Reductive amination of acetals by anilines in the presence of triethylsilane and iodine

Article abstract of DOI:10.3184/174751914X13946178288518

A mild and efficient method for N-alkylation of aromatic amines with various acetals such as aryl, alkyl, cyclic and acyclic acetals was developed. A number of aromatic amines bearing electron-donating or electron-withdrawing substituents were directly alkylated by acetals with excellent yields. The method uses a catalytic amount of I₂ and triethylsilane as the hydride source without a metal present. Monoalkylation with excellent chemoselectivity was observed.

Full text of DOI:10.3184/174751914X13946178288518

JOURNAL OF CHEMICAL RESEARCH 2014
VOL. 38 MAY, 261–264
RESEARCH PAPER 261
Reductive amination of acetals by anilines in the presence of triethylsilane
and iodine
Xue-Lin Zhang, Pan Yu, Yong-Wei Wu, Qin-Pei Wu* and Qing-Shan Zhang
School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081, P.R. China
A mild and efficient method for N‑alkylation of aromatic amines with various acetals such as aryl, alkyl, cyclic and acyclic
acetals was developed. A number of aromatic amines bearing electron‑donating or electron‑withdrawing substituents were
directly alkylated by acetals with excellent yields. The method uses a catalytic amount of I2 and triethylsilane as the hydride
source without a metal present. Monoalkylation with excellent chemoselectivity was observed.
Keywords: acetals, alkylation of amines, secondary amine, triethylsilane
Amines are a very important class of compounds in nature and
the chemical industry.1 They are used in many applications,
including numerous active pharmaceuticals,2 herbicides,
agrochemicals,3 conducting polymers4 and organic light‑
emitting diodes.5 Thus, various procedures for the formation
of C–N bonds6 have been developed. These procedures include
Hofmann alkylation,7 Buchwald–Hartwig8 and Ullmann
reactions,9 hydroamination of olefins and alkynes,10 reduction
of nitriles,11 imines12 and nitro compounds,13 alkylation of
amines using alcohols14 and reductive amination of carbonyl
compounds.15 However, most of these approaches require noble
metal catalysts, organic ligand additives16 and more than one
equivalent of base.6,17 These additives and expensive metal
catalysts render the process uneconomic and hazardous to the
environment. Thus, other methods are of particularly great
value and are therefore widely sought.
First, weattemptedtheuseofInCl3–Et3SiHintheN‑alkylation
of aniline with benzyl dimethyl acetal at room temperature.
The desired benzyl aniline was obtained at 73% yield (Table 1,
entry 1). To improve the efficiency, a series of Lewis acids were
screened. The results of this screening indicate that I2–Et3SiH
afforded the highest yield, 93% (Table 1, entries 1–12). Metal‑
based catalysts such as Hf(OTf) ZrCl HfCl4, Bi(OTf)3,
,
,
4
4
BiCl3 and SnCl2 led to slightly lower yields (Table 1, 84–87%,
entries 2, 4, 6, 9 and 11). The I2–Et3SiH combination has been
used as catalyst for a wide range of reactions.31,32 However, its
application in reductive N‑alkylation has not been reported.
The effect of solvents on the direct reductive N‑alkylation
reaction was also explored (Table 1, entries 13–21). MeCN was
found to provide the best yield (Table 1, entry 8). The yield
in ethanol was also satisfactory, higher than those in DMSO,
Recently, organosilanes have been successfully applied
in reductive amination of carbonyl compounds.15,18 These
compounds offer several advantages over traditional reducing
agents such as cyanoborohydride19 and organotin,20 which
generate toxic byproducts.21 Silanes are environmentally
friendly agents used in many kinds of reactions.22 Silane‑based
reduction systems usually contain metals, for example, IrCl3–
Et3SiH,23 TFA–PMHS,24 Ti(OiPr)4–PMHS,25 AlCl3–PMHS,26
Table 1 Optimising the reaction conditions for the one‑pot N‑alkylation of
aniline with acetalsa
O
Cat. (0.1 equiv.)
Et3SiH, solvent
Ph
NH2
+
Ph
N
Ph
Ph
Entry
O
H
Catalyst
Solvent
CH3CN
Time/h
Yield/%b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
InCl3
Bi(OTf)3
FeCl3
BiCl3
AlCl3
SnCl2
CuOTf
I2
Hf(OTf)4
HfCl4
ZrCl4
Cu(OTf)2
I2
I2
I2
I2
I2
I2
I2
I2
I2
16
7
6
16
12
8
8
5
6
5
5
8
7
6
7
8
6
6
73
85
71
87
78
85
63
93
84
75
87
69
39
36
77
<5
38
32
42
48c
15
Re2O7–Et3SiH,27 PhSiH3–ReIO2(PPh3)2,28 PhSiH3–Bu2SnCl2
20
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DMSO
CH3NO2
EtOH
1,4‑Dioxane
THF
CH2Cl2
Toluene
CH3CN/H2O(1:1)
–
and PhSiH3–MoO2Cl2.29 Yang and coworkers30 demonstrated
that Zn(ClO4)2·6H2O is an essential activator in the reductive
amination of carbonyl compounds catalysed by InCl3–Et3SiH.
In addition, one‑pot reactions of organic synthesis have
advantages over conventional stepwise or indirect reactions.
Direct alkylation of amines with acetals is more advantageous
over direct alkylation with aldehydes since it obviates the need
for deprotecting steps; acetals are extensively used as protecting
groups in synthetic chemistry.31 We report here our findings
that acetals can be used efficiently for direct N‑alkylation of
aromatic amines to form secondary arylamines in the presence
of I2–Et3SiH (Scheme 1).
NH2
O
H
I2 (10 mol%)
R1
N
+
O
R1
6
6
14
CH3CN, Et3SiH
R2
R2
Scheme 1 Secondary amine formation by direct N‑alkylation of aromatic
amines with acetals catalysed by iodine.
aReaction conditions: benzaldehyde dimethyl acetal (1.2 mmol), aniline
(1.0 mmol), triethylsilane (2.0 mmol), catalyst (0.10 mmol), solvent
(4.0 mL), under argon, room temperature.
bIsolated yields after flash chromatography.
* Correspondent. E‑mail: qpwu@bit.edu.cn
cUnder reflux.

262 JOURNAL OF CHEMICAL RESEARCH 2014
MeNO2, THF, dioxane, CH2Cl2, toluene and MeCN–H2O
(Table 1, entries 13–20, 5–48%). However, N‑alkylation was
not effective in the absence of solvent (Table 1, entry 21).
The optimised conditions were found to require 10 mol% I2
and 200 mol% Et3SiH in MeCN; using these conditions, we then
extended the range of acetals used for the direct N‑alkylation. As
shown in Table 2, aniline alkylation was efficiently performed
at high yields (60–95%) by using various aryl acetals. We also
found that aromatic acetals with electron‑donating or weakly
electron‑withdrawing substituents such as MeO, F, Cl and
Br (Table 2, entries 2–6) showed similar reactivity and gave
secondary arylamine products at excellent yields (>88%).
On the other hand, a strong electron‑withdrawing group such
as the p‑ or m‑nitro function did not form products at room
temperature; the expected secondary amines were produced
in moderate yields by refluxing (Table 2, entries 7 and 8).
Compared with the p‑nitro aromatic acetal, the m‑nitro aryl
acetal afforded a higher yield (73% vs 60%) of the desired
compound. Thus, electronic variations in aromatic acetals
affect the alkylation activity.
The substrate versatility of this novel protocol was further
demonstrated by cyclic acetals, which are more stable than
their corresponding acyclic acetals.31 In most cases, reductive
N‑alkylation of aniline with either five‑ or six‑membered‑ring
acetals was not observed at room temperature. Under reflux
conditions, the expected secondary amines were produced at
moderate to high yields (Table 2, entries 9–17, 65–89% yields).
Apart from the aromatic acetals, we also investigated
the reactivity of alkyl acetals. Alkylation of aniline
with cinnamaldehyde dimethyl acetal was successfully
accomplished (75% yield); the double bond was found to be
inert under this reductive process (Table 2, entry 18). The
reductive system IrCl3–Et3SiH,23 Pd–H2, or TFA–PMHS,18
however, is not compatible with substrates containing double
bonds in the reductive amination of aldehydes. Reductive
alkylation with other alkyl acetals such as dimethyl acetals
of phenylacetaldehyde was carried out smoothly under reflux
to give the corresponding secondary amines in good yields
(Table 2, entry 19). Alkylation with the ketal, 2‑methyl‑2‑
phenyl‑1,3‑dioxolane, was also successful (Table 2, entry 18).
Most importantly, none of the cases considered in the present
study gave overalkylation products; this observation suggests
that the direct alkylation system is highly efficient at producing
various secondary aromatic amines.
Furthermore, the reactions of various aromatic amines with
benzaldehyde dimethyl acetal were then examined. The results
revealthatallanilineswithvarioussubstituentsgavetheexpected
products in good to excellent isolated yields (Table 3, 77–95%).
A number of aromatic amines bearing either electron‑donating
or electron‑withdrawing substituents could be smoothly
alkylated to give the secondary amines in high yields (Table 3,
entries 1–5, 88–95%). Remarkably, a strong withdrawing
substituent such as nitro and acyl groups did not decrease the
yield (Table 3, entries 6 and 7, >91% yield). In contrast, the
catalytic system of AlCl3–PMHS26 and NaBH(OAc)315 catalysed
reductive amination of carbonyl compounds, leading to similar
products at much lower yields. These results clearly indicate
that the electronic effect of substituents in anilines has only a
slight impact on the N‑alkylation by acetals.
Table 2 Direct N‑alkylation of aromatic amines with various alkyl and
aromatic acetalsa
Entry
Acetal (R1)
C6H5
p‑CH3OC6H4
p‑CH3C6H4
p‑FC6H4
p‑ClC6H4
p‑BrC6H4
p‑NO2C6H4
Time/h
Yield/%b
1
2
3
4
5
6
7
8
5
5
5
5
5
5
5
5
93
90
93
90
88
97
60c
73c
m‑NO2C6H4
O
9
10
11
11
5
74
O
O
68c
68c
O
O
O
5
O
12
13
14
5
5
5
65c
66c
75c
O
O
MeO
We believe the mechanism involves I2 reacting with Et3SiH
to form HI and Et3SiI, followed by transformation of the acetal
into the carboxonium ion 1, which then reacts with the amine to
give the imine. This is then reduced by triethylsilane forming
the product. (Scheme 2).33
In conclusion, we have demonstrated a mild, metal‑free
approach for direct reductive N‑alkylation of aromatic
amines with acetals using the I2–Et3SiH reductive system.
O
O
l
C
O
O
15
16
5
5
67c
89c
l
C
O
Br
O
O
Br
Table 3 Direct N‑alkylation of various aromatic amines with acetalsa
O
O
17
18
5
5
79c
75
Entry
Acetal (R1)
Amine (R2)
Time/h
Yield/%b
t
OE
1
2
3
4
5
6
7
C6H5
C6H5
C6H5
p‑BrC6H4
C6H5
p‑CH3C6H4NH2
m‑CH3C6H4NH2
p‑CH3OC6H4NH2
p‑CH3OC6H4NH2
p‑ClC6H4NH2
5
5
5
5
5
5
5
90
95
88
88c
90
91
95
OEt
MeO
19
20
5
5
85c
67c
OMe
O
O
C6H5
C6H5
p‑NO2C6H4NH2
p‑COOEtC6H4NH2
aReaction conditions: acetal (1.2 mmol), aniline (1.0 mmol), triethylsilane
aReaction conditions: benzaldehyde dimethyl acetal (1.2 mmol), amine
(2.0 mmol), I2 (0.10 mmol), acetonitrile (4.0 mL), under argon, room
(1.0 mmol), triethylsilane (2.0 mmol), I2 (0.10 mmol), acetonitrile (4.0 mL),
temperature.
bIsolated yield.
cUnder reflux.
under argon, room temperature.
bIsolated yield.
cUnder reflux.

JOURNAL OF CHEMICAL RESEARCH 2014 263
N-(4-Nitrobenzyl)aniline: Yellow solid, m.p. 67–68 °C (lit.37 67–
69 °C); yield 60% (Table 2, entry 7); 1H NMR (400 MHz, CDCl3) δ 8.19
(d, J=8.4 Hz, 2H), 7.54 (d, J=8.4 Hz, 2H), 7.18 (t, J=7.6 Hz, 2H), 6.76
(t, J=7.3 Hz, 1H), 6.60 (d, J=8.5 Hz, 2H), 4.61 (br s, 1H), 4.48 (s, 2H).
N-(3-Nitrobenzyl)aniline: Yellow solid, m.p. 84–85 °C (lit.38 84–
O
O
O
I2
[H]
O
I-
Et3SiH
1
85 °C); yield 73% (Table 2, entry 8); H NMR (400 MHz, CDCl3) δ
1
8.24 (br s, 1H), 8.12 (d, J=8.4 Hz, 1H), 7.72 (d, J=7.3 Hz, 1H), 7.51 (t,
J=7.9 Hz, 1H), 7.18 (t, J=7.2 Hz, 2H), 6.75 (t, J=7.3 Hz, 1H), 6.61 (d,
J=8.3 Hz, 2H), 4.47 (s, 2H), 4.41 (br s, 1H).
Scheme 2 Reduction of benzaldehyde dimethyl acetal.
N-(3-Bromobenzyl)aniline:39 Light yellow oily liquid; yield 89%
(Table 2, entry 16);1H NMR (400 MHz, CDCl3) δ 7.53 (br s, 1H), 7.40
(d, J=7.9 Hz, 1H), 7.31 (d, J=7.6 Hz, 1H), 7.24–7.14 (m, 3H), 6.75 (t,
J=7.3 Hz, 1H), 6.63 (d, J=7.6 Hz, 2H), 4.32 (s, 2H).
This one‑pot reaction proceeds at high efficiency to give the
corresponding secondary amines, which are extremely useful
in the construction of biologically important compounds. The
reductive condition is mild enough to tolerate a broad range of
functional groups such as Cl, Br, I, F, MeO, NO2 and double
bonds.
N-(2-Bromobenzyl)aniline:40 Light yellow oily liquid; yield 79%
1
(Table 2, entry 17); H NMR (400 MHz, CDCl3) δ 7.58 (d, J=7.9 Hz,
1H), 7.42 (d, J=7.6 Hz, 1H), 7.26 (t, J=7.5 Hz, 1H), 7.22–7.10 (m, 3H),
6.74 (t, J=7.3 Hz, 1H), 6.63 (d, J=8.0 Hz, 2H), 4.41 (s, 2H), 4.31 (br s,
1H).
Experimental
N-Cinnamylaniline:34 Light yellow oily liquid; yield 75% (Table 2,
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 TLC using gel F 254 plates. The silica gel (300–400 meshes) is used
for column chromatography, and the distillation range of petroleum
ether is 60–90 °C. NMR spectra were recorded in CDCl3 on either a
Varian 400 MHz or Bruker 400 MHz Fourier‑transform spectrometer.
Chemical shifts are reported in ppm referenced to tetramethylsilane or
the CHCl3 solvent residual peak at 7.26 ppm for 1H and 77.23 ppm for
13C.
Acetal (1.2 mmol), amine (1.0 mmol), triethylsilane (0.319 mL,
2.0 mmol) and I2 (0.025 g, 0.1 mmol) 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 mL), washed with brine. The
aqueous layers was extracted with CH2Cl2 (3×10 mL), the combined
organic layers were dried over MgSO4, filtered, and evaporated
under reduced pressure. The residue was purified by column
chromatography on silica gel (petroleum ether) to afford the desired
product.
1
entry 18); H NMR (400 MHz, CDCl3) δ 7.38 (d, J=7.8 Hz, 2H), 7.32
(t, J=7.6 Hz, 2H), 7.28–7.17 (m, 3H), 6.80–6.58 (m, 4H), 6.45–6.31 (m,
1H), 3.95 (d, J=5.8 Hz, 2H), 3.89 (br s, 1H).
N-Phenethylaniline:36 Light yellow oily liquid; yield 85% (Table 2,
entry 19); 1H NMR (400 MHz, CDCl3) δ 7.37–7.31 (m, 2H), 7.30–7.17
(m, 5H), 6.74 (t, J=7.3 Hz, 1H), 6.65 (d, J=7.6 Hz, 2H), 3.86 (br s, 1H),
3.42 (t, J=7.1 Hz, 2H), 2.94 (t, J=7.0 Hz, 2H).
N-(1-Phenylethyl)aniline:34 Light yellow oily liquid; yield 67%
1
(Table 2, entry 22); H NMR (400 MHz, CDCl3) δ 7.39 (d, J=7.1 Hz,
2H), 7.33 (t, J=7.6 Hz, 2H), 7.27–7.21 (m, 1H), 7.15–7.08 (m, 2H), 6.67
(t, J=7.3 Hz, 1H), 6.54 (d, J=7.7 Hz, 2H), 4.50 (q, J=6.7 Hz, 1H), 4.21
(br s, 1H), 1.54 (d, J=6.7 Hz, 3H).
N-Benzyl-4-methylaniline:41 Light yellow oily liquid; yield 90%
1
(Table 3, entry 1); H NMR (400 MHz, CDCl3) δ 7.47–7.35 (m, 4H),
7.34–7.24 (m, 1H), 7.03 (d, J=7.9 Hz, 2H), 6.61 (d, J=8.3 Hz, 2H), 4.34
(s, 2H), 3.97 (br s, 1H), 2.28 (s, 3H).
N-Benzyl-3-methylaniline:36 Light yellow oily liquid; yield 95%
1
(Table 3, entry 2); H NMR (400 MHz, CDCl3) δ 7.41–7.33 (m, 4H),
7.31–7.25 (m, 1H), 7.08 (t, J=7.7 Hz, 1H), 6.56 (d, J=7.5 Hz, 1H), 6.51–
6.44 (m, 2H), 4.33 (s, 2H), 3.99 (br s, 1H), 2.28 (s, 3H).
N-Benzylaniline:34 Light yellow oily liquid; yield 93% (Table 2,
entries 1, 9 and 10), 74% (Table 2, entry 9) and 68% (Table 2, entry 10);
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 (100 MHz, CDCl3) δ 148.2, 139.5,
129.4, 128.8, 127.7, 127.4, 117. 8, 113.1, 48.5.
N-Benzyl-4-methoxyaniline:34 Light yellow oily liquid; yield 88%
(Table 3, entry 3); 1H NMR (400 MHz, CDCl3) δ 7.39 (d, J=6.8 Hz, 3H),
7.34 (d, J=7.8 Hz, 1H), 7.31–7.25 (m, 1H), 6.79 (d, J=8.9 Hz, 2H), 6.63
(d, J=8.9 Hz, 2H), 4.29 (s, 2H), 4.04 (br s, 1H), 3.75 (s, 3H).
N-(4-Bromobenzyl)-4-methoxyaniline:34 Light yellow oily liquid;
N-(4-Methoxybenzyl)aniline:34 Light yellow oily liquid; yield 90%
1
yield 88% (Table 3, entry 4); H NMR (400 MHz, CDCl3) δ 7.45 (d,
1
(Table 2, entries 2 and 13) and 66% (Table 2, entry 13); H NMR
J=8.1 Hz, 2H), 7.24 (d, J=8.1 Hz, 2H), 6.77 (d, J=8.5 Hz, 2H), 6.56 (d,
J=8.5 Hz, 2H), 4.24 (s, 2H), 3.73 (s, 3H).
(400 MHz, CDCl3) δ 7.31 (d, J=8.5 Hz, 2H), 7.19 (t, J=7.9 Hz, 2H),
6.89 (d, J=8.6 Hz, 2H), 6.74 (t, J=7.3 Hz, 1H), 6.66 (d, J=7.7 Hz, 2H),
4.27 (s, 2H), 4.21 (br s, 1H), 3.81 (s, 3H).
N-Benzyl-4-chloroaniline:36 Light yellow oily liquid; yield 90%
1
(Table 3, entry 5); H NMR (400 MHz, CDCl3) δ 7.35–7.29 (m, 4H),
N-(4-Methylbenzyl)aniline:35 Light yellow oily liquid; yield 93%
(Table 2, entries 3, 11 and 12), 68% (Table 2, entry 11) and 65%
7.27–7.18 (m, 1H), 7.05–7.10 (m, 2H), 6.53–6.50 (m, 2H), 4.26 (s, 2H),
4.17 (br s, 1H).
1
(Table 2, entry 12); H NMR (400 MHz, CDCl3) δ 7.26 (d, J=6.8 Hz,
N-Benzyl-4-nitroaniline: Yellow solid, m.p. 146–147 °C (lit.42 146–
2H), 7.20–7.13 (m, 4H), 6.71 (t, J=7.3 Hz, 1H), 6.63 (d, J=7.8 Hz, 2H),
4.28 (s, 2H), 3.98 (br s, 1H), 2.34 (s, 3H).
1
148 °C); yield 91% (Table 3, entry 7); H NMR (400 MHz, CDCl3) δ
8.15–8.03 (m, 2H), 7.44–7.31 (m, 5H), 6.57 (d, J=7.2 Hz, 2H), 4.86 (br
s, 1H), 4.43 (s, 2H).
N-(4-Fluorobenzyl)aniline36: Light yellow oily liquid; yield 90%
1
(Table 2, entry 4); H NMR (400 MHz, CDCl3) δ 7.35 (dd, J=8.4,
Ethyl-4-(benzylamino)benzoate:43 White solid, m.p. 90–91 °C;
5.5 Hz, 2H), 7.20 (t, J=7.8 Hz, 2H), 7.04 (t, J=8.7 Hz, 2H), 6.75 (t,
J=7.3 Hz, 1H), 6.64 (d, J=8.3 Hz, 2H), 4.31 (s, 2H), 4.07 (br s, 1H).
N-(4-Chlorobenzyl)aniline34: Light yellow oily liquid; yield 88%
(Table 2, entries 5, 14 and 15), 75% (Table 2, entry 14) and 67%
(Table 2, entry 15); 1H NMR (400 MHz, CDCl3) δ 7.34–7.31 (m, 4H),
7.19 (t, J=7.8 Hz, 2H), 6.75 (t, J=7.4 Hz, 1H), 6.62 (d, J=8.2 Hz, 2H),
4.32 (s, 2H), 4.18 (br s, 1H).
1
yield 95% (Table 3, entry 8); H NMR (400 MHz, CDCl3) δ 7.88 (d,
J=8.1 Hz, 2H), 7.35 (s, 4H), 7.33–7.26 (m, 1H), 6.59 (d, J=8.1 Hz, 2H),
4.54 (br s, 1H), 4.31 (q, J=7.1 Hz, 2H), 1.36 (t, J=7.1 Hz, 3H). 13C NMR
(100 MHz, CDCl3) δ 166.9, 151.8, 138.5, 131.6, 128.9, 127.6, 127.5,
119.1, 111.7, 60.3, 47.8, 14.6.
Electronic Supplementary Information
N-(4-Bromobenzyl)aniline:36 Light yellow oily liquid; yield 97%
1
Further experimental and spectroscopic details are available
through:
stl.publisher. ingentaconnect.com/content/stl/jcr/supp‑data.
(Table 2, entry 6); H NMR (400 MHz, CDCl3) δ 7.48 (m, 2H), 7.26
(d, J=8.3 Hz, 2H), 7.22–7.16 (m, 2H), 6.75 (t, J=7.3 Hz, 1H), 6.62 (d,
J=8.4 Hz, 2H), 4.31 (s, 2H), 4.15 (br s, 1H).

264 JOURNAL OF CHEMICAL RESEARCH 2014
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Financial support of this work from the National Natural
Science Foundation of China (21172019) is appreciated.
Received 20 December 2013; accepted 23 February 2014
Paper 1302357 doi: 10.3184/174751914X13946178288518
Published online: 6 May 2014
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