Greener Routes in Organic Preparations. Microwave Assisted
Synthesis of Banana-shape Liquid Crystals.
Kwang-Ting Liu,* Chung-Ching Hsieh, Te-Jung Hsu and Yi-Shan Liao
Department of chemistry, National Taiwan University, Taipei 106,
Taiwan, ROC
Abstract
During the course of our studies on the substituent effect in organic liquid crystals, the
use of microwave assisted technique has been found to be a convenient way of
synthesis which can also illustrate a number of principles of green chemistry.
In
the preparation of banana-shape molecules 1, 2, and 3, esterification, Heck reaction,
hydrogenolysis, oxidation, saponification, Schiff base formation, and Williamson
synthesis have been carried out with microwave irradiation for linking aromatic
moieties or for making long chain soft ends. Microwave assisted aromatic
substitutions, such as bromination, nitration, and Rosenmund-von Braun reaction,
were also employed. Although the over-all yield was only about 5-10% higher than
the traditional methods, the advantages in using microwave irradiation for
preparations can be demonstrated in terms of energy-saving, solvent-saving,
time-saving, as well as easy-separation. A comparison of the two methods will be
discussed.
X
O
O
O
O
O
O
Y
HO
O
OH
O
1
RO
OR
X
O
Br
Br
O
2
O
RO
O
OR
X
O
N
N
O
Y
H2N
O
NH2
RO
3
O
OR
K2CO3, RBr 300W, 10min
O
OC2H5
O
DMF 10 mL yield : 85-90%
OC2H5
o
K2CO3, RBr 90 C, 12hrs
DMF 60 mL yield : 78-84%
HO
10mmol
10% NaOH 300W, 10min
RO
O
EtOH 10 mL yield : 88-95%
10% NaOH
OH
reflux, 12hrs
A
RO
EtOH 50 mL yield : 82-87%
NaOH 2.2eq 250W, 15min
O
OH +
Br
O
H2O 25 mL yield :88%
NaOH 2.2eq reflux, 12hrs
HO
OH
B
O
EtOH 70 mL yield : 79%
DCC,DMAP 250W, 10min
O
THF 15 mL yield : 64%
O
B + HO
OH DCC, DMAP rt, 4hrs
THF 65mL
o
50 C, 12hrs
1,4-dioxane 65 mL yield : 85%
OBn
O
O
O
O
HO
OH
C
O
DCC, DMAP 200W, 10min
THF 10 mL yield : 65-70%
O
A+C
DCC, DMAP rt , 4hrs
THF 55mL yield : 57-66%
O
BnO
yield : 53%
Pd/C, H2
800W, 3min
1,4-dioxane 15 mL yield : 90%
Pd/C, H2
O
O
O
RO
O
O
O
O
OR
O
H
HNO3 1.1eq
300W, 10min
HOAc 5 mL
yield : 87%
HNO3 1.1eq rt, 10hrs
HOAc 25 mL yield : 79%
HO
Br2 1.1eq
O
H
H
HO
NO2
300W, 15min
O
HOAc 10 mL yield :88%
Br2 1.1eq
HO
O
rt, 12hrs
H
HO
Br
HOAc 40 mL yield : 79%
O
H
CuCN 1.1eq reflux, 16hrs
HO
Br
O
CuCN 1.1eq 850W, 30min
DMF 5 mL yield : 80%
H
HO
CN
DMF 45 mL yield : 76%
O
O
H
H
HO
HO
NO2
MW
Traditional
Yield
88%
80 %
Time
10 min
10 hrs
Solvent
5 mL
25 mL
Power (kW/hr)
0.17
0.18
O
O
H
H
HO
HO
Br
MW
Traditional
Yield
90 %
80 %
Time
15 min
12 hrs
Solvent
10 mL
40 mL
Power (kW/hr)
0.26
0.22
O
O
H
H
HO
HO
CN
Br
MW
Traditional
Yield
85 %
78%
Time
30 min
16 hrs
Solvent
5 mL
45 mL
Power (kW/hr)
1.38
1.65
Heck reaction 500W, 1 hr
DMF 10 mL
yield : 50-60%
O
O
Br
Heck reaction 72hrs, 100 oC
DMF 50 mL
yield : 48-55%
Br
KOH 2.2eq 300W, 10min
THF / Water 10/10 mL yield : 98%
KOH 2.2eq 2hrs reflux
THF / Water 50/50 mL yield : 95%
HO
OH
D
DMAP DCC 300W, 30min
O
THF 10 mL yield : 60-70%
A
D
DMAP DCC 48hrs, r.t.
THF 10 mL yield : 30-40%
O
O
O
RO
OR
O
O
O
O
RO
OR
MW
Traditional
15-23 %
14-16 %
Time
2 hrs
170 hrs
Solvent
50 mL
220 mL
Power (kW/hr)
1.85
4.67
Yield
(from C6H4Br2)
O
A
+
H
HO
DCC, DMAP
200W, 10min
THF 10 mL
yield : 65%~70%
DCC, DMAP
THF 55mL
O
H
O
rt , 4hrs
RO
yield : 57%~66%
O
acidic Al2O3, 300W, reflux 1h
H
O
O
+ HN
2
O
CHCl3 2 mL, yield : 80%~85%
NH2
acidic Al2O3, reflux 24h
RO
toluene 30 mL, yield : 80%~85%
O
N
N
O
O
O
RO
OR
O
N
N
O
O
O
RO
OR
MW
Traditional
52-60 %
46-56 %
Time
1.1 hr
28 hr
Solvent
12 mL
85 mL
Power (kW/hr)
1.10
1.01
Yield
(from HOC6H4CHO)
O
O
O
O
O
O
O
O
RO
OR
MW
Traditional
33-35 %
20-23 %
Time
58 min
56 hrs
Solvent
60 mL
360 mL
Power (kW/hr)
0.85
2.0
Yield
(from resorcinol)
K2CO3, RBr 300W, 10min
O
OC2H5
o
K2CO3, RBr 90 C, 12hrs
DMF 60 mL yield : 78-84%
HO
X
10% NaOH 300W, 10min
10% NaOH
reflux, 12hrs
EtOH 50 mL yield : 82-87%
RO
x
OH
A'
RO
X
O
DCC, DMAP 200W, 10min
THF 10 mL yield : 60-70%
DCC, DMAP rt , 4hrs
THF 55mL yield : 50-60%
OC2H5
O
EtOH 10 mL yield : 88-95%
A' + C
O
DMF 10 mL yield : 85-90%
O
O
O
O
O
O
O
RO
OR
X
X
X=Cl, NO2, OCH3, CN
CHCl3 2 mL, acidic Al2O3,
300W, reflux 1h, yield : 80%~85%
O
+
H
O
H2N
O
NH2
C12H25O
X
X = H, NO2, OCH3, Cl
O
N
N
O
O
O
RO
OR
X
X
X = H, NO2, OCH3, Cl
O
O
X
H
O
+
H 2N
NH2
CHCl3 2 mL, acidic Al2O3, 300W, reflux
RO
R = CnH2n+1, n = 10, 12
X
O
N
N
O
O
O
RO
OR
X
aldehyde/amine
(mole ratio)
time
(h)
yield
(%)
H
2.2
1
80-85
CH3
2.2
1
80-85
F
2.2
2
40-50
Cl
2.2
3
30-50