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.
O O
O O
RO OR
N N
O O
O O
RO OR
O O
O O
O O
O O
RO OR
Y Y
Br Br
H
2N NH
2HO OH
2
3 1 X
X
X
HO
OH O
+ Br
O
OH
250W, 15min O
NaOH 2.2eq
NaOH 2.2eq reflux, 12hrs H
2O 25 mL
EtOH 70 mL
yield :88%
yield : 79% B
HO
OC 2 H 5 O
RO
OC 2 H 5
300W, 10min O
K
2CO
3, RBr
K
2CO
3, RBr 90
oC, 12hrs DMF 10 mL
DMF 60 mL
yield : 85-90%
yield : 78-84%
10mmol
RO
OH
300W, 10min O
10% NaOH
reflux, 12hrs EtOH 10 mL
EtOH 50 mL
yield : 88-95%
yield : 82-87%
10% NaOH
A
HO OH
250W, 10min DCC,DMAP
DCC, DMAP rt, 4hrs THF 15 mL
THF 65mL
yield : 64%
yield : 53% BnO
O O
O O
OBn
800W, 3min Pd/C, H
250
oC, 12hrs 1,4-dioxane 15 mL yield : 90%
yield : 85%
1,4-dioxane 65 mL
Pd/C, H
2HO
O O
O O C OH B +
O O
O O
O O
O O
RO OR
200W, 10min DCC, DMAP
rt , 4hrs THF 10 mL
THF 55mL
yield : 65-70%
yield : 57-66%
DCC, DMAP
A + C
HO
H O
HO
H O
NO 2
300W, 10min HNO 3 1.1eq
rt, 10hrs HOAc 5 mL
HOAc 25 mL
yield : 87%
yield : 79%
HNO 3 1.1eq
HO
H O
HO
H O
Br
300W, 15min Br 2 1.1eq
rt, 12hrs HOAc 10 mL
HOAc 40 mL
yield :88%
yield : 79%
Br 2 1.1eq
HO
H O
HO
H O
CN
850W, 30min CuCN 1.1eq
reflux, 16hrs DMF 5 mL
DMF 45 mL
yield : 80%
yield : 76%
CuCN 1.1eq
Br
HO
H O
HO
H O
NO 2
MW Traditional
Yield 88% 80 %
Time 10 min 10 hrs
Solvent 5 mL 25 mL
Power (kW/hr) 0.17 0.18
HO
H O
HO
H O
Br
MW Traditional
Yield 90 % 80 %
Time 15 min 12 hrs
Solvent 10 mL 40 mL
Power (kW/hr) 0.26 0.22
HO
H O
HO
H O
Br CN
MW Traditional
Yield 85 % 78%
Time 30 min 16 hrs
Solvent 5 mL 45 mL
Power (kW/hr) 1.38 1.65
Br Br O
O Heck reaction 500W, 1 hr DMF 10 mL yield : 50-60%
72hrs, 100
oC DMF 50 mL yield : 48-55%
Heck reaction
300W, 10min THF / Water 10/10 mL yield : 98%
KOH 2.2eq
2hrs reflux
THF / Water 50/50 mL yield : 95%
KOH 2.2eq
HO OH
300W, 30min DMAP DCC
THF 10 mL yield : 60-70%
48hrs, r.t.
DMAP DCC
THF 10 mL yield : 30-40%
