Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 32

in combination with PhIO are also active; however, yields do not
exceed 71% with ee values from 7 to 63% 136 . Comparison of data for
stoichiometric and catalytic epoxidations using and
in the presence and absence of PhIO led to the conclusion
that the dioxo-species was the key oxidizing intermediate using this
oxidant 136 . Formation of was suggested as an intermediate,
based on stoichiometric styrene oxidation seen in the presence of pyrazole
(Hpz), eq. 31, where was detected by UV-vis spectroscopy
(by comparison with data for an authentic sample). The pyrazolate complex

was similarly isolated from the reaction of
with an alkene in the presence of pyrazole 123 . Kinetic studies on the
stoichiometric oxidation of alkenes by in the presence of

pyrazole revealed the rate law: with k values

of and at 25°C for styrene, cis-

respectively 136 . The rate-determining step for the
O-atom transfer for epoxidation of was given as
formation of the radical intermediate that explained generation of some
oxide (Fig. 11). Such long-lived radical intermediates
have been invoked in earlier catalytic epoxidation work using homochiral
Fe- and Mn-porphyrins 135,142-144 .
The system (see Fig. 10) was also thought to
involve the Ru(VI)-dioxo species as the most active catalyst 133 , but with an
N-oxide as

stilbene and trans- stilbene,

-donor an alternative mechanism has been suggested (see Fig.
12) 135 . With such oxidants, there is a strong dependence of the reactivity and
selectivity of the epoxidations on the solvent and nature of the oxidant 135,137,
145-147 . A general increase in enantioselectivity for epoxidations in benzene
vs. those in (e.g. with for epoxidation of
styrene 42 vs. 5% 135 , and with 68 vs. 58% 137 (see
Table 1)) has been considered to result from a strong interaction of Ru with
the aromatic solvent 137 , and there is, for example, NMR evidence for
formation of species 59 . Association of benzene with
species (or perhaps some other active Ru(IV) or Ru(V)

O