Indium Catalyzed Sequential Regioselective Remote C?H Indolylation and Rearrangement Reaction of Peroxyoxindole

Article abstract of DOI:10.1002/adsc.202100793

Indium-catalyzed sequential remote C?H functionalization (C-6 position) and C3-indolylation of peroxyoxindole using indole is described for the synthesis of terindolinone derivatives. Whereas, N-substituted 3-phenyl peroxyoxindole derivatives undergoes consecutive skeletal rearrangement to generate transient carbocation, which has been trapped with indole nucleophile to generate 2-(1H-indol-3-yl)-4-alkyl-benzo[b][1,4]oxazin-3(4H)-one derivatives. In contrast with Indium (III) Chloride, FeCl₃ ? 6H₂O facilitates oxidative cleavage of the peroxyoxindole (Hock cleavage) and further reaction with indole to afford biologically important trisindoline derivatives. A plausible mechanism has been proposed for these reactions with experimental evidences. (Figure presented.).

Full text of DOI:10.1002/adsc.202100793

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doi.org/10.1002/adsc.202100793
Indium Catalyzed Sequential Regioselective Remote CÀ H
Indolylation and Rearrangement Reaction of Peroxyoxindole
Moseen A. Shaikh,a Akash S. Ubale,a and Boopathy Gnanaprakasama,*
a
Department of Chemistry, Indian Institute of Science Education and Research, Pune-411008, India
E-mail: gnanaprakasam@iiserpune.ac.in
Manuscript received: June 25, 2021; Revised manuscript received: August 12, 2021;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202100793
formation towards the chemical synthesis were
Abstract: Indium-catalyzed sequential remote CÀ H
functionalization (C-6 position) and C3-indolylation
of peroxyoxindole using indole is described for the
synthesis of terindolinone derivatives. Whereas, N-
substituted 3-phenyl peroxyoxindole derivatives
undergoes consecutive skeletal rearrangement to
generate transient carbocation, which has been
trapped with indole nucleophile to generate 2-(1H-
indol-3-yl)-4-alkyl-benzo[b][1,4]oxazin-3(4H)-one
derivatives. In contrast with Indium (III) Chloride,
FeCl3 ·6H2O facilitates oxidative cleavage of the
peroxyoxindole (Hock cleavage) and further reac-
tion with indole to afford biologically important
trisindoline derivatives. A plausible mechanism has
been proposed for these reactions with experimental
evidences.
scarcely studied.[10] Furthermore, synthetic application
of peroxide as a coupling partner for the cross-coupling
reaction has not been reported in the literature.
Remarkably, in the last decade heterocycle substi-
tuted peroxides has gained significant attention in
chemical transformation.[11] As this peroxide are
chemically and thermally stable, this might attribute as
a coupling partner for the cross-coupling reactions and
unprecedented rearrangement reactions.[12] In addition,
it also exhibit various biological activities.[13] In 2017,
Stoltz and co-workers have shown the reactivity of
peroxyoxindole derivatives for the oxidative fragmen-
tation and skeletal rearrangement reactions.[14a] Sub-
sequently, we developed a Lewis acid catalyzed ring
expansion of oxindole derivatives via skeletal rear-
rangement of peroxyoxindole.[14b,c] Huo and co-workers
synthesized derivatives of benzoxazinone peroxides
and substituted the peroxy functionality with nucleo-
philes in the presence of diverse metals as the catalytic
system.[15]
Keywords: Peroxyoxindole; Indium (III) Chloride;
CÀ H functionalization; Trisindoline; Hock cleavage
Recently, Chen and co-workers described the syn-
thesis of 2-indolyl-3-peroxyindolenine via acid cata-
Peroxides are an imperative functional groups in lysed addition of indole across the imine bond, which
organic chemistry that can be found in many natural has been further explored for the diverse functionaliza-
products and pharmaceutical drugs.[1] Since peroxide tion of tetrahydro-β-carbolines via oxidative coupling
has weak oxygen-oxygen bond (ΔH 298 =158– rearrangement.[16] Although limited investigations are
°
194 kJmolÀ 1)[2] and low bond energy, intense applica- available for reactions and rearrangement of peroxyox-
tion can be found as a very attractive and important indole derivatives, exploration of these peroxides for
intermediate in many chemical transformations and the newer rearrangement and chemical reactions
rearrangement reactions.[3] As a result of this unique fascinate in contemporary synthetic methodology.
bond property, organic peroxides deliver a numerous Apart from peroxyoxindole, indolylation of 2-oxindole
fundamental rearrangement reactions like Baeyer- and other heterocycles are also important since this
Villiger,[4] Criegee,[5] Hock,[6] Kornblum-DeLaMare,[7] heterocycle represents as core structures in many
Smith,[8] etc. In contrast, Minisci and co-workers biologically active natural products and therapeutically
reported the substitution reaction of cumene peroxide important targets.[17] Thus, direct displacement of
with phenol.[9] Indeed, peroxides were used in multiple quaternary peroxides by indole or sequential reactions
transformations such as oxidation reagent, radical of peroxide is highly desirable in modern chemical
initiator and energetic materials. However, diverse synthesis as it deliver quaternary centre and scaffold of
applications of these peroxides in CÀ C and CÀ X bond biologically important natural products (Figure 1).[18]
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(Table 1, entries 11–15). Moreover, higher catalyst
loading (30 mol%) increases the yield of 3a (Table 1,
entry 16). The best-optimized condition obtained by
heating the reaction mixture of 3-(tert-butylperoxy)-3-
methylindolin-2-one (1a) and indole (2a) (3 eq.), in
the presence of 30 mol% of In(OTf)3 or InCl3 catalyst
°
at 100 C for 24 hrs, which afforded product 3a in
70% and 72% yield respectively (Table 1, entry 16 and
17). We used InCl3 as a catalyst for the further reaction
because it is less expensive than In(OTf)3.
Next, these optimized conditions were applied to
generalize the substrate scope for consecutive CÀ C
bond formation with CÀ H functionalization of 2-
oxindole at the C-6 carbon center (Scheme 1). Initially,
the reaction of 3-(tert-butylperoxy)-3-methylindolin-2-
one 1a with optimized condition gave 3’-methyl-
[3,3’:6’,3’’-terindolin]-2’-one 3a in 72% yield and the
compound was well-characterized by spectroscopic
Figure 1. Representative bioactive compound having 2-oxin-
dole core functionality.
To the best of our knowledge, there is no report for techniques and X-ray analysis (see ESI Figure 1). The
the consecutive remote CÀ H indolylation of peroxyox-
indole and C3- indolylation of 2-oxindole by a peroxy
displacement reaction. Further, there is no report for
the sequential rearrangement of peroxyoxindole and
incursion of the indole in the transient carbocation
intermediate. For the first time, herein we report
multiple new reactions promoted by a single Lewis
acid-catalyst (InCl3) for the remote CÀ H indolylation
and C3-indolylation of peroxyoxindole. Whereas, by
protecting NÀ H of the peroxyoxindole, a completely
different reaction arises which led to rearrangement
and indolylation of the transient carbocation. More-
over, FeCl3 promotes consecutive Hock cleavage and
further reaction with indole afforded biologically
important trisindoline derivatives.
We commenced our reaction studies using 3-(tert-
butylperoxy)-3-methylindolin-2-one 1a and indole 2a.
Initially, we have performed a control experiment in
the absence of catalyst, when peroxide and indole in
°
acetonitrile are stirred at 100 C for 24 hours, results in
no reaction (Table 1, entry 1). We then accomplished
the reaction using a catalytic amount of Sn(OTf)2
(10 mol%), gave 48% yield of 3’-methyl-[3,3’:6’,3’’-
terindolin]-2’-one 3a with deprotected peroxide 5a in
37% as a byproduct (Table 1, entry 2). When the
reaction was done with three equivalents of indole, the
yield of product was improved (Table 1, entry 3). This
suggests that an excess amount of indole is required
for higher yield. Other catalysts such as AgOTf,
Cu(OTf)2, and Pd(OAc)2 afford the reduced hydroxy
product 6a (Table 1, entries 5–7). Whereas In(OTf)3,
Sc(OTf)3 and amberlyst-15 gave good yield of product
3’-methyl-[3,3’:6’,3’’-terindolin]-2’-one 3a in 66%,
55% and 50% respectively (Table 1, entries 8, 9, 10).
These experiments reveal that In(OTf)3 is the best
catalyst for this transformation. Further, the role of
Scheme 1. Substrate scope for sequential remote CÀ H function-
solvent was also investigated, which suggested
acetonitrile is a superlative solvent for this conversion
alization and C-3 indolylation.a
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Table 1. Optimization of reaction conditions[a]
sr. no[a]
catalyst
solvent
yield
yield
yield
yield
(10 mol%)
3a (%)
4a (%)
5a (%)
6a (%)
1[b]
2[b]
3
–
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
Dioxane
Ethyl Acetate
THF
–
–
–
–
–
–
–
–
–
9
–
–
–
–
–
–
–
trace
45
42
17
–
Sn(OTf)2
Sn(OTf)2
Sn(OTf)2
AgOTf
48
60
57
–
–
–
66
55
50
7
37
27
38
–
–
–
4[c]
5
6
7
8
9
10
11
12
13
14
15
16[d]
17[d]
Cu(OTf)2
Pd(OAc)2
In(OTf)3
Sc(OTf)3
Amberlyst-15
In(OTf)3
In(OTf)3
In(OTf)3
In(OTf)3
In(OTf)3
In(OTf)3
InCl3
–
15
35
–
–
66
Complicated reaction mix.
Complicated reaction mix.
–
–
70
72
DMF
–
–
–
–
–
–
–
–
31
Toluene
MeCN
MeCN
trace
trace
trace
[a] Reaction conditions: [a]Catalyst (10 mol%), compound 1a (0.3 mmol), indole 2a (0.9 mmol), and solvent (2 mL) were stirred at
°
100 C for 24 hrs.
[b] Indole 2a (0.3 mmol).
[c]
°
80 C.
[d] Catalyst (30 mol%). The mentioned yields are isolated yields.
electron-donating group such as 2-methyl, 5-methoxy with 3-(tert-butylperoxy)-3-methylindolin-2-one 1a
on indole afforded moderate to good yield of the and indole 2a under optimized condition to afford
product (3b and 3c). Whereas in the case of N-methyl 54% yield of the product 3a along with 19% of 6a as
indole, the reaction proceeded smoothly and gave an a byproduct.
89% yield of 3d. Electron-withdrawing substituted
After obtaining effective results with 3-alkyl perox-
indoles like 6-chloro, 5-bromo, and 5-fluoro indoles yoxindole for the construction of CÀ C bond by
were reacted well with peroxyoxindole 1a, to deliver substitution of peroxide and CÀ H functionalization at
moderate yield of the products (3e–3g). Subsequently, C-6 carbon of oxindole, we turned towards the
the N-methyl peroxyoxindole 1b provided good yields rearrangement reaction peroxyoxindole followed by
of the corresponding products (3h–3l). Furthermore, indolylation to generate benzoxazinone with CÀ C bond
the reaction of N-butyl and N-benzyl protected formation. Recently, our group reported tin catalyzed
peroxide afforded 78% and 67% yield of the products. rearrangement of peroxyoxindole for the construction
The reaction also showed good results with 3-benzyl- of CÀ O bond.[14c] For the sequential rearrangement and
3-(tert-butylperoxy)-indolin-2-one 1c with 66%, 70% indolylation of peroxide, 3-(tert-butylperoxy)-1-meth-
and 71% yields of the products 3o, 3p and 3q yl-3-phenylindolin-2-one 1d and 3 equivalents of
respectively. However, this reaction with other hetero- indole 2a were reacted in presence of InCl3 (30 mol%)
arenes like pyrrole, benzofuran and imidazole were affording
2-(1H-indol-3-yl)-4-methyl-2-phenyl-2H-
failed to give the respective products. To our delight, a benzo[b][1,4]oxazin-3(4H)-one 7a in 77% isolated
gram-scale reaction was also successfully performed yield. The spectroscopic data confirmed the product
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7a. But we did not detect compound 3 in this reaction. the substrate scope, we studied this reaction with other
Furthermore, we reoptimized the reaction condition substituted peroxyoxindoles. The electron-donating
and we found that 1.5 equivalents of indole was and electron-withdrawing substitution on the benzyl
sufficient to deliver rearranged product 7a in 76% part of the peroxyoxindole result in a good to excellent
yield. Then, we set out the substrate scope for this yield of trisindoline derivatives 8b (Scheme 3). Fur-
transformation by using various indole and 3-phenyl thermore, indole having substitutions like 5-methoxy,
peroxyoxindole (Scheme 2). To our delight, the reac- 2-methyl, 6-chloro, N-methyl and N-allyl provided
tion of 3-(tert-butylperoxy)-1-methyl-3-phenylindolin- good to excellent yield of respective trisindoline
2-one 1d with indole having an electron-withdrawing products (8c–8g). This reaction was also successful
group like 5-bromo and 5-fluoro under optimized with N-butyl and N-benzyl peroxyoxindole to give 8h
reaction conditions afforded the desired products 7b and 8i in good yield, respectively.
and 7c in excellent yield, respectively. Then, the indole
To shed light on the mechanism, we performed the
bearing electron-donating functionalities such as 5- reaction of 4a and 2a under optimized reaction
methoxy, 2-methyl provided good yields of the conditions, in which the desired product 3a was not
corresponding products (Scheme 2, 7d–7e). Next, the observed (Scheme 4, entry a). Furthermore, TBHP was
N-protected indole gave 83% yield of the rearranged added in the reaction of 4a and 2a to check the role of
product 7f and the structure of 7f was confirmed by peroxy group on CÀ H functionalization, then product
the X-ray analysis (see ESI figure 2). Moreover, the 3a was not detected (Scheme 4, entry b). Next, a
substitution on phenyl of 3-(tert-butylperoxy)-1-alkyl- reaction of 5a or 6a with 2a under standard condition
3-phenylindolin-2-one gave moderate to good yield of afforded 3a. This suggests that product 3a can be
the products (Scheme 2, 7g–7h).
formed from 5a or 6a as an intermediate (Scheme 4,
On the other hand, 3-benzyl-3-(tert-butylperoxy)- entry c and d). These experiments clearly emphasize
indolin-2-one 1c undergoes Hock cleavage, which that both CÀ H functionalization and substitution take
delivers trisindoline derivatives in the presence of place simultaneously. Moreover, absence of peroxy
°
indole and FeCl3 ·6H2O as a catalyst at 100 C. Initially, group in 3-methyl-2-oxindole or 6-cholorooxindole
we have optimized this reaction (see supporting results no reaction with indole in the presence/absence
information table-1) and we found the best optimiza-
tion condition for this transformation is 3 equivalents
of indole 2a in the presence of 30 mol% Fe-catalyst at
°
100 C giving 87% yield of the product 8a. To extend
Scheme 2. Substrate scope for sequential rearrangement and
Scheme 3. Synthesis of trisindoline via sequential Hock cleav-
indolylation of peroxyoxindole.a
age and indolylation.a
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Scheme 5. Plausible mechanism for sequential remote CÀ H
functionalization and C-3 indolylation.
1
isobutylene is confirmed by H-NMR and GC-MS
spectra. Subsequently, protonation of intermediate D
and elimination of InCl3 generate complex E.[14b,c]
Furthermore, intermediate G can be formed from
intermediate E by elimination of H2O2,[9] which is
facilitated by the tautomerism involved with free NÀ H
of the peroxyindole in the presence of Lewis acid,
InCl3. Alternatively, intermediate G can also be formed
via the intermediate F by elimination of water assisted
by InCl3. Also, this intermediate G is confirmed by
HRMS analysis (see ESI, figure 3). According to
literature reports,[19] C-6 carbon of intermediate G has
the highest positive charge density than C-3. Hence
indole attack at C-6 carbon of complex G to afford
dearomatised complex I. Further, O2 might be gen-
erated in-situ from H2O2 under the experimental
condition.[20] Then, aromatization and peroxidation of
the complex I in the presence of H2O2 or O2 led the
intermediate J. Further, the intermediate J can be
transformed into the intermediate K in the presence of
Scheme 4. Experiments for mechanisms studies.
of external the oxidant TBHP. This indicates that InCl3. Finally, second indole attacks on C-3 of the
peroxide functionality is crucial for the CÀ H function- intermediate K led to the desired product 3. The
alization (Scheme 4, entry e). Similarly, dimethyl sub- product 4 is formed by the direct indole attack on the
stituted 2-oxindole or 3,3-dimethyl-2-oxindole under C3 position of the intermediate G.
the same condition resulted no reaction (Scheme 4,
The proposed mechanistic steps for rearrangement
entry f,g). Furthermore, in absence of indole, perox- reaction are similar to that of the substitution of
yoxindole gave product 5a and 6a (Scheme 4, entry i). peroxide (Scheme 6). However, in the case of rear-
From this, it was clear that the peroxy functionality rangement, complex D undergoes protonation to afford
(OÀ OÀ R) plays a vital role for the formation of intermediate E. Further coordination of E with InCl3
product 3.
furnished the Indium chelated complex L. Next, ring
Based on experimental observation and previous expansion led to carbocation intermediate M with
literature reports[9,14b,c,19] plausible mechanism for InCl3 regeneration of catalyst A.[14b,c] This might be due to
catalyzed synthesis 3’-methyl-[3,3’:6’,3’’-terindolin]-2’- resonance effect of phenyl ring which donate the
one 3 is illustrated in scheme 5. Initially, In(III) electron to the reactive center. This facilitate the ring
coordinates with peroxide 1 to form B. Then B expansion via C3–C4 carbon shift on to oxygen to
undergoes deprotonation produce complex D with the form the intermediate M. This carbocation M is
liberation of isobutylene C.[14b,c] The formation of trapped by indole H to afford rearranged product 7.
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peroxyoxindole undergoes sequential rearrangement
afforded 2-(1H-indol-3-yl)-4-methyl-2-phenyl-2H-ben-
zo[b][1,4]oxazin-3(4H)-one. Whereas in the case of
FeCl3 ·6H2O, 3-benzyl-3-(tert-butylperoxy)-indolin-2-
one undergoes oxidative (hock) cleavage followed by
indolylation delivers biologically active trisindoline
derivatives. All the reactions were supported with a
significant number of examples in 42–91% yields and
the products were completely characterized by spectro-
scopic data. The mechanism was justified with the
experimental evidences and based on the previous
literature reports.
Experimental Section
General experimental procedure for the synthesis of 3’-
alkyl-[3,3’:6’,3’’-terindolin]-2’-one (3): In a 20 mL re-sealable
vial was added InCl3 (0.09 mmol, 30 mol%), acetonitrile 2 mL,
peroxy compound (0.3 mmol, 1 equivalent) and finally indole
(0.9 mmol 3 equivalent). The tube was sealed with a cap using
°
crimper. The reaction mixture was heated at 100 C in a
preheated oil bath for 24 hrs. After 24 hrs added DCM and a
volatile component was evaporated using a vacuum and residue
was directly purified using silica gel chromatography (EtOAc:
hexane=20:80 to 50:50).
General experimental procedure for the rearrangement
reaction product (7): In a 20 mL re-sealable vial was added
InCl3 (0.06 mmol, 30 mol%), acetonitrile 2 mL, peroxy com-
pound (0.2 mmol, 1 equivalent) and finally indole (0.3 mmol
1.5 equivalent). The tube was sealed with a cap using crimper.
Scheme 6. Plausible mechanism for rearrangement and Hock
cleavage.
°
The reaction mixture was heated at 100 C in a preheated oil
bath for 24 hrs. After 24 hrs added DCM and a volatile
component was evaporated using a vacuum and residue was
directly purified using silica gel chromatography (EtOAc:
hexane=15:85 to 30:70).
Interestingly, based on previous literature reports, the
possible reaction mechanism for the Hock cleavage
product 8 and byproduct 9 is shown in Sche-
me 6.[9],[14c][16] In this case, iron (III) chloride chelates
with the peroxy (OÀ O) bond of 1 to produce complex
N. The migration of benzyl group on the oxygen of
peroxy (OÀ O) will generate isatin O and (tert-
butoxymethyl)benzene P. Then benzaldehyde 9 is
General experimental procedure for synthesis of trisindoline
(8) via Hock-rearrangement/cleavage: In a 20 mL re-sealable
vial (equipped with rubber septum and N2 balloon) was added
FeCl3 ·6H2O (0.06 mmol, 30 mol%), acetonitrile 2 mL, peroxy
compound (0.3 mmol, 1 equivalent) and finally indole
(0.9 mmol 3 equivalent). The tube was sealed with a cap using
°
crimper. The reaction mixture was heated at 100 C in a
formed as a cleaved product from P with the preheated oil bath for 24 hrs. After 24 hrs added DCM and a
volatile component was evaporated using a vacuum and residue
was directly purified using silica gel chromatography (EtOAc:
hexane=20:80 to 50:50).
elimination of isobutylene. Next, iron (III) is coordi-
nate with a carbonyl of isatin followed by indole attack
to generate complex Q. Finally, another molecule of
indole H is reacting with complex Q to afford the
desired product 8. Moreover, a reaction of isatin with
excess of indole in the presence of 30 mol% of FeCl3
The X-ray crystal structure: Crystallographic data are
deposited with the Cambridge Crystallographic Data Centre
(CCDC) under the following accession numbers: 3a (2054406)
afforded the product 8 (Scheme 4, entry h), support for and 7f (2060053). The data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via https://
www.ccdc.cam.ac.uk/data_request/cif/.
the proposed mechanism (Scheme 6).
In conclusion, the substituents on peroxyoxindole
delivered diverse reactions with indole in the presence
of Lewis acid as a catalyst. Thus, a sequential double
indolylation of peroxyoxindole via remote CÀ H
functionalization and C3-peroxy substitution for the
synthesis of terindolinone was achieved by inexpensive
InCl3 as a catalyst. Moreover, NÀ H protected 3-phenyl
Acknowledgements
This research was supported by the SERB (CRG/2018/003935),
India. M. A. S. thanks DST for INSPIRE fellowship. A. S. U
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thanks, UGC for fellowship. B. G. thanks SERB and IISER-
Pune for the research support
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