Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 19

terms of the dioxo species being a more powerful O-atom donor 61 ; a key
factor will be the nature of the ligand coordinated trans to the oxo ligand,
and to draw a general conclusion is likely untenable. The mechanistic details
of net O-atom transfer processes from metal-oxo species generally are of
intense current interest 14,15,32,80,84,85 .

3.2 Oxidation of thioethers

Selective of thioethers to sulfoxides is of industrial
importance 74,86-88 and has been accomplished for dialkyl sulfides using
complexes (porp = TMP, TDCPP) 74,89 . Kinetic and
data in benzene solution near ambient conditions were first interpreted in
terms of the chemistry shown in eq. 27. Under Ar, a stoichiometric reaction
gives the bis( O -bonded sulfoxide) product ( 14 ), while under or air the
labile sulfoxides are displaced to regenerate the trans -dioxo species and the

process becomes catalytic. The k values for the TMP system at 20°C are
0.0075, 0.012 and 0.11

respectively, for and

the differences resulting more from differences in than

values 74 ; as with the phosphine systems, the determined negative
values are consistent with O-atom transfer occurring via electronic coupling
induced by strong Ru=O vibrational motion 81 , the data implying an easier O-
atom transfer process with bulkier substrates.
Alkylaryl and diaryl sulfides do not react with the trans -dioxo
species 74,89,90 presumably because of their decreased nucleophilicity. Of note,
an Fe(TPP)Cl/PhIO system effects oxidation of dialkyl, alkylaryl and diaryl
sulfides, via a proposed ClFe(TPP)O intermediate 91 ; the axial ligand trans to
the oxo ligand is likely critical. The chemistry of eq. 27 would imply that
is a more efficient O-atom donor than
while data for phosphine oxidation (see Section 3.1) imply that the Ru-
monooxo species disproportionates more quickly than reacting with
phosphine. The kinetic data for thioether systems could be consistent with
the type of mechanism described for the phosphine systems, and a careful re-
evaluation of the thioether systems is needed. With Fe systems, of course,
disproportionation to a dioxo species is unknown. Relevant to this discussion
are the findings that LRu(porp)(O) species are better catalyst precursors than
the corresponding for oxidation of saturated hydrocarbons