Oxidative imination of toluenes catalyzed by Pd-Au/silica gel under mild reaction conditions

Article abstract of DOI:10.1039/c2cc31438j

A Pd-Au/SiO₂ catalyst was prepared for the oxidative imination of toluenes with up to 99% yields in the absence of dehydrating agents under mild conditions. Nanoparticles with a PdO layer and PdO-Au core may be the active structure for the reaction.

Full text of DOI:10.1039/c2cc31438j

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 7586–7588
www.rsc.org/chemcomm
COMMUNICATION
Oxidative imination of toluenes catalyzed by Pd–Au/silica gel under mild
reaction conditionsw
Xinjiang Cui,ab Feng Shi*a and Youquan Denga
Received 27th February 2012, Accepted 8th June 2012
DOI: 10.1039/c2cc31438j
A Pd–Au/SiO2 catalyst was prepared for the oxidative imination
of toluenes with up to 99% yields in the absence of dehydrating
agents under mild conditions. Nanoparticles with a PdO layer
and PdO–Au core may be the active structure for the reaction.
Aromatic imines are important motifs in the synthesis of many
natural and medicinal compounds with various structures and
properties.1 Commonly, the condensation of benzaldehydes
with amines is a straightforward and effective route for the
formation of imines. However, this process is usually carried out
under acidic conditions and limited by the use of dehydrating
agents or apparatus.2 Moreover, in industry, benzaldehydes are
mainly obtained as by-products from the oxidation of toluenes
to benzoic acids3 or via the chlorination–hydrolysis process.4
The rigorous reaction conditions or the use of chlorine make
these processes environmentally unfriendly and lead to high
energy consumption. Apart from the condensation reaction,
aromatic imines could also be synthesized through the oxidative
coupling of an amine with an alcohol,5 nitrobenzene reduction
and condensation with aldehyde,6 oxidative dehydrogenation of
an N,N-disubstituted amine7 and hydroamination of an alkyne,
etc.8 However, from the fundamental and practical points of
view, an ideal route for the clean synthesis of aromatic imines is
through the direct coupling of toluenes with amines by catalytic
activation of the primary C–H bonds, which remains one of the
most important and challenging subjects in the clean synthesis of
chemicals.9
Fig. 1 Toluene imination over Pd–Au/SiO2.
of benzaldehydes were difficult. Recently, Au based nano-
particles have emerged as an effective catalyst for many
oxidation reactions11 and the selective oxidation of alkanes12
has also been realized, although the activity and selectivity
were still low. Moreover, due to the lower oxophilicity and
larger electronegativity, the segregation of the metallic Au
phase could occur in an oxidizing atmosphere, which provided
a useful tool for Au–M bimetallic catalyst designation because
the catalytic behaviour of gold-containing bimetallic catalysts
could be fine-tuned by controlling Au–M structure.13 As a
successful example, the Au–Pd alloy was a good catalyst for
toluene oxidation to yield benzyl benzoate at 160 1C and
1 MPa O2.14 Meanwhile, surface Pd(O) was also an active
species in liquid phase aerobic oxidation reactions.15
Herein, we show the preparation and catalytic performance
of Pd–Au/SiO2 in the oxidative imination of toluenes with
sulfonamides and amines (Fig. 1). The catalysts were prepared
by a modified solvent thermal method. In a mixed solvent of
water and ethanol (V : V = 1 : 5), suitable amounts of poly-
vinylpyrrolidone (PVP) and cetyltrimethylammonium bromide
(CTAB) as surfactants were added and stirred until they had
dissolved completely. Then, the metal salt solution, ethyl silicate
(TEOS), and urea were added to the above solution and stirred
at 40 1C for 30 min. Subsequently, the mixture was transferred
into a Teflon-lined stainless steel autoclave and hydrothermally
treated at 140 1C for 12 h. The reaction mixture was cooled to
room temperature, centrifuged, washed with deionized water,
dried at 120 1C for 4 h and calcined at 400 1C for 4 h in air to
obtain the catalysts.
In order to realize this target, the crucial step is the controllable
oxidation of toluenes to the corresponding aldehydes because
over-oxidation to the acid will result in the formation of
ammonium salt with amine. With respect to this trans-
formation, several catalytic systems, such as Cu, Co, V–Ag,
Mn, Ce and N–OH imides, etc. have been reported in the past
decades.10 Unfortunately, the rigorous reaction conditions
used in the above systems, which are normally performed at
>150 1C, caused low selectivity and highly selective syntheses
a Centre for Green Chemistry and Catalysis, Lanzhou Institute of
Chemical Physics, CAS Middle Tian Road, 18, Lanzhou, China.
E-mail: fshi@licp.cas.cn; Fax: (+86)-931-8277088
b Graduate School of the Chinese Academy of Sciences, Beijing,
100049, China
w Electronic supplementary information (ESI) available: Catalyst
preparation procedure and characterization details. See DOI:
10.1039/c2cc31438j
TEM characterization showed that the co-addition of PVP,
CTAB and Pd was necessary to gain uniformly dispersed nano-
particles. The average particle sizes of samples Pd/SiO2–P–C,
Pd–Au/SiO2–P–C and Pd–Au/SiO2–P–C–1 were 4, 7 and 7 nm,
c
7586 Chem. Commun., 2012, 48, 7586–7588
This journal is The Royal Society of Chemistry 2012

View Article Online
Table 1 Catalyst screening for oxidative imination of toluenea
Pd/Au
(mol/mol)
Conversion
(%)b
TON/
TOFc
Entry
Catalyst
1
2
3
4
5
6
7
Pd/SiO2–P–C
Au/SiO2–P–C
Pd–Au/SiO2
Pd–Au/SiO2–P
Pd–Au/SiO2–C
Pd–Au/SiO2–P–C
Pd–Au/
0.42/0
0/0.21
35
5
13
4
30
83/3.5
24/1
21/0.9
6/0.3
48/2
0.42/0.21
0.41/0.21
0.41/0.21
0.42/0.21
0.16/0.32
92/(86d/83e)
30
148/6.2
63/2.6
SiO2–P–C–1
a
1 mmol sulfonamide, 5 mL toluene, 50 mg catalyst, 1 MPa O2,
b
120 1C, 24 h. Sulfonamide conversion determined by GC-FID.
TON = mmol sulfonamide converted per mol Pd + Au; TOF =
Fig. 2 (a) HR-TEM image of Pd–Au/SiO2–P–C (scale bar = 2 nm),
(b) HAADF image of Pd–Au/SiO2–P–C, (c) HAADF-STEM image of
Pd–Au/SiO2–P–C, (d) cross-sectional compositional line profiles of the
line in (c).
c
d
mmol sulfonamide converted per mol Pd + Au per hour. Isolated
e
yields. Reused at the 5th run.
Fig. S1a, e and g.w Larger particles formed (20–60 nm) in
Au/SiO2–P–C, Pd–Au/SiO2–P, Pd–Au/SiO2–C and Pd–Au/SiO2,
Fig. S1b–d and f.w Interestingly, a unique PdO/PdO–Au
structure formed in Pd–Au/SiO2–P–C, Fig. 2a, which contains
a core of PdO (101, d = 2.64 A) and Au (d = 2.20 A)
surrounded with PdO (101). The lattice distance of Au is
smaller than Au (111, d = 2.35 A) but bigger than Au (200,
d = 2.04 A), which suggests its interaction with PdO.
HR-TEM imaging revealed that the PdO–Au core diameter
was B5 nm and the PdO shell thickness was B1 nm, Fig. 2a.
HAADF-STEM and EDX spectroscopy line-scan characteri-
zation confirmed the observation from HR-TEM images, i.e.
the particle size was 6–8 nm, Fig. 2b and Fig. S2.w The EDX
line-scan over a randomly selected nanoparticle also indicated
the formation of a Au–PdO core and PdO shell, Fig. 2c and d
and Fig. S3.w The Au–PdO core of this particle was B5 nm
and the PdO shell was B2 nm. However, if more Au was
added to prepare a catalyst with a higher Au : Pd ratio, i.e.
sample Pd–Au/SiO2–P–C–1, the PdO shell disappeared and a
mixed structure of Au and PdO formed, Fig. S4e–h.w XPS
characterization showed that the binding energy of Pd3d5/2 in
Pd–Au/SiO2–P–C, i.e. 336.98 eV, was 0.14 eV, lower than that
of Pd/SiO2–P–C, i.e. 337.12 eV, which suggested the formation
of Au–Pd alloy in Pd–Au/SiO2–P–C, Fig. S8.w16
Powder X-ray diffraction (XRD) characterization revealed
the crystal structure of PdO and Au, Fig. S5.w Interestingly,
the co-addition of Pd and Au resulted in significant changes
in the diffraction peaks according to the molar ratio of Pd to
Au. When Pd : Au E 2 : 1, the major changes from the XRD
patterns were the shifting of the diffraction peak of Au (111)
from 38.11 to 38.81 and the shifting of the Au (200) diffraction
peak from 44.41 to 45.11. This might correspond to the lattice
distance of Au (d = 2.20 A) in the HR-TEM image, Fig. 2a. In
reverse, if Au : Pd E 2 : 1, the diffraction peaks of Pd (101)
disappeared, and Au (111) and Au (200) diffraction peaks
appeared again. It is also possible that the diffraction peak of
PdO is overlapped with Au (111).
Pd–Au/SiO2–C was used, in which only PVP or CTAB itself
was used as the surfactant in catalyst preparation, entries 3–5.
By applying Pd–Au/SiO2–P–C, the conversion reached 92%
with 86% isolated yield, entry 6. However, Pd–Au/
SiO2–P–C–1 showed lower catalytic activity and only B30%
conversion of benzenesulfonamide was obtained, entry 7. The
TON value of Pd–Au/SiO2–P–C reached 148 but only 6,
48 and 63 TON values were obtained for Pd–Au/SiO2–P,
Pd–Au/SiO2–C and Pd–Au/SiO2–P–C–1. By tracing the reac-
tion with GC-MS, benzaldehyde was observed as the inter-
mediate, which was also the major by-product, and no phenol
by-product formed. Notably, the catalyst could be easily
separated, and 83% isolated yield was maintained when it
was used for the 5th time, entry 6 and Table S1.w ICP-AES
measuring of the filtered solution showed that the Pd and Au
concentrations were out of the detection limits and no further
reaction could be observed if the catalyst was filtered off and
another portion of benzenesulfonamide added, Tables S1 and S2.w
Moreover, similar Au and Pd loadings were observed in the
used catalyst if comparing with the fresh sample, Table S2.w
Thus the catalytic activity was derived from Pd–Au/SiO2–P–C
but not leached Pd and Au in the solution. However, HRTEM
pictures of the used catalyst indicated its restructuring,
although no aggregation occurred, Fig. S6.w XRD characteri-
zation indicated the presence of PdO crystal and thus PdO was
not reduced, Fig. S7.w
The scope screening of the imination reaction of toluenes
was further investigated using Pd–Au/SiO2–P–C as catalyst,
Table 2. For the reaction of toluene with benzenesulfonamide,
86% isolated yield was obtained (3a). The reactions of toluene
with other sulfonamides were tested. It was clear that electron-
rich as well as electron-poor groups were all tolerated (3b–3e).
As a typical aliphatic sulfonamide, 88% yield was obtained
using methanesulfonamide as starting material (3f). In addi-
tion, 74–89% yields were obtained with toluenes and aromatic
sulfonamides contain different substituent groups as starting
materials (3g–3l).
Subsequently, the activities of these catalysts were tested,
Table 1. Only 35% and 5% conversions of benzenesulfonamide
were obtained if Pd/SiO2–P–C and Au/SiO2–P–C were employed,
entries 1 and 2. Similar results were obtained if Pd–Au/SiO2–P or
With the addition of a catalytic amount of K2CO3, the
imination of toluenes with amines could also be realized with
Pd–Au/SiO2–P–C as catalyst, Table 3. The catalyst system had
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 7586–7588 7587

View Article Online
Table 2 Oxidative imination of toluenes with sulfonamidesa,b
(e) X. G. Zhang, Q. Q. Min, C. Y. He and H. B. Zhou, Chem.
Commun., 2010, 46, 8029; (f) J. Barluenga, A. Jimenez-
Aquino, F. Aznar and C. Valdes, J. Am. Chem. Soc., 2009,
131, 4031.
2 (a) R. W. Layer, Chem. Rev., 1963, 63, 489; (b) X. F. Wu,
C. Vovard-Le Bray, L. Bechki and C. Darcel, Tetrahedron, 2009,
65, 7380.
3 H. V. Borgaonkar, S. R. Raverkar and S. B. Chandalia, Ind. Eng.
Chem. Prod. Res. Dev., 1984, 23, 455.
4 A. M. Potapov and S. R. Rafikov, Bull. Acad. Sci. USSR, Div.
Chem. Sci. (Engl. Transl.), 1982, 31, 1188.
5 B. Gnanaprakasam, J. Zhang and D. Milstein, Angew. Chem.,
Int. Ed., 2010, 49, 1468.
6 A. L. Korich and T. S. Hughes, Synlett, 2007, 2602.
7 K. C. Nicolaou, C. J. N. Mathison and T. Montagnon, Angew.
Chem., Int. Ed., 2003, 42, 4077.
8 T. E. Muller, K. C. Hultzsch, M. Yus, F. Foubelo and M. Tada,
Chem. Rev., 2008, 108, 3795.
a
1 mmol sulfonamide, 3 mL toluene, 50 mg Pd–Au/SiO2–P–C, 1 MPa O2,
b
9 (a) Z. X. Yu and Q. A. Li, Angew. Chem., Int. Ed., 2011, 50, 2144;
(b) F. W. Patureau and F. Glorius, Angew. Chem., Int. Ed., 2011,
50, 1977; (c) W. Kroutil, J. H. Schrittwieser, V. Resch, J. H. Sattler,
W. D. Lienhart, K. Durchschein, A. Winkler, K. Gruber and
P. Macheroux, Angew. Chem., Int. Ed., 2011, 50, 1068; (d) J. Q. Yu,
X. S. Wang, Y. Lu and H. X. Dai, J. Am. Chem. Soc., 2010,
132, 12203; (e) T. Lecourt, M. Boultadakis-Arapinis, P. Lemoine,
S. Turcaud and L. Micouin, J. Am. Chem. Soc., 2010, 132, 15477;
(f) M. T. Green and J. Rittle, Science, 2010, 330, 933;
(g) P. E. Floreancig and L. Liu, Angew. Chem., Int. Ed., 2010,
49, 3069; (h) S. Caddick, V. Chudasama and R. J. Fitzmaurice,
Nat. Chem., 2010, 2, 592–596; (i) R. G. Bergman, Nature, 2007,
446, 391.
c
120 1C, 24 h. Isolated yields. 135 1C, 1.5 MPa O2.
Table 3 Oxidative imination of toluenes with aminesa,b
10 (a) W. P. Ding, J. G. Lv, Y. Shen, L. M. Peng and X. F. Guo,
Chem. Commun., 2010, 46, 5909; (b) M. W. Xue, J. N. Yu, H. Chen
and J. Y. Shen, Catal. Lett., 2009, 128, 373; (c) X. Q. Li, J. Xu,
F. Wang, J. Gao, L. P. Zhou and G. Y. Yang, Catal. Lett., 2006,
108, 137; (d) Y. Ishii and S. Sakaguchi, Catal. Today, 2006,
117, 105; (e) R. L. Brutchey, I. J. Drake, A. T. Bell and
T. D. Tilley, Chem. Commun., 2005, 3736.
11 (a) A. Corma, A. Leyva-Perez and M. J. Sabater, Chem. Rev.,
2011, 111, 1657; (b) L. Prati, A. Villa and G. M. Veith, Angew.
Chem., Int. Ed., 2010, 49, 4499; (c) Z. G. Li, C. Brouwer and C. He,
Chem. Rev., 2008, 108, 3239; (d) R. M. Lambert, M. Turner,
V. B. Golovko, O. P. H. Vaughan, P. Abdulkin, A. Berenguer-
Murcia, M. S. Tikhov and B. F. G. Johnson, Nature, 2008,
454, 981; (e) M. C. Daniel and D. Astruc, Chem. Rev., 2004,
104, 293.
12 (a) G. J. Hutchings, N. F. Dummer, S. Bawaked, J. Hayward and
R. Jenkins, Catal. Today, 2011, 160, 50; (b) B. M. Weckhuysen and
B. P. C. Hereijgers, J. Catal., 2010, 270, 16; (c) C. Della Pina,
E. Falletta, L. Prati and M. Rossi, Chem. Soc. Rev., 2008, 37,
2077; (d) A. Corma and H. Garcia, Chem. Soc. Rev., 2008,
37, 2096; (e) R. Zhao, D. Ji, G. M. Lv, G. Qian, L. Yan,
X. L. Wang and J. S. Suo, Chem. Commun., 2004, 904;
(f) M. D. Hughes, Y. J. Xu, P. Jenkins, P. McMorn, P. Landon,
D. I. Enache, A. F. Carley, G. A. Attard, G. J. Hutchings, F. King,
E. H. Stitt, P. Johnston, K. Griffin and C. J. Kiely, Nature, 2005,
437, 1132.
a
1 mmol amine, 3 mL toluene, 50 mg Pd–Au/SiO2–P–C, 10 mol%
b
c
K2CO3, 1 MPa O2, 120 1C, 24 h. GC yields. 140 1C, 2 MPa O2.
a nice functional group tolerance on the structure of the amine
and 74–99% yields were obtained for the amination reactions
of toluene (5a–5h). The variation of substituent groups on the
aromatic ring of the toluene had a remarkable effect on the
coupling reaction. First, 84% and 87% yields were obtained
for the reactions of p-xylene and p-chlorotoluene with cyclo-
hexyl amine (5i, 5j). The incorporation of an electron-
withdrawing nitro group resulted in a much lower reactivity
to yield the imination product although higher temperature
and oxygen pressure were used (5k).
13 (a) T. Zhang, X. Y. Liu, A. Q. Wang, L. Li, C. Y. Mou and
J. F. Lee, J. Catal., 2011, 278, 288; (b) D. I. Enache, J. K. Edwards,
P. Landon, B. Solsona-Espriu, A. F. Carley, A. A. Herzing,
M. Watanabe, C. J. Kiely, D. W. Knight and G. J. Hutchings,
Science, 2006, 311, 362.
14 G. J. Hutchings, L. Kesavan, R. Tiruvalam, M. H. Ab Rahim,
M. I. bin Saiman, D. I. Enache, R. L. Jenkins, N. Dimitratos,
J. A. Lopez-Sanchez, S. H. Taylor, D. W. Knight and C. J. Kiely,
Science, 2011, 331, 195.
15 (a) S. E. J. Hackett, R. M. Brydson, M. H. Gass, I. Harvey,
A. D. Newman, K. Wilson and A. F. Lee, Angew. Chem., Int. Ed.,
2007, 46, 8593; (b) A. F. Lee, C. V. Ellis, J. N. Naughton,
M. A. Newton, C. M. A. Parlett and K. Wilson, J. Am. Chem.
Soc., 2011, 133, 5724; (c) A. F. Lee, S. F. J. Hackett, J. S. J.
Hargreaves and K. Wilson, Green Chem., 2006, 8, 549.
16 (a) A. F. Lee, C. V. Ellis, K. Wilson and N. S. Hondow, Catal.
Today, 2010, 157, 243; (b) C. W. Yi, K. Luo, T. Wei and
D. W. Goodman, J. Phys. Chem. B, 2005, 109, 18535.
In conclusion, for the first time, the oxidative imination of
toluenes was realized using a heterogeneous Au–Pd bimetallic
catalyst. This catalyst could be easily recovered and reused
several times without deactivation. It provides a successful
example to improve fine chemical synthesis, and extends the
application of heterogeneous catalysis.
We thank the National Natural Science Foundation of
China (21073208) for financial support.
Notes and references
1 (a) J. M. Chong and T. R. Wu, J. Am. Chem. Soc., 2006, 128, 9646;
(b) A. Corma, A. Leyva-Perez, J. R. Cabrero-Antonino and
A. Cantin, J. Org. Chem., 2010, 75, 7769; (c) S. E. Denmark and
T. W. Wilson, Nat. Chem., 2010, 2, 937; (d) T. Mita, J. Y. Chen,
M. Sugawara and Y. Sato, Angew. Chem., Int. Ed., 2011, 50, 1393;
c
7588 Chem. Commun., 2012, 48, 7586–7588
This journal is The Royal Society of Chemistry 2012