16
M.V. Kirillova et al. / Journal of Catalysis 272 (2010) 9–17
with a high selectivity of 96% after 3.5 h of reaction at 60 °C (entry
10). Further increase in the reaction time does not lead to some-
how better results. The selective TEMPO-mediated oxidations of
GLY to ketomalonic acid [79] or DHA [80] have been reported
but require the use of either the NaOCl/Brꢂ oxidant or electro-
chemical systems, respectively.
These and other research lines are planned to be explored in fu-
ture developments, aiming also at the search for new hydrosoluble
metal catalysts and efficient systems thereof for the selective oxi-
dative transformations of alkanes, alcohols and glycerol into valu-
able chemicals.
In most of the known heterogeneous systems [51–60], the oxi-
dation of glycerol does not stop at the stage of DHA, proceeding
more deeply with formation of various acid derivatives as the main
products. However, DHA is an important chemical with recognized
uses [51–55,81] in cosmetic industry (artificial suntans), pharma-
cological and nutritional compositions, and as an intermediate in
organic synthesis, whose commercial production is limited to bio-
logical oxidation of GLY [81]. The formation of DHA in rather good
yields, albeit with modest selectivities, has been previously
achieved in the continuous aerobic oxidation of glycerol on heter-
ogeneous metallic catalysts [57,82,83]. In our study, the obtained
DHA yields are comparable to those achieved in the recently re-
ported (i) H-transfer dehydrogenation of GLY to DHA, catalyzed
by organometallic iridium complexes [84] and (ii) heterogeneous
oxidation of GLY by the Au/CeO2/O2 system [85]. However, those
processes exhibited significantly lower selectivities to DHA in com-
parison with our work.
Acknowledgments
This work was supported by the Brazilian National Council on
Scientific and Technological Development (CNPq, Brazil; Grant
Nos. 552774/2007-3, 305014/2007-2), the State of São Paulo Re-
search Foundation (FAPESP, Brazil; Grant Nos. 2005/51579-2,
2006/03996-6), the Russian Foundation for Basic Research (Grant
No. 06-03-32344-a) and the Foundation for Science and Technol-
ogy (FCT), Portugal, its PPCDT (FEDER funded) and ‘‘Science
2007” programs. M.V.K., A.M.K. and G.B.S. express their gratitude
to the CNPq, FAPESP and the Faculdade de Química, Pontifícia Uni-
versidade Católica de Campinas for making it possible for them to
stay at this University as invited scientists. A.M.K. and M.V.K.
acknowledge respectively the Calouste Gulbenkian Foundation
(Portugal) and the FCT for travel grants. We are grateful to Mr.
A.J. Bonon for some GC–MS analyses.
Appendix A. Supplementary data
4. Conclusions
Supplementary data associated with this article can be found, in
In the present study, we have shown that the combination of
the hydrosoluble tetracopper(II) complex 1 with tert-butyl hydro-
peroxide oxidant gives rise to a versatile system for the oxidative
functionalization of various substrates that include alkanes, alco-
hols and glycerol. A very interesting feature of this system consists
in unusual product distribution patterns and selectivity parame-
ters obtained when using different types of alkane substrates. In
particular, the values of regio-, bond and stereoselectivity parame-
ters in the 1/TBHP system indicate that the oxidations can possibly
proceed in a hydrophobic pocket (cleft) containing some interme-
diate copper species; their nature is still to be established. This
contrasts significantly to the previously studied 1/H2O2 system
[42], wherein hydroxyl radicals play a crucial role in alkane oxy-
genations. Another difference concerns the fact that alkane oxida-
tions with catalyst 1 and TBHP do not require the addition of an
acid promoter, which is typically needed to activate 1 in the oxida-
tions with H2O2 [40–42]. The absence of an acid promoter eventu-
ally led to the preservation of an original structure of complex 1,
with strongly hindered copper-containing reaction centers. As a
consequence, the system exhibits a noticeable selectivity in the al-
kane oxidations.
The work also shows that the 1/TBHP system is suitable for the
selective oxidation of secondary alcohols to the corresponding ke-
tones, leading to product yields up to 82% and catalyst turnover
numbers up to 1200 (for isopropanol oxidation). In addition, these
reactions can proceed in the absence of MeCN solvent. The primary
alcohols appear to be more inert (maximum 22% product yield in
the oxidation of propanol), and their oxidations are less selective.
Moreover, we have extended the substrate versatility of catalyst
1 to the oxidation of glycerol, thus providing one of the first exam-
ples (see [78,86]) of its homogeneous metal-catalyzed oxidation. In
spite of the difficulties in achieving the selective transformation of
glycerol to dihydroxyaldehyde under mild conditions, all the tested
systems (1/H2O2, 1/TBHP and 1/Air/TEMPO) are rather promising
and exhibit different and interesting features. When operating
with an excess of GLY, the combination of 1 with H2O2 appears
to be particularly advantageous in terms of selectivity and oxidant
efficiency. However, further optimization of all these systems to
envisage higher conversions of GLY should be undertaken.
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