C-C cross-coupling of primary and secondary benzylic alcohols using supported gold-based bimetallic catalysts

Article abstract of DOI:10.1002/cssc.201200804

Clean alcohol-alcohol cross-coupling: A clean and efficient one-pot direct C-C cross-coupling of equimolar amounts of primary and secondary alcohols by a facile hydrogen autotransfer pathway is achieved over a robust and easily recovered hydrotalcite-supported Au-Pd bimetallic catalyst system. A variety of primary and secondary alcohols have been selectively converted into the corresponding b-alkylated ketones in good yields.

Full text of DOI:10.1002/cssc.201200804

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DOI: 10.1002/cssc.201200804
CÀC Cross-Coupling of Primary and Secondary Benzylic
Alcohols Using Supported Gold-Based Bimetallic Catalysts
Xiang Liu, Ran-Sheng Ding, Lin He, Yong-Mei Liu, Yong Cao,* He-Yong He, and
[a]
Kang-Nian Fan
[
1]
The formation of CÀC bonds, a vital approach to increase the
molecular complexity of a simple organic substrate, is one of
the central themes in modern synthetic chemistry. Although
a plethora of methods have been developed, the search for
new, mild, efficient, and straightforward routes that minimize
waste generation continues to be an active and challenging
geneous Au-catalyzed BH strategy that allows rapid and highly
selective CÀN bond construction by the direct coupling of al-
[15]
cohols with amines under mild, clean conditions. Herein, we
demonstrate that bimetallic Au–Pd NPs in combination with
[16]
a basic layered clay of hydrotalcite (HT) can work as a robust
and efficient multitask catalyst for the direct CÀC cross-cou-
pling of equimolar amounts of primary and secondary alcohols
under additive-free conditions. To the best of our knowledge,
this new one-pot, Au-based bimetallic catalytic protocol consti-
tutes the first base-free, recyclable solid catalytic system for
clean and efficient CÀC bond construction through alcohol
cross-coupling.
[
2]
subject. An elegant method such as the Guerbet reaction for
CÀC bond construction is the transition-metal-catalyzed direct
[
3–7]
cross-coupling of alcohols
by a borrowed hydrogen atom
[
8]
(
BH, also known as hydrogen autotransfer). In such a process,
readily available, green alcohols undergo sequential dehydro-
genation/aldol condensation/transfer hydrogenation to afford
higher alcohols or ketones with only water and/or H as by-
In a preliminary experiment, a model aldol condensation re-
action between benzaldehyde and acetophenone, the key step
to form the desired CÀC bond, was performed over various
noble-metal-free solid materials, and the results are summar-
2
products. To date, relevant studies have largely focused on var-
[
3–5]
ious Ru- or Ir-based homogeneous systems.
However, the
inherent problems of nonreusability and the necessity of spe-
cial handling precautions for air-sensitive metal complexes
have restricted the utility of these procedures. These limita-
tions reinforce the need for a ligand-free, reusable catalyst
system for this type of transformation. In the only two related
precedents to this work, Pd/C and Ag/Al O were used as effi-
ized in Table 1. Mg–Al/HT, TiO , and MgO showed high activi-
2
[
a]
Table 1. Aldol condensation of benzaldehyde and acetophenone.
2
3
cient catalysts for direct CÀC cross-coupling of secondary and
[
9]
primary alcohols. However, either a sacrificial hydrogen ac-
ceptor and/or a considerable amount of inorganic base was re-
quired to achieve a high selectivity. From a green and econom-
ic viewpoint, there is a great incentive to develop new simple
and efficient catalytic protocols that can facilitate direct alcohol
cross-coupling under mild, additive-free conditions.
Entry
Catalyst
HT
t
Yield
[%]
[
b]
[
h]
1
2
3
4
5
6
7
8
4
4
4
4
6
6
6
6
97
96
92
87
34
45
n.r.
n.r.
TiO
MgO
CeO
ZrO
Al O
2
Over the last decade, supported Au nanoparticles (NPs) have
emerged as promising new catalytic materials for green organ-
2
2
[
10]
2
3
ic transformations. Au was originally considered to be chem-
ically inert and hence a poor catalyst. However, if small Au par-
ticles or clusters are dispersed on a suitable support, Au be-
comes a highly active and selective catalyst capable of facilitat-
ing a broad range of synthetic reactions, which include selec-
SiO
La
2
2
O
3
[
a] Reaction conditions: benzaldehyde (1 mmol), acetophenone (1 mmol),
p-xylene (3 mL), catalyst (400 mg), 1208C, 1 bar N ; n.r.=no reaction.
b] Conversion and yield based on benzaldehyde consumption deter-
2
[
[
11]
[12]
tive oxidation,
multistep reactions.
studies on Au catalysis, we discovered an excellent hetero-
chemoselective reduction,
and one-pot
mined by GC using n-dodecane as the internal standard.
[
13,14a,b,15]
Very recently, from our continuing
[
14]
ties for the desired aldol condensation, and the yields of the
target product 1,3-diphenyl-2-propen-1-one (3a) were 92–97%
[
a] X. Liu, R.-S. Ding, L. He, Dr. Y.-M. Liu, Prof. Dr. Y. Cao, Prof. Dr. H.-Y. He,
Prof. K.-N. Fan
Department of Chemistry
(
Table 1, entries 1–3). CeO2 catalyzed the aldol condensation
moderately to give 3a in 87% yield (Table 1, entry 4). Other in-
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials
Fudan University
Handan Road 220, Shanghai 200433 (PR China)
Fax: (+86)21-65643774
organic solid materials, such as Al O , ZrO , SiO , and La O ,
2
3
2
2
2
3
had far inferior activities toward 3a (Table 1, entries 5–8). To ra-
tionalize these results, the surface acidity and basicity of the
catalysts were measured by CO2 and NH3 temperature-pro-
E-mail: yongcao@fudan.edu.cn
grammed desorption (CO /NH -TPD, Figure S1a–b and
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200804.
2
3
Table S1). In general, the solids that feature prominent surface
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basicity (HT, MgO, CeO ) can effectively promote the aldol reac-
2
[a]
Table 3. Reaction of 1a with 2a with various catalysts.
tions. Interestingly, surface-neutral TiO also exhibits excellent
2
activity for this reaction. However, most importantly and rele-
vantly, after loading with Au, Pt, or Pd, only the highly basic
HT is the most appropriate material that can catalyze the de-
[17]
sired aldol condensation (Figure S1c and Tables S1 and S2).
Based on these results, we focused on combining the capa-
bility of HT-supported metals to facilitate the dehydrogenation
of alcohols as well as the subsequent autotransfer reduction of
the C=C bond involved in the titled reaction. To this end, two
equivalents of benzyl alcohol (1a) was employed to promote
the reduction of the C=C bond of 3a in the presence of
a series of HT-supported metal catalysts (see the Supporting
[b]
[c]
Entry
Catalyst
Conversion
[%]
Yield [%]
3a 4a
[
c]
5a
6a
[d]
1
2
3
Au/HT
Pd/HT
100
100
100
100
97
100
100
100
n.r.
12 85
2
1
1
2
1
2
0
2
n.d.
<1
20
1
1
1
2
6
10
n.d.
0
1
1
5
2
1
1
n.d.
79
97
96
90
94
93
87
n.d.
Au–Pd(13:1)/HT
Au–Pd(13:1)/HT
Au–Pd(13:1)/HT
Au–Pd(8:1)/HT
Au–Pd(4:1)/HT
Au–Pd(5:2)/HT
HT
[
e]
4
5
6
7
8
9
Information) under N . As can be seen in Table 2, Au/HT has
2
[f]
[
a]
Table 2. Transfer reduction of 3a by 1a.
[
a] Reaction conditions: 1a (1 mmol), 2a (1 mmol), p-xylene (3 mL), cata-
lyst (metal: 1 mol%), 1208C, 1 bar N , 5 h; n.d.=not detected. [b] Data in
2
parentheses represent Au/Pd molar ratios, and the Au and Pd loading are
shown in Table S5. [c] Conversion and yield based on the consumption of
1
a determined by GC using n-dodecane as the internal standard. [d] Au
loading=0.50 wt% (based on ICP analysis). [e] Third run. [f] Fifth run.
Entry
1
2
3
4
5
6
Catalyst
Conversion
Yield [%]
4a
[
b]
[c]
[b]
[b]
[%]
5a
6a
[
d]
Au/HT
Pd/HT
97
96
75
70
36
24
83
74
62
55
28
16
97
77
75
69
29
23
n.d.
19
n.d.
1
7
1
gave an impressive conversion of 1a and 2a to achieve 4a in
a high yield of 85% at 1208C after 5 h (Table 3, entry 1).
As metal loading is especially influential for supported metal
catalysts, three Au/HT samples with different Au loadings were
prepared to explore the optimal catalyst. Interestingly,
[
e]
Ag/HT
Ru/HT
Rh/HT
Pt/HT
0.50 wt% Au/HT gave the highest yield of 4a, and higher or
[a] Reaction conditions: 1a (2 mmol), 3a (1 mmol), p-xylene (3 mL), cata-
lower Au loadings both decreased the yield of 4a (Table S3).
These results show that an optimal balance between the Au
NPs and the basic sites of the HT surface is essential to pro-
mote the desired reaction. Thus, the most active and selective
lyst (metal: 1 mol%, based on 3a), 1208C, 1 bar N , 4 h; n.d.=not detect-
2
ed. [b] Conversion and yield based on consumption of 1a determined by
GC using n-dodecane as the internal standard. [c] The yield of 4a is
based on the consumption of 3a. [d] Au loading=0.50 wt% (based on
ICP analysis). [e] 3 h.
0.50 wt% Au/HT catalyst was selected for further study. Subse-
quent experiments focused on the effect of the reaction tem-
perature, which revealed that the reaction rate increased dra-
matically as the reaction temperature increased from 80 to
110 8C but leveled off at 120–1358C (Table S4, entries 12 and
13). Among the solvents tested, p-xylene was the best, and
changing the solvent to mesitylene, o-xylene, and dioxane de-
creased the yield of 4a to 13, 8, and 2%, respectively.
a distinct advantage over other supported noble metals. Nota-
bly, Au/HT can effectively catalyze the chemoselective transfer
hydrogenation of 3a to give an 83% yield of the desired prod-
uct 1,3-diphenyl-propan-1-one (4a; Table 2, entry 1). Ag/HT
and Ru/HT also converted 3a, but their activities for this trans-
fer reduction transformation were much lower than that of Au/
HT (Table 2, entries 3 and 4). In the presence of Rh/HT or Pt/HT,
the yields of 4a were very low (Table 2, entries 5 and 6).
A high conversion of 1a was observed with Pd/HT as the cata-
lyst (Table 2, entry 2), however, only a moderate yield of 3a
was obtained because a substantial amount of toluene (6a)
To explore the reaction performed with Au/HT in detail, the
product evolution of the cross-coupling of 2a and 1a in
p-xylene under the conditions of entry 1 in Table 3 was
followed by continuous sampling by using GC. As shown in
Figure 1A, the desired product 4a was the main product in
the initial stage of the reaction. In addition, 2–11% of 5a was
detected during the whole course of the reaction, and there
was still approximately 12% of intermediate 3a present in the
final stage of the reaction. Taken together, these results dem-
onstrate that the present Au/HT-catalyzed cross-coupling may
proceed through the sequential reaction pathway depicted in
Scheme 1. Furthermore, we have confirmed by a set of kinetic
experiments that the initial reaction rates for each step of the
overall sequential reaction decreases in the order: r ꢀr >r
[18]
was produced through the undesired hydrogenolysis of 1a.
Having established suitable conditions for each step of the
sequential reaction, we were interested in the use of Au/HT to
promote the direct coupling of primary and secondary alcohols
to afford the corresponding higher ketones. For this purpose,
the reaction between 1-phenylethanol (2a) and 1a was per-
formed in p-xylene under N . Gratifyingly, Au supported on HT,
2
which possesses beneficial surface redox and basic properties,
a
b
c
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monometallic Pd/HT appears to be too active but not selective
for the desired transformation, we designed a series of Pd-con-
taining bimetallic Au–Pd/HT catalysts with various Au/Pd
atomic ratios (13:1–5:2), which were prepared by following the
same procedures used for the monometallic Au/HT catalyst.
Notably, the addition of a very small amount of Pd dramatically
improves the yield of 4a from 85 to 97% (Table 3, entry 3).
With an increasing Pd/Au atomic ratio from 1:13 to 2:5
(
Table 3, entries 3, 6–8), a gradual decrease in the yield of 4a
from 97 to 87% was observed. This may simply be because
the undesirable formation of toluene from the disproportiona-
tion of 1a is favored at higher Pd loadings. Thus, a 13:1 mix-
ture of Au/Pd provides the most effective catalyst for the titled
reaction, which exhibited both excellent selectivity and activity
towards the formation of 4a (Figure 1B). The advantage of the
Au–Pd(13:1)/HT catalyst becomes clear if the reaction profiles
obtained with the Au/HT catalyst (Figure 1A) and the Pd cata-
lyst (Figure S2) are compared. The XRD pattern and TEM image
of Au–Pd(13:1)/HT are shown in Figures S3 and S4, respectively,
which indicate that the metallic particles are finely dispersed
on the surface of HT and that most of the particles are 1–4 nm
in diameter. The energy-dispersive X-ray (EDX) spectrum (Fig-
ure S5) of a single NP shows the presence of both Au and Pd
on a single NP. EDX spectroscopy also shows that some of the
NPs are monometallic Au. X-ray photoelectron spectroscopy
(XPS) confirmed that the small Pd entities affect the electron
densities of Au and the binding energy of Au4f7/2 for Au–
Pd(13:1)/HT shifts to a higher energy than that of Au/HT by
Figure 1. Reaction profiles for the direct cross-coupling of 1a with 2a over
Au/HT (A) and Au–Pd(13:1)/HT (B). Reaction conditions: 1a (1 mmol), 2a
2
(1 mmol), p-xylene (3 mL), catalyst (metal: 1 mol%), 1208C, 5 h under N .
0.1 eV (Figure S6). Although it is difficult to clarify whether
a bulk alloy is formed between Au and Pd, the improved per-
formance of the bimetallic Au–Pd catalyst can be ascribed to
the beneficial role of a small amount of Pd to promote the
transfer hydrogenation of 3a (Scheme 1, step c).
Au–Pd/HT can be easily separated by filtration after the reac-
tion. Inductively coupled plasma (ICP) analysis confirmed that
no Au or Pd was present in the filtrate, which indicates that
the catalysis is truly heterogeneous in nature. Moreover,
Au–Pd/HT can be reused at least three times and still maintains
a complete conversion of 1a to give 4a in up to 96% yield
under the same reaction conditions (Table 3, entry 4), in which
the average turnover frequency (TOF) and the turnover
Scheme 1. Proposed sequence for the direct cross-coupling of primary and
secondary alcohols to produce higher ketones.
À1
number (TON, based on total metal) reached up to 20 h and
300, respectively. These values are one or two orders of magni-
(
Scheme S1), which accounts for the presence of appreciable
tude greater than those in catalyst systems reported previous-
À1
[9a]
amounts of benzaldehyde and chalcone intermediates during
the alcohol coupling reaction. In contrast to a previous study
ly, such as Ag/Al O (TOF=0.5 h and TON=25 at 1158C).
2 3
XRD, TEM, EDX, and XPS results showed no change in the dis-
persion of the Au NPs or the metallic state of Au before and
after reuse, which is in good agreement with the excellent re-
[
9a]
that used Ag/Al O , in which the rate-determining step of
2
3
the overall reaction was the initial alcohol dehydrogenation
mediated by supported Ag clusters, it seems that the control-
ling step in the present Au/HT-catalyzed system is the transfer
reduction of intermediate 3a by the gold hydride (Scheme 1,
step c).
[19]
tention of activity (Figures S3–S6).
Building on these results, we extended this direct Au–Pd-
mediated CÀC coupling protocol to a wider range of primary
and secondary alcohols. In these studies, two different strat-
egies were employed to explore the efficiency of the hetero-
geneous Au–Pd/HT catalytic system for the synthesis of diverse
ketones. In the first strategy, various structurally divergent pri-
mary alcohols were allowed to react with 2a for the coupling
reaction, and the results are summarized in Table 4. Au/HT
In an effort to further improve the selectivity towards 4a,
we selected metallic Pd as a promoter for the Au/HT catalyst.
Pd was chosen because Pd NPs are well known to catalyze
a wide range of reductive transformations, which include dehy-
[
6a,7d,e,g,9b]
drogenation and transfer hydrogenation.
Given that
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impressive yield of 89% in the presence of Au–Pd/HT.
Compound 4a (1.74 g) was isolated in high purity after simple
manipulation of the reaction mixture (isolated yield of ca.
[
a]
Table 4. Reaction of primary and secondary alcohols.
83%). Furthermore, the bimetallic Au–Pd/HT catalyst can also
catalyze the aerobic cross-coupling of 1a and 2a to produce
the corresponding chalcone 3a, which is known to have a di-
verse array of biological properties, such as antimalarial, anti-
inflammatory, antileishmanial, and anticancer activities, with
1
2
Entry
R
R
t
Conversion
[%]
Yield
[%]
[
b]
[b]
[h]
1
2
3
4
5
6
7
8
9
Ph
p-MeC
p-ClC
m-ClC
p-MeOC
o-MeOC
p-iPrC
p-CF
PhCH
2-pyridyl
nBu
n-pentyl
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
5
5
5
8
8
100
100
100
100
100
100
100
100
100
100
n.r.
97 (95)
93 (90)
90 (85)
87 (84)
80 (77)
72 (69)
89 (85)
71 (68)
82 (78)
82 (77)
n.d.
6
H
4
a high yield under 1 bar O (ca. 90%; Scheme 2).
2
6
H
4
6
H
4
6
H
H
4
6
4
10
8
6
H
4
3
C
6
H
4
8
2
36
20
24
24
8
10
1
1
12
13
14
15
16
n.r.
n.d.
2
Scheme 2. Aerobic cross-coupling reaction of 1a with 2a under O .
p-MeC
6
H
4
100
100
100
100
91 (88)
86 (83)
81 (77)
83 (76)
p-MeOC
p-FC
p-ClC
6
H
4
8
8
8
Ph
Ph
6
H
5
In conclusion, we have demonstrated the noteworthy fea-
tures of solid multifunctional Au–Pd/HT for direct and effective
CÀC cross-coupling of primary and secondary alcohols through
a facile BH pathway. This process has prominent advantages
compared with previous alcohol cross-coupling methods:
6
H
5
[
(
a] Reaction conditions: primary alcohol (1 mmol), secondary alcohol
1 mmol), p-xylene (3 mL), Au–Pd (1 mol%), 1208C, 1 bar N ; n.r.=no re-
action; n.d.=not detected. [b] Conversion and yield based on alcohol
2
consumption. Numbers in parenthesis refer to yields of isolated products.
(
a) high catalytic activity and selectivity, (b) no external H or
2
other sacrificial agent is necessary, (c) no need to apply a base,
and (d) no stabilizing ligands and/or co-catalysts are necessary.
The reaction chemistry demonstrated herein will promote the
application of the BH strategy in clean and atom-economical
chemical processes.
gave moderate to good yields (71–97%) with various electron-
donating groups, such as CH , CH O, iPr, and electron-
3
3
withdrawing groups, such as chloro- or trifluoromethyl-deriva-
tives of benzyl alcohol, and the reaction activity is independent
of the electronic character of the substituents (Table 4,
entries 1–8). Notably, m- or o-substituted alcohols had lower
reaction rates and yields than p-substituted alcohols, which
can be explained by steric effects (Table 4, entries 3–6). Chloro-
substituted benzyl alcohols were converted to the correspond-
ing ketones without dehalogenation (Table 4, entries 3 and 4),
which is often encountered with other catalytic procedures.
If 2-phenylethanol was employed as a substrate, a much
longer time was needed to complete the reaction (Table 4,
entry 9). Interestingly, an N-containing primary alcohol also re-
acted with 2a to give a moderate yield of the corresponding
N-containing ketone (Table 4, entry 10). Unfortunately, no de-
sired cross-coupling product was detected if aliphatic primary
alcohols, such as n-butyl and n-pentyl alcohols, were employed
as substrates even after prolonged reaction times (Table 4, en-
tries 11 and 12). In the second strategy employed for the direct
synthesis of ketones, the primary substrate was kept constant
Experimental Section
Preparation of Au/HT catalysts: HT was synthesized according to
[20]
a literature procedure. A modified deposition–precipitation (DP)
procedure was employed to prepare Au/HT. HT (1.0 g) was dis-
persed in an aqueous solution of HAuCl ·4H O (50 mL, 1.02ꢁ
4
2
À3
1
0 m) with vigorous stirring. After agitation for 2 h at 258C, the
mixture was filtered and washed thoroughly with deionized water
À
until no Cl was detected in the filtrate by using AgNO . The result-
3
ing compound was dried for 12 h under vacuum at rt and reduced
by using 5% H /Ar at 3008C for 2 h. Elemental analysis revealed
2
that the Au loading of Au/HT was 0.5 wt%.
Preparation of bimetallic Au–Pd/HT catalysts: Au–Pd/HT catalysts
were prepared by using the same procedure as that used to pre-
pare Au/HT. HT (1.0 g) was dispersed in an aqueous solution of
HAuCl and PdCl with varying Au/Pd atomic ratios (50 mL) with
4
2
vigorous stirring. The total metal loading was kept at approximate-
(i.e., only 1a was used) and it was allowed to react with differ-
ly 0.5 wt%. After agitation for 2 h at 258C, the mixture was filtered
ent secondary alcohols such as p-substituted 1-phenylethanol,
À
and washed thoroughly with deionized water until no Cl was de-
in which the substituents were CH , CH O, F, and Cl. Again, the
3
3
tected in the filtrate by using AgNO . The resulting compound was
3
corresponding higher ketones were obtained in good to excel-
lent yields (Table 4, entries 13–16).
dried for 12 h under vacuum at rt and reduced by using 5% H /Ar
at 3008C for 2 h.
2
The applicability of the heterogeneous Au–Pd/HT-based pro-
tocol is highlighted by the one-pot synthesis of 4a by the
direct cross-coupling of 1a and 2a under solvent-free condi-
tions. Notably, the in situ CÀC bond formation on a 10 mmol
scale proceeds smoothly at 1308C, and 4a was obtained in an
General procedure for the direct cross-coupling of alcohols:
A
mixture of primary alcohol (1 mmol), secondary alcohol
(1 mmol), catalyst (1 mol% metal), p-xylene (3 mL), and n-dodecane
(10 mL) as an internal standard was vigorously stirred at 1208C
under N (1 bar). The product was identified by GC–MS, and the
2
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spectra obtained were compared with those of standards. The con-
version and product selectivity were determined by using a GC-
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This work was support by the National Natural Science Founda-
tion of China (20873026 and 21073042), New Century Excellent
Talents in the University of China (NCET-09-0305), the National
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Keywords: alcohols
palladium
· cross-coupling · gold · ketones ·
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Received: October 25, 2012
Published online on March 26, 2013
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