FeCl3-PTSA co-catalysed highly regio- and stereo-selective synthesis of β-functionalised enamine derivatives

Article abstract of DOI:10.3184/174751912X13306054094882

β-Enaminone derivatives are useful synthetic precursors. β-N-Substituted (E)-enaminones and β-N-substituted (E)-aminoacrylates were synthesised with high regio- and stereo-selectivity via using the co-catalytic system of FeCl₃/PTSA, which also provides a new way to the formation of C-O bond.

Full text of DOI:10.3184/174751912X13306054094882

JOURNAL OF CHEMICAL RESEARCH 2012
RESEARCH PAPER 175
MARCH, 175–177
FeCl3-PTSA co-catalysed highly regio- and stereo-selective synthesis of
β-functionalised enamine derivatives
Weibing Liu,* Cui Chen and Qing Zhang
School of Chemistry and Life Science, Guangdong University of PetrochemicalTechnology, 2 Guangdu Road, Maoming 525000, P. R. China
β-Enaminone derivatives are useful synthetic precursors. β-N-Substituted (E)-enaminones and β-N-substituted
(E)-aminoacrylates were synthesised with high regio- and stereo-selectivity via using the co-catalytic system of
FeCl3/PTSA, which also provides a new way to the formation of C–O bond.
Keywords: high regio-selectivity; high stereo-selectivity, co-catalytic system, β-N-substituted (E)-enaminones, β-N-substituted
(E)-aminoacrylates
β-Enaminone derivatives1–10 are useful synthetic precursors11,12
and their utilisation in organic synthesis is of great interest13–22
due to the presence of the nucleophilic character of the
enamine moiety and the electrophilic character of the enone
moiety. A variety of biologically active compounds, such
as heterocycles,23–26 aminoacids,27,28 alkaloids,29–32 have been
prepared from β-N-substituted enaminones and aminoacrylates
taking advantage of their electronic properties. In this regard,
there are many reports for the synthesis of β-functionalised
enamines.33–36 However, some of them usually present signifi-
cant limitations, such as tedious workup, harsh nature of reac-
tion conditions, low yields, long reaction times, or poor scope
of substrates. In conjunction with our interest in exploring
the synthetic potential of β-functionalised enamines, we now
report an efficient and straightforward protocol under mild
conditions to construct β-N-substituted (E)-enaminones and
(E)-aminoacrylates (Scheme 1). The structure of the product
To explore the substrate scope and limitations of this
reaction, a range of substrates were examined under the
optimised reaction conditions. These results are summarised
in Table 2. Treatment of 1-phenylbutane-1,3-dione (1a) with
various amines 2 furnished the corresponding β-N-substituted
(E)-Enaminones 3 in good to excellent isolated yields (3aa–
af). As shown in Scheme 2, aromatic amines whether with
electron-withdrawing groups or with electron-donating groups
are all suitable for this protocol. Besides, the reaction appears
quite tolerant with respect to the electronic contribution of
the substituent on the benzene ring of aromatic amines.
For example, the reaction of naphthalen-1-amine (2c) and
4-methoxybenzenamine (2d) with 1-phenylbutane-1,3-dione
(1a) led to (E)-4-(naphthalen-1-ylamino)-4-phenylbut-3-en-2-
one (3ac) and (E)-4-(4-methoxyphenylamino)-4-phenylbut-3-
en-2-one in excellent yield. Interestingly, the amines bearing
an aliphatic substituent proved to be a suitable partner (3ab,
3af).
1
3ad has been confirmed by H NMR, 13C NMR and the
NMR-NOE further verified its configuration (Scheme 2).
Initially, we employed 1-phenylbutane-1,3-dione (1a) with
p-toluidine (2a) as the model reaction and tried to establish an
effective reaction system for the reaction. These results are
shown in Table 1. Among the Lewis acids tested, FeCl3 showed
the highest activity for this transformation (entries 2–4) and
the reaction would not work in the absence of Lewis acid (entry
1). Among the various solvents examined, 1,2-dichloroethane
(DCE) was successful (entries 5–8). When the reaction pro-
ceeded under refluxing condition using DCE as the solvent,
the effect on increasing the yield was not obvious. Surpris-
ingly, the reaction was found to proceed smoothly and led to
the desired product in 88% yield when 0.2 equiv. p-toluenesul-
fonic acid (PTSA) was added to the reaction (entry 9). An
increase in the amount of PTSA to 0.5 equiv., led to the same
yield of the product (entry 11). The reaction also resulted in
58% yield in the absence of FeCl3 (entry 10). A survey of the
reaction time and the amount of FeCl3 indicated that 2 h was
practical and 0.1 equiv. of catalyst was the optimum amount
(entries 9, 12–15). After screening, the optimal reaction condi-
tions were obtained; that is, the mixture of 1-phenylbutane-
1,3-dione (1a) with p-toluidine (2a), 0.1 equiv. of FeCl3 and
0.2 equiv. of PTSA reacted in DCE at 80 °C for 2 h (Table 1,
entry 9).
To demonstrate the generality of this method, we next
investigated the scope of this reaction to other 1,3-dicarbonyl
compounds, such as ethyl 3-oxo-3-phenylpropanoate and
1,1,1-trifluoropentane-2,4-dione. The reactions of ethyl 3-
oxo-3-phenylpropanoate with p-toluidine (2a), naphthalen-1-
amine (2c) and m-toluidine (2g) all lead to the corresponding
(E)-aminoacrylate in good yield with high region- and stereo-
selectivity (3ba, 3bc, 3bg). Additionally, 1,1,1-trifluoropen-
tane-2,4-dione was also a good partner of 1,3-dicarbonyl
compounds (3ca, 3cc, 3cg). The experimental results suggested
a mechanism through the formation of imine intermediate.
Scheme 2 The configuration of 3ad.
Scheme 1 Synthesis of β-functionalised enamine derivatives.
* Correspondent. E-mail: lwb409@yahoo.com.cn

176 JOURNAL OF CHEMICAL RESEARCH 2012
Table 1 Optimisation of reaction conditionsa
Further investigations concerning on the scope of this conden-
sation reaction, applications are ongoing in our laboratory.
Experimental
All the reactions were carried out at 80 °C in a Schlenk tube equipped
with magnetic stir bar. Solvents and all reagents were used as received.
NMR spectra and GC–MS was obtained using an electron ionisation
(EI)Agilent 6890N/5973 mass spectrometer. IR spectra were obtained
with a Bruker Vector 22 spectrometer. Microanalysis was obtained
using a Vario EL cube CHNOS elemental analyser. All the other
chemicals were purchased from Aldrich Chemicals.
Entry
Cat. (0.1 equiv.)
Solvent
Yieldb/%
1
2
—
Dioxane
Dioxane
Dioxane
Dioxane
DMSO
DMF
none
52
43
74
67
72
68
79
88
58
88
31
89
83
88
ZnCl2
CuCl2
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
—
3
4
Synthesis of (E)-4-(p-tolylamino)-4-phenylbut-3-en-2-one (3aa);
general procedure
5
6
7
Toluene
DCE
FeCl3 (16.2 mg, 0.1 mmol), p-toluenesulfonic acid (34.4 mg, 0.2 mmol),
DCE (4 mL), 1-phenylbutane-1,3-dione (1a) (162 mg, 1.0 mmol)
and p-toluidine (2a) (107 mg, 1.0 mmol) was added to a 10 mL
Schlenk tube. The mixture was stirred at 80 °C for 2 h. The solution
was directly subjected to isolation by PTLC (GF254), eluted with a
10:4 petroleum ether/ethyl acetate mixture, which furnished 3aa
(203.3 mg, 83%) as an orange oil.
(E)-4-(p-Tolylamino)-4-phenylbut-3-en-2-one (3aa): Orange oil;
1H NMR (CDCl3, 400 Hz) δ 13.03 (s, 1H), 7.92 (d, J = 8.0 Hz, 2H),
7.52–7.49 (m, 3H), 7.15 (d, J = 8.0 Hz, 2H), 7.05 (d, J = 8.0 Hz, 2H),
5.86 (s, 1H), 2.34 (s, 3H), 2.09 (s, 3H); 13C NMR (CDCl3, 100 Hz)
δ 188.3, 162.5, 140.0, 135.6, 130.7, 129.6, 128.1, 126.9, 124.7, 93.8,
20.8, 20.2; MS (EI) m/z (%): 250.95 (100.00); Anal. Calcd for
C17H17NO: C, 81.24; H, 6.82. Found: C, 81.11; H, 6.94%.
8
9c
DCE
10c
11d
12c,e
13c,f
14c,g
15c,h
DCE
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
DCE
DCE
DCE
DCE
DCE
a 1a (0.25 mmol), 2a (0.25 mmol), solvent (4 mL); b GC yield;
c PTSA (0.2 equiv.); d PTSA (0.5 equiv.); e reaction time: 0.5 h;
f reaction time: 4 h; g FeCl3 (0.05 equiv.); h FeCl3 (0.2 equiv.).
Table 2 Synthesis of β-N-substituted (E)-enaminones and
(E)-aminoacrylatesa
(E)-4-(Benzylamino)-4-phenybut-3-en-2-one (3ab): Orange oil;
IR νmax (KBr): 3442, 2926, 1720, 1598, 1444, 1294, 1064, 800, 738,
1
698 cm−1; H NMR (CDCl3, 400 Hz) δ 12.89 (b, 1H), 7.89 (d, J =
8.0 Hz, 2H), 7.43–7.29 (m, 8H), 6.17 (s, 1H), 4.64 (d, J = 4.0 Hz, 2H),
2.18 (s, 3H); 13C NMR (CDCl3, 100 Hz) δ 188.2, 161.2, 129.3, 128.7,
128.3, 127.8, 127.5, 94.8, 21.2; MS (EI) m/z (%): 91.05 (100.00),
251.00 (44.52); Anal. Calcd for C17H17NO: C, 81.24; H, 6.82. Found:
C, 81.36; H, 6.81%.
(E)-4-(Naphthalen-1-ylamino)-4-phenylbut-3-en-2-one(3ac):Orange
solid; m.p. 130–131oC; IR νmax (KBr): 3448, 3053, 1587, 1552, 1425,
1286, 1219, 925, 786, 692 cm−1; −1H NMR (CDCl3, 400 Hz) δ 13.6
(s, 1H), 7.88 (d, J = 8.0 Hz, 1H), 7.79 (d, J = 8.0 Hz, 2H), 7.57–7.53
(m, 2H), 7.51–7.44 (m, 6H), 7.31 (d, J = 8.0 Hz, 1H), 6.02 (s, 1H),
2.02 (s, 3H); 13C NMR (CDCl3, 100 Hz) δ 189.2, 164.1, 140.2, 134.4,
130.1, 128.5, 127.4, 125.5, 123.0, 94.3, 20.4; MS (EI) m/z (%): 286.93
(100.00); Anal. Calcd for C20H17NO; C, 83.59; H, 5.96. Found: C,
83.66; H, 6.10%.
(E)-4-(4-Methoxyphenlamino)-4-phenylbut-3-en-2-one (3ad): Brown
solid; m.p. 111–113 °C; IR νmax (KBr): 3437, 1610, 1508, 1433, 1328,
1244, 1184, 1107, 1028, 831, 759, 705 cm−1; 1H NMR (CDCl3,
400 Hz) δ 12.91 (s, 1H), 7.91–7.88 (q, 2H), 7.44–7.41 (m, 3H), 7.11–
7.07 (t, 2H), 7.89–7.86 (t, 2H), 5.86 (s, 1H), 3.80 (s, 3H), 2.05 (s, 3H);
13C NMR (CDCl3, 100 Hz) δ 188.6, 163.4, 158.0, 140.3, 131.0, 128.5,
127.2, 126.8, 114.5, 93.7, 55.7, 20.5; MS (EI) m/z (%): 105.05
(100.00), 267.05 (83.36); Anal. Calcd for C17H17NO2; C, 76.38; H,
6.41. Found: C, 76.81; H, 6.62%.
Entry
1
2
3
Yield/%b
1
2
1a
1a
1a
1a
1a
1a
1b
1b
1b
1c
1c
1c
2a
2b
2c
2d
2e
2f
2a
2c
2g
2a
2c
2g
3aa
3ab
3ac
3ad
3ae
3af
3ba
3bc
3bg
3ca
3cc
3cg
86
82
75
85
80
83
79
74
81
84
72
73
3
4
5
6
7
8
9
10
11
12
a 1 (1.0 mmol), 2 (1.0 mmol), DCE (4 mL), FeCl3 (0.1 equiv.),
PTSA (0.2 equiv.).
b Isolated yield
(E)-4-Phenyl-4-(pyridin-2-ylamino)but-3-en-3-one (3ae): Orange
oil; IR νmax (KBr): 3442, 2927, 1724, 1612, 1479, 1205, 1031, 856,
771, 694 cm−1; H NMR (CDCl3, 400 Hz) δ 13.47 (s, 1H), 8.30 (d,
1
Unstability of the imine intermediate quickly isomerised to
more stably conjugated enamine form (Scheme 3).
In summary, we have developed a facile and highly efficient
condensation reaction to synthesise β-N-substituted (E)-enami-
nones and (E)-aminoacrylates with high region- and stereo-
selectivity via using the co-catalytic system of FeCl3/PTSA.
1H), 7.93 (d, 2H), 7.85–7.82 (m, 2H), 7.53–7.51 (m, 1H), 7.49–7.46
(m, 3H), 6.96–6.93 (t, 2H), 5.93 (s, 1H), 2.60 (s, 3H); 13C NMR
(CDCl3, 100 Hz) δ 189.7, 161.4, 153.3, 148.3, 139.9,138.2, 131.5,
128.6, 127.5, 118.8, 115.4, 97.6, 23.1; MS (EI) m/z (%): 237.85
(100.00); Anal. Calcd for C15H14N2O; C, 75.61; H, 5.92. Found: C,
75.66; H, 5.74%.
Scheme 3 Reaction.

JOURNAL OF CHEMICAL RESEARCH 2012 177
(E)-4-(Hexylamino)-4-phenylbut-3-en-2-one (3af): Yellow oil; IR
νmax (KBr): 3410, 3062, 2931, 1708, 1597, 1444, 1307, 1078, 1022,
925, 725 cm−1; 1H NMR (CDCl3, 400 Hz) δ 11.46 (s, 1H), 7.85–7.82
(m, 2H), 7.38–7.34 (m, 3H), 5.63 (s, 1H), 3.32–3.27 (q, 2H), 2.05
(s, 3H), 1.63–1.61 (m, 2H), 1.44–1.29 (m, 6H), 0.88–0.86 (t, 3H);
13C NMR (CDCl3, 100 Hz) δ 188.1, 141.5, 130.6, 128.4, 127.1, 124.9,
93.1, 43.7, 31.7, 30.2, 26.8, 23.6, 19.7, 14.3; MS (EI) m/z (%): 105.05
(100.00), 245.10 (70.66); Anal. Calcd for C16H23NO; C, 78.32; H,
9.45. Found: C, 78.14; H, 9.55%.
Received 27 December 2011; accepted 12 February 2012
Paper 1101063 doi: 10.3184/174751912X13306054094882
Published online: 22 March 2012
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1
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The authors thank the Guangdong University of Petrochemical
Technology of China for financial support of this work.

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