784
S. Yadav et al. / Catalysis Communications 12 (2011) 781–784
Table 1
added. The reaction mixture was placed on the rotatory shaker at
180 rpm at 30 °C for 24 h. The product was extracted with 20 ml of n-
hexane. No ethylbenzene was detected in the extract by HPLC
indicating that it has been fully converted to the products namely
72% to (R)-1-phenylethanol and 28% to 1-phenylacetone.
Dependence of the yield of formation of (R)-1-phenylethanol from ethylbenzene by
Aspergillus niger pH, temperature and concentration of mycelia in its suspension.
S.N. pH Percentage Temperature Percentage Mycelia
Percentage
concentration yield
yield
(°C)
yield
(gm)
In order to resolve the question whether the mycelia of the same
fungal strain could be used for benzylic hydroxylation of other
alkylbenzenes, the biotransformations of methylbenzene and propyl-
benzene were also tested. The biotransformation of methylbenzene
gave 56% benzylalcohol and 40% benzaldehyde. Propylbenzene gave
only 47% of 1-phenylpropanol leaving 38% propylbenzene unreacted.
These results showed that other alkylbenzenes could also be
converted to their corresponding alcohols at benzylic positions
though the conversion yields need to be optimized by changing the
experimental conditions.
1
2
3
5.0 30
7.0 72
8.0 53
25
30
35
81
72
93
0.2
0.5
1.0
35
72
87
Since (R)-1-phenylethanol gets converted to 1-phenylacetone even
the standard sample of (R)-1-phenylethanol gives two peaks one due
to (R)-1-phenylethanol (71%) at retention time 6.0 min and the other
due to its oxidation products 1-phenylacetone (29%) at retention time
7.9 min. Like the standard sample of (R)-1-phenylethanol, the
biotransformation product also gave two peaks one due to (R)-1-
phenylethanol (72%) at a retention time 6.5 min and the other due to
its oxidation product 1-phenylacetone (28%) with a retention time
8.2 min. The starting material ethylbenzene was eluted with a
retention time 3.2 min. No peak corresponding to ethylbenzene in
the HPLC chromatogram of the extract of the biotransformation
productwas detected indicatingthat all theethylbenzene was converted
to the product.
Though we have not attempted to identify the enzyme involved in
the above conversions, cytochrome P450 mono-oxygenases is the
most likely enzyme. There are reports [9,11] also that cytochrome
P450 mono-oxygenase is involved in such conversions.
4. Conclusions
This short communication reports the biotransformation of
ethylbenzene to (R)-1-phenylethanol in 99% enantiomeric excess
with 72% yield using the fungal mycelia of A. niger MTCC-404.
Methylbenzene and propylbenzene were also converted to benzy-
lalcohol and 1-phenylpropanol respectively, but the conversion yields
were low and needed optimization of the experimental conditions to
achieve better yields. Such studies are in progress.
The results of 1H NMR, 13C NMR, IR and GC-MS analyses of
the biotransformation product clearly confirmed the presence of
1-phenylethanol.
1H NMR (300 MHz, CDCl3)
δ = 7:20–7:29ðm; 5H; HaromÞ; 4:74ðq; 1H; J = 6:3 Hz; CHOHÞ;
2:92ðbrS; 1H; OHÞ; 1:40 ðd; 3H; J = 6:3 Hz; CH3Þ:
Acknowledgments
13C NMR (75 MHz, CDCl3):
The financial support of CST, U.P., Lucknow through project No.
CST/SERPD/D-2895, dated 01.02. 2008 under which this research
work has been done is thankfully acknowledged. The scientific helps
rendered by Prof. V. K. Yadav, Department of Chemistry, Indian
Institute of Technology, Kanpur, U.P., India, in the determination of
enantiomeric excess of the product and by Prof. R. Gurunath of the
same department in GC-MS analysis of the product, are thankfully
acknowledged.
δ = 145:7; 128:2; 127:1; 125:3; 70:0; and 25:0
IR (Film):
3309; 3110; 2907; 1608; 1160; 764; and 710 cm−1
GC-MS: showed the characteristic fragments of 1-phenylethanol
with m/z values of 107, 79 and 77 along with the molecular ion peak
of 122.
The results of determination of enantiomeric excess of the bio-
transformation product (R)-1-phenylethanol are shown in Fig. 2, in
which Fig. 2(a) is the chromatogram of racemic ( )-1-phenylethanol
obtained using chiralcel OD column (4.6×250 mm) indicating clearly
50:50 percent peak areas of (R) and (S) forms of the racemic
mixture ( ) 1-phenylethanol. Fig. 2(b) shows the chromatogram of
the standard sample of (R)-1-phenylethanol showing 99% enantiomer-
ic excess of the (R) form. Fig. 2(c) shows the chromatogram of the
biotransformation product indicating that it is 99% (R) form.
The experiments were performed to see the effect of pH,
temperature and mycelia concentration on the percentage conversion
of ethylbenzene to 1-phenylethanol. The results are summarized in
Table 1. It becomes evident from the data listed in Table 1 that the
percent yield of the product 1-phenylethanol is dependent on the
experimental conditions. Due to our limitations for determination of
enantiomeric excess of the product under different experimental
conditions, enantiomeric excess of (R)-1-phenylethanol could not be
determined under all the experimental condition studied.
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