Highly efficient amide synthesis from alcohols and amines by virtue of a water-soluble gold/DNA catalyst

Article abstract of DOI:10.1002/anie.201102374

Gold takes to water: The synthesis of amides directly from alcohols and amines was realized by using a water-soluble Au/DNA nanohybrid as the catalyst. The interactions between the gold nanoparticles, DNA, and water lead to high catalytic efficiency under mild reaction conditions. The wide substrate scope includes less-basic aromatic amines, and this catalyst is recyclable.

Full text of DOI:10.1002/anie.201102374

DOI: 10.1002/anie.201102374
Heterogeneous Catalysis
Highly Efficient Amide Synthesis from Alcohols and Amines by Virtue
of a Water-Soluble Gold/DNA Catalyst**
Ye Wang, Dapeng Zhu, Lin Tang, Sujing Wang,* and Zhiyong Wang*
[
14]
Amides represent an important class of compounds and are
ubiquitous in the pharmaceutical industry, materials science,
geneous catalysts for a long time. DNA as a new template
for heterogeneous catalysts was investigated in our group
[
1]
[14e]
and chemical biology. Generally, the traditional syntheses of
amides are limited by toxicity issues and the harsh conditions
recently.
Herein, we report a water-soluble gold catalyst
immobilized on DNA (Au/DNA nanohybrid) and its appli-
cation in amide formation from various alcohols and amines
under mild reaction conditions. The high efficiency of the
reported catalyst was demonstrated by using less-basic
aromatic amines as substrates. The reaction time was
shortened to 12 hours and the reaction temperature was
decreased to 508C. More importantly, this Au/DNA catalyst
can be recycled.
[
2]
employed. Alternative methods, such as the Beckmann
[
3a,b]
[3c,d]
rearrangement,
tion of aryl halides,
of the terminal alkynes,
Staudinger reaction,
aminocarbonyla-
[3e–h]
[3i]
hydration of the nitriles, oxidation
[
3j,k]
and ligation of a-halo nitroalkanes
[
3l]
with amines have emerged to improve the preparation of
amides. Recently, amidation of aldehydes with amines under
the catalysis of metal complexes was reported. Considering
the stability and the availability of alcohols, chemists are now
focusing on the direct conversion of alcohols and amines into
amides, which is more atom economical and environmentally
[
4]
First of all, Au/DNA, Pd/DNA, Pt/DNA, and Ag/DNA
nanohybrids were prepared as reported previously, wherein
an inexpensive natural fish sperm DNA was used as the
[5–7]
[14e]
benign.
However, one limitation of these direct amidations
template and metal salts were used as precursors.
The
is the low yields of aromatic amine substrates. Also, the
requirements of special handling and high temperatures
hinder its application as one of the most straightforward
protocols for amide synthesis. Therefore the development of a
more efficient and feasible catalyst for such direct amidation
from alcohols and amines is highly desirable.
metal nanoparticles were chelated by DNA and the metal/
DNA complex was stable in air with good reversible solubility
in water and ethanol. Similarly, several water-soluble gold
nanoparticles supported on different templates, such as Au/
PVA, Au/PVP, Au/starch, and Au/gum arabic, were synthe-
sized. Gold catalysts supported on metal oxide were also
prepared. All of these gold catalysts were characterized by
transmission electron microscopy (TEM) and the average
particle diameters were 3–14 nm (3–5 nm for most of them).
The reaction of benzyl alcohol with aniline was then used as a
model reaction to optimize the catalysts. Generally, the water-
insoluble gold catalysts supported on metal oxide could
catalyze this amidation but the reaction yields were unsat-
isfactory (Table 1, entries 1–5). This can be ascribed to the
poor dispersion of the gold catalysts in water. As for the
water-soluble catalysts, the catalytic activity perhaps
depended on both the stability of the templates and the
binding between the templates and gold nanoparticles. For
instance, the low catalytic activity of Au/PVA, Au/Starch, and
Au/gum arabic could originate from the poor stability of these
templates under the reaction conditions (Table 1, entries 6–
8). C NMR spectra (see Figures S3–S5 in the Supporting
Information) showed that these polyalcohol templates were
oxidized under the reaction conditions. As a result, the gold
nanoparticles on the templates aggregated and lost their
catalytic activity. For another water-soluble Au/PVP, the low
catalytic activity (Table 1, entries 9) perhaps resulted from the
stronger binding (see Figure S9 in the Supporting Informa-
tion) between PVP and the gold nanoparticles although this
template was resistant to oxidation in the reaction. We
assumed that too strong of an interaction probably hindered
the osculation of the reaction substrate with the gold nano-
particles, thereby resulting in a lower catalytic activity of the
gold catalyst. When Au/DNA was employed as the catalyst,
the highest yield was achieved (Table 1, entry 10) and the
In contrast, heterogeneous catalysts have received more
and more attention because of the advantages of high
catalytic efficiency and easy recycling, which are important
[8]
for precious metal catalysts and flow chemistry processes.
Metal nanoparticles supported on different substrates have
been widely used as heterogeneous catalysts in recent
[9,10]
years.
However, little progress has been made on the
direct amidation from alcohols and amines by using sup-
ported heterogeneous catalysts. Shimizu et al. reported a
direct amidation by using a heterogeneous Ag/Al O nano-
2
3
[
11]
cluster. A few heterogeneous gold catalysts were employed
[
12a]
[12b,c]
in the amidation,
acylation,
and some homogeneous and heterogeneous gold
catalysts were used in the oxidative esterification directly
and the formylation of
[
12d,e]
amines
[
13]
from alcohols. Moreover, the catalytic efficiency and the
scope of the substrates for the direct amidation still need
further improvement. We have been focusing on the hetero-
1
3
[
*] Y. Wang, D. Zhu, L. Tang, Dr. S. Wang, Prof. Z. Wang
Hefei National Laboratory for Physical Sciences at the Microscale
CAS Key Laboratory of Soft Matter Chemistry and Department of
Chemistry, University of Science and Technology of China
Hefei, 230026 (China)
E-mail: zwang3@ustc.edu.cn
[
**] The authors are grateful to the National Natural Science Foundation
of China (90813008, 20972144, 20932002 and 21002096) and the
Graduate Innovation Fund of USTC.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102374.
Angew. Chem. Int. Ed. 2011, 50, 8917 –8921
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8917

Communications
Table 1: The amidation of benzyl alcohol with aniline catalyzed by
various catalysts.
Table 2: The amidation of aniline with different alcohols under the
catalysis of Au/DNA.
[
a]
[a]
1
[b]
Entry
R
Product
Yield [%]
[
b]
Entry
Catalyst
Yield [%]
1
2
3
4
5
6
7
8
Ph
4-MeC H
4-OMeC H
4-ClC H
4-BrC H
4-FC H
4
4-NO C H
4
4-CF C H
4
3-MeC H
6 4
2-MeC H
2-ClC H
2-BrC H
1-naphthyl
2-py
2-furyl
Me
1-butyl
benzyl
cyclohexyl
3aa
3ba
3ca
3da
3ea
3 fa
3ga
3ha
3ia
3ja
3ka
3la
3ma
3na
3oa
3pa
3qa
3ra
3sa
91
95
85
94
83
91
96
90
89
60
66
52
58
96
68
86
63
51
92
1
2
3
4
5
Au/TiO (3.08 nm)
26
30
36
39
12
11
10
41
23
61 (55 )
trace
trace
n.d.
2
6
4
Au/SiO (3.36 nm)
2
6
4
Au/CeO (3.39 nm)
2
6
4
Au/Al O (3.94 nm)
2
3
6
4
Au/NiO (4.80 nm)
Au/PVA (14.40 nm)
Au/starch (10.97 nm)
Au/gum arabic(5.26 nm)
Au/PVP (4.63 nm)
Au/DNA (4.03 nm)
Pd/DNA
Pt/DNA
Ag/DNA
KAuCl4
[
[
[
c]
d]
c]
6
6
7
8
9
1
1
1
1
1
2
6
3
6
9
[c]
1
1
1
1
1
1
16
17
18
19
0
1
2
3
4
5
[
e]
6
4
0
1
2
3
4
[c]
[c]
6
4
6
4
n.d.
[
d]
e]
[a] Reaction conditions: 1a (0.50 mmol) and 2a (0.25 mmol) in 2 mL
H O. The values in the parentheses following the catalyst indicate the
average diameter of the particle. [b] Yield determined by GC methods
2
[
using 1,3,5-trimethylbenzene as an internal standard. [c] Nanoparticles
were aggregated and the homogeneous solution was partly destroyed
after the reaction. [d] Nanoparticles were completely oxidized to gold
ions after the reaction. [e] Average yield of isolated product from using
three different batches of Au/DNA. n.d.=not detected, PVA=polyvinyl
alcohol, PVP=polyvinyl pyrrolidone.
[a] Reaction conditions: 1 (0.50 mmol) and 2 (0.25 mmol) in 1 mL H O.
2
[b] Yield of isolated product. [c] Au/DNA (Au loading=7.6 mol%) was
used. [d] EtOH (1p, 2.50 mmol) was used. [e] Reaction temperature was
08C. py=pyridyl.
8
catalyst remained stable in the reaction. IR and NMR tests
aromatic and aliphatic alcohols were good substrates. The
electron-withdrawing group on the aromatic ring favored this
amidation slightly (Table 2, entries 4, 6, 7, 8, and 14) whereas
steric bulk had a negative influence on this reaction (Table 2,
entries 10–12). The aromatic rings of the reaction substrates
could also be naphthyl, pyridyl, and furyl rings (Table 2, 13–
15). Generally, the aliphatic alcohols were successfully
employed in this reaction (Table 2, entries 16–19). For 2-
phenylethanol, which was more difficult to oxidize, a higher
temperature was needed (Table 2, entry 18). Importantly,
acetylation of aniline could be realized smoothly by using
EtOH as an acetyl source (Table 2, entry 16).
(
see Figures S6–S8 in the Supporting Information) showed
that the DNA template scarcely changed before and after the
amidation, and XPS showed that the interaction between the
DNA and gold nanoparticles was weaker than that between
the PVP and the gold nanoparticles (see Figure S9 in the
Supporting Information). These results implied that both the
water solubility of the catalysts and suitable interactions
between gold nanoparticles and the templates had an
important influence on the catalytic activity in this amidation.
Other metal/DNA nanohybrids and gold salts did not
effectively catalyze this amidation (Table 1, entries 11–14).
After catalyst screening, Au/DNA proved to be the best
catalyst for this direct amidation; the catalyst was water-
soluble and stable under the reaction conditions. Moreover, in
parallel experiments using different batches of Au/DNA,
good reproducibility of the yield for the isolated product was
achieved, thus demonstrating the stability of the composition
and the catalytic activity of the Au/DNA nanohybrid.
After this alcohol screening, the scope of the amine
substrates was examined when using benzyl alcohol. The
corresponding results are listed in Table 3. Previously, the
aromatic amine substrates gave the corresponding amides
[
5c,7a,b,e]
with a low yield.
Under the catalysis of Au/DNA,
various aniline derivatives gave the corresponding products
with good yields. The yields for substrates having aromatic
rings with 4-methoxy and 4-nitro substituents were lower
because of the low water solubility of the amines. Some
tBuOH was added in these two examples to increase the
solubility and enhance the reaction yields (Table 3, entries 2
and 4). As for aliphatic amines, it was necessary to raise the
reaction temperature to 808C (Table 3, entries 7 and 8) to
improve the yields. It was found that the scope could be
extended from primary to secondary amines (Table 3,
entries 9 and 11). However, it was difficult to obtain the
amide product for the secondary amines having steric bulk.
After the catalyst optimization, the reaction conditions
were optimized (see Table S1 in the Supporting Information).
After detailed optimization, the standard reaction conditions
were determined to be: 3.8 mol% of Au/DNA, 1.1 equiv-
alents of LiOH·H O, 1 mL of H O (solvent), reaction temper-
2
2
ature at 508C under an oxygen atmosphere for 12 hours. A
high yield (91%) of the isolated product for the model
reaction could be obtained under these standard reaction
conditions (Table 2, entry 1). The scope of the alcohol
substrates was then investigated. As shown in Table 2, both
8918
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 8917 –8921

Table 3: The amidation of benzyl alcohol with different amines under the
catalysis of Au/DNA.
atmosphere and high pH value were both essential for this
amidation. The imine was found to be the main by-product in
the reaction. Moreover, the reaction of aniline with benzal-
dehyde can be carried out smoothly to give the amide 3aa
with a good yield (79%) whereas the reaction of aniline with
benzoic acid did not work (see Scheme S1 in the Supporting
Information). These results indicated that oxidation was the
key step and that the aldehyde should be the intermediate of
this transformation. In the control experiments, it was noted
that water was crucial for this direct conversion of amides.
The amidation did not occur and no amide was observed in
the absence of water. When a different amount of water was
added to this reaction mixture, the amide yield can be
enhanced (see Table S2 in the Supporting Information).
When neat water was employed as the reaction solvent and
Au/DNA was used, the highest reaction yield was obtained
(see Table S3 in the Supporting Information). In terms of
[
a]
[
b]
Entry
1
2
R
Product
Yield [%]
2
3
R
4-MeC H
4-OMeC H4
4-ClC H4
4-NO C H
4
3-MeC H
2-MeC H
benzyl
1-butyl
Me
H
H
H
H
H
H
H
H
Me
Me
–
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
91
76
85
51
97
90
80
47
83
6
4
[
c]
2
3
4
5
6
7
8
9
1
1
1
6
6
[
c]
2
6
6
4
6
4
[
[
d]
d]
3aj
3ak
3al
[
d]
[e]
0
1
2
Ph
piperidine
NH ·H O
-
[5c,11–13,15,16]
91
50
these experimental results and the previous reports,
a proposed mechanism is described.
[
f]
–
3am
3
2
First the alcohol 1 is oxidized into aldehyde 4 under the
catalysis of Au/DNA (Scheme 1). Then the amine 2 attacks
aldehyde 4 to generate the hemiaminal 5, which can then be
converted into either an imine (6) by losing one water
molecule or an amide (3) by additional oxidation. Previous
works have reported that the imine 6 was the main product
when water-insoluble gold catalysts were employed under
[
[
a] Reaction conditions: 1 (0.50 mmol) and 2 (0.25 mmol) in 1 mL H O.
b] Yield of isolated product. [c] tBuOH (0.375 mol) was added to
2
improve the solubility of amine. [d] Reaction temperature was 808C.
[
2
e] Yield determined by GC methods to be <5%. [f] NH ·H O (2m,
3 2
.50 mmol) was used.
[10k,l]
For example, only trace amounts of product could be detected
anhydrous reaction conditions.
In this work, because of
by GC/MS when N-methylaniline was used as a substrate
the presence of water, the high pH value, and the Au/DNA
À
(
Table 3, entry 10). Notably, ammonia could also be
catalyst, OH and the hydroxides adsorbed at the gold/water
[
16]
employed as the reaction substrate to deliver benzamide
with a moderate yield of 50% (Table 3, entry 12).
interface could be generated.
This may promote the
oxidation of 5 to form the desired product 3. In contrast,
the presence of water can perhaps affect the dehydration
equilibrium towards 5 and thus most of 5 undergoes oxidation
to form 3 in the presence of the Au/DNA catalyst.
By taking advantage of the reversible solubility of Au/
DNA in water and ethanol, the catalyst could be easily
recovered by a simple phase separation. Table 4 details the
experimental results for the recyclability results of the
catalyst. From Table 4, the catalytic activity of this heteroge-
neous catalyst was almost the same after the first round. After
the fifth round, the catalyst still had a good catalytic activity
and catalyzed the reaction to give the corresponding amide
with a yield of 80%. The TEM image of Au/DNA after the
fifth round (see Figure S1k in the Supporting Information)
indicated that the gold nanoparticles were larger with an
average diameter of 9.70 nm, and a little aggregation was also
found.
The reaction mechanism was investigated in our labora-
tory. Under the standard reaction conditions, the oxygen
Scheme 1. Proposed mechanism of amidation from alcohols and
amines catalyzed by Au/DNA.
Table 4: The recycling of Au/DNA in the amidation of benzyl alcohol with
aniline.
In summary, a water-soluble gold catalyst was prepared
and employed in the direct amidation from alcohols and
amines. The unique stability, solubility, and the suitable
interaction between DNA and gold nanoparticles of this Au/
DNA nanohybrid led to both the high reaction yields and the
ability to recycle the catalyst. The reactions can be carried out
smoothly under mild reaction conditions in neat water and the
scope of the reaction substrates was extended to aromatic
amines to afford the amides with good to excellent yields. To
the best of our knowledge this is the first example of aromatic
alcohols reacting with aromatic amines to obtain the desired
[
a]
[
c]
Run
1
2
3
4
5
[
b]
Yield [%]
91
89
85
82
80
[a] Reaction conditions: 1a (0.50 mmol), 2a (0.25 mmol), initial Au
loading=3.8 mol% in 1 mL H O, 12 h. [b] Yield of isolated product.
2
[c] 18 h.
Angew. Chem. Int. Ed. 2011, 50, 8917 –8921
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8919

Communications
amides. The detailed structure of the Au/DNA nanohybrids in
water and other applications in organic reactions are cur-
rently under investigation in our laboratory.
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3+
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1
–2 h and then centrifuged at 5000 rmin for 5 min, the decantate
7
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2
times. The reaction mixture was then stirred under an O balloon at
2
5
08C for 12 h. After the reaction was finished, 3 times the volume of
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mixture. This mixture was left undisturbed for 2 h to allow precip-
À1
itation and then centrifuged at 5000 rmin for 5 min. The decantate
was poured off. The solid residue was dried and used as the catalyst
for the next round. The decantate was evaporated with a rotary
evaporator. The obtained residue was purified by column chroma-
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