Lipase-oxovanadium heterogeneous catalysis system: a robust protocol for the dynamic kinetic resolution of sec-alcohols

Article abstract of DOI:10.1002/cctc.202000292

Herein, we present a robust and eco-friendly dynamic kinetic resolution (DKR) protocol for secondary alcohols using a combined heterogeneous catalytic CAL?B/VOSO₄ system at 50 °C in the relatively green solvent heptane. This catalytic system is

Full text of DOI:10.1002/cctc.202000292

Accepted Article
Title: Lipase-oxovanadium heterogeneous catalysis system: a robust
protocol for the dynamic kinetic resolution of sec-alcohols
Authors: Laiza Almeida, Thayna Marcondes, Cintia Milagre, and
Humberto Milagre
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Lipase-oxovanadium heterogeneous catalysis system: a robust
protocol for the dynamic kinetic resolution of sec-alcohols
Laiza A. de Almeida,[a] Thayna H. Marcondes,[a] Cintia D. F. Milagre[a] and Humberto M. S. Milagre*[a]
This paper is dedicated to the memory of Professor José Augusto Rosário Rodrigues.
Abstract: Herein, we present a robust and eco-friendly dynamic
kinetic resolution (DKR) protocol for secondary alcohols using a
combined heterogeneous catalytic CAL-B/VOSO4 system at 50 °C in
the relatively green solvent heptane. This catalytic system is active
and chemo- and enantioselective for up to 5 cycles. A set of 13
aromatic and heteroaromatic secondary alcohols were evaluated to
determine the substrate scope. The performance of the combined
CAL-B/VOSO4 system was improved by employing a low-cost,
homemade Teflon tube to compartmentalize the catalysts in one-pot
conditions, making this system for up to 8 reaction cycles.
Scheme 1. Chemoenzymatic dynamic kinetic resolution.
Lipases are versatile biocatalysts used in DKR protocols because
of their commercial availability in a ready-to-use format, relatively
low cost, stability in organic solvents, and high activity and
stereoselectivity towards a wide range of substrates.[8,9] In a DKR
of sec-alcohols in organic media, lipases promote the
enantioselective transesterification of one enantiomer of the
racemic mixture in the presence of an acyl donor (Scheme 2). The
product is an optically active ester that can be subsequently
hydrolysed to the enantiopure alcohol.
Introduction
Optically active secondary alcohols and their derivatives are
essential building blocks for pharmaceuticals, agrochemicals, and
food products.[1] Several methodologies describe their
preparation, including
physicochemical approaches such as chromatographic
racemates,[2]
separation of chemo- and biocatalytic
stereoselective reduction from pro-chiral ketones[3] and
enantioselective resolution of racemates.[3a,4] Among these
methodologies, those catalysed by enzymes offer notable
advantages such as impressive chemo-, regio-, and
stereoselectivity, energy-efficient operations and more
environmentally friendly processes.[5] Despite of all these
methodologies, there is still room for greener and more robust
synthetic routes to enantiopure sec-alcohols.
a
multitude of synthetic and
In this context, chemoenzymatic dynamic kinetic resolution (DKR),
which combines the enzyme-catalysed kinetic resolution (KR) of
racemic sec-alcohols with the in situ chemocatalytic racemization,
affords the desired product as a single enantiomer in up to 100%
yield (Scheme 1), making it an attractive alternative for preparing
enantiopure sec-alcohols.[6,7]
Scheme 2. Chemoenzymatic DKR of sec-alcohol.
In an efficient chemoenzymatic DKR, the main challenge is to
combine both bio- and chemocatalysts since this protocol is
conducted in a one-pot manner, and those catalysts may require
specific reaction conditions.[6,7,10] Metal complexes, such as those
with ruthenium,[11,12] iridium,[13] palladium[6] and iron,[14] are useful
racemization agents. However, most are expensive, not eco-
friendly or not readily available and, in some cases, they require
conditions that are harmful to lipases, such as high temperatures
or the presence of strong bases. Therefore, substantial effort has
been devoted to developing biocompatible and more
environmentally benign racemization catalysts, such as solid
acids,[15] zeolites[16,17] and vanadium compounds.[18–24]
[a]
L. A. de Almeida, T. H. Marcondes, C. D. F. Milagre, H. M. S.
Milagre
Institute of Chemistry
São Paulo State University (Unesp)
Prof. Francisco Degni, 55 – Quitandinha, Araraquara, São Paulo,
Brazil 14800-060
E-mail: humberto.milagre@unesp.br
Supporting information for this article is given via a link at the end of
the document.
Vanadium catalysts of type O=V(L)n can efficiently racemize
allylic and benzyl secondary alcohols via an addition-elimination
mechanism involving carbocation formation (Scheme 3).[19]
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high substrate load, as well as by carrying out the reaction in a
larger scale. Lastly, to evaluate the efficiency of this method, the
DKR of a set of 12 other aromatic and heteroaromatic sec-
alcohols was explored.
Results and Discussion
Our study began by examining the VOSO4-catalysed
racemization step using (S)-1-phenylethanol ((S)-1) as a model
substrate aiming to increase the selectivity of the process
previously reported by Jacobs et al[19] and Souza et al.[24] As
shown in Table 1, fast racemization was achieved at 80 °C in all
the evaluated solvents, but with low selectivity (high by-product
formation). Decreasing the temperature resulted in better
selectivity with acceptable racemization rates. The optimum
conditions disclosed in this study, heptane at 50 °C (Table 1, entry
6), in addition to being greener than the conditions previously
reported in the literature,[19,24] are closer in those required by
enzymes, which will be crucial in the DKR.
Scheme 3. VOSO4-catalysed racemization of (S)-1-phenyletanol ((S)-1).
The Akai group used the homogeneous catalyst O=V(OSiPh3)3
and lipase B from Candida antarctica (CAL-B) in the DKR of
several allylic alcohols and obtained the corresponding chiral
esters in up to 99% yield and 99% ee.[18,20,21] However, depending
on the reaction time and oxovanadium catalyst activity, both the
metal and biocatalyst can be deactivated due to catalyst
interactions. To overcome this limitation, the Akai group used an
immobilization strategy in which the vanadium catalyst was
incorporated into a mesoporous silica matrix (V-MPS), allowing
low catalyst loading (1 mol%) and high recyclability.[21,22] In the
DKR of rac-1-phenylbut-2-en-1-ol, this V-MPS racemization
catalyst presented excellent recyclability (over six reaction cycles),
affording the (R)-product in quantitative chemical yield and
complete optical purity.[21] Despite these results, the high cost of
the MPS matrix and the immobilization procedure, which includes
the use of benzene as solvent and inert atmosphere,[21,22] are
limiting factors for the application of V-MPS.
Table 1. Racemization of (S)-1-phenylethanol catalysed by VOSO4.
Entry
Solvent
Toluene
T (°C)
80
Time (h)
1.0
ee (%)[a]
2.9
Sel (%)[b]
16
Considering that hydrated vanadyl sulfate (VOSO4.XH2O) is a
less expensive, readily available and less toxic metal complex,
this compound is a promising alternative for the development of
more economical and eco-friendly DKR processes, particularly
1
2
3
4
5
6
50
2.0
14
>99
78
when combined with an immobilized lipase to afford
a
80
0.5
1.0
heterogeneous catalytic system, which enables the reuse of both
catalysts. Jacobs et al. used VOSO4 as a heterogeneous
racemization catalyst and immobilized CAL-B in octane (80 °C)
for the DKR of rac-1-phenylethanol and obtained the (R)-product
in 93% yield and 99% ee.[19] They observed that VOSO4 is
incompatible with lipases and to circumvent this issue, the
vanadium compound was physically separated through a rotating
inox basket containing the lipase. Souza et al. also described the
DKR of rac-1-phenylethanol catalysed by VOSO4 and CAL-B
Octane
50
1.5
1.7
89
80
0.5
<1.0
4.6
43
Heptane
50
1.0
93
Conditions: (S)-1 (0.25 mmol), solvent (4 mL), VOSO4.XH2O (50 mg), vigorous
[a]
stirring.
Determined by chiral GC-FID analysis. [b] Determined by GC-FID
analysis with n-tetradecane (0.050 mmol mL-1) as internal standard.
using
a continuous flow approach that enabled catalyst
To demonstrate the recyclability of VOSO4, the racemization of
(S)-1 was carried out under the optimum reaction conditions with
consecutive 1 hour-cycles (Figure 1). The oxovanadium catalyst
remained active even after 10 cycles and it continued to afford low
ee values of (S)-1 (ee = 2-5%). An increase in by-product
formation was observed throughout the cycles (Sel = 91-63%).
The recyclability of VOSO4 was evaluated by Jacobs et al[19] at
higher temperatures (80 °C) using octane as the solvent, but in
that case the catalyst remained active for only 3 cycles of the
racemization of (S)-1. The optimization of the racemization
conditions performed here allowed a significant increase in
VOSO4 recyclability.
compartmentalization.[24] In this work, the DKR was performed in
toluene (70 °C) and the (R)-product was obtained with 96% yield
and 99% ee.
Due to the advantages outlined above, herein we report our effort
towards a thorough investigation of the chemoenzymatic DKR of
sec-alcohols using the CAL-B/VOSO4 system. The reaction
conditions (solvent and temperature) of the racemization were
optimized and the recyclability of VOSO4 in this step was
evaluated. Additionally, for the first time, the recyclability of the
CAL-B/VOSO4 heterogeneous system for the DKR of 1-
phenylethanol was studied, also evaluating the recyclability of
both catalysts alone to provide evidence about an incompatibility
between lipase and VOSO4. The robustness of the method
developed herein was achieved by performing the reaction with a
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acetate resulted in some inhibition of the racemization.[19] The
reaction with vinyl decanoate was the most efficient, resulting in
(R)-1b in 87% conversion and ee > 99% in 2 h. Thus, vinyl
decanoate was selected as the acyl donor for the DKR protocol.
We then evaluated the recyclability of the vanadium-lipase
system for DKR to develop a more sustainable and cost-effective
methodology. To demonstrate the recyclability of the CAL-
B/VOSO4 system, consecutive cycles of DKR reactions were
performed and the catalytic system was reused until a significant
decrease in the conversion was observed (Figure 2). At the end
of each cycle, the combined catalyst was recovered by simple
paper filtration and washed three times with heptane. Under these
conditions, the system remained stable for up to 5 cycles (c = 86-
83%, ee of (R)-1b = 99%).
Figure 1. VOSO4 recycling in the racemization of (S)-phenylethanol (1) (1 h for
each cycle; ● ee of (S)-1 at the end of each cycle, determined by chiral GC-FID
analysis; ▲ selectivity, determined by GC-FID analysis with n-tetradecane
(0.050 mmol mL-1) as internal standard).
The decrease in the conversion after the fifth cycle may be related
to an incompatibility between CAL-B and VOSO4 due to the
[19]
extended contact time
or an incompatibility between VOSO4
and the acyl donor. To better understand this limitation, we
evaluated the recyclability of CAL-B in the KR of rac-1 in the
absence of the racemization catalyst, and the lipase remained
active and enantioselective even after 6 cycles (Figure 3). In
addition, we evaluated the performance of VOSO4 in the
racemization of (S)-1 in the presence of vinyl decanoate. The
catalyst remained highly active in the presence of the acyl donor
even after 5 reaction cycles, reproducing previously described
observations (Figure 4). These results show that CAL-B and
VOSO4 are stable and remain active for several cycles when they
are not in the same reaction system. However, during the cycles
of DKR, the catalysts gradually lose their performance due to their
mutual incompatibility.
Having established the optimal condition for the vanadium-
catalysed racemization, VOSO4 and the immobilized Candida
antarctica lipase B (CAL-B) were combined to perform the DKR
of sec-alcohols. First, the DKR reaction was performed using rac-
1-phenylethanol (rac-1) as a model substrate to evaluate the acyl
donor effect since the acylating agent can significantly influence
the enzymatic resolution step (Table 2).[25–28]
Table 2. DKR of rac-1-phenylethanol (rac-1) catalysed by CAL-B/VOSO4
using different acyl donors.
eeP
(%)[b]
Entry
Acyl donor
Time (h)
Product
c (%)[a]
1
2
3
4
Vinyl acetate
3
3
2
8
(R)-1a
(R)-1a
(R)-1b
(R)-1b
75
43
82
61
78
Ethyl acetate
95
Vinyl decanoate
Ethyl decanoate
>99
>99
Figure 2. Recycling of CAL-B and VOSO4 in the DKR of rac-1 (2 h of each cycle;
● ee of the product (R)-1b at the end of each cycle, determined by chiral GC-
FID analysis; ■ conversion; ▲ selectivity). Conversion and selectivity were
determined by GC-FID analysis with n-tetradecane (0.050 mmol mL-1) as
internal standard. Conditions: rac-1 (0.25 mmol), vinyl decanoate (2 equivalents,
0.50 mmol), heptane (4 mL), CAL-B (20 mg), VOSO4.XH2O (50 mg), 50 °C,
vigorous stirring.
Conditions: rac-1 (0.25 mmol), acyl donor (2 equivalents, 0.50 mmol), heptane
[a]
(4 mL), CAL-B (20 mg), VOSO4.XH2O (50 mg), 50 °C, vigorous stirring.
Determined by GC-FID analysis with n-tetradecane (0.050 mmol mL-1) as
internal standard. [b] Determined by chiral GC-FID analysis.
The reactions using vinyl and ethyl decanoates (Table 2, entries
3 and 4) presented high enantioselectivities and afforded (R)-1-
phenylethyl decanoate ((R)-1b) with ee > 99%. These results
indicate that acyl donors with longer acyl chains lead to higher
enantioselectivities, which is corroborated by the literature.[27,28]
Besides this, the commonly used acyl donors such as vinyl
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Table 3. Increasing substrate load on DKR of rac-1-phenylethanol (rac-1):
Evaluation of catalytic tolerance.
Substrate
concentration
(M)
Time
(h)
c
eeP
(%)[b]
Sel.(%)[a]
E
Entry
(%)[a]
92
87
77
>200
>200
>200
1
2
3
0.0625
0.125
0.250
2
3
3
82
79
76
>99
>99
>99
Figure 3. Recycling of CAL-B in the KR of rac-1 (3 h for each cycle; ● ee of the
product (R)-1b at the end of each cycle, determined by chiral GC-FID analysis;
■ conversion, determined by GC-FID analysis with n-tetradecane (0.050 mmol
mL-1) as internal standard). Conditions: rac-1 (0.25 mmol), vinyl decanoate (2
equivalents, 0.50 mmol), heptane (4 mL), CAL-B (20 mg) 50 °C.
Conditions: rac-1 (0.25, 0.50 or 1.00 mmol), acyl donor (2 equivalents, 0.50,
1.00 or 2.00mmol), heptane (4 mL), CAL-B (20 mg), VOSO4.XH2O (50 mg),
50 °C, vigorous stirring. [a] Determined by GC-FID analysis with n-tetradecane
[b]
(0.050 mmol mL-1) as internal standard.
analysis.
Determined by chiral GC-FID
With these results in hands, the substrate scope was evaluated
based on 13 aromatic and heteroaromatic sec-alcohols (Table 4).
High yields (73 to 91%) and high enantiomeric excesses (up to
99%) were observed for the heteroaromatic and substituted
aromatic sec-alcohols. These results indicate that the
incompatibility between CAL-B and VOSO4 is an issue for
substrates that require extended reaction times. In these cases,
better conversions and selectivities can be obtained by physically
separating CAL-B and VOSO4, which has been mentioned in
previous works.[19,21,24] On the other hand, no conversions were
observed in the substitution of the side chain with groups larger
than ethyl (substrates 11-13, Table 4). According to the literature,
the size difference between the groups bound to the stereocenter
of the alcohol is important for lipase-catalysed kinetic
resolution.[9,29a] Reports in the literature point out that CAL-B is
less reactive towards substrates 11[29b,c] and 12[29d], while for
substrate 13 no kinetic resolution is observed.[29e]
Figure 4. Recycling of VOSO4 in the racemization of (S)-1 in the presence of
0.50 mmol of vinyl decanoate (1 h for each cycle; ● ee of (S)-1 at the end of
each cycle, determined by chiral GC-FID analysis; ▲ selectivity, determined by
GC-FID analysis with n-tetradecane (0.050 mmol mL-1) as internal standard).
Conditions: (S)-1 (0.25 mmol), vinyl decanoate (2 equivalents, 0.50 mmol),
heptane (4 mL), VOSO4.XH2O (50 mg), 50 °C, vigorous stirring.
Compartmentalization of incompatible catalysts is a strategy
inspired by natural systems, as compartments in cells allow
incompatible and concurrent catalytic transformations to occur
simultaneously for the synthesis of complex molecules.[30] Herein,
we decided to use this strategy to prolong the high performance
of the CAL-B/VOSO4 system to beyond the fourth cycle of use. To
do this, a homemade Teflon tube with micro holes was built (see
the Supporting Information), and one of the catalysts was placed
inside the tube to prevent physical contact between the two
catalysts. In the first test, we placed the CAL-B inside the Teflon
tube (Table 5).
To evaluate the tolerance of the catalyst system to high substrate
loads, we performed the DKR of rac-1-phenylethanol (rac-1),
keeping the other reaction conditions fixed and only varying the
substrate concentration from 0.0625 to 0.250 M (Table 3). The
results show that the catalytic system can hold up to 2 times the
increase in substrate loading, with a small drop in conversion and
selectivity. However, this effect is more pronounced at higher
substrate loading with a significant decrease in selectivity.
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Once the compartmentalization system was optimized, the next
step was to evaluate its recyclability after 8 cycles. As shown in
Figure 5, the system remained active throughout the experiments
and only a slight decrease in the conversion, from 94% in the first
cycle to 87% in the 8th cycle, was observed. The tube was
removed at the end of each cycle and washed three times with
heptane. CAL-B was recovered by simple paper filtration and
washed three times with heptane. No leaching was observed in
either situation (visual inspection).
Table 5. Results of the DKR of rac-1-phenylethanol (rac-1) with and without
compartmentalization of the catalysts.
These results show that this compartmentalization strategy
improves the recyclability of the CAL-B/VOSO4 system, as both
catalysts maintained their performance for 8 cycles (c = 94-87%,
ee of (R)-1b = 99%) as well as their high selectivity (no by-
products formation), confirming that it is possible to overcome
catalyst incompatibility issues in this DKR protocol using only a
low-cost and homemade tube. In addition, the simple
compartmentalization strategy adopted in this work facilitated the
removal and recovery of VOSO4 from the reaction system during
workup.
Time
(h)
c
eeP
Sel
Entry
E
(%)[a]
(%)[b]
(%)[a]
1
2
without tube
2
2
82
20
>99
>99
92
>200
>200
CAL-B into the
tube
>99
VOSO4 into the
tube
3
1
91
>99
98
>200
Conditions: rac-1 (0.25 mmol), acyl donor (2 equivalents, 0.50 mmol), heptane
(4 mL), CAL-B (20 mg), VOSO4.XH2O (50 mg) into a Teflon tube, 50 °C,
magnetic stirring. [a] Determined by GC-FID analysis with n-tetradecane (0.050
mmol mL-1) as internal standard. [b] Determined by chiral GC-FID analysis.
The DKR reaction was carried out in a 25-mL glass round-
bottomed flask containing the Teflon tube loaded with lipase,
using rac-1 as the substrate and the optimal reaction conditions
defined in this work (Table 5, entry 2). After 2 h, a 20% conversion
of rac-1 into (R)-1b (ee > 99%) and no by-product formation was
observed. This lower conversion rate could be attributed to poor
mass transfer of the substrate to the inside of the tube containing
the immobilized enzyme. Other than the lower conversion rate,
this result indicates that compartmentalization is promising for
optimization of the DKR since this strategy did not interfere with
the enantioselectivity of the process and it inhibited the formation
of by-product. In the second test, we placed the VOSO4 inside the
Teflon tube (Table 5, entry 3) and product (R)-1b was obtained
with 96% conversion, 98% selectivity and ee > 99% in only 1 h,
making the protocol developed in this work even more efficient.
The results for the DKR of rac-1 with and without
compartmentalization prove the importance of the physical
separation of the heterogeneous catalysts.
Figure 5. Recycling of CAL-B and VOSO4 in the DKR of rac-1 with
compartmentalization (VOSO4 into a Teflon tube; 1 h for each cycle; ● ee of the
product (R)-1b at the end of each cycle, determined by chiral GC-FID analysis;
■ conversion; ▲ selectivity). Conversion and selectivity were determined by
GC-FID analysis with n-tetradecane (0.050 mmol mL-1) as internal standard.
Conditions: rac-1 (0.25 mmol), vinyl decanoate (2 equivalents, 0.50 mmol),
heptane (4 mL), CAL-B (20 mg), VOSO4.XH2O (50 mg) into the tube, 50 °C,
vigorous stirring.
A schematic of the system under the optimized conditions is
shown in Scheme 4.
To evaluate the extension and applicability of the
compartmentalization system developed herein, we carried out
the DKR of the substrates that exhibited the lowest conversions
and selectivities (substrates 3, 6 and 10, Table 6) in the previously
non-compartmentalized tests. The DKR reactions performed with
the compartmentalization system resulted in higher selectivities,
similar to what was observed for model substrate rac-1.
a)
b)
Teflon tube
Solution of rac-alcohols, acyl
donor and heptane
VOSO4
CAL-B
Scheme 4. a) Schematic representation of DKR reaction setup with
compartmentalization of the catalysts; b) DKR reaction: tube with VOSO4 in
contact with the reaction media containing the substrate (a racemic sec-alcohol),
vinyl decanoate (acyl donor), immobilized CAL-B and heptane.
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Table 4. DKR of aromatic and heteroaromatic sec-alcohols catalysed by CAL-B and VOSO4.
Substrate
Time
2 h
Product
Yield (%)[a]
eeP. (%)[b]
E
Sel. (%)[c]
O
OH
(CH2)8CH3
O
85
>99
>200
>200
92
93
( R)
1
(R)-1b
O
OH
(CH2)8CH3
O
45 min
30 min
87
73
>99
>99
( R)
2
Me
(R)-2b
Me
O
OH
(CH2)8CH3
(R)-3b
O
>200
>200
194
87
96
91
( R)
3
MeO
MeO
O
OH
(CH2)8CH3
O
1 h
79
82
>99
( R)
4
F
O2N
F3C
( )-4b
R
F
O
OH
(CH2)8CH3
O
1 h
96
( R)
5
(R)-5b
O2N
O
OH
(CH2)8CH3
O
2 h
80
90
>99
>200
110
90
96
(R)
6
(R)-6b
F3C
30 min
91
30 min
88
91
>99
>200
97
O
OH
(CH2)8CH3
O
N
1 h
89
90
>99
N
( R)
9
(R)-9b
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2 h
74
n.r.
n.r.
n.r.
>99
n.d.
n.d.
n.d.
>200
77
24 h
24 h
24 h
-
-
-
-
-
-
Conditions: 0.25 mmol of rac-alcohol, 0.50 mmol of vinyl decanoate (2 equivalents), heptane (4 mL), VOSO4.XH2O (50 mg), CAL-B (20 mg), 50 °C, vigorous stirring.
[a] Isolated yield after flash chromatography. [b] Determined by chiral GC-FID analysis after hydrolysis of the product into the (R)-alcohol and compared to the racemic
standard. [c] Determined by GC-FID comparing the relative peak areas of substrate, product, and by-products. n.r. = no reaction observed, n.d. = not determined
Table 6. DKR of sec-alcohols with compartmentalization of the catalysts.
Substrate
Product
Time
Compartmentalization
without
Yield(%)[a]
83
eeP. (%)[b]
E
Sel. (%)[c]
O
>99b)
>200
87
OH
(CH2)8CH3
O
30 min
( R)
>99b)
>99
>200
>200
92
90
3
with
88
78
MeO
(R)-3b
MeO
O
without
OH
(CH2)8CH3
O
2 h
(R)
6
with
80
80
90
>99
>99
>99
>200
>200
>200
96
76
98
F3C
(R)-6b
F3C
without
with
2 h
Conditions: 0.25 mmol of rac-alcohol, 0.50 mmol of vinyl decanoate (2 equivalents), heptane (4 mL), VOSO4.XH2O (50 mg) into the tube, CAL-B (20 mg), 50 °C,
magnetic stirring. [a] Isolated yield after flash chromatography. [b] Determined by chiral GC-FID analysis after hydrolysis of the product into the (R)-alcohol and
compared to the racemic standard. [c] Determined by GC-FID comparing the relative peak areas of substrate, product, and by-products. n.r. = no reaction observed,
n.d. = not determined.
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10.1002/cctc.202000292
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General
To assess the robustness of the catalytic system, we performed
a large-scale DKR of rac-1-phenylethanol (rac-1) (Table 7) with a
substrate loading of 5.00 mmol (See the Supporting Information
for details). These experiments revealed that there were no
significant differences between the lowest and the large scale,
and high selectivity was also reached for the model substrate rac-
1, under these reaction conditions.
Ketones
and
aldehydes,
(S)-1-phenylethanol
((S)-1),
rac-1-
phenylpropanol (10), vinyl and ethyl decanoates, vanadyl sulfate hydrate
(VOSO4·XH2O), methylmagnesium bromide solution (3.0 M in diethyl
ether) and immobilized Candida antarctica lipase B (Novozym 435; CAL-
B; ≥ 5000 U/g, recombinant, expressed in Aspergillus niger and adsorbed
on a macroporous resin) were purchased from Sigma-Aldrich. All solvents
(p.a. grade), and reagents were used as received.
Table 7. Large-scale DKR of rac-1-phenylethanol (rac-1) with
compartmentalization of the catalysts: Evaluation of robustness of the protocol
Racemic alcohols 1-6 and 11-13 were synthesized via reduction of their
corresponding ketones with NaBH4 following a previously described
procedure.[31] Racemic alcohols 7-9 were obtained from their respective
aldehydes via Grignard reactions following
a previously described
procedure.[32] See the Supporting Information for details related to
procedures and for the full characterization data of the synthesized
alcohols.
GC-MS analysis was performed on an Agilent 7890B GC coupled to an
Agilent 5977A MS (electron impact ionization at 70 eV) with a (5%-phenyl)-
methylpolysiloxane column (30 m × 0.25 mm ID; HP5-MS) and using
helium as the carrier gas (1 mL min−1). The injector and interface
temperatures were 260 °C and 280 °C, respectively. The GC-MS
temperature program was as follows: 80 °C for 3 min, then ramp to 280 °C
at 30 °C min−1, then hold 3 min.
Substrate
load (mmol)
Yield
(%)[a]
eeP
Sel
Entry
1
Time (h)
2
E
(%)[b]
(%)[c]
0.25
5.00
91
88
>99
>99
98
98
>200
2[d]
2
>200
GC-FID analysis was performed on a Shimadzu GC-2010 Plus equipped
with an AOC-20i autosampler and using hydrogen as the carrier gas (1 mL
min−1). To determine the conversion and selectivity values, a (5%-
diphenyl)-dimethylpolysiloxane column (30 m × 0.25 mm ID; Rtx-5) was
used. In these analyses, the injector and interface temperatures were
260 °C and 280 °C, respectively. The GC-FID temperature program was
as follows: 80 °C for 3 min, ramp to 280 °C at 30 °C min−1, then hold 10
min. In DKR reactions of 1-phenylethanol, conversions and selectivities
were calculated by determining the concentration of 1-phenylethyl acetate
or 1-phenylethyl decanoate using a calibration curve with n-tetradecane as
internal standard. In other DKR reactions, selectivity values were
determined using the equation Sel (%) = [(AS + AP)/(AS + AP + AbyP)] × 100,
where AS, AP, and AbyP are the relative peak areas of substrate, product
and by-products, respectively, in the chromatograms. To determine the
enantiomeric excess (ee) values, two chiral columns were used: Hydrodex
β-3P (heptakis-(2,6-di-O-methyl-3-O-pentyl)-β-cyclodextrin, 25 m × 0.25
[a] Isolated yield after flash chromatography. [b] Determined by chiral GC-FID. [c]
Determined by GC-FID analysis with n-tetradecane (0.050 mmol mL-1) as
[d]
internal standard. Conditions:
rac-1 (5 mmol, 0.61 g), acyl donor (2
equivalents, 10 mmol, 1.98 g), heptane (80 mL), CAL-B (0.40 g), VOSO4.XH2O
(1.00 g) into the tube, 50 ºC, magnetic stirring.
Conclusions
In this work,
a robust DKR protocol for aromatic and
heteroaromatic sec-alcohols has been developed using the
heterogeneous catalysts CAL-B and VOSO4, and high
conversions, ee values and more important, selectivity, were
achieved. The recyclability of the CAL-B/VOSO4 system was
investigated for the first time, and this catalytic system remained
active for 4 DKR cycles. To further increase the recyclability, the
catalysts were compartmentalized through the physical
separation of VOSO4 from CAL-B by employing a low-cost,
homemade Teflon tube, which allowed 8 reaction cycles without
a loss in performance or selectivity. In addition to preventing
performance loss of the catalysts due to contact, the Teflon tube
facilitated the removal of the catalysts during reaction workup and
their reuse. Moreover, this methodology proved to be robust for
gram scale experiments. In summary, the combination of these
readily commercially available and non-toxic catalysts, CAL-B
and VOSO4, presents itself as an environmentally attractive
alternative for the resolution of aromatic and heteroaromatic sec-
alcohols.
mm ID) and Lipodex
E
(octakis-(2,6-di-O-pentyl-3-O-butyryl)-γ-
cyclodextrin, 25 m × 0.25 mm ID). In these analyses, the injector and
interface temperatures were 160 °C and 180 °C, respectively. For 1-
phenylethanol, 1-phenylethyl acetate and 1-phenylethyl decanoate, the ee
values were determined directly using the relative peak areas of their
enantiomers. For other DKR products, the ee were obtained indirectly after
determining the ee values of the correspondent alcohols obtained from
esters hydrolysis. See the Supporting Information for details related to the
temperature program for chiral GC-FID analysis.
1H NMR and 13C NMR (DEPTQ) spectra were acquired on a Bruker Fourier
600 (B0 14.1 T) using CDCl3 as the solvent, operating at a frequency of
600.13 MHz for the 1H nucleus and 150.90 MHz for the 13C nucleus.
Chemical shifts are expressed in ppm downfield from tetramethylsilane
(TMS). Signal multiplicities are indicated by the letters s (singlet), brs
(broad singlet), d (doublet), dd (doublet of doublets), doublet of doublets
of doublets (ddd), t (triplet), q (quartet), qnt (quintet) and m (multiplet).
Optical rotations were measured in CHCl3 solutions on a Perkin Elmer 341
LC polarimeter at the sodium D line (589 nm) and with a 1.00 dm quartz
cell.
Experimental Section
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10.1002/cctc.202000292
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Compartmentalization of the catalysts. To a 25-mL two-neck round
bottom flask were added 20 mg of immobilized Candida antarctica B lipase
(CAL-B) and 4.0 mL of heptane. A Teflon tube containing 50 mg of
VOSO4·XH2O was added into the reaction flask by fitting it into a septum.
The flask was closed with this septum connect to the tube, and then, using
syringes, 0.25 mmol of rac-alcohol and 0.50 mmol (99.1 mg) of vinyl
decanoate were added into the reaction system. The resulting suspension
was stirred at 50 °C using a magnetic bar. After maximum conversion of
the (R)-decanoates and lower by-product formation was achieved, the
tube with VOSO4 was removed, and CAL-B was filtered off by simple paper
filtration. For recycling analyses (performed for the DKR of rac-1-
phenylethanol), the catalysts were removed from the reaction flask,
washed with heptane (3 x 2 mL; VOSO4 was washed inside the tube) and
used in a new DKR cycle under the same reaction conditions. This
procedure was repeated for 8 consecutive cycles.
Racemization reactions
Solvent screening. To 0.25 mmol (30.5 mg) of (S)-1-phenylethanol was
added 4.0 mL of solvent (toluene, octane or heptane) and 50 mg of
VOSO4·XH2O. The resulting suspension was stirred at 50 °C or 80 °C
using a magnetic bar, and the reactions were monitored by chiral GC-FID
until reaching the lowest ee for the (S)-enantiomer.
Recycling of VOSO4. To 0.25 mmol (30.5 mg) of (S)-1-phenylethanol was
added 4.0 mL of heptane and 50 mg of VOSO4·XH2O. The resulting
suspension was stirred at 50 °C using a magnetic bar for 1 h. Then, the
catalyst (VOSO4) was filtered off by simple paper filtration, washed with
heptane (3 x 2 mL) and used in a new racemization cycle under the same
reaction conditions described. The procedure was repeated for 10 cycles.
DKR reactions
Large-scale DKR. To a 250-mL three-neck round bottom flask were
added 0.40 g of immobilized Candida antarctica B lipase (CAL-B) and 80.0
mL of heptane. A tube containing 1.00 g of VOSO4·XH2O was added into
the reaction flask by fitting it into a septum. The flask was closed with this
septum connect to the tube, and then, using syringes, 5.00 mmol of rac-1-
phenylethanol (0.61 g) and 10.0 mmol (1.98 g) of vinyl decanoate were
added into the reaction system. The resulting suspension was stirred at
50 °C using a magnetic bar and the reaction was monitored by GC-FID.
After maximum substrate conversion to the respective (R)-1-phenylethyl
decanoate and lower by-product formation was achieved, the tube with
VOSO4 was removed and CAL-B was filtered off by simple paper filtration.
Acyl donor screening. To 0.25 mmol (30.5 mg) of rac-1-phenylethanol
was added 4.0 mL of heptane and 0.50 mmol (2 equivalents) of acyl donor
(vinyl acetate, ethyl acetate, vinyl decanoate or ethyl decanoate). To the
resulting solution were added 20 mg of immobilized Candida antarctica
lipase B (CAL-B) and 50 mg of VOSO4·XH2O. The resulting suspension
was stirred at 50 °C using a magnetic bar, and the reactions were
monitored on GC-FID. After reaching maximum substrate conversions to
the (R)-products and lower by-product formation, the catalysts were
removed by simple paper filtration, and the reaction solution was
concentrated under reduced pressure. The residues were purified by flash
chromatography (heptane:ethyl acetate, 95 : 5) to afford (R)-1-phenylethyl
acetate and (R)-1-phenylethyl decanoate.
Acknowledgements
Recycling of CAL-B and VOSO4. To 0.25 mmol (30.5 mg) of rac-1-
phenylethanol was added 4.0 mL of heptane and 0.50 mmol (112 µL) of
vinyl decanoate. To this resulting solution were added 20 mg of
This work was funded by grants from FAPESP (2014/50249-8)
and GlaxoSmithKline. This study was financed in part by the
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
– Brasil (CAPES) – Finance Code 001. The authors are also
grateful to CAPES for maintaining the Portal de Periódicos
immobilized Candida antarctica lipase
B (CAL-B) and 50 mg of
VOSO4·XH2O. The resulting suspension was stirred at 50 °C using a
magnetic bar for 2 h. Then, the catalysts (CAL-B and VOSO4) were filtered
off by simple paper filtration, washed with heptane (3 x 2 mL) and added
in a new DKR cycle under the same reaction conditions. The procedure
was repeated for 6 consecutive cycles.
Keywords: dynamic kinetic resolution • heterogeneous catalysis
• lipase • sec-alcohols • vanadium
Evaluation of catalytic tolerance. To 0.25 mmol (30.5 mg), 0.50 mmol
(61.1 mg) or 1.00 mmol (122.2 mg) of rac-1-phenylethanol was added,
respectively, 0.50 mmol (99.1 mg), 1.00 mmol (198.3 mg) or 2.00 mmol
(396.6 mg) of vinyl decanoate and 4.0 mL of heptane. To the resulting
solutions were added 20 mg of immobilized Candida antarctica lipase B
(CAL-B) and 50 mg of VOSO4·XH2O. The resulting suspensions were
stirred at 50 °C using a magnetic bar, and the reactions were monitored
byGC-FID. After reaching maximum substrate conversions to (R)-1-
phenylethyl decanoate and lower by-product formation, the catalysts were
removed by simple paper filtration and the reaction solutions were
concentrated under reduced pressure.
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Table of Contents
FULL PAPER
L. A. de Almeida, T. H. Marcondes, C.
D. F. Milagre, H. M. S. Milagre*
Page No. – Page No.
Lipase-oxovanadium heterogeneous
catalysis system: a robust protocol
for the dynamic kinetic resolution of
sec-alcohols
A robust and eco-friendly dynamic kinetic resolution (DKR) protocol for secondary
alcohols using a combined heterogeneous catalytic CAL-B/VOSO4 system.
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