Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 33

intermediates) has been suggested to affect the solution structure of the
chiral cavity associated with chiral induction 135 . Further, the dependence of
enantioselectivity on the nature of the oxidant has been considered to
indicate that the structure of the active oxidant includes an axially
coordinated molecule of the O-atom donor; for example, a Ru(porp)(O)-
(pyNO) species has been proposed as an active intermediate in 2,3-dimethyl-
2-butene epoxidation catalyzed by (Fig. 12) 137 . Such possible
intermediates have been discussed prior to reports on the asymmetric
epoxidation (see Section 3.1.). The initial cycle starts with slow oxidation of
the olefin by to give and epoxide (step a), the
monooxo complex then reacting rapidly with N-oxide to form complex ( 15 )
(step b). ( 15 ) is then considered to effect very rapid epoxidation with
liberation of the pyridine (step c). Regeneration of from ( 15 )
(step d) was ruled out, as this step was slower than epoxidation via ( 15 ). The
role of coordinated pyNO was rationalized in terms of its effect on the chiral
environment on the trans oxo-coordination site 137 . Much improved chiral
induction resulted from the introduction of Cl-atoms into the meta-positions
of the bridged phenyl rings in (Fig. 10): thus, under the catalysis
conditions listed in Table 1, styrene was epoxidised with 79% ee at 551
turnover numbers, while cis- and yielded epoxides
with 57 and 69% ee at 244 and 487 turnovers, respectively. Trans -stilbene
was also oxidized to a mixture of epoxides with 38% ee at 242 turnovers.
The authors concluded that overall, the observed chiral induction was higher
for terminal and trans -olefins, versus cis -olefins 138 .