Synthesis of substituted amines and isoindolinones: Catalytic reductive amination using abundantly available AlCl3/PMHS

Article abstract of DOI:10.1039/c2gc36305d

AlCl₃ has been employed for highly chemoselective reductive amination of carbonyl compounds in ethanol using polymethylhydrosiloxane as an inexpensive, stable and safe reducing agent without an inert atmosphere. A large range of functional groups such as nitro, carboxylic acid, acetyl, nitrile, halogen, methoxy, alkene and heterocycles were well tolerated. AlCl₃ also catalyzed tandem amination-amidation of 2-carboxybenzaldehyde with different amines to afford N-substituted isoindolinones. The catalyst can be recycled at least three times without any significant effect on activity and selectivity.

Full text of DOI:10.1039/c2gc36305d

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PAPER
Synthesis of substituted amines and isoindolinones: catalytic reductive
amination using abundantly available AlCl /PMHS†‡
3
Vishal Kumar, Sushila Sharma, Upendra Sharma, Bikram Singh and Neeraj Kumar*
Received 17th August 2012, Accepted 9th October 2012
DOI: 10.1039/c2gc36305d
AlCl has been employed for highly chemoselective reductive amination of carbonyl compounds in
3
ethanol using polymethylhydrosiloxane as an inexpensive, stable and safe reducing agent without an inert
atmosphere. A large range of functional groups such as nitro, carboxylic acid, acetyl, nitrile, halogen,
methoxy, alkene and heterocycles were well tolerated. AlCl also catalyzed tandem amination–amidation
3
of 2-carboxybenzaldehyde with different amines to afford N-substituted isoindolinones. The catalyst can
be recycled at least three times without any significant effect on activity and selectivity.
eco-friendly solvents limits the scope of these methods. Most of
these methods are not efficient for reductive amination of aceto-
Introduction
Substituted amines are important building blocks for natural pro-
ducts, pharmaceuticals and agrochemicals (Fig. 1). Owing to
the immense importance of amines, their synthesis is an active
field in medicinal chemistry and modern organic synthesis. For
the synthesis of substituted amines, methods involving imine
reduction/direct reductive amination of carbonyl compounds
remain the simplest approach. Mainly, precious metal based
phenone derivatives, generally considered challenging substrates
1
4,5
for this reaction.
Aluminium is the most abundant metal in the earth’s crust and
2
has been explored as a catalyst for Lewis acid catalyzed organic
8
transformations, but has not been applied for reductive ami-
nation of carbonyl compounds. We have recently reported co-
balt(II) phthalocyanine (CoPc) catalyzed reductive amination of
carbonyl compounds using Ph SiH as a silylating agent in
2
catalysts such as Au, Pd, Pt, Rh, Ir, Ru, Re have been applied for
2
2
2,3
9
this transformation. Most of these precious metals are toxic
and at a high risk of depletion, which is the continuous driving
force for the development of abundantly available bio-relevant
metal based catalysts. Although a few methods involving the use
ethanol. Using an inexpensive metal with a green solvent was
inspiring; the use of an abundantly available, inexpensive, stable
and safe catalyst/reducing agent, such as AlCl /polymethyl-
10
3
hydrosiloxane (PMHS), could make the method highly attractive
from a green catalysis point of view. Also, polysiliconates
obtained as byproducts during hydrosilylation with PMHS are
precursors of porous derivatives useful for absorbent
4
5
of abundant metal catalysts such as Zn(ClO ) ·6H O, FeCl ,
4
2
2
3
6
7
Zn(OTf) , and Cu(OAc) have been developed, the use of
2
2
excess amounts of the catalyst, hydrogen sources and non-
11
properties.
In continuation of our work on development of green
reduction processes using abundantly available metal cata-
9,12
lysts,
here we disclose AlCl as a highly efficient catalyst for
3
chemoselective reductive amination of carbonyl compounds and
synthesis of N-substituted isoindolinones in ethanol using
PMHS without applying an inert atmosphere.
Results and discussion
In our recent report on CoPc catalyzed reductive amination of
carbonyl compounds, it was observed that the Lewis acidic char-
acter of CoPc played an important role in catalyzing the reac-
Fig. 1 Selected examples of therapeutically important substituted
amines.
9
tion. In view of this point, it was further envisaged that stronger
Lewis acidic Al based catalysts could catalyze this reaction more
efficiently. For this, firstly the reductive amination of benz-
aldehyde with aniline was carried out in the presence of different
hydrogen sources using various Al salts as the catalyst. Amongst
Natural Plant Products Division, CSIR-Institute of Himalayan
Bioresource Technology, Palampur, H.P. 176001, India.
E-mail: neerajnpp@rediffmail.com, neeraj@ihbt.res.in;
Fax: (+)91-1894-230433; Tel: (+)91-1894-230426
†
‡
1
IHBT Communication No. 2401.
Electronic supplementary information (ESI) available. See DOI:
0.1039/c2gc36305d
the different catalysts, AlCl (anhydrous) and AlCl ·6H O were
3
3
2
found to be most active at 70 °C with highest yield in the former
3410 | Green Chem., 2012, 14, 3410–3414
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a
a
Table 1 Optimization of catalyst and reducing agent
Table 2 Reductive amination of different aldehydes with amines
b
Yield
(%)
Entry Aldehyde
Amine
Product
b
Quantity
(mol%)
Reducing
agent
Yield
(%)
Entry
Catalyst
1
2
c
1
2
3
4
5
6
7
8
9
1
R = H
R = H
95
80
1
2
3
4
5
AlCl
AlCl
AlCl
AlCl
AlCl
AlCl
3
3
3
3
3
3
0.5
1.0
2.0
3.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
57
71
>99
98
99
NR
35
8
1
2
c
R = 2-NO
2
R = H
1
2
R = 4-NO
2
R = H
95
91
93
80
77
91
48
64
94
92
1
2
R = 4-Cl
R = H
1
2
R = 4-Br
R = H
1
2
c
R = 3-CN
R = H
6
1
2
R = H
R = 2-I
AlPc
1
2
R = H
R = 4-F
Al
Al
Al
Al
2
2
2
2
O
O
O
3
3
3
(acidic)
(neutral)
(basic)
1
2
R = H
R = 4-NO
2
12
14
6
1
2
c
0
R = H
R = 4-CO
2
H
1
2
c
1
1
R = H
R = 4-COCH
3
1
1
(SO
·6H
Al(NH )(SO
4 3
)
2
1
2
1
2
R = H
R = 4-OCH
3
12
13
14
15
16
17
18
19
20
21
AlCl
3
O
82
16
NR
97
94
96
<5
10
13
NR
c
4
4
)
13
89
—
d
d
d
AlCl
AlCl
AlCl
AlCl
AlCl
AlCl
AlCl
3
3
3
3
3
3
3
2.0
2.0
2.0
2.0
2.0
2.0
2.0
PhSiH
3
Ph
Ph
2
SiH
SiH
SiH
2
14
97
3
Et
3
(CH
)
3 2
CHOH
c
1
5
6
92
HCOONH
—
4
c
a
1
95
Reaction conditions: all reactions were carried out with benzaldehyde
(
1 mmol) and aniline (1 mmol) in EtOH (5 mL) at 70 °C for 12 h.
Yield based on GC-MS analysis of the reaction mixture. d The reaction
b
c
was carried out at room temperature. NR = no reaction. The reaction
was carried out for 1 h.
17
96
80
18
19
20
case (Table 1, entries 3 and 12). No reaction was observed at
room temperature (Table 1, entry 6). Aluminium phthalocyanine
(
AlPc) and other aluminium salts under the same reaction con-
d
d
51(91)
26(81)
ditions were not able to provide comparable yields of the desired
product (Table 1, entries 7–11 and 13). Comparable yields were
recorded with PMHS, PhSiH , Ph SiH and Ph SiH in the pres-
3
2
2
3
ence of AlCl (Table 1, entries 4 and 15–17). Although the reac-
3
tion with phenylsilanes was very fast as compared to PMHS,
PMHS was selected for further studies, considering the cost and
a
safety factors. Et SiH, isopropanol and ammonium formate gave
Reaction conditions: aldehyde (1 mmol), amine (1 mmol), AlCl
3
b
3
(
2 mol%), PMHS (2 mmol), EtOH (5 mL) at 70 °C for 12 h. Yield
the desired product in very low yields (Table 1, entries 18–20).
As expected, no reaction was observed in the absence of a cata-
lyst or a reducing agent showing the necessity of both (Table 1,
entries 14 and 21).
c
based on GC-MS analysis of the reaction mixture. The reaction was
d
carried out with 1 g of aldehyde and isolated yield is reported. Yield
given in parentheses corresponds to the reaction carried out using
PhSiH (1.5 equiv.) as the reducing agent.
3
With optimized reaction conditions, the scope of the method
was explored by carrying out the reductive amination of different
aldehydes with various amines (Table 2). The –NO group is
2
3
i,13
highly susceptible to reduction,
however, in the present case
desired products were obtained in high yields (Table 2, entries
4–6). The method was also successful in reductive amination of
a heteroaromatic aldehyde, 2-furaldehyde (Table 2, entry 15).
Excellent yield and chemoselectivity (>99%) was observed in
the case of a polysubstituted aldehyde, 2,3,4-trimethoxybenz-
aldehyde, with aniline (Table 2, entry 16).
Other amines were also tested for reductive amination under
the present reaction conditions. Halogen-substituted amines such
as 2-iodo- and 4-fluoroaniline were efficiently utilized for the
reductive amination of benzaldehyde (Table 2, entries 7 and 8)
the reductive amination of 2-nitro and 4-nitrobenzaldehyde with
aniline afforded the desired products in good to high yields
leaving the aromatic nitro group unaltered (Table 2, entries 2 and
3
). One of the major problems associated with reductive ami-
1
4
nation is intolerance of CvC bonds, however, in the present
case >99% chemoselectivity was observed with conjugated as
well as isolated CvC bonds containing aldehydes (Table 2,
entries 13 and 14). Functional groups such as chloro, bromo and
cyano present on the aldehyde remained unaffected and the
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a
without the formation of any byproduct. The reaction of benz-
aldehyde with 4-nitroaniline afforded relatively low yield (48%)
with high chemoselectivity (Table 2, entry 9). Good to high
yields were obtained in the case of functional groups such as car-
boxylic acid and acetyl on amines, which are generally not con-
sidered in reductive amination due to their low reactivity and
incompatibility with reducing agents (Table 2, entries 10 and
3
Table 4 AlCl catalyzed synthesis of N-substituted isoindolinones
b
3
a–d
Entry
Amine
Yield (%)
11).
The reaction of benzaldehyde with 4-methoxyaniline
gave an excellent yield of the product (Table 2, entry 12). Ali-
phatic primary amines were also tested and the desired products
were obtained in very good yields (Table 2, entries 17 and 18).
Secondary amines (aliphatic as well as aromatic) were found to
1
2
3
4
5
6
7
R = H
R = 4-OCH
R = 4-CH
R = 4-I
R = 4-NO
R = 4-COCH3
R = 2-CH
96
96
91
72
78
92
3
be incompatible with the AlCl /PMHS system giving a low yield
3
3
of the product, whereas the AlCl /PhSiH3 system showed
c
3
2
c
efficient conversion to the desired products (Table 2, entries 19
and 20).
c
3
74
One of the major advantages of the present method is the
reductive amination of acetophenones, which are otherwise con-
sidered difficult substrates for this reaction resulting in low
8
62
1
5
9
95
98
yields of the products or require higher quantities of the cata-
1
6
lyst. Under the present reaction conditions, the reductive ami-
nation of different acetophenones with amines afforded the
desired products in good to excellent yields (Table 3, entries
1
0
1
–5). Also, very good yields of products were obtained in the
reaction of aliphatic ketones and aniline (Table 3, entries 6 and
). When the method was applied on larger scales (up to gram
a
Reaction conditions: ketone (1 mmol), amine (1 mmol), AlCl (2 mol%),
3
b
PMHS (2 mmol), EtOH (5 mL) at 70 °C for 12 h. Isolated yield.
7
c
Yield based on GC-MS analysis of the reaction mixture.
scale), with some important substrates containing functional
groups such as nitro, carboxylic acid, ketone, alkene, hetero-
cycle, 2,3,4-trimethoxy and bromo (Table 2, entries 2, 10, 11,
1
3, 15 and 16, Table 3, entries 3 and 4), good isolated yields
Table 5 Recyclability of the catalyst
were obtained in all the cases.
Furthermore, the scope of the method was extended for the
1
7
synthesis of biologically important N-substituted isoindoli-
nones by tandem amination–amidation of 2-carboxybenzalde-
1
8
hyde. Under the present reaction conditions, a wide range of
amines was investigated for tandem amination–amidation of
Cycle
1st
2nd
92
3rd
94
4th
65
a
Yield (%)
>99
a
Table 3 Reductive amination of ketones
a
Yield based on GC-MS analysis of the reaction mixture.
b
Entry Ketone
Amine
Product
Yield (%)
2
-carboxybenzaldehyde to synthesize the corresponding N-sub-
1
2
1
2
3
4
5
R = H
R = H
73
98
83
74
70
stituted isoindolinones (Table 4). Interestingly, both aromatic and
aliphatic amines produced the corresponding isoindolinones in
good to excellent yields. The reaction with aniline afforded the
desired isoindolinone in excellent yield (Table 4, entry 1). Aro-
matic amines having electron-donating substituents such as
1
2
R = H
R = OCH
3
3
3
3
1
2
c
R = 4-NO
2
R = OCH
1
2
c
R = 4-Br
R = OCH
1
2
R = 4-OCH
3
R = OCH
6
96
4
-OCH , 4-CH and 4-I gave good yields of the desired products
3 3
(Table 4, entries 2–4). In the reaction of 4-nitroaniline high che-
moselectivity was observed and the desired product was obtained
in good yield (Table 4, entry 5). Amines with strongly electron-
7
89
withdrawing substituents like 4-COCH generally give low
3
13
yields of the product, but, in the present case, the reaction of
4-acetylaniline proceeded smoothly to give high yields of the
product (Table 2, entry 6). The effect of steric hindrance was
seen in the reaction of o-substituted anilines giving the desired
product in comparatively lower yields (Table 4, entries 7 and 8).
a
3
Reaction conditions: ketone (1 mmol), amine (1 mmol), AlCl (2 mol%),
b
PMHS (2 mmol), EtOH (5 mL) at 70 °C for 12 h. Yield based on
GC-MS analysis of the reaction mixture. The reaction was carried out
with 1 g of ketone and isolated yield is reported.
c
3412 | Green Chem., 2012, 14, 3410–3414
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amination of carbonyl compounds with amines. Use of an
eco-friendly hydrogen source and solvent with a low loading
of recyclable abundant metal catalyst under ambient
reaction conditions makes the present method superior to
earlier reported methods in terms of environment safety and
economy. In addition, there was no requirement of an inert
atmosphere making the current method more economical and
simple. Other remarkable advantages of this methodology
include high isolated yields, clean reactions and easy work-up
procedures.
3
Scheme 1 Preferential interaction of Al(III) with Et N leading to no
reaction.
Experimental section
Excellent yields were obtained with aliphatic amines (Table 4,
entries 9 and 10).
The reusability study of the catalyst on the model reaction
showed that it can be reused up to three cycles without any loss
in catalytic activity (Table 5).
General
AlCl3 was purchased from Fisher Scientific India Pvt. Ltd
(Acros Organics). Silica gel (60–120 mesh) used for column
chromatography was purchased from Sisco Research Labora-
tories Pvt. Ltd India and all other chemicals were purchased
from Spectrochem, India, Merck, Germany, and Sigma-Aldrich,
USA and were used without further purification. NMR spectra
were recorded on a Bruker Avance-300 spectrometer. Mass
spectra were recorded on a QTOF-Micro of Waters Micromass
and Maxis-Bruker. The GC-MS analysis was carried out on a
Shimadzu (QP 2010) series Gas Chromatogram-Mass Spec-
trometer (Tokyo, Japan), an AOC-20i auto-sampler coupled, and
a DB-5MS capillary column (30 m × 0.25 mm i.d., 0.25 μm).
The initial temperature of the column was 70 °C held for 4 min
and was programmed to 230 °C at 4 °C min− , then held for
As far as the mechanism of the reaction is concerned, it was
proposed in our recent report that Lewis acid–base interaction
between CoPc and an imine helps in the activation of the imine
9
towards reduction. Expecting a similar mechanism in the
present case, the model reaction was carried out in the presence
of triethylamine and no reaction was observed (Scheme 1),
which may be due to preferential interaction of Et N with Al(III).
3
To further confirm the interaction between AlCl and an imine
3
(4-(methoxybenzylidene)-4-methoxyaniline, 1), a UV-Vis study
was carried out. Addition of AlCl to an ethanolic solution of 1
3
1
resulted in a bathochromic shift in λmax of 1 from 335 nm to
1
5 min at 230 °C; the sample injection volume was 2 μl in GC
3
75 nm and the color of the solution changed from colorless to
grade dichloromethane. Helium was used as the carrier gas at a
flow rate of 1.1 ml min− on split mode (1 : 50). UV-Vis spectra
were recorded on a UV-Vis 2450 spectrophotometer from
Shimadzu.
dark yellow confirming the coordination of 1 with AlCl , which
3
1
was consistent with the previous reports (for spectra see
19,20
ESI‡).
When Et N was added to the mixture of the imine (1)
3
and AlCl , the yellow color of the solution as well as the band at
3
λmax 375 nm in the UV-Vis spectrum disappeared and a new
band at 342 nm for free imine was observed due to preferential
coordination of Et N with Al(III). The observation of low yield
General experimental procedure for reductive amination of
3
in the case of amines with strong electron-withdrawing groups
carbonyl compounds catalyzed by the AlCl /PMHS system
3
such as –NO , –CO H etc. may be due to lesser Lewis basic
2
2
To a stirred suspension of AlCl (0.02 mmol) in ethanol (4 mL)
3
character of the corresponding imines, which further supports
the crucial role of Lewis acid–base interaction in catalyzing the
reaction (Table 2, entries 9 and 10). Also, Lewis acids are known
to activate hydrosilanes for hydride transfer. To confirm this, the
were added carbonyl compound (1.0 mmol), amine (1.0 mmol)
and PMHS (2.0 H equiv.) at room temperature and then the
temperature was raised to 70 °C. On completion of the reaction
(as monitored by TLC), the reaction mixture was dried under
reaction of PMHS with AlCl in ethanol was carried out for 12 h
3
1
vacuum and the crude product was analyzed directly by GC-MS.
For the purification of the desired product column chromato-
graphy was carried out (n-hexane : ethyl acetate).
and analyzed by H NMR. The appearance of new peaks at δ 3.8
(2H) and 1.2 (3H) corresponding to Si–OEt and the decrease in
intensity of the peak at δ 4.7 for Si–H confirmed the activation
of PMHS by AlCl . This observation provides evidence for the
3
consumption of PMHS under the action of AlCl . On the basis
3
General experimental procedure for synthesis of N-substituted
isoindolinones catalyzed by the AlCl /PMHS system
3
of the above observations, it can be proposed that AlCl activates
3
imine through Lewis acid–base interaction and also helps in the
hydride transfer to imine.
To a stirred suspension of AlCl (0.02 mmol) in ethanol (4 mL)
3
were added 2-carboxybenzaldehyde (1.0 mmol), amine
(1.0 mmol) and PMHS (2.0 H equiv.) at room temperature and
then the temperature was raised to 70 °C. On completion of the
reaction (as monitored by TLC), the reaction mixture was dried
under vacuum and the product was purified by crystallization
with absolute ethanol.
Conclusions
An AlCl catalyzed novel method has been reported for the syn-
3
thesis of substituted amines and isoindolinones through reductive
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Experimental procedure for recyclability of the catalyst
5 S. Enthaler, ChemCatChem, 2010, 2, 1411–1415.
6
7
8
S. Werkmeister, S. Fleischer, S. Zhou, K. Junge and M. Beller, Chem-
SusChem, 2012, 5, 777–782.
S. Werkmeister, K. Junge and M. Beller, Green Chem., 2012, 14, 2371–
The recyclability of the catalyst was studied by carrying out the
reductive amination of benzaldehyde with aniline as a test reac-
tion. On completion of the reaction (as monitored by TLC), the
reaction mixture was dried under vacuum and the crude product
was extracted with ethyl acetate (2 × 5 mL). The residue left was
dried under vacuum for 15 min. Successive reactions were
carried out by sequential addition of fresh substrates, PMHS and
ethanol to the crude remains after extracting the product.
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3414 | Green Chem., 2012, 14, 3410–3414
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