Acylation of (R,S)-1-phenylethanol with ethyl acetate over an immobilized enzyme

Article abstract of DOI:10.1007/s11164-010-0127-4

Acylation of (R,S)-1-phenylethanol, which is a product of acetophenone hydrogenation, over a Pd-supported catalyst, was studied in ethyl acetate with an immobilized lipase. It was demonstrated that in the presence of hydrogen and Pd/C in the reaction medium the (R,S)-1-phenylethanol acylation rate is not hindered, whereas the selectivity was strongly altered in the latter case, leading to significant amounts of side products. Springer Science+Business Media B.V. 2010.

Full text of DOI:10.1007/s11164-010-0127-4

Res Chem Intermed (2010) 36:245–252
DOI 10.1007/s11164-010-0127-4
Acylation of (R,S)-1-phenylethanol with ethyl acetate
over an immobilized enzyme
P a¨ ivi M a¨ ki-Arvela
Dmitry Yu. Murzin
•
Serap Sahin
•
Kari Er a¨ nen
•
Received: 22 April 2009 / Accepted: 28 October 2009 / Published online: 10 March 2010
Ó Springer Science+Business Media B.V. 2010
Abstract Acylation of (R,S)-1-phenylethanol, which is a product of acetophenone
hydrogenation, over a Pd-supported catalyst, was studied in ethyl acetate with an
immobilized lipase. It was demonstrated that in the presence of hydrogen and Pd/C
in the reaction medium the (R,S)-1-phenylethanol acylation rate is not hindered,
whereas the selectivity was strongly altered in the latter case, leading to significant
amounts of side products.
Keywords Acylation Á Lipase Á Hydrogen Á Pd/C catalyst
Introduction
Cascade chemo–bio catalytic systems for synthesis of optically pure compounds
could be of great industrial interest providing, for example, the possibility of
enzymatic separation of the racemic products obtained in a heterogeneous catalytic
reaction. Synthesis of R-1-phenylethyl acetate starting from acetophenone hydro-
genation was recently achieved over a heterogeneous Pd catalyst in combination with
acylation over an immobilized lipase [1]. In the first reaction step (R,S)-1-
phenylethanol was formed; this was acylated in the second step to R-1-phenylethyl
acetate (Fig. 1). The main challenge was to suppress the debenzylation and
hydrogenolysis of (R,S)-1-phenylethanol over acidic catalysts such as Pd/C. In the
hydrogenolysis reaction styrene is formed as a primary product and it is rapidly
hydrogenated to ethylbenzene. In the acylation step the beneficial reaction conditions
are relatively low temperatures (below 70 °C) and the use of nonpolar solvents [1],
whereas in polar solvents the enzymatic activity decreased dramatically [2]. The
P. M a¨ ki-Arvela (&) Á S. Sahin Á K. Er a¨ nen Á D. Yu. Murzin
Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre,
˚
˚
Abo Akademi University, 20500 Turku/Abo, Finland
e-mail: Paivi.Maki-Arvela@abo.fi
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P. M a¨ ki-Arvela et al.
O
CH3
HO
CH3
H C
O
3
CH3
+
H2
O
HO
CH3
Q
+
Pd/Al O
immobilised lipase
2
3
I
A
B
P
+
H2
+
H2
Pd/Al O
- H O + H2
2
3
2
-
acetic acid
HO
CH3
CH3
+
H2
-
H O
2
C
D
Fig. 1 Cascade synthesis of R-1-phenylethyl acetate (P) starting from hydrogenation of acetophenone
A) to R-1-phenylethanol (B) and S-1-phenylethanol (C), followed by acylation of R-1-phenylethanol to
(
R-1-phenylethyl acetate. Ethanol (I) is formed as a stoichiometric product to R-1-phenylethyl acetate.
Ethylbenzene (D) is formed by rapid hydrogenation of styrene, obtained by dehydration of (R,S)-1-
phenylethanol or by debenzylation of R-1-phenylethyl acetate
reason for this behavior is stripping of water from the enzyme [3]. Additionally
ethanol is formed in equimolar amounts in the acylation of R-1-phenylethanol, which
can, in turn, reduce the enzyme activity.
Dynamic kinetic resolution through racemization of 1-S-phenylethanol would be
even more beneficial for cascade catalysis affording yields higher than 50%. Besides
zeolites [4, 5], Ru/Al O [6] and Ru/HAP [7] have also been reported to be active in
2
3
the racemization of secondary alcohols. Furthermore, racemization of chiral amines
has been performed over Pd/C [8]. Unfortunately zeolites are not suitable in
combination with the hydrogenation of acetophenone in organic phases because of
the dehydration of (R,S)-1-phenylethanol.
Because, previously, the cascade system has only seldom been studied for
combinations of heterogeneous and enzymatic catalysts, transformations of the
intermediate product in the hydrogenation of acetophenone, (R,S)-1-phenylethanol,
are also of interest for better mechanistic understanding of cascade reactions. There
are several open questions, for example the effect of hydrogen atmosphere and
ethylbenzene, which is a side product in acetophenone hydrogenation, on the
activity of lipase. These questions are addressed in this communication.
Experimental methods
The transformation of (R,S)-1-phenylethanol (Fluka,[98%, 305 mg, 2.5 mmol) was
performed in an autoclave at 50 °C under 5.6 bar total pressure of hydrogen (AGA,
9
9.999%) or argon (AGA, 99.999%). Acylation of (R,S)-1-phenylethanol was
1
23

Acylation of (R,S)-1-phenylethanol with ethyl acetate
247
conducted in the presence of lipase (Novozym 435, which is a lipase (lipase B) from
Candida antarctica produced by submerged fermentation of a genetically modified
Aspergillus microorganism and adsorbed on a macroporous resin Lewatit resin [9]) and
2
in the presence or absence of 5 wt% Pd/C (Aldrich, D = 42%, surface area 1214 m /g
cat.
(
BET), acidity 34.6 lmol/g , 15 lm mean particle size). In a typical experiment
cat
1
25 mg Pd/C and 125 mg lipase were loaded into the reactor together with the liquid
containing 0.02 M (R,S)-1-phenylethanol in 125 ml ethyl acetate. Thereafter, the
reaction mixture was heated to the desired temperature and pressurized either with
hydrogen or argon. The reaction kinetics were measured under vigorous stirring
(650 rpm). In some experiments ethylbenzene (Fluka,[99%, 30 mg, 0.28 mmol) was
added to the reaction mixture. Samples were withdrawn as a function of time and the
products were analyzed by gas chromatography with a chiral column (CP Chirasil Dex
250 lm 9 250 lm 9 25 m). The samples were analyzed using the temperature
(
program: 100 °C (1 min)—0.30 °/min—130 °C—15 °/min—200 °C (10 min). The
temperature of the injector and the split ratio were 280 °C and 100:1, respectively.
Results and discussion
In cascade catalysis for transforming acetophenone, via hydrogenation and
acylation, to R-1-phenylethyl acetate the importance of the support acidity has
been recognized [1]. It was not however, clear, whether the by-product formed,
ethylbenzene could inhibit the acylation reaction. In order to obtain more
information about the interrelationship between the supported metal catalyst and
the enzyme, the following experiments were carried out using the intermediate
product (R,S)-1-phenylethanol as reactant:
1
2
transformation of (R,S)-1-phenylethanol with lipase under hydrogen atmosphere;
transformation of (R,S)-1-phenylethanol under argon in the presence of 30 mg
ethylbenzene;
3
4
transformation of (R,S)-1-phenylethanol over Pd/C in the absence of the enzyme
under hydrogen atmosphere; and
transformation of (R,S)-1-phenylethanol over Pd/C–lipase catalyst system under
hydrogen.
In a reference experiment (R,S)-1-phenylethanol was converted to R-1-phenyl-
ethyl acetate over lipase at 50 °C under 5.6 bar hydrogen pressure using ethyl
acetate both as solvent and as acylation agent. In this experiment the initial reaction
rate for (R,S)-1-phenylethanol was 0.02 mmol/min/gcat and the conversion of (R,S)-
1
-phenylethanol was 21% after 300 min of reaction time (Table 1, entry 1). When
acetophenone hydrogenation was studied separately in the presence of lipase under
.6 bar hydrogen at 50 °C in ethyl acetate over Pd/C large amounts of ethylbenzene
39% after 300 min corresponding 100% to conversion of acetophenone) were
5
(
formed. Thus the role of ethylbenzene on enzyme activity and selectivity was
studied by adding ethylbenzene to the reaction mixture containing (R,S)-1-
phenylethanol and a lipase catalyst under inert gas atmosphere of argon. The
question to be answered was whether ethylbenzene could inhibit the activity of the
123

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P. M a¨ ki-Arvela et al.
Yield of Yield of
Table 1 Catalytic transformation of (R,S)-1-phenylethanol over different catalysts
Entry Catalyst Atmosphere Conversion (%) Amount of Amount of
a
R-1-PE (%) S-1-PE (%) EB (%) R-PEAc
a
a
a
after 300 min
1
2
3
4
a
Lipase
Lipase
Hydrogen
Argon
21
26
46
31
33
33
44
40
47
46
36
41
\1
19
20
0
b
c
0
Pd/C, lipase Hydrogen
20
19
Pd/C
Hydrogen
0
At 20% conversion
b
Ethylbenzene added at 10 mol%; its amount remained constant
Styrene ca. 1%
c
R-1-PE, S-1-PE, EB, and R-PEAc denote R-1-phenylethanol, S-1-phenylethanol, ethylbenzene, and R-
phenylethyl acetate
enzyme catalyst. Higher acylation rate of (R,S)-1-phenylethanol (0.03 mmol/min/gcat.
)
was observed under argon atmosphere (Fig. 2a) when ethylbenzene was added into
the reaction mixture than in the experiment under hydrogen in the absence of
ethylbenzene (see above). This result indicated that presence of ethylbenzene at
least did not retard the acylation activity of lipase. Furthermore, conversion of (R,S)-
1-phenylethanol after 300 min reaction time under argon was higher than under
hydrogen, 26 and 21%, respectively (Table 1, entries 1 and 2).
It should be noted that water and ethanol are known to affect the activity of
enzymes, leading to activity changes during the reaction. Such changes were not
noticed in this study. The reason could be that the reactions were conducted in
excess acylation agent, therefore the amounts of water and ethanol formed are rather
small. In addition enantioselectivity of immobilized enzymes is less sensitive to
water than that of unsupported enzymes.
The lipase catalyst had high selectivity for acylation of (R,S)-1-phenylethanol
under argon and under hydrogen. The selectivities to R-1-phenylethyl acetate were
very high, about 95 and 93%, respectively, at 20% conversion levels corresponding
to yields of 20 and 19%. In both cases where lipase was used as a catalyst the
amount of S-1-phenylethanol decreased only slightly during the reaction (Fig. 2b),
because traces of ethylbenzene and styrene were formed (Table 1, entries 1 and 2).
Thus the main reaction over lipase catalyst was acylation of R-1-phenylethanol to
R-1-phenylethyl acetate.
The next step was to study the effect of Pd/C present in the system under hydrogen
atmosphere on the transformation of (R,S)-1-phenylethanol. Over Pd/C catalyst about
9
mol% (R,S)-1-phenylethanol was converted to styrene and further to ethylbenzene,
whereas over the Pd/C-lipase system the only product formed was ethylbenzene. The
initial transformation rates with and without lipase, were 0.014 mmol/min/gcat and
0
3
.008 mmol/min/g , respectively. The corresponding conversion levels after
cat
00 min reaction time were 46 and 31% (Table 1, entries 3, 4) indicating that the
initial reaction rate increased in the presence of enzyme catalyst, because of its
1
23

Acylation of (R,S)-1-phenylethanol with ethyl acetate
249
(a)
6
5
4
3
2
1
0
0
0
0
0
0
0
0
60
120
180
240
300
360
Time (min)
(b)
R-1-phenylethanol (mol-%)
60
50
40
30
20
10
0
0
10
20
30
40
50
60
S-1-phenylethanol (mol-%)
Fig. 2 a Conversion of (R,S)-1-phenylethanol and b yield of S-1-phenylethylacetate as a function of the
yield of R-1-phenylethylacetate in ethyl acetate. Symbols: (d) over lipase catalyst under hydrogen, (m)
lipase under Ar, ethylbenzene added into the reaction mixture, (u) Pd/C and lipase under hydrogen, (j)
Pd/C under hydrogen. The hydrogen or argon partial pressure was 5.6 bar and the reaction temperature
50 °C
interaction with R-1-phenylethanol. No R-1-phenylethyl acetate was, however,
formed in the presence of Pd/C and lipase (see below).
The molar amounts of R and S-1-phenylethanol from these four experiments are
depicted in Fig. 2b. In the presence of the enzymatic catalyst only the amount of
123

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P. M a¨ ki-Arvela et al.
S-1-phenylethanol reduced slightly because of the hydrogenolysis reaction. In the
presence of Pd/C without lipase the amounts of R and S-1-phenylethanol decreased
at the same rate. When, however, both Pd/C and lipase were present in the reactor,
the decrease in the amount of S-1-phenylethanol was faster than that of R-1-
phenylethanol indicating clearly that the latter interacted with an enzyme. This
statement can be confirmed from the kinetic data as follows: the ratio of the
decrease in the amount of R-1-phenylethanol to the decrease in the amount of
(a)
3
2
2
1
1
0
5
0
5
0
5
0
0
10
20
30
40
50
60
Conversion (%)
(b)
5
4
3
2
1
0
0
0
0
0
0
0
10
20
30
40
50
60
Conversion (%)
Fig. 3 Yield of a R-1-phenylethylacetate and b ethylbenzene as a function of conversion in the acylation
of (R,S)-1-phenylethanol. Symbols: (d) over lipase catalyst under hydrogen, (m) lipase under Ar,
ethylbenzene added into the reaction mixture, (u) Pd/C and lipase under hydrogen, (j) Pd/C under
hydrogen. The hydrogen or argon partial pressure was 5.6 bar and the reaction temperature 50 °C
1
23

Acylation of (R,S)-1-phenylethanol with ethyl acetate
251
S-1-phenylethanol over Pd/C-lipase and Pd/C systems was calculated and
compared. These ratios for the former and for the latter cases were 1.7 and 1.0,
confirming that S-1-phenylethanol underwent the hydrogenolysis reaction initially
faster than the corresponding R isomer in the presence of both catalysts, because
larger amounts of ethylbenzene were formed already at the beginning of the reaction
than in the case of lipase only (Fig. 3b).
In the acylation of (R,S)-1-phenylethanol over lipase only traces of styrene and,
subsequently, ethylbenzene were formed (Fig. 3b) showing that lipase does not
catalyze dehydration of secondary alcohols. The effect of ethylbenzene on lipase
activity was studied by carrying out the acylation of (R,S)-1-phenylethanol under
argon atmosphere. The amount of ethylbenzene remained constant indicating that
acylation of R-1-phenylethanol proceeded with lipase regardless of the presence of
ethylbenzene. When using the Pd/C-lipase combination, however, no R-1-phenyl-
acetate was formed (Fig. 3a, Table 1, entry 4). Furthermore the formation of
ethylbenzene was enhanced over the Pd/C–lipase system compared with the
experiment with only Pd/C. Ethylbenzene could be formed via dehydration of (R,S)-
1
-phenylethanol followed by its rapid hydrogenation to ethylbenzene, or, alterna-
tively, by debenzylation of R-1-phenylethyl acetate over Pd. It can be thus stated
that the properties of the support material are decisive for the one-pot synthesis of
R-1-phenylethyl acetate starting from hydrogenation of acetophenone.
Conclusions
The acylation of (R,S)-1-phenylethanol, which is an intermediate product in cascade
synthesis of R-1-phenylethyl acetate starting from acetophenone hydrogenation, was
studied separately in order to elucidate the reaction mechanism. This reaction
proceeded well in the presence of lipase under hydrogen atmosphere and under
argon and in the presence of ethylbenzene, showing that ethylbenzene did not
inhibit the acylation reaction. On the other hand the acylation of (R,S)-1-
phenylethanol in the presence of a heterogeneous palladium catalyst on slightly
acidic carbon support, Pd/C, led to significant amounts of ethylbenzene, which
could be formed via dehydration of (R,S)-1-phenylethanol followed by its rapid
hydrogenation to ethylbenzene, or alternatively by debenzylation of R-1-phenyl-
ethyl acetate over Pd.
˚
Acknowledgements This work is part of the activities at the Abo Akademi Process Chemistry Centre
˚
(AA-PCC) within the Finnish Centre of Excellence Programme (2000–2011) appointed by the Academy
of Finland.
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