NaF regulated aqueous phase synthesis of aromatic amides and imines catalyzed by Au/HT

Article abstract of DOI:10.1039/c4cy00210e

An Au/HT catalyst was found to be an efficient heterogeneous catalyst for the coupling reaction of aromatic alcohols and amines. The amides and imines can be selectively synthesized with up to 99% yield with or without the addition of NaF as the cocatalyst. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cy00210e

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NaF regulated aqueous phase synthesis
of aromatic amides and imines catalyzed
by Au/HT†
Cite this: Catal. Sci. Technol., 2014,
4, 1710
Received 28th February 2014,
Accepted 14th March 2014
a
a
Qianqian Wang,ab Youquan Deng and Feng Shi
*
DOI: 10.1039/c4cy00210e
www.rsc.org/catalysis
An Au/HT catalyst was found to be an efficient heterogeneous
catalyst for the coupling reaction of aromatic alcohols and amines.
The amides and imines can be selectively synthesized with up to
99% yield with or without the addition of NaF as the cocatalyst.
and non-reusability.16 A more efficient and feasible heteroge-
neous catalyst for the coupling reaction of alcohols and amines
is highly desirable.18 In 2009, Satsuma and co-workers reported
a heterogeneous Ag/Al2O3 catalyst, which can catalyze the
amide synthesis from secondary amines and alcohols with a
good yield, and the catalyst can be recycled.17 Since then,
Au/DNA,19 PICB-Au,20 OMS-2,21 Au/PVP,22 Au/HT,23 Au–Pd/resin24
and Au/HAP25 have been reported. Although there are gratify-
ing achievements in the direct coupling of amines and alcohols
catalyzed by heterogeneous catalysts, one limitation is the low
yield when aromatic amines and aromatic alcohols are used as
starting materials. In the existing literature reports, normally a
high alcohol/amine ratio is needed for the coupling reactions
of alcohols and amines to form substituted amides in heteroge-
neous catalyst systems.
Hydrotalcite, especially Mg–Al hydrotalcite, is widely used
as a catalyst in different reactions.26,27 In addition hydrotalcite
supported nanometal catalysts have gradually played an impor-
tant role in the field of catalysis, such as oxidation of alcohols
in the presence of oxygen.28,29 Here, we report an Au/HT cata-
lyst system, which was prepared by a precipitation–deposition–
reduction method, for the synthesis of alkyl amides and
imines. Importantly, alkyl amides and imines can be selectively
generated by tuning the catalyst systems with or without the
addition of NaF as the cocatalyst.
In this work, water was chosen as the reaction medium
because it is considered as a “green solvent” in organic chemis-
try, and it has attracted extensive attention in the last years.30,31
Additionally, since the first report on the insoluble Diels–Alder
reactions using water as a solvent,31 “on-water chemistry” has
also been investigated.32–34 In the current work, the synthesis
of amides or imines might be an in-water or on-water organic
reaction.
The structure and physicochemical properties of Au/HT-1
(Mg/Al = 1:1), Au/HT-2 (Mg/Al = 3:1) and Au/HT-3 (Mg:Al = 5:1)
catalysts were characterized by XRD, XPS, TEM, BET and
ICP-AES. From the BET analysis (Table S1†), Au/HT-2 exhibited
Nitrogen-containing compounds such as amides and imines
play a key role in organic and biological chemistry.1 Generally,
the alkyl amides are produced through the reaction of amides
and alkyl halides with the addition of stoichiometric amounts
of inorganic bases or by the reaction of a carboxylic acid or ester
and an amine.2–5 These methods often suffer from drawbacks
such as the generation of inorganic waste.6 For the production of
imines, the condensation of amines with aldehydes or ketones is
an effective way.7 However, this method is usually limited by the
use of dehydrating agents or apparatus. In the last years, the
direct synthesis of amides and imines via oxidative coupling of
alcohols with amines has developed rapidly due to its high atom
efficiency.5,8–12 This procedure involves the following steps. First,
the alcohol is converted into the corresponding aldehyde by
catalytic oxidation. Afterwards, the hemiaminal is generated
via the coupling of amine and aldehyde. Finally, the hemiaminal
is converted to amides or imines.
In 2007, Milstein reported the transformation of alcohols
and amines into amides and hydrogen catalyzed by a ruthenium
pincer complex.13 Subsequently, a series of Ru catalyst systems
were developed.14,15 On the other hand, homogeneous catalysts
show high activity for imine formation from alcohols and
amines, too.16,17 Although homogeneous catalysts are very effi-
cient for generating amides or imines selectively, there are dis-
advantages such as ligand dependence, complicated operation
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Center for Green
Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy
of Sciences, Lanzhou, 730000, China. E-mail: fshi@licp.cas.cn; Fax: +86 931 8277088;
Tel: +86 931 4968142
b Graduate School of the Chinese Academy of Sciences, Beijing, 100049, China
† Electronic supplementary information (ESI) available: Catalyst preparation
procedures and characterization details. See DOI: 10.1039/c4cy00210e
1710 | Catal. Sci. Technol., 2014, 4, 1710–1715
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the largest BET area accompanied by the smallest pore size and
pore volume compared with Au/HT-1 and Au/HT-3. In addition,
an ordered mesoporous structure was formed in Au/HT-2, which
may have contributed to the high surface area and is beneficial
for the preparation of the active catalyst. According to the XRD
diffraction patterns (Fig. 1), the typical diffraction peaks at 11.7,
23.5, 34.7, and 46.4, which are related to the (003), (006), (009)
and (018) reflections of hydrotalcite, were observable. Another
featured peak at 38.4 is related to the crystal lattice of Au (111).
Noteworthy, the Au (111) diffraction peak in sample Au/HT-2
was much weaker, which suggests that the gold species in
Au/HT-2 are less crystallized. X-ray photoelectron spectroscopy
(XPS) was used to clarify the chemical state of the gold species
formed on the catalyst surface (Fig. 2). It can be seen that the
binding energies of Au4f7/2 for Au/HT-1, Au/HT-2 and Au/HT-3
are 83.6, 83.3 and 83.6 eV, respectively. Another peak is attrib-
uted to Mg2s and the binding energies are 89.2, 88.9 and
88.8 eV. Therefore, metallic gold formed on the catalyst sur-
face. TEM images showed that the structure of the catalysts
comprises nano-Au particles anchored on the surface of
hydrotalcite (Fig. 3). It can be seen that all of the supported
nano-Au particles are evenly distributed, and the average par-
ticle sizes are 7.7, 20.1 and 14.0 nm. So the properties of the
supports influence the structure of the final catalyst samples
remarkably. ICP-AES analysis showed that the loadings of
gold in these catalysts are 3.4 wt%, 3.7 wt% and 3.4 wt%.
Fig. 2 XPS spectra of the prepared catalysts.
Next, we chose the amidation of benzylamine and benzyl
alcohol as the model reaction to optimize the reaction condi-
tions (alcohol : amine = 1.5, mol/mol). As given in (Table 1),
N-phenylbenzamide is not obtained when using Au/HT-2 as
the catalyst and the major product is N-benzylideneaniline
(entry 1). This result is similar to a previous report showing
that alcohol imination can be realized with Au/Mg2Al–HT as
the catalyst and with molecular oxygen as the oxidant.35 It
has been shown that amides can be synthesized efficiently with
the addition of Cs2CO3.17 However, our reaction was not signif-
icantly improved and the selectivity to amide product was only
37% (entry 2). Other bases such as NaOH and Na2CO3 were also
tested and the selectivity increased to 96% and 84% but the con-
versions of aniline were not high enough (entries 3, 4). Interest-
ingly, the conversion and selectivity increased remarkably when
Fig. 3 TEM pictures of the prepared catalysts Au/HT-1 (a, b), Au/HT-2
(c, d) and Au/HT-3 (e, f).
NaF was used as the additive and 95% aniline conversion was
obtained (entry 5). With the addition of KF and NaBF4 as cocat-
alysts, the amidation of aniline with benzyl alcohol can be car-
ried out as well (entries 6, 7). The coupling reaction did not
occur when NaCl was used as an additive which indicates that
the reaction is promoted by fluoride (entry 8). When the ratios
of Mg to Al of the catalyst support were changed to 1 : 1 and
5 : 1, the conversions and selectivities of the amidation
Fig. 1 XRD diffraction patterns of the prepared catalysts.
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Table 1 Synthesis of amides from aniline and benzyl alcohola
Table 2 Synthesis of amides from alcohols and aminesa
Entry Cat.
Additive Con. % of 1b Sel. % of 3b Sel.% of 4b
Entry
1
Amine
Alcohol
Yield of 3b (%)
86
1
Au/HT-2
Au/HT-2 Cs2CO3
Au/HT-2 NaOH
—
80
79
45
16
36
96
84
98
95
86
3
84
74
4
6
2
2c
3
2
3
70
72
4
5
6
Au/HT-2 Na2CO3 87
Au/HT-2 NaF 95
Au/HT-2 KF·2H2O 90
5
7
8
9
Au/HT-2 NaBF4
Au/HT-2 NaCl
Au/HT-1 NaF
Au/HT-3 NaF
Au/HT-2 NaF
83
9
14
97
18
34
50
4
5
46
62
69
76
95
82
66
50
10
11d
a
Aniline (0.5 mmol), benzyl alcohol (0.75 mmol), Au/HT (50 mg),
6
7
60
81
b
additive (2 mmol), 2 mL H2O, O2, 40 °C, 12 h. The conversion and
selectivity were detected by GC-FID according to the peak area.
c
Cs2CO3: 20 mol%. d The catalyst was reused in the second run.
8
9
73
77
reaction decreased remarkably (entries 9, 10). Therefore, 3 : 1
is a suitable Mg/Al ratio. In order to explore the catalytic
behavior of leached catalyst species, the catalyst was removed
by filtration after it reacted for 12 h and the aniline conver-
sion was maintained at 77% after reaction for another 12 h.
Therefore, the real catalyst is the supported nano-Au. Addi-
tionally, only 50% amide was formed when the catalyst was
reused for the second run. ICP-AES analysis showed that the
Au loading was 3.1 wt% after use. Possibly the catalyst struc-
ture variation but not Au leaching is responsible for the
deactivation.
Then, the generality of the amidation reaction of amines
and alcohols was investigated under the optimized reaction
conditions and the results are summarized in Table 2. First,
N-phenylbenzamide was synthesized with 86% yield (entry 1).
Then, we investigated the scope of the amidation of benzyl
alcohol and anilines containing different functional groups.
Clearly, the catalyst can catalyze the amidation reactions of var-
ious aromatic amines, which contain electron-donating as well
as electron-withdrawing substituents, to synthesize the corre-
sponding amides with moderate to good isolated yields (46%–
81%, entries 2–7). However, a lower yield was obtained when
o-toluidine was used as the starting material, which might be
caused by steric hindrance. Upon using benzyl alcohol deriva-
tives as starting materials, the catalyst showed good catalytic
activity with 47–78% isolated yields of the desired products
(entries 8–16). Further, alcohols with electron-donating sub-
stituents such as methyl, methoxyl and isopropyl were more
favorable for the amidation reactions than those with
electron-withdrawing substituents such as chlorine, bromine,
fluorine and trifluoromethyl. Similarly, the catalysts exhibited
lower activity in the amidation of o-methylbenzyl alcohol
compared with m-methylbenzyl alcohol and p-methylbenzyl
alcohol (entries 8–10). 4-(Trifluoromethyl)benzyl alcohol
and 3,4-(methylenedioxy)aniline were converted into the
10
11
48
78
12
13
14
15
77
63
68
73
16
50
47
17
a
Reaction conditions: amine (0.5 mmol), alcohol (0.75 mmol), Au/HT-2
(50 mg), NaF (2 mmol), H2O (2 mL), O2, 40 °C, t = 24 h. b Isolated yields.
corresponding amides with 50% and 47% isolated yields
(entries 16 and 17).
During the optimization of the reaction conditions for the
amidation reaction, we found that benzyl alcohol and aniline
can be converted into N-benzylideneaniline in the absence
of NaF (Table 1, entry 1). It inspired us that our catalyst might
be suitable for the imination of amines and alcohols without
the addition of NaF. As expected, the anilines with various sub-
stituents such as methyl, methoxyl, chlorine, and bromine were
converted into the corresponding imines with benzyl alcohol
(Table 3, entries 1–9). Moreover, the electron-donating substituents
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on the aromatic ring are slightly favorable for the imination
reaction compared with the electron-withdrawing substituents
while steric effects have a negative influence. 80% yield was
obtained when 2,5-dimethylaniline was used as the starting
material (entry 9). In addition, the reactions of anilines with
different aromatic alcohols progressed well with 82–97% yields
(entries 10–12).
In order to explore the reaction mechanism, several con-
trol reactions were used (Scheme 1). Full conversion of ani-
line was obtained with 24% amide and 72% imine under the
given reaction conditions using benzaldehyde as the starting
material catalyzed by Au/HT-2 and NaF, and no amidation
product was observed when NaF or gold was excluded. Simi-
larly, almost no reaction occurred when benzyl alcohol and
aniline were used as starting materials with HT and NaF as
catalysts. These results suggest that the co-presence of nano-
Au and fluoride is crucial for amide formation. Moreover, the
amidation reaction might happen promptly after the forma-
tion of hemiaminal on the surface of Au/HT in the presence
of fluoride. For the imination reaction, it can be carried out
in the absence of fluoride (Scheme 2).
Scheme 1 The mechanism investigation of alcohol amidation and
imination with alcohol.
Table 3 Synthesis of imines from alcohols and aminesa
Entry
1
Amine
Alcohol
Yield of 4b (%)
98
2
3
85
99
Scheme
2 The proposed reaction mechanism of amidation and
imination reactions catalyzed by Au/HT-2 with or without the addition
of NaF.
4
5
84
63
Conclusions
In summary, an active Au/HT catalyst was prepared by a pre-
cipitation–deposition–reduction method for the coupling
reaction of amines and alcohols. The amidation and
imination reactions can be realized efficiently with moderate
to good yields with or without the addition of NaF as the
co-catalyst. In addition, the mechanism of the reaction was
investigated, too.
6
7
94
71
8
70
9
80
82
87
97
Experimental section
General
10
11
12
All of the solvents and chemicals were obtained commercially
and were used as received. Mass spectra were, in general,
recorded using an HP 6890/5973 GC-MS. High-resolution
TEM analysis was carried out using a JEM 2010 operating at
200 KeV. The catalyst samples after pretreatment were dis-
persed in methanol, and the solution was mixed ultrasonically
a
Reaction conditions: amine (0.5 mmol), alcohol (0.75 mmol), Au/HT-2
b
(50 mg), H2O (2 mL), O2, 40 °C, t = 24 h. Yields were determined by
GC with biphenyl as the internal standard.
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at r.t. A part of solution was dropped on the grid for the mea-
surement of TEM images. XRD measurements were conducted
by a STADI P automated transmission diffractometer (STOE)
equipped with an incident beam curved germanium mono-
chromator selecting Cu Kα1 radiation and a 6° position sensi-
tive detector (PSD). The XRD patterns were scanned in the 2θ
range of 10–80°. For the data interpretation the software
WinXpow (STOE) and the database of Powder Diffraction File
(PDF) of the International Centre for Diffraction Data (ICDD)
were used. The XPS measurements were performed using a
VG ESCALAB 210 instrument provided with a dual Mg/Al anode
X-ray source, a hemispherical capacitor analyser and a 5 keV Ar+
ion gun. All spectra were recorded using non-monochromatic
Mg Ka (1253.6 eV) radiation. Nitrogen adsorption–desorption
isotherms were measured at 77 K using a Micromeritics 2010
instrument. The pore–size distribution was calculated using
the Barrett, Joyner and Halenda (BJH) method from desorption
isotherms. The Cu and Al contents of the catalyst were mea-
sured by inductively coupled plasma-atomic emission spec-
trometry (ICP-AES) using an Iris advantage Thermo Jarrel
Ash device.
was obtained by column chromatography (ethyl acetate/petro-
leum ether = 9/1, v/v; silica gel: 200–300 mesh; white solid;
m.p. 162–163 °C). For the reusability test, the catalyst was
separated by centrifugation, washed with ethanol and water,
and was used for the next run using the same procedure as
mentioned above.
General procedure for the imination of amines and alcohols
All of the reactions were carried out in a Schlenk tube. Typi-
cally, 0.5 mmol of aniline, 0.75 mmol of benzyl alcohol,
50 mg of Au/HT (3.7 wt% Au, 1.9 mol% to aniline) and
2 mmol of NaF were added successively. Then, the reaction
mixture was stirred (300 rpm) at 40 °C under 1 atm oxygen
(O2 balloon) for 24 h. Then it was cooled down to room tem-
perature and quantitatively analyzed by GC-FID (Agilent
7890A) using biphenyl as the standard material.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21303228).
Typical procedure for HT-2 preparation36
Notes and references
7.69 g (30 mmol) of Mg(NO3)2·6H2O and 3.75 g of (10 mmol)
Al(NO3)3·9H2O were dissolved into 100 mL of deionized
water. Then the solution was dropwise added into 100 mL of
Na2CO3 solution (0.3 mol L−1) after 1 h under magnetic stir-
ring and the pH value of the solution was maintained at 10
by adding aqueous solution of NaOH (1 M). After aging for
1 h at 65 °C, the white solid was filtered and washed with
deionized water (2 L). The obtained solid was dried at 100 °C
in air overnight and ~3.3 g of HT-2 was obtained. HT-1 and
HT-3 were prepared using a similar procedure.
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50 mg of Au/HT (3.7 wt% Au, 1.9 mol% to aniline) and
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