Efficient three-component Strecker reaction of acetals and aromatic amines catalysed by hafnium tetrachloride at room temperature

Article abstract of DOI:10.3184/174751913X13814077309487

A straightforward, mild, efficient, one-pot method has been found for the synthesis of á-aminonitriles via three-component Strecker reaction using acetals or cyclic acetals, curious aromatic amines and trimethylsilyl cyanide (TMSCN) catalysed by hafnium tetrachloride at room temperature. It is with good to excellent yields under mild conditions. This developed approach has been successfully applied for the synthesis of a wild rang of á-aminonitriles with a variety of functional groups.

Full text of DOI:10.3184/174751913X13814077309487

690 RESEARCH PAPER
NOVEMBER, 690–693
JOURNAL OF CHEMICAL RESEARCH 2013
Efficient three-component Strecker reaction of acetals and aromatic amines
catalysed by hafnium tetrachloride at room temperature
Xue-Lin Zhang, Qin-Pei Wu* and Qing-Shan Zhang
School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081, P.R. China
A straightforward, mild, efficient, one-pot method has been found for the synthesis of α-aminonitriles via three-component
Strecker reaction using acetals or cyclic acetals, curious aromatic amines and trimethylsilyl cyanide (TMSCN) catalysed by
hafnium tetrachloride at room temperature. It is with good to excellent yields under mild conditions. This developed approach
has been successfully applied for the synthesis of a wild rang of α-aminonitriles with a variety of functional groups.
Keywords: acetals, Strecker reaction, α-aminonitriles, trimethylsilyl cyanide
Acetals play a key role in natural products,1 and in synthetic
Table 1 Three-component Strecker reaction of benzaldehyde dimethyl
acetal, p-toluidine and trimethylsilyl cyanide (TMSCN) with different
and medicinalchemistry,2 and the medicinal chemistry. In
catalystsa
particular, they are extensively used as protecting groups in
Entry
1
2
3
4
Catalyst
HfCl4
Hf(OTf)4
ZrCl4
InCl3
Time/h
0.5
1
1
2
Yield/%b
91
76
75
88
synthetic chemistry.3 They undergo a large number of reactions
such as the Mukaiyama aldol reaction of acetals with enol silyl
ethers which was reported in 1974.4 Since then, there are many
examples reported of the displacement of only one alkoxy
group, as well as transprotection reactions.5
5
6
FeCl3
I2
6
7
57
82
The Strecker reaction has been one of the most important
multicomponent reactions in organic chemistry since it was
first reported 1850.6,7 It is used in C–C bond formation to make
α-aminonitriles.8 α-Aminonitriles are very useful, compounds
which possess important biological activities.9 α-Aminonitriles
are generally prepared by the nucleophilic addition of the
cyanide anion to imines using a variety of cyanating agents6,7
under Strecker-type reaction conditions.10 Me3SiCN is an
effective and safe source of cyanide anions and is commonly
employed as cyanide source in the presence of various Lewis
acids, metal complexes, solid supported acids and organic
catalysts.11–16
7
AlCl3
8
53
8
BiCl3
8
81
9
Bi(OTf)3
SnCl2
CuCl
CuBr
CuI
12
23
24
24
24
23
84
88
42
76
87
31
10
11
12
13
14
CuOTf
aReaction conditions: benzaldehyde dimethyl acetal (1.2 mmol), p-toluidine
(1.0 mmol), TMSCN (1.3 mmol), catalyst (0.20 mmol), acetonitrile (4.0 mL), r.t.
bIsolated yields after flash chromatography.
However, there are many short-comings in the reported
methods, such as long reaction time, the formation of large
amounts of toxic by-products, the need for large amount of
catalysts and harsh reaction conditions.17 Several alternatives
have been developed to overcome these drawbacks as well as
devising novel processes to efficiently catalyse the Strecker
reaction.18 The reaction has been mostly reported to take
place efficiently using aldehydes. Three-component Strecker
reactions using acetals as substrates have rarely been reported.
We now report an efficient method for the synthesis of
α-aminonitriles involving the reaction of the corresponding
acetals, aromatic amines, and TMSCN using HfCl4 as the
catalyst under mild conditions (Scheme 1). To the best of
our knowledge, this is the first report of the synthesis of
α-aminonitriles employing acetals.
Initially, in order to obtain the best reaction conditions,
various Lewis acids were screened in the three-component
Strecker reaction using benzaldehyde dimethyl acetal
(1.2 equiv.), p-toluidine (1.0 equiv.), and TMSCN (1.3 equiv.)
in acetonitrile under an argon atmosphere at room temperature.
The results are summarised in Table 1. The three-component
Strecker reaction proceeded smoothly and generated the
desired product in 91% yield, representing one of the best
results when 20 mol% of HfCl4 was used as the catalyst without
any cocatalyst or activator at very short time (0.5 h) (Table 1,
entry 1). When using Hf(OTf)4, ZrCl4 and InCl3, although the
reaction time was short, the yields were lower on contrary with
HfCl4 (Table 1, entries 2–4). Compared with HfCl4, the reaction
time of FeCl3, I2, AlCl3, BiCl3 and Bi(OTf)3 was longer, and
the yields were much lower (Table 1, entries 5–9). SnCl2 and
Cu salts (such as CuCl, CuBr, CuI, CuOTf) were inferior and
generated the desired product in 88, 42, 76, 87 and 31 yields,
respectively, and the reaction time was much longer than HfCl4
(Table 1, entries 10–14). HfCl4 was therefore chosen as the
catalyst for other substrates.
We next screened the effect of solvent on three-component
Strecker reaction of the model substrates by using 20 mol%
of HfCl4 as the catalyst under an argon atmosphere at room
temperature (Table 2). Among the solvents tested, it was
observed that a much better yield was obtained when the
reaction was carried out in acetonitrile at room temperature
compared to other solvents. Acetonitrile was the most suitable
reaction medium for the three-component Strecker reaction
(Table 2, entry 1). Among the other solvents screened, toluene,
N
C
O
HfCl4 (20 mol%)
R2
R2
+
+
NH2
TMSCN
N
R1
rt
CH
3CN,
R1
O
H
Scheme 1 Strecker reaction of acetals, aromatic amines, and TMSCN in CH3CN.
* Correspondent. E-mail: qpwu@bit.edu.cn
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JOURNAL OF CHEMICAL RESEARCH 2013 691
Table 2 Screening the effect of various solvents for the three-component
Strecker reaction of benzaldehyde dimethyl acetal, p-toluidine and
trimethylsilyl cyanide (TMSCN)a
Table 3 Three-component Strecker reactions of acetals, aromatic amines
and TMSCN using HfCl4 catalysta
Amine
Entry
Acetal (R1)
Time/h
Yield/%b
(R2)
Entry
Solvent
Acetonitrile
Toluene
Ethanol
THF
1,4-dioxane
Dichloromethane
DCE
Time/h
0.5
1
1
1
1.5
1.5
2
Yield/%b
91
80
28
79
31
82
78
1
2
3
4
5
6
7
C6H5
p-CH3C6H4
p-CH3OC6H4
p-FC6H4
p-ClC6H4
p-BrC6H4
m-NO2C6H4
O
p-H
p-H
p-H
p-H
p-H
p-H
p-H
0.5
0.5
0.5
0.5
0.5
0.5
0.5
91
86
88
96
91
92
93
1
2
3
4
5
6
7
8
Nitromethane
2
87
8
p-H
p-H
p-H
0.5
0.5
0.5
76
76
70
aReaction conditions: benzaldehyde dimethyl acetal (1.2 mmol), p-toluidine
(1.0 mmol), TMSCN (1.3 mmol), HfCl4 (0.064 g, 0.2 mmol), solvent
(4.0 mL), r.t.
O
O
9
O
O
O
bIsolated yields after flash chromatography.
Cl
10
ethanol, and THF were inferior and generated the corresponding
products in 80%, 28%, and 79% yields, respectively after a
prolonged reaction time (Table 2, entries 2–4). Note that when
1,4-dioxane and dichloromethane were used as the solvents, the
yields were very different although the reaction time was the
same (Table 2, entries 5 and 6). When DCE was used as the
solvent, the yield was close to that of nitromethane at the same
reaction time (Table 2, entries 7 and 8). Thus, all the reactions
were performed in acetonitrile under an argon atmosphere with
20 mol% of HfCl4 at room temperature without any cocatalyst
or activator.
Br
O
O
11
p-H
0.5
67
O
O
12
13
14
p-H
p-H
p-H
0.5
0.5
0.5
75
74
81
O
O
O
Cl
O
Subsequently, a variety of α-aminonitriles were prepared
from various acetals, and aromatic amines using the previously
optimised reaction conditions. The results are summarised in
Table 3. At the beginning of the investigation into the acetal
substrate scope, aniline and TMSCN were used as model
substrates and a variety of acetals and cyclic acetals were
examined in the three-component Strecker reactions (Table 3,
entries 1–14). As can be seen from Table 3, acyclic acetals were
often much more reactive than cyclic acetals. Acyclic acetals,
with both electron-donating and electron-withdrawing groups
attached to the benzene ring, underwent the three-component
Strecker reaction and smoothly, generated the corresponding
products in good to excellent yields (Table 3, entries 1–7).
Then, the three-component Strecker reaction was examined
using cyclic acetals (Table 3, entries 8–14). Aromatic cyclic
acetal possessing no substituent group afforded good yields
(Table 3, entry 8). Five-membered cyclic acetals with electron-
donating groups attached to the benzene rings did not decrease
the reactivity, but electron-withdrawing groups did (Table 3,
entries 9–11). Interestingly, six-membered cyclic acetals with
electron-withdrawing groups attached to the benzene rings gave
the desired product more smoothly than those with electron-
donating groups (Table 3, entries 12–14). Unfortunately, when
(1,1-dimethoxy-ethyl) benzene was used as the substrate, the
yield was much lower than acetals (Table 3, entry 15).
In order to expand the scope of the amine substrates,
several aromatic amines were examined for the synthesis
of α-aminonitriles using benzaldehyde dimethyl acetal and
TMSCN as the model substrates. Good to excellent yields
of the desired products were obtained in each case (Table 3.
The reactions were completed within 0.5 h affording 81–95%
yields. The results indicated that aromatic amines with both
electron-donating and electron-withdrawing groups attached to
the benzene rings underwent the reaction to furnish the desired
products in good yields (Table 3, entries 16–21). With aliphatic
alkynes such as octyne and decyne only a trace amount of the
product was obtained. However, when secondary amines, such
as Bn2NH were used as an amine substrate, a good yield of
O
O
15
p-H
0.5
43
16
17
18
19
20
21
22
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
p-CH3
m-CH3
p-CH3O
p-Cl
p-NO2
p-COOEt
Bn2NH
0.5
0.5
0.5
0.5
0.5
0.5
5
91
81
87
94
95
95
74
aReaction conditions: acetal (1.2 mmol), aromatic amine (1.0 mmol),
TMSCN (1.3 mmol), HfCl4 (0.064 g, 0.20 mmol), acetonitrile (4.0 mL),
room temperature.
bIsolated yields after flash chromatography.
product was obtained under the standard reaction conditions,
but with longer reaction time (Table 3, entry 22).
To elucidate the reaction pathway, we hoped to intercept
the reaction intermediate. Yang and coworkers19 demonstrated
that [In–H] generated in the InCl3/Et3SiH/MeOH system is
an active agent for the reductive amination of aldehydes with
various amines. Accordingly, Et3SiH-InCl3 is able to trap the
iminium ion if it occurs in the reaction solution. Under our
standard reaction conditions, Et3SiH (2.0 equiv.) was added to
the reaction solution of benzaldehyde dimethyl acetal, aniline,
and TMSCN. Both N-benzylaniline (the reductive amination
product) and the corresponding α-aminonitrile 2 were obtained
of 32% and 63% isolated yields, respectively (Scheme 2). The
N-benzylaniline was formed from the reduction of benzyl
iminium ion 1, involving substitution of both the methoxy
groups of benzaldehyde dimethyl acetal with aniline. At the
same time, a α-aminonitrile product 2 was formed by the attack
of TMSCN on the intermediate 1. These results show that this
protocol proceeds via an iminium ion intermediate 1.
In conclusion, we have developed a simple and efficient
method for the three-component Strecker reaction of acetals,
aromatic amines and TMSCN in acetonitrile through C–C bond
activation at room temperature to yield α-aminonitriles with
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692 JOURNAL OF CHEMICAL RESEARCH 2013
NHPh
Ph
Et iH
3S
Ph
l-
32
%
H
PhNH2
Hf l
C 4
,
O
O
N
N
C
Ph
N
H
C
TM
N
SC
1
63
%
2
Scheme 2 Intercepting the reaction intermediate with iminium ion.
2-(4-Bromophenyl)-2-(phenylamino)acetonitrile:
White
solid,
good to excellent yields using HfCl4 as the catalyst. The process
was simple. Aromatic acetals are particularly effective in the
reaction, which provides an extension to the HfCl4 catalysed
three-component Strecker reaction. The reaction is efficient
and has a high atom-economy in this mild three-component
Strecker reaction. The scope, mechanism, stereoselectivity,
and synthetic applications of this reaction are under further
investigation.
m.p. 87–88 °C (lit.20 87–88 °C); yield 92% (Table 3, entry 6); 1H NMR
(400 MHz, CDCl3) δ 7.67–7.55 (m, 2H), 7.48 (d, J= 8.5 Hz, 2H), 7.33–7.25
(m, 2H), 6.92 (t, J=7.4 Hz, 1H), 6.76 (d, J=7.9 Hz, 2H), 5.40 (s, 1H), 4.06
(s, 1H).
2-(3-Nitrophenyl)-2-(phenylamino)acetonitrile: Light yellow solid,
m.p. 89–90 °C (lit.20 86–88 °C); yield 93% (Table 3, entry 7); 1H NMR
(400 MHz, CDCl3) δ 8.42 (s, 1H), 8.22 (d, J=8.2 Hz, 1H), 7.90 (d,
J=7.7 Hz, 1H), 7.59 (t, J=8.0 Hz, 1H), 7.19 (dd, J=14.6, 6.9 Hz, 2H), 6.86
(t, J=7.4 Hz, 1H), 6.69 (d, J= 8.0 Hz, 2H), 5.49 (s, 1H), 4.09 (s, 1H).
2-(3-Bromophenyl)-2-(phenylamino)acetonitrile22: White solid, m.p.
73–74 °C; yield 67% (Table 3, entry 11); 1H NMR (400 MHz, CDCl3) δ
7.77 (s, 1H), 7.57 (t, J=8.6 Hz, 2H), 7.37–7.25 (m, 3H), 6.93 (t, J= 7.3 H z,
1H), 6.77 (d, J= 8.3 Hz, 2H), 5.42 (s, 1H), 4.08 (s, 1H).
2-Phenyl-2-(phenylamino)propanenitrile: White solid, m.p. 140–
141 °C (lit.20 140–142 °C); yield 43% (Table 3, entry 15); 1H NMR
(400 MHz, CDCl3) δ 7.57 (dd, J=5.7, 3.8 Hz, 2H), 7.33 (dd, J= 10.5,
4.5 Hz, 2H), 7.20 (s, 1H), 7.09–7.02 (m, 2H), 6.74 (t, J=7.3 Hz, 1H), 6.48
(d, J= 8.1 Hz, 2H), 4.24 (s, 1H), 1.89 (s, 3H).
Experimental
Reactions were performed under an argon atmosphere at room
temperature. The materials were used as purchased. Unless otherwise
stated, all solvents and reagents were commercially available and
used as purchased without further purification. Reactions were
monitored by thin-layer chromatography using gel F 254 plates. The
silica gel (300–400 mesh) was used for column chromatography, and
the distillation range of petroleum ether was 60–90 °C. NMR spectra
was recorded in CDCl3 on either a Varian 400 MHz or Bruker 400 MHz
Fourier-transform spectrometer. Chemical shifts were reported in ppm
referenced to TMS or the CHCl3 solvent residual peak at 7.26 ppm for
1H and 77.23 ppm for 13C.
Acetal (1.2 mmol), aromatic amine (1.0 mmol), TMSCN (0.165 mL,
1.3 mmol), and HfCl4 (0.064 g, 0.2 mol) were added to a flask (25 mL),
followed by addition of acetonitrile (4.0 mL) under argon. The mixture
was stirred at room temperature and monitored by TLC. The solution
was then diluted with dichloromethane (5.0 mL), washed with brine.
The aqueous layer was extracted with CH2Cl2 (3×10 mL), the combined
organic layer was dried over MgSO4, filtered, and evaporated under
vacuum. The residue was purified by column chromatography on silica
gel (petroleum ether) to afford the desired product.
2-Phenyl-2-(p-tolylamino)acetonitrile: White solid, m.p. 102–103 °C
(lit.20 104–106 °C); yield 91% (Table 3, entry 16); H NMR (400 MHz,
1
CDCl3) δ 7.61 (dd, J=7.6, 1.9 Hz, 2H), 7.47 (m, 3H), 7.09 (d, J= 8.2 H z,
2H), 6.71 (d, J= 8.5 Hz, 2H), 5.41 (s, 1H), 3.93 (s, 1H), 2.29 (s, 3H).
2-Phenyl-2-(m-tolylamino)acetonitrile: White solid, m.p. 94–95 °C
1
(lit.23 94–96 °C); yield 81% (Table 3, entry 17); H NMR (400 MHz,
CDCl3) δ 7.59 (dd, J=7.6, 1.9 Hz, 2H), 7.45 (m, 3H), 7.16 (t, J= 7.7 H z,
1H), 6.72 (d, J=7.4 Hz, 1H), 6.59 (d, J=8.3 Hz, 2H), 5.41 (s, 1H), 3.97 (s,
1H), 2.32 (s, 3H).
2-((4-Methoxyphenyl)amino)-2-phenylacetonitrile: White solid,
m.p. 94–95 °C (lit.24 75–77 °C); yield 87% (Table 3, entry 18); 1H NMR
(400 MHz, CDCl3) δ 7.51–7.42 (m, 1H), 7.27 (m, 4H), 7.01–6.88 (m, 1H),
6.70 (d, J=8.9 Hz, 1H), 6.66–6.55 (m, 2H), 5.23–5.03 (m, 1H), 3.77–3.51
(m, 4H).
2-((4-Chlorophenyl)amino)-2-phenylacetonitrile: White solid, m.p.
107–108 °C (lit.23 91–92 °C); yield 94% (Table 3, entry 19); H NMR
(400 MHz, CDCl3) δ 7.54–7.41 (m, 2H), 7.31 (d, J= 5.5 Hz, 3H), 7.16–7.04
(m, 2H), 6.55 (d, J= 8.8 Hz, 2H), 5.24 (s, 1H), 3.91 (s, 1H).
2-Phenyl-2-(phenylamino)acetonitrile: White solid, m.p. 76–77 °C
(lit.20 76–78 °C); yield 91% (Table 3, entry 1), 76% (Table 3, entry 8) and
75% (Table 3, entry 12); 1H NMR (400 MHz, CDCl3) δ 7.53 (dd, J= 7.5,
1.9 Hz, 2H), 7.44–7.36 (m, 3H), 7.24–7.18 (m, 2H), 6.83 (t, J= 7.4 H z, 1H ),
6.71 (d, J= 8.2 Hz, 2H), 5.36 (s, 1H), 3.96 (s, 1H).
1
2-((4-Nitrophenyl)amino)-2-phenylacetonitrile: White solid, m.p.
128–129 °C (lit.25 128–129 °C); yield 95% (Table 3, entry 20); 1H NMR
(400 MHz, CDCl3) δ 8.09 (d, J=9.1 Hz, 2H), 7.51 (dd, J=6.4, 3.0 Hz, 2H),
7.42 (d, J=4.6 Hz, 3H), 6.69 (d, J=9.2 Hz, 2H), 5.45 (d, J= 7.4 H z, 1H ),
4.78 (d, J= 7.1 H z, 1H ).
2-(Phenylamino)-2-(p-tolyl)acetonitrile: White solid, m.p. 77–78 °C
(lit.20 69–71 °C); yield 86% (Table 3, entry 2), 76% (Table 3, entry 9) and
74% (Table 3, entry 13); 1H NMR (400 MHz, CDCl3) δ 7.46 (d, J= 8.0 H z,
2H), 7.26 (t, J=8.2 Hz, 4H), 6.89 (t, J=7.8 Hz, 1H), 6.76 (d, J= 8.5 H z,
2H), 5.37 (d, J=7.9 Hz, 1H), 3.99 (d, J= 7.7 Hz, 1H), 2.38 (s, 3H).
2-(4-Methoxyphenyl)-2-(phenylamino)acetonitrile: White solid,
m.p. 95–96 °C (lit.20 93–94 °C); yield 88% (Table 3, entry 3); 1H NMR
(400 MHz, CDCl3) δ 7.44 (d, J=8.7 Hz, 2H), 7.23–7.18 (m, 2H), 6.90 (d,
J=8.7 Hz, 2H), 6.83 (t, J=7.4 Hz, 1H), 6.70 (d, J=8.2 Hz, 2H), 5.29 (s,
1H), 3.90 (s, 1H), 3.77 (s, 3H).
Ethyl-4-((cyano(phenyl)methyl)amino)benzoate: White solid, m.p.
109–110 °C; yield 95% (Table 3, entry 21); 1H NMR (400 MHz, CDCl3)
δ 7.99 (d, J=8.7 Hz, 2H), 7.69–7.58 (m, 2H), 7.49 (d, J=5.3 Hz, 3H), 6.77
(d, J=8.7 Hz, 2H), 5.51 (s, 1H), 4.52 (s, 1H), 4.35 (q, J=7.1 Hz, 2H), 1.39
(t, J=7.1 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ 166.47, 148.33, 133.25,
131.70, 129.92, 129.59, 127.34, 121.99, 117.63, 112.98, 60.66, 49.56, 14.49.
HRMS (ESI) calcd forC17H17N2O2 [M + H+]: 281.12845, found 281.12845.
2-(Dibenzylamino)-2-phenylacetonitrile: White solid, m.p. 102–
103 °C (lit.26 96.4 °C); yield 74% (Table 3, entry 22); 1H NMR (400 MHz,
CDCl3) δ 7.50 (d, J=7.4 Hz, 2H), 7.28 (m, 11H), 7.17 (dd, J= 11.9, 4.9 H z,
2H), 4.82 (s, 1H), 3.80 (d, J=13.4 Hz, 2H), 3.33 (d, J= 13.4 H z, 2H ).
N-Benzylaniline27: Light yellow liquid; yield 32% (Scheme 2); 1H NMR
(400 MHz, CDCl3) δ 7.45–7.35 (m, 4H), 7.32 (d, J=6.9 Hz, 1H), 7.26–7.17
(m, 2H), 6.76 (t, J=7.3 Hz, 1H), 6.67 (d, J=8.2 Hz, 2H), 4.36 (s, 2H), 4.14
(br s, 1H). 13C NMR (101 MHz, CDCl3) δ 148.21, 139.53, 129.42, 128.79,
127.68, 127.39, 117.78, 113.06, 48.51.
2-(4-Fluorophenyl)-2-(phenylamino)acetonitrile: White solid, m.p.
1
98–99 °C (lit.21 98–100 °C); yield 96% (Table 3, entry 4); H NMR
(400 MHz, CDCl3) δ 7.65–7.56 (m, 2H), 7.31–7.25 (m, 2H), 7.19–7.12 (m,
2H), 6.92 (t, J=7.4 Hz, 1H), 6.78 (d, J=7.7 Hz, 2H), 5.42 (s, 1H), 4.02 (s,
1H).
2-(4-Chlorophenyl)-2-(phenylamino)acetonitrile: White solid, m.p.
110–112 °C (lit.20 114–116 °C); yield 91% (Table 3, entry 5), 70% (Table 3,
entry 10) and 81% (Table 3, entry 14); 1H NMR (400 MHz, CDCl3) δ 7.48
(d, J=8.5 Hz, 2H), 7.37 (d, J=8.6 Hz, 2H), 7.25–7.18 (m, 2H), 6.85 (t,
J=7.4 Hz, 1H), 6.70 (d, J= 8.5 Hz, 2H), 5.35 (s, 1H), 3.97 (s, 1H).
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Financial support of this work from the National Natural
Science Foundation of China (21172019) is appreciated.
Received 23 July 2013; accepted 23 August 2013
Paper 1302078 doi: 10.3184/174751913X13814077309487
Published online:11 November 2013
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