Samarium(III) triflate catalyzed conjugate addition of amines to electron-deficient alkenes

Article abstract of DOI:10.1055/s-2007-990876

Amines undergo smooth nucleophilic addition to α,β-unsaturated compounds in the presence of a catalytic amount of samarium(III) triflate at ambient temperature to produce the corresponding β-amino compounds in excellent yields. This method is simple, convenient, and works efficiently under mild conditions. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-990876

SHORT PAPER
3447
Samarium(III) Triflate Catalyzed Conjugate Addition of Amines to
Electron-Deficient Alkenes1
C
onjugate
.
A
ddition of
S
A
mines to Ele
.
c
tron-Defic
i
Y
ent
A
lkenes adav,* A. Ramesh Reddy, Y. Gopal Rao, A. V. Narsaiah, B. V. Subba Reddy
Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Fax +91(40)27160512
Received 5 December 2006; revised 23 July 2007
In continuation of our ongoing research program to devel-
op various new synthetic methodologies9 herein, we wish
to disclose our results on conjugate addition of various
amines to electron-deficient alkenes using 10 mol% of sa-
Abstract: Amines undergo smooth nucleophilic addition to a,b-un-
saturated compounds in the presence of a catalytic amount of sa-
marium(III) triflate at ambient temperature to produce the
corresponding b-amino compounds in excellent yields. This meth-
od is simple, convenient, and works efficiently under mild condi- marium(III) triflate under mild reaction conditions
tions.
(Table 1).
Key words: amines, a,b-unsaturated compounds, conjugate addi-
tion, b-amino derivatives
In a typical experiment, benzylamine was treated with
ethyl acrylate in dichloromethane in the presence of 10
mol% of samarium(III) triflate. The reaction went to com-
pletion within two hours and the product, ethyl 3-(benzyl-
The aza-Michael reaction is widely recognized as one of
the most important C–N bond-forming reactions in organ-
ic synthesis.2 b-Amino ketones are versatile building
blocks for the synthesis of various complex natural prod-
ucts, antibiotics, b-amino alcohols, and chiral auxiliaries.3
In recent years, the synthesis of b-amino acid derivatives
with various substitution patterns in the carbon chain has
become a field of increasing interest in organic synthesis.
The most common method for the preparation of b-amino
ketones is the Mannich reaction. However, the classical
Mannich reaction often suffers from the drawbacks of
long reaction times and harsh reaction conditions,4 which
limit its use in the synthesis of complex molecules. An al-
ternative approach for the preparation of b-amino esters is
the conjugate addition of amines to a,b-unsaturated ester
derivatives. Generally, these reactions are performed un-
der basic or acidic conditions;5 however, to avoid the side
reactions that are normally encountered in the presence of
strong acid or base, a number of alternative methods have
been developed.6 In the past years, Lewis acids such as
metal halides and metal triflates and microwave-acceler-
ated reactions have been shown as the best promoters for
these addition reactions, especially concerning simplicity,
atom economy, and efficiency.7 However, many of these
methods involve the use of expensive catalysts, stoichio-
metric amounts of reagents, poor regioselectivity, and ex-
tended reaction times. Therefore, the introduction of a
new and efficient protocol for the addition of amines to
a,b-unsaturated compounds under mild and convenient
conditions is still required. Samarium(III) triflate is well
known in the literature as a mild, water-tolerant, and effi-
cient Lewis acid catalyst for various organic transforma-
tions.8
amino)propanoate (3a) was obtained in 96% yield. In a
similar manner, the reaction of benzylamine with acry-
lonitrile gave the Michael product 3q in 92% yield. The
reaction proceeds smoothly with 10 mol% of the catalyst
at room temperature and is completed within 2.5 hours.
Encouraged by the results obtained with benzylamine, we
turned our attention to various aliphatic, heterocyclic and
aromatic amines. Interestingly, various a,b-unsaturated
compounds such as ethyl acrylate, acrylonitrile, acryl-
amide, (E)-b-nitrostyrene, chalcone, and methyl vinyl ke-
tone underwent 1,4-addition with a variety of amines to
furnish the corresponding b-amino compounds. However,
aromatic amines such as aniline and 1-aminonaphthalene
failed to produce Michael adducts under these reaction
conditions even after a long reaction time (10 h). The re-
actions are relatively faster with cyclic amines compared
to acyclic amines. Similarly, acrylates react more rapidly
than other unsaturated systems. In general, the reactions
are completed in a short period (1.5–3.5 h) and the prod-
ucts are obtained in high yields ranging from 85% to 96%.
In the absence of catalyst, the reactions are slow and the
products are obtained in low yields. For example, treat-
ment of pyrrolidine with b-nitrostyrene in the absence of
catalyst furnished the corresponding 1,4-adduct 3r in 20%
yield after 24 hours whereas the same reaction in the pres-
ence of 10 mol% samarium(III) triflate within 2.5 hours
gave the product in 89% yield. In all cases, the reactions
proceed smoothly at ambient temperature to give the cor-
responding Michael adducts in excellent yields. The scope
and generality of this process is illustrated with respect to
various amines and a,b-unsaturated systems and the re-
sults are presented in Table 1.
In conclusion, this article describes an efficient protocol
for the conjugate addition of various amines to electron-
deficient alkenes to produce b-amino compounds using a
catalytic amount of samarium(III) triflate. This method
avoids the use of special precautions like stringent reac-
SYNTHESIS 2007, No. 22, pp 3447–3450
x
x.
x
x
.
2
0
0
7
Advanced online publication: 31.10.2007
DOI: 10.1055/s-2007-990876; Art ID: Z25306SS
© Georg Thieme Verlag Stuttgart · New York

3448
J. S. Yadav et al.
SHORT PAPER
recorded on a Gemini-200 spectrometer (200 MHz) in CDCl3 using
TMS as internal standard. Mass spectra were recorded on a Finni-
gan MAT 1020 mass spectrometer operating at 70 eV. Column
chromatography was performed using E. Merck 60–120 mesh silica
gel.
Table 1 Conjugate Addition of Amines to Alkenes Catalyzed by
Samarium(III) Triflate
Sm(OTf)3,
CH2Cl2, r.t.
R3
R1
R2
R3
EWG
NR1R2
+
NH
EWG
2-Substituted Ethylamines 3; General Procedure
1
2
3
To a mixture of a,b-unsaturated compound 2 (2 mmol) and amine 1
(2 mmol) in CH2Cl2 (10 mL) was added Sm(OTf)3 (0.2 mol%) at r.t.
The resulting mixture was stirred at the same temperature for a
specified period (Table 1). The progress of the reaction was moni-
tored by TLC. After completion of the reaction as indicated by TLC,
the mixture was diluted with CH2Cl2 (20 mL) and washed with H2O
and brine, and the organic layer was dried (anhyd Na2SO4) and con-
centrated under reduced pressure. Thus obtained crude products
were purified by column chromatography (silica gel, 60–120 mesh,
EtOAc–hexane, 2:8).
Amine 1
R1
Alkene 2
Product Time Yield
R2
EWG
R3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
Ph
H
H
H
Ph
Ph
H
(h)
2.0
2.5
3.0
3.5
1.5
2.0
1.5
2.5
2.5
3.0
2.5
3.0
2.0
2.0
2.5
3.0
2.5
2.5
3.5
(%)
96
93
92
91
92
92
90
88
89
86
89
90
89
88
90
85
92
89
86
Bn
H
H
CO2Et
CO2Et
CO2Et
3a
3b
3c
3d
3e
3f
CH2CH2Ph
–(CH2)2O(CH2)2–
–(CH2)2NPh(CH2)2– CO2Et
Ethyl 3-(Benzylamino)propanoate (3a)
Et
Et
Et
CO2Et
CO2Et
CO2Et
CO2Et
IR (neat): 3254, 3041, 2968, 2837, 1728, 1612, 1591, 1504, 1472,
1355, 1286, 1131, 1058, 1016, 985, 867, 821, 794, 739 cm–1.
4-pyridylmethyl
–(CH2)4–
CH(Me)Ph
1H NMR: d = 1.23 (t, J = 7.0 Hz, 3 H), 2.15 (br s, 1 H), 2.50 (t,
J = 6.2 Hz, 2 H), 2.85 (t, J = 6.2 Hz, 2 H), 3.70 (s, 2 H), 4.15 (q,
J = 7.0 Hz, 2 H), 7.22–7.40 (m, 5 H).
MS (EI): m/z (%) = 207 (M+, 25), 178 (18), 134 (100), 106 (31), 77
(56), 52 (15)
3g
3h
3i
H
–(CH2)2NMe(CH2)2– CO2Et
CH(Pr)Ph
Bn
H
CO2Et
3j
Ethyl 3-(Phenethylamino)propanoate (3b)
Colorless liquid.
Me CO2Et
3k
3l
IR (neat): 3430, 3027, 2930, 2852, 1730, 1601, 1457, 1375, 1253,
1171, 1121, 1030, 748, 699, 638, 580 cm–1.
1H NMR: d = 1.23 (t, J = 6.5 Hz, 3 H), 2.15 (s, 1 H), 2.47 (t, J = 6.0
Hz, 2 H), 2.75–2.95 (m, 6 H), 4.10 (q, J = 6.0 Hz, 4 H), 7.12–7.30
(m, 5 H).
–(CH2)2O(CH2)2–
CN
Et
Et
CN
3m
3n
3o
3p
3q
3r
3s
–(CH2)4–
4-pyridylmethyl
CN
MS (EI): m/z (%) = 221 (95), 220 (10), 209 (20), 208 (100), 205
(10), 160 (12) 140 (10), 102 (10), 88 (10), 73 (13).
Et
CN
–(CH2)2O(CH2)2–
CONH2
CN
Ethyl 3-(4-Phenylpiperazin-1-yl)propanoate (3d)
IR (neat): 3444, 2975, 2821, 1930, 1732, 1618, 1599, 1500, 1455,
1380, 1231, 1056, 1015, 927, 864, 760, 694 cm–1.
1H NMR: d = 1.26 (t, J = 7.0 Hz, 3 H), 2.45–2.55 (m, 8 H), 2.68 (t,
J = 7.5 Hz, 2 H), 3.48 (s, 2 H), 4.12 (q, J = 7.0 Hz, 2 H), 7.18–7.30
(m, 5 H).
Bn
H
–(CH2)4–
NO2
Bn
Bn
H
COPh
COMe
CONH2
CONH2
NO2
H
–(CH2)4–
H
3t
2.30 89
MS (EI): m/z (%) = 262 (M+, 40), 233 (21), 189 (100), 161 (15), 119
(26), 91 (10), 84 (36), 77 (85), 65 (14), 52 (12), 42 (20).
3u
3v
3w
3x
3y
3.0
3.5
3.0
2.5
2.5
85
87
86
88
87
Bn
Ethyl 3-{N-Ethyl-N-[(pyridine-4-yl)methyl]amino}propanoate
(3f)
Brown liquid.
4-pyridylmethyl
–(CH2)2O(CH2)2–
4-pyridylmethyl Et
Et
IR (neat): 3440, 3051, 2943, 2837, 1728, 1645, 1605, 1573, 1461,
1339, 1258, 1169, 1108, 1071, 968, 842, 741 cm–1.
1H NMR: d = 1.08 (t, J = 6.5 Hz, 3 H), 1.26 (t, J = 6.5 Hz, 3 H),
2.40–2.60 (m, 4 H), 2.81 (t, J = 6.0 Hz, 2 H), 3.60 (s, 2 H), 4.15 (q,
J = 6.5 Hz, 2 H), 7.24 (d, J = 6.0 Hz, 2 H), 8.58 (d, J = 6.0 Hz, 2 H).
NO2
COMe
tion conditions, stoichiometric amounts of catalyst, and
highly toxic reagents. The simple experimental procedure
combined with easy of isolation of products makes this
method a convenient and general strategy for the aza-
Michael reaction.
MS (EI): m/z (%) = 236 (M+, 15), 207 (18), 163 (100), 135 (32), 134
(20), 106 (45) 78 (25), 53 (15), 40 (10).
Ethyl 3-(Pyrrolidin-1-yl)propanoate (3g)
Light red liquid.
IR (neat): 3432, 2928, 1727, 1630, 1518, 1458, 1343, 1216, 1163,
1108, 1024, 961, 845, 761, 732 cm–1.
1H NMR: d = 1.30 (t, J = 7.5 Hz, 3 H), 1.78–1.88 (m, 4 H), 2.48–
2.60 (m, 6 H), 2.82 (t, J = 7.5 Hz, 2 H), 4.15 (q, J = 7.5 Hz, 2 H).
Melting points were recorded on a Buchi R-535 apparatus and are
uncorrected. IR spectra were recorded on a Perkin-Elmer FT-IR
240-c spectrophotometer using KBr optics. H NMR spectra were
1
Synthesis 2007, No. 22, 3447–3450 © Thieme Stuttgart · New York

SHORT PAPER
Conjugate Addition of Amines to Electron-Deficient Alkenes
3449
MS (EI): m/z (%) = 171 (M+, 20), 142 (25), 98 (100), 70 (15), 46
1-(2-Nitro-1-phenylethyl)pyrrolidine (3r)
(10).
Thick syrup.
IR (KBr): 3430, 3052, 2947, 2841, 1635, 1568, 1510, 1495, 1420,
1371, 1267, 1215, 1173, 1121, 1085, 1006, 953, 861, 739 cm–1.
1H NMR: d = 2.10–2.30 (m, 4 H), 2.70–2.90 (m, 4 H), 4.15 (dd, J =
6.0, 2.0 Hz, 1 H), 4.35 (dd, J = 6.0, 2.0 Hz, 1 H), 4.90 (t, J = 7.0 Hz,
1 H), 7.20–7.55 (m, 5 H).
Ethyl 3-(1-Phenylethylamino)propanoate (3h)
Liquid.
IR (neat): 3446, 3062, 3027, 2973, 2929, 2863, 1731, 1451, 1373,
1256, 1182, 1121, 1029, 953, 861, 760 cm–1.
1H NMR: d = 1.25 (t, J = 6.5 Hz, 3 H), 1.32 (d, J = 6.0 Hz, 3 H), 1.85
(br s, 1 H), 2.42 (t, J = 6.0 Hz, 2 H), 2.60–2.80 (m, 2 H), 3.75 (q,
J = 6.5 Hz, 1 H), 4.12 (q, J = 6.5 Hz, 2 H), 7.15–7.30 (m, 5 H).
MS (EI): m/z (%) = 220 (20), 174 (10), 160 (100), 90 (25), 77 (15),
65 (10), 51 (22).
MS (EI): m/z (%) = 221 (M+, 10), 207 (80), 179 (10), 161 (12), 135
3-(Benzylamino)-1,3-diphenylpropan-1-one (3s)
(20), 119 (55), 106 (100), 92 (25), 78 (32), 51 (40), 43 (35).
Colorless liquid.
IR (neat): 3338, 3060, 3028, 2924, 2853, 1682, 1643, 1603, 1493,
1450, 1336, 1283, 1214, 1178, 1075, 1022, 981, 855, 750 cm–1.
1H NMR: d = 3.25–3.35 (m, 1 H), 3.55–3.70 (m, 1 H), 4.25–4.35 (m,
1 H), 4.70 (s, 2 H), 7.20–7.50 (m, 10 H), 7.55–8.05 (m, 5 H).
MS (EI): m/z (%) = 317 (15), 316 (40), 315 (M+, 10), 308 (15), 299
(32), 298 (100), 224 (18), 210 (15), 147 (15), 119 (25), 77 (18), 51
(20).
Ethyl 3-(4-Methylpiperazin-1-yl)propanoate (3i)
Light yellow liquid.
IR (neat): 3418, 2936, 2800, 1735, 1508, 1459, 1410, 1372, 1287,
1206, 1183, 1115, 1088, 1010, 961, 836, 795, 742 cm–1.
1H NMR: d = 1.25 (t, J = 6.5 Hz, 3 H), 2.25 (s, 3 H), 2.38–2.55 (m,
10 H), 2.68 (t, J = 6.0 Hz, 2 H), 4.15 (q, J = 6.5 Hz, 2 H).
MS (EI): m/z (%) = 200 (M+, 100), 171 (12), 127 (40), 99 (10), 84
(10), 56 (10).
N-Ethyl-2-nitro-1-phenyl-N-[(pyridin-4-yl)methyl]ethanamine
(3w)
Ethyl 3-(N-Phenyl-N-propylamino)propanoate (3j)
Low-melting solid.
Colorless liquid.
IR (KBr): 3334, 3078, 3049, 2930, 2846, 1681, 1625, 1605, 1573,
1510, 1495, 1415, 1341, 1280, 1215, 1169, 1072, 1015, 980, 873,
739 cm–1.
IR (neat): 3432, 3051, 2942, 2839, 1730, 1605, 1570, 1462, 1378,
1259, 1170, 1125, 1034, 952, 840, 739 cm–1.
1H NMR: d = 0.88 (t, J = 7.0 Hz, 3 H), 1.35 (t, J = 7.0 Hz, 3 H),
1.50–1.70 (m, 2 H), 2.36–2.42 (m, 2 H), 2.60–2.70 (m, 2 H), 3.55 (t,
J = 6.0 Hz, 1 H), 4.10 (q, J = 7.0 Hz, 2 H), 7.20–7.35 (m, 5 H).
MS (EI): m/z (%) = 249 (M+, 100), 220 (22), 176 (45), 148 (24), 119
(15), 104 (10), 77 (62), 51 (20).
1H NMR: d = 1.05 (t, J = 6.5 Hz, 3 H), 2.40 (q, J = 6.5 Hz, 2 H), 3.60
(s, 2 H), 4.35 (dd, J = 6.0, 20 Hz, 1 H), 4.55 (dd, J = 6.0, 20 Hz, 1
H), 5.05 (t, J = 6.5 Hz, 1 H), 7.20–7.60 (m, 7 H), 8.50 (d, J = 6.0 Hz,
2 H).
MS (EI): m/z (%) = 285 (M+, 18), 239 (20), 225 (100), 196 (15), 119
(10), 106 (45), 92 (25), 78 (10), 77 (55), 65 (10), 53 (15), 51 (20).
Ethyl 3-(N-Benzyl-N-methylamino)propanoate (3k)
Colorless liquid.
4-(2-Nitro-1-phenylethyl)morpholine (3x)
Light red solid; mp 51–52 °C.
IR (neat): 3424, 3052, 2926, 2843, 1735, 1587, 1458, 1386, 1212,
1123, 1029, 745, 700, 604 cm–1.
1H NMR: d = 1.27 (t, J = 7.5 Hz, 3 H), 2.18 (s, 3 H), 2.50 (t, J = 7.5
Hz, 2 H), 2.72 (t, J = 7.5 Hz, 2 H), 3.50 (s, 2 H), 4.12 (q, J = 7.5 Hz,
2 H), 7.15–7.30 (m, 5 H).
MS (EI): m/z (%) = 222 (M+, 100), 216 (20), 194 (12), 149 (10), 134
(20), 121 (12), 92 (15), 77 (15), 66 (10), 51 (35).
IR (KBr): 3426, 3032, 2922, 2854, 1634, 1560, 1493, 1452, 1366,
1276, 1227, 1112, 1035, 1002, 868, 741 cm–1.
1H NMR: d = 2.30–2.55 (m, 4 H), 3.50–3.70 (4 H), 4.30 (dd, J = 6.5,
2.0 Hz, 1 H), 4.50 (dd, J = 6.5, 2.0 Hz, 1 H), 4.95 (t, J = 7.5 Hz, 1
H), 7.20–7.60 (m, 5 H).
MS (EI): m/z (%) = 237 (60), 236 (M+, 20), 235 (10), 197 (35), 177
(10), 176 (90), 169 (12), 161 (10), 132 (15), 131 (100), 90 (15), 77
(12), 65 (18), 51 (20).
Ethyl 3-(Morpholin-4-yl)propanoate (3l)
IR (neat): 2841, 1722, 1615, 1592, 1506, 1463, 1410, 1376, 1230,
1108, 1036, 1005, 981, 912, 871, 824, 766, 742, 691 cm–1.
4-{N-Ethyl-N-[(pyridin-4-yl)methyl]amino}butan-2-one (3y)
Light yellow liquid.
1H NMR: d = 1.30 (t, J = 7.0 Hz, 3 H), 2.45–2.52 (m, 6 H), 2.68 (t,
J = 6.8 Hz, 2 H), 3.70 (t, J = 4.5 Hz, 4 H), 4.20 (q, J = 7.0 Hz, 2 H).
MS (EI): m/z (%) = 187 (M+, 18), 158 (22), 114 (100), 86 (15), 58
IR (neat): 3438, 3053, 2937, 1718, 1678, 1615, 1583, 1506, 1459,
1351, 1292, 1210, 1172, 1105, 1083, 1015, 973, 847, 734 cm–1.
1H NMR: d = 1.10 (t, J = 6.5 Hz, 3 H), 2.10 (s, 3 H), 2.40 (q, J = 6.5
Hz, 2 H), 2.65 (t, J = 6.0 Hz, 2 H), 2.70 (t, J = 6.0 Hz, 2 H), 3.60 (s,
2 H), 7.24 (d, J = 6.0 Hz, 2 H), 8.58 (d, J = 6.0 Hz, 2 H).
(30).
3-{N-Ethyl-N-[(pyridin-4-yl)methyl]amino}propanenitrile (3o)
Liquid.
MS (EI): m/z (%) = 206 (M+, 25), 177 (10), 163 (100), 135 (30), 134
IR (neat): 3445, 3056, 2927, 2350, 1637, 1416, 1217, 1080, 761
cm–1.
(12), 106 (62), 92 (15), 78 (20), 53 (10).
1H NMR: d = 1.10 (t, J = 6.0 Hz, 3 H), 2.40 (t, J = 6.0 Hz, 2 H), 2.60
(q, J = 6.0 Hz, 2 H), 2.80 (t, J = 6.0 Hz, 2 H), 3.60 (s, 2 H), 7.30 (d,
J = 7.0 Hz, 2 H), 8.58 (d, J = 7.0 Hz, 2 H).
MS (EI): m/z (%) = 189 (M+, 10), 149 (100), 141 (15), 121 (10), 111
(15), 106 (30), 92 (60), 83 (20), 70 (25), 65 (18), 57 (50), 51 (12),
43 (45).
Acknowledgement
A.R.R. and Y.G.R. thank the Council of Scientific and Industrial
Research, New Delhi.
Synthesis 2007, No. 22, 3447–3450 © Thieme Stuttgart · New York

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J. S. Yadav et al.
SHORT PAPER
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Synthesis 2007, No. 22, 3447–3450 © Thieme Stuttgart · New York