Aerobic oxidative coupling of alcohols and amines over Au-Pd/resin in water: Au/Pd molar ratios switch the reaction pathways to amides or imines

Article abstract of DOI:10.1039/c3gc41117f

A facile switch of the reaction pathways of aerobic oxidative coupling of alcohols and amines from amidation to imination was realized for the first time by tuning the Au/Pd ratios in ion-exchange resin supported Au-Pd alloy catalysts (Au-Pd/resin). Amides were obtained with high yields on Au₆Pd/resin while imines were obtained over AuPd₄/resin. Various alcohols and amines underwent oxidative coupling smoothly in water to afford the desired products with good to excellent yields. Further investigation on the reaction mechanism suggested the synergistic effect between Au and Pd determined the adsorption strength of the aldehyde intermediate, which in turn dictated the reaction pathways. That is, on Au-rich alloys (e.g., Au₆Pd) absorbed aldehyde species was formed, followed by further oxidation to yield amides, while on Pd-rich alloys (e.g., AuPd₄), free aldehyde was generated, which then underwent condensation with amines to produce imines. The discovery might provide avenues to develop new efficient catalysts for the green synthesis of special chemicals.

Full text of DOI:10.1039/c3gc41117f

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Aerobic Oxidative Coupling of Alcohols and Amines over Au-Pd/resin in
Water: Au/Pd Molar Ratios Switch the Reaction Pathways to Amides or
Imines
Leilei Zhang,a,b Wentao Wang,a Aiqin Wang,a Yitao Cui,c Xiaofeng Yang,a Yanqiang Huang,a Xiaoyan
5
Liu,a Wengang Liu,b Jin-Young Son,c Hiroshi Oji,c and Tao Zhang*,a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Facile switch of the reaction pathways of aerobic oxidative coupling of alcohols and amines from
amidation to imination was realized for the first time by tuning the Au/Pd ratios in ion-exchange resin
10 supported Au-Pd alloy catalysts (Au-Pd/resin). Amides were obtained with high yields on Au6Pd/resin
while imines over AuPd4/resin. Various alcohols and amines underwent oxidative coupling smoothly in
water to afford the desired products with good to excellent yields. Further investigation on the reaction
mechanism suggested the synergistic effect between Au and Pd determined the adsorption strength of
aldehyde intermediate, which in turn dictated the reaction pathways. That is, on Au-riched alloys (e.g.,
15 Au6Pd) absorbed aldehyde species was formed, and followed by further oxidation to yield amides while
on Pd-riched alloys (e.g., AuPd4) free aldehyde was generated, which then underwent condensation with
amines to produce imines. The discovery might provide avenue to develop new efficient catalysts for
green synthesis of special chemicals.
promoted the selective production of imine4d. The results
suggested that the selectivity of reaction catalyzed by gold could
be tuned by changing the second metal.5 However, the respective
mechanism of Co and Pd in this reaction is to be clarified yet,
50 which would be highly desirable for the rational design of new
catalysts. Moreover, some intrinsic defects may exist in PICB-
supported metal catalysts: (1) the PICB catalyst is not readily
available and involves a multi-step and expensive synthesis
procedure; and (2) possibly due to the weak interaction between
55 PICB and metal species, the leaching of metal species appeared
unavoidable in water, which necessitated the employment of
organic solvent.6 From a environmentally benign point of view,
developing efficient and durable catalyst that can be used for
organic transformations in aqueous media is highly desirable.
Introduction
20
Amides and imines represent two important classes of N-
containing intermediates and functional groups for organic
synthesis.1 Amides play a major role in the pharmaceutical
industry, biological system and material science1a-1c while imines
are widely applied as electrophiles in many organic
25 transformations such as cyclization, reduction, addition and
condensation1d,1e. Traditional synthesis of amides and imines
involves the reactions of carboxylic acid or derivatives such as
acid chlorides and anhydrides with amine, which suffers from
toxicity issues and harsh reaction conditions.2 Lately, new
30 methodologies towards the green synthesis of amides and imines
have been developed,3 such as oxidative coupling of aldehydes
and amines,3a hydrolyzation of nitriles,3b and dehydrogenative
coupling of alcohols and amines3c-3e. Among others, aerobic
oxidative coupling of alcohols and amines offers prominent
35 advantages of mild reaction conditions, high efficiency, and wide
scope of the substrates.4 This reaction is a tandem process
composed of three consecutive steps: (i) oxidative
dehydrogenation of alcohol to aldehyde, (ii) oxidative coupling of
aldehyde and amine to form hemiaminal, and (iii) further
40 oxidation of hemiaminal to yield amide or dehydration to yield
imine, as shown in Scheme 1. Kobayashi and co-workers
successfully explored this reaction by employing their unique
PICB (polymer-incarcerated carbon black)-supported gold
bimetallic catalysts.4a,4d According to their reports, Au-Co/PICB
45 catalyzed the selective formation of amide4a while Au-Pd/PICB
O
OH
(i)
R2
(ii)
HNR2R3
R2
(iii)
R1
OH
R1
R1
N
R3
R1
N
R3
O
O2 H2O
O2 H2O
R3 = H
H2O
(iii)
R2
R1
N
60
Scheme 1 pathways of oxidative coupling of alcohols and amines.
Supported Au and Au-containing bimetallic nanoparticles have
shown great potentials as catalysts in green synthesis of fine
chemicals.7,8 In particular, Au-Pd alloy catalysts often outperform
65 monometallic counterparts owing to the synergistic interplay
between Au and Pd in terms of ensemble effect and ligand
effect.9 Ensemble effect arises when Au and Pd atoms,
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Green Chemistry
Page 2 of 6
individually or together, activate distinct substrates in a reaction
Oxidative coupling of alcohols and amines were carried out in
mechanism while ligand effect is derived from the modification 40 aqueous media under O2 balloon. First, the reaction of benzyl
View Article Online
DOI: 10.1039/C3GC41117F
of electronic structure of Au and Pd through charge transfer
between dissimilar neighbouring atoms.10 It is obvious that the
composition, namely, the Au/Pd molar ratios of Au-Pd alloys
plays a critical role in dictating the synergistic effect and hence
alcohol 1a and aniline 2a was tested to investigate the effect of
Au/Pd ratio and the results are shown in Table 1. It was found
that both the activity and selectivity changed remarkably with
Au/Pd ratios. For monometallic gold and gold-rich alloy catalysts
5
tuning the catalytic performance.10e,11 Therefore, it would be 45 (Au/Pd > 1, Table 1, entries 1-4), the main product was amide 3a
highly desirable to employ strong ensemble and/or ligand effect
to achieve selective organic synthesis, or even better, to switch
10 the reaction pathways by tuning the Au/Pd molar ratios in Au-Pd
alloys. However, currently, no reports have been deliverd that the
and the best result was achieved with Au6Pd/resin (Table 1, entry
3). On the other hand, for monometallic Pd and Pd-rich alloy
catalysts (Au/Pd ≤ 1, Table 1, entries 5-8), the main product
turned out to be imine 4a and the best result was obtained with
reaction pathways could be switched merely by the Au/Pd molar 50 AuPd4/resin (Table 1, entry 7). Thus, we have provided a new
ratios in organic transformations.
opportunity to switch the reaction pathways of oxidative coupling
of alcohols and amines, from amides to imines merely by tuning
the Au/Pd molar ratios. After optimizations of the reaction
conditions (Table S2-S7, ESI†), 91% and 80% isolated yields of
Herein, we used cheap and commercially available anion-
15 exchange resin as the support to prepare a series of Au-Pd alloy
catalysts (denoted as Au-Pd/resin hereafter) and then employed
them for the oxidative coupling of alcohols and amines in water. 55 amide 3a and imine 4a could be obtained over Au6Pd/resin and
To our delight, our Au-Pd/resin catalysts were highly active and
robust for this reaction. More importantly, by tuning Au/Pd molar
20 ratios, the reaction pathways could be switched facilely from
amidation to imination. Amides and imines were obtained with
AuPd4/resin, respectively. It was also noted that the catalytic
activity went up first with a decrease of Au/Pd ratio until Au/Pd =
6/1, and then dropped significantly with further decreasing the
Au/Pd ratio. This trend implies that Au is much more active than
good to excellent yields over Au6Pd/resin and AuPd4/resin, 60 Pd in the coupling reaction. Accordingly, we speculate that Au
respectively. To the best of our knowledge, this is the first
example in organic transformations that the reaction pathways
25 can be switched by the Au/Pd molar ratios of Au-Pd alloys.
acts as the main active component and Pd functions as a switch to
direct the reaction pathway.
Table 1 Catalytic performance of Au-Pd/resin.
Results and discussion
(c)
Au/resin
AuPd /resin
4
Pd/resin
Selectivity (%)a
Conversion
Au Pd/resin
6
Entry Catalysts
(%)a
3a
4a
1
Au/resin
47
63
77
51
93
91
97
71
6
8
2
28
2
3
4
5
6
7
8
Au10Pd/resin
Au6Pd/resin
Au2Pd/resin
AuPd/resin
AuPd2/resin
AuPd4/resin
Pd/resin
10 20 30 40 50 60 70 80
2θ (degree)
28
33
66
12 (60b)
12 (42b)
1 (16b)
4 (32b)
0 (5b)
0 (0b)
95 (67b)
99 (94b)
99 (99b)
a
65
Determined by GC using 1,3,5-trimethylbenzene as an internal
standard and calculated based on 2a. 1a (1.0 mmol), 2a (0.5 mmol),
b
catalyst (2 mol%), H2O (1 mL), 5 h.
After establishing suitable catalysts and optimum reaction
conditions, we focused on the issues of scope and functional
70 group tolerance. Amidation reaction was further studied over the
best catalyst Au6Pd/resin and various alcohols 1 and amines 2
were tested (Table 2). As for alcohols 1, benzyllic alcohols
bearing both electron-withdrawing and -donating groups (Table 2,
entries 2-5, 29) were successfully converted to the corresponding
75 amides 3b-3e and 3ac with high yields. The Au6Pd/resin was also
tolerant to alcohols when the aromatic rings were naphthyl (Table
2, entry 30) and pyridyl (Table 2, entries 6, 21, 26), and the
amidation reaction proceeded very well to afford the desired
products 3ad, 3f, 3u and 3z. Specifically, for biomass derived
80 ethanol, n-butanol and furyl alcohols (Table 2, entries 7, 9, 11,
17), the corresponding amides 3g, 3i, 3k and 3q were also
achieved with good to excellent yields. This would open up a new
Fig. 1 HAADF images of Au6Pd/resin (a) and AuPd4/resin (b); XRD
patterns of Au-Pd/resin (c); HAXPES spectra of Au 3d5/2 of Au6Pd/resin
30 (d).
Au-Pd/resin catalysts were prepared with ion exchange-NaBH4
reduction method. Fig. 1((a), (b), (c)) shows HAADF images and
XRD patterns of samples with different Au/Pd ratios. Au-Pd
nanoparticles are highly dispersed on the support, and the average
35 particle size decreased with decreasing the Au/Pd ratio, indicating
Pd helps to stabilize alloy nanoparticles against aggregation.10a,10d
The gradual shift of (111) reflection towards higher angles with a
decrease of Au/Pd ratio suggests the formation of Au-Pd alloy.
2
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Page 3 of 6
Green Chemistry
route to the synthesis of valuable fine chemicals from renewable
secondary amines that are sluggish to perform amidation (Table 2,
biomass-derived materials. Importantly, for aliphatic alcohols that 20 entries 27-30), good to excellent yields of the corresponding
amides 3aa-3ad were obtained. Ammonia couldVaielwsoArbticelegOonolinde
substrate (Table 2, entries 31-32) though moderate yields of
DOI: 10.1039/C3GC41117F
Table 2 Oxidative coupling reactions over Au-Pd/resin.a
benzamides 3ae-3af were achieved. Gram-scale experiment was
also performed with ethanol and aniline, and acetanilide, an
25 important intermediate in pharmaceutical industry, was achieved
with 96% yield (Scheme S1, ESI†), indicating our Au6Pd/resin
catalyst could be tolerant to large-scale production and therefore
showed great potentials for industrial application.
The reaction pathway was completely switched to imination
O
1
2
3
4
9
5
O
O
O
O
Ph
Ph
30 when Au6Pd/resin was replaced by AuPd4/resin while the other
reaction conditions were almost identical. Various aromatic
alcohols 1 and amines 2 bearing a broad scope of substitution
groups on the benzene ring (Table 2, entries 33-41) could be
converted into the desired imines 4b-4i. Pyridyl alcohol (Table 2,
35 entry 42) was also tolerated and the reaction proceeded very well.
It is noted that though the selectivity of imines determined by GC
was very high, only moderate to good isolated yields of imines
were obtained when the products were purified by
chromatography because the products decomposed to some
40 extent over silica gel.
Hot filtration test was carried out to rule out the possible
contribution of homogeneous catalysis.12 The Au6Pd/resin was
separated from the reaction system by filtration at a conversion of
52% and the filtrate was examined by ICP-AES. The results
45 showed that only negligible amounts of Au (0.4 ppm
corresponding to 0.02 wt% of initial charge) and Pd (0.1 ppm
corresponding to 0.09 wt% of initial charge) were detected in the
solution and no further reaction took place when new batch of
substrates was added into the filtrate. Evidently, the oxidative
50 coupling reaction indeed took place on the surface of solid
catalyst. The catalysts also exhibited good recyclability and
stability. For instance, the Au6Pd/resin could be recovered by
simple filtration and reused for 6 times (Table 3) and could be
stored for 7 months under air atmosphere without significant loss
55 of catalytic activity.
Ph
Ph
Ph
N
N
N
N
N
H
H
H
H
H
O
3d; 75%
3e; 95%
3a; 91%
3b; 95%
3c; 83%
7
8b
6
10
15
20
24
28
32
O
O
O
O
O
Ph
Ph
Ph
Ph
Ph
N
N
H
N
N
H
N
H
H
H
N
O
3j;
3h;
54%
94%
3f;
3g;
95%
67%
3i;
99%
11
16
21
25
12
17
13
14
O
O
O
O
O
Ph
Ph
Ph
Ph
Ph
N
N
H
N
N
N
3
4
6
H
H
H
H
3k;
99%
3l;
3m;
99%
79%
3n;
70%
3o;
99%
18
19
O
O
O
O
O
Ph
Ph
N
N
Ph
N
H
Ph
N
H
H
H
Ph
N
H
O
3r;
3s;
91%
81%
3p; 79%
3q; 69%
3t;
62%
22
23
Ph
O
O
O
O
O
N
H
N
N
O
Ph
26
N
H
H
N
H
O
3x; 98%
3v; 70%
3w; 97%
3u; 96%
27
31
O
O
N
O
O
N
H
N
N
H
N
O
3z;
91%
3aa;
3y; 83%
91%
O
3ab;
95%
30
29
O
O
O
NH2
N
N
NH2
O
O2N
3ae; 49%
3ac;
3ad;
3af;
54%
57%
95%
35
33
34
36
37
Ph
O
N
Table 3 Recovery and reuse of Au6Pd/resin.
Ph
N
Ph
40
N
Ph
N
Ph
N
4c; 50%
4a; 80%
4e; 61%
4b;
4d;
65%
63%
39
41
42
N
38
Ph
Ph
N
N
N
N
N
O
Run
Yield (%)
1
99
2
99
3
97
4
96
5
89
6
84
4h; 60%
4f;
4g;
41%
83%
4i; 53%
4j; 53%
5
a
For experimental details, see Electronic Supplementary Information
(ESI†). b Obtained from coupling of aniline and 2-buten-1-ol.
How are the reaction pathways switched from amidation to
are difficult to oxidize, the amidation reaction could also proceed 60 imination with decreasing the Au/Pd ratios? To address this issue,
smoothly (Table 2, entries 9-17). Allylic alcohol (Table 2, entry 8)
we designed following control experiments. First, the oxidation
of benzyl alcohol was performed on catalysts with different
Au/Pd ratios (Table S8, ESI†). The selectivity of benzaldehyde
increased with a decrease of Au/Pd ratio, and no benzaldehyde
10 was also investigated. To our surprise, saturated N-
phenylbutyramide 3h was obtained when 2-buten-1-ol was
employed as the starting alcohol. Obviously, hydrogen transfer
occurred during the oxidative process, which is quite different 65 was formed over Au6Pd/resin. This result was well consistent
from the result obtained with Au/Co4a catalysts. This result will
15 provide some hints for the underlying mechanism and therefore
be discussed in the following part. Besides alcohols, a series of
amines 2a were also investigated (Table 2, entries 18-32) and the
with the observation that no aldehyde was detected in
Au6Pd/resin system while a quantity of free aldehyde was always
detected in the case of AuPd4/resin during the oxidative coupling
reaction. Second, the coupling reaction of benzaldehyde and
reaction proceeded smoothly. Notably, for sterically hindered 70 aniline was performed, and it resulted in the exclusive formation
This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 |
3

Green Chemistry
Page 4 of 6
of imine either with Au6Pd/resin or without any catalyst (Scheme
on Pd atoms and thereby hinder further oxidative
S3-S4, ESI†). Taking these results together, we can propose that
the reaction pathway to amidation or imination depends on the
dehydrogenation. Meanwhile, the isolation of Au by Pd atoms
View Article Online
also results in the weak adsorption of the aldehyde intermediates
DOI: 10.1039/C3GC41117F
adsorption strength of aldehyde intermediate on catalyst surface, 55 on Au atoms. Taken together, the aldehyde intermediate desorbs
5
which in turn relies on the Au/Pd ratio. In the case of gold-rich
alloy catalyst (e.g., Au6Pd), an adsorbed aldehyde species would
be formed at the initial stage of the reaction due to the strong
affinity of Au to acyl group.13 The adsorbed aldehyde then
reacted with amine to generate adsorbed hemiaminal, followed
readily from the AuPd4 catalyst surface, which directs the
reaction towards imination pathway.
10 either by further oxidation to yield amide or dehydration to
imine.4 Nevertheless, dehydration process is significantly
inhibited in aqueous media, thus amidation turns out to be the
main reaction pathway on the Au6Pd/resin. In contrast, free
aldehyde was preferentially formed with Pd-rich alloy (e.g.,
15 AuPd4) probably owing to the isolation of Au atoms by Pd which
weakens the adsorption of aldehyde on Au atoms. Upon
formation, the free aldehyde then underwent condensation with
amine smoothly to produce imine. The possible mechanisms of
amidation and imination are shown in Scheme 2.
Scheme 3 Schematic illustration of synergistic interplay
60 between Au and Pd in amidation of 2-buten-1-ol and aniline over
Au6Pd/resin.
Conclusions
We have developed ion exchange resin supported Au-Pd nano-
alloy catalysts that afforded high activity, tunable selectivity,
65 wide substrate variety, and good reusability for oxidative
coupling of alcohols and amines to synthesis of amides and
imines. The reaction pathways could be facilely switched by the
Au/Pd molar ratios; amides were obtained over Au6Pd/resin while
imines on AuPd4/resin. Through deliberate design of control
20
Scheme 2 Proposed reaction pathways of amidation and imination
over Au-Pd/resin.
To gain insight into the synergistic interplay between Au and
Pd, further control experiments and hard x-ray photoemission
25 spectroscopy (HAXPES) characterizations were carried out.14 As 70 experiments as well as the catalyst characterization, we also
mentioned before, transfer hydrogenation occurred during
amidation of 2-buten-1-ol and aniline (Table 2, entry 8). To
clarify the dependence of hydrogen transfer capability on the
Au/Pd ratios, this reaction was further investigated over a series
provided insightful understanding of the reaction mechanisms
responsible for the switch-like behavior of the Au/Pd ratio. The
synergistic interplay between Au and Pd revealed in this work
may offer a new guide for the design of other gold-bimetallic
30 of catalysts with different Au/Pd ratios (Table S9, ESI†). The fact 75 catalysts towards the green synthesis of fine chemicals.
that hydrogen transfer occurred on all Pd-containing catalysts
except Au/resin suggested that β-H of alcohol was abstracted
Acknowledgments
directly on Pd rather than Au atoms and the Pd-H species was
This work is supported by grants from the National Science
probably formed.15 The HAXPES results of Au6Pd/resin (Fig.
Foundation of China (21176235, 21203182, 21202163), Hundred
35 1(d)) showed the Au atoms were negatively charged. These
Talents Program of Dalian Institute of Chemical Physics (DICP).
experimental results, combined with the conclusion in literature
80 HAXPES experiment was performed at Spring-8 with the
that Au had strong affinity to acyl group,13 helps us to understand
approval of JASRI (Proposal No. 2013A1400).
the synergistic effect in Au-Pd alloy. For the Au6Pd catalyst
(Scheme 3), Au served as the main active sites to adsorb acyl
40 group-containing intermediates and O2, while addition of Pd
Notes and references
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, 116023,
facilitated the abstraction of β-H, a step that is usually sluggish to
proceed directly over Au.16 Moreover, electron transfer from Pd
to Au made the Au atoms slightly negatively charged, and
consequently promoted the activation of O2 on Au17 to generate
85 China.
b University of Chinese Academy of Sciences, Beijing, 100049, China.
c
Industrial Application Division, Japan Synchrotron Radiation Research
45 superoxo-like species,18 which in turn facilitated the
H
Institute, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo, 679-5198, Japan.
† Electronic Supplementary Information (ESI) available: Experimental
90 details, 1H NMR and 13C NMR spectra are provided. See
DOI: 10.1039/b000000x/
elimination from Pd-H species through a H2O2 route.19 Through
such synergistic interplay between Au and Pd, the Au6Pd catalyst
exhibited the best performance for amidation. Distinct from the
Au6Pd catalyst, in the case of AuPd4 catalyst, the catalyst surface
50 might be covered by H atoms to a great extent during the reaction,
which will weaken the adsorption of the aldehyde intermediates
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Green Chemistry
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Green Chemistry
Page 6 of 6
View Article Online
DOI: 10.1039/C3GC41117F
Facile switch of the reaction pathways from amidation to imination was realized by tuning the
Au/Pd ratios in Au-Pd/resin.