Changhui Liu et al. / Chinese Journal of Catalysis 37 (2016) 979–986
981
= 8.9 Hz, 2H), 6.81 (d, J = 8.7 Hz, 2H), 6.42 (d, J = 15.7 Hz, 1H),
6.19–5.92 (m, 1H), 4.40 (t, J = 7.2 Hz, 1H), 4.21–4.07 (m, 2H),
3.87 (s, 3H), 3.78 (s, 3H), 2.97–2.76 (m, 2H), 1.17 (t, J = 7.1, 3H).
13C NMR (150 MHz, CDCl3, 25°C) δ = 192.9, 169.7, 163.9, 159.0,
131.9, 131.1, 130.0, 129.2, 127.3, 124.1, 113.9, 113.9, 61.4, 55.5,
55.3, 54.3, 32.5, 14.1. IR (KBr) v: 2933, 2839, 1735, 1677, 1602,
1511, 1421, 1252, 1175, 1031, 968, 842 cm−1. HRMS m/z (ESI)
calculated for C22H24NaO5 [M + Na]+ 391.1521 found 391.1518.
(E)‐Methyl 2‐acetyl‐5‐(4‐(tert‐butyl)phenyl)pent‐4‐enoate
gated. The reaction was performed in nitromethane (CH3NO2)
at 80 °C. As shown in Table 1, in the presence of AlCl3 catalyst,
only trace amounts of the desired product 2a was detected
with the starting material selectively converting to the dimeri‐
zation product 3a in abundance (entry 1). Under identical con‐
ditions, FeCl3·6H2O was shown to be a less effective catalyst for
the dehydration of 1a (entry 2). Toluenesulfonic acid (p‐TSA)
was also observed to be unreactive for this reaction (entry 3).
To circumvent issues with reactivity, we studied the reaction
system in the presence of an additive. It was evidenced that the
addition of 5 mol% of PPh3 in the presence of the AlCl3 Lewis
acid yielded an increase in the desired product to 93% (entry
4). However, PPh3 cannot catalyze this transformation alone
and requires the presence of a Lewis acid (entry 5). Alternative
combinations of Lewis bases or Brönsted bases were also ex‐
amined (entries 6–12). Tributyl phosphine (PBu3) and
1,2‐bis(diphenylphosphino)ethane (DPPE) showed significant‐
ly improved efficiency compared with pyridine, o‐phenanthro‐
line and 1,4‐diazabicyclo[2.2.2]octane (DABCO). To reduce cost,
PPh3 was used henceforth in our study. AlCl3 was observed to
be the most efficient Lewis acid, as when subjecting the reac‐
tion to other Lewis acids, such as p‐TSA, FeCl4·6H2O or I2 in the
presence of PPh3 a dramatic drop in the conversion of 1a re‐
sulted (entries 13–15). Further investigations were also con‐
ducted as a function of the solvent. Among the various solvents
tested, nitromethane clearly yielded more of the desired prod‐
uct, with toluene, acetonitrile and ethanol observed to be less
efficient (64%–76%) (entries 16–18). Increasing the reaction
temperature from 80 to 100 °C resulted in a decrease of the
reaction selectivity, even though the reaction time was halved
(entry 19). The test matrix revealed the optimal conditions to
1
(6i): colorless oil. H NMR (600 MHz, CDCl3, TMS, 25 °C) δ =
7.32 (d, J = 8.4 Hz, 2H), 7.26 (d, J = 8.3 Hz, 2H), 6.44 (d, J = 15.8
Hz, 1H), 6.07 (dt, J = 15.7, 7.2 Hz, 1H), 3.74 (s, 3H), 3.60 (t, J =
7.4 Hz, 1H), 2.74 (dd, J = 10.3, 4.4 Hz, 2H), 2.25 (s, 3H), 1.30 (s,
9H). 13C NMR (150 MHz, CDCl3, 25 °C) δ = 202.5, 169.7, 150.6,
134.2, 132.6, 125.9, 125.5, 124.8, 59.5, 52.5, 34.6, 31.6, 31.3,
29.3. IR (KBr) v: 2959, 1745, 1719, 1436, 1361, 1266, 1217,
1152, 970, 803, 560 cm−1. HRMS m/z (ESI) calculated for
C18H24NaO3 [M + Na]+ 311.1623 found 311.1643.
(E)‐2‐Methoxyethyl 2‐acetyl‐5‐(p‐tolyl)pent‐4‐enoate (6j):
colorless oil. 1H NMR (400 MHz, CDCl3, TMS, 25°C) δ = 7.21 (d, J
= 7.8 Hz, 2H), 7.09 (d, J = 7.8 Hz, 2H), 6.43 (d, J = 15.8 Hz, 1H),
6.24–5.89 (m, 1H), 4.37–4.19 (m, 2H), 3.63 (t, J = 7.3 Hz, 1H),
3.57 (t, J = 4.6 Hz, 2H), 3.33 (d, J = 0.8 Hz, 3H), 2.74 (t, J = 7.3 Hz,
2H), 2.31 (s, 3H), 2.26 (s, 3H). 13C NMR (100 MHz, CDCl3, 25°C)
δ = 202.3, 169.3, 137.2, 134.2, 132.6, 129.2, 126.1, 124.5, 70.2,
64.2, 59.5, 58.9, 31.6, 29.2, 21.1. IR (KBr) v: 3024, 2925, 1742,
1717, 1513, 1450, 1361, 1129, 1032, 971, 793 cm−1. HRMS m/z
(ESI) calculated for C17H22NaO4 [M + Na]+ 313.1416 found
313.1422.
(E)‐Ethyl 2‐acetyl‐5‐(4‐fluorophenyl)hex‐4‐enoate (6k):
colorless oil,1H NMR (600 MHz, CDCl3, TMS, 25°C) δ = 7.33 (dd,
J = 5.5, 3.2 Hz, 2H), 6.99 (s, 2H), 6.03 (s, 1H), 4.28–4.16 (m, 2H),
3.51 (s, 1H), 2.62 (ddd, J = 22.5, 17.3, 10.3 Hz, 3H), 2.44 (t, J =
18.8 Hz, 2H), 1.40–1.35 (m, 3H), 1.33–1.28 (m, 4H). 13C NMR
(150 MHz, CDCl3, 25 °C) δ = 202.6, 169.4, 139.4, 139.4, 136.7,
129.4, 129.3, 127.3, 127.2, 123.3, 122.9, 115.2, 115.1, 115.0,
114.9, 61.5, 59.5, 29.2, 27.5, 16.1, 14.1. 19F NMR (565 MHz,
CDCl3, 25°C) δ = −115.7, −116.0, −116.1. IR (KBr) v: 2983, 2933,
1740, 1716, 1601, 1509, 1363, 1228, 1158, 1097, 969, 849
cm−1. HRMS m/z (ESI) calculated for C16H19FNaO3 [M + Na]+
301.1216 found 301.1213.
(E)‐Ethyl 2‐acetyl‐5‐(4‐fluorophenyl)pent‐4‐enoate (6l):
colorless oil. 1H NMR (400 MHz, CDCl3, TMS, 25°C) δ = 7.27 (td,
J = 5.4, 2.6 Hz, 2H), 6.97 (t, J = 8.7 Hz, 2H), 6.39 (t, J = 17.8 Hz,
1H), 6.15–5.96 (m, 1H), 4.33–4.06 (m, 2H), 3.58 (t, J = 7.3 Hz,
1H), 2.73 (t, J = 7.1 Hz, 2H), 2.26 (s, 3H), 1.26 (t, J = 7.1 Hz, 3H).
13C NMR (100 MHz, CDCl3, 25°C) δ = 202.4, 169.2, 163.4, 161.0,
133.1, 131.5, 129.6, 128.6, 127.7, 127.6, 127.5, 127.4, 125.5,
115.5, 115.5, 115.3, 115.2, 61.5, 59.5, 31.4, 29.2, 14.1. 19F NMR
(377 MHz, CDCl3, 25°C) δ = −114.8, −114.9, −115.4. IR (KBr) v:
3521, 2963, 2928, 1715, 1511, 1380, 1229, 1181, 1101, 1034,
818, 539 cm−1. HRMS m/z (ESI) calculated for C15H17FNaO3 [M
+ Na]+ 287.1059 found 287.1071.
Table 1
Dehydration of 1a over various catalyst systems.
Yield a (%)
Conv. of
1a (%)
97
21
15
95
0
71
43
Entry
Acid
Base
Solvent
2a
5
2
3a
91
1
1
2
3
4
5
6
7
AlCl3
FeCl3·6H2O
p‐TSA
AlCl3
—
—
—
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
trace trace
93 trace
PPh3
PPh3
NEt3
Pyridine
—
AlCl3
AlCl3
0
0
52 trace
10 trace
8
9
AlCl3
AlCl3
AlCl3
AlCl3
o‐Phenanthroline CH3NO2
62
41
29
91
89
12
18
26
81
87
19
98
0
5
trace
90 trace
85
trace trace
trace trace
15 trace
35
47
3
P(O)Ph3
DABCO
PBu3
DPPE
PPh3
PPh3
PPh3
PPh3
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
PhMe
10
11
12
13
14
15
16
17
18
19 b
AlCl3
2
p‐TSA
FeCl3·6H2O
I2
AlCl3
AlCl3
AlCl3
AlCl3
64
76 trace
trace
79 trace
2
PPh3
PPh3
CH3CN
EtOH
8
PPh3
CH3NO2
3. Results and discussion
Reactionconditions: 0.4 mmol, Lewis acid 0.02 mmol, Lewis base 0.02
mmol, solvent 1.0 mL, 80 °C, 2 h.
a GC yield. b 100 °C, 1 h.
Initially, dehydration of 1a to form alkene 2a was investi‐