Aerobic oxidation of amines to imines catalyzed by bulk gold powder and by alumina-supported gold

Article abstract of DOI:10.1016/j.jcat.2008.08.012

Both bulk gold powder (～50 μm particle size) and alumina-supported gold (50-150 nm) are highly active catalysts for the aerobic oxidative dehydrogenation of amines (CH-NH) to imines (C{double bond, long}N) under the mild conditions of 1 atm O₂ and 100 °C. Reactions using the 5% Au/Al₂O₃ catalyst make efficient use of the gold metal and offer a practical synthesis of imines from amines. These studies add to the growing list of reactions that are catalyzed by bulk gold metal.

Full text of DOI:10.1016/j.jcat.2008.08.012

Journal of Catalysis 260 (2008) 1–6
Contents lists available at ScienceDirect
Journal of Catalysis
www.elsevier.com/locate/jcat
Aerobic oxidation of amines to imines catalyzed by bulk gold powder
and by alumina-supported gold
Bolin Zhu a, Mihaela Lazar b, Brian G. Trewyn a, Robert J. Angelici a,∗
a
Ames Laboratory and Department of Chemistry, Iowa State University, Ames, IA 50011-3111, USA
National Institute for Research and Development of Isotopic and Molecular Technologies, 65-103 Donath Street, 400293 Cluj Napoca, Romania
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Both bulk gold powder (∼50 μm particle size) and alumina-supported gold (50–150 nm) are highly active
catalysts for the aerobic oxidative dehydrogenation of amines (CH–NH) to imines (C=N) under the mild
Received 10 June 2008
Revised 14 August 2008
Accepted 21 August 2008
Available online 2 October 2008
◦
conditions of 1 atm O2 and 100 C. Reactions using the 5% Au/Al2O3 catalyst make efficient use of the
gold metal and offer a practical synthesis of imines from amines. These studies add to the growing list
of reactions that are catalyzed by bulk gold metal.
© 2008 Elsevier Inc. All rights reserved.
Keywords:
Amine
Gold
Heterogeneous catalysis
Imine
Oxidation
Oxygen
1. Introduction
bulk gold powder-catalyzed aerobic oxidation of secondary amines
to imines (Eq. (4)) [10]:
Although catalysis by supported nano-sized gold metal particles
(<5 nm) is a field of great interest [1–6], we recently reported
that bulk gold powder, consisting of particles much larger than
nanoparticles (50–150 μm), can catalyze reactions of isocyanides
(Eqs. (1) and (2)) [7,8] and carbon monoxide (Eq. (3)) [9a] with
amines and oxygen to produce carbodiimides and ureas in high
yields under mild conditions:
Au
+ (1/2)O2 −→
+ “H2O”.
(4)
All of these reactions [11] are especially notable because they are
catalyzed by a bulk form of gold metal that was previously known
to be a poor catalyst [1] of other reactions. As an extension of the
reaction in Eq. (4), herein we report a gold powder-catalyzed ox-
idative dehydrogenation and coupling of primary amines to give
imines. In addition, we describe the use of alumina-supported
gold (Au/Al2O3) as a catalyst for the oxidative dehydrogenation
of secondary and primary amines to imines. This catalyst was
prepared by the incipient wetness impregnation method and con-
tained relatively large gold particles (50–150 nm). Like gold pow-
der, Au/Al2O3 is an excellent catalyst for the oxidative dehydro-
genation of amines. In fact, the catalytic activity of 5 mg of gold in
the Au/Al2O3 catalyst is greater than that of 1000 mg of bulk gold
powder. The results described in this paper show that Au/Al2O3 is
a useful catalyst for the practical synthesis of imines by the oxida-
tive dehydrogenation of secondary and primary amines. They also
demonstrate that the gold particles in these catalysts need not be
nano-sized in order to be active in these reactions.
Au
◦
60
C
ꢀ
ꢀ
R–N≡C + H2N–R + (1/2)O2 −→ R–N=C=N–R + “H2O”,
(1)
(2)
1 atm
O
Au
◦
60
C
ꢀ
ꢀ
NR2 ,
R–N≡C + NHR2 + (1/2)O2 −→
RHN
C
1 atm
(3)
Bulk gold particles also catalyze the oxidation of CO and glyc-
erol under alkaline conditions [9b]. We also have described the
2. Experimental
The amines (pyrrolidine, piperidine, hexamethyleneimine) were
dried and purified as described previously [12]. Toluene, diben-
Corresponding author. Fax: +1 515 294 0105.
E-mail address: angelici@iastate.edu (R.J. Angelici).
*
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.08.012

2
B. Zhu et al. / Journal of Catalysis 260 (2008) 1–6
zylamine, 1,2,3,4-tetrahydroisoquinoline, N-benzylaniline, n-hexyl-
amine, benzylamine, 4-chlorobenzylamine, 4-methylbenzylamine,
N-benzylideneaniline, and N-benzylidenebenzylamine were pur-
chased from Sigma Aldrich or Fisher and used as received. Authen-
tic samples of N-hexylidenehexylamine [13], N-(4-methylbenzyl)-
4-methylbenzaldimine, N-(4-chlorobenzyl)-4-chlorobenzaldimine,
and 3,4-dihydroisoquinoline [14] were prepared as described pre-
viously, and authentic samples of other imines mentioned in
this work were synthesized as described previously [10]. The O2
(99.5%) was used as received. The shiny gold powder was prepared
from HAuCl4 by reduction with hydroquinone as described pre-
viously [15]; it had a surface area of 0.35 m2/g [16]. As described
previously [9], attempts to identify nano-sized gold particles in this
powder using SEM (see below) and by studying the reaction of CO
and O2 showed no evidence of gold nanoparticles in the gold pow-
der. The gold powder (10 g) was cleaned before being reused in
subsequent catalytic reactions by treatment with piranha solution
(40 mL, a 3:1 mixture of concentrated H2SO4 and 30% aqueous
H2O2. Caution: H2SO4 must be added to H2O2 in a large beaker
under slow stirring). After vigorous foaming and gas evolution for
the first 5 min, the mixture was stirred for another 2 h, and then
diluted with 200 mL of distilled water. After the mixture was fil-
tered on a coarse frit, the gold was washed 10 times with 40 mL of
water, followed by five washings in 40 mL of methanol. After dry-
ing under a nitrogen stream, the Au powder was heated overnight
support were gold. The total surface areas of the Al2O3 and 5%
Au/Al2O3 were determined by the BET method using N2 adsorp-
tion at 77 K on a Sorptomatic 1990 instrument (Thermo Electron
Corporation).
2.4. General procedure for the catalytic reactions
A mixture of amine (0.20 mmol) and gold powder (1.0 g) (or 5%
Au/Al2O3, 100 mg) in toluene (5 mL) was prepared in a glass tube
(2.5 ×18 cm, ∼85 mL volume). The O2 gas (about 1.0 L) was intro-
duced into a rubber balloon that was attached to a syringe needle,
which was inserted into the septum covering the tube opening.
◦
The mixture was stirred vigorously (magnetic stir bar) at 100 C
for 24 h, and then worked up by filtration to remove the catalyst.
Products of the reactions were identified by comparison of their
mass spectra and GC retention times with those of authentic sam-
ples. Yields were determined by GC using authentic samples of the
products as calibrants.
2.5. Preparative-scale oxidative dehydrogenation of
1,2,3,4-tetrahydroisoquinoline
The amine (1.0 g, 8.0 mmol) and 5% Au/Al2O3 (250 mg) in
toluene (10 mL) were placed in a glass tube (2.5 × 18 cm, ∼85 mL
volume) that was capped by a septum. A syringe needle at-
tached to a rubber balloon containing O2 (∼2.0 L) was inserted
into the septum. The mixture was stirred vigorously (magnetic
◦
in air in a 110 C oven. The gold powder that was used in the cat-
alytic reactions was re-used and treated with H2O2/H2SO4 many
times.
◦
stir bar) at 100 C for 80 h. After cooling to room temperature,
the Au/Al2O3 catalyst was removed by filtration. The yield of the
3,4-dihydroisoquinoline was determined by GC using an authentic
sample as the calibrant.
2.1. Preparation of Au/Al2O3
The Au/Al2O3, with 5% Au (ww), was prepared overnight by
incipient wetness impregnation of 4.0 g of γ -Al2O3 (specific sur-
face area of 158 m2/g) with an aqueous solution of 0.42 g of
HAuCl4·3H2O in 1.5 mL of distilled water. The sample was allowed
to dry overnight in air at room temperature. After drying in an
3. Results and discussion
3.1. SEM characterization of gold powder
◦
oven at 100 C for 1 h, the solid was reduced under a flow of
◦
The bulk gold powder examined by SEM had been used in sev-
eral catalytic reactions and had been treated with H2O2/H2SO4
solution several times. SEM was used because the gold powder
particles were too large to be analyzed by TEM or STEM. Fig. 1
shows two SEM images of the bulk gold powder at 500× (a) and
50,000× (b). The particles have an average particle size of approx-
imately 50 μm. The particles do not have a uniform shape but
are relatively monodispersed in regard to size. High magnification
(Fig. 1b) shows gold particles with ridges and much smaller parti-
cles (∼100 nm, black arrows) on the surface of the large particles.
These small particles and ridges are possibly the most catalytically
active sites.
H2 with heating at a rate of 3 C/min from room temperature to
◦
◦
300 C and then holding at 300 C for 1 h. In order to remove
−
Cl , the catalyst was washed several times with hot water until
a AgNO3 test gave no AgCl precipitate. Finally, the solid was dried
and calcined in air at 700 C for 68 h.
◦
2.2. Recycling of Au/Al2O3
After being used in a catalytic reaction, Au/Al2O3 (1.0 g) was
cleaned by stirring (for 10 min) three times with 50 mL of CH2Cl2,
three times with 50 mL of ethanol, and three times with 50 mL of
◦
hexane. It was then dried in an oven at 150 C overnight.
3.2. Characterization of 5% Au/Al2O3
2.3. Characterization of gold powder and Au/Al2O3
The specific surface area of the Au/Al2O3 catalyst, which was
determined by the BET method, is 74.7 m2/g. This is substantially
smaller than that (158 m2/g) of the Al2O3 itself. This decrease is
likely due to sintering of the alumina under the 700 C thermal
treatment; these high temperatures have been observed previously
to cause sintering of alumina [17].
Typical STEM images of the gold particles on the 5% Au/Al2O3
catalyst are shown in Fig. 2. Images were recorded in the scanning
transmission mode to achieve higher contrast between the gold
particles and the alumina support. The gold particles (white dots
in Fig. 2) are fairly uniformly distributed over the alumina sup-
port (gray, darker regions). The white dots were identified as gold
particles by EDX analysis, as illustrated in Fig. 3. The Cu peaks in
this figure are due to the Cu grid that is used to support the sam-
The gold powder and alumina-supported gold samples were
analyzed by scanning electron microscopy (SEM) and scanning
transmission electron microscopy (STEM), respectively. The SEM
measurements were performed on a JEOL 840A microscope op-
erated at 10 kV accelerating voltage and 0.005 nA of beam cur-
rent. The STEM measurements with energy dispersive X-ray anal-
ysis (EDX) were performed on a Tecnai G2 F20 microscope op-
erated at 200 kV. The samples were sonicated in isopropyl alco-
hol for five minutes; then, a drop of the suspension was placed
on a lacy carbon copper grid (from Ted Pella, Inc.) and dried at
ambient conditions. The average gold particle size and size dis-
tribution on the alumina support were determined by counting
55 particles. Energy dispersive X-ray analysis was used to establish
that the contrasting particles on the surface of the larger alumina
◦

B. Zhu et al. / Journal of Catalysis 260 (2008) 1–6
3
(a)
(b)
Fig. 1. Scanning electron micrographs of gold powder: (a) 500× magnification and (b) 50,000× magnification. Black arrows point to small Au particles on the surface of the
much larger Au particles.
Fig. 2. Scanning transmission electron micrographs of the 5% Au/Al2O3 catalyst. White rectangle represents the area of the EDX analysis (Fig. 3). White arrows point to the
Au particles on the surface of the alumina.
Fig. 3. Typical energy dispersive X-ray analysis of Au particles on the surface of the alumina (Fig. 2).
ple in the STEM and EDX studies. The gold particles are generally
larger than 50 nm, as illustrated in the Au particle size distribu-
tion in Fig. 4. Although particles less than 20 nm could have been
detected by the STEM instrument, none was observed. The shapes
of the particles are quite irregular.
cles (∼1.9 nm) were observed in Au/Al2O3 that was treated at only
◦
250 C for 2 h [22].
3.3. Gold-catalyzed aerobic oxidation of secondary amines to imines
The relatively large sizes of the Au particles in the Au/Al2O3
catalyst are due, in part, to the use of the impregnation method
of preparation [18]. Others have also observed that Au/Al2O3 pre-
pared by this method contains relatively large particles [1,19,20].
Another factor leading to the large Au particles in the present
We reported previously the gold powder-catalyzed oxidative
dehydrogenation of secondary amines, using O2 as the oxidant,
to give imines (Eq. (4)) [10] under a standard set of conditions
(0.20 mmol amine, O2 (∼1.0 L at ∼1 atm), gold powder (1.0 g) in
◦
5 mL of toluene at 100 C for 24 h). Yields from these reactions
◦
case is the high temperature treatment (700 C for 68 h) of the
are given in column 4 of Table 1. The benzyl amines (entries 1–3)
give the simple imine products (Eq. (4)), but the cyclic amines (en-
tries 4–6) give products resulting from the coupling and oxidative
dehydrogenation of two amine molecules (Eq. (5)):
Au/Al2O3. Sintering of small gold particles would be expected as
the melting point of Au is known to decrease as the size of the
particle decreases [21]. It should be noted that smaller gold parti-

4
B. Zhu et al. / Journal of Catalysis 260 (2008) 1–6
Table 1
Gold-catalyzed aerobic oxidation of secondary amines to imines according to Eqs. (4) and (5)
Entry
Substrate
Product
Yield (%)
Au powder cat.a
Au/Al2O3 cat.b
97 (1c)
1
2
3
64
87 (85d)
89 (81e)
18
9
4
5
93
75
98
95
6
22f
33
a
◦
Reaction conditions: amine (0.2 mmol), O2 (∼1.0 L at ∼1 atm), gold powder (1.0 g) in 5 mL of toluene solvent at 100 C for 24 h [10].
b
c
d
e
f
◦
Reaction conditions: amine (0.2 mmol), O2 (∼1.0 L at ∼1 atm), 5% Au/Al2O3 (100 mg), in 5 mL of toluene solvent at 100 C for 24 h.
Only Al2O3 (100 mg) as catalyst.
0.4 mmol of pyridine added.
◦
Reaction conditions: amine (8.0 mmol, 1.0 g), O2 (∼2.0 L at ∼1 atm), 5% Au/Al2O3 (250 mg), in 10 mL of toluene solvent at 100 C for 80 h.
◦
At 60 C in 5 mL of acetonitrile solvent for 40 h [10].
Scheme 1. Proposed mechanism for the oxidative dehydrogenation of cyclic sec-
ondary amines (illustrated for piperidine).
tetrahydroisoquinoline gave N-benzylidenebenzylamine and 3,4-
dihydroisoquinoline in 97 and 89% yields, respectively (Table 1,
entries 1 and 2). On the other hand, N-benzylaniline produced
N-benzylideneaniline in only poor yield (18%), which is presum-
ably due to the much less basic nitrogen in this amine, which
reduces its tendency to adsorb to the gold surface. The 5-, 6-, and
7-membered cyclic amines (entries 4–6) gave the coupled imine
products (Eq. (5)) in good yield especially for entries 4 and 5,
which gave almost quantitative yields. To exclude the possibility
that the Al2O3 support was catalyzing these reactions, the reac-
tion of dibenzylamine with O2 was performed with only Al2O3
(100 mg) as the catalyst (entry 1); the very low yield (1%) of imine
shows that the Al2O3 support is not the catalyst. In an experiment
designed to determine whether pyridine in the reaction mixture
would reduce the product yield by competitively adsorbing to the
gold powder [23], the reaction of 1,2,3,4-tetrahydroisoquinoline
was performed in the presence of two equivalents of pyridine (en-
try 2); however, the added pyridine did not affect the yield of the
3,4-dihydroisoquinoline.
Fig. 4. Gold particle size distribution in 5% Au/Al2O3 as determined by TEM (55
randomly chosen particles measured).
(5)
A mechanism proposed [10] for this reaction (Scheme 1), based
on additional studies, suggested the following steps: (a) oxidative
dehydrogenation of the cyclic amine to give the imine, (b) addition
of another molecule of amine across the imine double bond, and
(c) oxidative dehydrogenation of the resulting diamine to give the
product.
We have now investigated the possibility that Au/Al2O3 can
catalyze the same reaction under the same conditions but us-
ing only 5 mg of gold metal in 100 mg of the 5% (w/w) gold-
loaded Au/Al2O3 catalyst (rather than 1000 mg of gold pow-
der). As is evident in column 5 of Table 1, yields of the imine
products are as high or higher with the Au/Al2O3 catalyst than
with gold powder. The benzyl amines dibenzylamine and 1,2,3,4-
For the purpose of demonstrating that these catalytic reactions
could be used to prepare larger amounts of imine products, 1.0 g
◦
of 1,2,3,4-tetrahydroisoquinoline was heated at 100 C in 10 mL of
toluene with 250 mg of 5% Au/Al2O3 under 2 L of O2 at 1 atm
pressure for 80 h. Although the reaction time was longer than that

B. Zhu et al. / Journal of Catalysis 260 (2008) 1–6
5
Table 2
Gold-catalyzed aerobic oxidation of primary amines to imines according to Eq. (6)
Entry
Substrate
Product
Yield (%)
Au powder cat.a
Au/Al2O3 cat.b
29
1
2
C6H13 –N=CHC5H11
5
56 (49c, 4d)
92 (3e, 71f)
3
61
96
59
4
7
a
◦
Reaction conditions: amine (0.2 mmol), O2 (∼1.0 L at ∼1 atm), gold powder (1.0 g), in 5 mL of toluene solvent for 24 h at 100 C.
b
c
d
e
f
◦
Reaction conditions: amine (0.2 mmol), O2 (∼1.0 L at ∼1 atm), 5% Au/Al2O3 (100 mg), in 5 mL of toluene solvent at 100 C for 24 h.
0.4 mmol of H2O added.
0.4 mmol of bromobenzene added.
Only Al2O3 (100 mg) as catalyst.
With re-used catalyst.
(24 h) used for the smaller quantities, a high yield (81%) of the
imine product was obtained.
3.4. Gold-catalyzed aerobic oxidation of primary amines to coupled
imines
During the course of our studies of the oxidative dehydrogena-
tion of secondary amines, we observed that primary benzyl amines
undergo coupling and oxidative dehydrogenation to give imines ac-
cording to Eq. (6):
Au
Scheme 2. Possible mechanisms for the gold-catalyzed aerobic oxidation of primary
amines to imines according to Eq. (6).
2R–CH2NH2 + (1/2)O2 −→ R–CH=NCH2R + “H2O” + “NH3”.
(6)
The reactions were performed under the same conditions (0.2
mmol amine, 1 L of O2 at 1 atm in 5 mL of toluene at 100 C
◦
Pt/TiO2 [13], ZnS [24], and Hg [25,26] catalysts. The reaction of
HgO and I2, as oxidizing agent, with primary amines gives the cou-
pled imines [14]. Copper (I and II) salts catalyze the oxidation (O2)
of benzylamines to the N-benzylidenebenzylamines, as in Eq. (6)
[27,28]. Also, heteropolyoxometallate clusters containing Mo and
V catalyze the oxidation in Eq. (6) [29,30]. The two mechanisms
(Scheme 2) that have been proposed for the oxidation of primary
amines (Eq. (6)) both proceed by way of oxidative dehydrogena-
tion of the amine to the imine intermediate RCH=NH. In path A,
this intermediate is attacked by a second molecule of the primary
amine to give an aminal which loses NH3 to give the coupled
imine product RCH=NCH2R; this path was proposed for the oxida-
tive dehydrogenation using I2 and HgO [14]. In path B, the initially
formed imine reacts with trace amounts of H2O to give the alde-
hyde RCH=O, which subsequently reacts with a second molecule
of the amine to give the imine product; this mechanism was pro-
posed for the reaction (Eq. (6)) using the Na6PMo2V2O40 cata-
lyst [29]. In the gold metal-catalyzed reactions described herein,
it is likely that the first step is the oxidative dehydrogenation
because this gold-catalyzed reaction occurs with a variety of sec-
ondary amines [10]. The next step could be either reaction with
amine (path A) or with H2O (path B). In the toluene reaction sol-
vent, little water would be present. To determine whether or not
H2O might promote the reaction in path B, 0.4 mmol of H2O was
added to the reaction of 0.2 mmol of benzylamine (entry 2, Ta-
ble 2). The yield of product actually decreased from 56% to 49%,
which indicates that a substantial increase in the H2O concentra-
tion does not accelerate the reaction. Although this result is not
definitive, path A is most consistent with our results for the gold-
catalyzed reactions (Eq. (6)).
for 24 h) using 1.0 g of gold powder or 100 mg of 5% Au/Al2O3. As
shown in Table 2, low yields of the imine product (entry 1) were
obtained from the aliphatic n-hexylamine. On the other hand, high
yields of the imine were achieved with the benzyl amines (entries
2–4). In the benzylamine series, the highest yield was obtained
from the benzylamine with the electron-donating p-methyl group,
and the lowest yield was obtained with the electron-withdrawing
p-chloro group. These effects can be understood by assuming that
the most electron-donating group favors amine adsorption on the
gold surface. Of course, it is possible that the p-substituent affects
other steps in the mechanism. The slower rate of the p-chloro-
derivative may be due to deactivation of the catalyst, as bromoben-
zene added to the gold powder-catalyzed reaction of benzylamine
(entry 2) greatly inhibits the reaction.
In all of the reactions, the product yields under the standard
conditions are significantly higher with the Au/Al2O3 catalyst than
with the gold powder (Table 2). It should be noted that when
the reaction of benzylamine was conducted under an argon at-
mosphere, rather than O2, or in the absence of a gold catalyst,
only a trace of the imine product was detected. Also, when Al2O3
alone was used as the catalyst, only a low yield (3%, entry 2) of
product was observed. When the Au/Al2O3 catalyst was re-used a
second time after it was washed with CH2Cl2, EtOH, and hexanes,
the yield was lower (71%). The lower activity of the recycled cat-
alyst may be due to incomplete cleaning of the gold surface or
pulverization of the Au/Al2O3 catalyst to smaller particles, which
is observed visually during stirring with a magnetic stirbar.
Prior to the studies reported herein, there were a few previous
investigations of the conversion of primary amines (often benzy-
lamines) to coupled imines. There are photolytic conversions in
the absence of an oxidizing agent, with the formation of H2, over
To our knowledge, the only previously reported gold-catalyzed
oxidative dehydrogenation of primary amines is that using nano-

6
B. Zhu et al. / Journal of Catalysis 260 (2008) 1–6
gold particles in the form of Au/TiO2 [31]. With this catalyst,
neat n-hexylamine reacts with O2 (1 atm) at 130 C to give N-
hexylbutanoic amide (Eq. (7)):
from the previously reported reactions of isocyanides (C≡N–R) and
C≡O with amines and O2 (Eqs. (1)–(3)). The use of small amounts
of gold in Au/Al2O3, while achieving high yields of products, makes
these catalysts useful for the synthesis of imines on a preparative
scale.
Although Au/Al2O3 with large gold particles is highly effec-
tive for the oxidative dehydrogenation of amines, it would be of
considerable interest to compare its catalytic activity with that of
Au/Al2O3 catalysts that contain nanogold particles (<5 nm) [22].
Another future study might examine the selectivity of the Au/Al2O3
catalyst in O2 oxidations of amines that also contain other oxidiz-
able functional groups.
◦
O
Au/TiO2
CH3(CH2)5NH2 + O2 −→ CH3(CH2)4
NH(CH2)5CH3 .
(7)
C
◦
130
C
Thus, nanogold catalyzes a quite different reaction of primary
amines and O2 than that catalyzed by bulk gold (Eq. (6)). However,
specially prepared palladium black catalyzes the dehydrogenation
◦
of α-phenylethylamine A (Eq. (8)) at 100 under an argon atmo-
sphere [32]:
Acknowledgments
This research was supported by the U.S. Department of Energy
under contract No. DE-AC02-07CH11358 with Iowa State University.
M.L. thanks P. Marginean and V. Almasan for useful discussions.
(8)
References
Presumably, NH3 is the other product of the reaction. The pro-
posed mechanism involves initial dehydrogenation of A to give
an intermediate imine hydride species [Ph(Me)C=NH]PdH2 which
undergoes attack at the imine carbon by a second molecule of
amine A. The resulting aminal (analogous to the intermediate in
path A of Scheme 2) then breaks down to products B, C, and NH3.
Product C arises from the transfer of the two hydrogen atoms
from Pd. The formation of B from the aminal intermediate oc-
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in path A of Scheme 2. Thus, the Au- and Pd-catalyzed reactions
appear to proceed by similar imine and aminal intermediates, but
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not performed under an O2 atmosphere. If they had been, and they
followed path A (Scheme 2) proposed for the Au-catalyzed reac-
tion, B would have been the only expected product.
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4. Conclusion
These investigations show that bulk gold powder and sup-
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ness impregnation, are excellent catalysts for the oxidative dehy-
drogenation of secondary and primary amines. Both catalysts con-
sist of particles that are much larger than those (<5 nm) found in
nanogold catalysts. The gold powder particles were about 50 μm in
diameter, while the Au particles on Au/Al2O3 were much smaller
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to give an aminal that undergoes oxidative dehydrogenation to the
final coupled imine product (Scheme 1). Primary amines also first
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