One-pot reductive amination of araldehydes by aniline using borohydride with CeCl3·7H2O as catalyst

Article abstract of DOI:10.3184/174751915X14356747417290

A one-pot, two-step reductive amination of araldehydes or acetophenones with anilines using NaBH₄as a cheap hydride source and catalysed by CeCl₃·7H₂O has been achieved in EtOH at room temperature in good yields.

Full text of DOI:10.3184/174751915X14356747417290

390 RESEARCH PAPER
VOL. 39 JULY, 390–393
JOURNAL OF CHEMICAL RESEARCH 2015
One-pot reductive amination of araldehydes by aniline using borohydride with
.
CeCl3 7H2O as catalyst
Xun Zhua,b*, Xiuqin Zhoua andWei Zhanga
a Department of Chemical Engineering, Yancheng Institute of Industry Technology, Yancheng 224005, P. R. China
b School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. China
A one-pot, two-step reductive amination of araldehydes or acetophenones with anilines using NaBH4 as a cheap hydride source and
.
catalysed by CeCl3 7H2O has been achieved in EtOH at room temperature in good yields.
Keywords: one-pot, reductive amination, cerium(III) chloride
Substituted amines are important building blocks in a multitude
of biologically active natural products and pharmaceuticals.1,2
For example, they are used as organic intermediates for the
synthesis of bactericides, drugs, herbicides, rubber accelerators,
corrosion inhibitors, and surface-active agents. Owing to the
immense importance of amines, their synthesis is an active
field in medicinal chemistry and modern organic synthesis.3,4
As a consequence, various methods for the efficient formation
of a C–N bond have been widely investigated. As one of the
most important and effective C–N bond-forming processes,
reductive amination is a fundamental transformation both
in laboratory synthesis and industrial production.5-7 Direct
reductive amination of carbonyl compounds remain the simplest
approach.8-10 Typically, reductive amination of aldehydes or
ketones, in which a mixture of a carbonyl compound and an
amine is treated with a reductant in a “one-pot” fashion, is one
of the most useful methods for the preparation of secondary
or tertiary amines which avoids the isolation of an unstable
imine or iminium intermediate.11-13 As shown in Scheme 1,
one of the synthetic methods to N-benzyl-N-phenylamines
3 utilized condensation between aldehydes 1 and anilines 2
followed by reduction of the imine intermediate 4. Several
hydrogen sources such as sodium cyanoborohydride,14 sodium
triacetoxyborohydride,15 sodium- or zinc borohydride16 in the
presence of Brønsted or Lewis acids17,18 have been utilised
to form the intermediate imines and to activate these C=N
intermediates for a preferential reduction. However, these
various borohydride derivatives suffer from limitations such
as expense, high toxicity and low selectivity,19 which limit
the scope of these methods. For this reason, wide commercial
availability of substrates, generally mild reaction conditions
and in some cases exceptionally high functional group
tolerance which promotes efficient one-pot synthesis has
become an important research area in organic chemistry.20
Several methods which effect direct reductive amination have
been recently developed, such as ZnCl2–NaBH4,21 NiCl2–
NaBH4,22 Ti(OiPr)4–polymethylhydrosiloxane,23 Ti(OiPr)4–
NaBH4,24 silica gel–ZnBH4,25 trifluoro acetic acid–Et3SiH26
and pyridine–BH3.27
In previous work, we found that a Lewis acid can not only
promote the formation of a C–C bond, but can also promote the
formation of an imine.28-30 Since imine formation is the rate-
determining step for in situ reductive aminations, addition of a
mild Lewis acid as catalyst can increase the rate. Ravishankar
et al.31 have recently reported a one-pot, solvent-free reductive
amination of cinnamaldehydes and araldehydes by anilines
.
using sodium borohydride and catalysed by CeCl3 7H2O.31 We
.
now report that, using NaBH4 with CeCl3 7H2O as catalyst
in solvent ethanol at room temperature, aldehydes and
acetophenones can be reductively aminated by anilines with
higher efficiency.
Results and discussion
.
Initially, we studied the influence of CeCl3 7H2O on the
condensation reaction of benzaldehyde with aniline. The
condensation reaction was carried out with NaBH4 in EtOH
at room temperature and the results are summarised in Table
.
1. Clearly CeCl3 7H2O plays an important role in catalysing
the reaction, as without any catalyst, the yield of N-benzyl-
N-phenylamine was very low and benzyl alcohol (45%) was
.
obtained as the main product (entry 1). When CeCl3 7H2O
was added, reduction of benzaldehyde to benzyl alcohol was
observed; 1, 2, 5 and 7 mol% of catalyst was investigated
.
(entries 2–7) and that 5 mol% of CeCl3 7H2O was sufficient to
catalyse the reaction. Among the solvents screened, including
EtOH, MeOH, CH2Cl2 and AcOEt, EtOH was the best solvent
which afforded the amine in 93% yield using 2 equiv. NaBH4 at
room temperature (entry 6).
2
NH
R
2
2
2
O
1
R
N
R
1
R
Catalyst
4
1
2
R
Reducing
agent
N
H
1
R
3
Scheme 1
* Correspondent. E-mail: xunzhu.yiit@gmail.com

JOURNAL OF CHEMICAL RESEARCH 2015 391
The influence of several hydride sources on the reaction
outcome was studied. In general, the hydride sources were
found to be most active at 70°C in EtOH (Table 2). No reaction
was observed at room temperature (entry 5). By comparison
of the yields when using different hydride sources, such as
NaBH4, PhSiH3, Ph2SiH2 and Ph3SiH, we found that Et3SiH
gave the N-benzyl-N-phenylamine in very low yield (entry 4).
However, further studies were performed with NaBH4, because
of its great availability, low price and ready reaction at room
temperature.
One of the major advantages of the present method is that
it can be used for the reductive amination of acetophenones,
which are considered difficult substrates for this reaction.24
The reductive amination of acetophenone was found to require
higher quantities of the catalyst and the yields of the products
are low.32 Using our optimal conditions for the longer time of
12 h, the reductive amination of acetophenone with amines
afforded the desired products in good yields (entries 14–16).
Reductive amination of propinaldehyde with aniline
reactions was carried out in the presence of 5 mol%
.
To explore the scope and limitations of the present catalyst
system, reductive aminations of various carbonyl compounds
and amines were performed. The reaction was in EtOH in
CeCl3 7H2O affording the corresponding secondary amine in
a moderate yield (entry 17). An excellent yields was obtained
when benzaldehyde was reductively aminated with an aliphatic
amines (entry 18).
.
the presence of 5 mol% of CeCl3 7H2O under air for 8 h, as
can be seen in Table 3. Excellent yields were obtained when
benzaldehydes were substituted with electron-withdrawing
as well as electron-donating groups (entries 1–9), while
difficulties arose with nitro groups in an o-position or a
m-position (entries 10 and 11). Functional groups such as –Cl,
–Br, –I, –NO2, and C=C present on the aldehyde remained
unaffected.
Next we investigated the influence of the amine component
in combination with benzaldehyde. The reactions of an aniline
containing an electron-withdrawing group decreases the
reaction yield (entry 12), wheras an aniline containing an
electron-donating group increased product yield slightly (entry
13).
Table 3 Yields of reductive aminations of various araldehydes and
acetophenones by anilines using NaBH4 and CeCl3•7H2O under the
optimised conditionsa
.
CeCl 7H O
3
2
2
R
O
(5 mol%)
3
3
+
2
R
NH
R
2
1
1
R
N
R
R
EtOH, NaBH
4
H
1
2
3
Entry R1
R2 R3
Product
3a
Yield/%b
92
92
89
88
91
85
88
82
83
62
70
83
1
2
3
p-NO2C6H5
p-ClC6H5
p-BrC6H5
p-IC6H5
H
H
H
H
H
H
H
H
H
H
H
H
H
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
p-BrC6H5
p-OCH3C6H5
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
4
5
6
7
C6H5CH=CH
p-CH3C6H5
C6H5
Table 1 Optimisation of the amounts of NaBH4, catalyst and solvent in
the reductive amination of benzaldehyde (1, R1 = H) with aniline (2, R2 =
H) (Scheme 1) a
.
Entry CeCl3 7H2O /mol% Solvent
NaBH4/equiv. Conversion /%b
8
9
p-OCH3 C6H5
p-HOC6H5
m-NO2C6H5
o-NO2C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
1
2
3
4
5
6
7
8
9
0
1
2
5
5
5
7
5
5
5
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
CH2Cl2
AcOEt
1
1
1
1
1.2
2
2
2
2
2
10 (45 c)
50
54
60
67
93
94
90
78
10
11
12
13
14
15
16
17
18
90
CH3 C6H5
CH3 p-BrC6H5
CH3 p-OCH3C6H5
75 c
70c
79 c
60
3p
3q
3r
C3H7
C6H5
H
H
C6H5
C4H9
10
72
91
aReaction conditions: benzaldehyde (1 mmol), aniline (1.2 mmol), CeCl3 7H2O (mol%), and
NaBH4 in solvent (5 mL), 8 h, at room temperature.
bYield determined by GC.
.
.
aReaction conditions: araldehyde (1 mmol), aniline (1.2 mmol), CeCl3 7H2O (5 mol%), and
NaBH4 (2 mmol) in EtOH (5 mL), 8 h, at room temperature.
b Yield of isolated product.
cYield of benzyl alcohol.
c Reaction time was 12 h.
Table 2 Influence of the hydride source and the Lewis acid on the yield of the reductive amination of benzaldehyde (1, R1 = H) with aniline (2, R2 = H)
(Scheme 1)
Entry
Lewis acid
Hydride source
PhSiH3 (2 equiv.)
Ph2SiH2 (2 equiv.)
Ph3SiH (2 equiv.)
Time /h
4
4
4
4
12
12
4
10
8
Temp /oC
Solvent
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
THF
Yield/%a
95
92
67
35
NR 33
99 33
9517
88 19
93
1
2
3
4
5
6
7
8
9
CeCl3·7H2O (5 mol%)
CeCl3·7H2O (5 mol%)
CeCl3·7H2O (5 mol%)
CeCl3·7H2O (5 mol%)
AlCl3
AlCl3
Zn(OTf)2
ZnCl2
CeCl3·7H2O (5 mol%)
0
70
70
70
rt
70
60
rt
Et3SiH (2 equiv.)
(5 mol%)
(5 mol%)
(5 mol%)
(5 mol%)
Poly (methylhydro-siloxane) (2 equiv.)
Poly (methylhydro-siloxane) (2 equiv.)
PhSiH3 (1.5 equiv.)
NH3BH3 (1.5 equiv.)
NaBH4 (2 equiv.)
THF
EtOH
rt
a Yield was determined by GC-MS. NR= less than 5% conversion.

392 JOURNAL OF CHEMICAL RESEARCH 2015
Experimental
N-Benzyl-4-bromoaniline (3l)：Yellow solid (209 mg, 83%), m.p.
o
o
51–53 C (EtOH) (lit.34 52–53 C]; 1H NMR：δ 7.40–7.35 (m, 4H),
7.18–7.14 (m, 1H), 6.74–6.72 (m, 2H), 6.58–6.56 (m, 2H), 6.26 (s, 1
H), 4.26 (s, 2H).
All chemicals (AR grade) were obtained from commercial sources
and used without further purification. Gas chromatography analysis
was performed on an Agilent GC-6820 chromatograph equipped
with a 30 m×0.32 mm×0.5 μm HP-Innowax capillary column and a
flame ionisation detector. GC-MS spectra were recorded on Thermo
Trace DSQ GC-MS spectrometer using TRB-5MS (30 m×0.25
mm×0.25 μm) column. Melting points were determined using a XT-4
apparatus and are not corrected. 1H NMR spectra were obtained on a
Bruker Avance III 400 (400 MHz) spectrometer in DMSO-d6 using
TMS as internal standard. Chemical shifts (δ) are given in ppm and
coupling constants (J) in Hz. Progress of the reactions was followed
by TLC using silica-gel polygrams SIL G/UV 254 plates. Column
chromatography was performed using Silicycle (40–60 mm) silica
gel. All products are known compounds and were characterised by
comparison of their physical and spectral properties with literature
data.
N-Benzyl-4-methoxyanilines (3m)：White solid (191 mg, 90%),
o
o
m.p. 47–50 C (EtOH) (lit.35 48–50 C); 1H NMR：δ 7.40–7.38 (m,
2H), 7.35–7.31 (m, 2H), 7.25–7.22 (m, 1H), 6.69–6.61 (m, 4H), 6.24
(s, 1 H), 4.29 (s, 2H), 3.55 (s, 3H).
N-(1-Phenylethyl) aniline (3n): Colourless oil (147 mg, 75%);
1H NMR：δ 7.38–7.36 (m, 2H), 7.31–7.27 (m, 2H), 7.19–7.16 (m, 1H),
6.99–6.95 (m, 2H), 6.50–6.43 (m, 3H), 6.11 (s, 1H), 4.46–4.44 (t,
J=8 Hz, 1H), 1.42 (d, J=8 Hz, 3H).
4-Bromo-N-(1-phenylethyl) aniline (3º)：Yellow oil (162 mg, 70%);
1H NMR：δ 7.39–7.35 (m, 2H), 7.30–7.26 (m, 2H), 7.20–7.16 (m, 2H),
7.08–7.00 (m, 3H), 6.11 (s, 1H), 4.45–4.43 (t, J=8 Hz, 1H), 1.42 (d,
J=8 Hz, 3H).
4-Methoxy-N-(1-phenylethyl) aniline (3p)：White solid (179
1
mg, 79%), m.p. 61–63 oC (EtOH) (lit.36 61–62 oC); H NMR：δ
7.38–7.27(m, 5H), 6.77–6.73 (m, 2H), 6.65–6.62 (m, 2H), 6.11 (s,
1H), 4.45–4.45 (m, 1H), 3.57 (s,3H), 1.42 (d, J=8 Hz, 3H).
N-Butyl-anilines (3q): Colourless oil (0.089 mg, 60%); 1H NMR：δ
7.09–7.06 (m, 2H), 6.81–6.78 (m, 1H), 6.63–6.60 (m, 2H), 6.08 (s,
1H), 4.08–4.06 (t, J=8 Hz, 2H), 2.81 (m, 2H ), 1.99–1.91 (m, 2H),
1.10 (t, J=8 Hz, 3H).
Synthesis of secondary amines catalysed by the CeCl3·7H2O–NaBH4
system; general procedure
Benzaldehyde (1.0 mmol), amine (1.2 mmol) and CeCl3·7H2O (0.02
mmol) in ethanol (5 mL) were stirred for 20 min at room temperature,
then NaBH4 (2 mmol) was added. On completion of the reaction (as
monitored by TLC), the reaction mixture was dried under vacuum
and the product was extracted with ethyl acetate (3 × 10 mL).
Evaporation of the solvent gave a crude product which was purified
on a small silica gel column with EtOAc: petroleum ether (10:1) as
eluent.
We are grateful to JiangSu Provincial Natural Science
Foundation of China (no BK20141260) and Six Talented
Summit funding of Jiangsu Province of P. R. China (no 2011-
JZ-017) for financial support.
N-(4-Nitrobenzyl)-aniline (3a): White solid (209 mg, 92%),
o
o
1
m.p. 66–68 C (EtOH) (lit.13 67–68 C); H NMR：δ 7.78–7.74 (m,
2H),7.66–7.62 (m, 2H), 6.77–6.73 (m, 2H), 6.63–6.61 (m, 2H),
6.56–6.52 (m, 1H), 6.26 (s, 1H), 4.26 (s, 2H).
Received 8 May 2015; accepted 10 June 2015
Paper 1503354 doi: 10.3184/174751915X14356747417290
Published online: 9 July 2015
N-(4-Chlorobenzyl)-aniline (3b): Yellow oil (200 mg, 92%);
1H NMR：δ 7.30–7.30 (m, 4H), 7.07–7.03 (m, 2H), 6.58–6.51 (m, 3H),
6.26 (s, 1H), 4.26 (s, 2H).
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