Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 34

Attractive from an environmental point of view, nitrous oxide has been
employed as an oxidising agent. acyclic and
steroidal substrates have been epoxidized using Ru(TMP)-based
systems 148,149 . Stoichiometric epoxidation of is
effected by generated via reaction of with
but catalytic oxidation of cholesteryl esters has been reported 149 .
Although the conditions are relatively severe: 0.2 mmol of olefins, 5.0 mol
% at 10 atm in chlorobenzene at 140°C, the yields and
selectivities to the steroidal epoxidation are good; only were
formed with yields from 48 to 99%. Use of polar solvents (THF, EtOH,
or use of other complexes (porp = TPP, OEP, 4-

MeO-TPP) resulted in no epoxidation 149 .
Application of Ru-porphyrin complexes for catalytic, homogeneous
alkene epoxidation is limited because of small turnover numbers, only
moderate enantioselectivities in the case of chiral systems, the cost of
porphyrin and the O-atom donor and often limited stability of
the porphyrin ring under the oxidizing conditions. In attempts to improve
possible applicability, Ru-porphyrins have been “heterogenized” on various
supports 150-152 , following methodology developed previously for Fe- and Mn-