Chemistry Reference
In-Depth Information
Technology exists both for the generation of halo-
gens in situ and for ex situ halogen recovery [65].
dichlorophenyl) porphyrin (MnTDCPP) was found,
along with high-yielding preparative methods.
Whenever organic ligands are present in oxidation
catalysts, some degradation over long time periods
almost invariably occurs. However, in the case of
porphyrins, this has been mitigated by electron-
withdrawing
meso
-substituents such as are present in
TDCPP, and in later generations by b-substitution
also [67]. With H
2
O
2
as the oxidant, homolytic rather
than heterolytic cleavage of the M-O-O-H interme-
diate can occur (Fig. 11.9).
Free hydroxyl radical is an indiscriminate oxidant
that will attack even the most unreactive ligands,
which is why H
2
O
2
often is found to be more aggres-
sive to porphyrins than hypochlorite, even though
the latter is a more powerful oxidant
per se
. The
electron-withdrawing substituents used to stabilise
porphyrins (and to make the oxene a more power-
ful electrophile) also make heterolytic cleavage less
favourable and hence actually encourage the forma-
tion of oxidising radicals.
Manganese porphyrins mainly depend on a co-
catalyst to assist heterolytic O-O cleavage by donat-
ing electrons as an axial ligand below the porphyrin
plane and by assisting in proton transfer to the OH
group to create O
+
H
2
as the leaving group. Imidazoles
are the traditional choice, reflecting the role of his-
tidine in natural peroxidases. However, in 'free'
systems these are degraded rapidly. Alternatives such
as ammonium acetate [68] and aliphatic amine N-
oxides [69] appear more practicable. The porphyrin
MnTDCPP oxidises terminal
n
-olefins such as non-
1-ene with high turnover and little loss of catalyst—
in fact, catalyst doses are so small as to make disposal
3 State of Progress on Main Catalytic
Systems
[11b]
3.1 Redox metal and oxo-metal complexes
Most work in this area has been done with iron and
manganese porphyrins [66], aimed largely at oxygen
transfer to form M
(
n
+2)+
=O oxene intermediates (see
Fig. 11.8). These were not thought at all relevant as
commercial catalysts until relatively recently, when
the relative robustness of Mn(III) tetrakis(
meso
-2,6-
b
b
meso
b
b
axial
N
N
meso
Fe
meso
N
axial
N
b
b
meso
b
b
Fig. 11.
8
Metalloporphyrin structure.
ox
.
O
-
R
(n+1)+
M
O
L
O
homolytic
ROOH
-H
+
n+
n+
M
M
heterolytic
ox
-
L
L
O
(n+2)+
M
Fig. 11.9
The O-O cleavage pathways
in metalloporphyrins.
L
