Gold supported on hydroxyapatite as a versatile multifunctional catalyst for the direct tandem synthesis of imines and oximes

Full text of DOI:10.1002/anie.200900802

Communications
DOI: 10.1002/anie.200900802
Heterogeneous Tandem Catalysis
Gold Supported on Hydroxyapatite as a Versatile Multifunctional
Catalyst for the Direct Tandem Synthesis of Imines and Oximes**
Hao Sun, Fang-Zheng Su, Ji Ni, Yong Cao,* He-Yong He, and Kang-Nian Fan
Tandem catalysis that enables one-pot multistep reactions
holds great potential for increasing the efficiency of chemical
synthesis.[1] Performing multiple reactions simultaneously in a
single reaction vessel offers possibilities for reduced waste
and increased safety as well as manipulation of equilibria.[2]
Although many transition-metal-based catalysts have been
developed, the focus has been largely on homogeneous rather
than heterogeneous catalysis.[3] Indeed, several heterogene-
ous multifunctional processes involving different synthetic
transformations in the context of one-pot multistep catalysis
have been reported.[4] However, relevant reports are limited
to a few heterogenized homogeneous metal complexes, which
generally suffer from product contamination and limited
recyclability.[5] In general, truly heterogeneous systems capa-
ble of catalyzing two or more mechanistically different
reactions in tandem are scarce.[6] Moreover, most of these
systems require the use of large excesses of reagents or
expensive activators and are not effective for the construction
of complex organic compounds.[7] In this respect, the develop-
ment of a single heterogeneous multitask catalyst that is
highly effective for sustainable tandem synthesis still remains
a great challenge.
Imines and oximes are versatile synthetic intermediates
for dyes, fragrances, pharmaceuticals, fungicides, and agricul-
tural chemicals.[8] Although several oxidation procedures that
use stoichiometric amounts of reagents for the synthesis of
imines and oximes from amines are known, only a few
catalytic procedures have been reported. A number of
ruthenium or palladium complexes have been used for the
oxidation of amines with dioxygen,[9] iodosylbenzene,[10] or
persulfate ions[11] as oxidants. However, these systems are not
generally useful because of their low turnover numbers
(TONs) and frequencies (TOFs), the formation of significant
amounts of by-products, severe deactivation of the catalysts,
and narrow applicability to a limited number of amines.
Recently, a tandem catalytic process to produce imines
directly from alcohols and amines with manganese octahedral
molecular sieves (OMS-2) was reported.[6c] However, the
reaction requires elevated temperatures (above 1008C) and
prolonged reaction times (up to 24 h) to achieve high yields of
the products (greater than 90%).
Application of supported gold nanoparticles as catalysts
for organic transformations is of growing interest.[12]
Although catalytic tandem reactions for efficient organic
synthesis are recognized as an atom-economical route to
sustainable chemistry, the possibilities offered by supported
gold catalysts have scarcely been explored to date.[13] Very
recently, we reported the design of a versatile bifunctional Au/
b-Ga2O3 catalyst that is highly effective for direct one-step
oxidative esterification of a range of alcohols, aldehydes, and
acetals.[14] The benefit of utilizing gallium oxide as a support
has been attributed to a unique cooperative interaction
between gold and surface Lewis acidic sites on the b-Ga2O3
support. Herein, we report that gold nanoparticles supported
on hydroxyapatite[15] possessing a unique amphoteric charac-
ter are efficient multifunctional catalysts for rapid direct
synthesis of imines and oximes from amines under mild
conditions by a facile tandem oxidation–condensation path-
way. Prior to this work, there have been no examples in the
literature of recyclable multitask gold catalysts for the direct
tandem synthesis of imines or oximes.
Hydroxyapatite (HAP, Ca10(PO4)6(OH)2) was synthesized
according to literature procedures.[16] Surface acidity and
basicity measurements by NH3- and CO2-TPD and pyridine
adsorption coupled with FTIR measurements (see details in
the Supporting Information, Table S1, Figure S1 and S2;
TPD = temperature-programmed desorption) reveal an
abundance of basic surface sites of medium strength and a
limited number of Lewis acid sites in the as-synthesized HAP
material. When gold nanoparticles were deposited onto the
HAP (see Supporting Information for the detailed prepara-
tion procedure), X-ray photoelectron spectroscopy (XPS) of
the Au 4f7/2 core level showed a contribution from metallic
Au0 at a binding energy of 83.9 eV. Almost identical XRD
patterns were obtained for Au/HAP and HAP, indicating a
good maintenance of the crystallinity of hydroxyapatite. TEM
analysis of the Au/HAP catalyst reveals randomly dispersed
particles, and energy-dispersive X-ray (EDX) analyses con-
firm that the particles corresponded to gold with an average
diameter of about 3.6 nm. The introduction of gold nano-
particles significantly modifies the acid–base distribution
(Table S1 in the Supporting Information), increasing the
overall number of both acidic and basic sites on HAP.
Initially, benzyl alcohol and aniline (1:1 molar ratio) were
used as model substrates to study the activity of various
catalysts, including previously reported solid catalysts
(Table 1). Among various catalysts tested, Au/HAP showed
[*] H. Sun, F. Z. Su, J. Ni, Prof. Y. Cao, Prof. H. Y. He, Prof. K. N. Fan
Shanghai Key Laboratory of Molecular Catalysis and
Innovative Materials, Department of Chemistry, Fudan University
Shanghai 200433 (China)
Fax: (+86)21-6564-3774
E-mail: yongcao@fudan.edu.cn
[**] The authors thank the NSF of China (20633030, 20721063, and
20873026), the State Key Basic Research Program of PRC
(2003CB615807), Science & Technology Commission of Shanghai
Municipality (08DZ2270500, 07QH14003), and Shanghai Education
Committee (06SG03) for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900802.
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Chemie
the highest catalytic activity for the direct tandem synthesis of
benzylimine (Table 1, entry 1). Compared to palladium or
ruthenium nanoparticles supported on HAP[17] as reference
catalysts, gold is far superior to other noble metals for the
unique in terms of its ability to significantly accelerate the
direct condensation of benzaldehyde with aniline under mild
conditions (Table S2 in the Supporting Information), which is
a process known to be catalyzed by Lewis acids.[21] On the
basis of these observations, a delicate cooperation between
metallic gold and the acid/base sites of the hydroxyapatite
surface may play a key role in determining the efficiency and
compatibility of the Au/HAP catalyst for not only the rate-
determining alcohol oxidation but for the subsequent con-
densation step of the tandem reaction (Scheme S1 in the
Supporting Information). The later process would be cata-
lyzed by the acid sites on Au/HAP. This scenario is reinforced
by the fact that the dehydrative condensation reaction was
strongly inhibited when Au/HAP was impregnated with a
basic oxide such as MgO.
The present system was applicable to larger-scale produc-
tion (10 mmol for both substrates, heating at reflux, bath
temperature 1008C) in the absence of solvent, and the
corresponding imine could be isolated in 95% yield after
5 h (Table 1, entry 10). The efficient tandem oxidation of
benzyl alcohol catalyzed by Au/HAP in the presence of
aniline, together with the synthetic utility of the product
imines,[22] prompted us to develop a one-pot three-component
approach as an alternative catalytic route for the tandem
synthesis of a-aminophosphonates. These compounds are
currently obtained by a number of multistep synthetic
approaches[23] involving nucleophilic addition of phosphites
to imines. Interestingly, when a mixture of benzyl alcohol
(10 mmol), aniline (10 mmol), and dimethyl phosphite
(10 mmol) was stirred under dioxygen (1 atm) at 1008C for
6 h in the presence of multifunctional Au/HAP under solvent-
free conditions, the corresponding a-aminophosphonate
product was isolated in 86% yield (Scheme 1). Given that
Table 1: Direct tandem synthesis of benzylimine from benzyl alcohol and
aniline by various catalysts.[a]
Entry
Catalyst
Conv. [%]
Sel. [%]
Aldehyde
Imine
1
2
3
4
5
6
7
8
Au/HAP
99
49
15
53
10
36
1
13
8
99
n.r.
n.r.
>99
99
90
74
99
88[c]
99
97
58
96
–
<1
<1
10
25
<1
10
<1
3
Ru/HAP[b]
Pd/HAP[b]
[b]
Au/CeO2
Au/Fe2O3
Au/TiO2
Au/C
Au/b-Ga2O3
K-OMS-2[d]
Au/HAP[e]
none
[b]
9
42
4
–
10
11
12
HAP
–
–
[a] Reaction conditions: alcohol (1 mmol), amine (1 mmol), catalyst
(metal: 1 mol%), toluene (10 mL), 608C, O2 bubbling (20 mLminÀ1),
3 h. Selectivity based on the conversion of alcohol to imine. Main by-
product was aldehyde. n.r.=no reaction. [b] Au/b-Ga2O3,[14] Ru/HAP,[17a]
Pd/HAP,[17b] and Au/CeO2
were prepared according to literature
[18]
procedures. [c] As noted by Grirrane et al.,[19] in the case of Au/TiO2,
aniline also underwent the oxidative reaction, leading to the formation of
azobenzene. We confirmed that in the present case, the yield of the azo
compounds is ca. 23% based on aniline conversion. [d] K-OMS-2
(50 mg) prepared according to reference [6c]. [e] Reaction conditions:
10 mmol each substrate, Au/HAP (Au: 0.23 mol%), no solvent, 1008C,
O2 bubbling (20 mLminÀ1), 5 h.
tandem oxidation (Table 1, entries 2, 3). The tandem reaction
hardly proceeded in the absence of catalysts (Table 1,
entry 11) or in the presence of parent HAP (Table 1,
entry 12). In the presence of Au/CeO2,[18] the selectivity for
benzylimine was very low, and benzaldehyde was obtained as
a main by-product (Table 1, entry 4). Gold supported on b-
Ga2O3 (Au/b-Ga2O3) and gold reference catalysts (Au/TiO2,
Au/Fe2O3, and Au/C) supplied by the World Gold Council
were not effective for the tandem oxidation reaction (Table 1,
entries 5–8). In the case of Au/TiO2, the undesired formation
of appreciable amounts of azobenzene as a consequence of
the direct aerobic oxidation of aniline was observed.[19]
Previously reported solid catalysts such as K-OMS-2[6c]
showed only very little activity under otherwise identical
conditions (Table 1, entry 9).
To gain insight into the origin of the enhanced activity
achieved using HAP as support, we examined the course of
the tandem conversion of benzyl alcohol over time along with
the corresponding oxidation in the absence of aniline. The
time course plot for the alcohol oxidation alone over Au/HAP
closely resembles that for the tandem reaction (Figure S6 in
the Supporting Information). Conversely, a significantly
retarded conversion has been identified for other supported
gold systems.[20] Furthermore, Au/HAP was also found to be
Scheme 1. Three components form a-aminophosphonates with Au/
HAP in a solvent-free system. The product is isolated in 86% yield.
a-aminophosphonates are prominent core scaffolds in bio-
logically active compounds,[24] this atom-economical route
opens up new possibilities for the application of supported
gold catalysts to the green synthesis of pharmaceutically
important compounds.
To demonstrate the general applicability of the Au/HAP
catalyst for direct imine synthesis and the scope of the
process, various alcohols and amines were investigated. As
depicted in Table 2, various aromatic alcohols react with
aniline to give the desired products in excellent yields. Benzyl
alcohols with electron-donating groups react smoothly
(Table 2, entries 2, 3), while substitution with electron-with-
drawing groups on the benzene ring decreases the reactivity
(Table 2, entries 4, 5). High yields are still generally obtained
in the latter case except for with 4-nitrobenzyl alcohol, which
contains a strongly electron-withdrawing group (Table 2,
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Table 2: Tandem synthesis of imines from various alcohols and amines
using Au/HAP.[a]
alcohol oxidation is the rate-determining step in the direct
tandem synthesis of imines from alcohols and amines.
It is essential to verify that the observed catalytic
conversion is caused by solid Au/HAP rather than leached
gold species; to this end, control experiments were carried
out. Catalytic transformation of benzyl alcohol and aniline
was performed under the conditions in Table 2. After
completion of the reaction, Au/HAP was removed by
filtration. When further substrate was added to the filtrate
and the mixture was again heated at 608C, no further
formation of benzylimine was detected. Inductively coupled
plasma (ICP) analysis showed that the amount of Au species
present in the filtrate was below the detection limit (2.5 ppb).
TEM and XPS analysis of the surface of the catalyst reveal no
change in the Au dispersion before and after use (see Table S3
and Figure S7 in the Supporting Information). Furthermore,
the Au/HAP catalyst could be reused with almost the same
catalytic performance (Table 2, entry 17). These findings rule
out any contribution to the observed catalytic conversion
from soluble leached gold species.
Entry
R1
R2
t [h]
Conv.
[%][b]
Sel.
[%][b]
1
2
3
4
Ph
4-MeC6H4
4-MeOC6H4
4-ClC6H4
4-NO2C6H4
cinnamyl
n-heptyl
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
3
4
99
98
97
99
76
95
75
99
97
99
99
99
95
99
95
95
95
>99
97
95
97
97
96
99
99
98
99
90
98
97
99
97
99
98
4.5
7
12
7.5
16
3
2.5
2.5
9
1.5
1
5[c]
6
7[d]
8
PhCH2
4-MeC6H4
4-MeOC6H4
4-NO2C6H4
n-hexyl
n-butyl
cyclohexyl
(+)-a-methylbenzyl
n-butyl
Ph
9
10
11[d]
12
13
14
15
16[e]
17[f]
3
2.5
3
4-HOCH2C6H4
Ph
The Au/HAP catalyst was also applied to the tandem
synthesis of oximes from alcohols and hydroxylamine.
Various kinds of alcohols, including aromatic (Table 3,
entries 1–3), unsaturated (Table 3, entry 4), and heterocyclic
(Table 3, entry 5) alcohols, could be converted into the
3
[a] Reaction conditions: alcohol (1 mmol), amine (1 mmol), Au/HAP
(Au: 1 mol%), toluene (10 mL), 608C, O2 bubbling (20 mLminÀ1).
[b] Determined by GC using 1,3,5-trimethylbenzene as an internal
standard and confirmed by GC-MS; conversion (%) and selectivity (%)
based on the conversion of alcohols to imines. [c] 0.5 mmol substrates
(Au: 2 mol%). [d] 0.25 mmol substrates (Au: 4 mol%). [e] 1 mmol
alcohol (Au: 1 mol%), 2.5 mmol butylamine. [f] Result for the fifth run
using the same catalyst material.
Table 3: Tandem synthesis of oximes by using Au/HAP.[a]
Entry
R
t [h]
Yield [%][b]
entry 5), although longer reaction time is needed. Au/HAP
also displays high activity in the tandem oxidation of allylic
alcohols for imine synthesis. For example, cinnamyl alcohol
was selectively oxidized to the corresponding imine with 95%
conversion in 7.5 h (Table 2, entry 6). Furthermore, unacti-
vated aliphatic alcohols, which are notoriously difficult to
oxidize, could also be smoothly oxidized to the corresponding
imines in high yield (Table 2, entry 7). However, the catalytic
activity for most aliphatic alcohols was significantly lower
than for benzylic and allylic alcohols (Table 2, entries 2–7).
Furthermore, the reaction also proceeds successfully with
other structurally diverse amines. The use of (+)-a-methyl-
benzylamine gave the expected amine with no loss of optical
activity ([a]D = 60.1 degcm3 gÀ1 dmÀ1 (c = 0.5 gcmÀ3, EtOH);
1
2
3
Ph
3.5
2.5
5
6.5
5
95(89)
97
92
94
93
4-MeOC6H4
4-ClC6H4
cinnamyl
furfuryl
4
5[c]
[a] Reaction conditions: alcohol (0.5 mmol), hydroxylamine (0.5 mmol),
Au/HAP (Au: 2 mol%), solvent (10 mL), 908C, O2 bubbling
(20 mLminÀ1). The number in the parenthesis is the yield for the third
run using the same catalyst under identical reaction conditions.
[b] Determined by GC using 1,3,5-trimethylbenzene as an internal
standard and confirmed by GC-MS. [c] 0.25 mmol substrates (Au:
4 mol%).
corresponding oximes in high yields in the presence of only
one equivalent hydroxylamine. This reaction is also catalyzed
heterogeneously, and the Au/HAP catalyst could be reused
without significant loss of catalytic activity. Although the
tandem one-pot synthesis of oximes is very useful, stoichio-
metric amounts of reagents such as Na2SO4 (10 equiv with
respect to substrates) and ZnO (2 equiv) have generally been
used with an excess of hydroxylamine (more than 3 equiv).[27]
While Taylor and co-workers have reported an efficient one-
step synthesis of oximes from alcohols in the presence of
activated manganese dioxide,[28] this system only gives high
yields of oximes in the presence of an additive, such as 4 ꢀ
molecular sieves as the dehydrating agent. In contrast, the
present system has the following significant advantages: 1) It
needs only one equivalent of hydroxylamine. 2) It functions
[a]D[25] = 54.4 degcm3 gÀ1 dmÀ1
(c = 0.5 gcmÀ3,
EtOH);
Table 2, entry 15). Diols can be converted efficiently into
bis(imines) (Table 2, entry 16). The results in Table 2
(entries 9, 10) also indicate that the presence of electron-
donating substituents on the benzene rings have a positive
effect on the tandem oxidation–imine formation reaction.
Moreover, a surprising increase in formation rate could be
achieved when aliphatic amines were used instead of aniline
(Table 2, entries 12, 13). For example, when the reaction
proceeds with n-butylamine and benzyl alcohol, it is complete
after only 1 h (Table 2, entry 13). These results suggest that
the high basicity of alky amines can provide an environment
that facilitates the proton abstraction step in the oxidation of
alcohols,[26] thus further confirming that the preliminary
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4) The Au/HAP catalyst can be reused. 5) The Au/HAP
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In conclusion, we report herein the noteworthy features of
the multitask Au/HAP reagent for the heterogeneously
catalyzed tandem oxidation of alcohols and amines to
imines (or oximes) as well as the one-pot three-component
syntheses of a-aminophosphonates in solvent-free conditions.
Further extensions of this multifunctional catalyst to a variety
of functional transformations, as well as studies to clarify
other factors that control tandem reactions, are currently
under investigation.
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Published online: May 11, 2009
Keywords: gold · heterogeneous catalysis · hydroxyapatite ·
.
imines · oximes
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