Oxidation of organoselenium compounds. A study of chemoselectivity of phenylacetone monooxygenase

Article abstract of DOI:10.1016/j.molcatb.2011.07.018

Organoselenium acetophenones oxidation using enzymatic reactions has been developed and chemoselectivity of phenylacetone monooxygenase (PAMO) with selenium-containing ketones has been explored. We discovered that this biocatalyst prefers selenium oxidation, which leads to selenoxide in excellent conversion, over Baeyer-Villiger oxidation.

Full text of DOI:10.1016/j.molcatb.2011.07.018

Journal of Molecular Catalysis B: Enzymatic 73 (2011) 63–66
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Journal of Molecular Catalysis B: Enzymatic
journal homepage: www.elsevier.com/locate/molcatb
Oxidation of organoselenium compounds. A study of chemoselectivity of
phenylacetone monooxygenase
Leandro H. Andrade∗, Eliane C. Pedrozo, Henrique G. Leite, Patrícia B. Brondani
Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, SP 05508-900, São Paulo, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Organoselenium acetophenones oxidation using enzymatic reactions has been developed and chemose-
lectivity of phenylacetone monooxygenase (PAMO) with selenium-containing ketones has been explored.
We discovered that this biocatalyst prefers selenium oxidation, which leads to selenoxide in excellent
conversion, over Baeyer–Villiger oxidation.
Received 21 April 2011
Received in revised form 24 July 2011
Accepted 25 July 2011
Available online 30 July 2011
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Selenium
Phenylacetone monooxygenase
Chemoselectivity
Selenoxide
1. Introduction
reactions, due to their wide applications under mild condi-
tions. An interesting class of enzymes that catalyzes a variety
Organoselenium compounds are very useful synthetic tools in
organic synthesis, they are regularly employed in the prepara-
tion of many organic compounds including natural products [1].
Among the various types of organoselenium compounds, selenox-
ides are frequently used as versatile intermediates. In addition,
these compounds are known as mild oxidant reagents of several
organic compounds oxidation (olefins, thiols, sulfides, phosphines,
hydrazines, amines, alcohols, catechols) and selenoxide can also
be used as catalysts in hydrogen peroxide activation for bromide
anions oxidation [2]. Moreover, selenoxides that have a ␤-hydrogen
atom suffer the well-known ␤-elimination reaction, which has
been used to prepare alkenes and ␣,␤-unsaturated ketones [3]. In
the absence of a ␤-hydrogen atom, the selenoxide is stable and
easily isolable.
Selenoxides are generally prepared from the corresponding
selenides using common oxidants such as hydrogen peroxide [4],
ozone [5], tert-butyl hydroperoxide [6], oxyridines [7], sodium
metaperiodate [8], m-CPBA [9], (dichloroiodo)benzene [7], N-
chlorosuccinimide [10], tert-butyl hypochloride [9] or nitrogen
oxides [11]. These methods usually are carried out with great
amounts of organic solvents [12]. Despite of the organoselenium
compounds versatility, very few studies about the enzymatic oxi-
dation of selenium have been published to date [13]. In addition,
enzyme-catalyzed transformations are environmentally friendly
of oxidations are the Baeyer–Villiger monooxygenases (BVMOs):
flavin-containing and NAD(P)H-dependent enzymes that catalyze
the incorporation of one oxygen into organic substrates [14].
BVMOs are known to perform the aldehydes and ketones oxi-
dation to their corresponding esters, the heteroatoms oxygenation
(sulfur, phosphorus, nitrogen, boron, selenium) and even epoxi-
dation reactions [15]. BVMOS have gained strong interest, mainly
due to its high performance for selective oxidation [16]. One exam-
ple is the chemoselective transformation of heterocyclic ketones
into lactones leaving the hetoroatom (ex. sulfur) untouched [17].
Usually, sulfoxidation reaction is chosen as an easy model to eval-
uate the BVMOs activity [16]. Among the BVMOs, phenylacetone
monooxygenase (PAMO) is a very interesting biocatalyst, which
offers unique and attractive features such as thermostability and
organic solvent tolerance [18]. Therefore, this study was aimed
at evaluating the chemoselectivity of PAMO in organoselenium
acetophenones biooxidation. In addition, another purpose of this
study is also to determine the behavior of the selenium-containing
ketones towards catalytic conditions using a monooxygenase.
2. Experimental
2.1. General
Oxidation reactions were performed using purified enzymes.
Recombinant phenylacetone monooxygenase (PAMO) from Ther-
mobifida fusca [19] was overexpressed and purified as previously
described (gift from Prof. M.W. Fraaije) [20]. Glucose-6-phosphate
∗
Corresponding author. Tel.: +55 11 3091 2287; fax: +55 11 3815 5579.
E-mail address: leandroh@iq.usp.br (L.H. Andrade).
1381-1177/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcatb.2011.07.018

64
L.H. Andrade et al. / Journal of Molecular Catalysis B: Enzymatic 73 (2011) 63–66
O
O
O
RBr, NaBH4
MeOH, 0 ºC
NaNO2, HCl 2M
KSeCN, 0 ºC
H2N
NCSe
RSe
3a, 4-SeMe (55%)
2a, 4-SeCN (64%)
2b, 3-SeCN (28%)
2c, 2-SeCN (68%)
1a, 4-NH2
1b, 3-NH2
1c, 2-NH2
3b, 4-SeCH2Ph (71%)
3c, 3-SeMe (48%)
3d, 3-SeCH2Ph (45%)
3e, 2-SeMe (45%)
3f, 2-SeCH2Ph (41%)
Scheme 1.
dehydrogenase from Leuconostoc mesenteroides, glucose-6-
phosphate and NADPH were purchased from Sigma–Aldrich.
The organoselenium acetophenones (3a–f) were synthesized as
previously described [21].
2.3.4. 1-(3-(Benzylseleninyl)phenyl)ethanone (4d), orange oil
1H NMR (200 MHz, CDCl3) ı = 2.51 (s, 3H), 3.95 (d, J = 11.4 35 Hz,
1H), 4.19 (d, J = 11.2 Hz, 1H), 6.88–6.93 (m, 2H), 7.21–7.27 (m, 3H),
7.52–7.61 (m, 2H), 7.81 (d, J = 0.8 Hz, 1H), 8.05 (dd, J = 1.6 and 5.6 Hz,
1H). 13C NMR (50 MHz, CDCl3) ı = 26.59, 58.63, 126.02, 128.54,
129.44, 129.88, 130.30, 130.74, 137.58, 40 140.29, 196.74. HRMS
[ESI(+)], calcd [M+H]+: 307.0237, found: 307.0241.
2.2. General procedure for oxidation of the organoselenium
acetophenones (3a–f) using monooxygenase (PAMO)
2.3.5. 1-(2-(Methylseleninyl)phenyl)ethanone (4e), orange solid
1H NMR (200 MHz, CDCl3) ı = 2.72 (s, 6H), 7.68 (dt, J = 1.4 and
6.2 Hz, 1H), 7.90 (dt, J = 1.2 and 6.4 Hz, 1H). 13C NMR (50 MHz,
CDCl3) ı = 26.37, 38.33, 126.06, 130.76, 130.89, 134.67, 199.74.
HRMS [ESI(+)], calcd [M+H]+: 230.9924, found: 230.9915.
To an erlenmeyer flask (100 mL) containing the starting mate-
rial solution (0.047 mmol in 190 ␮L of DMSO) was added Tris/Cl
aqueous buffer at pH 9.0 (50 mM, 19 mL), glucose-6-phosphate
(0.1 mmol), NADPH (0.002 mmol), glucose-6-phosphate dehydro-
genase (0.62 mM) and PAMO (2.5 mM). Reactions were shaken at
30 ◦C for 24 h, and then extracted with dichloromethane (3 × 5 mL).
The organic phase was dried over MgSO4, and the solvent was
evaporated under vacuum. The crude reaction was purified by
preparative thin-layer chromatography (eluent: ethyl acetate).
2.3.6. 1-(2-(Benzylseleninyl)phenyl)ethanone (4f), orange solid
1H NMR (200 MHz, CDCl3) ı = 2.70 (s, 3H), 4.02 (d, J = 11 Hz,
50 1H), 4.14 (d, J = 11 Hz, 1H), 7.06–7.18 (m, 5H), 7.55–7.7 (m,
2H), 7.94–8.1 (m, 2H). 13C NMR (50 MHz, CDCl3) ı = 26.45, 58.77,
127.13, 127.81, 128.12, 129.80, 130.54, 198.50. HRMS [ESI(+)], calcd
[M+H]+: 307.0237, found: 307.0225.
2.3. General procedure for oxidation of the organoselenium
acetophenones (3a–f) using sodium periodate [22]
3. Results and discussion
Methanol (3 mL) and water (1 mL) were mixed in a 50 mL
round-bottomed 5 flask containing organoselenium acetophe-
nones (3a–f). The reaction was cooled at 0 ◦C and sodium periodate
(1 mmol) was then added. The mixture was shaken at 0 ◦C for 1 h
and dichloromethane (5 mL) was added, and the resulting solution
10 was washed with aqueous NaCl saturated solution (2 × 5 mL).
The organic phase was dried over MgSO4, and the solvent was
evaporated under vacuum. The crude reaction was purified by
preparative thin-layer chromatography (eluent: ethyl acetate) pro-
viding the desired product (65–90%).
Initially, in order to evaluate the chemoselectivity between
selenium oxidation and Baeyer–Villiger (BV) oxidation catalyzed
by PAMO, six selenium-containing acetophenones (3a–f) were
selected as substrates. The organoselenium acetophenones (3a–f)
were prepared from commercially available ortho-, meta- and para-
amino-acetophenones (1a–c) (Scheme 1). The in situ preparation
of diazonium salt from 1a–c followed by the addition of KSeCN
afforded the selenocyanate acetophenones (2a–c) (28–68%). Then,
the alkylation of the selenium atom was carried out with NaBH4
and the appropriate alkyl halide to give the organoselenium ace-
tophenones (3a–f) (41–71%) [21].
With organoselenium acetophenones in hands, we started the
investigation on PAMO-catalyzed reactions (Scheme 2). The oxi-
dation reactions were coupled to a second enzymatic reaction
catalyzed by glucose-6-phosphate dehydrogenase (G6PDH), in
order to regenerate the consumed NADPH [23]. All oxidations were
carried out in a Tris/Cl aqueous buffer at pH 9.0 (Table 1). The buffer
was saturated with oxygen for 5 min; otherwise the conversions
would be low.
The organoselenium acetophenones (3a–d) oxidation reaction
gave the corresponding selenoxides (4a–d) in high conversion after
24 h reactions. In all these reactions, PAMO was chemoselective
by only catalyzing selenium oxidation, leaving the ketone moiety
intact. While a NAD(P)H-regenerating system was employed, no
product from a Baeyer–Villiger reaction was observed.
On the other hand, the PAMO-catalyzed reaction of 4-
hydroxyacetophenone, which contains an electron donor group,
yield ester (4-hydroxyphenyl acetate) in excellent conversion
(>99%) (see supporting material). As the methylselane and
2.3.1. 1-(4-(Methylseleninyl)phenyl)ethanone (4a), orange solid
1H NMR (200 MHz, CDCl3) ı = 2.65 (s, 6H), 7.85 (d, J = 8.6 Hz,
2H), 8.11 (d, J = 8.6 Hz, 2H). 13C NMR (50 MHz, CDCl3) ı = 26.74,
37.43, 125.79, 129.35, 139.45, 147.18, 197.03. HRMS [ESI(+)], calcd
[M+Na]+: 252.9744, found: 252.9735.
2.3.2. 1-(4-(Benzylseleninyl)phenyl)ethanone (4b), orange solid
1H NMR (200 MHz, CDCl3) ı = 2.64 (s, 3H), 3.98 (d, J = 11.4 Hz,
1H), 4.23 (d, J = 11.4 Hz, 1H), 6.94–6.97 (m, 2H) 7.24–7.30 (m, 3H),
7.49 (d, J = 8.4, 2H), 7.99 (d, J = 8.4, 2H). 13C NMR (50 MHz, CDCl3)
ı = 26.76, 59.08, 125.30, 126.50, 128.71, 128.78, 139.40, 197.18.
HRMS [ESI(+)], calcd [M+H]+: 307.0237, found: 307.0233.
2.3.3. 1-(3-(Methylseleninyl)phenyl)ethanone (4c), orange oil
1H NMR (200 MHz, CDCl3) ı = 2.67 (s, 6H), 7.65–7.72 (m, 1H), 30
7.98 (d, J = 7.2 Hz, 1H), 8.11 (d, J = 5.6 Hz, 1H), 8.30 (s, 1H). 13C NMR
(50 MHz, CDCl3) ı = 26.74, 125.28, 129.75, 130.14, 130.97, 138.23,
196.77. HRMS [ESI(+)], calcd [M+H]+: 230.9924, found: 230.9919.

L.H. Andrade et al. / Journal of Molecular Catalysis B: Enzymatic 73 (2011) 63–66
65
O
O
PAMO
RSe
RSe
O
4a-f
3a-f
R = Me or Bz
NADP+
NADPH
D-Glucose-6P
D-Gluconate-6P
G6PDH
Scheme 2.
O
O
NaIO4, MeOH
H2O
RSe
Table 1
Oxidation reaction of organoselenium acetophenones (3a–f) mediated by PAMO.a
RSe
65-90%
O
4a-f
3a-f
Entry
Substrate
Product
Conversion
(%)b
R = Me, Bz
Scheme 3.
O
O
O
1
>99
>99
benzylselane groups are moderate electron donor groups, we
expected that PAMO was able to oxidize the carbonyl group
(BV-oxidation) instead selenium atom. However, PAMO eas-
ily transformed the selenium moieties into the corresponding
selenoxides, which contain an electron-withdrawing group, pre-
venting the subsequent BV-oxidation. When the starting materials
were 3e and 3f, no reaction was observed. These results can be
attributed to the proximity of the two moieties (organoselane and
ketone group), which result in steric hindrance (Table 1, entries 5
and 6). Selenoxides leads to racemization process in the presence of
water, even in basic condition. Therefore, we could not determine
the enantiomeric excess value for those compounds [24].
MeSe
MeSe
O
3a
4a
4b
O
2
BzSe
BzSe
O
3b
O
O
O
MeSe
MeSe
3
4
76
We also performed the chemical oxidation of 3a–f in order to
obtain the organoselenoxides (4a–f). The oxidation of 3a–f was car-
ried out with sodium periodate, methanol and water at 0 ◦C, which
led to the selenoxides in 65–90% yield (Scheme 3).
3c
4c
4d
O
O
O
BzSe
BzSe
4. Conclusions
>99
3d
In conclusion, the present results demonstrate that oxidation
mediated by PAMO is a good way to oxidize selenides to selenoxides
from organoselenium acetophenones. The oxidation reactions were
chemoselective and most of the conversions were very efficient
(up to >99% conversion). This study provides a new enzyme-based
approach to perform mild oxidation of selenium-containing com-
pounds using aqueous medium and biocatalyst (monooxygenase).
O
O
5
(–)c
SeMe
SeMe
O
O
3e
3f
4e
4f
O
Acknowledgements
6
(–)c
The authors thank CNPq, CAPES and FAPESP for financial
support, and also to Prof. M.W. Fraaije for kindly providing pheny-
lacetone monooxygenase samples.
SeBz
SeBz
O
a
Reactions were carried out by using selenide 3a–f (0.047 mmol in 190 ␮L of
DMSO) in presence of Tris–HCl buffer at pH 9.0 (50 mM, 19 mL), G6P (0.1 mmol,
27 mg), NADPH (0.002 mmol, 1.6 mg), G6PDH (0.62 mM) and PAMO (2.5 mM).
Appendix A. Supplementary data
b
Conversion was measured by 1H NMR (the reaction mixture was analyzed by
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcatb.2011.07.018.
NMR).
c
No reaction was observed.

66
L.H. Andrade et al. / Journal of Molecular Catalysis B: Enzymatic 73 (2011) 63–66
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