Various substituents on
1, including fluoro (1b), methyl (1d), ethyl (1e), t-
butyl (1f), and methoxy (1g, 4-methoxyphenyl =
PMP) groups, were observed to tolerate the catalytic
conditions well. Representative indoles bearing a
methoxy (2b) or chloro (2c) functionality at the C4-
position were appropriate reaction partners; i.e., they
reacted with β-alkynyl ketones 1 to afford the
corresponding products 3h-3p with yields ranging
from 40% to 82%. Unsubstituted indole (2d) was also
engaged in this benzannulation and furnished the
desired products 3q-3t in 41-58% yields.
o
(AgTFA, 10 mol %) as a catalyst at 40 C under air
conditions (Table 1, entry 1). As a result, two
different
annulation
products,
namely
benzo[b]carbazole 3a and C3-naphthylated indole 4a,
were isolated in 32% and 10% yields, respectively.
Next, we aimed to improve the yield of each product
by realizing the chemoselectivity of the reaction.
Besides AgTFA, several other silver salts such as
AgOTf, AgOAc and AgNO3 were screened: AgOTf
displayed a better catalytic performance for the
formation of product 3a than did AgTFA, and drove
the conversion into 3a with 63% yield along with a
much lower yield of 4a (entry 2); in contrast, the
latter two did not show any improvement in the
efficiency of the transformation (entries 3 and 4).
Various solvents were tested. The use of aprotic
solvents including toluene, dichloromethane (DCM),
dimethyl sulfoxide (DMSO), and acetonitrile
(CH3CN) had no positive impact on the yield of 3a
(entries 5-9), but DMSO was beneficial for the
generation of product 4a, albeit with still a relatively
low yield, of 26% (entry 5). Unexpectedly, when the
temperature was lowered to 0 oC, the reaction
2
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10.1002/adsc.202000491
Advanced Synthesis & Catalysis
Table 1 Optimization of the reaction conditions for forming 3a, 4a and 5a[a]
Yield[b] (%)
Entry Cat. (mol%)
Solvent
Temp/oC
3a/4a/5a
1
AgTFA (10)
THF
40
40
40
40
40
40
40
40
40
0
32/10/0
2
AgOTf (10)
THF
63/8/0
3
AgOAc (10)
THF
23/12/0
4
AgNO3 (10)
THF
trace/trace/0
25/26/0
5
AgOTf (10)
toluene
DCM
6
AgOTf (10)
17/12/0
7
AgOTf (10)
DMSO
MeCN
1,4-dioxane
THF
trace/trace/0
trace/trace/0
trace/trace/0
trace/0/98
51/28/0
8
AgOTf (10)
9
AgOTf (10)
10
11
12
13
14
15
16
AgOTf (10)
AgOTf (10)/Sc(OTf)3 (20)
AgOTf (10)/Sc(OTf)3 (20)
AgOTf (10)/Sc(OTf)3 (20)
AgOTf (10)/Sc(OTf)3 (20)
AgOTf (10)/Sc(OTf)3 (20)
Sc(OTf)3 (20)
THF
40
80
80
100
110
100
THF
40/37/0
toluene
toluene
toluene
toluene
31/49/0
trace/60/0
trace/55/0
0/0/0
[a] Reaction conditions: 1a (0.5 mmol), 2a (0.75 mmol), catalyst (x mol%), and solvent (3 mL), air conditions, 3 h.
[b] Isolated yield.
withdrawing (chloro and fluoro), electron-neutral (H),
and electron-
Scheme 3. Substrate scope for the synthesis of 4.
Scheme 4. Substrate scope for the synthesis of 5.
After the successful syntheses of products 3, we
turned our attention to evaluating AgOTf/Sc(OTf)3-co-
catalyzed annulation reactions for synthesizing C3-
naphthylated indoles 4 by exploiting the optimized
reaction conditions (Table 1, entry 14; Scheme 3). β-
Alkynyl ketones bearing various substituents linked
by the arylalkynyl moiety (R1) with electron-
donating (methyl, ethyl and methoxy) groups were all
accommodated, confirming the efficiencies of the
benzannulation reactions, as products 4a-4q were
obtained with 46%-92% yields. The reaction
proceeded reliably in the presence of different
functional groups (methyl, methoxy and chloro) at
the C5 position of the indole ring. Similarly,
unsubstituted indole (2d) was also suitable for this
3
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10.1002/adsc.202000491
Advanced Synthesis & Catalysis
reaction. And N-methyl indole (2e) worked well
under the standard conditions, furnishing product 4q
with an 89% yield. Next, we focused on the silver-
enabled 6-endo-dig oxo-cyclization/nucleophilic
addition cascade of β-alkynyl ketones 1 with indoles
for the syntheses of isochromen-1-yl-substituted
indoles 5. These reactions proceeded readily under
role of silver cation, the reaction of 1c with 2a was
conducted in the presence of HOTf, but the target 3c
was not observed (Scheme 5b), demonstrating that
silver cation may activate the carbon-carbon triple
bond of β-alkynyl ketone 1 to accelerate its further
transformation. In order to understand reaction
pathway for forming 3, the resulting product 5a was
o
the standard conditions (Table 1, entry 10; Scheme 4), subjected with the reaction in THF at 40 C in the
enabling Ag catalysis to offer indole products 5a-5g
in good to excellent yields. As described above, β-
alkynyl ketones with both electron-withdrawing
(fluoro, chloro and bromo) and electron-donating
(methyl and ethyl) groups were compatible.
presence of AgOTf, and the corresponding
benzo[b]carbazole 3a was obtained in 78% yield.
These results may support product 5 to be
intermediate for accessing product 3 (Scheme 5c) and
the high temperature favors ring-opening of pyran
ring and electrocyclization in the presence of Ag
catalyst.
Similar tests were also carried out using indole
components. In general, the current controllable
tandem annulation strategies represent reliable and
chemoselective
approaches
for
the
direct
constructions of richly decorated indoles and their
fused derivatives. The structures of these heterocycles
were confirmed using NMR spectroscopy and HRMS.
Furthermore, in the cases of 3j and 5c, their structures
were unambiguously determined using single crystal
X-ray diffraction (Figures 1-2).[12]
Scheme 5. Control experiments.
Figure 1. The ORTEP drawing of 3j.
To gain a mechanistic insight into the formation of 4,
the preformed 3-phenylnaphthalen-1-ol 6 was treated
with indole 4d under the standard conditions (Table 1,
entry 14). It did not give the desired product 4l
(Scheme 5a), indicating that 3-phenylnaphthalen-1-ol
is not an intermediate for the generation of 4 and the
dehydration of a β-alkynyl ketone with an indole
occurs prior to its carbocyclization. To confirm the
Figure 2. The ORTEP drawing of 5c.
4
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10.1002/adsc.202000491
Advanced Synthesis & Catalysis
Scheme 6. Proposal reaction pathways
Example for the synthesis of 3a: Under air conditions,
CF3SO3Ag (12.8 mg, 10 mol %), 5-methyl-1H-indole (2a,
98.3 mg, 0.75 mmol, 1.5 equiv), 1-(2-((4-
chlorophenyl)ethynyl)phenyl)ethan-1-one (1a, 127 mg, 0.5
mmol) and THF (3.0 mL) were added into a 10-mL
reaction vial. Then the reaction vial was sealed and stirred
at 40 °C. Until TLC (petroleum ether: ethyl acetate 5:1)
revealed that conversion of the starting material 1a was
complete (3 hours), the reaction mixture was dissolved in
acetone. After that, the reaction mixture was concentrated
by performing vacuum distillation and was purified by
carrying out flash column chromatography (silica gel,
mixtures of petroleum ether / acetic ester, 30:1, v/v) to
afford the desired pure product 3a as a yellow solid.
By combining the above experimental results with
literature precedents of silver-catalyzed oxo-
cyclizations,[7],[8],[9], we derived what appears to be
reasonable
mechanisms
for
these
three
transformations (Scheme 6). According to our
proposed mechanisms, first, Ag-catalyzed 6-endo-dig
oxo-cyclization of β-alkynyl ketone 1 would generate
isobenzopyrylium intermediate A, which at a reaction
o
temperature of 0 C apparently prefers to undergo
nucleophilic addition with indole to give product 5
and regenerate silver catalyst. At higher temperatures,
a ring opening reaction of product 5 yields
intermediate B in the presence of silver catalyst, and
then
intermediate
B
undergoes
thermal
General procedure for the synthesis of products 4
electrocyclization to afford dihydrobenzo[b]carbazole
intermediate C[5g,6c] and release silver catalyst.
Subsequent oxidation of C by O2 from air gives final
the product 3. In the presence of AgOTf and
Sc(OTf)3,[13] Sc(OTf)3 would, according to the
proposed mechanisms, activate carbonyl group of β-
alkynyl ketone 1, which would become trapped by
indole to form tertiary alcohol anion intermediate D,
followed by dehydration of D and release of Sc(OTf)3
to form 1,5-enyne intermediate E.[14] AgOTf would,
according to the mechanisms, then activate the
Example for the synthesis of 4a: Under air conditions,
CF3SO3Ag (12.8 mg, 10 mol %), Sc(SO3CF3)3 (49.2 mg,
20 mol%), 5-methyl-1H-indole (2a, 98.3 mg, 0.75 mmol,
1.5 equiv) and dry toluene (2.0 mL) were added into a 10-
mL reaction vial. Then, the reaction vial was sealed. After
that,
1-(2-((4-chlorophenyl)ethynyl)phenyl)ethan-1-one
(1a, 127 mg, 0.5 mmol) in dry toluene (1.0 ml) was added
into the reaction system at 110 °C via a syringe pump the
course of 2.0 h. Until TLC (petroleum ether: ethyl acetate
7:1) revealed that conversion of the starting material 1a
was complete (3 hours), the reaction mixture was dissolved
in acetone. After that, the reaction mixture was
concentrated by performing vacuum distillation and was
purified by carrying out flash column chromatography
(silica gel, mixtures of petroleum ether / acetic ester, 50:1,
v/v) to afford the desired pure product 4a as a colourless
oil.
carbon-carbon triple bond of
E
to promote
intramolecular 6-endo-dig cyclization,[15] offering
intermediate G —followed finally by a proton
transfer (PT) in G and regeneration of the Ag catalyst
to produce 4.
General procedure for the synthesis of products 5
Conclusion
Example for the synthesis of 5a: Under air conditions,
CF3SO3Ag (12.8 mg, 10 mol %), 5-methyl-1H-indole (2a,
98.3 mg, 0.75 mmol, 1.5 equiv) and THF (2.0 mL) were
added in a 10-mL reaction vial. Then, the reaction vial was
sealed. After that, 1-(2-((4-chlorophenyl)ethynyl)phenyl)
ethan-1-one (1a, 127 mg, 0.5 mmol) in THF (1.0 ml) was
added into the reaction system at 0 °C via a syringe pump
over the course of 1.0 h. The system was stirred at this
temperature until TLC (petroleum ether: ethyl acetate 4:1)
revealed that conversion of the starting material 1a was
complete (3 hours). The resulting reaction mixture was
dissolved in acetone, then concentrated by performing
vacuum distillation, and after that was purified by carrying
out flash column chromatography (silica gel, mixtures of
petroleum ether / acetic ester, 30:1, v/v) to afford the
desired pure product 5a as a white solid.
In conclusion, starting from easily available -
alkynyl ketones and indoles, we have established
tunable catalytic annulation reactions for the selective
syntheses of skeletally diverse benzo[b]carbazoles,
and C3-substituted indoles with generally good yields
under mild conditions. The silver-catalyzed
benzannulation reaction between -alkynyl ketones
and indoles afforded tetracyclic benzo[b]carbazoles.
o
At lower reaction temperatures (0 C or rt), this
silver- catalysis enabled a different route to be
followed to isochromen-1-yl-substituted indoles
through an oxo-cyclization and nucleophilic addition
cascade. Notably, by using Sc(OTf)3 and AgOTf as a
combined
catalytic
system,
the
complete
chemoselectivity was achieved and C3-naphthylated
indoles were furnished via benzannulation reactions.
The present transformations provide chemoselective
and practical protocols for the collection of skeletally
diverse indole-containing heterocycles from the same
substrates, opening new avenues for realizing the
control of the chemoselectivity.
Acknowledgements
We are grateful for financial support from the NSFC (No.
21871112 and 21971090)
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10.1002/adsc.202000491
Advanced Synthesis & Catalysis
Crone, S. F. Kirsch, K.-D. Umland, Angew. Chem., Int.
Ed. 2010, 49, 4661-4664.
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10.1002/adsc.202000491
Advanced Synthesis & Catalysis
FULL PAPER
Selective Syntheses of Benzo[b]carbazoles and C3-
Substituted Indoles via Tunable Catalytic
Annulations of β-Alkynyl Ketones with Indoles
Adv. Synth. Catal. Year, Volume, Page – Page
Dan Wang, Shi-Chao Wang, Wen-Juan Hao,* Shu-
Jiang Tu, Bo Jiang*
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