SnCl2·2H2O-Catalyzed Solvent-Free Synthesis of α-Amino Ketones and Tetrasubstituted Pyrazines

Article abstract of DOI:10.1055/s-0035-1561663

Solvent-free reaction of various anilines with α-hydroxy ketones catalyzed by SnCl₂·2H₂O provides α-amino ketones in excellent yields. While a similar reaction with aliphatic amines is applicable for the synthesis of substituted pyrazines, SnCl₂·2H₂O permits versatility in the solvent-free reaction of α-hydroxy ketones with ammonium acetate to give the corresponding substituted pyrazines in good to excellent yields.

Full text of DOI:10.1055/s-0035-1561663

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© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–D
letter
A
F. Tamaddon, A. DehghaniTafti
Letter
Synlett
SnCl2·2H2O-Catalyzed Solvent-Free Synthesis of α-Amino Ketones
and Tetrasubstituted Pyrazines
Fatemeh Tamaddon*
Arefeh DehghaniTafti
Department of Chemistry, Yazd University, Yazd 89195-741,
Iran
ftamaddon@yazduni.ac.ir
Received: 20.04.2016
Accepted after revision: 11.05.2016
Published online: 22.06.2016
oxidation of the products.10 Recently, Marques et al. have
synthesized 2,3,5,6-tetraphenylpyrazine (1a) using CeCl3 in
refluxing EtOH, although this synthesis was not successful
using SnCl2·2H2O.11 A patent has also disclosed a synthesis
of 1a and tetra(4-chlorophenyl)pyrazine using a complex
catalyst system in organic solvent.12 Synthesis of pyrazines
through multicomponent reactions (MCR), that offer the
simpler one-pot alternatives to access complex products for
combinatorial purposes13 is an important approach and
coupling the advantages of MCR with solvent-free condi-
tions14 for the synthesis of pyrazines is even more desirable.
SnCl2·2H2O-catalyzed C–C, C–O, and C–N bond transfor-
mations are valuable organic reactions developed with this
low-cost and water-tolerant catalyst.15 As a part of our cur-
rent researches on heterogeneous reactions,16 we present
herein the rapid synthesis of either α-amino ketones from
α-hydroxy ketones or pyrazines via a simple [2+2+1+1]
pseudo-four-component strategy under thermal and micro-
wave conditions (Scheme 1).
DOI: 10.1055/s-0035-1561663; Art ID: st-2016-d0279-l
Abstract Solvent-free reaction of various anilines with α-hydroxy ke-
tones catalyzed by SnCl2·2H2O provides α-amino ketones in excellent
yields. While a similar reaction with aliphatic amines is applicable for
the synthesis of substituted pyrazines, SnCl2·2H2O permits versatility in
the solvent-free reaction of α-hydroxy ketones with ammonium acetate
to give the corresponding substituted pyrazines in good to excellent
yields.
Key words pseudo-four-component reaction, α-amino ketone, α-hy-
droxy ketone, SnCl2·2H2O, aromatic amines, pyrazine
α-Amino ketones are key structural units of natural
products,1 synthetic intermediates,2 and clinically used
medicines such as bupropion and brephedrone for treat-
ment of psychological disorders.3 These bifunctional pre-
cursors have been used for the synthesis of quinazolines,
imidazoles, pyrazines, indoles, and pyrroles.2 Among the
procedures available for the general synthesis of α-amino
ketones,4 the traditional SN2 reaction of N-nucleophiles
with α-halo ketones4a or α-hydroxy ketones4b are the most
documented. Due to the problematic nature of direct sub-
stitution of the hydroxyl group with N-nuclophiles,5 need
for compatibility of the nucleophilic substitution condi-
tions with the catalyst6 and dimerization of α-amino ke-
tones to pyrazines using Brønsted acids,7 synthesis of α-
amino ketones under more convenient conditions remains
desirable.
Ar
O
Ar
Ar
O
SnCl2.2H2O
Ar1NH2
+
solvent-free, 80 °C
or microwave
NHAr1
Ar
OH
Ar
Ar
O
O
Ar
Ar
N
N
Ar
Ar
NH4OAc
SnCl2.2H2O
+
solvent-free, 80 °C
or microwave
NH4OAc
OH
HO
Ar
Ar
Scheme 1 SnCl2·2H2O-catalyzed synthesis of α-amino ketones and
substituted pyrazines
Pyrazines are important secondary metabolites or radi-
cal scavengers in natural food flavors and pharmaceuticals
with antifibrosis, antitumor, antibacterial, and antioxidant
activities.8 These diverse properties have led to develop-
ment of synthetic protocols for pyrazines,9 mainly focused
on condensation of diamines with diketones and further
Initially, the catalytic performance of various metal-
based catalysts was compared in the synthesis of 1,2-diphe-
nyl-2-(phenylamino)ethanone (2a) via the reaction of ben-
zoin and aniline as model substrates under various condi-
tions (Table 1). As the results indicate, the maximum yield
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D

B
F. Tamaddon, A. DehghaniTafti
Letter
Synlett
O
of product 2a was with 10 mol% SnCl2·2H2O under solvent-
free conditions at either 80 °C or under microwave irradia-
tion (Table 1, entries 9–11). Due to the formation of benzil
via the undesired aerobic oxidation of benzoin, the yield of
2a was lower in other cases (Table 1, entries 8 and 12–16).
A scale-up of the model reaction with catalytic amount of
SnCl2·2H2O was also successful on a 50 mmol scale (Table 1,
entries 18 and 19).
Cl
Ph
Cl
H
Ph
Sn
Ph
N
N
C
O
Sn
Ph
H
C
O
H
dehydration
H
O
H
O
N
Ph
H
Ph
H
C
C
Ph
Sn
Ph
Ph
Cl
Cl
Cl
Cl
keto–enol
tautomerization
H
Ph
O
N
SnCl2.2H2O
+
Table 1 Optimization of the Model Reactiona
C
H
C
Ph
Ph
O
Scheme 2 The postulated mechanism for SnCl2·2H2O-catalyzed nucleo-
O
philic substitution
conditions
NH2
+
HN
OH
2a
The superiority of SnCl2·2H2O for promotion of the
model reaction may be due to the template effect of Sn2+ for
locating the carbonyl group of the benzoin into the proxim-
ity of the amine nitrogen as depicteded in Scheme 2.
Entry
Catalyst (mol%)
Solvent
Temp
(°C)
Time
(min)
Yield
(%)b
1
2
–
–
80
80
150
190
180
120
90
53c
30c
45c
60c
78
ZnO (5)
–
Table 2 SnCl2·2H2O-Catalyzed Synthesis of α-Amino Ketonesa
3
Fe3O4 (5)
–
80
Ar
Ar
O
Ar
Ar
O
SnCl2.2H2O (10 mol%)
solvent-free, 80 °C
4
TiO2 (5)
–
120
80
1
+
Ar NH2
5
SnO2 (5)
–
NHAr1
OH
2a–r
6
CeO2 (5)
–
80
60
86c
50c
77c
92
7
SnCl2·2H2O (5)
SnCl2·2H2O (10)
SnCl2·2H2O (10)
SnCl2·2H2O (10)
SnCl2·2H2O (10)
SnCl2·2H2O (10)
SnCl2·2H2O (10)
SnCl2·2H2O (10)
SnCl2·2H2O (10)
SnCl2·2H2O (10)
SnCl2·2H2O (15)
SnCl2·2H2O (10)
SnCl2·2H2O (10)
CeCl3·7H2O (10)
CeCl3·7H2O (20)
ZnCl2 (5)
–
80
360
55
Entry Arb
Ar1
Ph
Product Time
Yield mp (°C)
found
(min) (%)c
8
–
70
1
2
Ph
2a4i
2b19
2c19
2d20
2e
30
30
92
91
89
83
88
92
90
92
95
92
85
90
88
85
85
92
80
90
87–90
9
–
80
30
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
–
MW
MW
90
1
92
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4-MeC6H4
143–145
157–160
187–189
125–127
93–95
–
2
90
3
4-ClC6H4
4-O2NC6H4
3-ClC6H4
3-BrC6H4
3-O2NC6H4
3-MeC6H4
4-HOC6H4
4-AcC6H4
4-BrC6H4
1-naphthyl
Ph
25
–
165
310
190
110
180
360
30
80c
60c
52c
65c
73c
67c
91d
90d
53c
70c
65c
79c
35c
25c
4
60
H2O
80
5
35
6
2f20
2g
40
MeOH
60
EtOH
80
7
60
138–140
140–141
162–164
158–160
160–163
85–87
8
2h20
2i
60
CHCl3
80
–
–
–
–
–
–
–
–
–
80
9
105
100
40
80
10
11
12
13
14
15
16
17
18
2j
MW
80
3
2k
2l20
2m
2n
150
40
180
120
120
120
180
350
80
4-ClC6H4
4-MeC6H4
4-ClC6H4
4-ClC6H4
4-MeC6H4
4-MeC6H4
111–113
116–117
125–127
133–135
128–130
147–149
80
Ph
75
MgCl2 (5)
120
80
4-MeC6H4
4-ClC6H4
4-MeC6H4
4-ClC6H4
2o
45
FeCl3 (5)
2p
20
2q20
2r
95
LiCl (10)
80
a Reaction was performed with benzoin (2 mmol) and aniline (2 mmol).
45
b Isolated yields.
a Reaction was performed with benzoin (2 mmol) and amine (2 mmol).
b Benzoin derivatives were prepared from benzaldehydes via the benzoin
condensation.21
c Benzil (PhCOCOPh) by-product was formed by aerobic oxidation of benzo-
in.
d Reaction was run in 50 mmol scale.
c Isolated yields.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D

C
F. Tamaddon, A. DehghaniTafti
Letter
Synlett
To define the generality of the SnCl2·2H2O-catalyzed
synthesis of α-amino ketones, various benzoins and amines
were reacted under optimized reaction conditions17 to give
excellent yields of pure products in most cases (Table 2). As
the results show, reaction of aromatic amines with benzo-
ins gave the desired products; although anilines bearing
electron-withdrawing groups required extended reaction
times (Table 2, entries 4, 7, 10).
An NMR-based study of the reaction between benzoin
and benzylamine under similar conditions confirmed the
formation of the undesired product 1,4-dibenzyl-2,3,5,6-
tetraphenyl-1,4-dihydropyrazine (3) in 90% yield (Scheme
3).18
The results obtained for the synthesis of substituted
pyrazine highlights the catalytic efficiency of the
SnCl2·2H2O in multicomponent reactions, representing a
good catalytic system for either synthesis of α-amino ke-
tones or the tandem synthesis of tetraaryl pyrazines via a
pseudo-four-component [2+1+1+2] synthetic strategy.
In conclusion, we have developed an efficient
SnCl2·2H2O-catalyzed approach for either synthesis of α-
amino ketones by nucleophic substitution of anilines with
benzoins or pseudo-four-component synthesis of pyrazines
under solvent-free conditions. All reactions gave excellent
yields of products without purification being necessary.
Acknowledgement
Bn
Ph
Ph
O
Ph
Ph
O
SnCl2⋅2H2O (10 mol%)
We acknowledge support from the Research Council of Yazd University.
HN
Ph
Ph
H2N–Bn
2
+ 2
+
solvent-free, 80 °C
NH
Bn
OH
O
Supporting Information
Ph
Ph
N
Supporting information for this article is available online at
self-condensation
60 min
http://dx.doi.org/10.1055/s-0035-1561663.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
o
nrtogI
f
rmoaitn
Bn
N
Bn
Ph
Ph
3
References and Notes
Scheme 3 Formation of 1,4-dibenzyl-2,3,5,6-tetraphenyl-1,4-dihydro-
pyrazine (3)
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Formation of pyrazine 3, which is possible by self-con-
densation of the in situ formed α-amino ketone, prompted
us to investigate the catalytic performance of SnCl2·2H2O in
a pseudo-four-component reaction of benzoin and ammo-
nium acetate18 to give 1a in 90% yield.
This [2+1+1+2] synthetic strategy and MCR-based reac-
tion was then generalized to a high-yielding synthesis of
tetrasubstituted pyrazines from a range of benzoins and
ammonium acetate18 (Table 3).
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Table 3 Synthesis of 2,3,5,6-Tetraarylpyrazinesa
O
Ar
Ar
N
Ar
Ar
SnCl2.2H2O (10 mol%)
solvent-free, 80 °C
Ar
Ar
2
2 NH4OAc
+
OH
N
1a–f
Entry
Ar
Ph
Time (min) Product Yield (%)b mp (°C) found
1
2
3
4
5
6
30
80
80
40
30
50
1a16f
1b
90
85
89
91
92
90
252–254
280–283
276–279
302–304
229–232
243–245
4-MeC6H4
4-MeOC6H4
4-ClC6H4
4-FC6H4
1c
1d9o
1e
4-BrC6H4
1f
a Reaction was performed with the requisite benzoin (2 mmol) and NH4OAc
(2.2 mmol).
b Isolated yield.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D

D
F. Tamaddon, A. DehghaniTafti
Letter
Synlett
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(17) General Procedure for Synthesis of α-Amino Ketones
SnCl2·2H2O (10 mol%) was added to a stirred mixture of benzoin
(2 mmol) and amine (2 mmol) in a round-bottom flask at 80 °C.
The resulting mixture was stirred for the given time and prog-
ress of the reaction was followed by TLC (EtOAc–hexane, 20:80).
After completion of the reaction, EtOAc was added (50 mL), and
the mixture was filtered to remove the catalyst. Then the
organic layer was washed with 10% NaHCO3 and water, dried
over Na2SO4, filtered, and concentrated to give the product.
Although additional purification was not required in most
cases, the crude products may be recrystallized from H2O–EtOH
(60:40).
Analytical Data for 2-[(4-Acetylphenyl)amino]-1,2-dipheny-
lethanone (2j)
Pale yellow solid; yield for reaction in 2 mmol scale: 300 mg
(92%); mp 158–160 °C. FT-IR (KBr): 3375 (NH stretch), 1676,
1664 (C=O stretch), 1600, 1520, 1479, 1447, 1273, 1170 cm–1
.
1H NMR (400 MHz, DMSO-d6): δ = 2.40 (s, 3 H), 5.49 (s, 1 H),
6.59 (d, J = 7.6 Hz, 1 H), 6.87 (d, J = 8.8 Hz, 2 H), 7.21–7.66 (m, 8
H), 7.71 (d, J = 8.8 Hz, 2 H), 8.17 (d, J = 7.2 Hz, 2 H) ppm. 13C NMR
(100 MHz, DMSO-d6): δ = 26.42, 61.07, 112.67, 126.89, 128.41,
128.52, 128.86, 129.24, 129.32, 129.34, 130.00, 130.09, 134.22,
135.15, 137.90, 137.92, 151.59, 195.75, 197.11 ppm. Anal. Calcd
for C22H19NO2: C, 80.22; H, 5.81; N, 4.25; O, 9.71. Found: C,
80.25; H, 5.98; N, 4.27; O, 9.80.
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(18) General Procedure for Synthesis of Substituted Pyrazines
To a mixture of benzoin (2 mmol) and NH4OAc (2.2 mmol) was
added SnCl2·2H2O (10 mol%), and the mixture was stirred for
the requisite time at 80 °C. After completion of the reaction
(TLC), EtOH (3 mL), and water (7 mL) were added, the mixture
was stirred, and the product was isolated by filtration. The pyra-
zine products were analytically pure and showed the expected
characterization data without additional purification.
Analytical Data
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2,3,5,6-Tetraphenylpyrazine (1a)
Yellow solid; yield for reaction at 2 mmol scale: 346 mg (90%);
mp 252–254 °C. FT-IR (KBr): 3054, 1628 (C=N stretch), 1599,
1521, 1461, 1393, 1175, 1025 cm–1. 1H NMR (400 MHz, CDCl3):
δ = 7.26–7.34 (m, HAr, 12 H), 7.65 (t, J = 6.8 Hz, HAr, 8 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 148.47, 138.52, 129.94, 128.62,
128.52, 128.30 ppm.
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1477. (b) Jafari, A. A.; Amini, S.; Tamaddon, F. J. Iran. Chem. Soc.
2013, 10, 677. (c) Tamaddon, F.; Pouramini, F. Synlett 2014, 25,
1127. (d) Tamaddon, F.; Kargar-Shooroki, H.; Jafari, A. A. J. Mol.
Catal A: Chem. 2013, 368. (e) Tamaddon, F.; Farahi, M. Synlett
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1,4-Dibenzyl-2,3,5,6-tetraphenyl-1,4-dihydropyrazine (20)
Yellow solid; yield for reaction at 2 mmol scale: 510 mg (90%);
mp 157–160 °C. FT-IR (KBr): 1601, 1448 (C=C stretch), 1027
(C–N stretch) cm–1 1H NMR (400 MHz, DMSO-d6): δ = 5.18 (s,
.
4 H), 6.77 (d, J = 6.8 Hz, 3 H), 7.15–7.25 (m, HAr, 9 H), 7.30–7.41
(m, HAr, 3 H), 7.43–7.49 (m, HAr, 12 H), 7.67–7.69 (m, HAr, 3 H)
ppm. 13C NMR (100 MHz, DMSO-d6): δ = 48.16, 106.28, 126.10,
126.58, 126.76, 127.68, 128.59, 129.02, 129.08, 129.32, 129.37,
129.43, 130, 68, 131.06, 131.25, 131.31, 135.00, 137.36, 137.79
ppm. Anal. Calcd for C42H34N2: C, 89.01; H, 6.05; N, 4.94. Found:
C, 88.98; H, 6.03; N, 4.97.
(19) Nápoles-Escutary, F. A.; Vale Capdevilla, R. M.; Casals Hung, M.;
Jaciel, L. O.; Joyce, J. Rev. Cubana Quim. 2012, 24, 261.
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Lett. 2015, 56, 1887.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D