Ultrathin platinum nanowire catalysts for direct C-N coupling of carbonyls with aromatic nitro compounds under 1 bar of hydrogen

Article abstract of DOI:10.1002/chem.201100818

Traditionally important in the pharmaceutical, agrochemical, and synthetic dye industries, C-N coupling has proved useful for the preparation of a number of valuable organic compounds. Here, a new method for the direct one-pot reductive C-N coupling from carbonyl and aromatic nitro compounds is described. Employing ultrathin platinum nanowires as the catalyst and hydrogen as the reducing agent, N-alkylamines were achieved in high yields. Debenzylation products were not detected after prolonged reaction times. Time-dependent analysis, ReactIR spectroscopy and DFT calculations revealed that the C-N coupling proceeded through a different mechanism than traditional "reductive amination." N-Alkylamines were directly obtained by intermolecular dehydration over platinum nanowires under a hydrogen atmosphere, instead of intramolecular water elimination and imine hydrogenation.

Full text of DOI:10.1002/chem.201100818

FULL PAPER
DOI: 10.1002/chem.201100818
À
Ultrathin Platinum Nanowire Catalysts for Direct C N Coupling of
Carbonyls with Aromatic Nitro Compounds under 1 Bar of Hydrogen
Lei Hu,[a] Xueqin Cao,[a] Danhua Ge,[a] Haiyan Hong,[a] Zhiqiang Guo,[a] Liang Chen,[b]
Xuhui Sun,[c] Jianxin Tang,[c] Junwei Zheng,[d] Jianmei Lu,*[a] and Hongwei Gu*[a]
Abstract: Traditionally important in
the pharmaceutical, agrochemical, and
and hydrogen as the reducing agent, N-
alkylamines were achieved in high
yields. Debenzylation products were
not detected after prolonged reaction
times. Time-dependent analysis, Reac-
tIR spectroscopy and DFT calculations
À
revealed that the C N coupling pro-
À
synthetic dye industries, C N coupling
ceeded through a different mechanism
than traditional “reductive amination.”
N-Alkylamines were directly obtained
by intermolecular dehydration over
platinum nanowires under a hydrogen
atmosphere, instead of intramolecular
water elimination and imine hydroge-
nation.
has proved useful for the preparation
of a number of valuable organic com-
pounds. Here, a new method for the
À
direct one-pot reductive C N coupling
À
Keywords: amination · C N cou-
from carbonyl and aromatic nitro com-
pounds is described. Employing ultra-
thin platinum nanowires as the catalyst
pling · heterogeneous catalysis ·
nanowires · platinum
Introduction
aromatic nitro compounds) with aldehydes (or ketones)[4] is
another attractive procedure for N-alkylamine formation
which includes two synthetic steps: C=N double-bond for-
mation and hydrogenation of the imine derivatives. Several
catalysts have demonstrated their efficiency in reductive
amination reactions. Yamane et al.[5] reported a direct reduc-
tion over Au/Fe2O3 under a 2 MPa hydrogen atmosphere.
Sreedhar et al.[6] reported that gum-acacia-stabilized Pd
nanoparticles could be used as the catalyst with excellent
The importance and widespread use of N-alkylamines in
dye, pharmaceutical, and agrochemical industries have led
À
C N coupling to become a core interest in organic synthe-
sis.[1] The reaction of amines with alkyl halides or similar al-
À
kylation agents is one of the most common methods of C N
coupling.[2] However, these processes have significant draw-
backs such as alkylation agent toxicity and a lack of mono-
alkylation selectivity. The alkylation of amines with alcohols
through a hydrogen auto-transfer or oxidation process[3] is
an important reaction class that follows three steps to gener-
ate the N-alkylamines. Reductive amination of amines (or
yield. Decaborane Pd/C and HCOOÀNH4+/Pd/C systems
[7]
À
have also been used for C N bond formation. These meth-
ods are encouraging; however, the Au/Fe2O3 system needs
high pressure and the Pd catalysts system causes debenzyla-
tion,[8] which is an side reaction to N-alkylamine formation.
Herein, we report on the use of ultrathin platinum nano-
wires (Pt NW) as nonsupported catalysts for N-alkylation
from aromatic nitro compounds and various carbonyls in a
one-pot reaction under 1 bar of hydrogen. Time-dependent
[a] L. Hu, Prof. Dr. X. Cao, D. Ge, H. Hong, Z. Guo, Prof. Dr. J. Lu,
Prof. Dr. H. Gu
Key Laboratory of Organic Synthesis of Jiangsu Province
College of Chemistry,
Chemical Engineering and Materials Science
Soochow University, Suzhou, 215123 (P. R. China)
Fax : (+86)512-65880905
À
analyses and DFT calculations revealed that C N bonds are
directly formed through intermolecular dehydration instead
of C=N intermediate formation through intramolecular
water elimination and C=N double-bond hydrogenation.
This mechanism is different than previously reported molec-
ular catalysts or “Pd” nanocatalysts. Debenzylation products
were not detected even after prolonged reaction times, up
to 24 h.
E-mail: hongwei@suda.edu.cn
lujm@suda.edu.cn
[b] Prof. Dr. L. Chen
Institute of Materials Technology & Engineering
Chinese Academy of Sciences
Ningbo, 315201 (P. R. China)
[c] Prof. Dr. X. Sun, Prof. Dr. J. Tang
Functional Nano & Soft Materials Laboratory (FUNSOM) &
Jiangsu Key Laboratory for Carbon-Based
Functional Materials and Devices
Results and Discussion
Suzhou, Jiangsu, 215123 (P. R. China)
[d] Prof. Dr. J. Zheng
Ultrathin Pt NW catalysts were obtained from the acidic
etching of FePt NW under an air atmosphere. Compared
with the starting material, FePt NW (Fe atomic ratio
ꢀ50%, Figure S1, Supporting Information), Fe not was de-
Institute of Chemical Power Sources
Soochow University, Suzhou, 215123 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100818.
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Figure 1. A) TEM and B) high resolution TEM images of the as-synthe-
sized Pt NW. (Insert: Pt NW atomic lattice).
tected in the ultrathin Pt NW by inductive coupling plasma
(ICP) and X-ray photoelectron spectroscopy (XPS, Fig-
ure S2 in the Supporting Information). Figure 1A shows
representative transmission electron microscopic (TEM)
images of the as-synthesized Pt NW several hundred nano-
meters in length and less than 2 nm in average diameter
(Figure 1B). The high-resolution TEM image in Figure 1B
(insert) shows that the Pt NW are single crystals, and the
plane distance is 0.23 nm, which is in agreement with that of
the (111) plane of the face-centered cubic (fcc) phase of
bulk Pt crystals.[9] The powder X-ray diffraction (XRD) pat-
tern (Figure S3, Supporting Information) also showed that
(111) is the predominant plane.
Figure 2. A) Optical images of the nanowire catalysts before and after
the reactions; B) TEM image of the Pt nanowire catalysts after 6 reaction
cycles; C) stabilities of Pt NW catalysts. Reaction conditions: nitroben-
zene (1.1 mmol), benzaldehyde (1.0 mmol), and toluene (2 mL) in H2
(1 bar) with Pt NW (0.005 mmol). Product yields were determined by
GC using tert-butylbenzene as an internal standard.
The catalytic activity of the Pt NW was first investigated
with nitrobenzene and benzaldehyde as reactants under a
hydrogen atmosphere (1 bar). Table S1 (Supporting Infor-
mation) compares the catalytic performance in different sol-
vents. In methanol, ethanol, or toluene (Table 1, entry 1),
the reaction gives high yields (>90%) after 12 h. A com-
The Pt NW catalysts could be conveniently recycled and
reused. As shown in Figure 2A, Pt NW served as heteroge-
neous catalysts and could be recovered by simple centrifuga-
tion. This recycled catalyst could be directly reused without
losing any catalytic activity (Figure 2C). The Pt NW were
relatively stable. Evident morphology changes were not
found (Figure 2B), and Pt leaching was not detected by ICP
after six cycles. It is interesting to note that the catalytic ac-
tivities were highly depended on the morphology of the cat-
alysts. When using Pt nanoparticles (Figure S4, ꢀ3 nm in di-
À
pletely green process for the C N coupling (Table S1,
entry 4, Supporting Information) was also tested, and a yield
of about 88% of N-benzylbenzenamine was obtained when
using water as the solvent.
À
ameter), the yields of C N and C=N products were 48.8 and
3.9%, respectively (Table 1, entry 2), but when using Pt
nanorods (Figure S5, Supporting Information, ꢀ2 nm in di-
Table 1. The control experiments for the preparation of N-benzylbenzen-
amine from direct reductive amination in toluene at 1008C.
À
ameter and ꢀ20 nm in length), the yields of C N and C=N
products were 67.6 and 22.3% respectively (Table 1,
entry 3). Compared to nanoparticles or nanorods, Pt NW
show much higher catalytic activity. This was also demon-
strated in the nitrobenzene hydrogenation and several other
kinds of reactions.[10]
Substrate 1
Substrate 2
Product
t
Yield
[%][e]
[h]
1[a]
2[b]
3[c]
12
24
24
92.4
48.8
67.6
4[a]
5[a]
6[d]
24
24
99.4
97.6
95.0
The scope of the reaction was then explored using a
number of aromatic nitro and benzaldehyde reactants. As
shown in Table 2, various aromatic nitro compounds were
À
coupled with benzaldehyde to form C N bonds under ambi-
ent hydrogen pressure in toluene. High yields from o-, m-,
and p-methyl nitrobenzene (Table 2, entries 1–3) showed
good regioselectivity, and the yields of the corresponding N-
alkylamines were higher than 85%. The functional groups
0.8
7[a]
24
4.6
À
showed little influence on the formation of the C N cou-
pling products (Table 2, entries 4–9).
[a] Pt nanowires, 1 bar of H2. [b] Pt nanoparticles, 1 bar of H2. [c] Pt
nanorods, 1 bar of H2. [d] Pt nanowires, 1 bar of N2. [e] GC yield.
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Ultrathin Platinum Nanowire Catalysts
FULL PAPER
À
À
Table 2. The Pt NW catalyzed reductive C N coupling of benzaldehyde
with different aromatic nitro compounds and amines under a H2 (1 bar)
atmosphere.[a]
Table 3. The Pt NW catalyzed reductive C N coupling of nitrobenzene
with different carbonyl compounds under a H2 (1 bar) atmosphere.[a]
R1R2C=O
Product
t
Yield
[%][b]
À
R NO2
Product
t
Yield
[%][b]
[h]
[h]
1
24
24
24
24
24
24
24
24
94.9 (94)
93.1
1
22
89.5
2
2
3
4
5
6
7
8
9
22
22
24
24
24
22
24
24
85.0
3
89.1
92.2 (92)
75.9
4
95.0 (95)
94.7
5
94.7 (94)
71.8 (67)
92.6
6
90.8 (90)
88.3
7[c]
8
71.2 (68)
91.4 (87)
83.9 (79)
9
24
76.9 (75)
[a] Reaction conditions: Carbonyl compound (1 mmol), nitrobenzene
(1.1 mmol), toluene (2 mL), and Pt NW catalyst (0.005 mmol) at 1008C.
[b] GC yield. [c] 608C. The values in parentheses are the yields of the iso-
lated products.
[a] Reaction conditions: benzaldehyde (1 mmol), nitro aromatic com-
pound (1.1 mmol), toluene (2 mL), and Pt NW catalyst (0.005 mmol) at
1008C. [b] GC yield. The values in parentheses are the yields of the iso-
lated products.
We conducted a series of experiments to gain information
on the reaction mechanism (Table 1, entries 4–7). Under the
same reaction conditions (1 bar of H2, 1008C, Pt NW as the
catalyst), a 99.4% yield of N-benzylbenzenamine was ach-
ieved in the reaction between benzaldehyde and aniline, in-
stead of nitrobenzene. We separated this reaction into two
steps. First, aniline reacted with benzaldehyde without the
catalyst giving N-benzylidenebenzenamine in 95% yield.
Second, reduction of N-benzylidenebenzenamine occurred
to form N-benzylbenzenamine in the fresh catalytic system
with only 4.6% yield (1 bar of H2, 1008C, 24 h, Pt NW as
the catalyst, Figure S7, Supporting Information). This result
demonstrated that N-benzylbenzenamine was not formed by
N-benzylidenebenzenamine hydrogenation.
In light of these results, various aldehydes were subjected
to the reaction for N-alkylamine formation. Both aromatic
and aliphatic aldehydes were investigated (Table 3). Irre-
spective of the electronic nature of the substituent, aromatic
aldehydes always reacted smoothly to give the correspond-
ing products in excellent yields (Table 3, entries 1–5). Re-
gardless of whether the aliphatic aldehyde compounds were
linear or a-branched, the reactions always gave good yields
without any apparent byproducts (Table 3, entries 6–8). Ace-
tophenone, which is known to be an inert reactant,[11] was
À
successfully converted to the C N product with a yield of
76.9% (Table 3, entry 9).
À
The time-dependent conversion plot of the reductive C N
À
coupling is shown in Figure 3A. Only trace amounts of N-
benzylidenebenzenamine, phenylmethanol, and aniline were
detected in the reaction by gas chromatograph (GC) analy-
sis. Debenzylation products were not detected when the re-
action time was prolonged to 24 h (Figure S6, Supporting In-
formation). ReactIR technology (Figure 3B) was also em-
Compared with the previously reported C N bond forma-
tion in the “reductive amination” procedure, the reaction
underwent a new pathway in our nanocatalytic system.
Based on this result, a proposed mechanism for the direct
À
reductive C N coupling is given in Scheme 1. Under a hy-
drogen atmosphere, nitrobenzene (A) was first reduced to
nitrosobenzene and then to N-phenylhydroxylamine (C)[12]
with the help of the Pt NW catalyst. N-Phenylhydroxyl-
À
ployed to monitor the reaction. The intensity of the C N
À1
À
bond (1329 cm ) increased, but the intensities of the NO2
group (1352 cmÀ1) and the C=O group (1709 cmÀ1) de-
amine (C) reacts with benzaldehyde (B)[13] to give a dihy-
ACHTUNGTRENNUNG
creased. Figure 3C shows the initial conversion speeds
droxy intermediate, then this dihydroxy intermediate elimi-
nates a water molecule over the Pt nanowire to form phe-
nyl(phenylamino)methanol (D) under a hydrogen atmos-
phere. When hydrogen and Pt were removed, this dihydroxy
(from ReactIR) of the NO2 and C=O groups, that is
À1
0.02 molLÀ1 min . The C N bond formation speed was
À
0.004 molLÀ1 minÀ1, which is shown in Figure 3D.
Chem. Eur. J. 2011, 17, 14283 – 14287
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J. Lu, H. Gu et al.
À
À
À
Figure 3. A) GC and B) ReactIR profiles for C N coupling reactions; the kinetic plots for the reactions of C) NO2, C=O and D) C N bonds. Reaction
conditions: nitrobenzene (5.5 mmol), benzaldehyde (5 mmol), toluene (10 mL), catalyst (0.025 mmol), and H2 (1 bar) at 1008C. Product yields were de-
termined by GC using tert-butylbenzene as an internal standard.
intermediate was intramolecularly dehydrated to obtain N-
benzylideneaniline and N-oxide.[14,13c] N-Phenylhydroxyl-
AHCTUNGTREGaNNUN mine (C) also can be reduced to aniline (G) under these
reaction conditions. However, the aniline product (G) can
also reacted with benzaldehyde (B)[15] to give phenyl(pheny-
lamino)methanol (D). The phenyl(phenylamino)methanol
(D) is quickly transformed to N-benzylbenzenamine (F)
through intermolecular dehydration and subsequent H addi-
tion on the Pt NW surface under a hydrogen atmosphere.
ReactIR technology was employed to monitor the conver-
sion. By analysis of two bands at 1411 (dOH) and 1533 cmÀ1
(dNH), phenyl(phenylamino)methanol (D, Scheme 1) was
proved to be the intermediate product of the reaction (Fig-
ure S8, Supporting Information), and most of the nitroben-
zene and benzaldehyde was first converted to phenyl(pheny-
lamino)methanol (D). However, trace amounts of N-benzy-
lidenebenzenamine (E) could also be obtained through in-
tramolecular elimination of water from phenyl(phenylami-
no)methanol (D). These two steps are competitive reactions
(from D to F and from D to E, Scheme 1).[16] DFT calcula-
tions offer a simple explanation for this proposed mecha-
nism. Hydrogen molecules are extremely easier to dissociate
to form the active hydrogen species at a low energies
(0.7 eV) on the surface of Pt (111). The binding energy of
À
Scheme 1. The proposed mechanism for the direct reductive C N cou-
pling of nitrobenzene and benzaldehyde.
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Ultrathin Platinum Nanowire Catalysts
FULL PAPER
À
this active hydrogen species to the Pt (111) surface (Pt H)
is about 2.6 eV, which is much lower than the bonding
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In summary, we have described a new and efficient synthetic
methodology for N-alkylamine formation from aromatic
nitro and carbonyl compounds. Pt NW were used as cata-
À
lysts and applied in C N coupling reactions. The reactions
were carried out under very mild conditions to generate a
diverse range of N-alkylamine compounds with remarkably
high yields. The new mechanism, in contrast with the tradi-
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ReactIR and GC analyses and DFT calculations. The simple
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Synthesis of Pt nanowires: Ultrathin Pt nanowires were achieved by
acidic etching FePt nanowires, which were synthesized according to the
procedure described by S. H. Sun et al.[17] FePt nanowires (100 g) were
first treated by bubbling air at 1008C in methanol (100 mL), to which
HCl (15 mL) solvent mixture was added. The solution was heated and
stirred at 608C for 1 hour. The resulting precipitate was obtained follow-
ing 10 min of centrifugation (3000 rpm). The dark solid was washed at
least twice with methanol and stored in hexane.
À
Typical procedure for catalytic C N bond formation: Catalyst testing
was carried out in a sealed tube. Pt nanowires (0.005 mmol) in hexane
(0.2 mL) were added and the hexane was evacuated under reduced pres-
sure. Aromatic nitro compounds (1.1 mmol), carbonyls (1 mmol), toluene
(2 mL), and 1-tert-butylbenzene (0.1 mL) were added to the reaction
tube and then sealed. The reaction tube was evacuated and flushed with
hydrogen three times. The reaction was conducted at a certain tempera-
ture and under a hydrogen atmosphere (Supporting Information). The re-
sulting product mixtures were analyzed by GC (VARIAN CP-3800 GC,
HP-5 capillary column, FID detector) and GC-MS (VARIAN 450-GC &
VARIAN 240-GC; equipped with a CP8944 capillary column, 30 m ꢁ
0.25 mm, and a FID detector). Some N-alkylamine compounds were pu-
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Acknowledgements
H.W.G. acknowledges the financial support from the National Natural
Science Foundation of China (No. 21003092), NSF of Jiangsu Province
(BK2008158) and the New Faculty Start Foundation of Soochow Univer-
sity. L.H. acknowledges financial support from scientific innovation re-
search of college graduate in Jiangsu province (CXZZ11_0102). The au-
thors thank Prof. Xiaobing Wan and Prof. Fan Xu for the in situ ReactIR
analyses.
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Received: March 17, 2011
Published online: November 10, 2011
Chem. Eur. J. 2011, 17, 14283 – 14287
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14287