Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 24

of the Ru-dioxo could become the rate-determining step and a dependence
on would be expected; alternatively, genuine saturation kinetics might
be operative, implying the build-up of a kinetically important 'Ru-dioxo-
olefin' ternary intermediate (cf. the benzhydrol system in Section 3.6.).
Irradiation of the cyclooctene system with visible light from a tungsten lamp
gives a five-fold increase in the epoxidation rate 40 , and this could result from
photolytic decarbonylation of Ru(TMP)CO 109 .
The isolable ethylene complex is stable to torr at
20°C, but the cyclohexene analogue readily dissociates the coordinated
alkene 77 . The competition of alkene binding versus reactivity to give
dioxo species is reminiscent of mechanisms in catalytic homogeneous
hydrogenation, where hydride and unsaturated routes have been identified 110 ;
the former operates by olefin attack on a metal-hydride, and the latter by
attack of on a metal-alkene species.
Epoxides coordinate to Ru(II)-porphyrins, and Ru(TDCPP)(CO)-
(styrene oxide) has been characterized by X-ray analysis 84 . The bent
geometry of the coordinated epoxide ring (Fig. 8) may be similar to the
transition state geometry for olefin epoxidation, with a side-on approach of
the alkene allowing for favourable interactions between its filled
and the metal-oxygen orbitals; solution data show that the
coordinated epoxide rotates about the Ru-O axis 84,105 . The corresponding
thioepoxide and aziridine complexes also have bent geometries analogous to
that of the epoxide 40 . Several other mechanistic possibilities, invoking a
metallaoxetane, carbon radical, carbocation, ion-pair, or charge transfer
species as an intermediate or transition state, have been proposed for
epoxidations catalyzed by 1st-row metalloporphyrin monooxo species 13,32,111 .
A further complication in the catalyzed olefin epoxidation is a catalyzed
cis-trans isomerization of the coordinated epoxide 84,105 . For example, in a
reaction that is 1st-order in metal complex, catalyzes
isomerization of cis- or oxide in benzene to give a 1:5
equilibrium mixture of the cis and trans forms. The non-hindered tetra- p -