Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 27

in effects the stoichiometric epoxidation of
olefins such as norbornene, styrene and cis- and trans -stilbene according to
eq. 29; cis -stilbene yields cis- and trans -epoxides in a 1:2.7 ratio, and trans-
stilbene gives mainly

trans -epoxide 67 . The supposed Ru(IV)=O co-product

reacts with present to form inactive and no catalysis

occurs even up to 10 atm The 2nd-order rate constant for reaction 29

at 25°C 67 . In pyridine solution, a with norbornene was

reaction similar to 29 generates and two equivalents of epoxide.

In EtOH, the monooxo species is thought to be stabilized as
Ru(OEP)O(EtOH) and a very slow catalytic epoxidation, observed at 1 atm
was attributed to slow of Ru(IV)=O to Ru(VI)-dioxo
species 67 ; whether disproportionation of the Ru(IV)=O occurs under these
conditions (eq. 16) is unclear.
As noted for the thioether oxidations, introduction of halogen substituents
into the porphyrin, especially in positions to prevent sterically formation of
Ru species, can increase and/or prolong catalytic activity. More
generally, extensive halogenation of TPP-type porphyrins at the meso-
phenyl rings and positions gives rise to the so-called third
generation metalloporphyrins 9,92,94 , and excitement became intense when
such Fe-porphyrins were found to effect catalytic of light
alkanes with high activity, and a mechanism involving high-valent Fe=O
intermediates was proposed 92 ; the mechanism was shown subsequently to be
free-radical in nature, involving hydroperoxide decomposition pathways
(eqs. 6, 7) 121 . Such substituents on the porphyrin ring can also modify the
conformation of the porphyrin, and out-of-plane distortion can enhance
catalytic activity, as first suggested for some Fe(III)-porphyrin systems. 121
One rationale for increased activity via out-of-plane distortion is an imposed,
so-called “unidirectionality of electron transfer” 122 .
The highly distorted (X-Ray data revealed that its
precursor, Ru(DPP)(CO)(py), also exhibits both saddle and ruffle
distortion) 123 catalyzes in benzene-MeCN (9:1) some epoxidation of
norbornene, styrene, cyclohexene, cyclooctene and cis -stilbene under 1 atm
with turnovers of 8 - 40 in 4 h before deactivation of catalyst 123 ; the
numbers are comparable with those for and greater than for
and There were also co-oxidation products:
benzaldehyde (7 turnovers) and trace phenylacetylaldehyde were formed
from styrene, cyclohex-2-en-1-ol (20 turnovers) and cyclohex-2-en-1-one
(11 turnovers) from cyclohexene, and trace benzaldehyde and trans- stilbene