3+
presence of Mn heterolytic bond cleavage of the O–O bond
is the preferred route leading to a higher selectivity to
2+
-methylbenzyl alcohol. In contrast, Co induces an acceler-
2
ation of the radicalic pathway by a homolytic O–O bond
cleavage. Note that the higher maximal concentration of
2
+
2
-methylbenzyl hydroperoxide in the presence of Co
is in
line with the increased tendency for the formation of radicals.
In the absence of catalysts, 2-methylbenzyl hydroperoxide
decomposed only above 2 mol% and the ratio of aldehyde
to alcohol was 0.7.
Fig. 4 Reaction network of the selective oxidation of o-xylene.
The present results show that hydrophobic organofunctio-
2
+
3+
nalized Co and Mn containing polysiloxanes are highly
efficient catalysts for the radical chain initiation and the
decomposition of the intermediate 2-methylbenzyl hydro-
peroxide during the selective liquid phase oxidation of
o-xylene under solvent free conditions. These catalysts are
highly stable and did not leach metal cations to a detectable
degree (see ESIw). Ongoing research shows that these robust
catalysts can be applied for the selective catalytic oxidation of
a wide variety of substrates.
Fig. 4 summarizes the reaction network. The novel material
activates the substrate by hydrogen abstraction and, thus,
catalyzes the formation of the primary product 2-methyl-
benzyl hydroperoxide. However, it also catalyzes its decom-
position to the secondary products, o-tolualdehyde and
2
-methylbenzyl alcohol. o-Toluic acid is only formed from
o-tolualdehyde. Further oxidation of o-toluic acid was not
observed, as the presence of the carboxy group reduces the
activity of the other methyl group. The –I effect of the carboxy
group reduces the electron density of the aromatic ring in
o-toluic acid and, in consequence, reduces the stability of a
potential benzyl radical, which would be the intermediate of
further oxidation. In the case of o-xylene the reactivity of
o-toluic acid is estimated to be a tenth of the o-xylene
Notes and references
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2
+
3+
Co
and Mn
catalysts different ratios of aldehyde and
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¨
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2
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This journal is c The Royal Society of Chemistry 2011