Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 49

there was no spectroscopic evidence for this intermediate. Such species are
commonly proposed for alcohol oxidation using other Ru complexes 171-175 .
For the porphyrin dioxo system, the “more sensitive” (Ru=O) IR region

was investigated, but the data were inconclusive 169 . A primary kinetic
isotope effect at 20°C of ~15 (e.g., for and
for with no isotope effect found for oxidation of
is consistent with the rate-determining hydride transfer. Large
kinetic isotope effects of between 8 176 and 50 177 have been reported for
cleavage of bonds in alcohol oxidation by non-porphyrin trans-
and complexes, respectively. A Hammett plot of log
against for oxidation the p -substituted MeO-, H- and F-benzhydrols gave
a linear relationship with a slope (the F-system being least active),
implying that transfer of electron density from the to the Ru oxidant
occurs in the formation of the transition state; the Ru=O...H bonding within
(

v

25 ) is akin to H-bonding but involving a hydride H-atom and an

electrophilic O-atom 169 .
The presence of a small and reasonably constant quantity of
(see Section 3.3.) was detected by NMR during the
stoichiometric oxidation of and its formation was suggested to
take place concomitantly with that of the ketone via a net loss of from
the intermediate and protonated ketone (Fig. 25). Production
of the bis(alkoxide) was written as involving a ligand exchange reaction