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Isomerization of Some Allylbenzenes
in a Microwave Batch Reactor

Le Ngoc Thach and Christopher R. Strauss
Journal of Chemistry, 38(1), 76-79 (2000)

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Abstract

In this paper we would like to present our study of the isomerization of some allylbenzenes as: allylbenzene, eugenol, methylchavicol and safrole. It is the first time this reaction take place in the microwave batch reactor under aqueous acidic condition. The high yields of 1-propenylbenzene, was obtained in a short reaction times.

Table 1
Conversions from the isomerization of allylbenzene

Temp.
Acid Conc. (g/100ml)
Time
(min)
Yield (%)
0.1
0.5
1
5
10
cis-
trans-
total
220°C
-
-
-
-
-
15
0
0
0
220°C
-
-
+
-
-
15
03
15
18
230°C
-
-
+
-
-
15
05
21
26
240°C
-
-
+
-
-
15
07
37
44
240°C
-
-
-
+
-
15
06
34
40
250°C
-
-
-
-
-
15
01
09
10
250°C
+
-
-
-
-
15
04
20
24
250°C
-
+
-
-
-
15
07
39
46
250°C
-
-
+
-
-
15
08
47
55
250°C
-
-
-
+
-
15
09
52
61
250°C
-
-
-
-
+
15
08
50
58
250°C
-
-
+
-
-
60
10
61
71
250°C
-
-
-
+
-
60
07
46
53
250°C
0.1 M NaOH
15
10
40
50
250°C
0.5 M NaOH
60
13
58
71

The isomerization of allylbenzene, was studied at 220 to 250C, with concentrations of sulfuric acid ranging between 0 and 10 g per 100 ml and reaction times up to 1 hour. The product mixture comprised cis- and trans-methylstyrene, with the maximum conversion being 71% after 60 minutes at 250C in 1% w/v acid (see Table 1).

Various plots of the conversion against the concentration of acid, the temperature and the reaction time were symmetrical, indicating that the MBR would facilitate kinetics studies. For eugenol, acidic media were less effective at inducing isomerization, than was base. Concentration of 0.4 M NaOH at 220C, afforded cis- and trans-isoeugenol in 0.73% conversion after only 15 minutes.

With methyichavicol, a lower concentration of base sufficed: With a solution of 0.2 M NaOH at 230C the conversion was 81%.

Several studies on isomerization of allylbenzenes have been carried out1-5 with or without phase transfer, catalysis6-7. Encouraging results were obtained earlier for the isomerization of eugenol to isoeugenol, in "dry media" under microwave irradiation at atmospheric pressure8. For such isomerizations in aqueous media, we now have explored the use of a microwave batch reactor (MBR), which operates in the laboratory at up to 100 atm and 260C with software control9,10, and wish to report our results.

Table 2
Conversion of safrole to isomerized products

Temp.
Base Conc.
Time
(min)
Yield (%)
0.1M
0.2M
cis-
trans-
total
200°C
-
-
15
0
0
0
200°C
-
-
15
09
26
35
220°C
+
-
15
19
67
86
220°C
-
+
15
18
74
92
220°C
+
+
30
19
78
97
235°C
-
+
15
18
67
85

With safrole, the basic media were also effective, the conversion in 0.2 M NaOH, being 97% at 220C after 30 minutes (see Table 2).

In conclusion, the isomerization of some allylbenzenes has been studied in the MBR under aqueous conditions. Reactions usually required addition of acid or base, however, with good conversions obtained within a relatively short time.

Experimental

General Methods

The MBR was fabricated in laboratories of CSIRO. Conversions were estimated by GC-MS using a HP 5890 instrument.

Example preparations: Isomerization of allylbenzene

Allylbenzene (2 g) was added to an aqueous solution of either H2SO4 or NaOH (60 ml) at strengths outlined in table 1 and table 2, in a PTFE reaction vessel for the MBR. These mixtures were heated with stirring under the conditions designated. The reaction mixture was extracted, the extract dried and analysed by GC and GC-MS.

References

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  3. Gino Bontempelli, Daniele, Giberto Schiavon, Mario Fiorani. Transition Met. Chem., 12, 292-295 (1987)
  4. Gabriela Barak, Yoel Sasson. Bull. Soc. Chim. Fr., 3, 584 (1988)
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