dianisidine), forming resonance-stabilized radicals. The latter undergo spontaneous
inter- or intramolecular C–C, C–O and/ or C–N coupling to yield dimers or polymers [1466–1468]. In certain cases, dimers, such as biaryls and aryl-ethers, have
been obtained (Scheme 2.173) [1469, 1470].
Oxidative Halogenation
A class of peroxidases – haloperoxidases – specializes in the peroxidation of
halides (Cl
À , Br
À
, I
À but not F
À
), thereby creating reactive halogenating species
(such as hypohalite), which in turn form haloorganic compounds
[1471, 1472]. These reactions are described in Sect. 2.7.1.
Oxygen Transfer
From a synthetic viewpoint, selective oxygen transfer via the peroxygenase-path is
particularly intriguing, because it is comparable to those catalyzed by
monooxygenases with one significant advantage – it is independent of redox
cofactors, such as NAD(P)H. Among the various types of reactions – C–H bond
oxidation, alkene epoxidation and heteroatom oxidation – the most useful transformations are described below.
Hydroxylation of C–H Bonds Heme-dependent chloroperoxidase (CPO) from
the marine fungus Caldariomyces fumago has been found to effect the hydroxylation of C–H bonds. The large-scale production of CPO is facilitated by the fact
that it is an extracellular enzyme, which is excreted into the fermentation medium
[1473–1475] and its crystal structure has been solved [1474]. In order to become
susceptible towards hydroxylation by CPO, the C–H bonds have to be activated
by a π-electron system. In the allylic position, hydroxylation is not very efficient
[1476], but benzylic or propargylic hydroxylation is readily effected to furnish the
corresponding sec-alcohols in high e.e. (Scheme 2.174) [1477, 1478]. CPO is very
sensitive with respect to the substrate structure as the stereochemistry of products
was reversed from (R) to (S) when the alkyl chain was extended from methyl to an
R
R
O -
O
-
N
N
R
N-OH
Ar
OH
OCH 3
OH
OCH 3
dimer
dimer / polymer
horseradish
peroxidase
horseradish
peroxidase
+
+
n
guaiacol
2 H 2 O
H 2 O 2
2 H 2 O
H 2 O 2
or
O
OCH 3
n
Ar
Scheme 2.173 Peroxidase-catalyzed oxidative coupling of aromatics and arylether formation
2.3 Oxidation Reactions
201
inter- or intramolecular C–C, C–O and/ or C–N coupling to yield dimers or polymers [1466–1468]. In certain cases, dimers, such as biaryls and aryl-ethers, have
been obtained (Scheme 2.173) [1469, 1470].
Oxidative Halogenation
A class of peroxidases – haloperoxidases – specializes in the peroxidation of
halides (Cl
À , Br
À
, I
À but not F
À
), thereby creating reactive halogenating species
(such as hypohalite), which in turn form haloorganic compounds
[1471, 1472]. These reactions are described in Sect. 2.7.1.
Oxygen Transfer
From a synthetic viewpoint, selective oxygen transfer via the peroxygenase-path is
particularly intriguing, because it is comparable to those catalyzed by
monooxygenases with one significant advantage – it is independent of redox
cofactors, such as NAD(P)H. Among the various types of reactions – C–H bond
oxidation, alkene epoxidation and heteroatom oxidation – the most useful transformations are described below.
Hydroxylation of C–H Bonds Heme-dependent chloroperoxidase (CPO) from
the marine fungus Caldariomyces fumago has been found to effect the hydroxylation of C–H bonds. The large-scale production of CPO is facilitated by the fact
that it is an extracellular enzyme, which is excreted into the fermentation medium
[1473–1475] and its crystal structure has been solved [1474]. In order to become
susceptible towards hydroxylation by CPO, the C–H bonds have to be activated
by a π-electron system. In the allylic position, hydroxylation is not very efficient
[1476], but benzylic or propargylic hydroxylation is readily effected to furnish the
corresponding sec-alcohols in high e.e. (Scheme 2.174) [1477, 1478]. CPO is very
sensitive with respect to the substrate structure as the stereochemistry of products
was reversed from (R) to (S) when the alkyl chain was extended from methyl to an
R
R
O -
O
-
N
N
R
N-OH
Ar
OH
OCH 3
OH
OCH 3
dimer
dimer / polymer
horseradish
peroxidase
horseradish
peroxidase
+
+
n
guaiacol
2 H 2 O
H 2 O 2
2 H 2 O
H 2 O 2
or
O
OCH 3
n
Ar
Scheme 2.173 Peroxidase-catalyzed oxidative coupling of aromatics and arylether formation
2.3 Oxidation Reactions
201