ethyl analog, albeit at a slow reaction rate. The selectivity of CPO-catalyzed
propargylic hydroxylation was found to be sensitive with respect to the polarity
and the alkyne chain length [1479]. In addition, hydroxylation of aromatic C–H
bonds seems to be possible, as long as electron-rich (hetero)aromatics, such as
indol are used [1480, 1481].
Epoxidation of Alkenes Due to the fact that the asymmetric epoxidation of
alkenes using monooxygenase systems is impeded by the toxicity of epoxides to
microbial cells, the use of H 2 O 2 -depending peroxidases represents a valuable
alternative.
Chloroperoxidase-catalyzed epoxidation of alkenes proceeds with excellent
enantioselectivites (Scheme 2.175) [1482, 1483]. For styrene oxide it was demonstrated that all the oxygen in the product is derived from hydrogen peroxide, which
proves the validity of direct oxygen-transfer via the peroxygenase-path (Scheme
2.171) [1484]. Unfunctionalized cis-alkenes [1485] and 1,1-disubstituted olefins
[1486, 1487] were epoxidized with excellent selectivities. On the other hand,
aliphatic terminal and trans-1,2-disubstituted alkenes were epoxidized in low yields
and moderate enantioselectivities [1488].
Sulfoxidation Heteroatom oxidation catalyzed by (halo)peroxidases has been
observed in a variety of organic compounds. N-Oxidation in amines, for instance,
can lead to the formation of the corresponding aliphatic N-oxides or aromatic
nitroso or nitro compounds. From a preparative standpoint, however, sulfoxidation
of thioethers is of greater importance since it was shown to proceed in a highly
stereo- and enantioselective fashion. Moreover, depending on the source of the
haloperoxidase, chiral sulfoxides of opposite configuration could be obtained
(Scheme 2.176).
Chloroperoxidase from Caldariomyces fumago is a selective catalyst for the
oxidation of methylthioethers to furnish (R)-sulfoxides. Initial results were
H
H
R
2
R
1
OH
R 2
R 1
chloroperoxidase
(R) or (S)
H 2 O 2
H 2 O
R
1
R
2
Configuration
e.e. [%]
Ph
Me
(R)
9 7
Ph
Et
(S)
8 8
Et-C≡CMe
(R)
9 1
n-Pr-C≡CMe
(R)
8 7
AcO-CH 2 -C≡CMe
(R)
9 5
AcO-(CH 2 ) 2 -C≡CMe
(R)
8 3
Br-CH 2 -C≡CMe
(R)
9 4
Br-(CH 2 ) 2 -C≡CMe
(R)
9 4
Scheme 2.174 Benzylic and propargylic C–H hydroxylations
202
2 Biocatalytic Applications
propargylic hydroxylation was found to be sensitive with respect to the polarity
and the alkyne chain length [1479]. In addition, hydroxylation of aromatic C–H
bonds seems to be possible, as long as electron-rich (hetero)aromatics, such as
indol are used [1480, 1481].
Epoxidation of Alkenes Due to the fact that the asymmetric epoxidation of
alkenes using monooxygenase systems is impeded by the toxicity of epoxides to
microbial cells, the use of H 2 O 2 -depending peroxidases represents a valuable
alternative.
Chloroperoxidase-catalyzed epoxidation of alkenes proceeds with excellent
enantioselectivites (Scheme 2.175) [1482, 1483]. For styrene oxide it was demonstrated that all the oxygen in the product is derived from hydrogen peroxide, which
proves the validity of direct oxygen-transfer via the peroxygenase-path (Scheme
2.171) [1484]. Unfunctionalized cis-alkenes [1485] and 1,1-disubstituted olefins
[1486, 1487] were epoxidized with excellent selectivities. On the other hand,
aliphatic terminal and trans-1,2-disubstituted alkenes were epoxidized in low yields
and moderate enantioselectivities [1488].
Sulfoxidation Heteroatom oxidation catalyzed by (halo)peroxidases has been
observed in a variety of organic compounds. N-Oxidation in amines, for instance,
can lead to the formation of the corresponding aliphatic N-oxides or aromatic
nitroso or nitro compounds. From a preparative standpoint, however, sulfoxidation
of thioethers is of greater importance since it was shown to proceed in a highly
stereo- and enantioselective fashion. Moreover, depending on the source of the
haloperoxidase, chiral sulfoxides of opposite configuration could be obtained
(Scheme 2.176).
Chloroperoxidase from Caldariomyces fumago is a selective catalyst for the
oxidation of methylthioethers to furnish (R)-sulfoxides. Initial results were
H
H
R
2
R
1
OH
R 2
R 1
chloroperoxidase
(R) or (S)
H 2 O 2
H 2 O
R
1
R
2
Configuration
e.e. [%]
Ph
Me
(R)
9 7
Ph
Et
(S)
8 8
Et-C≡CMe
(R)
9 1
n-Pr-C≡CMe
(R)
8 7
AcO-CH 2 -C≡CMe
(R)
9 5
AcO-(CH 2 ) 2 -C≡CMe
(R)
8 3
Br-CH 2 -C≡CMe
(R)
9 4
Br-(CH 2 ) 2 -C≡CMe
(R)
9 4
Scheme 2.174 Benzylic and propargylic C–H hydroxylations
202
2 Biocatalytic Applications