Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

CHAPTER 2
mustard gas) with ambient air at room temperature (Scheme 2‐16) [226]. The activity of the truly
heterogeneous catalyst was ascribed to an interaction of redox active Ag and V. In this study, also
homogeneously dissolved AgNO3 catalyzed the ambient oxidation. The same group published an
example for a highly active homogeneous Au(III) catalyst [227] for which further examples exist
[228]. Homogeneous Cu complexes were also reported to oxidize thioethers with molecular oxygen
(e.g. [229]) or H2O2 (potentially enantioselectively [230, 231]). Immobilized analogous Cu complexes
were further reported [232, 233], however, examples of “classical” heterogeneous Cu catalysts for
sulfide oxidation are rare and limited in scope (e.g. aqueous H2S with Cu/carbon [234]). This and the
high amount of potent homogeneous catalysts leave substantial potential for further research.
2.3.9 Oxidation of silanes
Scheme 2‐16: Oxidation of 2‐chloroethyl ethylsulfide [226].
The oxidation of silanes to silanols over supported Ag and Au catalysts has been reported only
recently (Table 2‐9). Mitsudome et al. tested Ag and Au nanoparticles on various supports (e.g. TiO2,
SiO2 and Al2O3) for the oxidation of silanes [87, 235]. Hydroxyapatite turned out to be the best
support material both for Ag and Au. The oxidation with water as oxidant and solvent had various
advantages also including facile product separation. Both catalysts exhibited selectivities exceeding
99 %, so virtually no disiloxane usually being a common byproduct in silane oxidation could be found.
While gold was identified to be the most universal catalyst oxidizing both aliphatic (Table 2‐9, entry
1a) and aromatic silanes (Table 2‐9, entry 1b), silver was selective only to aromatic silanes (entry 2a,
Scheme 2‐17). The selectivity was ascribed to the aromatic moiety being necessary for binding of the
substrate to the silver particles. Silver was more active than gold catalyzing the reaction already at
room temperature (entry 2b) and ‐40 °C. The maximum TONs of 1600 were superior to other
homogeneous and heterogeneous systems. The high activity in the oxidation of aliphatic silanes is
not a feature unique to gold. Though the use of THF was necessary, Pt nanoparticles in PHMS
(poly(methylhydro)siloxane) were found to be as universal as gold at significantly higher reaction
rates than silver also using water as an oxidant (entry 3a and 3b) and equally high selectivities [236].
Cu is principally also a feasible catalyst material. Dissolved Cu salts catalyzed the aerobic oxidation
via a Cu(II)/Cu(I) redox cycle [237, 238]. Reoxidation of Cu(I) with molecular oxygen is facilitated by
the higher stability of Cu(II) in complexes which would also be an important design parameter for
heterogeneous catalysts.
S
Ag
Cl 5PV2Mo10O40 S
O 2
54
O
Cl