Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 22

Both O-atoms of can be transferred to an olefin in a
stoichiometric reaction to generate 2 moles of epoxide, and the species also
catalyzes of olefins at ambient conditions in benzene with high
selectivity and yields with norbornene; the order of olefin reactivity
is: norbornene > > cyclooctene >
while epoxidation of cis- and proceeds with retention
of configuration 60 . The suggested catalytic cycle shown in Figure 6 implies
the key disproportionation of a Ru(IV)=O intermediate to the
and Ru(II) species (see also Section 2, eq. 16, and Section 3.1, eq. 25) 60 ,

while a plausible route for reoxidation of Ru(II) is shown in eq. 14 40,74 .In

catalyzed epoxidations of cis -but-2-ene in benzene, using in the

presence of excess a 10:1 ratio of to was produced,
and the 2 different pathways outlined in Figure 7 were considered in order to
rationalize the data 40 : the dioxo species could be formed by
disproportionation, or by cleavage of a dimer. Separate
experiments had shown that the dioxo species undergoes oxygen exchange
slowly with (see Fig. 7), and that epoxides do not undergo exchange;
thus the pathway would be expected to generate a 1:1 mixture of
the and epoxides, while the disproportionation pathway
would yield in principle 100% The conclusion was that the
observed 10:1 ratio thus favored the disproportionation pathway 40 , which is
reasonable, with the presumption that undergoes rapid exchange,
and that the does not. Although these data suggest that the
fastest step in the epoxidations (Fig. 6) is the disproportionation reaction
(ref. 40, see Fig. 7), incorporation from added to a reaction
mixture does not necessarily imply the intermediacy of a metal-oxo species;
other intermediates such as M-OOH or M(O-atom donor) could also have
exchangeable O-atoms 103 .
These selective of olefins, a commercially important
process 104 , are based on the somewhat exotic hindered porphyrins, but are
highly significant. Such O-atom transfer via an

or air-generated metal-