Copper(II)-Catalyzed Synthesis of Indoloquinoxalin-6-ones through Oxidative Mannich Reaction

Article abstract of DOI:10.1002/ejoc.201501532

A Cu-catalyzed synthesis of indoloquinoxalin-6-one has been developed that starts from o-indolyl-N,N-dialkylamines through sp³ C-H bond oxidation α to the nitrogen atom with di-tert-butyl peroxide as oxidant. Other heterocycles, such as pyrrole

Full text of DOI:10.1002/ejoc.201501532

DOI: 10.1002/ejoc.201501532
Full Paper
C–H Activation
Copper(II)-Catalyzed Synthesis of Indoloquinoxalin-6-ones
through Oxidative Mannich Reaction
[
a]
[a]
[a]
[a]
[a]
Anupal Gogoi, Prasenjit Sau, Wajid Ali, Srimanta Guin, and Bhisma K. Patel*
Abstract: A Cu-catalyzed synthesis of indoloquinoxalin-6-one as pyrrole, imidazole and benzimidazole derivatives also reacted
has been developed that starts from o-indolyl-N,N-dialkyl- successfully to give their respective fused quinoxalin-6-one de-
3
amines through sp C–H bond oxidation α to the nitrogen atom rivatives. In this process, one of the methyl groups is trans-
with di-tert-butyl peroxide as oxidant. Other heterocycles, such formed into a carbonyl group.
Introduction
The classic Mannich reaction, which leads to the synthesis of
dialkylamines.[5] If the same indolyl moiety is anchored at the
ortho position of N,N-dialkylanilines, the stereochemical orienta-
tion will inhibit C3 attack and a 6-endo-dig cyclization reaction
takes place by C2 attack. To examine the feasibility of this
envisaged intramolecular CDC reaction, 2-indolyl-N,N-dimethyl-
amine (1a) was treated with CuBr (10 mol-%) and aqueous tert-
butyl hydroperoxide (TBHP; 70 %; 3 equiv.) in dimethyl sulfoxide
substituted ꢀ-amino carbonyl compounds, involves three com-
o
ponents: an enolizable aldehyde (or a ketone); an amine (1 or
o
2
); and formaldehyde. This important C–C bond-forming proc-
ess comprises two steps: formation of an imine or iminium ion,
which is obtained by the condensation of an amine with form-
aldehyde; and nucleophilic attack of an enolizable aldehyde or
a ketone at the reactive carbon end of the imine or iminium
(DMSO) at 80 °C. Interestingly, the reaction resulted in the for-
mation of indolo[1,2-a]quinoxalin-6-one (1′a) in 19 % yield
2
(
Table 1, Entry 1) through the expected intramolecular C(sp )–
[
1]
ion. In general, an imine or iminium ion generated through
an alternative route will react with a suitable nucleophile and
likely provide similar results.
3
C(sp ) bond-forming reaction and numerous other products.
This process is accompanied by concomitant installation of a
carbonyl functionality at the expense of the remaining two sp3
C–H bonds (Scheme 1, Path B).
An appropriate redox-active transition metal in conjunction
with a suitable oxidant can oxidize N,N-dialkylanilines to the
corresponding iminium ion, which, if trapped with a nucleo-
phile, will install the desired functionality at the carbon atom
adjacent to nitrogen. This oxidative Mannich concept forms
the basis of transition metal catalyzed inert sp C–H functionali-
[
2]
3
[
2]
zation reactions α to nitrogen atoms in N,N-dialkylanilines.
The in-situ generated iminium carbon can be attacked either
intermolecularly or intramolecularly with a suitable nucleophile.
The intermolecular cross-coupling reaction with tertiary amines
has undergone significant advances to encompass a wide range
[
2,3]
of nucleophiles,
although the intramolecular version of this
[
1f,4]
reaction has been little studied.
The motivation to explore
the intramolecular oxidative Mannich reaction led to our recent
report on Cu-catalyzed synthesis of 3-aroylindoles from 2-alk-
ynyl-N,N-dialkylamines, in which the alkynyl group serves as an
internal nucleophile (Scheme 1, Path A).[ However, other such
internal nucleophiles may behave similarly to the alkynyl group.
Scheme 1. Intramolecular oxidative cyclization reaction of tertiary amines.
4a]
The indolo- or pyrrole-fused quinoxalinone moiety is found
The indolyl moiety is known to act as the coupling partner in in compounds that display a variety of pharmacological proper-
2
3
intermolecular CDC reactions, in which it forms a C(sp )–C(sp ) ties, such as anticancer, anxiolytic, antimicrobial, analgesic, and
[
6]
bond by C3 attack at the iminium ion generated from N,N- antiallergic activities. Some of the biologically active mol-
ecules that contain quinoxalin-6-one as the core unit are shown
in Figure 1. Although an important scaffold, only a few reports
are available in the literature for their synthesis that include:
[
a] Department of Chemistry, Indian Institute of Technology Guwahati,
Guwahati 781039, Assam, India
E-mail: patel@iitg.ernet.in
http://www.iitg.ac.in/patel/
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejoc.201501532.
(
i) two-step reaction of indole-2-carboxylates with 2-fluoroni-
[
7]
trobenzenes; (ii) intramolecular Cu-catalyzed N-arylation reac-
tion of pyrrole and indole carboxamides and carboxylates
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Table 1. Screening of the reaction conditions.[a]
Results and Discussion
To determine the most suitable reaction conditions for this
transformation, various reaction parameters, such as catalysts,
oxidants, solvents, and temperatures were screened and the re-
I
sults are summarized in Table 1. Various Cu [CuBr, CuCl, Cu O]
2
II
and Cu salts [Cu(OAc) , CuBr , CuCl , Cu(OTf) ] tested (Table 1,
2
2
2
2
Entries 1–7) revealed Cu(OAc) to be the most effective catalyst
2
(Table 1, Entry 6).
Entry
Catalyst
mol-%]
Oxidant
[equiv.]
Temp.
[°C]
Solvent
Yield[b]
[%]
[
By changing the oxidant from aqueous TBHP to decane TBHP
(3 equiv.) an inferior yield (15 %) of the product was obtained
(Table 1, Entry 8). No doubt the use of TBHP at 80 °C gave
expected product 1′a with disappearance of starting material
[
c]
1
2
3
4
5
6
7
8
9
CuBr (10)
CuCl (10)
TBHP (3)
80
80
80
80
80
80
80
80
80
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
19
16
21
12
10
24
18
15
29
8
33
36
46
52
21
16
47
12
<5
[
c]
TBHP (3)
[
c]
Cu
CuBr
CuCl
2
O
TBHP (3)
[
c]
2
(10)
(10)
TBHP (3)
1a, but it is also associated with mono-demethylation of the
[
c]
2
TBHP (3)
[
c]
starting material and a demethylated cyclized product, which
led to difficulties in separating the individual components. The
use of di-tert-butyl-peroxide (DTBP) as the oxidant slightly in-
creased the product yield, whereas H O was almost inactive
towards the desired transformation (Table 1, Entries 9 and 10).
No doubt, the reaction was slower and the starting material
remain unreacted at 80 °C even after 20 h, although formation
of demethylated side products was not observed when DTBP
was used as oxidant. An increase in the reaction temperature
to 100 °C and 120 °C resulted in an increased yield of product,
Cu(OAc)
2
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
TBHP (3)
[
c]
Cu(OTf)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
2
TBHP (3)
[
d]
2
2
2
2
2
2
TBHP (3)
DTBP (3)
2
2
[
e]
10
11
12
13
H
2
O
2
(3)
80
DTBP (3)
DTBP (3)
DTBP (5)
DTBP (6)
DTBP (6)
DTBP (6)
DTBP (6)
DTBP (6)
DTBP (6)
100
120
120
120
120
120
120
120
120
14
Cu(OAc)
2
(10)
(10)
(10)
(10)
(10)
15
16
17
18
19
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
–
2
2
2
2
MeCN
toluene
dioxane
DMSO
3
3 and 36 % respectively (Table 1, Entries 11 and 12). The reac-
tion produced better yields, 46 and 52 % upon increasing the
oxidant quantity to 5 and 6 equiv., respectively (Table 1, En-
tries 13 and 14). Overall, DTBP was found to be advantageous
relative to commonly used oxidant TBHP for this transforma-
tion. Finally, various other polar [dimethylformamide (DMF),
acetonitrile) and non-polar solvents (toluene, dioxane) were
screened (Table 1, Entries 15–18). Apart from toluene, which
produced a comparable yield to DMSO, other solvents show no
positive effects on the reaction. In the absence of copper salt
the reaction produces only a trace of the desired product
[
1
a] Reaction conditions: N,N-dimethyl-2-(indolyl)aniline (1a; 0.25 mmol) for
3 h. [b] Isolated yield. [c] 70 % aqueous solution. [d] Decane solution (5–6
). [e] 50 % aqueous solution.
M
linked with a pendant haloarene;[8] (iii) Pd-catalyzed intramolec-
ular cyclization reaction of indole carboxamides that bear a suit-
able o-halosubstituted aryl group at the amide nitrogen; and
[
9]
(
iv) Stevens rearrangement of a spiro-quinoxaline-derived
(<5 %), which suggests it is essential (Table 1, Entry 19). The
[
10]
ammonium ylide.
However, no method has been reported
addition of copper salt increases the rate of formation of the
iminium ion thereby increasing the product yield. Thus, the op-
timal reaction conditions were found to be Cu(OAc) (10 mol-
based on a C–H functionalization strategy. Herein, we have de-
veloped an elegant method for the synthesis of indolo[1,2-
a]quinoxalin-6-one that follows a C–H functionalization proto-
col.
2
%) and DTBP (6 equiv.) in DMSO at 120 °C, which were adopted
for all following reactions.
After determining the optimized conditions for the reaction,
the substrate scope of the methodology was explored. The ef-
2
fects of various substituents on the indolyl ring (R ) were exam-
ined by keeping the aniline part fixed. Substitution on the aryl
2
ring R with electron-donating groups, such as 5-Me (1b) and
5
-OMe (1c), and electron-withdrawing groups, such as 5-Cl (1d)
and 5-F (1e), all afforded corresponding indoloquinoxalin-6-
ones (1′b–1′e) in 46–59 % yields (Scheme 2). However, the
yields were better and the times taken were shorter for sub-
strates that possessed electron-donating groups 1′b (56 %) and
1′c (59 %) than for substrates that possessed electron-with-
drawing groups 1′d (46 %) and 1′e (49 %). The structure of 1′b
[
11]
was confirmed by X-ray crystallography (Figure 2).
If the
1
amine-bearing aryl ring R of 2-indolyl-N,N-dimethyl aniline is
substituted with an electron-donating group, such as 4-Me, and
2
aryl moiety R is either without a substituent (2a) or substi-
Figure 1. Some biologically active quinoxalin-6-ones.
tuted, such as 5-Me (2b) and 5-Cl (2d), then the reactions all
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yielded corresponding indoloquinoxalin-6-ones in moderate
yields (Scheme 2). The yield improved more if both of the rings
contained electron-donating groups, such as 4-Me as for 2′b
(63 %) than for substrates that contained electron-neutral
groups, such as 2′a (54 %), or electron-withdrawing groups,
2
such as 2′d (51 %), in aryl ring R . Similar reactivity trends were
1
observed when aryl ring R was substituted with an electron-
donating group, such as 3,4-diMe, and the substituents on aryl
2
ring R were varied from electron-neutral (3a), electron-donat-
ing (3b), and electron-withdrawing (3d). Substitution of aryl
1
ring R with a moderately electron-withdrawing group, such as
4-Cl, resulted in relatively lower yields of expected products
irrespective of the nature of the substituents on the other ring,
as demonstrated with substrates 4a, 4c, and 4d (Scheme 2).
Figure 2. ORTEP view of 1′b.[11]
2g, and 4g contained an imidazole moiety, a five membered
di-nitrogen heterocycle, for which electrophilic substitution is
favored at the C-2 position were specifically designed. Com-
pounds 1g, 2g, and 4g underwent intramolecular oxidative het-
erocyclization under the present reaction conditions to give
corresponding imidazoloquinoxalin-6-ones 1′g, 2′g, and 4′g, re-
spectively, in moderate yields (Scheme 3). The success of this
oxidative Mannich reaction was finally applied to benzimid-
azole-based precursors 1h, 2h, and 4h, all of which afforded
respective oxidative cyclized products 1′h, 2′h, and 4′h, respec-
tively, in modest yields (Scheme 3).
Scheme 2. Substrate scope for indoloquinoxalin-6-ones.[a,b] [a] Reaction con-
ditions: o-indolyl-N,N-dimethylamine (0.25 mmol), Cu(OAc) (0.025 mmol),
2
and DTBP (1.5 mmol) in DMSO (1 mL) at 120 °C. [b] Yield of isolated pure
product.
Electrophilic aromatic substitution in indoles takes place at
C-3 on the five-membered ring. But as seen above, with a favor-
able intramolecular process electrophilic substitution takes
place at the C-2 position on the nitrogen-bearing heterocycle
(
Scheme 2). Pyrrole, which is a reactive heterocycle, undergoes
substitution at the C-2 position. Thus, an analogous fused pyr-
role system should react to give pyrroloquinoxalin-6-ones.
Fused pyrroles 1f, 2f, and 4f reacted under optimized condi-
tions to give pyrroloquinoxalin-6-ones 1′f, 2′f, and 4′f, respec-
tively, in modest yields (Scheme 3). Other starting materials 1g,
Scheme 3. Substrate scope for quinoxalin-6-ones.[a,b] [a] Reaction conditions:
Substrate (0.25 mmol), Cu(OAc)
2
(0.025 mmol), and DTBP (1.5 mmol) in DMSO
(1 mL) at 120 °C. [b] Isolated yield of pure product.
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To gain insight into the reaction mechanism, an experiment Path II). Loss of a hydrogen radical gives iminium ion intermedi-
was carried out in the presence of radical scavenger TEMPO ate H, which loses a molecule of isopropylene to give the ex-
(
6 equiv.) under otherwise identical conditions. Retardation in pected product.
the expected product formation was observed along
Thus, both Path I and Path II mechanisms operate in tandem.
with formation of numerous side products, which indicates Further, addition of external water (1, 2, and 3 equiv.) to the
that the mechanism involves radicals. Based on literature re- reaction medium under otherwise identical conditions did not
[
2e–2h,3a–3c,4a,4f,12]
ports
and the yields pattern obtained for sub- improve the product yield. This indirectly supports the dual
stituted indoloquinoxalin-6-one derivatives (Scheme 2), a plau- mechanism proposal because there are sufficient nucleophiles
sible mechanism is proposed for this transformation (water and tertiary butyl radicals) present in the medium. The
(
Scheme 4). Aminyl radical cation A is generated by a single mechanism with other heterocycles, such as pyrrole, imidazole
II
electron transfer that is facilitated by Cu in combination with and benzimidazole, is expected to follow the same pathways.
peroxide. Subsequent abstraction of the hydrogen radical alpha
to nitrogen on aminyl radical cation A produces iminium ion
Conclusions
intermediate B. Intermediate B undergoes intramolecular cycli-
In conclusion, we have developed a Cu-catalyzed method for
zation reaction by nucleophilic attack from the C2 position of
the synthesis of indoloquinoxaline-6-ones that starts from o-
indolyl-N,N-dimethylarylamines through an intramolecular oxi-
dative coupling reaction with DTBP as the oxidant. The process
the indolyl ring to produce intermediate C. Rearomatization of
intermediate
C
generates 5-methyl-5,6-dihydroindolo[1,2-
a]quinoxaline (D). Similar iminium intermediate E is also gener-
ated by a single electron transfer/proton abstraction process.
This process is then followed by nucleophilic attack of water at
the iminium carbon to produce intermediate F, which finally
oxidizes to product 1′a (Scheme 4, Path I). The reaction of sub-
strate 1a under an atmosphere of nitrogen afforded 5-methylin-
dolo[1,2-a]quinoxalin-6(5H)-one (1′a) without affecting the
yield. This result suggests that atmospheric oxygen is not the
source of carbonyl oxygen. In another experiment, substrate 1a
3
involves formally the cleavage of three sp C–H bonds and one
2
sp C–H bond with concomitant installation of C–C and C–O
bonds to form indoloquinoxalin-6-ones. The use of a cheap cat-
alytic system and extension of the methodology to other heter-
ocyclic systems establishes the practical applicability of the
present protocol.
Experimental Section
General Procedure for the Synthesis of 2-(1H-Indol-1-yl)-N,N-
dimethylaniline (1a): A mixture of 1H-Indole (2 mmol), cuprous
1
8
was treated H O (20 equiv.) under otherwise identical condi-
2
1
8
tions. O-incorporated 5-methylindolo[1,2-a]quinoxalin-6(5H)-
one (1′′a) was obtained as confirmed by HRMS analysis of the oxide (0.15 mol), and potassium hydroxide (3 mmol) was placed
reaction mixture (see the Supporting Information). This result into a 25 mL round-bottomed flask and charged with N gas (bal-
confirms that water is the source of carbonyl oxygen, which is loon). To this, DMSO (2 mL) and then 2-iodo-N,N-dimethylaniline
2
(
1 mmol) was added. The resultant mixture was then put into a
often present in commercial-grade DMSO. However, in anhy-
drous DMSO and under a nitrogen atmosphere the reaction
was found to be equally effective. This result suggests that, in
addition to water as the nucleophile, perhaps a tertiary-butyl
radical obtained from DTBP through homolytic cleavage attacks
iminium intermediate E to furnish intermediate G (Scheme 4,
preheated oil bath at 120 °C and stirred for 24 h. The reaction mix-
ture was cooled to room temperature, diluted with ethyl acetate
(20 mL) and then filtered through a pad of Celite. Water (5 mL) was
added to the filtrate and the organic layer was collected. The aque-
ous layer was extracted with ethyl acetate (10 mL). The organic
phase was dried with anhydrous Na SO and concentrated in vacuo.
2
4
The crude product was purified by column chromatography with
silica gel (hexane/ethyl acetate, 50:1) to give 2-(1H-indol-1-yl)-N,N-
dimethylaniline (1a; 58 %, 137 mg).
However, the aforementioned method provides inferior yields for
chloro derivatives 4a–4g and they were synthesized by means of
[
13]
an alternative method.
General Procedure for the Synthesis of 5-Methylindolo[1,2-
a]quinoxalin-6(5H)-one (1′a): To a solution of N,N-dimethyl-2-(in-
dolyl) aniline (1a; 59.08 mg, 0.25 mmol) in DMSO (1 mL) was added
Cu(OAc)2 (4.54 mg, 0.025 mmol), followed by DTBP (219 mg,
1
.5 mmol) and the resultant mixture was heated (oil bath; 120 °C)
for 13 h. The resultant reaction mixture was admixed with water
5 mL) and the product was extracted with ethyl acetate (2 ×
0 mL). The organic phase was dried with anhydrous sodium sulf-
(
2
ate, decanted, and concentrated in vacuo. The crude product was
purified with silica gel (hexane/ethyl acetate, 9:1) to give 5-methyl-
indolo[1,2-a]quinoxalin-6(5H)-one (1′a; 52 %, 32.27 mg).
Acknowledgments
B. K. P. acknowledges the support of this research by the De-
partment of Science and Technology (DST), New Delhi (SB/S1/
Scheme 4. Plausible mechanism for the formation of indoloquinoxalin-6-one.
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Received: December 5, 2015
Published Online: February 12, 2016
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