Chemistry Reference
In-Depth Information
In biological systems, a number of copper-containing enzymes are
known to catalyze metabolically relevant oxidation reactions using
molecular oxygen as the terminal oxidant. One such metalloenzyme is
galactose oxidase (GO), a mononuclear copper enzyme, that catalyzes the
aerobic oxidation of primary alcohols to aldehydes with concomitant
reduction of dioxygen to hydrogen peroxide.
8
The copper(
II
)centeris
surrounded by four protein-derived ligands (Tyr272, His495, His496 and
His581) and a labile aqua ligand, resulting in a square-pyramidal co-
ordination geometry.
9,10
Structural studies on the 'inactive' form of GO
revealed an unusual tyrosinate ligand (Tyr272) covalently connected to a
cysteine (Cys228) (Figure 2.1).
9
In the 'active' form of GO, Tyr272 becomes
one-electron oxidized to form a tyrosyl radical. The copper(
II
)-phenoxyl
radical intermediate is the key feature of the mechanism of the two-
electron oxidation of alcohols.
11
Inspired by the coupling of an organic cofactor and reversible
copper(
I
)/copper(
II
) redox chemistry in the unique O
2
-dependent reaction
catalyzed by GO, a variety of synthetic systems have been developed.
12-18
These systems exhibit versatile reactivity towards alcohols and provide a
valuable insight into the development of alcohol oxidation catalysts. Over
the past several decades, extensive studies have been carried out on copper-
based catalysts for the aerobic oxidation of alcohols.
2,3,5,19
This chapter
presents an overview of the advances made in this direction.
d
n
4
r
4
n
g
|
0
.
HO
O
O
N
N
N
N
N
N
Cu
II
Cu
II
-1e
RCH
2
OH
Cu
II
O
H
2
O
H
2
O
O
O
O
- H
2
O
H
S
S
R
H
S
'Inactive'
'Active'
Proton-coupled
electron transfer
Tyr
495
-H
2
O
2
Tyr
272
O
2
H
2
O
His
581
HO
HO
N
N
N
N
Cu
II
O
Cu
I
- RCHO
O
His
496
Cu
H
H
2
O
O
Cys
228
H
R
H
S
S
Figure 2.1 Proposed catalytic cycle for the aerobic oxidation of primary alcohol
catalyzed by galactose oxidase.
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