Selective oxidations of organoboron compounds catalyzed by Baeyer-Villiger monooxygenases

Article abstract of DOI:10.1002/adsc.201100029

The applicability of Baeyer-Villiger monooxygenases (BVMOs) in organoboron chemistry has been explored through testing chemo- and enantioselective oxidations of a variety of boron-containing aromatic and vinylic compounds. Several BVMOs, namely: phenylacetone monooxygenase (PAMO), M446G PAMO mutant, 4-hydroxyacetophenone monooxygenase (HAPMO) and cyclohexanone monooxygenase (CHMO) were used in this study. The degree of chemoselectivity depends on the type of BVMO employed, in which the biocatalysts prefer boron-carbon oxidation over Baeyer-Villiger oxidation or epoxidation. Interestingly, it was discovered that PAMO can be used to perform kinetic resolution of boron-containing compounds with good enantioselectivities. These findings extend the known biocatalytic repertoire of BVMOs by showing a new family of compounds that can be oxidized by these enzymes. Copyright

Full text of DOI:10.1002/adsc.201100029

FULL PAPERS
DOI: 10.1002/adsc.201100029
Selective Oxidations of Organoboron Compounds Catalyzed by
Baeyer–Villiger Monooxygenases
a
b
b
Patrꢀcia B. Brondani, Gonzalo de Gonzalo, Marco W. Fraaije,
a,
and Leandro H. Andrade *
a
Instituto de Quꢀmica, Universidade de S¼o Paulo, Av. Prof. Lineu Prestes 748, SP 05508-900 S¼o Paulo, Brazil
Fax : (+55)-11-3815-5579; phone: (+55)-11-3091-2287; e-mail: leandroh@iq.usp.br
Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Nijenborgh 4, 9747 AG,
Groningen, The Netherlands
b
Received: January 20, 2011; Revised: April 20, 2011; Published online: August 10, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100029.
Abstract: The applicability of Baeyer–Villiger mono- tion or epoxidation. Interestingly, it was discovered
oxygenases (BVMOs) in organoboron chemistry has that PAMO can be used to perform kinetic resolu-
been explored through testing chemo- and enantiose- tion of boron-containing compounds with good enan-
lective oxidations of a variety of boron-containing ar- tioselectivities. These findings extend the known bio-
omatic and vinylic compounds. Several BVMOs, catalytic repertoire of BVMOs by showing a new
namely: phenylacetone monooxygenase (PAMO), family of compounds that can be oxidized by these
M446G PAMO mutant, 4-hydroxyacetophenone enzymes.
monooxygenase (HAPMO) and cyclohexanone mon-
ooxygenase (CHMO) were used in this study. The
degree of chemoselectivity depends on the type of Keywords: Baeyer–Villiger reaction; Baeyer–Villiger
BVMO employed, in which the biocatalysts prefer monooxygenases; boron; boron compounds; oxida-
boron-carbon oxidation over Baeyer–Villiger oxida- tion
Introduction
mono
ACHTUNGTNERNGNUo xygenases aiming at carbon-boron bond oxida-
[3b,5]
tion to afford the corresponding alcohols.
This is
The chemo-, regio-, and stereoselective introduction in contrast with the fact that boron-containing com-
of oxygen into organic molecules represents a key pounds are versatile intermediates in synthetic organ-
chemical transformation with broad applicability in ic chemistry. A variety of chemical approaches has
[
1]
organic synthesis. These features have significantly been reported to synthesize organoboron com-
[
6]
increased the interest in exploring oxidative reactions pounds. In order to increase the synthetic potential
catalyzed by enzymes. One of the most versatile class of oxidative biocatalysts for the preparation of boron-
of enzymes that catalyzes a variety of oxidations are containing compounds, in this work we explored four
the Baeyer–Villiger monooxygenases (BVMOs): BVMOs, namely: phenylacetone monooxygenase
[
7]
flavin-containing and NAD(P)H-dependent enzymes (PAMO) from Thermobifida fusca,
its M446G
[
8]
that catalyze the incorporation of one oxygen atom PAMO mutant, 4-hydroxyacetophenone monooxy-
into organic substrates. BVMOs are known for per- genase (HAPMO) from Pseudomonas fluorescens
forming the oxidation of aldehydes and ketones to ACB and cyclohexanone monooxygenase (CHMO)
their corresponding esters, the oxygenation of heter- from Acinetobacter sp. Achiral aromatic and vinylic
[
2]
[
9]
[
10]
oatoms (sulfur, nitrogen, phosphorus, boron, seleni- boron compounds as well as racemic ones have been
[
3]
um) and even epoxidation reactions. In many of evaluated as target substrates in BVMO-catalyzed ox-
these reactions, the transformations of substrates idation reaction.
occur with high enantio- and/or regioselectivity, while
[
4]
using environment friendly conditions.
A literature survey revealed to us that only very
few boron compounds have been evaluated with
Adv. Synth. Catal. 2011, 353, 2169 – 2173
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Patrꢀcia B. Brondani et al.
FULL PAPERS
Results and Discussion
and 7). On the other hand, in all HAPMO-catalyzed
reactions, both oxidations occurred (boron oxidation
Initially, in order to evaluate the chemoselectivity be- and BV oxidation). The esters 8 or 9 (3- or 4-hydroxy-
tween the CÀB bond oxidation and the Baeyer–Villig- phenyl acetate, respectively) obtained from the BV
er (BV) reaction, five boron-containing acetophe- oxidation undergo hydrolysis to give the correspond-
nones (1 and 2: meta- and para-substituted boronic ing catechols (resorcinol 10 or hydroquinone 11, re-
acids; 3 and 4: meta- and para-substituted boronic spectively). Finally, CHMO showed a high chemose-
esters; 5: para-substituted potassium trifluoroborate) lectivity in favor of boron oxidation, but low activity,
were selected as substrates. Reactions with 3- and 4- affording only the 3- or 4-hydroxyacetophenones (6
hydroxyacetophenones (6 and 7) were also performed or 7) with poor conversions. Besides the hydroxylated
as reference (Table 1). Purified BVMOs were used as compounds that are obtained from a boron-carbon
[
3b,6]
biocatalysts in the oxidation reactions. Besides, a sec- oxidation using BVMO, B(OH) or HOBPin
can
ondary enzymatic system containing phosphite dehy- be produced as by-products. In another set of reac-
drogenase was employed for NADPH regeneration. tions, we tested all BVMOs with 3- and 4-hydroxyace-
3
When PAMO was used as biocatalyst, the boron tophenones 6 and 7 as substrates with the aim of eval-
oxidation was observed for all substrates affording uating the BV reaction. The PAMO- and M446G
the corresponding phenols (8–11). However, the BV PAMO-catalyzed oxidation of 3-hydroxyacetophe-
oxidation was achieved only for the 4-substituted sub- none 6 gave no BV reaction product, but it was ob-
strates (Table 1, compounds 2, 4, 5 and 7). All the oxi- served for 4-hydroxyacetophenone 7. These results
dations led to only one final product with excellent are identical to those obtained with boron-containing
conversions: 3-hydroxyacetophenone 6 for the 3-sub- acetophenones 1–5. HAPMO was discovered to be an
stituted compounds and 4-hydroxyphenyl acetate 9 excellent biocatalyst for BV oxidation of 3- and 4-sub-
for the 4-substituted ones. The M446G PAMO mutant stituted acetophenones (hydroxy- or boron-substitut-
presented a similar behavior for all substrates in ed compounds 1–7), which resulted in resorcinol 10
which the boron oxidation was observed. However, and hydroquinone 11 as products, respectively. Final-
for the 4-substituted compounds (2, 4, 5 and 7), the ly, when CHMO was employed as biocatalyst, no BV
M446G PAMO mutant showed to be less effective for reaction was observed for 3- and 4-substituted aceto-
BV oxidation when compared with the wild-type phenones as substrates (compounds 1–7).
enzyme, resulting in a final mixture of the ketones
However, CHMO was found to be highly chemose-
and the esters as indicated in Table 1 (entries 2, 4, 5 lective for CÀB bond oxidation of boron-containing
[a]
Table 1. Oxidation of boron-containing acetophenones 1–5 and 3- and 4-hydroxyacetophenones catalyzed by BVMOs.
[b]
Entry
Substrate
Products (Yield) with BVMO
M446G PAMO HAPMO
PAMO
CHMO
[
c]
c]
c]
c]
c]
[c]
[c]
[c]
[c]
[c]
[c]
1
2
3
4
5
6
7
1
2
3
4
5
6
7
6 (>99%)[
9 (>99%)[
6 (>99%)
6 (>99%)
10 (>99%)
11 (>99%)
10 (>99%)
6 (>99%)
7 (>99%)
[d]
7 and 9 (>99%, 1:4)
[c]
[e]
6 (>99%)
–
[
[c]
[d]
[d]
9 (>99%)[
7 and 9 (>99%, 5:4)
7 and 9 (>99%, 1:2)
11 (74%)
7 (43%)
[d]
[c]
[c]
[c]
[c]
9 (>99%)
11 (>99%)
10 (>99%)
11 (>99%)
7 (>99%)
[e]
[e]
[e]
–
–
–
–
[c]
[c]
[e]
9 (>99%)
7, 9 and 11 (>99%, 5:2:2)
[
a]
b]
See Experimental Section.
[
All products were obtained commercially or synthetically, they were then injected on GC and GC-MS for comparison
purposes.
[
[
c]
This compound was detected as sole product by GC and GC-MS analysis, after extraction with organic solvent.
Conversion was determined by GC and GC-MS analysis. The ratio between the products formed is also shown in brack-
d]
ets, when necessary.
No reaction was observed.
[
e]
2170
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Adv. Synth. Catal. 2011, 353, 2169 – 2173

Selective Oxidations of Organoboron Compounds Catalyzed by Baeyer–Villiger Monooxygenases
[a]
Table 2. Oxidation of vinyl boron compounds 12–17 catalyzed by BVMOs.
Entry Substrate
Products (Yield) with BVMO
M446G PAMO
PAMO
HAPMO
CHMO
[b]
[b]
[b]
[b]
1
–
–
–
(À)
[
[
b]
b]
[b]
[b]
[b]
2
3
–
–
–
–
(À)
[b]
[b]
[b]
–
–
(À)
[
c]
[c]
[c]
4
5
6
A
T
N
T
E
N
N
(>99%)
(>99%, 3:2)
A
T
N
T
E
U
G
ACHTUNGTRENNUNG( >99%,1:4)
[
c]
c]
[c]
[c]
[c]
[c]
[c]
A
H
U
G
R
N
U
G
(>99%, 7:3)
A
H
U
G
R
N
U
G
ACHTUNGTRNENUNG( >99%)
[
[c]
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
ACHTUNGTRNENUNG( >99%)
[a]
[b]
[c]
See Experimental Section.
No reaction was observed.
Conversion was determined by GC and GC-MS analysis. The ratio between the products formed is also shown in brack-
ets, when necessary.
acetophenones (substrates 1, 2, 4 and 5) affording the biocatalyst was chosen due to its high activity and ex-
corresponding hydroxyacetophenones in up to com- clusive chemoselectivity in favor of boron oxidation.
plete conversions after 24 h (Table 1, entries 1, 2, 4, The results revealed that the turnover rates (k =
cat
À1
À1
and 5). These findings point out that BVMOs can cat- 2.6 s and 1.1 s for boronic acid 1 and boronic ester
alyze the BV oxidation of 3- or 4-hydroxyacetophe- 3, respectively) were similar to those observed for the
nones as well as boron-containing acetophenones. In best substrates reported for the M446G PAMO
order to identify the intermediates described above, mutant. The boronic acid 1 and boronic ester 3 are
the oxidation of the compounds 2 and 4 by BVMOs also well recognized by the enzyme (K =1.4 and
M
over time was monitored using GC and GC-MS anal- 0.4 mM for 1 and 3, respectively).
ysis (1, 4, 8 and 24 h, see Supporting Information).
We have also explored the BVMO-catalyzed oxida-
For both compounds, the boron oxidation is the first tion of vinyl boron compounds 12–17 (Table 2). These
reaction to give the corresponding 3- or 4-hydroxya- reactions were performed in order to evaluate chemo-
cetophenones and boron by-products [B(OH) or selectivity of BVMO between boron oxidation and
3
[3b,6]
[3]
HOBPin].
the possible epoxidation reaction. It was observed
With the aim of gaining a better view on the cata- that no epoxidation was achieved for any of the sub-
lytic performance of BVMOs in the boron atom oxi- strates. Moreover, no reaction at all was observed for
dation, we decided to perform steady-state kinetic the aliphatic boron compounds 12, 13 and 14 used as
studies for the M446G PAMO mutant with com- substrates (Table 2, entries 1, 2 and 3). Nevertheless,
pounds 1 and 3 (see Supporting Information). This all the aromatic compounds were transformed by the
Adv. Synth. Catal. 2011, 353, 2169 – 2173
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Patrꢀcia B. Brondani et al.
FULL PAPERS
BVMOs, revealing the exclusive boron oxidation pref- Table 3. Enzymatic kinetic resolution of chiral boron-com-
pounds (23 and 24) catalyzed by PAMO.
erence (Table 2, entries 4, 5 and 6). When PAMO and
its M446G mutant were employed as biocatalysts in
the oxidations of substrates 15 and 16, benzyl alcohol
18 (entries 4 and 5) was obtained as sole product with
complete conversion. For these enzymatic oxidations,
phenylacetaldehyde was initially produced by a
BVMO-catalyzed boron oxidation and then the BV
oxidation and subsequent hydrolysis led to the final
product, benzyl alcohol 18. On the other hand, the
HAPMO oxidation of compounds 15 and 16 as well
as the CHMO oxidation of 15 led to a mixture of 2-
phenylacetaldehyde (19) (a boron oxidation product),
and surprisingly, to 2-phenylethanol (20), a reduction
product of phenylacetaldehyde. In order to evaluate
the role of CHMO and HAPMO in the above-men-
tioned reduction process, we carried out another set
of enzymatic reactions, but excluding the respective
BVMOs from the incubation mixtures. The results
pointed out that such reductions did not occur. This
suggests that the employed BVMO preparations pre-
sumably still contained trace amounts of a reductase
from Escherichia coli, which can be the responsible
[
a]
[a]
[b]
Entry pH Time Conv.
ee (S)-26
[%]
ee (R)-24
[%]
E
[
h]
[%]
1
2
9
7.5
3
5
48
49
77
91
70
85
23
33
[a]
The ee values were determined by HPLC analysis using a
chiral stationary phase column. In the case of 24, chemi-
cal oxidation (H O /NaOH) was performed and then the
2
2
ee values were determined.
[
[
b]
c]
The E values were calculated by using the formula E=
ln
A
H
U
G
R
N
U
G
ACHTUNGTRNNGE[U (1+ee )/(1+ee /ee )].
s
s
p
s s p
Absolute configuration was determined by comparison
[
11]
with data described in ref.
biocatalyst for the reduction of phenylacet
ACHTUNGTRENNUNGa ldehyde to
2-phenylethanol. However, we cannot rule out a role
of BVMOs in this reduction. While the enzymatic oxi- the corresponding (S)-alcohol. The (R)-borane was
dation of trans-substituted vinyl boronic ester 15 cata- recovered in a process with a decent enantioselectivi-
[
13]
lyzed by HAPMO led to phenylacetaldehyde (19) as ty (E=23)
and good conversion after 3 h. More-
major product, the oxidation catalyzed by CHMO over, in order to evaluate the pH influence in the ki-
produced mainly 2-phenylethanol (20). The PAMO- netic resolution process, we performed the enzymatic
and CHMO-catalyzed oxidations of cis-substituted reaction in a low pH (pH 7.5) (Table 3). The results
vinyl boronic ester 16 allowed us to obtain exclusively revealed that the enantioselective behavior is superior
benzyl alcohol (18).
at more neutral pH, as previously shown for this bio-
The reaction of gem-substituted vinyl boronic ester catalyst when performing the BV oxidation of racemic
[
14]
1
7 with PAMO, M446G PAMO and CHMO led only aldehydes and ketones. (S)-26 can be obtained with
to the respective boron oxidation product (acetophe- 91% ee, while a 49% conversion was achieved in a
none 21). However, when the oxidation reaction was process with a good enantioselectivity (E=33).
catalyzed by HAPMO some amount of phenol 22,
produced by hydrolysis of phenyl acetate (a BV oxi-
dation product from acetophenone) was achieved. In Conclusions
order to identify the intermediates described above,
additional reactions were carried out and monitored In summary, we have found that for a set of substrates
in time (after 1 and 4 h, see Supporting Information). with two oxidizable moieties, boron oxidation cata-
Based on the intriguing results above, which re- lyzed by the studied BVMOs occurs rather than the
vealed that BVMOs can be effective in boron oxida- BV oxidation or epoxidation reaction. Based on these
tions, we decided to evaluate the enzymatic kinetic promising synthetic properties of BVMOs concerning
resolution of chiral boron-compounds (23 and 26) cat- boron chemistry, one of the studied biocatalysts,
alyzed by PAMO (Table 3).
PAMO, has also been examined in the enzymatic ki-
Compounds 23 and 24 were synthesized from the netic resolution of racemic boron compounds. This re-
[
12]
reduction of 14 and 17 catalyzed by Rh, respective- vealed that PAMO is very well suited to perform
ly. The oxidative kinetic resolution reaction was car- enantioselective boron oxidations, demonstrating for
ried out by the same methodology applied for com- the first time that a BVMO can be employed in the
pounds 1–5 and 12–17 using pH 9.0. After 24 h no re- kinetic resolution of boron compounds, affording
action was observed for 4,4,5,5-tetramethyl-2-(octan- chiral alcohols and chiral boron compounds in high
2
-yl)-1,3,2-dioxaborolane 23.
However, the enzymatic oxidation of 24 showed ex- BVMOs may be very elegant tools in boron chemistry
cellent results in which the (S)-borane was oxidized to through their chemo- and stereoselective behavior.
enantiomeric excess. This study demonstrates that
2172
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2169 – 2173

Selective Oxidations of Organoboron Compounds Catalyzed by Baeyer–Villiger Monooxygenases
Experimental Section
[4] For some recent reviews, see: a) G. de Gonzalo, M. D.
Mihovilovic, M. W. Fraaije, ChemBioChem 2010, 11,
2
9
1
208–2231; b) M. M. Kayser, Tetrahedron 2009, 65,
47–974; c) M. D. Mihovilovic, Curr. Org. Chem. 2006,
0, 1265–1287; d) N. M. Kamerbeek, D. B. Janssen,
General Procedure for Enzymatic Oxidation
Reactions
ꢂ
To a flask (2 mL, Eppendorf ) containing a solution of the
W. J. H. van Berkel, M. W. Fraaije, Adv. Synth. Catal.
003, 345, 667–678.
starting material (1M in DMSO, 5 mL), Tris/Cl buffer
2
(50 mM, pH 9.0 or 7.5, 440 mL), phosphite solution
(500 mM, 20 mL), NADPH (100 mM, 10 mL), PTDH
(100 mM, 5 mL) and enzyme (BVMO) (100 mM, 20 mL) were
[
5] a) G. de Gonzalo, D. E. Torres PazmiÇo, G. Ottolina,
M. W. Fraaije, G. Carrea, Tetrahedron: Asymmetry
2
miÇo, G. Ottolina, M. W. Fraaije, G. Carrea, Tetrahe-
dron: Asymmetry 2005, 16, 3077–3083.
006, 17, 130–135; b) G. de Gonzalo, D. E. Torres Paz-
added. Reaction mixtures were shaken at 200 rpm and 308C
(
(
PAMO and M446G PAMO mutant) or 150 rpm and 248C
HAPMO and CHMO) for the times established. The reac-
[
[
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tions were then stopped, the mixture extracted with EtOAc
3ꢃ0.5 mL), dried over Na SO and analyzed by GC, GC-
(
2
4
MS or chiral HPLC. The absolute configurations of chiral
compounds were established by comparison of HPLC chro-
matograms with the patterns described in previous experi-
[11]
ments for the known configurations. Control experiments
in absence of enzyme were performed for all substrates
tested, and no reaction was observed.
Steady-State Kinetics
[
Steady-state kinetic parameters for substrates 1 and 3 with
M446G PAMO mutant were estimated using substrate con-
centrations from 0.5 mM to 5 mM, 0.1 mM of NADPH,
0.05 mM of BVMO in Tris/HCl buffer (pH 9.0) and 1%
DMSO. Kinetic measurements by measuring the absorbance
decrease at 340 nm were performed on a Perkin–Elmer
Lambda Bio40 spectrophotometer at 258C. The obtained
data were fitted using the SigmaPlot program, version 10.0
for Windows.
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Acknowledgements
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Fraaije and Gonzalo de Gonzalo thank the EU-FP7 Oxy-
green project for financial support.
1
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2173