Supported copper triflate as an efficient catalytic system for the synthesis of highly functionalized 2-naphthol Mannich bases under solvent free condition

Article abstract of DOI:10.1016/j.tetlet.2012.06.022

Various heterogeneous catalysts (Lewis acid) have been prepared and screened for the synthesis of Betti bases in an attempt to reduce the environmental hazards associated with the conventional homogeneous Lewis acid system. And we found especially Cu(OTf)

Full text of DOI:10.1016/j.tetlet.2012.06.022

Tetrahedron Letters 53 (2012) 4376–4380
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Tetrahedron Letters
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Supported copper triflate as an efficient catalytic system for the synthesis
of highly functionalized 2-naphthol Mannich bases under solvent free condition
Someshwar D. Dindulkar a, Vedavati G. Puranik b, Yeon Tae Jeong a,
⇑
a Department of Image Science and Engineering, Pukyong National University, Busan 608 737, Republic of Korea
b Center for Materials Characterization, National Chemical Laboratory, Pune 400 008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Various heterogeneous catalysts (Lewis acid) have been prepared and screened for the synthesis of Betti
bases in an attempt to reduce the environmental hazards associated with the conventional homogeneous
Lewis acid system. And we found especially Cu(OTf)2ÁSiO2 catalyzes the three-component coupling of
aldehyde, 2-naphthol, and alicyclic amine to generate Betti base with high efficiency under neat condi-
tions without additional co-catalyst or additive in air. The reaction is not sensitive to water and occurs
smoothly in water as well.
Received 7 May 2012
Revised 4 June 2012
Accepted 6 June 2012
Available online 12 June 2012
Keywords:
Three-component coupling
Betti bases
Ó 2012 Elsevier Ltd. All rights reserved.
Supported copper triflate
Single-crystal XRD
Solvent-free
The Mannich reaction is a classic method for the synthesis of
many natural substances, pharmacophore and one of the most
important methods in organic synthesis for providing carbon–
carbon bond formation and its products are of considerable impor-
tance in industry.1a However, recently chemists are more interested
to synthesize the Mannich product using one-pot multicomponent
with environmentally friendly approach that maximizes the utility
of known multicomponent reactions.1b,c One-pot multicomponent
reactions have gained significant importance and attracted much
attention on progress in synthesis of complex building blocks.2a
These reactions are single step one-pot transformations proceeding
through more than two subsequent reactions in which the product
of the first is a substrate for the second. During the past few decades,
many one-pot multi-component reactions have been reported for
the making of new carbon–carbon, carbon–hetero atom, and het-
ero–hetero atom bonds.2b–d Especially, the ability of these unified
multicomponent reactions that provide poly-functionalized hetero-
cyclic scaffolds in single operation and in stereospecific manner is of
great importance in organic synthesis and make them useful in
medicinal chemistry.3a,b Recent development of solvent-free multi-
component reactions has attracted increasing interest from chem-
ists. In this initiative, aided with the recent development of new
strategies, the chemists have been showing more concern toward
minimizing the environmental pollution caused by solvents and
developing eco-compatible synthetic procedures.4a,b
One of the important reactions in the field of multicomponent
synthesis is the Betti reaction which was discovered at the
beginning of the 20th century by Mario Betti. Mario Betti was a
distinguished Italian chemist, he has discovered the synthesis of
1-(a-aminoalkyl)-2-naphthols from a simple and straightforward
condensation between 2-naphthol, aryl aldehydes, and ammonia,
or amines, the formed product called Betti base.5a,b Initially Betti
had concluded that the reaction proceeded via the nucleophilic at-
tack of 2-naphthol carbon on imine produced from benzaldehyde
and aniline. Later he proved that 2-naphthol should be a good car-
bon nucleophile toward the imine.6
The synthesis of 2-naphthol Mannich bases from this one of the
modified Mannich reactions gives versatile synthetic building
blocks which have a wide range of application such as useful
precursor for the preparation of many nitrogen-containing phar-
macophores and key intermediate for many multistep organic
synthesis.7a,b Some of the 2-naphthol Mannich bases and related
molecules are well reported as biologically active ingredients,
medicaments.7c Moreover, we are also interested to emphasize
on the recent reported patents which proved that this 2-naphthol
Mannich bases (Fig. 1) are biologically multipurpose precursors,
which can be useful in treatment of many infectious diseases such
as Giardiasis and also showed their effectiveness in treatment of
body pain as an analgesic drug.7d,e Besides, compounds of this type
were reported in the literature as good potent oxytocic nature.7h
Furthermore, Betti bases were used as ligands in the enantioselec-
tive addition of diethylzinc to aromatic aldehydes and showed
highly efficient asymmetric induction in this addition.7f Also it
has been proved that the (S)-Betti base is an excellent chiral
⇑
Corresponding author. Tel.: +82 51 629 6411; fax: +82 51 629 6408.
E-mail address: ytjeong@pknu.ac.kr (Y. T. Jeong).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.022

S. D. Dindulkar et al. / Tetrahedron Letters 53 (2012) 4376–4380
4377
of biologically active Betti products. So herein we wish to report
the multicomponent reaction for the synthesis of 1-( -aminoal-
a
1)
2)
3)
kyl)-2-naphthols from 2-naphthol, aldehyde, and alicyclic amine
using Cu(OTf)2ÁSiO2 as reusable catalyst at room temperature to
40 °C under neat condition in quantitative yield (Scheme 1). After
series of studies we achieved excellent yields in very short dura-
tion at milder condition. It has been demonstrated that solvent-
free reaction condition is an efficient technique for various organic
reactions. Due to high concentration of the reactant leads to signif-
icant decrease in reaction time, increased yields, and easier workup
procedure. Our studies indicated that this reaction proceeds with a
catalytic amount of catalyst with short reaction time, and easy
workup procedure is the one of the advantages of this protocol.
In the light of previously reported literature survey for the syn-
thesis of this interesting moiety, there are very few reports avail-
able using different Lewis acid, Brønsted acid surfactant, and
neutral and efficient non-ionic surfactant catalyst.10a–d Homoge-
neous Lewis acids are most commonly used catalysts for the fast
and efficient catalytic system in the synthesis of verity of organic
transformation. In addition, separation of the Lewis acid from the
reaction products produces a large volume of acidic waste due to
the water-quenching step needed to neutralize the acid. Hence
we focused our initial investigation to find out a best heteroge-
neous catalytic system for this precious one-pot multicomponent
Betti reaction. In our literature survey, the usage of silica supported
R
OH
OH
Cl
OH
R
N
O
R =
N
O
N
,
NC
4)
5)
OH
N
N(CH2)5CH3
OH
O
NC
Figure 1. Biologically active reported Betti bases and their related structure.
auxiliary for the synthesis of enantiopure (2S,6R)-dihydropinidine
and (2S,6R)-isosolenopsins.7g Due to multi applicability in biology
as well as in chemistry, this adventitious moiety encourages us
to develop an efficient route for their synthesis.
The usage of heterogeneous metal Lewis catalyst instead of tra-
ditional homogeneous metal Lewis and Brønsted acid catalysts,
could possess a more environmentally friendly alternative. More-
over the use of metal heterogeneous catalysts under solvent-free
conditions is becoming very popular as it has many advantages,
such as faster rate of reaction, reduced hazardous solvent pollution,
reusability, air/water compatibility, low cost, and remarkable abil-
ity to suppress side reactions in acid sensitive substrates that make
them valuable and advantageous catalyst in synthetic processes.
Copper triflate (Cu(OTf)2), is less sensitive to air and moisture. The
presence of triflates (OTf) has advantages of being water tolerant
and less sensitive toward atmospheric exposure than halide Lewis
acids. The supported copper triflate has various advantages due to
its low toxicity, low price, ease of handling, reusability, and experi-
mental simplicity. The supported Cu(OTf)2ÁSiO2 catalyst exhibited
both Lewis and Brønsted acidic surfaces due to the presence of
residual moisture within Cu(OTf)2, as well as the surface silanol
groups of the silica, and the methanol used as solvent in the catalyst
preparation.8a Sage et al. reported that Cu(OTf)2ÁSiO2 is an efficient
catalyst for cationic polymerization of styrene as compared to tradi-
tional vinyl polymerization catalyzed by homogeneous Lewis acid
catalyst. Faster reaction rate and higher molecular weight polymers
were observed compared to the homogeneous catalyst.8a Also the
supported copper triflate as an adventitious catalytic system for
the thioacetalization of carbonyl compounds under solvent free
condition has been reported.8b
metal Lewis acid as catalyst in the synthesis of 1-(
naphthols is unprecedented.
a-aminoalkyl)-2-
Initially, in search of best catalytic system for this one-pot syn-
thesis, optimization of various reaction parameters like different
metal Lewis acid catalysts, temperature, and solvent was carried
out (Table 1). In order to establish the real effectiveness of the cat-
alyst for the synthesis of Betti base, a test reaction was performed
without catalyst using 2-naphthol, benzaldehyde, and piperidine at
room temperature in air. It was found that only a trace amount of
product was obtained in the absence of catalyst even after 12 h
(Table 1, entry 2). Even though we increased temperature up to
100 °C for this catalyst free reaction, there was no appreciable
improvement in yield (Table 1, entry 2). In search of effective,
eco-friendly, and efficient reusable catalytic system for this reac-
tion, same test reaction was performed with different supported
metal Lewis acid catalysts such as Cu–Sn (200 mesh, 100 mg),
Cu(OTf)2ÁSiO2, Zn(OTf)2ÁSiO2, TiO2ÁSiO2, BF3ÁSiO2, and ZnCl2ÁSiO2
(Table 1). To study the role of SiO2, reaction was tested with SiO2
(100 mg) only, and we observed comparable good yield as com-
pared to catalysts free reaction (Table 1, entry 4). Among all
screened catalysts Cu(OTf)2ÁSiO2 gave the best result in view of
yield and reaction time (Table 1, entry 5). In contrast TiO2ÁSiO2,
BF3ÁSiO2, and ZnCl2ÁSiO2 did not afford the desired product in good
yields. Careful analysis of screened supported Lewis acid catalyst,
shows that silica supported metal triflate such as Cu(OTf)2ÁSiO2
and Zn(OTf)2ÁSiO2 were effective than other silica supported metal
Lewis acid catalyst, but promising results were obtained with silica
supported copper triflate catalyst in lesser time with better yield.
Once we found Cu(OTf)2ÁSiO2 as best catalyst for this reaction,
temperature and solvent optimization was done. We screened var-
In continuation of our ongoing effort to the development of
newer environmentally benign methodology for the synthesis of
useful precursor in the field of biology, industry, and key interme-
diate for the multistep synthesis,9a–d we decided to investigate the
efficiency of supported metal Lewis acid catalyst for the synthesis
R1
R
N
R2
OH
R1
R2
Cu(OTf)2.SiO2
OH
CHO
+
R
+
N
H
Neat or Water, rt
1
2
3
4a-x
24 examples
Scheme 1. Supported copper triflate catalyzed synthesis of Betti base.

4378
S. D. Dindulkar et al. / Tetrahedron Letters 53 (2012) 4376–4380
Table 1
Table 2
Screening of various types of heterogeneous catalysts and solvents for the synthesis of
The reuse of Cu(OTf)2ÁSiO2 in the synthesis of compound 4a
compound 4a.a
Entry
Reaction cycle
Yielda
1
2
3
Ist (fresh run)
IInd cycle
IIIrd cycle
92
88
81
N
OH
+
CHO+
a
OH
Isolated yield.
N
H
and reusability was ascertained with minimal losses of Cu.11a,b This
prompted us to study the reusability of the Cu(OTf)2ÁSiO2 catalyst
for at least two more cycles. Accordingly, after the first fresh run
with 92% yield, the catalyst was recovered by dissolving reaction
mixture in hot ethanol and catalyst was removed by filtration.
The recovered catalyst was dried under vacuum at 60 °C for 12 h
and tested up to two more reaction cycles. Recycling and reuse
of the catalyst showed minimal decreases in yields. The product
4a was obtained in 92%, 88%, and 81% yields after successive cycles.
(Table 2, entries 1–3), thus proving the catalyst’s reusability. Care-
ful analysis of Table 2 shows that there is no significant loss of cat-
alytic activity of this supported catalyst, thus proving that the
catalyst shows very less leaching of copper during the course of
reaction in the solution. The stability and recovery of this sup-
ported catalyst has also been proved by Sage et al.8a
Once the effective catalytic amount of the catalyst was proven,
we extended our study to investigate generality and efficiency of
developed protocol using Cu(OTf)2ÁSiO2 (10 mol %) under solvent-
free condition at 40 °C with 2-naphthol, functionalized aromatic
aldehydes, and different alicyclic amines to prepare a series of
substituted aminobenzylnaphthols (Table 3). Various aromatic
aldehydes with different substituents at ortho, meta, or para-posi-
tions show equal ease toward the product formation in high yields.
The obtained results were shown in Table 3. Careful analysis of Ta-
ble 3 reveals that the reactions were compatible with various func-
tionalized aldehydes and secondary amines using this catalytic
system. In contrast, aromatic aldehydes having groups like Cl, F,
Br, and MeO showed better reactivity and the reactions were com-
pleted in shorter time. Heteroaryl aldehydes like nicotinaldehyde
and thiophene-2-carbaldehyde afforded the desired product in
quantitative yields. We eventually achieved excellent yields in very
short duration at milder condition. Surprisingly best yield was ob-
tained with alicyclic amines like morpholine and thiomorpholine
as compared to piperidine and pyrrolidine.
Entry Catalyst (10 mol %)
Solvent
Toluene
Toluene 100
Toluene rt
Temp
(°C)
Time
(h)
Yieldb
1
2
3
—
—
rt
12
12
12
Trace
630
52
Cu–Sn (200 mesh,
100 mg)
4
5
6
7
8
SiO2
Toluene 100
12
12
12
12
12
12
12
12
7
12
12
1
0.75
1
62
78
74
54
37
56
53
73
81
56
82
84
92
93
69
93
Cu(OTf)2ÁSiO2
Zn(OTf)2ÁSiO2
TiO2ÁSiO2
Toluene
Toluene
Toluene
Toluene
Toluene
THF
CH3CN
Water
Ethanol
Toluene
Neat
rt
rt
rt
rt
rt
rt
rt
rt
rt
80
rt
40
60
40
40
BF3ÁSiO2
9
ZnCl2ÁSiO2
10
11
12
13
14
15
16
17
18
19
Cu(OTf)2ÁSiO2
Cu(OTf)2ÁSiO2
Cu(OTf)2ÁSiO2
Cu(OTf)2ÁSiO2
Cu(OTf)2ÁSiO2
Cu(OTf)2ÁSiO2
Cu(OTf)2ÁSiO2
Cu(OTf)2ÁSiO2
Cu(OTf)2ÁSiO2 (05 mol %)
Cu(OTf)2ÁSiO2 (20 mol %)
Neat
Neat
Neat
Neat
2
1
a
Reactions and conditions: benzaldehyde (0.6 mmol), piperidine (0.5 mmol), 2-
naphthol (0.5 mmol) at room temperature.
b
Isolated yields.
ious solvents, such as toluene, EtOH, CH3CN, water and THF at
room temperature. Among the tested different organic solvents
only toluene and acetonitrile were found to give moderate yields.
It is worthy of note, that when we performed the reaction in aque-
ous media, reaction was completed with good yield as compared to
conventional organic solvents (Table 1, entry 12).
It is observed that under solvent condition, it required longer
times (7–12 h) to afford comparable yields. A similar model reac-
tion was carried out using 10 mol % of Cu(OTf)2ÁSiO2 at room tem-
perature without solvent. It was observed that reaction proceeded
in an efficient manner in few minutes with increased yield (84%)
under solvent free condition at room temperature (Table 1, entry
15). Practically due to high concentration of reactant, rate of reac-
tion was faster in neat condition. When we found solvent free
condition gives better yield in lesser time, we decided to study
the effect of temperature on the rate of reaction with different
temperature optimization. Surprisingly, the best result was
obtained by using 10 mol % Cu(OTf)2ÁSiO2 at room temperature in
1 h (84%), affording a high yield of 92% in 45 min at 40 °C under
neat condition.
Once found the best catalyst and solvent condition, we studied
the effect of varying amount of Cu(OTf)2ÁSiO2 catalyst for this one-
pot multicomponent system. In order to evaluate the appropriate
catalyst loading, a model reaction was carried out using 5 and
20 mol % of Cu(OTf)2ÁSiO2 at 40 °C without solvent (Table 1, entries
18 and 19). It was found that 10 mol % of catalyst shows maximum
yield in minimum time. A larger loading amount of the catalyst
(20 mol %) neither increases the yield nor shortens the conversion
time. So, 10 mol % of catalyst was found to be the optimal quantity
and sufficient to push the reaction forward. It was found that in-
stead of 1.0 equiv aldehyde the use of 1.2 equiv of aldehyde pro-
vides a better yield. A few examples of supported Cu species
have been reported as efficient catalyst in organic transformation
The reported as well as synthesized novel compounds were fur-
ther characterized by their spectral properties (1H, 13C NMR, and
HRMS). Moreover, single crystal X-ray diffraction analysis was
done for compounds 4a and ORTEP diagram of the compound 4a
is shown in Figure 2. All the data were corrected for Lorentzian,
polarisation and absorption effects. SHELX-97 (ShelxTL)12a was
used for structure solution and full matrix least squares refinement
12b
on F2.
The single-crystal X-ray analysis of 4a clearly indicates
that C11 has ‘R’ relative configuration. The naphthol ring is almost
planar which is at an angle of 72.75° with respect to the benzene
ring shown in Figure 3 and molecular packing diagram provided
as Figure 4 (Figs. 3 and 4 given as Supplementary data).
In conclusion, the main advantages of this present methodology
are the simple work-up, easy recovery of catalyst, no need for
anhydrous condition, no base, or any additional activator required,
and the residue was crystallized from ethanol to give the pure
product without further purification. A possible mechanism of this
one pot reaction is expected on the basis of reported literature.6 It
is assumed that, after formation of iminium ion from benzaldehyde
and secondary amines followed by the nucleophilic attack of
2-naphthol carbon on iminium carbon, subsequent shifting of
hydrogen atom leads to the formation of 1-(
a-aminoalkyl)-2-
naphthols (Scheme 2). A variety of silica supported Lewis acid cat-

S. D. Dindulkar et al. / Tetrahedron Letters 53 (2012) 4376–4380
4379
Table 3
Synthesis of diversified aminoalkyl naphthol derivatives via Scheme 1a
Table 3 (continued)
Entry
R
Amine
Time (h)
0.5
Yieldb
Entry
R
Amine
Time (h)
Yieldb
92
N
N
4u
72
80
74
83
N
H
S
4a
0.75
N
H
4v
4w
4x
1.18
0.75
0.66
N
H
4b
4c
4d
4e
1
95
82
90
88
N
H
Cl
S
S
N
H
S
2.5
1.16
1
N
H
EtO
N
H
N
H
MeO
MeO
a
Reactions and conditions: benzaldehyde (0.6 mmol), piperidine (0.5 mmol),
2-naphthol (0.5 mmol), and 10 mol % Cu(OTf)2ÁSiO2 at 40 °C under neat condition.
N
H
b
Products were isolated by recrystallization from ethanol except products 4c, 4e,
MeO
MeO
4f, and 4g purified using column chromatography.
N
H
4f
2
94
OMe
4g
4h
4i
0.66
0.75
3
91
95
87
N
H
Cl
Cl
Br
N
H
N
H
O
4j
1.33
0.83
1.16
90
85
88
Br
F
N
H
4k
4l
N
H
O
F
F
N
H
S
F
F
F
4m
4n
0.83
0.5
93
92
N
H
S
F
F
N
H
F
N
H
4o
0.75
84
Figure 2. ORTEP of compound 4a. Ellipsoids are drawn at 50% probability.
F
F
N
H
4p
4q
4r
0.33
1.16
2
94
90
88
F
F
F
R1
HN
R1
N
R
H
N
H
+
O
R
R2
R2
-HO
C
H
R1
N
H
R1
N
F
+
R
R
N
H
R2
R2
OH
H
-H2O
O
MeO
O
F
N
H
4s
4t
0.5
86
90
Br
F
F
Scheme 2. A possible mechanism for the synthesis of 1-(a-aminoalkyl)-2-
naphthols.
1.33
N
H

4380
S. D. Dindulkar et al. / Tetrahedron Letters 53 (2012) 4376–4380
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ZnCl2ÁSiO2, were applied for the multi-component one-pot synthe-
sis of Betti bases by a modified Mannich condensation. The use of
10 mol % Cu(OTf)2ÁSiO2 at rt to 40 °C was found to be a simple and
straightforward protocol for the synthesis of polyfunctionalized
Betti bases using three-components, viz., substituted benzalde-
hydes, alicyclic amine, and 2-naphthol.
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Acknowledgment
This research work was supported by the Second Phase of Brain
Korea (BK21) Program.
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13. General: Chemicals were purchased from Aldrich and Alfa aesar Chemical
Companies. NMR spectra were recorded in ppm in CDCl3 on a Jeol JNM ECP 400
NMR instrument using TMS as internal standard. HR-MS were recorded on Jeol
JMS-700 mass spectrometer. Melting points are taken in open capillaries and
are uncorrected; Electrothermal-9100 (Japan) instrument was used to
determine the melting point of the compounds.
Supplementary data
General chemicals and experimental procedure are given in ref-
erence section.13–15 All new compounds are well characterized by
their spectral properties (1H and 13C NMR and HRMS) and copies
of NMR spectra were provided as Supplementary data. All crystal-
lographic parameters and data of compound 4a are provided. The
complete set of structural parameters for compound 4a (CCDC
No.874639) in CIF format is available as an Electronic Supplemen-
tary Publication from the Cambridge Crystallographic Data Centre
www.ccdc.cam.ac.uk/conts/retrieving.html.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
06.022.
14. Preparation of supported copper triflate:
References and notes
A
supported Cu(OTf)2 catalyst were prepared by adopting the literature
precedent Sage et al.8a Copper trifluoromethanesulfonate (0.18 g, 0.5 mmol)
was added to methanol (50 ml) in 100-ml 3-neck round bottom flask
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a
equipped with a reflux condenser and a magnetic stirrer bar. The silica (230–
400 mesh, 5 g, pretreated at 600 °C for 18 h) was added and the resulting slurry
was stirred at room temperature under N2 atmosphere for 2.5 h. The solvent
was then evaporated in vacuo at 80 °C for 1 h. The resulting solid product
obtained was light blue depending on the loading.
15. Typical procedure for the 1-(a-aminoalkyl)-2-naphthol derivatives: A mixture of
2-naphthol (1.0 equiv), amine (1.0 equiv), and aldehyde (1.2 equiv) was
stirred at room temperature to 40 °C without any solvent in the presence
of 10 mol % of catalyst for certain period as indicated in Table 3. After
completion of reaction indicated by TLC, the reaction mixture was dissolved
in hot ethanol and the catalyst was recovered by filtration and product was
recrystallized from ethanol. 1H and 13C NMR data of the known compounds
were in good agreement with those in the literature. All new compounds
were completely characterized by their analytical and spectral data.
Spectroscopic data for new compounds are given and scan spectra were
provided as Supplementary data.