A rhodium(ii) catalysed domino synthesis of azepino fused diindoles from isatin tethered: N -sulfonyl-1,2,3-triazoles and indoles

Article abstract of DOI:10.1039/c9cc08377d

An efficient and convenient protocol for the synthesis of a novel class of azepino fused diindoles from isatin tethered N-sulfonyl-1,2,3-triazoles and indoles has been disclosed. The reaction proceeds via denitrogenative aza-vinyl rhodium carbene formation to give a carbonyl ylide, which with indole results in 1,3-dipolar cycloaddition followed by sequential semipinacol rearrangement/ring expansion/oxidation to produce azepino fused diindoles. The reaction shows a broad substrate scope giving up to 81% yield. Furthermore, reversible catalytic hydrogenation and photophysical studies were carried out to demonstrate the application of these molecules.

Full text of DOI:10.1039/c9cc08377d

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A rhodium(II) catalysed domino synthesis of
azepino fused diindoles from isatin tethered
Cite this:DOI: 10.1039/c9cc08377d
N-sulfonyl-1,2,3-triazoles and indoles†
Received 25th October 2019,
Accepted 16th December 2019
a
a
Nilesh Kahar,
Pankaj Jadhav,a R. V. Ramana Reddyb and Sudam Dawande
*
DOI: 10.1039/c9cc08377d
rsc.li/chemcomm
An efficient and convenient protocol for the synthesis of a novel suitable substrates13 such as annulation reactions,14 X–H
class of azepino fused diindoles from isatin tethered N-sulfonyl- (C, N, O, S) insertions,15 carbene transfer reactions to (hetero)-
1,2,3-triazoles and indoles has been disclosed. The reaction aromatics,16 carbonyl ylide formation and subsequent reactions.17
proceeds via denitrogenative aza-vinyl rhodium carbene formation Moreover, N-sulfonyl-1,2,3-triazoles have also been used in the
to give a carbonyl ylide, which with indole results in 1,3-dipolar synthesis of fused azepines. Earlier, Yoo18 and Li19 reported an
cycloaddition followed by sequential semipinacol rearrangement/ intermolecular cycloaddition protocol to obtain fused azepines
ring expansion/oxidation to produce azepino fused diindoles. The from a-imino carbenes generated by denitrogenation of triazoles
reaction shows a broad substrate scope giving up to 81% yield. in the presence of Rh(II) and Rh(III) catalysts, respectively (Scheme 1, a).
Furthermore, reversible catalytic hydrogenation and photophysical Also, Shi16a,b and Sarpong20 described intramolecular cyclizations of
studies were carried out to demonstrate the application of these tethered N-sulfonyl-1,2,3-triazoles to obtain fused azepine derivatives
molecules.
using rhodium(II) catalysts (Scheme 1, b).
Polycyclic indoles are ubiquitous units of numerous natural
products, biologically active molecules, and functional materials.1
Specifically, azepino fused indoles such as azepino[1,2-a]indole,2
azepino[4,3-b]indole3 and 2,30-diindole like indirubins4,5 have
gained considerable attention due to their regular appearance in
important molecular frameworks. Given the importance of these
polycyclic indoles, appreciable efforts have been taken for their
synthesis.6–9 On the other hand, transition metal-catalysed carbo-
nyl ylide generation from diazo compounds and their subsequent
dipolar cycloadditions serve as one of the most privileged
synthetic tools in the synthesis of complex organic molecules.10
These methods have been successfully employed by various
researchers for the construction of polycyclic indole frameworks
from diazo compounds and indoles.11
In recent years, the pioneering contribution by Gevorgyan
and Fokin12 showed that the N-sulfonyl-1,2,3-triazoles act as a
competent substitute for the diazo compounds. These on
treatment with Rh(II) salts result in a-imino carbene complexes
Scheme 1 Rhodium catalysed azepine synthesis.
Thus, inspired by these findings, we envisaged that the
which in situ undergo a wide range of transformations with N-sulfonyl-1,2,3-triazoles derived from N-propargyl isatins in
the presence of a rhodium(II) catalyst would deliver electro-
a Department of Chemistry, Institute of Chemical Technology, Mumbai,
Maharashtra 400019, India. E-mail: sg.dawande@ictumbai.edu.in
b Department of Chemistry, Indian Institute of Technology, Indore,
Madhya Pradesh 453552, India. E-mail: ramireddy@iiti.ac.in
† Electronic supplementary information (ESI) available. CCDC 1911349 and
1911350. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c9cc08377d
philic a-imino carbene leading to intramolecular carbonyl
insertion to produce a ylide, which in situ could undergo 1,3-
dipolar cycloaddition with indole to give fused diindole species.
To investigate our hypothesis, we studied the rhodium(II)
catalysed domino reaction of isatin tethered triazole-1-((1-tosyl-1H-
1,2,3-triazol-4-yl)methyl)indoline-2,3-dione 1a with N-methylindole
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Table 1 Optimization of the reaction conditionsa
Table 2 The substrate scope of the Rh(II)-catalysed reactiona
Entry Catalyst (x mol%) Solvent Temp. (1C) Time (h) Yieldb (%)
1
2
3
4
5
6
7
8
9
Rh2(OAc)4 (2)
Rh2(OAc)4 (2)
Rh2(OAc)4 (5)
Rh2(esp)2 (5)
Rh2(Oct)4 (5)
Rh2(TFA)4 (5)
Rh2(Oct)4 (5)
Rh2(Oct)4 (5)
Rh2(Oct)4 (5)
Rh2(Oct)4 (5)
DCM
DCE
DCE
DCE
DCE
DCE
CHCl3
Toluene 80
EtOAc
DCE
50
80
80
80
80
80
80
12
6
6
6
6
6
6
6
6
6
10
28
55
72
78
—
65
60
56
54
80
120
10
a
Reaction conditions: 1a (0.11 mmol), 2a (0.1 mmol), Rh(II) catalyst
b
(X mmol), solvent (2 mL). Isolated yield.
2a (Table 1). Initially, the treatment of triazole 1a (0.11 mmol) with
indole 2a (0.1 mmol) using 2 mol% [Rh2(OAc)4] as catalyst in DCM
(2 mL) as solvent at 50 1C led to the formation of product 3a in 10%
yield after 12 hours (Table 1, entry 1). When the reaction was
carried out in DCE as a solvent at 80 1C the product 3a was formed
in 28% yield (Table 1, entry 2). Furthermore, with 5 mol% of
[Rh2(OAc)4], the yield was improved to 55% (Table 1, entry 3). The
product formed was confirmed by NMR spectroscopic analysis.
Delighted by this result, we screened the reaction with 5 mol%
Rh(II) salts like [Rh2(esp)2], [Rh2(Oct)4] and [Rh2(TFA)4] in DCE as
solvent (Table 1, entries 4–6). With these observations, we studied
the effect of polar and non-polar solvents on the reaction in the
presence of 5 mol% of [Rh2(Oct)4] as a catalyst (Table 1, entries 7–9).
Inversely, with elevation in temperature to 120 1C, the yields were
reduced to 54% (Table 1, entry 10). Thus, the optimal reaction
conditions include treatment of isatin triazole 1a (1.1 equiv.) with
indole 2a (1.0 equiv.) using 5 mol% of [Rh2(Oct)4] as a catalyst at
80 1C in 2 mL DCE as a solvent.
After identifying the absolute reaction conditions, we investi-
gated the generality of the cascade reaction for the synthesis of
several azepino fused diindole derivatives by the reaction of sub-
stituted isatin triazoles with various indoles using 5 mol%
[Rh2(Oct)4] as a catalyst (Table 2). The optimized reaction condi-
tions were found to be consistent when isatin triazole 1a was
a
Reaction conditions: 1a (0.22 mmol), 2a (0.2 mmol), Rh2(Oct)4 catalyst
(0.01 mmol), DCE (4 mL).
The reaction of C-7 formyl indoles, with isatin triazole
treated with different N-substituted indoles giving the products derivatives furnished the products 3o–3q in 62–65% yields.
3b–3e in 73–81% yields. The structures of compounds 3d and 3e Both tosyl and mesyl triazoles were equally effective in delivering
were confirmed by single-crystal X-ray diffraction analysis (XRD). the products 3r–3u with C-5-bromo indoles giving a 68–76% yield.
The treatment of 5-methyl isatin derived triazoles with N-alkylated Similarly, C-5 substituted isatin derivatives and C-5-bromo indoles
indoles afforded the products 3f–3h in 67–80% yields. Further- also provided the azepino fused diindole products 3v–3x in good
more, the reaction of triazoles obtained from 5-halo-isatins (Br, yields. Under optimal conditions, the reaction of N-methyl-7-
Cl, F) with indoles produced the products 3i–3l in moderate yields, azaindole with triazole 1a was sluggish and resulted in only
indicating that electronically diverse isatin triazoles do not have any 54% of product 3y.
significant impact on the reaction. Then the scope of the reaction
The reaction of triazole 1a with 1,2-dimethyl indole 2i under
was examined using isatin triazole obtained from mesyl (Ms) azide. the optimized reaction conditions efficiently results in 1,3-dipolar
This triazole on treatment with N-alkyl indoles delivered the cycloaddition/semipinacol rearrangement to produce octa-
products 3m–3n with 71% and 69% yields, respectively.
hydroazepino diindole 4 in 88% yield as diastereoisomers
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Scheme 2 The reaction of triazole 1a with 1,2-dimethyl indole.
(Scheme 2). In contrast, 1,3-dimethyl indole failed to give the
product.
To explore applications and study the mechanism of the
reaction, the newly obtained azepino fused diindole 3a was
subjected to hydrogenation reaction using 10 mol% PtO2
catalyst in 2 ml CDCl3 at 1 atmospheric pressure. Gratifyingly,
the 3a readily underwent hydrogenation resulting in the
disappearance of the blue color of the solution to give colorless
product 5 which when exposed to air resulted in product 3a
through reoxidation. On the repetition of the redox cycle, the
products 5 and 3a were reproduced (Scheme 3).
Scheme 4 Plausible mechanism of Rh(II)-catalysed reaction.
intramolecular carbonyl oxygen insertion gives carbonyl ylide
II, which leads to 1,3-dipolar cycloaddition with indole 2 giving
diindole fused oxabicyclo[2.2.1]heptane III. The intermediate
III further results in a sequential ring-opening/expansion
cascade through a semipinacol type rearrangement to give IV
which on proton transfer forms V. The resulting dihydroaze-
pino fused diindole on in situ oxidation produces product 3.
In summary, we have successfully developed an effecient
domino reaction for the synthesis of novel azepino fused
diindoles through Rh(II) catalysed reaction of isatin tethered
N-sulfonyl-1,2,3-triazoles with indoles. Diversely substituted
azepino fused diindoles were obtained in good to moderate yields.
Besides, we demonstrated catalytic hydrogenation-dehydrogenation
reactions of the newly obtained products as well as studied their
photophysical properties.
Scheme 3 Redox reaction of azepino fused diindoles.
The photophysical properties of 3e, 3l, and 3i were evaluated
using UV-Vis-NIR absorption and photoluminescence (PL)
spectroscopies. All the compounds exhibited almost identical
spectra with a relatively broad and intense absorption in the
UV-vis-NIR region. The sharp absorption bands between 280–
450 nm are ascribed to the p–p* transitions of the conjugated
systems, and the broad bands in the range of 450–860 nm are
assigned to the intramolecular charge transfer (ICT). In the PL
spectra, the shapes of the emission peaks were almost the same
for 3e, 3l, and 3i with an emission maximum in the 405–432 nm
region for all derivatives (Fig. 1).
SGD and RVRR acknowledge the financial support from the
Department of Science & Technology (DST-SERB) under the
schemes (YSS/2015/001994) and (PDF/2017/003036).
Conflicts of interest
There are no conflicts to declare.
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