Facile synthesis of benzoindoles and naphthofurans through carbonaceous material-catalyzed cyclization of naphthylamines/naphthols with nitroolefins in water

Article abstract of DOI:10.1039/c5ob00129c

A facile and efficient approach has been established for the synthesis of benzoindole and naphthofuran derivatives via the metal-free cyclization reaction of nitroolefins with naphthylamines/naphthols. Various substituted benzoindoles and naphthofurans are obtained in good to excellent yields. Moreover, the ability to recycle the carbonaceous material makes this method quite cost-effective and environmentally benign compared to traditional acid-catalyzed methods. Theoretical studies indicated that the reaction between naphthylamine and nitroolefin catalyzed by this solid acid was thermodynamically controlled at 60 °C, resulting in the formation of the benzoindoles.

Full text of DOI:10.1039/c5ob00129c

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Facile synthesis of benzoindoles and
naphthofurans through carbonaceous material-
catalyzed cyclization of naphthylamines/naphthols
with nitroolefins in water†
Cite this: DOI: 10.1039/c5ob00129c
Furen Zhang, Chunmei Li, Chen Wang and Chenze Qi*
A facile and efficient approach has been established for the synthesis of benzoindole and naphthofuran
derivatives via the metal-free cyclization reaction of nitroolefins with naphthylamines/naphthols. Various
substituted benzoindoles and naphthofurans are obtained in good to excellent yields. Moreover, the
ability to recycle the carbonaceous material makes this method quite cost-effective and environmentally
benign compared to traditional acid-catalyzed methods. Theoretical studies indicated that the reaction
between naphthylamine and nitroolefin catalyzed by this solid acid was thermodynamically controlled at
60 °C, resulting in the formation of the benzoindoles.
Received 22nd January 2015,
Accepted 19th March 2015
DOI: 10.1039/c5ob00129c
www.rsc.org/obc
metal catalyst, additives, hazardous or expensive reagents,
tedious work-up processes, the requirement of special appar-
Introduction
Indole derivatives constitute a very important class of hetero-
cycles owing to their importance as a ‘privileged pharmaco-
logical structure’ distributed in numerous natural products,
pharmaceutical agents, and agrochemicals.1 Among them,
benzoindole is particularly noteworthy because it presents a
key structural motif of numerous potential drug precursors2
possessing antiarrhythmic,2c central nervous system depres-
sant,2c anti-inflammatory,2c antihypertensive,2c dopamine
antagonist3 and anti-cancer (e.g. CC-1065 and duocarmycins)
activities.4 In addition, recent studies on carbocyanine and
4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes with
benz[e]indole5 and benz[g]indole6 moieties showed that they
can fluoresce in the near-infrared spectral region and have
been used for bio-monitoring. Owing to these interesting func-
tions, the chemistry of benzoindoles has been developed sig-
nificantly and considerable efforts have been devoted to
develop more efficient synthetic methodologies or to improve
the existing synthetic routes to access this scaffold, as well as
to introduce substituents onto the core.7
atus, and difficulty in recovery and reusability of the catalyst,
which adversely affect the economics as well as the ecofriendly
nature of the reaction. Thus, it remains highly desirable to
develop more environmentally benign synthetic methodologies
for the construction of highly valuable heterocycles containing
the benzoindole ring fragment, requiring milder conditions as
well as avoiding the need for toxic metal catalysts and inaccess-
ible starting materials. Recently, with the discovery of ‘new
forms of carbonaceous materials’8 and the investigation of
their applications,9 carbonaceous material catalysts for the
synthesis of fine chemicals have attracted considerable interest
from both environmental and economical points of view. The
extensive application of such heterogeneous catalysts in syn-
thetic organic chemistry can make the synthetic process more
efficient and the used catalyst can be easily recycled.
In the course of our research on the development of
efficient methods for heterocycle synthesis from common
starting materials,10 herein, we wish to report a novel, simple
and very efficient protocol for the synthesis of benzoindoles (3)
and naphthofurans (5) via carbonaceous material (C–SO3H)11
catalyzed cyclization reactions of naphthylamines (1)/
naphthols (4) in water at 60 °C (Scheme 1). To the best of our
knowledge, the synthesis of benzoindole derivatives by the het-
erogeneous catalytic cyclization of easily accessible naphthyl-
amines and nitroolefins in water has not been reported so far.
This methodology allows access to benzoindoles bearing mis-
cellaneous substituents at the C2 and C3 positions by variation
of the nitroolefins.
Despite these advances, most of these approaches suffer
from one or more shortcomings, such as use of a transition-
School of Chemistry and Chemical Engineering, Zhejiang Key Laboratory of
Alternative Technologies for Fine Chemicals Process, Shaoxing University, Shaoxing,
Zhejiang Province 312000, China. E-mail: qichenze@usx.edu.cn;
Fax: +86 (575) 88345682; Tel: +86 (575) 88345682
†Electronic supplementary information (ESI) available. CCDC 1033243. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5ob00129c
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water, even at an enhanced temperature of 100 °C for 24 h
(Table 1, entries 1–2). Gratifyingly, when zeolite (HY) and
Amberlyst-15 were used as a catalyst in water at 60 °C, the reac-
tion occurred and gave the target compound in 38% and 49%
yields (entries 3–4). The carbonaceous material obtained from
just furaldehyde (carbon) showed almost no activity due to
there being little functionality on the surface. However, the
novel carbonaceous material (C–SO3H) exhibited a remarkably
high activity for the formation of benzoindole derivatives. To
improve the reaction, various catalysts, including traditional
Brønsted acids and Lewis acids, were evaluated as potential
promoters for the reaction (entries 7–10); in all cases, 10 mol%
of catalyst was used and the reaction was carried out in water
at 60 °C. Lewis acids Sc(OTf)3 and Y(OTf)3 are similarly
effective for the reaction, and the expected product 3a was
obtained in 72% and 66% yields, respectively (entries 9–10).
However, Brønsted acids, such as formic acid (HCOOH) and
p-toluenesulfonic acid (TsOH), are less active than the examined
Lewis acids (entries 7–8). Subsequently, C–SO3H was chosen as
a catalyst for solvent screening. Several protic or aprotic sol-
vents with variable polarities, such as methanol (MeOH),
ethanol (EtOH), N,N-dimethylformamide (DMF), tetrahydro-
furan (THF), toluene and dichloromethane (DCM), were
employed as reaction media (entries 11–16). The results indi-
cated that the polarity and the protonation ability of the sol-
vents had an obvious impact on the yield of 3a. The first two
protic solvents (MeOH and EtOH) afford results similar to that
with water as the solvent. However, several aprotic solvents
gave product 3a in relatively low isolated yields (entries 13–16).
Taking into account efficiency, economy, sustainability and
safety, we chose water as the optimum reaction medium. Next,
the effect of catalyst loading on the reaction was evaluated and
the results indicated that increasing the amount of C–SO3H to
20 mg did not obviously improve the yield but decreasing the
amount of C–SO3H to 5 mg resulted in a significant decrease
in the yield (entries 17–18). Furthermore, to identify the
optimum reaction temperature, the reaction was carried out
at temperatures ranging from room temperature to 100 °C in
water in the presence of C–SO3H (10 mg). We found that
when varying the temperature from 50 °C to room tempera-
ture, the yield also declined significantly (entries 19–21).
When the reaction was performed at room temperature, no
desired product was found. Instead, the Michael adduct 6
was obtained in 48% yield (Scheme 2). When the reaction
temperature was elevated to 40 °C and 50 °C, respectively,
only the desired product 3a was obtained (entries 19–20).
Moreover, when further increasing the temperature to 80 °C,
no significant improvement in yield was observed (entry 22).
However, with a further increase of the temperature to
100 °C, the reaction gave a relatively low yield, probably
because of the instability of the substrate (entry 23).
This interesting observation indicated that the temperature
of the reaction may chemoselectively control the reaction
pathways. Extensive screening showed that the optimized
conditions are 10 mg C–SO3H as a catalyst, in water, at 60 °C
(entry 5).
Scheme 1 Synthesis of benz[e]indole and benz[g]indole derivatives.
Results and discussion
In order to identify the optimal reaction conditions, the reac-
tion of 2-naphthylamine 1a with nitroolefin 2a, which was
derived from the reaction of nitromethane with benz-
aldehyde,12 was chosen as a model reaction. The reaction
mixture, which was composed of a 1 : 1 mixture of 1a to 2a,
was examined under various conditions; the results are pre-
sented in Table 1.
The optimization process revealed that none of the desired
product was observed in the absence of an acid catalyst in
Table 1 Optimization of reaction conditionsa
Entry
Catalyst (mg)
Solvent
T (°C)
Yieldb (%)
1
—
—
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
MeOH
EtOH
DMF
THF
Toluene
DCM
H2O
H2O
H2O
H2O
H2O
H2O
H2O
60
Reflux
60
60
60
60
60
60
60
60
60
60
60
60
60
Reflux
60
60
—
—
38
49
86
23
30
22
72
66
83
79
17
24
31
38
87
43
75
43
—
86
71
2c
3
Zeolite (HY) (10)
Amberlyst-15 (10)
C–SO3H (10)
Carbon (10)
TsOH (10)d
HCOOH (10)d
Sc(OTf)3 (10)d
Y(OTf)3 (10)d
C–SO3H (10)
C–SO3H (10)
C–SO3H (10)
C–SO3H (10)
C–SO3H (10)
C–SO3H (10)
C–SO3H (20)
C–SO3H (5)
C–SO3H (10)
C–SO3H (10)
C–SO3H (10)
C–SO3H (10)
C–SO3H (10)
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
50
40
rt
80
Reflux
a Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), catalyst (x mg),
solvent (3 mL), 8 h. b Isolated yields. c This reaction was carried out for
24 h. d Catalyst 10 mol%.
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Table 2 Scope of the reaction leading to 3a,b
Scheme 2 Michael addition reaction of 2-naphthylamine 1a with
nitroolefin 2a at room temperature.
With the optimal reaction conditions established, we then
examined the generality and limitations of our synthetic proto-
col by using readily available and common starting materials.
Various nitroolefins were prepared and subjected to the cycli-
zation reaction. As revealed in Table 2, a wide range of func-
tional groups on the phenyl substituent of the nitroolefin,
such as fluoro, bromo, chloro, nitro, and methyl, were well tole-
rated and gave the expected products 3a–3h in good yields. To
our delight, a less reactive heteroaromatic group, namely thio-
phene, still displayed high reactivity and led to the desired
product in 81% yield. The electronic properties of the aryl ring
of the nitroolefins had a little influence on the efficiency of the
reaction. Compared to electron-rich groups (i.e. –Me), nitroole-
fins having electron-poor groups (i.e. –F, –Br and –NO2) gave
the products in relatively higher yields. Then, we also carried
out the synthesis of the benzoindoles with methyl substitution
using β-methylnitrostyrenes with different substitution pat-
terns including electron-rich as well as electron-deficient
groups. Pleasingly, the reaction could proceed smoothly to
provide the corresponding products 3j–3q in 70–85% yields.
Interestingly, when (Z)-(2-bromo-2-nitrovinyl)benzene was
used, again we obtained product 3a in 77% yield, probably
because of the large ionic radius and the strong inductive
effect of the bromine atom, which greatly facilitates its depar-
ture. However, when (E)-(2-nitrobut-1-en-1-yl)benzenenone was
used, none of the target product was obtained under similar
conditions, probably because of its poor stability, which influ-
ences the formation of the desired product.
a Reaction conditions: 1 (0.5 mmol), 2 (0.5 mmol), C–SO3H (10 mg),
water (3 mL), 60 °C, 8 h. b Isolated yields.
Then, we examined the reaction of 1-naphthylamine 1b
with various nitroolefins under the optimized conditions.
Satisfyingly, this reaction also could proceed smoothly and the
desired products 3r–3z were isolated in 67–85% yields.13 In
general, 2-naphthylamine produced benzoindole derivatives in
higher yields than 1-naphthylamine.
In order to further expand the scope of the present method,
the replacement of naphthylamine with naphthol was exam-
ined. This is particularly attractive because naphthofurans are
an important class of heterocyclic compounds and are found
in many biologically active compounds14 and organic func-
tional materials.15 To our delight, under the above optimized
conditions, the reaction proceeded smoothly. The naphtho-
furan derivatives 5 were obtained in moderate to good yields
(Table 3). It is noteworthy that, recently, Hajra and co-workers
reported an elegant one-pot synthesis of naphthofurans from
naphthols and nitroolefins using indium triflate catalyst in
1,2-dichloroethane (DCE) under reflux.16 In addition, we also
noted that the great majority of procedures employed in the
construction of naphthofurans are based on transition metal-
catalyzed cyclization strategies and usually require multistep
procedures.17 Compared to the reported method, the use of
C–SO3H as a catalyst (10 mg) in water at 60 °C makes the present
method quite simple, more green, sustainable and practical.
In view of these results, we were interested in whether the
catalytic system would work for aniline (1c) and phenol (4c)
instead of naphthylamine and naphthol, respectively, while
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Table 3 Scope of the reaction leading to 5a,b
Fig. 1 X-ray crystal structure of compound 3u.
Fig. 2 Recyclability of carbonaceous material catalyst.
solid, after drying in a vacuum oven at 120 °C for 4 h, could be
reused easily. The recovered catalyst was used repetitively for 5
cycles with only a marginal loss in its catalytic activity. The
slight loss observed in the catalytic activity after the 3rd run
could be due to minor losses in the recovery process (Fig. 2).
To probe the mechanism of this type of cyclization reaction,
a mixture of 1a and 2a were treated in the presence of 10 mg
C–SO3H at 60 °C in water with TLC monitoring for the for-
mation of an intermediate. Unfortunately, the desired inter-
mediate 7 (Scheme 2) could not be obtained, probably because
of its high reactivity. To our delight, when 1b was used under
the optimized conditions, an intermediate 2-(2-nitro-1-phenyl-
ethyl)naphthalen-1-amine 9 was isolated in 42% yield along
with the desired product 3r (23%). In addition, we found the
intermediate 9 could further be transformed into the final
product 3r under the optimized reaction conditions (Scheme 3).
On the basis of these experiments, a plausible mechanism
for the C–SO3H catalyzed cyclization reaction of naphthyl-
amines with nitroolefins is shown in Scheme 4. Initially, the
intermediate A was generated from the Michael addition of
1-naphthylamine 1b to nitroolefin 2a and further undergoes
imine-enamine tautomerization to give radical intermediate B
in the presence of C–SO3H, which is further isomerized to 9.
a Reaction conditions: 4 (0.5 mmol), 2 (0.5 mmol), C–SO3H (10 mg),
water (3 mL), 60 °C, 10 h. b Isolated yields.
modifying the conditions slightly. Unfortunately, the reaction
failed to afford the desired product, even at an enhanced
temperature of 100 °C for 36 h. The probable reason for these
results was the low nucleophilicity of the ortho position of
aniline and phenol.
The results show the scope and generality of the reaction
with respect to a range of nitroolefin and naphthylamine or
naphthol substrates. It should be noted that nitroolefins
bearing halogen substituents (e.g., 4-Cl, 2,4-Cl, 4-Br) on the
phenyl rings were well tolerated with the corresponding halo-
substituted naphthofuran/benzoindole derivatives being iso-
lated in high yields, which provide the possibility for further
functionalization. Indeed, the protocol provides a straightfor-
ward pathway to construct substituted benzoindoles and
naphthofurans, which are generally prepared via metal-cata-
lyzed coupling reactions. The structures of all the synthesized
compounds were based on their spectroscopic data. Further-
more, the target compound 3u was further determined by
X-ray crystallographic analysis (Fig. 1).18
Then, intramolecular nucleophilic addition of
B would
produce adduct C. Finally, the intermediate C would lead to
the indole 3r moiety after elimination of H2O and HNO.
Next we carried out theoretical calculations to understand
the mechanistic origin of the interesting regioselectivity with
1-naphthylamine (1b) and (E)-(2-nitrovinyl)benzene (2a) as
Subsequently, the reusability of the carbonaceous material
was examined by using the model reaction of 2-naphthylamine
1a and nitroolefin 2a. After the reactions, the reaction mixture
was filtered and then washed with MeOH and water, and the
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Scheme 3 Control experiment.
Fig. 3 Energy profile for the reaction of 1-naphthylamine (1b) with
(E)-(2-nitrovinyl)benzene (2a). Bond distances in transition states are
in angstroms.
Scheme 4 Plausible pathway for the formation of 3r.
Path A and Path B are 17.8 and 28.2 kcal mol−1, respectively.
The energy barrier of 28.2 kcal mol−1 is significantly higher
than that for Path B of the reaction between 1b and 2a
(23.8 kcal mol−1). Therefore, the reaction between aniline (1c)
and (E)-(2-nitrovinyl)benzene (2a) can only undergo Path A but
cannot undergo Path B even at 60 °C. All these results are con-
sistent with our experiments.
model substrates. The calculated results are shown in Fig. 3.
Their energies are set as the zero reference of the energy
profile. In Path A, the nucleophilic attack of the N1 atom of 1b
on the C1 atom of 2a through transition state TSA requires an
activation energy of 17.3 kcal mol−1. After this step, a zwitterio-
nic intermediate INTA with an energy of 10.4 kcal mol−1 is
formed, and then quickly converts to the product 8 through
proton transfer. The energy of 8 is −4.0 kcal mol−1. In Path B,
the nucleophilic attack of the C2 atom of 1b on the C1 atom of
2a through transition state TSB requires an activation energy
of 23.8 kcal mol−1 to form a zwitterionic intermediate INTB
with an energy of 18.4 kcal mol−1. INTB then converts to the
product 9 through proton transfer. The energy of PB is
−8.0 kcal mol−1. Comparing Path A and Path B, it is apparent
that Path A is kinetically favored whereas Path B is thermo-
dynamically favored. The energy barrier of Path A is 17.3 kcal
mol−1, indicating that Path A can take place easily at room
Conclusions
In summary, we have established a novel, simple and efficient
method for the construction of a variety of potentially biologi-
cally active benzoindole and naphthofuran derivatives using
carbonaceous material as a solid acid catalyst in water at
60 °C. The advantages of our presented methodology are the
easily available starting materials (aldehydes, nitroalkanes,
naphthylamines and naphthols) and the reusability of the
catalyst. Theoretical studies show that the reaction between
naphthylamines and nitroolefins catalyzed by carbonaceous
material is thermodynamically controlled at 60 °C, resulting in
the formation of the desired products.
temperature. Path B has an energy barrier of 23.8 kcal mol−1
,
and thus does not occur at room temperature, but requires
elevated temperature (i.e. 60 °C in our experiments). Because
the energy barrier for the reverse reaction of Path A is only
21.3 kcal mol−1, Path A is reversible at 60 °C. Therefore, the
reaction between 1-naphthylamine (1b) and (E)-(2-nitrovinyl)-
benzene (2a) is thermodynamically controlled at 60 °C, result-
ing in the formation of the product 9 (Fig. 3).
Experimental section
General
All the chemicals were commercially available and used
without further purification, unless otherwise stated. Analyti-
cal thin layer chromatography (TLC) was performed using
Moreover, we also calculated the reaction between aniline
(1c) and (E)-(2-nitrovinyl)benzene (2a). The energy barriers for
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Merck silica gel GF254 plates. Flash column chromatography give the pure product. 1-(4-bromophenyl)-2-methylnaphtho[2,1-b]-
was performed on silica gel (200–300 mesh). Melting points furan (5h), white solid, mp: 132–134 °C; 1H NMR (400 MHz,
were measured on an X-4 melting point apparatus. 1H NMR CDCl3, 25 °C, TMS): δ = 7.95 (d, J = 8.0 Hz, 1H), 7.76 (d, J =
spectra were recorded on a 400 MHz instrument (Bruker 8.4 Hz, 1H), 7.65–7.74 (m, 4H), 7.39–7.45 (m, 3H), 7.32–7.37
Avance 400 Spectrometer). Chemical shifts (δ) are given in (m, 1H), 2.44 ppm (s, 3H); 13C NMR (100 MHz, CDCl3, 25 °C,
ppm relative to TMS as the internal reference, with coupling TMS): δ = 151.4, 151.3, 133.2, 132.2, 130.7, 128.9, 127.7, 125.8,
constants (J) in Hz. 13C NMR spectra were recorded at 124.8, 124.1, 123.0, 121.9, 121.7, 117.8, 112.1, 12.3 ppm;
100 MHz. Chemical shifts were reported in ppm with the HRMS (ESI) m/z calcd for C19H14BrO [M + H]+: 337.0223,
internal chloroform signal at 77.0 ppm as a standard. HRMS found: 337.0228.
(ESI) was measured with a Bruker Daltonics APEXII instru-
ment. The single-crystal X-ray diffraction measurement was
carried out on a Rigaku Saturn CCD diffractometer at 100(2) K Computational methods
using graphite monochromated Mo Kα radiation (λ
=
All the calculations were carried out using Gaussian 09 pro-
grams. The solvation phase geometry optimizations and fre-
quency calculations were performed using the B3LYP
functional with the 6-31G+(d) basis set in collaboration with
the SMD solvation model. Methanol was used as the solvent.
Single-point energy calculations were then performed on the
stationary points using the M06-2X method with a larger basis
set, 6-311++G(2df,2p), and the same solvation model used in
the optimization calculation. All the energies correspond to
the reference state of 1 mol L−1, 298.15 K.
0.71073 Å). The structure was solved using direct methods and
refined with full-matrix least squares on F2 using the
SHELXTL-97 program package.
General procedure for the synthesis of 3
In a 10 mL reaction vial, 1-naphthylamine/2-naphthylamine
(0.5 mmol), nitroolefin (0.5 mmol), carbonaceous material (C–
SO3H) (10 mg) and water (3.0 mL) were mixed and then
capped. The mixture was stirred for a given time at 60 °C.
Upon completion, as shown by TLC monitoring, the reaction
mixture was cooled to room temperature. The resulting solid
precipitate was filtered and dried along with the catalyst.
Crude product was further purified by column chromatography
on silica gel (eluting with ethyl acetate–petroleum ether =
1 : 20–1 : 4) to give the pure product. 1-phenyl-3H-benzo[e]-
Acknowledgements
We are grateful for the financial support from the Natural
Science Foundation of China (no. 21172149), the Science Tech-
nology Department of Zhejiang Province (no. 2010R50014-17)
and the Research Funds of Shaoxing University (no.
2014LG1006). We are also grateful to Wenzhou University for
the help of HRMS analysis.
1
indole (3a), brown oil; H NMR (400 MHz, CDCl3, 25 °C, TMS):
δ = 8.48 (br, s, 1H), 8.18 (d, J = 8.0 Hz, 1H), 7.96 (d, J = 8.8 Hz,
1H), 7.67–7.69 (m, 3H), 7.52–7.56 (m, 3H), 7.47–7.50 (m, 1H),
7.35–7.45 (m, 2H), 7.19 ppm (d, J = 2.0 Hz, 1H); 13C NMR
(100 MHz, CDCl3, 25 °C, TMS): δ = 137.1, 132.9, 130.2, 129.8,
128.8, 128.7, 128.3, 126.8, 125.4, 123.8, 123.4, 123.2, 121.6,
121.3, 119.5, 112.9 ppm; HRMS (ESI) m/z calcd for C18H13N
[M + H]+: 244.1121, found: 244.1125. 2-methyl-1-phenyl-3H-benzo[e]-
indole (3j), brown solid, mp: 52–54 °C; 1H NMR (400 MHz,
CDCl3, 25 °C, TMS): δ = 8.17 (br, s, 1H), 7.94 (t, J1 = 6.8 Hz, J2 =
7.2 Hz, 2H), 7.63 (d, J = 8.4 Hz, 1H), 7.55–7.57 (m, 4H),
7.45–7.50 (m, 2H), 7.38–7.42 (m, 1H), 7.29–7.33 (m, 1H),
2.35 ppm (s, 3H); 13C NMR (100 MHz, CDCl3, 25 °C, TMS): δ =
137.2, 131.3, 131.2, 130.4, 129.8, 128.7, 128.5, 128.3, 126.8,
125.1, 123.3, 122.9, 122.4, 120.9, 117.2, 112.4, 11.9 ppm;
HRMS (ESI) m/z calcd for C19H15N [M + H]+: 258.1277, found:
258.1273.
Notes and references
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General procedure for the synthesis of 5
In a 10 mL reaction vial, 1-naphthol/2-naphthol (0.5 mmol),
nitroolefin (0.5 mmol), carbonaceous material (C–SO3H)
(10 mg) and water (3.0 mL) were mixed and then capped. The
mixture was stirred for a given time at 60 °C. Upon completion
as shown by TLC monitoring, the reaction mixture was cooled
to room temperature. The resulting solid precipitate was fil-
tered and dried along with the catalyst. The crude product was
further purified by column chromatography on silica gel
(eluting with ethyl acetate–petroleum ether = 1 : 30–1 : 8) to
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was carried out in ethanol at room temperature. X-ray crys-
tallographic analysis was performed with a Bruker APEX-II
CCD, mounted in inert oil and transferred to the cold gas
stream of the diffractometer. Crystal structure determi-
nation of complex 3u – crystal data: C19H15N, M = 257.32,
monoclinic, a = 10.182(3), b = 19.806(7), c = 8.310(3) Å, U =
1346.7(8) Å3, T = 100(2) K, space group Cc, Z = 4, 1.269
reflections measured, 4446 unique (Rint = 0.0246), which
were used in all calculations. The final wR(F2) was 0.0847
(all data).
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