Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 26

times of up to 3-6 days 113,114 . The cholesteryl acetate epoxidation revealed a
2 h induction period, with subsequent maximum rates roughly proportional
to the initial [Ru]; complete epoxidations after 5 h were realized for a
substrate : catalyst ratio of ~25. Reuse of the catalyst gave much lower rates
but the product selectivities were retained; catalyst deactivation was
attributed to the formation of hydroxylic species via protonation of the oxo
ligands 113 . Although slow, the epoxidations are synthetically useful.
With steroids having additional C=C bonds, either elsewhere in the
nucleus or in the C17 side-chain, epoxidation still predominates at the 5,6-
position; for conjugated 5,7-dienes, the epoxidation is regioselective at C5-
C6 but with loss of stereoselectivity 115 .
There is no effective epoxidation of cholest-4-ene-3-one, which has a
carbonyl conjugated with the olefin bond, but ketalization of the conjugated
carbonyl shifts the double bond to the 5,6-position and epoxidation occurs as
described above 114 . The non-conjugated cholest-5-ene-3-one yields a mixture
of epimeric 6-hydroxy-4-ene-3-ones, where the C=C bond has been shifted,
and a 4-ene-3,5-dione 116 ; this reaction was insensitive to the addition of a
radical inhibitor, indicating a non-radical process. Ru(TMP)CO also
catalyzes equally well this same reaction, but the true catalyst was again the
trans- dioxo species formed from the carbonyl via reaction with a 6-
hydroperoxy-4-ene-3-one (cf. Fig. 5), formed by radical-initiated, incipient
autoxidation of the cholest-5-ene-3-one.
The reactivity and high of the epoxidations have
been rationalized generally in terms of steric interactions between the
catalyst and substrate 117 . Steroids containing a Me group on C6, or a double
bond in ring C or D, are not epoxidized because of non-bonded interactions
between the steroid and the porphyrin ring for a side-on approach of the
alkene moiety, with the mean plane of the steroid orthogonal and not planar
to the Ru=O bond (Fig. 9; cf. Fig. 8). A rationale for the
has emerged from the structures of cholesteryl ethyl carbonate and its
epoxide, and is based on conformational differences between the two
structures along the C5-C10 bond 117 . Molecular modeling indicates that
epoxidation on the 'folds' the A-B junction and allows for an easier
approach of the substrate by releasing steric strain that results from
interactions between the C3-ester and porphyrin mesityl; epoxidation on the
has no effect on the A-B junction and is relatively disfavoured.
NMR data confirm that only the interacts with the metal in

Ru(TMP)CO 117 .
The activity and stereoselectivity of other related metalloporphyrin
systems depend on the metal and on the oxidant used; Fe(III)- or Mn(III)-
TMP complexes are weakly active or inactive for the aerobic
oxidations 9,14,40,118-120 .