Imidazolium-based ionic liquid-catalyzed hydrosilylation of imines and reductive amination of aldehydes using hydrosilane as the reductant

Article abstract of DOI:10.1039/c7ra04245k

The first imidazolium-based ionic liquid-catalyzed hydrosilylation of imine and reductive amination of aldehydes with primary amines using a catalytic amount of 1-butyl-3-methylimidazolium tetrachloride iron [BMIm][FeCl₄] and Ph₂SiH₂ as a reductant were performed under mild conditions. Good yields of secondary amines with high chemoselectivity and a tolerance for a wide range of functional groups were obtained.

Full text of DOI:10.1039/c7ra04245k

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Imidazolium-based ionic liquid-catalyzed
hydrosilylation of imines and reductive amination
Cite this: RSC Adv., 2017, 7, 31795
of aldehydes using hydrosilane as the reductant†
a
*
Bin Li,
Lu Chen
Shilin Zhang,a Weizhen Wu,a Lecheng Liang,ab Shaohua Jiang,a
*
a
and Yibiao Lia
The first imidazolium-based ionic liquid-catalyzed hydrosilylation of imine and reductive amination of
aldehydes with primary amines using a catalytic amount of 1-butyl-3-methylimidazolium tetrachloride
iron [BMIm][FeCl4] and Ph2SiH2 as a reductant were performed under mild conditions. Good yields of
secondary amines with high chemoselectivity and a tolerance for a wide range of functional groups were
obtained.
Received 14th April 2017
Accepted 3rd June 2017
DOI: 10.1039/c7ra04245k
rsc.li/rsc-advances
The formation of amines is one of the most important trans- between ions due to the designable cations and anions from
formations in chemistry because amines are versatile building ILs.20 However, only few studies have been reported on IL-
blocks for various organic molecules and essential precursors to catalyzed hydrosilylations. Recently, Liu's group has shown
a variety of biologically active compounds.1 Although several that using a stoichiometric amount of imidazolium-based ionic
methods are known for amine synthesis, catalytic reduction of liquids [BMIm]Cl, high efficiency for the synthesis of formam-
imines or direct reductive amination of carbonyl derivatives in ides of amines and carbon dioxide under hydrosilylation
the presence of primary amines via sequential condensation/ conditions was obtained (Scheme 1, 1a).21
catalytic reduction are some of the most efficient methods
Following our continuous efforts to perform selective
through hydrogenation,2,3 or hydrogen transfer reactions.4,5 reduction via hydrosilylation,22 herein, we report the rst simple
Recently, transition metal-catalyzed hydrosilylations have commercially available 1-alkyl-3-methylimidazolium-based
attracted attention as a competitive alternative method to the ionic liquids and their use in catalytic amount for hydro-
classical stoichiometric reduction, hydrogenation or hydrogen silylation of imines or reductive amination of aldehydes and
transfer reaction due to their mild conditions and good che- primary amines (Scheme 1, 1b).
moselectivities.6,7 In contrast, although some catalysts have
Initial studies focused on the reduction of imine 1a with
been included in the amine synthesis via hydrosilylation of several stoichiometric amounts of commercially available
imine8–12 and the synthetic method of more green direct imidazolium-based ionic liquids and diphenylsilane (1.5 equiv.)
reductive amination from primary amines with carbonyl deriv- as the hydride donor (Scheme 2). Preliminary tests on the ionic
atives under hydrosilylation conditions has some examples in liquid 1 (IL 1: [AMIm]OAc) 1-allyl-3-methylimidazolium acetate
the literature,13–17 the IL-catalyzed hydrosilylation of imines or indicated low conversion to the secondary amine 4a at room
direct reductive amination has not yet been developed.
temperature. In contrast, 1-butyl-3-methylimidazolium tetra-
Ionic liquids (ILs), composed of organic cations and organic/ chloride iron (IL 2: [BMIm]FeCl4) was very efficient for this
inorganic anions, have attracted attention as a competitive reaction, affording secondary amine 4a in 83% yield within 16 h
alternative green solvent to the classical solvents mainly at room temperature. The other ve imidazolium-based ionic
because of its unique features such as high thermal and
chemical stability, negligible vapor pressure, easy separation,
and tunable properties.18 ILs have been applied in many areas,
especially in catalysis.19 On the other hand, ILs can provide
specic functions as a result of cooperative or synergistic effects
aSchool of Chemical & Environmental Engineering, Wuyi University, Jiangmen 529020,
Guangdong Province, P. R. China. E-mail: binli@wyu.edu.cn; wyuchemcl@126.com
bGuangdong Wamo New Material Technology Co., Ltd, Jiangmen 529020, Guangdong
Province, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
Scheme
1 Comparison of different ionic liquid-catalyzed
10.1039/c7ra04245k
hydrosilylation.
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the formation of the secondary amine 4a (Table 1, entries 4–7).
As PMHS is a less expensive and greener hydrosilylation reagent
than other hydrosilanes, it was then evaluated for optimizing
the conditions. When the reaction temperature increased to
ꢀ
80 C, a 94% yield of amine 4a was obtained (Table 1 entry 8).
Decreasing the amount of IL 2 to 20 mol% led to 66% yield
(Table 1, entry 13). However, when this reaction was performed
with Ph2SiH2 (1.5 equiv.) instead of PMHS (4 equiv.) in the
presence of 20 mol% of IL 2 [BMIm]FeCl4, 91% yield of amine
4a was still obtained (Table 1, entry 15). Decreasing the amount
of IL 2 to 10 mol% led to a lower yield using Ph2SiH2 as
a hydrogen source (Table 1, entry 16).
Scheme 2 Imidazolium-based ionic liquid-catalyzed hydrosilylation
The scope and limitations of this rst imidazolium-based-
catalyzed hydrosilylation of imines with Ph2SiH2 were then
explored using 20 mol% of [BMIm][FeCl4] to produce the
secondary amines 4 at 80 ꢀC under an air atmosphere (Table 1,
entry 24). Various imines, 1, were applied to synthesize the
secondary amines 4, as shown in Table 2.
As shown in Table 2, secondary amine 4a, which was
produced from imine 1a with a p-methyl group on the imine R2
aryl, was isolated in 85% yield (entry 1). The reaction could be
of imines.
liquids IL 3–7 were also examined for catalyzing this reaction,
but lower catalytic activity was observed. These ndings indi-
cated that different cations and anions have a signicant
inuence on the activity of the ILs and on the catalytic
hydrosilylation.
Aer the ionic liquids were tested in this catalytic system,
IL 2 was determined to be the best catalyst and applied in this
catalytic hydrosilylation of imine. The reaction could proceed in
acetonitrile, toluene, even under neat conditions, but provided
the desired product in lower yield (Table 1, entries 1–3). Among
the variety of silanes tested in ethanol at room temperature
for 16 h in the presence of 1 equiv. of [BMIm]FeCl4, PhSiH3,
TMDS (1,1,3,3-tetramethylhydroxysiloxane), and PMHS (poly-
methylhydrosiloxane) were optimal hydrogen sources towards
Table 2 [BMIm][FeCl4]-catalyzed hydrosilylation of iminesa
Entry
Product
Yield (%)
Table 1 Optimization of [BMIm][FeCl4] catalyzed hydrosilylation of
1
2
3
4
5
6
7
8
4a: R1 ¼ H
85
84
88
36
52
94
70
75
iminea
4b: R1 ¼ p-OMe
4c: R1 ¼ m-Me
4d: R1 ¼ p-NO2
4e: R1 ¼ p-CN
4f: R1 ¼ H
4g: R1 ¼ p-Br
4h: R1 ¼ p-CO2Me
Amount of IL
Entry 2 (mmol)
Solvent
(mL)
Temp. GC-yieldb
9
10
4i: R1 ¼ H
86
90
Silane
(ꢀC)
(%)
4j: R1 ¼ p-Me
1
2
3
4
5
6
7
8
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.4
0.3
0.2
0.1
—
Ph2SiH2
Ph2SiH2 CH3CN (2 mL)
Ph2SiH2 Toluene (2 mL) r.t.
—
r.t.
r.t.
51
50
Trace
97
25
83
91
94
91
93
89
75
66
11
12
4k: R1 ¼ H
70
72
PhSiH3
Ph3SiH
PMHS
TMDS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
EtOH (2 mL)
EtOH (2 mL)
EtOH (2 mL)
EtOH (2 mL)
EtOH (2 mL)
EtOH (2 mL)
EtOH (2 mL)
EtOH (2 mL)
EtOH (2 mL)
EtOH (2 mL)
EtOH (2 mL)
r.t.
r.t.
r.t.
r.t.
80
90
80
80
80
80
80
80
80
4l: R1 ¼ p-Me
9
13
4m
4n
77
75
10
11
12
13
14
15
16
—
14
0.1
0.05
Ph2SiH2 EtOH (2 mL)
Ph2SiH2 EtOH (2 mL)
91 (85c)
86
a
a
Imine (0.5 mmol), [BMIm][FeCl4], silane (0.75 mmol), solvent (2 mL),
Typical conditions: [BMIm][FeCl4] (20 mol%), aldimine (0.5 mmol),
Ph2SiH2 (0.75 equiv), EtOH (2 mL), 80 ꢀC, 16 h.
16 h. b Determined by GC. c Isolated yield of 4a.
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carried out with p-OMe and m-Me substituents on the imines R1 m-Me, p-NO2, p-CN, and p-Br group on the amine R1 aryl in
aryl in 84% and 86% isolated yield, respectively (entries 2 and moderate to good yields (48–86%) (entries 1–7). Several anilines
3). However, the hydrosilylation was more difficult to perform bearing a functional group, such as p-OMe, p-Cl, and o-F, were
with the nitro group and cyano group, where only 36% and 52% tested under similar conditions, leading to the corresponding
yield of the corresponding amines were obtained (entries 4 and amines 4i, 4q, and 4r in 78%, 75%, and 88% isolated yields,
5), respectively. No dehalogenation occurred in this hydro- respectively (entries 8–10). Additionally, the aliphatic aldehyde
silylation of imine 1g and the corresponding amine 4g was could also be applied to synthesize a secondary amine under
generated in 70% yield (entry 7). More importantly, hydro- these hydrosilylation conditions (entry 11). The reductive
silylation tolerated the functional ester group of imine 1h, and product dibenzylamine 4t obtained from the reaction of ben-
the corresponding amine 4h (75%) was directly obtained zylamine with benzaldehyde was produced and isolated in 76%
without alteration of the carbonyl moieties (entry 8). The results yield.
of entries 9–12 indicated that the electronic effects of the
For the imidazolium-based catalytic hydrosilylation, Wang
donating group on the imine R2 aryl resulted in slightly better and co-workers reported that imidazolium preferred to activate
reactivity than those of the withdrawing group. The steric effect the Si–H bond of hydrosilane as the rst step, as obtained from
of the o-methyl group on the imine R2 aryl did not hamper the the calculated results.23 Moreover, the Liu's group also
reaction (entry 13). Moreover, the hydrosilylation of the cyclo- demonstrated that the ionic liquid [BMIm]Cl favored the acti-
hexane carboxaldehyde 1n successfully led to the corresponding vation of the Si–H bond of phenylsilane then proceeded for the
amine 4n with 75% isolated yield (entry 14).
next transformation.21 To obtain more clear information about
In addition, direct reductive amination from carbonyl the mechanism of this catalytic system, some control experi-
derivatives in the presence of primary amines via in situ ments were performed (Scheme 3). When [BMIm]Cl instead of
generated imine and catalytic reduction shows a more green [BMIm][FeCl4] was used as a catalyst, the secondary amine 4a
process in the amine synthesis than reduction of imines, which was also obtained, but with 37% GC-yield under the same
eliminated one condensation and imine isolation step. The conditions. Moreover, 76% GC-yield of secondary amine 4a was
reductive amination of aldehydes 2 and anilines 3 with Ph2SiH2 detected using only FeCl3 as a catalyst. However, while mixing
was also explored using 20 mol% of [BMIm][FeCl4] under both [BMIm]Cl and FeCl3, the GC-yield of the secondary amine
similar conditions, but with some molecular sieves (Table 3). 4a was increased to 88%. These results indicate that imidazo-
Various aromatic aldehydes 2 successfully reacted with aniline lium and FeCl3 interact to effect the transformation.
3a to produce the secondary amine 4 bearing the p-OMe, p-Me,
Based on the previous works21,23 and the abovementioned
control experiments, we proposed a mechanism (Scheme 4) for
the [BMIm][FeCl4]-catalyzed reductive amination. First, [BMIm]
[FeCl4] interacts with silane silicone, making the Si–H bond of
diphenylsilane insert into the in situ generated imine much
easier, resulting in the formation of the N-silylamine; the
resulting product is then generated via hydrolysis with H2O
or EtOH.
Table 3 [BMIm][FeCl4]-catalyzed reductive amination of aldehydes
and anilines under hydrosilylation conditionsa
In summary, we developed the rst imidazolium-based ionic
liquid-catalyzed hydrosilylation of imine using catalytic amount
of 1-butyl-3-methylimidazolium tetrachloride iron [BMIm]
[FeCl4]. Moreover, this catalytic system was also applied in the
reductive amination of aldehydes to access secondary amines
with diphenylsilane. Furthermore, many functional groups,
such as nitro, cyano, halide, and ester, were tolerated under this
Entry
Product
Yield (%)
1
2
3
4
5
6
7
8
9
4a: R1 ¼ H
80
83
86
85
48
55
82
78
75
88
4b: R1 ¼ p-OMe
4o: R1 ¼ p-Me
4c: R1 ¼ m-Me
4d: R1 ¼ p-NO2
4e: R1 ¼ p-CN
4p: R1 ¼ p-Br
4i: R2 ¼ p-OMe
4q: R2 ¼ p-Cl
4r: R2 ¼ o-F
10
11
4s
4t
80
76
12
a
Typical conditions: [BMIm][FeCl4] (20 mol%), aldehydes (0.6 mmol),
˚
aniline (0.5 mmol), Ph2SiH2 (0.75 equiv.), 4 A molecular sieves (200
mg), EtOH (2 mL), 80 ꢀC, 16 h.
Scheme 3 Control experiments.
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Acknowledgements
We thank the Support received from the Natural Science
Foundation of Guangdong Province (No.: 2014A030310211), the
Foundation of the Department of Education of Guangdong
Province (No.: YQ2015162), SRF for ROCS, SEM (No.: [2015]311),
and the Science Foundation for Young Teachers of Wuyi
University (No.: 2014zk04, No.: 2015td01 and 2016zk03).
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