Cu(OTf)2 loaded protonated trititanate nanotubes catalyzed reaction: A facile method for the synthesis of furo[2,3-: B] quinoxalines

Article abstract of DOI:10.1039/c8nj00287h

An efficient method is developed for the synthesis of furo[2,3-b]quinoxalines using novel Cu(OTf)₂ loaded protonated trititanate nanotube catalysts via A₃-coupling followed by 5-endo-dig cyclization from o-phenylenediamines, ethylglyoxalate and phenylacetylenes. This method is beneficial and advantageous as it facilitates high yield in conventional heating and a short reaction time besides offering reusable heterogeneous catalysts. These catalysts can be recovered by centrifugation and their activity remain largely unchanged for five successive runs. On the other side, these catalysts are prepared using simple hydrothermal and wet-impregnation methods and are characterized using XRD, HR-TEM and N₂-adsorption-desorption techniques.

Full text of DOI:10.1039/c8nj00287h

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Cu(OTf)2 loaded protonated trititanate nanotubes catalyzed
reaction: A facile method to furo[2,3-b]quinoxalines synthesis
Received 00th January 20xx,
Accepted 00th January 20xx
Bhoomireddy Rajendra Prasad Reddy, Sirigireddy Sudharsan Reddy and Peddiahgari Vasu
Govardhana Reddy*
DOI: 10.1039/x0xx00000x
An efficient method is developed for the synthesis of furo[2,3-b]quinoxalines using novel Cu(OTf)2 loaded protonated
trititanate nanotubes catalysts viaA3-coupling followed by 5-endo-dig cyclization from o-phenylenediamines,
ethylglyoxalate and phenylacetylenes. This method is beneficial and advantageous as it facilitates high yield in
conventional heating and short reaction time besides offering reusable heterogeneous catalyst. The catalyst can be
recovered by centrifugation and its activity remains largely unchanged for five successive runs. On the other side, the
catalyst is prepared by simple hydrothermal and wet-impregnation methods and is characterized by XRD, HR-TEM and N2-
adsorption-desorption techniques.
www.rsc.org/
the methods involved transition metal catalyzed Sonogashira
coupling of 2, 3-dichloroquinoxalines, hydrolysis and
subsequent cyclization steps.2-4 Chupakinet al. synthesized by
the Sꢀꢁ and Sꢀꢂꢃꢄꢅ reactions of 2-substituted quinoxalines with
acetophenones.5In another way, to the best of our knowledge
only one literature method is available for the synthesis of
furo[2,3-b]quinoxalines from one-pot three component
reaction of o-phenylenediamines, ethyl glyoxalate and
terminal alkynes using a copper triflate catalyst.6
Introduction
Functionalized fused N-heteroaromatics have played a key role
in the early stage of drug discovery and many of them have
been marketed as successful drugs,1 e.g., best-seller antibiotic
levofloxacin. For instance, some furo[2,3-b]quinoxalines
showed promising pharmacological properties in vitro and in
vivo as the potential inhibitors of sirtuins.22-Alkyl furo[2,3-
b]quinoxalines were significant for the inhibition of the growth
of human cervical cancer (Hela) and hepatocellular liver
carcinoma (HepG2) cells.2In view of the biological importance
of furo[2,3-b]quinoxalines, the synthesis of these heterocycles
using cost-effective and easily accessible starting materials is
an important area of current research.
In recent years there has been an increasing expansion in the
field of nanocatalysis, thus we have explored protonated
trititanate nanotubes (HTNT) as an efficient, recyclable solid-
acid nanocatalysts in multicomponent reactions for the
synthesis of tertiary α-aminophosphonates,7 secondary α-
aminophosphonates8 and alkylaminophenols.9Further, we
have also reported the catalytic activity of CuI supported on
HTNT catalysts for the synthesis of propargylaminesvia A3-
coupling reaction.10From the similarity between the
mechanism of our CuI supported on HTNT catalyzedA3-
coupling10and Narenderet al. approach to furo[2,3-
b]quinoxalines synthesis,6we can assume that the CuI and
Cu(OTf)2 supported on HTNT catalysts may also possess a high
Fig. 1 Structure of drugs based on the furo[2,3-b]quinoxaline
activity for the synthesis of
furo[2,3-b]quinoxalines from o-
moiety
phenylenediamine, ethyl glyoxalate and terminal alkyne.
Based on the above assumption, in the present study, we
reported a convenient and practical protocol for the synthesis
of the series of furo[2,3-b]quinoxalines through Cu(OTf)2
supported HTNT catalyzed one-pot three component A3-
coupling reaction followed by5-endo-dig (diagonal) cyclization
process using o-phenylenediamines, ethyl glyoxalate and
terminal alkynes (Table 3 & 4).
A very few literature methods have been reported describing
the preparation of furo[2,3-b]quinoxalines,2-6 in which most of
Results and Discussions
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To
examine
the
catalytic
performances
of Table 1 Catalytic performance of Cu-based catalysts in
Cu(OTf)2/HTNTcatalysts, we began our study using o-
phenylenediamine, ethylglyoxalate 50% solution in toluene
and phenylacetylene in acetonitrile at refluxing temperature.
Initially, we have tried the reaction in absence of catalyst, but
4
synthesis.a
the expected product
4 was not obtained and even in the
presence of catalysts such as CuI, CuOTf-toluene, HTNT alone
and CuI/HTNT-5 (Table 1, entries 1 - 3 & 7, 8). Next, copper (II)
salts such as Cu(OAc)2 and CuBr2 were used to promote the
Entry
1
2
3
4
5
6
7
8
Catalyst
t [h]
Yieldb [%]
-
24
24
24
24
24
5
24
24
0
0
0
23
20
45
0
CuI
CuOTf-toluenec
Cu(OAc)2
reaction, these produced product
after 24 h (Table 1, entries 4 & 5). When we switched over the
reaction with Cu(OTf)2, product was obtained in about 45%
4 in 23 and 20% respectively
CuBr2
4
Cu(OTf)2
yield for 5 h (Table 1, entry 6). From this, we found that
Cu(OTf)2 was an essential requisite for this reaction. Further
we tried the reaction with Cu(OTf)2/HTNT-2 heterogeneous
HTNTd
CuI/HTNT-5e
Cu(OTf)2/HTNT-2f
Cu(OTf)2/HTNT-3g
Cu(OTf)2/HTNT-4h
Cu(OTf)2/HTNT-5i
Cu(OTf)2/HTNT-6j
0
9
2
1
50
54
63
70
70
catalyst and the product
4 was obtained in 50% yield within 2
10
11
12
30 mink
20 mink
20 mink
h (Table 1, entry 9). Supplementary investigation revealed that
the yield was affected by the amount of Cu(OTf)2 present in
the Cu(OTf)2/HTNT catalyst. Among the catalyst systems
examined, Cu(OTf)2/HTNT-5 showed the best catalytic
performance and no change in yield and reaction time was
observed with additional increase of Cu(OTf)2 loading in HTNT
(Table 1, entries 10 - 13). Therefore, Cu(OTf)2/HTNT-5 was
selected for further optimizations. The possible reason for
superior catalytic activity of Cu(OTf)2/HTNT-5 might be that the
Cu(OTf)2 molecules are well dispersed on the large surface of
HTNT, allowing more acetylene molecules to be in contact with
the Cu(OTf)2. Another important feature of the catalyst was
the support (HTNT) having a high concentration of acid sites,7-
13
a
Reaction conditions: O-phenylenediamine (1 mmol), ethylglyoxalate
50% solution in toluene (2 mmol), phenylacetylene (1 mmol), catalyst
(50 mg) and acetonitrile (5 mL) at refluxing temperatures.b Isolated
yields; cused 100 mg of CuOTf-toluene; d HTNT = protonated trititanate
e
nanotubes; 5 wt% CuI-loaded HTNT; 2 wt% Cu(OTf)2-loaded HTNT;
i
3 wt% Cu(OTf)2-loaded HTNT; 4 wt% Cu(OTf)2-loaded HTNT; 5 wt%
j
Cu(OTf)2-loaded HTNT; 6 wt% Cu(OTf)2-loaded HTNT;
f
g
h
k
time in
minutes.
10
which facilitated the imine formation and 5-endo-dig
cyclization by enolyzing the intermediate
catalyzed process (Fig. 4).
E since it is an acid
Table 2 Exploring the best reaction conditions.a
Encouraged by this result, we attempted to optimize the
reaction conditions (like solvent, catalyst loading and
temperatures) to attain the best possible yield. At first, the
solvent effect was tested. Toluene provided a higher yield
(85%) than other organic solvents which were tested (Table 2,
entries 1 - 4 & 7). When the reaction was performed in water,
very low yield (<10%) of the product was obtained possibly
due to a heterogeneous mixture of the reacting substrates
(Table 2, entry 5). Solvent-free conditions also gave lower
yields (Table 2, entry 6). Subsequently, catalyst amount was
optimized since it played a crucial role in the reaction. It was
found that 40 mg of Cu(OTf)2/HTNT-5 is the optimal amount
for this reaction (Table 2, entry 9). Finally, the effect of
temperature was examined. At room temperature the reaction
gave only a trace amount of product (Table 2, entry 13), but
upon raising the temperature, a gradual increase in the yield of
the product was observed (Table 2, entries 14 - 16). An
excellent yield (93%) of the product was observed when the
reaction was performed at 100 °C (Table 2, entry 16). Further,
elevating the reaction temperature to 110 °C did not result a
better yield than the earlier (Table 2, entry 17). Based on the
above results from Table 2, the best condition of entry 16 is
Entry
Solvent
Catalyst Cu(OTf)2 T [oC]
loading content
Yieldb
[%]
[mg]
50
50
[mol%]
0.69
0.69
0.69
0.69
0.69
0.69
0.69
0.62
0.55
0.48
0.41
0.27
0.55
0.55
0.55
0.55
0.55
1
2
3
4
5
6
7
8
Acetonitrile
THF
Reflux
Reflux
80
Reflux
80
80
80
80
80
80
80
80
r.t
50
70
100
110
70
63
65
Trace
<10
35
85
85
85
81
73
57
Trace
40
73
93
93
1,4-Dioxane 50
DCM
50
50
50
50
45
40
35
30
20
40
40
40
40
40
Water
---c
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
9
10
11
12
13
14
15
16
17
The bold values are the most effective conditions for the model
reaction.a Reaction conditions: O-phenylenediamine (1 mmol), ethyl
glyoxalate 50% solution in toluene (2 mmol), phenylacetylene (1
mmol), catalyst: Cu(OTf)2/HTNT-5, solvent (5 mL) and quenched after
20 min; b isolated yield; c solvent-free conditions.
opted to prepare the furo[2,3-b]quinoxalines4 - 21
.
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Further, extended these results to
a variety of o- 1,2-diamine (Table 3, entry 11), failed to give any discernible
phenylenediamines (1), ethylglyoxalate (2) and acetylenes (3) results. In addition to o-phenylenediamines, naphthalene-1,8-
(Table 3 & 4). The electron-rich diamines offered slightly better diamine (Table 3, entry 12) was also applied to this
yields than electron-deficient diamines (Table 3, entries 4 - 9). transformation, as expected it is ineffective to produce
Notably, regioselectivities were observed in this furoquinoxaline. Further, the scope of acetylenes was
transformation
when
employing mono
substituted evaluated on this transformation (Table 4, entries 13 - 21). The
phenylenediamines with a mixture of two separable isomers (7 -CH3 and -OCH3 substituted phenylacetylenes produced
and 6 substituted furoquinoxalines in 90:10 ratio with respect desired furoquinoxalines smoothly in high yields. In contrast,
to the isolated yields) (Table 3, entries 5, 7, 8 & 10).6 But in the the aliphatic acetylenes failed to produce the target
case of 3, 4- diaminotoluene an inseparable mixture of
5 was furoquinoxalines under similar reaction conditions (Table 4,
obtained (Table 3, entry 5). The reactionwith 4-nitrobenzene- entry 16).
Table 3Substrate scope of the o-phenylenediamine.a
a
Reaction conditions: o-phenylenediamine (1 mmol), ethyl glyoxalate 50% solution in toluene (2 mmol), phenylacetylene (1 mmol), catalyst:
Cu(OTf)2/HTNT-5 (40 mg), toluene (5 mL), temperature (100 oC) and quenched after 20 min.
Isolated yields in parentheses. # = Isolated yields of a mixture of two isomers.
Table 4Substarte scope of the terminal alkynes.a
aReaction conditions: o-Phenylenediamine (1 mmol), ethyl glyoxalate 50% solution in toluene (2 mmol), phenylacetylene (1 mmol), catalyst:
Cu(OTf)2/HTNT-5 (40 mg), toluene (5 mL), temperature (100 oC) and quenched after 1 h.
Isolated yields in parentheses. # = Isolated yields of a mixture of two isomers.
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One of the major advantages of heterogeneous catalysts is absence of active Cu in the filtrate and no leaching of Cu(OTf)2
their recyclability. The Cu(OTf)2/HTNT-5 catalyst recycle study from HTNT support.
was performed using model reaction under optimized
conditions to obtain furo[2,3-b]quinoxaline (4) (Fig. 2). After
each run, the catalyst was separated by centrifugation, washed
o
twice with ethyl acetate and dried in oven at 100 C for 6 h.
Eventually it was reused for another five consecutive reaction
cycles. A significant drop in yield was observed in the sixth
cycle. The decrease in yield is mainly due to the partial
leaching of catalytically active Cu(II) species from the support.
To find out the stability of the catalyst, we have characterized
by TEM studies (Fig. 5d) and N2-adsorption desorption
isotherm (Fig. 7b) after sixth catalytic cycle.
Fig. 4 Possible reaction mechanism for the tandem A3-coupling
and 5-endo-dig cyclization
We proposed
a possible reaction mechanism for this
transformation; it appears to be tandem C-C bond formation
followed by a 5-endo-dig cyclization reaction (Fig. 4). Initially,
Cu (II) supported on HTNT involve the formation of a π-metal-
alkyne complex
terminal proton’s acidity to facilitate its abstraction and
concurrently generate a copper-alkylidene complex on the
surface of the HTNT; at the same time an addition of the
copper-alkylidene complex to the iminium ion , which is
generated in situ from the reaction between aldehyde and
amine to give the propargylamine The ensuing
propargylamine further attacks the ester functionality
intramolecularly and generated the intermediate . Since
is easily enolizable in an acidic medium, it
cyclized intermediate 3-(alkynyl)-3,4-
and further cleavage of the
A, which is very crucial step in increasing
B
C
Fig. 2 Yields of furo[2,3-b]quinoxaline (4) after different
number of cycles.
D
.
D
E
intermediate
provides
E
dihydroquinoxalin-2(1H)-one
F
metal π-complex occurs followed by oxidation furnishing the
target furoquinoxaline. Further studies, however, are required
to confirm the exact reaction mechanism.
Different techniques were chosen for the characterization of
the optimized catalysts. TEM image of HTNT (Fig. 5a) showed
bundle of one dimensional random aligned tubes having
length ranging between 200 - 400 nm with open ends. The
TEM image of single nanotube (Fig. 5a, inset) displays
cylindrical shape with hollow inside. It consists of 4 - 5 layers
on the wall; inner and outer diameters of the tubes have 3 - 5
nm and 8 - 12 nm, respectively. TEM image of Cu(OTf)2/HTNT-
5 (Fig. 5b & 5c) showed clear evidence of Cu(OTf)2 adhered to
the inner- and outer-wall of the trititanate nanotube supports,
without damaging the tubular structure. The size of Cu(OTf)2 is
found to be 5 - 8 nm. TEM image of recovered Cu(OTf)2/HTNT-
5 (Fig. 5d) showed no major change in the morphology of
HTNT support, no aggregation of Cu(OTf)2 molecules, however
a partial loss of Cu(OTf)2 molecules from HTNT was observed.
Fig. 6 Shows XRD patterns of HTNT and Cu(OTf)2/HTNT-5. The
HTNT showed characteristic diffraction peaks at 2θ = 10.2⁰
(200), 24.1⁰ (202), 28.3⁰ (112) and 48.2⁰ (303) indicating the
monoclinic structure of layered H2Ti3O7 (JCPDS No. 47-0561
Fig. 3 Sheldon's hot filtration test for the synthesis of furo[2,3-
b]quinoxaline( ).
4
Next, we studied the heterogeneity of Cu(OTf)2/HTNT-5
catalyst by Sheldon's hot filtration test for a model reaction to
afford furo[2,3-b]quinoxaline (4). To that, the synthesis of 4 in
o
toluene at 100 C for 20 min was performed and after 10 min
the catalyst was filtered out from the reaction mixture. Then
the reaction was continued with filtrate under optimized
conditions and there was no progress of reaction even after 20
min (Fig. 3). The negative results were indicating that the
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and 31-1329) and these observations matches well with our average pore diameter of 6.0 nm while that of HTNT has 286
earlier publications.7-10 XRD pattern of Cu(OTf)2/HTNT-5 m2.g-1 surface area with an average pore diameter of 10.5 nm
exhibited additional peaks along with HTNT peaks around 2θ = (the surface area and pore diameter values of HTNT taken
26.6⁰ (002), 42.3⁰ (110) and 49.89⁰ (112), may be the from our previous publications).7-10 The decrease in the surface
characteristic reflections of Cu(OTf)2. Based on TEM and XRD area and pore diameter in Cu(OTf)2/HTNT-5 when compared
analysis of Cu(OTf)2/HTNT-5, the existence of Cu(OTf)2 on with the support HTNT can be attributed to the deposition of
HTNT support can be presumed.
Cu(OTf)2 on the surface and pores of the HTNT. The decrease
of the surface area is in agreement with observed structural
transformations (Fig. 5) and previous data.11N2-adsorption
desorption isotherm of recovered Cu(OTf)2/HTNT-5 showed
same kind of isotherm like fresh Cu(OTf)2/HTNT-5 whereas BJH
pore size distribution plot showed distinctive mesopores with
peak values at ca. 5.0 - 14.0 nm. The BET surface area of
Cu(OTf)2/HTNT-5 was calculated to be 97.3 m2.g-1 with an
average pore diameter of 4.5 nm. The decrease in surface area
and pore diameter of the catalyst may be due to the blockage
of mesopores with reactants which were used in the reaction.
Fig. 5 TEM images of (a) HTNT, (b, c) Cu(OTf)2/HTNT-5 and (d)
recovered Cu(OTf)2/HTNT-5 after six cycles
.
Fig. 7 N2-adsorption desorption isotherm and corresponding
BJH pore size distribution curve (inset) of (a) Cu(OTf)2/HTNT-5
and (b) Recovered Cu(OTf)2/HTNT-5 after sixth cycle
.
Conclusions
We have developed a new heterogeneous [Cu(OTf)2-loaded
HTNT] catalytic system for the efficient synthesis of furo[2,3-
b]quinoxalines from o-phenylenediamines, ethylglyoxalate and
phenylacetylenes via A3-coupling reaction followed by 5-endo-
dig cyclization. Affording high yields in conventional heating,
short reaction time, high regioselectivities and reusable
catalytic system are the added advantages of this method.
Other organic transformations with this catalyst and more
characterization studies of the catalysts are currently
underway in our laboratory and will be reported in due course.
Experimental Section
Hydrothermal synthesis of HTNT
Fig. 6 XRD pattern of HTNT and Cu(OTf)2/HTNT-5.
In a typical synthesis process, TiO2-LAB (2.5 g) dispersed in 10
M NaOH aqueous solution (200 mL) and heated at 130 oC/20 h
in teflon-lined autoclave (capacity 250 mL). The white
precipitate thus obtained was subjected to washing twice with
distilled water (800 mL) followed by dil. HCl (0.1 N, 200 mL),
N2-adsorption desorption isotherm of Cu(OTf)2/HTNT-5 (Fig.
7a) showed type IV isotherm with type H3 hysteresis loop,
indicating the mesoporous nature of catalysts. The
corresponding BJH pore size distribution plot indicates (Fig. 7a
inset), Cu(OTf)2/HTNT-5 has distinctive mesopores with peak
values at ca. 5.0 - 17.0 nm. The BET surface area of
Cu(OTf)2/HTNT-5 was calculated to be 115.7 m2.g-1 with an
o
finally with ethanol (200 mL) and dried at 80 C for 12 h. The
obtained material was denoted as HTNT.7-10
Synthesis of Cu(OTf)2/HTNT catalysts
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Wet-impregnation method was adopted for copper loading
onto HTNT by using an appropriate amount of Cu(OTf)2 in
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Acknowledgements
We are grateful for the financial support from the Council of
Scientific and Industrial Research (CSIR (02(248)/15/EMR-II).
B.R.P. Reddy thanks to the UGC-CSIR, New Delhi, India for the
award of Junior Research Fellowship. The authors are
greatfulto Prof. M. V. Shankar, Department of Material Science
and Nano Technology, Yogi Vemana University, Kadapa for
providing catalysts and encouraging fruitful scientific
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6 | NewJ. Chem., 2018, 00, 1-6
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New Journal of Chemistry
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DOI: 10.1039/C8NJ00287H
Graphical Abstract
Cu(OTf)2 loaded protonated trititanate nanotubes catalyzed reaction: A facile
method to furo[2,3-b]quinoxalines synthesis
Bhoomireddy Rajendra Prasad Reddy, Sirigi Reddy Sudharsan Reddy, and Peddiahgari Vasu
Govardhana Reddy*
An efficient synthesis of furo[2,3-b]quinoxalines via a Cu(OTf)2 loaded protonated trititanate
nanotubes catalyzed one-pot three-component reaction of o-phenylenediamines,
ethylglyoxalate and phenylacetylenes has been reported. This transformation proceeds
through a copper-catalyzed A3-coupling followed by 5-endo-dig cyclization leading to
furo[2,3-b]quinoxalines in good yields.