Zinc-catalyzed reduction of imines by triethylsilane

Article abstract of DOI:10.1080/00397910802521246

Low-cost zinc is employed as a catalyst along with triethylsilane (TES) in a simple, straightforward, chemoselective reduction of various aldimines and ketimines to the corresponding secondary amines at room temperature and pressure.

Full text of DOI:10.1080/00397910802521246

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Zinc-Catalyzed Reduction of
Imines by Triethylsilane
K. Anil Kumar a , T. S. Sreelekha a , K. N.
Shivakumara a , K. C. Prakasha a & D. Channe Gowda
a
a Department of Studies in Chemistry, University of
Mysore, Manasagangotri, Mysore, India
Available online: 19 Mar 2009
To cite this article: K. Anil Kumar, T. S. Sreelekha, K. N. Shivakumara, K. C. Prakasha
& D. Channe Gowda (2009): Zinc-Catalyzed Reduction of Imines by Triethylsilane,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 39:8, 1332-1341
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Synthetic Communications1, 39: 1332–1341, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802521246
Zinc-Catalyzed Reduction of Imines
by Triethylsilane
K. Anil Kumar, T. S. Sreelekha, K. N. Shivakumara,
K. C. Prakasha, and D. Channe Gowda
Department of Studies in Chemistry, University of Mysore,
Manasagangotri, Mysore, India
Abstract: Low-cost zinc is employed as a catalyst along with triethylsilane (TES)
in a simple, straightforward, chemoselective reduction of various aldimines and
ketimines to the corresponding secondary amines at room temperature and pressure.
Keywords: Hydrogenation, imines, secondary amines, triethylsilane (TES), zinc dust
The conversion of carbonyl derivatives to secondary amines via imines is
a very convenient and explicit route to the synthesis of substituted
amines. However, difficulties may be encountered partly as a result of
imine instability, which makes it cumbersome to easily access the imine
in its pure form. Even though additions to the C¼N bonds of imines,
hydrazones, and oximes are well known, exploitation of these reactions
is often limited by the poor electrophilicity of the C¼N carbon.[1]
To date, several protocols have been utilized for the reduction of imines
to secondary amines,[2–14] but some of them have limitations based on che-
moselectivity, low yield, economic considerations, chemical efficiency, and
waste disposal. Catalytic hydrogenation is also commonly used,[15] but the
success of the reaction depends on the solvent, catalyst, and substrate.
Catalytic hydrogenation employs highly diffusible, low-molecular-weight,
flammable hydrogen gas, requires pressure equipment; and is often
Received June 18, 2008.
Address correspondence to Prof. D. Channe Gowda, DOS in Chemistry,
University of Mysore, Manasagangotri, Mysore 570 006, India. E-mail:
dchannegowda@yahoo.co.in
1332

Reduction of Imines by Triethylsilane
1333
problematic because of harsh reaction conditions. Nowadays, silicon-based
reducing agents are extensively used in organic synthesis.[16] Thus, triethyl-
silane (TES) has been the most popular of the silicon-based reducing agents,
and Si-H containing system can provide the hydride for many or most of the
reductions.
Recently metal-mediated reactions have wide scope in organic synthesis
because of their simple workup and selectivity. Several methods have been
developed based on the use of a variety of metals such as magnesium,[17]
indium,[18] zinc,[19] and tin.[20] The utility of zinc for the synthesis of b,c-
unsaturated ketones by a reaction of an acid chloride with allyl bromide[21]
and homoallylic alcohols has been demonstrated.[22] Further, the zinc-
mediated preparation of triphenylphosphonium ylides,[23] amide bond
formation,[24] Friedel–Crafts acylation,[24] carbamate formation,[24] and est-
erification of acid chloride[24] has been demonstrated. Zinc has also been
employed as a catalyst along with different hydrogen donors in many
functional group interconversions.[25] In this context, we report the zinc-
catalyzed rapid and selective transfer hydrogenation of imines to corre-
sponding secondary amines using TES at room temperature in methanol
(Scheme 1).
The results given in Table 1 reveal the viability of using the Zn and
TES system for the reduction of imines. The course of reaction was
monitored by thin-layer chromatography (TLC) and infrared (IR) spec-
tra. The workup and isolation of the products were easy. Thus all the
compounds reduced by this system were obtained in moderate to good
yields and most of the reactions were completed within 30 min. The pro-
ducts were characterized by comparison of their boiling or melting
points, TLC, IR, and 1H NMR spectra with authentic samples. The
disappearance of strong absorption bands between 1690 and 1640 cmꢀ1
due to C¼N stretching and the appearance of a strong absorption bands
between 3500 and 3200 cmꢀ1 for the –NH group clearly show that the
imines were reduced to corresponding secondary amines. In case of nitro
imines, both nitro and imine undergo reduction to yield primary and
secondary amines respectively.
To ensure the role of zinc, a controlled experiment was carried out
using imines with TES without zinc dust, which failed to yield any
Scheme 1. Reduction of imines to secondary amines using triethylsilane.

1334
K. A. Kumar et al.
Table 1. Zinc-catalyzed reduction of imines to secondary amines using
triethylsilane
Melting point
(ꢁC)
Time
(min)
Yieldc
(%) Found
Entry
1
Substrate
Producta,b
Lit.
20
85 36–37 35–38[29]
2
25
80 48–50 50–50.5[30]
3
4
20
26
82 38–39 39.5–40[31]
75
Oily
—
—
5
24
78 34–36
6
7
8
25
26
30
75
Oily 19–20[29]
76 45–46 46.5–47[30]
73
Oily
—
(Continued )

Reduction of Imines by Triethylsilane
1335
Table 1. Continued
Melting point
(ꢁC)
Time
(min)
Yieldc
(%) Found
Entry
9
Substrate
Producta,b
Lit.
23
75 83–84 84–85[32]
10
11
20
28
80 68–70 70–71[29]
75 55–56 60–61[30]
12
13
25
35
70
70
Oily
Oily
—
—
a1H NMR spectra were obtained on a 300-MHz Bruker FT-NMR spectrometer
in CDCl3 as the solvent and TMS as internal standard.
bIsolated products gave IR spectra in agreement with their structures.
cIsolated yields are based on a single experiment, and yields were not optimized.
reduced product. This clearly confirms that zinc catalyzes the reaction. In
anticipating metal=alcohol reduction, experiments were performed in the
absence of a hydrogen source (i.e., TES), by refluxing the substrate with
methanol and zinc dust for 4–5 h. Even after a longer duration, the start-
ing material was isolated in quantitative yield, which clearly shows the
requirement of TES as a hydrogen source for the reduction. Here, metha-
nol serves as a solvent. After the completion of reaction (Scheme 1), zinc

1336
K. A. Kumar et al.
was recycled almost quantitatively (>95%) after activation by treating it
with acetic acid or 0.01 N HCl.
In summary, the TES=Zn method is useful for preparing a variety of
secondary amines from suitable carbonyl compounds and amines. We
have chosen TES as a reducing agent because of its ease of handling
and high reactivity, which decomposes to an environmentally friendly
by-product. The use of zinc dust instead of Pd=C,[26] PdCl2,[27] ruthenium
Grabbs, or Raney Ni not only reduces the cost and contains risk but also
enhances the chemoselectivity, particularly with halogens, which are not
compatible under Pd=C-, PdCl2-, or Raney Ni–catalyzed transfer hydro-
genation conditions. This procedure gave rise to the formation of second-
ary amines in moderate to good yields, had operational simplicity, had
few by-products, used mild reaction condition, and offered an alternative
to traditional synthetic procedures.
EXPERIMENTAL
The imines were either obtained from a commercial source or prepared from
the corresponding carbonyl compound and amines as previously reported.[28]
In cases where the imine was obtained as an E=Z mixture, no attempts were
made to separate such imines, and they were used as such for the reduction.
Zinc dust powder was purchased from Loba Chemie Pvt. Ltd. (Mumbai,
India). TES and 60- to 120-mesh silica gel (for column chromatography) were
purchased from E. Merck Ltd. (India). All the solvents used were analytical
grade or were purified according to standard procedures. TLC was carried
out on precoated silica-gel plates (Merck) using chloroform=methanol
(90:10) as a solvent system. The melting points were determined by a open
capillary apparatus and are uncorrected. IR spectra were recorded on a Jasco
1
4100 Fourier transform infrared (FTIR) spectrometer. H NMR spectra
were obtained on a 300-MHz Bruker FT-NMR spectrometer using CDCl3
as a solvent and tetramethylsilane (TMS) as an internal standard.
Typical Procedure for the Reduction of Imines
Neat TES (20 mmol) was added dropwise from a dropping funnel to a
stirred solution of substrate (5 mmol) and zinc dust (6 mmol) in methanol
(10 ml) under a nitrogen atmosphere at room temperature. After the com-
pletion of reaction (monitored by TLC), the reaction mixture was filtered
through celite, the organic layer was evaporated, and the residue was dis-
solved in dichloromethane or ether and washed with saturated KHSO4
and brine solution. The organic layer was dried over anhydrous Na2SO4,

Reduction of Imines by Triethylsilane
1337
and the solvent was evaporated under reduced pressure. The products
were purified by column chromatography 60- to 120-mesh silica gel using
a suitable eluting system (90:10 hexane–ethylacetate, 90:10 chloroform–
hexane, 80:20 chloroform–methanol, 85:15 chloroform–methanol, 90:10
chloroform–methanol) at a flow rate of 20 drops per minute.
Analytical Data of Synthesized Compounds
Compound 1, N-Phenylbenzylamine
IR (neat): n ¼ 3418, 2927, 1468, 1340, 1303 cmꢀ1
;
1H NMR (CDCl3,
300 MHz, TMS) d ¼ 7.05–7.15 (m, 7H, Ar-H), 6.56 (t, 1H, Ar-H), 6.42
(d, 2H, Ar-H), 4.33 (s, 2H, Ar-CH2), 4.04 (s br, 1H, -NH).
Compound 2, Benzyl-(4-methoxyphenyl)-amine
IR (neat): n ¼ 3398, 2930, 2852, 1463, 1341, 1255, 1041, 820 cmꢀ1
;
1H NMR (CDCl3, 300 MHz, TMS) d ¼ 7.06–7.15 (m, 5H, Ar-H), 6.55
(d, 2H, Ar-H), 6.30 (d, 2H, Ar-H), 4.32 (s, 2H, Ar-CH2), 3.92 (s, 1H,
-NH), 3.70 (s, 3H, Ar-OCH3).
Compound 3, Benzyl-(2-chlorophenyl)-amine
IR (neat): n ¼ 3406, 2925, 2850, 1470, 1334, 1291, 1255, 1041, 810 cmꢀ1
;
1H NMR (CDCl3, 300 MHz, TMS) d ¼ 7.05 (m, 6H, Ar-H), 6.92 (t, 1H,
Ar-H), 6.52 (t, 1H, Ar-H), 6.37 (d, 1H, Ar-H), 4.32 (s, 2H, Ar-CH2), 3.80
(s, 1H, -NH).
Compound 4, Benzyl-(2-methoxyphenyl)-amine
IR (neat): n ¼ 3401, 2936, 2856, 1460, 1336, 1289, 1251, 1025, 841 cmꢀ1
;
1H NMR (CDCl3, 300 MHz, TMS) d ¼ 7.06–7.14 (m, 5H, Ar-H), 6.60
(t, 1H, Ar-H), 6.55 (d, 1H, Ar-H), 6.47 (t, 1H, Ar-H), 6.32 (d, 1H,
Ar-H), 4.12 (s, 2H, Ar-CH2), 3.85 (s, 1H, -NH) 3.75 (s, 3H, Ar-OCH3).
Compound 5, 4-chlorobenzyl-(3-methylphenyl)-amine
IR (neat): n ¼ 3408, 2930, 2916, 2870, 2863, 2856,1454, 1336, 720 cmꢀ1
;
1H NMR (CDCl3, 300 MHz, TMS) d ¼ 7.14 (d, 2H, Ar-H), 7.01 (d,
2H, Ar-H), 6.92 (t, 1H, Ar-H), 6.38 (d, 1H, Ar-H), 6.24 (s, 1H, Ar-H),
4.21 (s, 2H, Ar-CH2), 3.78 (s, 1H, -NH), 2.42 (s, 3H, Ar-CH3).

1338
K. A. Kumar et al.
Compound 6, N-(4-Methylphenylmethyl)aniline
1
IR (neat): n ¼ 3408, 2960, 2921, 2869, 2852, 1457, 1336, 1288 cmꢀ1; H
NMR (CDCl3, 300 MHz, TMS) d ¼ 7.05 (tt, 2H, Ar-H), 6.94 (d, 4H,
Ar-H), 6.58 (t, 1H, Ar-H), 6.43 (dd, 2H, Ar-H), 4.28 (s, 2H, Ar-CH2),
3.98 (s br, 1H, -NH), 2.35 (s, 3H, Ar-CH3).
Compound 7, N-(4-Methoxybenzyl)benzenamine
1
IR (neat): n ¼ 3398, 2930, 2852, 1463, 1341, 1255, 1041, 820 cmꢀ1; H
NMR (CDCl3, 300 MHz, TMS) d ¼ 7.05 (t, 2H, Ar-H), 6.96 (d, 2H,
Ar-H), 6.66 (d, 2H, Ar-H), 6.59 (t, 1H, Ar-H), 6.44 (d, 2H, Ar-H), 4.26
(s, 2H, Ar-CH2), 3.90 (s, 1H, -NH), 3.81 (s, 3H, Ar-OCH3).
Compound 8, N-Benzyl-N-(1-phenylethyl)-amine
1
IR (neat): n ¼ 3400, 2935, 2848, 1650, 1461, 1146, 730 cmꢀ1; H NMR
(CDCl3, 300 MHz, TMS) d ¼ 7.20 (t, 2H, Ar-H), 7.14 (t, 2H, Ar-H),
7.12 (d, 2H, Ar-H), 7.08 (t, 1H, Ar-H), 7.07 (t, 1H, Ar-H), 7.06 (d, 2H,
Ar-H), 3.77 (q, 1H, -CH), 3.58 (s, 2H, Ar-CH2), 1.51 (s br, 1H, -NH),
1.32 (d, 3H, -CH3).
Compound 9, N-(3-Aminobenzyl)benzenamine
IR (neat): n ¼ 3401, 3369, 1650, 1485, 1345 cmꢀ1
;
1H NMR (CDCl3,
300 MHz, TMS) d ¼ 7.04 (t, 2H, Ar-H), 6.89 (t, 1H, Ar-H), 6.58 (t, 1H,
Ar-H), 6.43 (d, 2H, Ar-H), 6.42 (d, 1H, Ar-H), 6.27 (d, 1H, Ar-H),
6.26 (s, 1H, Ar-H), 4.35 (s, 2H, Ar-CH2), 4.10 (s br, 1H, -NH), 3.32
(s, 2H, Ar-NH2).
Compound 10, N-(4-Chlorobenzyl)-4-chlorobenzenamine
IR (neat): n ¼ 3413, 2918, 2860, 1475, 1334, 820, 723cmꢀ1
;
1H NMR
(CDCl3, 300 MHz, TMS) d ¼ 7.15 (d, 2H, Ar-H), 7.05 (d, 2H, Ar-H), 7.00
(d, 2H, Ar-H) 6.37 (d, 2H, Ar-H), 4.31 (s, 2H, Ar-CH2), 4.09 (s, 1H, -NH).
Compound 11, N-(4-Methylbenzyl)-4-methylbenzenamine
1
IR (neat): n ¼ 3405, 2930, 2870, 2845, 1343, 1256, 1039 cmꢀ1; H NMR
(CDCl3, 300 MHz, TMS) d ¼ 6.94 (d, 4H, Ar-H), 6.84 (d, 2H, Ar-H),
6.31 (d, 2H, Ar-H), 4.29 (s, 2H, Ar-CH2), 4.02 (s, 1H, -NH), 2.34
(s, 6H, Ar-2CH3).

Reduction of Imines by Triethylsilane
1339
Compound 12, N-Benzyl-1-(4-chlorophenyl)ethanamine
1
IR (neat): n ¼ 3408, 2935, 2848, 1650, 1461, 1146, 730 cmꢀ1; H NMR
(CDCl3, 300 MHz, TMS) d ¼ 7.22 (d, 2H, Ar-H), 7.14 (t, 2H, Ar-H),
7.07 (t, 1H, Ar-H), 7.06 (d, 4H, Ar-H), 3.74 (q, 1H, -CH), 3.55 (s, 2H,
Ar-CH2), 1.51 (s br, 1H, -NH), 1.28 (d, 3H, -CH3).
Compound 13, N-Benzyl-1-(4-methoxyphenyl)ethanamine
1
IR (neat): n ¼ 3410, 2932, 2840, 1648, 1466, 1140, 735 cmꢀ1; H NMR
(CDCl3, 300 MHz, TMS) d ¼ 7.14 (t, 2H, Ar-H), 7.07 (t, 1H, Ar-H),
7.06 (d, 2H, Ar-H), 7.01 (d, 2H, Ar-H), 6.72 (d, 2H, Ar-H), 3.76 (s, 3H,
Ar-OCH3), 3.72 (q, 1H, -CH), 3.56 (s, 2H, Ar-CH2), 1.76 (s br, 1H, -NH),
1.30 (d, 3H, -CH3).
ACKNOWLEDGMENT
The authors thank the Council of Scientific and Industrial Research
(CSIR), New Delhi, India, for financial assistance.
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