A mild method for the synthesis of β-enaminones and β-enamino esters using KH2P04 as catalyst

Article abstract of DOI:10.3184/030823408X382117

β-Enaminones and β-enamino esters have been produced by the direct condensation of amines with β-diketones and β-ketoesters using KH₂P0₄ as catalyst under mild, solvent-free conditions.

Full text of DOI:10.3184/030823408X382117

JOURNAL OF CHEMICAL RESEARCH 2008
DECEMBER, 707-710
RESEARCH PAPER 707
A mild method for the synthesis of p-enaminones and p-enamino esters
using KH2P04 as catalyst
Fang Xu*, Hong-Xia Lv, Jin-Ping Wang, You-Ping 1ian and Jian-Jun Wang
Ke~ Lab?rato:,y of Macro~olecular
Science of Shaanxi Province, School of Chemistry and Materials Science, Shaanxi Normal
University, XI an, Shaanxl, 710062, P. R. China
~-Enaminones and .~-enamino esters have been p~oduced by the direct condensation of amines with ~-diketones
and ~-ketoesters uSing KH2P04 as catalyst under mild, solvent-free conditions.
Keywords: ~-enaminones, ~-enaminoesters, potassium dihydrogen phosphate, solvent-free condition
~-Enaminones and ~-enamino esters are used as intermediates
in organic synthesis and as precursors for the synthesis
of a variety of heterocyclesl-3 including pharmaceutical
compounds,4 such as antibacterial, anticonvulsant and
antitumor agents.5,6Due to the importance of these compounds
Scheme
1
as intermediates, a simple high yielding one-pot approach
for this transformation is higWy desirable. Classically,
~-enaminones and ~-enaminoesters are prepared by the direct
condensation of 1,3-dicarbonyl compounds with amines under
reflux in an aromatic solvent with the azeotropic removal
of water.7 Several improved methods for the preparation of
~-enaminones and ~-enamino esters have been reported using
protic acids,8,9 Lewis acids,IO-14iodine,15 silica gel,16 and
sulfated zirconia,15 as catalysts. More recently, [EtNH3]N03,18
HClOc Si0219 as well as silica chloride2° also have been
used to effect this transformation. Although these methods
have improved reaction condition or shorter reaction time a
gene~al procedure is lacking. There is still a need to devel~p
a sUltable method for the synthesis of ~-enaminones and
~-enamino esters conveniently.
Either protic acid or Lewis acid can effect the above
transformation. The cheaper protic acid catalyst involve harsh
conditions. The more easily handled and environmentally
friendly Lewis acid catalysts are associated with a higher
cost We wished to develop a new catalyst which not only
possessed the higher effect and inexpensive chamcter of protic
acid catalyst, but was also easy to control and environmentally
friendly typical of the Lewis acid catalyst Potassium dihydro-
gen .phosphate, usually used as the buffer in analytical
chemistry, possesses the property of protic acid and a Lewis
acid, with the advantages of low cost, ease of handling and
insensitive to air moisture. The special properties of a metal
hydrogen phosphate salt prompted an investigation of the use
of potassium dihydrogen phosphate in organic reactions. We
wish to report a simple, convenient and efficient method for
the chemo-selective enamination of 1,3-dicarbonyl compounds
catalysed by KH2P04. This method not only afforded the
products in excellent yields but also avoided the problems
associated with catalyst cost, and safety. To the best of our
knowledge, such efficient and practical method for the synthesis
of target compounds has not been reported previously.
Table
1
Screening of various phosphate salt catalysts for
enamination of acetylacetone and 4-methylaniline
Entry
Catalyst (5 mol%l
Time/min
Yield a/%
1
2
3
4
5
None
KH2P04
NaH2P04·2H20
Na2HP04·12H20
Na3(P04b,12H20
40
40
40
40
40
68
98
85
60
50
alsolated yields after column chromatography.
The effect of the amount of catalyst on the reaction of
acetylacetone and 4-methylaniline was also investigated
(Table 2). The results show that the yields increased as the
amo~t of catalyst increased from 0% to 5%. A higher catalyst
loadmg (5-10%) did not bring any obvious increase in yield.
On the contrary, it caused the yield to slightly decrease.
The best catalyst loading for this reaction is 5% based on
acetylacetone.
An initial study was performed treating acetylacetone and
4-methylaniline under solvent-free conditions in the presence
ofa catalytic amount ofKH2P04 (5 mol%) at 50°C. Product
3C was isolated in 98% yield after 30 minutes. However, the
same reaction works carried out in an organic solvent, such as
C2H50H, THF, CH2CI2,DMF, gave lower yields ofthe desired
product even over a prolonged reaction time. Various aromatic
amines were used in the condensation with ~-dicarbonyl
compounds to give the corresponding ~-enaminones and
~-enamino esters in good to nearly quantitative yield under
solvent-free condition. The results are sunmIarised in Table 3.
In general, for aromatic primary amines the condensation
reactions usually afforded the corresponding ~-enaminones
and aromatic ~-enamino esters in over 80% yields in a
short time. However, anilines with electron-donating groups
afforded a higher yields than those aniline with an electron-
withdrawing substituent It should be pointed out that in
the reaction of ~-diketones and ~-ketoesters with aromatic
amines the corresponding ~-enaminones and ~-enamino
Results and discussion
Potassium dihydrogen phosphate catalysed condensation
of ~-enaminones and ~-ketoesters 1 with primary amines 2
affording ~-enaminones and ~-enamino esters 3 (Scheme 1).
The research began by comparing the catalytic activity of
different metal hydrogen phosphate and phosphate salts towards
the reaction between acetylacetone and 4-methylaniline under
solvent free condition (Table 1). Among the salts that were
tested, KH2P04 proved to be the most efficient in giving
excellent yields (Table 1, entry 2).
Table
2
The effect of mol% of KH2P04 on reaction of
acetylacetone with 4-methylaniline
Entry
KH2P04 (mol%l
Time/min
Yield a/%
1
2
3
4
5
0
1
3
5
10
~
~
30
30
30
30
90
93
98
97
* Correspondent. E-mail: fengxu@snnu.edu.cn
alsolated yields after column chromatography

JOURNAL OF CHEMICAL RESEARCH 2008
708
Table
3
Enamination
3a-p
of ~-dicarbonyl
R
compounds
catalysed by KH2P04 under solvent-free conditions
R2
Yield b/%
88
Product
Time/h
1.0
R'
o
HN-O
CH3
CH3
CH3
CH3
CH3
CH3
C6H5
3a
3b
3c
A A
rO
2
3
CH2C6H5
4-CH3C6H4
0.5
0.5
95
98
NH
0
A A
o
HN-Q-
A A
o
HN
~
-Q-CI !.
4
5
3d
CH3
CH3
CH3
CH3
4-CIC6H3
4-BrC6H4
2
80
92
A A
o
HN
~
-Q-sr !.
1.0
3ec
A A
~
:::,...
6
7
3f
CH3
CH3
CH3
CH3
1-Naphthyl
2-Naphthyl
1.0
1.0
88
91
~
HN
0
A A
r .--:
3gC
o
HN
.--:
-&
A A
:::,... NH
Q
0
8
9
3h
3i
OC2H5
OC2H5
CH3
CH3
C6H5
1.0
0.5
87
96
~OC2H5
U
~
NH
0
4-CH3C6H4
~OC2H5
CI'Q
:::,... NH
3jC
3k
0
10
11
OC2H5
OC2H5
CH3
CH3
4-CIC6H3
4-BrC6H4
1.5
1.0
79
88
~OC2H5
sr'Q
:::,... NH
0
A)lOC2H5
~
:::,...
OQ
12
31
OC2H5
CH3
1-Naphthyl
1.0
90
HN
0
~OC2H5
:::,...:::,... NH
CO
0
13
14
3mc
OC2H5
Ph
CH3
CH3
2-Naphthyl
1.0
0.5
87
97
A)lOC2H5
o
HN-Q-
3n
4-CH3C6H4
Ph
A AP
15
16
Ph
Ph
CH3
CH3
CH2C6H5
0.5
0.5
95
96
30
3p
HN
0
PhA A
o
NHCH2CH2OH
CH2CH2OH
PhA A
aA11 products were identified by comparison
blsolated yields after column chromatography.
cNew compound.
oftheir physical and spectral data with those of authentic samples.

JOURNAL OF CHEMICAL RESEARCH 2008 709
1385,1357,1271,1231,1163,1094,1058,1022,976,
788, 752,698;
esters were always obtained having a (Z)-configuration of the
carbon-carbon double bond. This is due to the formation of
an intramolecular hydrogen bond, as shown by the IH NMR
spectra following the procedure reported by Das et al.l9 This
reaction is very clean and free from side reactions which are
normally observed in other catalyst.
IH NMR (CDC13, 300 MHz) 81.26 (t, 3H, J = 6.9 Hz, OCH2CH3),
2.06 (s, 3H, CH3), 4.13(q, 2H, J= 6.8 Hz, OCH2CH3), 4.69(s, lH,
C=CH), 7.04-7.31(m, 5H, CJIs), 10.42 (s, lH, NH); Anal. Calcd for
C12HISN02 (205): C, 70.22; H, 7.37; N, 6.82; Found: C, 70.20; H,
7.42; N, 6.75%.
(Z)-3-(4-Methyl-phenylamino)-but-2-enoic
acid ethyl ester (3i):
Wine red oil: IR (KBr cm-I) 3259, 2979, 2927, 1653, 1608, 1577,
1517, 1488, 1440, 1384, 1357, 1270, 1230, 1162, 1095, 1058, 1019,
807, 787; IH NMR (CDC13, 300 MHz) 81.26 (t, 3H, J = 6.9 Hz,
OCH2CH3), 1.92 (s, 3H, CH3), 2.35 (s, 3H, C6~CH3)' 4.13 (q,2H,
J= 6.9 Hz, OCH2CH3), 4.66 (s, lH, C=CH), 6.95 (d, 2H,J= 7.7 Hz,
CJI4)' 7.09 (d, 2H,J= 7.7 Hz, C6~)' 10.33 (s, lH, NH)Anal. Calcd
for C13H17N02(219) C, 71.21; H, 7.81; N, 6.39; Found: C, 71.18; H,
7.90; N, 6.35%.
Experimental
Starting materials were obtained from commercial suppliers and used
without further purification. Melting points were determined with
an X-5 apparatus in open glass capillaries and were uncorrected. IR
spectra were recorded on EQUINX 55 FT-IR spectrometer using KBr
pellets. NMR spectra were collected on an AVANCE 300 MHz with
TMS as an internal standard. Elemental analyses were performed on
Vario ELIII instrument. Silica gel (200-300 mesh size) was used as a
stationary phase for column chromatography.
Typical procedure for the synthesis of~-enaminone and ~-enamino
esters catalysed by KH2P04: KH2P04 (0.25 mmol) was added to
a mixture of the 1,3-dicarbonyl compound (5 mmol) and amine
(5 mmol). The mixture was stirred under solvent-free conditions
at 50°C. After the reaction was complete (monitored by TLC), the
reaction mixture was diluted with CH2C12(5 ml) and filtered. The
filtrate was concentrated and the gummy mass was subjected to
column chromatography over silica gel using petroleum ether-EtOAc
as eluent to obtain pure ~-enaminone and ~-enamino esters.
(Z)-3-(4-chloro-phenylamino)-but-2-enoic
acid ethyl ester (3j):
White solid: m.p. 70-71 °C; IR (KEr cm-I) 3278, 3071, 2979, 2925,
1647, 1610, 1584, 1481, 1439, 1437, 1387, 1351, 1260, 1168, 1062,
1012, 981, 856, 789; IH NMR (CDC13, 300 MHz) 81.29 (1, 3H,
J = 7.1 Hz,OCH2CH3), 1.95 (s, 3H, CH3), 4.16 (q, 2H, J = 7.1 Hz,
OCH2CH3), 4.71 (s, lH, C=CH), 7.00 (d, 2H, J = 8.3 Hz, CJI4)'
7.28 (d, 2H, J = 8.6 Hz, CJI4)' 10.35 (s, lH, NH); Anal. Calcd for
C12HI4CIN02(239): C, 60.13; H, 5.89; N, 5.84; Found: C, 60.12; H,
5.90 N, 5.83%.
(Z)-3-(4-Bromo-phenylamino)-but-2-enoic acid ethyl ester (3k):
Buff solid: m.p. 53-54°C; IR (KEr cm-I) 3275, 3068, 2978, 1646,
1609,1578,1479,1437,1387,1352,1260,1162,1005,853,
790, 719;
IH NMR (CDC13, 300 MHz) 81.28 (t, 3H, J= 7.1 Hz,OCH2CH3),
2.01 (s, 3H, CH3), 4.15 (q, 2H, J = 7.0 Hz, OCH2CH3), 4.72 (s,lH,
C=CH), 6.95(d, 2H,J= 8.1 Hz, C6~)' 7.43(d, 2H,J= 8.1 Hz, CJI4)'
10.35 (s, lH, NH); Anal. Calcd for C12HI~rN02 (283): C, 50.72; H,
4.97; N, 4.93; Found: C, 50.75; H, 4.80; N, 4.98%.
Characterisation data for compounds 3a-p
(Z)-4-(Phenylamino)-pent-3-en-2-one (3a): White solid: m.p. 50-
51°C; IR (KEr em-I)3420, 2926, 1595, 1570, 1510, 1436, 1355, 1316,
1281, 1186,905,820,748; IH NMR (CDC13,300 MHz) 8 1.99 (s, 3H,
CH3C=C), 2.21 (s, 3H, COCH3), 5.27 (s, lH, C=CH), 6.67-7.35 (m,
5H, CJIs), 12.48 (s, lH, NH); Anal. Calcd for Cl l H13NO (175): C,
75.40; H, 7.48; N, 7.99; Found: C, 75.42; H, 7.50; N, 7.90%.
(Z)-4-(benzylamino)-pent-3-en-2-one(3b): Yellow oil: IR (KEr em-I)
(Z)-4-(Naphthalen-I-ylamino)-but-2-enoic
acid
ethyl
ester
(31): Red oil: IR (KBr cm-I) 3244, 3047, 2977, 2931, 1651, 1607,
1484, 1435, 1382, 1336, 1272, 1157, 1087, 1058, 1018, 976, 784;
IH NMR (CDC13, 300 MHz) 8 1.30 (t, 3H, J = 6.8 Hz, OCH2CH3),
2.00 (s, 3H, CH3), 4.21 (q, 2H, J = 6.7 Hz, OCH2CH3), 4.82 (s,
lH, C=CH), 7.19-8.06(m, 7H, Ar), 10.63 (s, lH, NH); Anal. Calcd
for CIJI17N02 (255): C, 75.27; H, 6.71; N, 5.49; Found: C, 75.30;
H, 6.77; N, 5.45%.
(Z)-4-(Naphthalen-2-ylamino)-but-2-enoic acid ethyl ester (3m):
Pink solid: m.p. 67-68°C; IR (KBr cm-I) 3257, 3057, 2983, 2930,
1646, 1599, 1490, 1436, 1384, 1339, 1257, 1156, 1057, 1019, 974,
901, 865, 830, 784; IH NMR (CDC13, 300 MHz) 8 1.30 (t, 3H,
J = 7.0 Hz, OCH2CH3), 2.16 (s, 3H, CH3), 4.18 (q, 2H, J = 7.0 Hz,
OCH2CH3), 4.75 (s, lH, C=CH), 7.21-7.80 (m, 7H, Ar), 10.58 (s,
lH, NH); Anal. Calcd for CIJI17N02 (255): C, 75.27; H, 6.71; N,
5.49; Found: C, 75.29; H, 6.78; N, 5.50%.
3427,3062,3029,2921,2854,
1610, 1573, 1511, 1439, 1534, 1294,
1236, 1104, 1071, 1025, 984, 736; IH NMR (CDC13, 300 MHz) 8
1.72 (s, 3H, CH3C=C), 1.84 (s, 3H, COCH3), 4.25 (d, 2H,J= 6.0 Hz,
CJISCH2), 4.89 (s,l H, C=CH), 7.10-7.16 (m, 5H, C6HS)' 11.04 (s,
lH, NH); Anal. Calcd for C12HISNO (189): C, 76.16; H, 7.99; N,
7.40; Found: C, 76.15; H, 8.00; N, 7.39%.
(Z)-4-(4-Methyl-phenylamino)-pent-3-en-2-one (3c): Wine red solid:
m.p. 6~9°C;
IR (KEr em-I) 3437, 3023, 2991,2920, 2854, 1606,
1564, 1518, 1498, 1438, 1354, 1311, 1280, 1215, 1185, 1017, 922,
827,761; IH NMR (CDC13,300 MHz) 1.91 (s, 3H, CH3C=C), 2.06 (s,
3H, COCH3), 2.32 (s, 3H, CJI4CH3)' 5.17 (s, lH, C=CH), 6.96-7.13
(m, 4H, C6~) 12.41 (s, lH, NH); Anal. Calcd for C12HISNO(189): C,
76.16; H, 7.99; N, 7.40; Found: C, 76.11; H, 7.85; N, 7.44%.
(Z)-4-(4-chloro-phenylamino)-pent-3-en-2-one (3d): Pale white
solid: m.p. 60-61°C; IR (KEr em-I) 3448, 2991, 2925, 1612, 1565,
1501,1432,1403,1314,1276,1212,1184,1089,1010,839,
IH NMR (CDC13, 300 MHz) 8 1.94 (s, 3H, CH3C=C), 2.17 (s, 3H,
COCH3), 5.21 (s, lH, C=CH), 7.04 (d, 2H,J= 7.8 Hz, C~), 7.30 (d,
2H, J = 7.8 Hz, C~,), 12.43 (s, lH, NH); Anal. Calcd for Cl l H12CINO
(209): C, 63.01; H, 5.77; N, 6.68; Found: C, 63.03; H, 5.79; N, 6.65%.
(Z)-4-(4-Bromo-phenylamino)-pent-3-en-2-one
m.p. 55-56°C; IR (KBr cm-I) 3440, 2991, 2926, 1596, 1570, 1509,
1435, 1356, 1315, 1323, 1279, 1185, 1019,905,819,749; IH NMR
(Z)-3-(4-Methyl-phenylamino )-I-phenylbut-2-en-I-one
(3n):
Yellow solid: m.p. 88-89°C; IR (KBr em-I) 3441, 2918, 2849, 1601,
1569, 1504, 1438, 1380, 1320, 1284, 1195, 1064, 1024, 929, 831,
806,715; IH NMR (CDC13, 300 MHz) 82.11 (s, 3H, CH3), 2.35 (s,
3H, C6~CH3)' 5.87 (s, lH, C=CH), 7.06 (d, 2H, J= 8.2 Hz, CJI4)'
7.16 (d, 2H, J= 8.1 Hz, C6~),7.22-7.92 (m, 5H, C6HS)' 13.03 (s,
lH, NH); Anal. Calcd for C17H17NO(251): C, 81.24; H, 6.82; N,
5.57; Found: C, 81.25; H, 6.83; N, 5.58%.
796, 754;
(3e): White solid:
(Z)-3-(benzylamino)-I-phenylbut-2-en-I-one
m.p. 63-64°C; IR (KBr cm-I) 3055, 3023, 2920, 1602, 1542, 1519,
1445,1368,1308,1062,1023,970,874,790; IH NMR (CDC13, 300
(30): Beige solid:
(CDC13, 300 MHz) 8 1.94 (s, 3H, CH3C=C), 2.13 (s, 3H, COCH3),
5.21 (s, lH, C=CH), 6.98 (d, 2H, J = 8.1 Hz, CJI4)' 7.44 (d, 2H,
J = 8.1 Hz, C~), 12.43 (s, lH, NH); Anal. Calcd for Cl l H12BrNO
(254): C, 51.99; H, 4.76; N, 5.51; Found: C, 51.98; H, 4.77; N, 5.52%.
MHz) 82.06 (s, 3H, CH3), 4.53 (s, 2H, CH2), 5.74 (s, lH, C=CH),
7.25-7.35 (m, 5H, C6HsCH2),7.38-7.88 (m, 5H, CJIs),l1.76 (s, lH,
NH); Anal. Calcd for C17H17N02(251): C,81.24; H, 6.82; N, 5.57;
Found: C, 81.25; H, 6.84; N, 5.56%.
(Z)-4-(Naphthalen-I-ylamino)-pent-3-en-2-one
m.p. 51-53°C; IR (KBr cm-I) 3471, 3056, 1599, 1550, 1500, 1429,
1382, 1280, 1156, 1080, 1020,984,912,781,727; IH NMR (CDC13,
(31): Buff solid:
(Z)-3-(2-hydroxyethylamino)-I-phenylbut-2-en-I-one
(3p): Beige
solid: m.p. 80-81 °C; IR (KBr em-I) 1597, 1546; lH NMR (CDC13,
300 MHz) 8:1.99 (s, 3H, CH3), 3.42 (brs, lH, OH), 3.45 (q,J= 5.1 Hz,
2H, NHCH2), 3.79 (1,J= 4.4 Hz, 2H CH20H,), 5.63 (s, lH, C=CH),
7.26-7.84 (m, 5H, CJIs), 11.46 (s, lH, NH); Anal. Calcd for
C12HISNOi205): C, 70.24; H, 7.32, N, 6.83; found C,70.19; H, 7.30;
N,6.86%.
300 MHz) 8 1.82 (s, 3H, CH3C=C), 2.15 (s, 3H, COCH3), 5.31(s,
lH, C=CH), 7.26-8.04(m, 7H, Ar), 12.75 (s, lH, NH); Anal. Calcd
for CIsHIsNO (225): C, 79.97; H, 6.71; N, 6.22; Found: C, 79.90; H,
6.80; N, 6.18%.
(Z)-4-(Naphthalen-2-ylamino)-pent-3-en-2-one
m.p. 100°C; IR (KEr cm-I) 3430, 3057, 3014, 2898, 1615, 1586,
1515,1467,1435,1380,1348,1283,1175, 1121, 1028,958,861,826,
(3g): Pink solid:
This work was supported by the Scientific Research
Foundation for ROCS, State Education Ministry of China and
the start-up fund ofShaanxi Normal University.
752; IH NMR (CDC13, 300 MHz) 8 2.02 (s, 3H, CH3C=C), 2.18 (s,
3H, COCH3), 5.24(s, lH, C=CH), 7.23-7.82(m, 7H, Ar), 12.65 (s,
lH, NH); Anal. Calcd for CIsHIsNO (225): C, 79.97; H, 6.71; N,
6.22; Found: C, 79.92; H, 6.75; N, 6.18%.
(Z)-3-(Phenylamino)-but-2-enoic acid ethyl ester (3h): Yellow
oil: IR (KBr em-I) 3257, 3184, 2979,1653,1620,1592,1495,1440,
Received 3 September 2008; accepted 13 October 2008
Paper 08/0147 doi: 10.3184/030823408X382117
Published online: 17 December 2008

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