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
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 4390 –4393