Chemistry Reference
In-Depth Information
ascribed this to the instability of the material under
alkaline conditions. Similarly, Pagliaro
et al
. [115]
showed that reaction of
D
-methylglucose to the
uronic acid was carried out efficiently using their
TEMPO system (strictly speaking prepared from a
non-templated sol-gel synthesis and thus unlikely to
be structured regularly) and NaOBr. Bolm & Frey
[114] found that the immobilised TEMPO would
selectively oxidise primary alcohols to aldehydes in
very good yields, with little over-oxidation.
Other papers dealing with enantioselective epoxi-
dations will be discussed in Section 3.5.
An alternative oxidation pathway is followed with
the catalysts studied by Tsuruya
et al
. [116]. They
have published details of a Cu-impregnated MCM-
41 catalyst that caused the oxidative dimerisation
of the same phenol in air (Fig. 7.27). The different
catalyst/oxidant combination leads to complete
selectivity towards dimerisation products, without
any hydroxylation being observed. The Ti-peroxide
system is known to produce Ti-OOH species that
are capable of transferring oxygen to electron-rich
substrates, whereas the role of Cu(II) is to carry out
a one-electron oxidation of the phenol, followed
by dimerisation, (some) subsequent oxidation to the
quinonoid product and presumably aerial oxidation
of Cu(I) to Cu(II). The exact direction that such re-
actions take can be influenced profoundly by the
choice of reaction conditions (e.g. solvent, ligands
around the metal) [117,118].
Thus, the use of Cu(II)-HMS and hydrogen per-
oxide has been found to be active in the hyroxyla-
tion of phenol itself [119]. The authors found that
the combination of the Cu-containing material and
aqueous hydrogen peroxide effected the oxidation
of phenol to dihydroxybenzenes in a conversion of
36%. No organic co-solvent was used. Indeed, the
use of solvents (methanol or acetone) caused the
reaction to stop completely. Interestingly, the Ti-
containing analogue of this material displayed no
activity in the hydroxylation reaction, in contrast to
the findings of Pinnavaia
et al
. This curious result
may be explicable on the basis of the aqueous envi-
ronment of the catalysts—Pinnavaia's study utilised
acetone as the solvent (possibly to aid the trans-
fer/activation of hydrogen peroxide) and it is well
known that TS-1 is at its most active when a co-
solvent such as methanol is used [11-13].
Arene hydroxylation
Several authors have investigated the hydroxylation
of aromatics to give phenols. This important oxida-
tion type is, like epoxidation, one of the typical TS-
1 oxidations. Interestingly, little appears to have
been done with Ti-containing micelle-templated
silicas in the hydroxylation of aromatics. However,
other metal-containing materials have been investi-
gated with some success.
Pinnavaia
et al
. [87,90] described the use of Ti-
containing materials prepared via different routes for
the oxidation of 2,6-di-
t
-butylphenol to the corre-
sponding quinone (Fig. 7.27). Using hydrogen per-
oxide as oxidant, they showed that Ti-HMS
(prepared via the neutral amine templating route
[43]) outperformed Ti-MCM-41. Anatase (TiO
2
) was
almost inert and the small pore size of TS-1 pre-
cluded any oxidation. Decomposition of hydrogen
peroxide was minimal in each case (<10%).
Fig. 7.27
Oxidation of phenols using two different oxidation
catalysts (MTS
=
micelle-templated silica).
OH
O
OH
Ti-MTS
Cu-MTS
O
OH
