An unusual chemoselective hydrogenation of quinoline compounds using supported gold catalysts

Article abstract of DOI:10.1021/ja3066978

The pursuit of modern sustainable chemistry has stimulated the development of innovative catalytic processes that enable chemical transformations to be performed under mild and clean conditions with high efficiency. Herein, we report that gold nanoparticles supported on TiO₂ catalyze the chemoselective hydrogenation of functionalized quinolines with H₂ under mild reaction conditions. Our results point toward an unexpected role for quinolines in gold-mediated hydrogenation reactions, namely that of promoter; this is in stark contrast to what prevails in the traditional noble metal Pd-, Pt-, and Ru-based catalyst systems, in which quinolines and their derivatives typically act as poisons. As a result of the remarkable promotional effect of quinoline molecules to H₂ activation over supported gold, the transformation can proceed smoothly under very mild conditions (even at temperatures as low as 25 °C). Of practical significance is that various synthetically useful functional groups including halogens, ketone, and olefin remain intact during the hydrogenation of quinolines. Moreover, the protocol also shows promise for the regiospecific hydrogenation of the heterocyclic ring of a variety of other biologically important heteroaromatic nitrogen compounds, such as isoquinoline, acridine, and 7,8-benzoquinoline, in a facile manner. Apart from its importance in catalytic hydrogenation, we believe that this intriguing self-promoted effect by reactant molecules may have fundamental implications for the broad field of gold catalysis and form the basis for development of new catalytic procedures for other key transformations.

Full text of DOI:10.1021/ja3066978

Article
pubs.acs.org/JACS
An Unusual Chemoselective Hydrogenation of Quinoline
Compounds Using Supported Gold Catalysts
Dong Ren, Lin He, Lei Yu, Ran-Sheng Ding, Yong-Mei Liu, Yong Cao,* He-Yong He, and Kang-Nian Fan
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Handan Road
220, Shanghai 200433, P. R. China
*
S Supporting Information
ABSTRACT: The pursuit of modern sustainable chemistry has stimulated
the development of innovative catalytic processes that enable chemical
transformations to be performed under mild and clean conditions with high
efficiency. Herein, we report that gold nanoparticles supported on TiO2
catalyze the chemoselective hydrogenation of functionalized quinolines with
H under mild reaction conditions. Our results point toward an unexpected
2
role for quinolines in gold-mediated hydrogenation reactions, namely that of
promoter; this is in stark contrast to what prevails in the traditional noble
metal Pd-, Pt-, and Ru-based catalyst systems, in which quinolines and their
derivatives typically act as poisons. As a result of the remarkable promotional
effect of quinoline molecules to H activation over supported gold, the
2
transformation can proceed smoothly under very mild conditions (even at
temperatures as low as 25 °C). Of practical significance is that various
synthetically useful functional groups including halogens, ketone, and olefin remain intact during the hydrogenation of
quinolines. Moreover, the protocol also shows promise for the regiospecific hydrogenation of the heterocyclic ring of a variety of
other biologically important heteroaromatic nitrogen compounds, such as isoquinoline, acridine, and 7,8-benzoquinoline, in a
facile manner. Apart from its importance in catalytic hydrogenation, we believe that this intriguing self-promoted effect by
reactant molecules may have fundamental implications for the broad field of gold catalysis and form the basis for development of
new catalytic procedures for other key transformations.
INTRODUCTION
surprising activity and selectivity for a broad array of organic
reactions, which is believed to be essential for the chemical
■
7
Heterogeneous catalytic hydrogenation is a powerful tool for
achieving controlled reduction of many types of organic
compounds under mild conditions. The importance of this
industry to move toward sustainability. The pioneering work by
Bond et al. in the early 1970s first reported the excellent
selectivity of supported Au catalysts prepared by conventional
impregnation method for the hydrogenation of unsaturated
1
reductive transformation has been originally highlighted by the
awarding of the 1912 Nobel Prize in Chemistry to Sabatier fo2r
8
hydrocarbons, albeit with a very limited activity. Despite such
his work on hydrogenation of alkenes over nickel catalysts.
impressive results it was not until almost three decades later,
that Claus’s and Corma’s groups have demonstrated that
supported Au NPs with size of 3−5 nm can promote the
selective hydrogenation of carbonyl or nitro groups in the
presence of other reducible functions even at very high
Since then, an extensive number of heterogeneous metal
catalysts have been introduced. The mostly used ones are the
supported Pt and Pd catalysts, along with Raney nickel and a
3
few supported Cu catalysts. These catalysts can hydrogenate
many different functional groups with high activity. However,
when dealing with multifunctional molecules, the problem
often arising is that the classical hydrogenation catalysts reduce
sensitive functions simultaneously, leading to an undesired
9
conversion values. These Au-catalyzed hydrogenation pro-
cesses contributed substantially to catalytic hydrogenation by
enhancing the chemoselectivity to an unprecedented level.
Nevertheless, they are still not practically useful owing to the
lack of sufficient acitvity. In fact, as a consequence of the limited
4
selectivity. In such cases, the use of older, noncatalytic
manufacturing processes (by employing stoichiometric reduc-
ing agents, such as sodium hydrosulfite, iron, tin, or zinc in
ammonium hydroxide), prevails despite the concomitant
10
capability of Au toward H activation and dissociation, Au-
2
catalyzed hydrogenation reactions typically occur under
demanding conditions (commonly >100 °C), and the hydro-
genation delivery rates (mols of substrate converted per mol
metal per second) of the Au catalyst are generally 1−2 orders of
5
formation of large quantities of unwanted waste. From both
green and synthetic points of view, it is imperative to develop
ultraselective and highly efficient heterogeneous catalytic
reduction methodologies for clean chemical synthesis.
6
Catalysts based on supported gold nanoparticles (Au NPs)
Received: July 10, 2012
have attracted considerable attention as a material with a
Published: September 30, 2012
©
2012 American Chemical Society
17592
dx.doi.org/10.1021/ja3066978 | J. Am. Chem. Soc. 2012, 134, 17592−17598

Journal of the American Chemical Society
Article
magnitude lower than those of the traditional platinum group
Table 1. Hydrogenation of 6-Chloroquinoline to 6-Chloro-
1,2,3,4-tetrahydroquinoline under Various Conditions
9c,11
a
metal (PGM)-based catalysts.
Hence, the development of
new effective Au-based hydrogenation processes that can
deliver high activity while preserving high selectivity under
mild conditions constitutes a major challenge.
1,2,3,4-Tetrahydroquinolines are ubiquitous in numerous
biologically active natural products and pharmacologically
12
g
relevant therapeutic agents. While Au-catalyzed approaches
select. (%)
13
to tetrahydroquinoline derivatives have been reported,
a
g
entry
catalysts
T (°C) t (h) conv. (%)
2
3
4
chemo- and regioselective hydrogenation approach would be
superior. The direct hydrogenation of readily available
quinolines offers a straightforward and promising approach to
access tetrahydroquinolines in terms of its simplicity and high
b
1
Au/TiO -WGC
80
80
80
80
80
80
60
25
60
60
60
60
60
60
60
60
60
4
4
43
100
74
100
100
61
0
0
0
0
2
c
2
Au/TiO -M
2
d
3
Pd/C
Pt/C
4
28
31
30
0
11
16
24
0
d
4
4
81
53
1
4
atom efficiency. Unfortunately, the use of this truly green tool
has proven extremely difficult, due to the aromatic nature of the
quinoline ring and the potential poisoning of the catalysts. Until
now, relevant protocols are largely based on several Ir-, Ru- and
d
5
Ru/Al O3
4
37
46
2
6
7
Au/HSA-TiO2
Au/HSA-TiO2
Au/HSA-TiO2
Pd/HSA-TiO2
Pt/HSA-TiO2
Ru/HSA-TiO2
Pd/HAP
1.5
3
100
100
92
100
100
100
86
0
0
e
8
16
3
0
0
15
Rh-based homogeneous systems. Despite their utility, these
procedures still suffer from the inherent problems associated
with the reusability and/or the indispensable use of cocatalysts.
Recently, a handful of heterogeneous catalytic systems based on
traditional noble metals (Pd, Pt, Ru etc.) have been
9
92
12
7
2
1
1
1
1
1
1
1
0
1
2
3
4
5
6
3
97
92
1
3
93
88
11
17
−
−
−
−
0
1
3
86
79
4
HSA-TiO2
3
n.r.
n.r.
n.r.
n.r.
98
−
−
−
−
−
−
−
−
0
1
6
developed. These catalysts need to be used with caution
because they are vulnerable to the poisoning effect of strongly
adsorbed quinolines and/or their hydrogenated derivatives.
Moreover, clear drawbacks of these systems are their low
tolerance for functional groups and limited substrate scope.
Given the vital importance of this type of transformation, the
search for new simple, efficient, and general catalytic system
that can facilitate selective quinoline hydrogenation under mild
and ligand-free conditions remains a challenging but rewarding
task.
HAuCl4
3
Au O
3
2
3
0
Au powder
3
f
1
7
Au/HSA-TiO2
3
100
a
6
%
-chloroquinoline (0.5 mmol), toluene (3 mL), catalyst (metal: 1 mol
), 2 MPa H ; n.r. = no reaction. Au/TiO -WGC sample was
b
2
2
c
provided by the World Gold Council. Au/TiO -M was supplied by
2
d
e
Mintek. Pd/C, Pt/C, and Ru/Al O were provided by Alfa Aesar. 6-
2
3
chloroquinoline (0.5 mmol), toluene (3 mL), catalyst (metal: 5 mol
%), 2 MPa H2. Fifth run. Conversion (Conv.) and selectivity
(Select.) were based on 6-chloroquinoline consumption (GC analysis).
f
g
We report herein an unusual chemoselective hydrogenation
of quinolines to the corresponding 1,2,3,4-tetrahydroquinolines
using supported Au NPs. As a result of the remarkable reactant-
promoted effect of quinoline compounds to hydrogen
activation over highly dispersed Au NPs, the transformation
can proceed smoothly under mild conditions (even at
temperatures as low as 25 °C). Furthermore, a wide range of
substituted quinolines can be exclusively hydrogenated to the
desired products, keeping various synthetically useful functional
groups including halogens, ketone, and olefin intact. The
excellent performance of the present catalytic system has
allowed us to extend this methodology to the regiospecific
hydrogenation of the heterocyclic ring of various other
biologically important heteroaromatic nitrogen compounds,
such as isoquinoline, acridine, in a facile manner.
particle size (average size ca. 2.7 nm, Figure S3) facilitated the
complete conversion of 1 under identical conditions (Table 1,
entry 2). Applying Pd/C, Pt/C, or Ru/Al O (provided by Alfa
2
3
Aesar), the desired halo-substituted 1,2,3,4-tetrahydroquinoline
was also formed (Table 1, entries 3−5). Nevertheless, in these
cases, an unavoidable concomitant formation of dehalogenation
products readily occurs. Given the fact that the rate of nitro or
carbonyl group reduction over Au (<5 nm) was significantly
lower than those over traditional PGM-based catalysts, the
comparable or even superior performance obtained in the
present Au-catalyzed hydrogenation of substituted quinolines
was remarkable.
After these initial promising results, we further examined a
series of Au deposited on other mineral supports, such as
Al O , ZnO, CeO and activated carbon. These catalysts,
however, were not found to be particularly active, although in
all cases the desired chloro-substituted 1,2,3,4-tetrahydroquino-
line can be exclusively produced (Table S1, entries 2−5).
Considering that TiO (P25) in both Au/TiO -WGC and Au/
RESULTS AND DISCUSSION
■
2 3 2
In exploratory experiments, we selected the hydrogenation of 6-
chloroquinoline (1) to corresponding 6-chloro-1,2,3,4-tetrahy-
droquinoline (2) as the model reaction to study the catalytic
activity and selectivity of several commercially available noble
metal-based catalysts (Table 1, entries 1−5). The hydro-
genation of 1 without C−X bond cleavage (X = halogen, by
hydrogenolysis) is of critical importance, as the aryl halide
products obtained are useful reagents in metal-catalyzed cross-
2
2
2
−1
TiO -M has rather low surface areas (45 m g ), we envision
2
that the catalytic performance might be optimized by using
higher surface area support materials. Gratifyingly, very small
Au NPs (with mean diameter of 2 nm, Figure 1) dispersed on
high surface area (HSA) TiO (S ∼ 124 m g , Figure S1)
turned out to give a much more effective catalyst. In particular,
17
2
−1
coupling reactions. Under 2 MPa H at 80 °C, the benchmark
2
2
BET
Au/TiO -WGC catalyst (with a Au particle mean diameter of
2
3
.5 nm, supplied by World Gold Council, Figure S3), can afford
over Au/HSA-TiO , the quantitative conversion of 1 into 2 was
2
exclusive formation of 2 with moderate activity (43%
conversion) within 4 h (Table 1, entry 1). To our delight,
Au/TiO -M (supplied by Mintek) catalyst with smaller Au
achieved in a very short reaction time (1.5 h) (Table 1, entry
18
6). When the reaction temperature was lowered to 60 °C,
Au/HSA-TiO still produces 2 as the sole product within 3 h
2
2
1
7593
dx.doi.org/10.1021/ja3066978 | J. Am. Chem. Soc. 2012, 134, 17592−17598

Journal of the American Chemical Society
Article
Table 2. Au/HSA-TiO -catalyzed Hydrogenation of Various
2
a
Quinolines
Figure 1. Representative TEM image and size distribution of Au/
HSA-TiO (0.5 wt %).
2
(
entry 7). Most remarkably, at temperatures as low as 25 °C,
the hydrogenation of 1 proceeds smoothly, although a longer
reaction time and relatively larger amounts of the Au catalyst
were required (Table 1, entry 8). Nevertheless, the perform-
ance is encouraging since it represents the mildest condition in
the Au-based hydrogenation process to date.
In subsequent experiments performed under 2 MPa H at 60
2
°
C, a clear advantage of the Au/HSA-TiO catalyst in terms of
2
conversion and selectivity over other noble metals (Pd, Pt, Ru)
19
with identical particle size supported on HSA-TiO was also
2
noticed (Table 1, entries 9−11). Note that previously reported
solid catalysts, such as hydroxyapatite (HAP)-supported Pd
16a
NPs (Pd/HAP),
showed good activity but much inferior
selectivity relative to Au/HSA-TiO under the present reaction
2
conditions (Table 1, entry 12). Blank experiments without
catalyst or using the Au-free HSA-TiO gave no conversion,
2
thus further confirming that the highly dispersed Au NPs were
indispensable for the desired transformation (Table 1, entry
1
3). In addition, the use of the catalyst precursor HAuCl as
4
0
well as with other Au compounds, including Au O and Au
2
3
powder (average particle size of ca. 150 nm), did not promote
the reaction at all (Table 1, entries 14−16).
To verify whether the observed catalysis is truly heteroge-
neous or not, the reaction mixture was hot filtered at 40%
conversion of 1 at 60 °C. Further stirring of the filtrate under
the above reaction conditions did not yield any additional
product. An inductively coupled plasma method with detection
limit of 0.007 ppm was used to confirm that no Au leached into
2
0
the filtrate. Furthermore, the recovered Au/HSA-TiO2
catalyst could be reused at least five runs without appreciable
loss of the original catalytic activity (Table 1, entry 17). These
results rule out any possible contribution of homogeneous
catalysis by leached Au species.
a
Substrate (0.5 mmol), toluene (3 mL), Au/HSA-TiO (Au: 1 mol
2
b
c
%
), 60 °C, 2 MPa H2. 80 °C. Conversion was based on
d
heteroaromatic nitrogen compounds consumption. Numbers in
parentheses refer to yields of isolated products.
Having established that Au/HSA-TiO2 was an efficient
catalyst for the hydrogenation of quinolines, we extended our
studies to various structurally different substituted quinolines.
The results were summarized in Table 2. The hydrogenation of
quinoline was highly regioselective, producing 1,2,3,4-tetrahy-
droquinoline exclusively (Table 2, entry 1). No trace of 5,6,7,8-
tetrahydroquinoline or decahydroquinoline, byproducts fre-
quently formed under hydrogenation catalysis, was observed.
Reactions of quinolines bearing a methyl group at the 2- and 4-
position proceeded successfully to give the corresponding
tetrahydro-8-hydroxyquinolines (Table 2, entries 6−8), which
22
can inhibit leukotriene formation in macrophages. Further-
more, in the more challenging reactions, where quinolines with
other reducible groups (halogens, ketone, and olefin) were
employed, the corresponding valuable functional tetrahydro-
quinolines were obtained in excellent yield (Table 2, entries 9−
12), which has no precedent in heterogeneous quinoline
hydrogenation.
21
1
,2,3,4-tetrahydroquinolines (Table 2, entries 2 and 3).
Encouraged by these results, we applied this attractive
protocol to the regioselective hydrogenation of heterocyclic
ring of a series of other biologically important heteroaromatic
nitrogen compounds. Isoquinoline, acridine, and 7,8-benzoqui-
noline were tested. As depicted in Table 3, nearly quantitative
Quinolines with electron-donating groups (−OMe and
−
NH ) on the benzene ring react smoothly (Table 2, entries
2
4
and 5). Remarkably, the hydrogenation of 8-hydroxyquino-
lines afforded the corresponding biologically active 1,2,3,4-
1
7594
dx.doi.org/10.1021/ja3066978 | J. Am. Chem. Soc. 2012, 134, 17592−17598

Journal of the American Chemical Society
Article
Table 3. Au/HSA-TiO -Catalyzed Hydrogenation of
Heteroaromatic Nitrogen Compounds
XPS data presented in Figure S5, only metallic Au was
2
a
identified on the surface of the Au/HSA-TiO catalyst. The fact
2
+
δ+
that no bands for Au or Au can be seen from the DRIFT
spectrum (Figure S6) of adsorbed CO further confirmed the
sole presence of metallic Au species in this material. Moreover,
Au L -edge XANES data (Figure 2) of the Au/HSA-TiO were
3
2
a
Substrate (0.5 mmol), toluene (3 mL), Au/HSA-TiO (Au: 1 mol
2
b
c
%
), 60 °C, 2 MPa H2. 80 °C. Conversion was based on
d
heteroaromatic nitrogen compounds consumption. Numbers in
parentheses refer to yields of isolated products.
yields were obtained in most cases. Isoquinoline was reduced to
the corresponding tetrahydroisoquinoline (Table 3, entry 1). 6-
bromoisoquinoline was converted to the corresponding 6-
bromotetrahydroisoquinoline without any dehalogenation
Figure 2. Au L -edge XANES spectra of the (a) HAuCl , (b) Au foil,
and (c) Au/HSA-TiO2.
3
4
(Table 3, entry 2). More complex fused ring compounds
including acridine and 7,8-benzoquinoline were hydrogenated
to their corresponding tetrahydroderivatives (Table 3, entries 3
and 4). Note that the order of individual initial rate was acridine
clearly different from that of HAuCl (as a reference compound
4
for ionic Au3+ species) but rather similar to that of Au foil,
which further indicated that Au species in the Au/HSA-TiO2
sample exists in its elemental state.
>
quinoline > 7,8-benzoquinoline > isoquinoline, which reflects
both steric and electronic effects. Similar tendencies were
observed and explained in the previous reported homogeneous
reduction of polynuclear heteroaromatic compounds using
To clarify the origin of the superior activity achieved in
0
present Au -mediated hydrogenation system, we studied the
26
H −D exchange reaction over a variety of catalysts. For the
2
2
23
(
Ph P) RhCl complexes.
3 3
sake of comparison, the measurements were applied as close to
the reaction conditions as possible. Thus, the rates of HD
formation in terms of the TOF values were estimated at 60 °C
over the samples with and without the presence of preadsorbed
A heterogeneous direct hydrogenation without solvents
would be more desirable for practical applications. Notably,
this Au/HSA-TiO -catalyzed system can facilitate the 100
2
27
mmol scale hydrogenation of quinoline under neat conditions
at 140 °C (Scheme 1). The hydrogenation was complete within
quinoline or its hydrogenated derivatives (Figure 3). It is
revealed that, as opposed to the case without preadsorption or
in presence of preadsorbed tetrahydroquinoline, the H −D
2
2
2
Scheme 1. Au/HSA-TiO -Catalyzed 100 mmol Scale
exchange proceeds much more rapidly over the Au/HSA-TiO
2
Hydrogenation of Quinoline
4
8 h with 100% selectivity in the presence of 0.02 mol % Au, in
which the turnover number (TON, based on total Au) of the
Au catalyst approached 5000 with an excellent average turnover
frequency (TOF) of approximately 104 h . When the reaction
−1
was carried out on this scale, the Au/HSA-TiO catalyst can
2
also be reused without loss of activity.
At this stage, we became interested in elucidation of the
nature of active Au in Au/HSA-TiO sample, since the chemical
2
state of Au was generally assumed as one of the key factors in
24
determining the catalytic activity. Detailed spectroscopic
characterizations of the Au/HSA-TiO sample were done with
X-ray photoelectron spectroscopy (XPS) and diffuse reflectance
2
Figure 3. Rate for HD formation over various noble metal-based
catalysts. The TOF values were calculated based on the mol of HD
molecules formed per mol metal (Au, Pd, Pt, or Ru) per min. The
error bars represent the standard deviation of triplicate tests.
infrared Fourier transform spectroscopy (DRIFTS) as well as
25
X-ray absorption near-edge structure (XANES) studies. From
1
7595
dx.doi.org/10.1021/ja3066978 | J. Am. Chem. Soc. 2012, 134, 17592−17598

Journal of the American Chemical Society
Article
catalyst preadsorbed with quinoline molecules. This is in strong
contrast to that occurs with the corresponding traditional Pd-,
Pt-, and Ru-based catalysts, in which the rate of HD formation
was heavily decreased when the samples were preadsorbed with
relevant N-heterocylic compounds. These results unambigu-
ously demonstrate that quinoline, a poison for traditional noble
3
metal-containing hydrogenation catalysts, can act as a
promoter for the Au-catalyzed hydrogenation processes via
facilitating the activation of H2.
As for the activation of H for chemoselective reduction of
2
polar bonds (CO or CN) with homogeneous metal−
ligand bifunctional catalysts, it is widely accepted that the
+
reaction initiates with the heterolytic cleavage of H to yield H
2
−
in a OH or NH ligand and H in metal hydrides, and the
resulting H /H pair preferentially transfers to the polar
bonds. This type of ionic mechanism might be applicable also
for heterogeneous metal catalysts. Very recently, Garcia and co-
workers showed direct IR evidence on heterolytic dissociation
+
−
2
8
of H on ceria-supported Au NPs at 150 °C, leading to a proton
2
29
bonded to a ceria oxygen and a hydride attached to Au. On
the other hand, it is well established that on exposure of H to
2
metal particles dispersed on oxide materials, such as Pt/Al O3
2
and Rh/Al O , the formation of additional OH groups on the
2
3
3
0
support via hydrogen spillover will readily occur. Adopting
these models, the dissociation of H over Au/HSA-TiO would
yield H on the support and H species on Au NPs, albeit the
characteristic vibrations associated with the Au hydrides were
2
2
δ+
δ−
31
not detected by IR spectroscopy in our case.
To measure the rate of H cleavage step over various Au-
2
based catalysts, H −D exchange of D with surface OH groups
2
2
2
of the catalysts at 60 °C was monitored by DRIFTS. The
results are shown in Figure 4. For Au/HSA-TiO , a
2
simultaneous loss and gain in the IR band intensity for the
−
1
−1
Ti−OH (3686 cm ) and Ti−OD (2694 cm ) stretching
32
vibrations were observed. The OD formation rate for Au/
HSA-TiO was 3 orders of magnitude higher than that for
2
Figure 4. H
2
−D
2
exchange of D
2
with surface OH groups of the
HSA-TiO (result not shown). This gives direct evidence of the
catalysts at 60 °C. (A) DRIFT spectra recorded after a 20 min
exposure to 10% D in He. (B) Integrated intensity of OD band with
respect to exposure time.
2
Au NPs-catalyzed cleavage of the H−H bond. From the kinetic
curves, the initial rate of the OD formation decreased
significantly when Au particle size increased from 2 to 3.5
nm. For the catalysts with the same Au loading (0.5 wt %) and
identical mean Au particle size (1.9−2.1 nm), the OH−D2
2
Scheme 2. Model Explaining the Activation of H
the Substrate through Heterolytic Cleavage
by Au and
2
exchange reaction rate ranks in the order of Au/HSA-TiO
2
2
−1
2
−1
2
(
124 m g ) > Au/TiO −P25 (45 m g ) > Au/CeO (50 m
2
2
−1
g ). This trend is in line with the catalytic data for quinoline
hydrogenation, as shown in Tables S1 and S2, suggesting that a
high abundance of surface hydroxyl groups on HSA-TiO is
2
beneficial for D dissociation.
2
Based on the above-mentioned facts and bearing in mind that
basic ligands of transition-metal complexes promote heterolytic
28
cleavage of H to give metal-hydride species, we propose a
2
concerted effect between quinoline molecules and the
supported Au NPs. As depicted in Scheme 2, the key aspect
present Au-mediated hydrogenation reactions. Recent density
functional theory (DFT) calculations have confirmed that flat
and tilted orientations of the quinoline ring have lower
adsorption energy on Au (7 and 10 kcal·mol− for flat and
tilted species, respectively) compared to adsorption on Pt (ca.
of quinoline is to facilitate the crucial heterolytic H activation
2
at the Au-support interface. Namely, the quaternizable nitrogen
atom of quinoline can serve as a basic ligand to promote the
1
heterolytic H cleavage, providing a favorable situation for
2
provoking hydrogen activation under very mild conditions.
Although the precise way by which the quinoline-assisted H2
cleavage at the Au-support interface occurs remains to be
clarified at this stage, it is important to emphasize that the
moderate interaction between quinoline and Au could be the
key factor accounting for the essential role of quinolines in the
−1
33
4
0 kcal·mol for flat species). These facts, coupled with the
findings in the present study, suggest that self-cooperation of
Au surface and functional reactants may open new possibilities
for development of new catalytic procedures for other key
transformations.
1
7596
dx.doi.org/10.1021/ja3066978 | J. Am. Chem. Soc. 2012, 134, 17592−17598

Journal of the American Chemical Society
Article
XANES Measurements. The X-ray absorption data at the Au L3-
edge of the samples were recorded at room temperature in
CONCLUSIONS
■
In conclusion, we have described an exceedingly efficient
approach for mild and clean hydrogenation of a wide range of
quinolines and related heteroaromatic nitrogen compounds to
the corresponding tetrahydroderivatives using supported Au
NPs. Of practical significance is that various synthetically useful
functional groups including halogens, ketones, and olefins
remain intact during quinoline hydrogenation. Quinoline
compounds, a class of well-known poisons for the traditional
noble metal-based hydrogenation catalysts, can act as
promoters for the Au catalysts, contributing to the superior
activity and selectivity achieved with these processes. We
believe this intriguing and remarkable promotional effect by
reactants may have fundamental implications for the broad field
of Au catalysis and be useful to design new powerful catalytic
systems for green and sustainable organic synthesis.
transmission mode using ion chambers or in the fluorescent mode
with silicon drift fluorescence detector at beamline BL14W1 of the
Shanghai Synchrotron Radiation Facility (SSRF), China. The station
was operated with a Si (311) double crystal monochromator. During
the measurement, the synchrotron was operated at energy of 3.5 GeV
and a current between 150 and 210 mA. The photon energy was
calibrated with standard Pt metal foil.
DRIFT Spectroscopic Measurements. The DRIFTS experi-
ments were carried out on a Nicolet Magna-IR 760 Fourier transform
spectrometer (ThermoNicolet, Madison, WI) using an MCT detector.
Prior to the chemisorption (10 vol% D /He), the sample (typically 20
2
−1
mg) was pretreated in a stream of He (35 mL·min ) at 250 °C for 2 h
and then cooled under He to 25 °C. Unless stated otherwise, infrared
(
IR) spectra were recorded against a background of the sample at the
reaction temperature under flowing He. IR spectra were recorded with
coaddition 64 scans in single-beam spectra or absorbance spectra by
−1
applying a resolution of 4 cm .
EXPERIMENTAL SECTION
ASSOCIATED CONTENT
Supporting Information
Complete experimental procedures and characterization details.
Supporting Information for this article is available free of charge
via the Internet at http://pubs.acs.org.
■
■
Preparation of HSA-TiO Support. HSA nanocrystalline TiO
*
S
2
2
powders predominantly in the anatase phase were prepared by the
hydrolysis of tetrabutyl titanate [TTBT, Ti(OBu) ]. Typically, 17.2 g
4
Ti(OBu)4 was dropwise added to 800 mL water under vigorous
stirring at room temperature, and then the mixture was further stirred
for 2 h. The precipitate obtained was filtered and washed with water.
The resultant powder was dried at 100 °C for 12 h and then calcined
in a tubular oven at 400 °C in air for 4 h. The BET surface area of the
AUTHOR INFORMATION
Corresponding Author
yongcao@fudan.edu.cn
■
2
−1
resultant material was 124 m ·g .
Preparation of Au/HSA-TiO . Using a routine deposition
2
Notes
precipitation (DP) method, 0.5 wt % Au/HSA-TiO2 catalyst was
prepared. An appropriate amount of aqueous solutions of chloroauric
The authors declare no competing financial interest.
acid (HAuCl ) was heated to 80 °C under vigorous stirring. The pH
was adjusted to 8.0 by dropwise addition of NaOH (0.2 M), and then
4
ACKNOWLEDGMENTS
The authors thank the NSF of China (21073042, 21273044),
New Century Excellent Talents in the University of China
■
1
.0 g of HSA-TiO was dispersed in the solution. The mixture was
2
stirred for 2 h at 80 °C, after which the suspension was cooled to room
temperature. Extensive washing with deionized water was then
followed until it was free of chloride ions. The samples were dried
under vacuum at room temperature for 12 h and calcined in air at 250
(
NCET-09-0305), the State Key Basic Research Program of
PRC (2009 CB623506), Foster Outstanding Students in Basic
Sciences Campaign (J1103304), and Science & Technology
Commission of Shanghai Municipality (08DZ2270500) for
financial support.
°
C for 2 h.
General Procedure for Catalytic Hydrogenation of Quino-
lines. A mixture of substrate (0.5 mmol), catalyst (metal: 1 mol %),
and toluene (3 mL) was charged into a Parr autoclave (25 mL
capacity, SS-316). Then the autoclave was sealed, and 2 MPa H was
REFERENCES
2
■
charged into it after internal air being degassed completely. The
resulting mixture was vigorously stirred (1200 rpm with a magnetic stir
bar) at a given temperature. The products were confirmed by the
(
1) (a) Augustine, R. L. Heterogeneous Catalysis for the Synthetic
Chemist; Marcel Dekker: New York, 1996. (b) Patai, S. The Chemistry
of Double-bonded Functional Groups; Wiley: Chichester, U.K., 1997.
c) Kulkarni, A.; Toeroek, B. Curr. Org. Synth. 2011, 8, 187.
2) (a) Sabatier, P. La Catalyse en Chimie Organique, Catalysis in
Organic Chemistry (translated by Reidl., E. E.); Van Norstrand:
Princeton, NJ, 1913. (b) Rideal, E. K. J. Chem. Soc. 1951, 1640.
(3) Nishimura, S. Handbook of Heterogeneous Catalytic Hydrogenation
for Organic Synthesis, Wiley: New York, 2001.
(4) (a) Blaser, H. U.; Siegrist, U.; Steiner, H.; Studer, M. In Fine
Chemicals through Heterogeneous Catalysis; Sheldon, R. A., van Bekkum,
H., Eds.; Wiley-VCH: Weinheim, Germany, 2001. (b) Blaser, H. U.;
Steiner, H.; Studer, M. ChemCatChem 2009, 1, 210.
(5) (a) Kovar, R. F.; Armond, F. E. (U.S. Air Force). U.S. Patent
3975444, 1976. (b) Suchy, M.; Winternitz, P.; Zeller, M. (Ciba-Geigy).
WO Patent 91/02278, 1991. (c) Butera, J.; Bagli, J. (American Home
Products). WO Patent 91/09023, 1991.
1
comparison of their GC retention time, mass, and H NMR
(
spectroscopy. The conversion and product selectivity were determined
by a GC-17A gas chromatograph equipped with a HP-5 column (30 m
(
×
0.25 mm) and a flame ionization detector (FID). The
tetrahydroquinoline products can be isolated by silica gel chromatog-
raphy (hexanes/ethyl acetate).
Isotope H −D Exchange Reaction. The H −D exchange
2
2
2
2
reaction was carried out in a fixed bed quartz reactor with an inner
diameter of 4 mm at 60 °C. The feed gas containing 20 vol% H , 20
2
−1
vol% D , and balancing He is at a total flow rate of 60 mL·min ,
2
5
corresponding to a gas hourly space velocity (GHSV) of 1.8 × 10 mL
−1
−1
h
gcat . Under these conditions, the H −D exchange conversions
2
2
were always kept below 15% to avoid mass-transfer limitations.
Reaction products (H , HDm and D ) were analyzed with an online
2
2
mass spectrometer (Balzers, QMS 200 Omnistar). Trace amounts of
HD formed in the experimental setup without catalyst and the product
concentrations were corrected for this. Prior to catalytic test, the
catalysts were heated in He flow (10 mL·min ) at 200 °C for 0.5 h,
followed by cooling to the desired temperature under He flow. For the
introduction of quinoline or tetrahydroquinoline to the samples, the
liquid compound (25 μmol) was injected under the He flow.
(6) (a) Schmidbaur, H. Gold: Progress in Chemistry, Biochemistry, and
Technology; Wiley: New York, 1999. (b) Astruc, D. Nanoparticles and
Catalysis; Wiley-VCH: Weinheim, Germany, 2008. (c) Jin, R.; Egusa,
S.; Scherer, N. F. J. Am. Chem. Soc. 2004, 126, 9900. (d) Yeung, C. M.
Y.; Yu, K. M. K.; Fu, Q. J.; Thompsett, D.; Petch, M. I.; Tsang, S. C. J.
Am. Chem. Soc. 2005, 127, 18010. (e) Tsunoyama, H.; Sakurai, H.;
Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 9374. (f) Li, Y.;
−1
1
7597
dx.doi.org/10.1021/ja3066978 | J. Am. Chem. Soc. 2012, 134, 17592−17598

Journal of the American Chemical Society
Article
Zaluzhna, O.; Xu, B.; Gao, Y.; Modest, J. M.; Tong, Y. Y. J. Am. Chem.
Soc. 2011, 133, 2092. (g) Wittstock, A.; Zielasek, V.; Biener, J.; Friend,
(24) Han, Y. F.; Zhong, Z.; Ramesh, K.; Chen, F.; Chen, L. J. Phys.
Chem. C 2007, 111, 3163.
C. M.; Bau
̈
mer, M. Science 2010, 327, 319.
(25) EXAFS and XANES has recently even been used to in situ
monitoring of Au-catalyzed reactions, see: Hashmi, A. S. K.;
Lothschutz, C.; Ackermann, M.; Doepp, R.; Anantharaman, S.;
Marchetti, B.; Bertagnolli, H.; Rominger, F. Chem.Eur. J. 2010, 16,
(
7) (a) Rudolph, M.; Hashmi, A. S. K. Chem. Soc. Rev. 2012, 41,
2
2
448. (b) Zhang, Y.; Cui, X.; Shi, F.; Deng, Y. Chem. Rev. 2012, 112,
467. (c) Stratakis, M.; Garcia, H. Chem. Rev. 2012, 112, 4469.
8
012.
(
2
d) Della Pina, C.; Falletta, E.; Prati, L.; Rossi, M. Chem. Soc. Rev.
008, 37, 2077. (e) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem.,
Int. Ed. 2006, 45, 7896. (g) Haruta, M. Nature 2005, 437, 1098.
f) Gartner, F.; Losse, S.; Boddien, A.; Pohl, M. M.; Denurra, S.; Junge,
H.; Beller, M. ChemSusChem 2012, 5, 530.
8) Bond, G. C.; Sermon, P. A.; Webb, G.; Buchanan, D. A.; Wells, P.
B. J. Chem. Soc., Chem. Commun. 1973, 444.
9) (a) Claus, P.; Brueckner, A.; Mohr, C.; Hofmeister, H. J. Am.
(26) Bus, E.; Miller, J. T.; van Bokhoven, J. A. J. Phys. Chem. B. 2005,
1
09, 14581.
(
27) Note that the H −D exchange rates measured for Pd or Au
(
2
2
catalysts are consistent with previously reported results in the
literature. See: (a) Benkhaled, M.; Descorme, C.; Duprez, D.;
Morin, S.; Thomazeau, C.; Uzio, D. Appl. Catal., A 2008, 346, 36.
(
(
1
b) Bus, E.; Miller, J. T.; Bokhoven, J. A. Van J. Phys. Chem. B 2005,
09, 14581.
28) (a) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40.
b) Bullock, R. M. Chem.Eur. J. 2004, 10, 2366.
29) Juarez, R.; Parker, S. F.; Concepcion, P.; Corma, A.; Garcia, H.
Chem. Sci. 2010, 1, 731.
30) (a) Shishido, T.; Hattori, H. Appl. Catal., A 1996, 146, 157.
b) Miller, J. T.; Meyers, B. L.; Modica, F. S.; Lane, G. S.; Vaarkamp,
(
Chem. Soc. 2000, 22, 11430. (b) Corma, A.; Serna, P.; Garcia, H. J. Am.
Chem. Soc. 2007, 129, 6358. (c) Corma, A.; Serna, P. Science 2006,
(
(
313, 332.
(
(
10) (a) Barrio, L.; Liu, P.; Rodriguez, J. A.; Campos-Martin, J. M.;
Fierro, J. L. G. J. Chem. Phys. 2006, 125, 164715. (b) Boronat, M.;
Concepcion, P.; Corma, A. J. Phys. Chem. C 2009, 113, 16772.
(
(
(
2
(
1
11) Ide, M. S.; Hao, B.; Neurock, M.; Davis, R. J. ACS Catal. 2012,
, 671.
12) (a) Mitchinson, A.; Nadin, A. J. Chem. Soc., Perkin Trans. 1 2000,
7, 2862. (b) Katritzky, A. R.; Rachwal, S.; Rachwal, B. Tetrahedron
996, 52, 15031. (c) Sridharan, V.; Suryavanshi, P.; Menendez, J. C.
M.; Koningsberger, D. C. J. Catal. 1993, 143, 395. (c) Benseradj, F.;
Sadi, F.; Chater, M. Appl. Catal., A 2002, 228, 135.
(
31) It is important to remark that the cleavage of H−H bond is
involved in the rate-determining step on the basis of kinetic isotope
1
effect measurements with H and D . See the Supporting Information
2
2
Chem. Rev. 2011, 111, 7157.
13) (a) Hashmi, A. S. K.; Rudolph, M.; Bats, J. W.; Frey, W.;
for details.
32) Bron, M.; Teschner, D.; Knop-Gericke, A.; Jentoft, F. C.;
(
(
Rominger, F.; Oeser, T. Chem.Eur. J. 2008, 14, 6672. (b) Hashmi, A.
Krohnert, J.; Hohmeyer, J.; Volckmar, C.; Steinhauer, B.; Schlogl, R.;
Claus, P. Phys. Chem. Chem. Phys. 2007, 9, 3559.
S. K.; Frost, T. M.; Bats, J. W. J. Am. Chem. Soc. 2001, 122, 11553.
(
14) (a) Baralt, E.; Smith, S. J.; Hurwitz, J.; Horvath, I. T.; Fish, R. H.
(33) Ferri, D.; Burgi, T.; Baiker, A. J. Phys. Chem. B 2001, 105, 3187.
J. Am. Chem. Soc. 1992, 114, 5187. (b) Pitts, M. R.; Harrison, J. R.;
Moody, C. J. J. Chem. Soc., Perkin Trans. 1 2001, 955.
(
15) (a) Wang, W. B.; Lu, S. M.; Yang, P. Y.; Han, X. W.; Zhou, Y. G.
J. Am. Chem. Soc. 2003, 125, 10536. (b) Dobereiner, G. E.; Nova, A.;
Schley, N. D.; Hazari, N.; Miller, S. J.; Eisenstein, O.; Crabtree, R. H. J.
Am. Chem. Soc. 2011, 133, 7547. (c) Lu, S. M.; Bolm, C. Adv. Synth.
Catal. 2008, 350, 1101. (d) Zhou, H.; Li, Z.; Wang, Z.; Wang, T.; Xu,
L.; He, Y.; Fan, Q.-H.; Pan, J.; Gu, L.; Chan, A. S. C. Angew. Chem., Int.
Ed. 2008, 47, 8464. (e) Wang, C.; Li, C. Q.; Wu, X. F.; Pettman, A.;
Xiao, J. L. Angew. Chem., Int. Ed. 2009, 48, 6524.
(
16) (a) Hashimoto, N.; Takahashi, Y.; Hara, T.; Shimazu, S.;
Mitsudome, T.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K. Chem. Lett.
010, 39, 832. (b) Bell, T. W.; Khasanov, A. B.; Drew, M. G. B. J. Am.
2
Chem. Soc. 2002, 124, 14092. (c) Gulyukina, N. S.; Beletskaya, I. P.
Russ. J. Org. Chem. 2010, 46, 781. (d) Zhandarev, V. V.; Goshin, M. E.;
Kazin, V. N.; Ramenskaya, L. M.; Mironov, G. S.; Shishkina, A. L. Russ.
J. Org. Chem 2006, 42, 1093. (e) Fache, F. Synlett 2004, 15, 2827.
(
17) Bissantz, C.; Dehmlow, H.; Martin, R. E.; Obst Sander, U.;
Richter, H.; Ullmer, C. (F. Hoffmann-La Roche AG, Switzerland). U.S.
Patent 581227, 2009.
(
18) The benefit of using Au/HSA-TiO catalyst becomes obvious
2
when comparing its activity with activities of larger Au NPs (with
average Au particle size, ca. 3.5 nm,) deposited on HSA-TiO and Au
2
NPs (with average Au particle size, ca. 1.9 nm) supported on
2
−1
conventional TiO (P25, 45 m g ) (Table S2, entries 1−3).
2
(
19) 0.5 wt% Pd/HSA-TiO (Pd ∼ 2 nm), 0.5 wt% Pt/HSA-TiO
2
2
(
Pt ∼ 2 nm), and 0.5 wt% Ru/HSA-TiO (Ru ∼ 2.3 nm) catalysts
2
were prepared by impregnation technique.
20) For a recent example of the importance of such ICP analysis in
(
the field of Au catalysis, see comments in: Hashmi, A. S. K.;
Lothschutz, C.; Dopp, R.; Ackermann, M.; Becker, J. D.; Rudolph, M.;
Scholz, C.; Rominger, F. Adv. Synth. Catal. 2012, 354, 133.
(
21) Sun, Y.; Fu, H.; Zhang, D.; Li, R.; Chen, H.; Li, X. Catal.
Commun. 2010, 12, 188.
22) Biller, S. A.; Misra, R. N. (E. R. Squibb and Sons, Inc., New
York). U.S. Patent 4843082, 1989.
23) Fish, R. H.; Tan, J. L.; Thormodsen, A. D. J. Org. Chem. 1984,
9, 4500.
(
(
4
1
7598
dx.doi.org/10.1021/ja3066978 | J. Am. Chem. Soc. 2012, 134, 17592−17598