Practical synthesis of silyl-protected and functionalized propargylamines using nanostructured Ag/TiO2 and Pt/TiO2 as active recyclable catalysts

Article abstract of DOI:10.1007/s11696-018-0604-6

Abstract: Herein we report the use of Ag/TiO₂ and Pt/TiO₂ nanocatalysts to promote the synthesis of a series of silyl-protected and functionalized propargylamine derivatives through alkyne–amine–aldehyde (A³) coupling unde

Full text of DOI:10.1007/s11696-018-0604-6

Chemical Papers
https://doi.org/10.1007/s11696-018-0604-6
ORIGINAL PAPER
Practical synthesis of silyl‑protected and functionalized
propargylamines using nanostructured Ag/TiO2 and Pt/TiO2 as active
recyclable catalysts
Yasser M. A. Mohamed1 · Hossam A. El Nazer1 · Eirik Johansson Solum2
Received: 16 July 2018 / Accepted: 25 September 2018
© Institute of Chemistry, Slovak Academy of Sciences 2018
Abstract
Herein we report the use of Ag/TiO2 and Pt/TiO2 nanocatalysts to promote the synthesis of a series of silyl-protected and
functionalized propargylamine derivatives through alkyne–amine–aldehyde (A3) coupling under microwave condition using
water as solvent in short reaction times with high yielding production. A comparative study between the two catalysts was
also demonstrated to prove that Pt/TiO2 nanocomposite has higher potential efect for propargylamines synthesis more than
Ag/TiO2. The main features of the investigated method for propargylamines synthesis were easy handling, low catalyst
loading as well as the feasibility of catalyst recyclability. The prepared compounds are potentially benefcial with possible
pharmacological and biological properties, which that could be used in several disciplines including medicinal and agricul-
tural applications.
Graphical abstract
Keywords Metal-doped TiO2 · Nanocatalysis · Propargylamine · Multicomponent reaction · Recyclability
Introduction
* Yasser M. A. Mohamed
y.m.a.mohamed@outlook.com
Multicomponent reactions are an important protocol in mod-
ern synthetic chemistry, and are utilized in the synthesis of
highly functionalized organic products (Haji 2016; Abbiati
and Rossi 2014). However, some important factors, such as
reaction conditions, temperature and catalyst, must be taken
into considerations for enabling multicomponent reactions to
* Hossam A. El Nazer
dr_hosamnazer@yahoo.com
1
Photochemistry Department, National Research Center,
Dokki, Giza 12622, Egypt
2
Faculty of Health Sciences, NORD University, 7800 Namsos,
Norway
Vol.:(0123456789)
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Chemical Papers
produce one single product (Wender 2014). One of the best
examples of this type of reactions is the one-pot synthesis of
propargylamine as main product from the catalytic coupling
reaction of aldehyde, amine and alkyne (Abbiati and Rossi
2014). Recently, propargylamine moieties have been found
in some bioactive pharmaceuticals such as rasagiline (Old-
feld et al. 2007) and selegiline (Riederer and Lachenmayer
2003) (Fig. 1).
supported metal catalysts was explored in the literature for
various organic transformations (Oi et al. 2016). In this pro-
tocol, diferent molecules containing propargylamine moiety
have been synthesized conducting alkyne–aldehyde–amine
coupling (A3 coupling) reaction utilizing Ag/TiO2 and Pt/
TiO2 in water as heterogeneous catalysts under both ther-
mal and microwave irradiation. In addition, we investigated
the efect of metal doping on the catalytic activity of TiO2
nanoparticles. Furthermore, by carrying out the reactions in
water this protocol ofers advantages, from the green chem-
istry point of view, compared to those performed in organic
solvents (Kashid and Balakrishna 2018; Shah et al. 2018;
Loukopoulos et al. 2017; GhavamiNejad et al. 2016; Trivedi
and Singh 2015; Zhao and Seidel 2015; Sharma et al. 2014).
A literature survey on some reported approaches for the syn-
thesis of propargylamines is shown in Table 1.
Furthermore, propargylamines were used as versatile
building blocks in organic synthesis (Vessally et al. 2016).
Hence, due to the importance of these compounds, a vari-
ety of synthetic approaches have been investigated for the
synthesis of these motifs. Several examples using Cu(I) salts
(Gommermann et al. 2003; Li and Li 2004; Xu and Li 2009;
Okamura et al. 2010), Cu(II) salts (Meyet et al. 2012), Zn(II)
salts (Zani et al. 2006; Sarode et al. 2016; Kano et al. 2016),
In(III) salt (Sakai et al. 2005), Au(III) salts (Lo et al. 2006),
Ag(I) salt (Wei et al. 2003), [IrCl(COD)]2/MgI2 (Fischer
and Carreira 2004), [Ru(bpy)3]/CuI (Zhang et al. 2017) as
well as nanocatalysts such as Au0 NPs (Borah et al. 2014),
and Ag0 NPs have been reported (Borah and Das 2016).
However, these methods sufer from drawbacks such as long
reaction time (e.g., 1–6 days) (Gommermann et al. 2003;
Meyet et al. 2012; Zani et al. 2006; Lu et al. 2009), the util-
ity of strong bases such as alkyllithium reagents (Qi et al.
1988; Riederer and Lachenmayer 2003; Merino et al. 1996),
the use of auxiliary substances with the catalyst (Gommer-
mann et al. 2003; Fischer and Carreira 2004; Zhang et al.
2017; Cardoso et al. 2013). Hence, the search for an efcient
approach for the synthesis of these compounds is required.
Lately, there has been an increasing interest in nanocata-
lysts as active heterogeneous catalysts (Nguyen et al. 2017;
Okamura et al. 2010). One of the defects which occurred
during the reaction processes is the agglomeration of cata-
lyst particles that may block the active sites of the catalyst,
which efect on its instability. Titanium dioxide (TiO2) was
found to be good supported metal catalysts due to its high
surface area, which can be stabilizing the catalysts in its
mesoporous framework, which leads to strong metal support
interaction and high chemical stability (Bagheri et al. 2014).
Hence, due to the mentioned properties, the activity of TiO2
Continuing to our studies for the development and appli-
cation of heterogeneous catalysts in new synthetic routes in
organic reactions (Mohamed and Solum 2017; Mohamed
and Hansen 2013; Attia et al. 2017; Petrof et al. 2017),
herein we set an eco-friendly reaction condition that has
the following features: (1) recyclable heterogeneous nano-
catalyst which is easy to prepare and to handle (2) the use
of microwave irradiation which shortens the reaction time;
(3) no co-catalysts or auxiliary substances have been used,
(4) the reaction is carried out in water as solvent, and (5)
obtaining variety of propargylamine derivatives with high
yields. To the best of our knowledge, Ag and Pt doped on
TiO2 nanocatalysts have not previously been employed in the
A3 coupling reaction. It was demonstrated in this report that
Pt/TiO2 is a more practical nanocatalyst under microwave
irradiation condition in water, compared to Ag/TiO2. The
Pt/TiO2 catalyst surface was in addition easier to regenerate
which leads to far greener outcomes.
Experimental
All solvents, amines and paraformaldehyde were pur-
chased from supplier and used without further puri-
fication. Anatase titanium dioxide (Aldrich) having
the specific surface area of (35–65 gm−2) was used.
2-(prop-2-yn-1-yl)isoindoline-1,3-dione (1c) and (prop-
2-yn-1-yloxy)benzene (1d) were prepared according to
the literature procedures (Menon et al. 2009; Al-Qaisi
et al. 2014). Melting points (mp) were measured by an
electrothermal (variable heater) melting point apparatus
and were uncorrected. Thin layer chromatography (TLC)
was performed on silica gel plates 60 GF254, cellulose
1
plates (20 × 20 cm). H NMR (300 MHz) and 13C NMR
(75 MHz) spectra were recorded on Bruker-Avance DPX
(300 MHz for 1H and 75 MHz for and 13C, respectively).
CDCl3 was used as partial deuterated-NMR solvent.
Fig. 1 Structures of some bioactive propargylamines
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Table 1 Some literature reports
for catalyzed A3 coupling
reaction
Catalyst
CuBr
Rx. conditions
Catalyst (20
Isolated yield
Refs.
Sharma et al. (2014)
mol%), toluene,
100 oC, 4 h
Nano-Ag2O
Catalyst (2
mol%), MW
condition,
Trivedi et al. (2015)
CH3Cl, 5 min
Catalyst (4
mol%),
cocatalyst (3
mol%), CH2Cl2,
0 oC, 12 h
Cu(I)/Acid-
Thiourea
Catalyst
Zhao et al. (2015)
ZSM-5-AgNPs
Catalyst (100
mg, 5% Ag),
glycol, 75 oC
condition, 30
min
GhavamiNejad et. al
(2016)
Cu(II)
Coordination
Polymers
Catalyst (2
mol%), 2-
Loukopoulos et al.
(2017)
propanol, 90 oC,
5 h
Catalyst (0.1
mol%), CH3CN,
MW condition,
5 min
Catalyst (20
mg), ethylene
glycol, MW
condition, 15
min
Kashid et al. (2018)
Shah et.al. (2018)
[{Cu(µ I) Cu}(P
–
Ph3)42]
CuNPs@ZnO–
PTh
Pt/TiO2
nanocomposite
Catalyst (0.25
mol%), MW
condition,
In this report
water, 6 min
Chemical shifts (δ) are given in parts per million (ppm)
downfield from tetramethylsilane. High-resolution mass
(ESI/MS) spectra were recorded on Agilent; 6230 Series
Accurate-Mass Time-of-Flight (TOF) LC/MS. The used
microwave oven is a Milestone Italy; StartSynth model
accompanied with Pack2B Basic Single Vessel Kit reac-
tor. The microstructure of the prepared powders was
examined using high resolution transmission electron
microscope (HRTEM), JEOL, JEM-2100 working volt-
age at 200 kV. The metal content was observed by energy-
dispersive X-ray spectroscopy (EDX). The EDX spectra
of the TiO2 doped with Pt and Ag were recorded on a
JOEL Model JED-2300 instrument.
Preparation of metal‑doped TiO2
TiO2 (5 g), ethanol (500 mL) and the appropriate amounts
of metal salts (corresponding to 0.5 wt% metal loading)
were added to 1 L of distilled water and placed in a 1.5-L
cylindrical photoreactor equipped with a medium pressure
Hg lamp (500 W). The suspension was bubbled by helium
atmospheric pressure for 30 min before irradiation. The
suspensions were irradiated for 6 h (Bellardita et al. 2017).
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0.46; HRMS (ESI) m/z calcd. for C10H20NOSi [M + H]+
198.1314, found 198.1311.
General procedure for the synthesis
of propargylamines
1‑Methyl‑4‑(3‑(trimethylsilyl)prop‑2‑yn‑1‑yl)piperazine (4d)
A mixture of a terminal alkyne (2.0 mmol), paraformalde-
hyde (180.2 mg, 2.0 mmol), amine (2 mmol) and Pt/TiO2
(5.0 mg, 0.25 mol%) loaded in distilled H2O (3 mL) and
heated under microwave condition at 140 °C and 250 W
for appropriate reaction time (the reaction was monitored
by TLC). After the completion of the reaction, the mixture
was allowed to cool down and then extracted with EtOAc
(3 × 20 mL). The combined organic extracts were dried
(MgSO4) and fltered. The solvent was evaporated under
reduced pressure and the resulting crude product was puri-
fed by column chromatography (silica gel) to obtain the
corresponding products 4a–4 l.
1
(Kashid and Balakrishna 2018): Yellow oil, yield 90% H
NMR (300 MHz, CDCl3): δ (ppm) 3.28 (s, 2H), 2.58–2.56
(m, 8H), 2.31 (s, 3H), 0.18 (s, 9H); 13C NMR (75 MHz,
CDCl3): δ (ppm) 107.9, 85.2, 59.4, 46.4, 36.3, 35.0, 0.45;
HRMS (ESI) m/z calcd. for C11H23N2Si [M+H]+ 211.1631,
found 211.1628.
1‑(3‑(Trimethylsilyl)prop‑2‑yn‑1‑yl)pyrrolidine (4e)
1
Colorless oil, yield 95%; H NMR (300 MHz, CDCl3): δ
(ppm) 3.23 (s, 2H), 2.80–2.62 (m, 4H), 1.95-1.76 (m, 4H),
0.13 (s, 9H); 13C NMR (75 MHz, CDCl3): δ (ppm) 86.3,
84.2, 52.9, 43.6, 23.7, 0.42; HRMS (ESI) m/z calcd. for
C10H20NSi [M+H]+ 182.1365, found 182.1358.
Catalyst recovery procedure
After completion of the reaction, Pt/TiO2 nanocatalyst was
collected by fltration and then washed with water (30 mL)
and CHCl3 (30 mL). The recovered catalyst was reused in
another A3 coupling reaction.
N‑benzyl‑N‑methyl‑3‑(trimethylsilyl)prop‑2‑yn‑1‑amine
(4f)
1
Colorless oil, yield 87%; H NMR (300 MHz, CDCl3): δ
(ppm) 7.27–7.22 (m, 2H), 7.20–7.18 (m, 3H), 3.62 (s, 2H),
3.53 (s, 2H), 2.35 (s, 3H), 0.10 (s, 9H); 13C NMR (100 MHz,
CDCl3, 25 °C) δ 141.2, 128.9, 128.2, 126.6, 86.9, 83.6, 60.6,
45.0, 41.9, 0.47; HRMS (ESI) m/z calcd. for C14H22NSi
[M+H]+ 232.1522, found 232.1518.
Characterization data for the synthesized
propargylamines 4a–4 l
N,N‑dimethyl‑3‑(trimethylsilyl)prop‑2‑yn‑1‑amine (4a)
(Bieber and Da Silva 2004): Colorless oil; yield 98%; 1H
NMR (CDCl3, 300 MHz): δ (ppm) 3.48 (s, 2H), 2.36 (s,
6H), 0.14 (s, 9H); 13C NMR (CDCl3, 75 MHz): δ (ppm)
85.2, 82.6, 56.1, 44.3, 0.38; HRMS (ESI) m/z calcd. for
C8H18NSi [M + H]+ 156.1209, found 156.1216.
N,N‑dimethyl‑3‑phenylprop‑2‑yn‑1‑amine (4 g)
(Xu et al. 2011): Colorless oil, yield 93%; 1H NMR (CDCl3,
300 MHz): δ (ppm) 7.42–7.44 (m, 2H), 7.27–7.31 (m, 3H),
3.48 (s, 2H), 2.35 (s, 6H); 13C NMR (CDCl3, 75 MHz): δ
(ppm) 130.9, 128.6, 128.2, 122.7, 88.1, 83.6, 50.1, 44.3;
HRMS (ESI) m/z calcd. for C11H14N [M + H]+ 160.1126,
found 160.1121.
1‑(3‑(Trimethylsilyl)prop‑2‑yn‑1‑yl)piperidine (4b)
1
(Aguilar et al. 2010) Colorless oil, yield 91%; H NMR
1‑(3‑Phenylprop‑2‑yn‑1‑yl)piperidine (4 h)
(300 MHz, CDCl3): δ (ppm) 3.25 (s, 2H), 2.61–2.57 (m,
4H), 1.67–1.61 (m, 4H), 1.46–1.43 (m, 2H), 0.15 (s,
9H); 13C NMR (75 MHz, CDCl3): δ (ppm) 84.6, 82.2,
56.3, 49.7, 25.9, 24.6, 0.41. HRMS (ESI) m/z calcd. for
C11H22NSi [M + H]+ 196.1522, found 196.1519.
(Gao et al. 2012): Yellow oil, yield 86%; 1H NMR (CDCl3,
300 MHz): δ (ppm) 7.43–7.40 (m, 2H), 7.31–7.28 (m,
3H),3.51 (s, 2H), 2.63–2.58 (m, 2H), 1.64 (dt, J = 10.9,
5.4 Hz, 4H), 1.47–1.44 (m, 4H); 13C NMR (CDCl3, 75 MHz)
δ 131.7, 128.3, 128.0, 123.2, 86.2, 83.1, 62.9, 59.4, 51.2,
26.3, 24.2; HRMS (ESI) m/z calcd. for C14H18N [M+H]+
200.1439, found 200.1431.
4‑(3‑(Trimethylsilyl)prop‑2‑yn‑1‑yl)morpholine (4c)
(Kashid and Balakrishna 2018): Colorless oil, yield 93%;
1H NMR (300 MHz, CDCl3): δ (ppm) 3.81–3.76 (m, 4H),
3.23 (s, 2H), 2.50–2.53 (m, 4H), 0.17 (s, 9H); 13C NMR
(75 MHz, CDCl3): δ (ppm) 85.7, 83.1, 59.8, 47.6, 36.3,
(4‑(Dimethylamino)but‑2‑yn‑1‑yl)isoindoline‑1,3‑dione (4i)
(Jeon et al. 2003): White solid, mp 95–97 °C, Lit. 94–96 °C.
yield 91%; 1H NMR (CDCl3, 300 MHz): δ (ppm) 7.82–7.85
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(m, 2H), 7.76–7.73 (m, 2H), 4.48 (t, J=1.9 Hz, 2H), 3.24 (t,
J=1.9 Hz, 2H), 2.36 (s, 6H); 13C NMR (CDCl3, 75 MHz):
δ (ppm) 166.4, 135.0, 132.6, 122.7, 85.4, 80.1, 49.8, 44.7,
28.2; HRMS (ESI) m/z calcd. for C14H15N2O2 [M + H]+
242.1134, found 242.1120.
2‑(4‑(Piperidin‑1‑yl)but‑2‑yn‑1‑yl)isoindoline‑1,3‑dione (4j)
(Al-Qaisi et al. 2014): White solid, mp 87–88 °C, Lit.
83–84 °C, yield 90%; 1H NMR (CDCl3, 300 MHz): δ (ppm)
7.86–7.82 (m, 2H), 7.75–7.71 (m, 2H), 4.47 (t, J=1.9 Hz,
2H), 3.21 (t, J=1.9 Hz, 2H), 2.51–2.47 (m, 4H), 1.59–1.42
(m, 4H), 1.02–0.75 (m, 2H); 13C NMR (CDCl3, 75 MHz):
δ (ppm) 167.4, 134.7, 132.1, 128.6, 121.8, 86.5, 83.3, 62.6,
59.2, 26.4, 24.6; HRMS (ESI) m/z calcd. for C17H19N2O2
[M+H]+ 283.1447, found 283.1438.
N,N‑dimethyl‑4‑phenoxybut‑2‑yn‑1‑amine (4 k) (Xu et al.
2011)
Fig. 2 XRD pattern of a bar TiO2, b TiO2 doped with Ag, c TiO2
doped with Pt
1
Colorless oil, yield 92%; H NMR (CDCl3, 300 MHz): δ
(ppm) 7.33–7.29 (m, 2H), 7.71–7.69 (m, 3H), 4.63 (s, 2H),
3.22 (s, 2H), 2.32 (s, 6H); 13C NMR (CDCl3, 75 MHz): δ
(ppm) 156.8, 129.1, 122.6, 114.7, 84.3, 82.0, 62.1, 49.0,
44.5; HRMS (ESI) m/z calcd. for C12H16NO [M + H]+
190.1232, found 190.1227.
62.69o (Shi et al. 2014). The crystals phases of TiO2 are
slightly efected by the addition of metal dopants.
The surface structure, particle morphology, and crystal-
lite sizes of the nanocatalysts were investigated by TEM and
EDX (Fig. 3).
1‑(4‑Phenoxybut‑2‑yn‑1‑yl)piperidine (4 l)
In both cases of the Ag- and Pt-doped nanocomposites,
the TiO2 particle sizes are quite uniform in a nearly spherical
morphology which is in agreement with previous investiga-
tion by Yi et al. (Yi et al. 2014) who reported that metal-
doped TiO2 could be formed in homogeneous morphologies
that may have impact in enhancement the catalytic efciency.
The representative TEM images had been performed for Ag/
TiO2 and Pt/TiO2 to show that Ag and Pt are doped into the
TiO2 framework (Fig. 3a, b). It was showed that the crystal
sizes of Ag- and Pt-doped TiO2 were ranging 16–25, 25–29
and 6–8 nm (Fig. 3a, b). In addition, EDX analysis showed
the presence of (Ti, O and Ag) with (43.5 wt%, 21.42 wt%
and 1.95 wt%) (Fig. 3c) and (Ti, O and Pt) with (43.64 wt%,
21.47 wt% and 4.75 wt%) (Fig. 3d), respectively.
(Jokubaityte et al. 1980): Colorless oil, yield 86%; 1H NMR
(CDCl3, 300 MHz): δ (ppm) 7.78–7.76 (m, 3H), 7.56–7.51
(m, 2H), 4.48 (t, J=1.9 Hz, 2H), 3.25 (t, J=1.9 Hz, 2H),
2.57–2.54 (m, 4H), 1.57–1.54 (m, 4H), 1.45–1.42 (m, 2H);
13C NMR (CDCl3, 75 MHz): δ (ppm) 156.6, 129.5, 123.0,
113.6, 87.2, 84.3, 58.9, 49.3, 44.8; HRMS (ESI) m/z calcd.
for C15H20NO [M+H]+ 230.1545, found 230.1539.
Results and discussion
Characterization of catalysts
Loading of TiO2 samples by metal nanoparticles was carried
out by a photodeposition technique (Bellardita et al. 2017).
Platinum chloride and silver nitrate were used as precursors
for Pt and Ag, respectively. The X-ray difraction (XRD)
peaks that are corresponding to bare TiO2, Ag/TiO2 and Pt/
TiO2 photocatalysts are shown in (Fig. 2), confrmed on the
formation of the as prepared nanocomposites with a single
phase anatase structure without detecting any secondary
phase.
Catalytic activity
To demonstrate the catalytic efciency of TiO2, Ag/TiO2
and Pt/TiO2 nanocomposites as catalyst precursors for
the well-known aldehyde–amine–alkyne coupling reac-
tion (A3 coupling), a set of experiments were carried
out. Trimethylsilylacetylene (1a), paraformaldehyde (2)
and dimethylamine solution (40 wt% in water) (3a) were
selected as model substrates to obtain propargylamine 4a
(Scheme 1). The catalysts TiO2, Ag/TiO2 and Pt/TiO2,
respectively, were used in 0.1 mol % under thermal heating
The XRD spectra showed the main characteristic peaks of
anatase phase at 2θ=25.28o, 37.80o, 48.05o, 53.89o, 55.06o,
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Fig. 3 TEM images of a Ag/TiO2, b Pt/TiO2 and EDX pattern of c Ag/TiO2, d Pt/TiO2
Scheme 1 A model A3 coupling
reaction
using DMF, DMSO or THF, but no encouraging results
were obtained. However, by carrying out the reaction by
water as solvent under thermal condition at 100 °C, the
adduct 4a was obtained in 67 and 54% isolated yields,
respectively. Furthermore, it was found that using TiO2
under thermal or microwave condition, the reaction was
not performed and the starting materials were collected.
To achieve high catalytic performance of Ag/TiO2 and Pt/
TiO2 catalysts, microwave irradiation was used, yielding
the corresponding propargylamine 4a, after 19 and 24 min
with 82% and 75% isolated yields, respectively (Table 2).
From these results, it was observed that Pt/TiO2 has a
potential higher catalytic performance compared to Ag/
TiO2.
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Table 2 The catalytic performance of Pt/TiO2, Ag/TiO2 and TiO2
parallel catalytic reactions. Interestingly, it was demon-
strated that when the concentration of Ag/TiO2 or Pt/TiO2
nanocatalysts was increased, product 4a was obtained after
short time in high yield (Table 3).
towards A3 coupling reaction
Catalyst
Rx time (min)
Yield %
Thermal
MW condition Thermal
condition
MW condition
It was observed from the abovementioned results, a
further increase of Ag/TiO2 or Pt/TiO2 concentrations to
0.25 mol % leads to improvement in the reaction time and
yields. From the results, it was worth noting that the Pt/
TiO2 catalyst showed superior catalytic activity than Ag/
TiO2 because it is hypnotized that the Pt/TiO2 nanocatalyst
could accelerate the process of formation of alkynide ion
faster than Ag/TiO2. A possible mechanistic pathway has
been suggested (Scheme 2).
condition
Pt/TiO2
Ag/TiO2 5 h
TiO2 48 h
3 h
19 min
24 min
120 min
67
82
75
n.d.
54
n.d.a
an.d.=not detected, it means no reaction was performed
Table 3 The efect of catalyst loading
In this reaction, when Pt-supported TiO2 nanocompos-
ite was irradiated by microwave irradiation, the generated
electron could be transferred and trapped to the Pt nanopar-
ticles due to the lower Fermi energy level of the noble metal
(Schmickler and Santos 2010). The enriched electron den-
sity on Pt would favor the formation of iminium ion, while
the abstraction of hydrogen from alkyne to furnish alkynide
ion was performed over TiO2 (Panagiotopoulou et al. 2013).
The formed ions were reacted rapidly in fast step to form
propargylamine.
Catalyst
Catalyst loading
(mol %)
Rx time (min)
Yield %
Ag/TiO2
0.1
0.15
0.2
0.25
0.1
0.15
0.2
24
20
17
9
19
13
8
75
80
87
92
82
89
95
98
Pt/TiO2
0.25
6
Catalytic recyclability
Both catalysts Ag/TiO2 and Pt/TiO2 were separated and
recovered by washing with water and CHCl3. After drying,
0.25 mol % of each catalyst was loaded in the model reaction
(see scheme 1) and the catalyst recovery process was per-
formed up to four catalytic runs as shown in Figs. 4 and 5.
As seen above, it appears that the reaction in the presence
of Pt/TiO2 compared to Ag/TiO2 can be readily recycled
Efect of catalyst loading
It is worth to note that when Ag/TiO2 and Pt/TiO2 nanocom-
posites were used in 0.1 mol %, the product was obtained
in 82 and 75%, respectively, under microwave irradiation.
Therefore, to achieve maximum yield for the A3 coupling
reaction, the loading of both catalysts was increased in
Scheme 2 Proposed reaction
mechanism
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Fig. 4 Catalytic recyclability of
Ag/TiO2 nanocomposite
Fig. 5 Catalytic recyclability of
Pt/TiO2 nanocomposite
and reused four times without any observable decrease in
its activity which indicated on the reusability and stability
of Pt/TiO2 catalyst. To detect the crystal phase evolution
of the Pt/TiO2 nanocatalyst after the fourth catalytic cycle,
XRD of the recycled catalyst was determined. The XRD
pattern showed peaks at 2θ=25.26°, 37.85°, 48.07°, 53.88°,
55.06°, 62.66° (Fig. 6), which are assigned to Pt/TiO2 with
a single phase anatase. From this result, it was proved that
the surface of the catalyst had not altered upon further use
(see Fig. 2).
Evaluation of the catalytic activity of Pt/TiO2
nanocomposite in a series of A3 coupling reactions
A variety of silyl-protected propargylamines were prepared
Fig. 6 XRD pattern of the recycled Pt/TiO2 nanocatalyst
from, trimethylsilylacetylene (1a), paraformaldehyde (2) and
Scheme 3 Pt/TiO2 catalyzed
A3 coupling for the synthesis of
silyl-protected propargylamines
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Table 4 Experimental results
Amine (3b-f)
Product (4b-f)
Rx time
Yield %
5
91
8
7
93
90
6
7
95
87
amine 3b–f that depicted in scheme 2. Propargylamine prod-
ucts 4b–f were obtained in 87–95% yield (Scheme 3, Table 4).
That provides a new efcient synthetic route for production
of propargylamines. To achieve complete conversions of the
starting materials to the products, the reactions were monitored
by thin layer chromatography (TLC) analysis.
compatible for various types of substrates and it was demon-
strated that Pt-supported TiO2 nanoparticles is an efcient cat-
alyst for the synthesis of diferent propargylamine derivatives.
Conclusion
In another set of experiments, various A3 coupling reactions
were performed under the optimized condition (Scheme 4).
The results are summarized in Table 5. The reactions of phe-
nylacetylene (1b), N-phthalic anhydride–acetylene (1c) and
phenoxyacetylene (1d) with paraformaldehyde and with sec-
ondary amine; dimethylamine solution (3a) or piperidine (3b)
were carried out under microwave irradiation within 6–9 min
to furnish the corresponding propargylamine 4 g–4 l in high
yields (86–93%) (Table 5).
In this report, we examined the catalytic performance of
Ag/TiO2 and Pt/TiO2 under microwave irradiation in func-
tionalized propargylamine. It was demonstrated that Pt/
TiO2 catalyst had higher catalytic activity than Ag/TiO2.
An efficient route for the synthesis of propargylamine
derivatives had been investigated with short reaction time,
high yield and performing the reaction in water as solvent.
In addition, the catalyst was easy to recover and reuse in
another reaction cycle under the same reaction condition.
The investigated procedure can open the access for design-
ing multifunctional complex molecules via a green route
due to its simplicity.
From the aforementioned results, it was observed that dif-
ferent terminal alkynes and secondary amines were selected to
be employed in the catalytic A3 coupling reactions catalyzed
by Pt/TiO2 nanocomposite which indicate that the catalyst is
Scheme 4 Pt/TiO2 catalyzed
A3 coupling for the synthesis of
diverse functionalized propar-
gylamines
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Chemical Papers
Table 5 Experimental results
Alkyne
Amine
Product
Rx time
Yield %
8
93
7
7
86
91
9
90
92
86
6
8
Acknowledgements The authors are grateful to the National Research
Center (Egypt) and NORD University (Norway) for providing the
facilities.
TiO2 catalysts prepared by low frequency ultrasound treatment.
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Compliance with ethical standards
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Conflict of interest The authors declare no confict of interest.
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