Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 28

oxide (3 turnovers) from cis -stilbene. did not oxidize

under these conditions. Stoichiometric alkene oxidation by
in in the presence of pyrazole was also studied; allylic
C-H oxidation was seen for cyclohexene, while styrene and cis -stilbene gave
solely styrene oxide and cis -stilbene oxide, respectively. the
final Ru product, was characterized by X-Ray 123 . The Stoichiometric alkene
oxidations obeyed a rate law of the form =

trans- stilbene

k [Ru(porp)][alkene], with k

values for norbornene and styrene (in at 25.9°C) of 3.8 and
respectively, but whether this referred to a radical process or an O-
atom transfer process was uncertain. Use of perhalogenated Ru porphyrins
certainly leads to radical autoxidation processes 65,124 . This is reflected in the
fact that is significantly less active catalytically than the
carbonyl for of cyclohexene 124 ; a total
turnover of 20 to radical products (epoxide, 2-cyclohexen-1-ol, 2-
cyclohexen-1-one) was achieved using M in
at ambient conditions. In contrast, can catalyze
similar radical of cyclohexene (and styrene) with turnovers up
to 300; cyclooctene gave only epoxide, but such high product selectivity is
not unusual for this substrate even in radical, autoxidation processes 125 . Of
note, these systems were photo-initiated with visible light. Closely
related are the autoxidations catalyzed by or the
carbonyl precursor 65 . Use of M of these complexes in neat hydrocarbon
substrate at ~90°C gives extremely efficient catalyzed autoxidations;
turnovers of up to are found for non-selective cyclohexene oxidation
(trace cyclohexene hydroperoxide was also detected), while cyclooctene
gives >80% selectivity to epoxide 65 . Corresponding autoxidations using
TDCPP systems were completely inhibited by addition of a radical inhibitor
such as BHT 65 . The TDCPP-C 8 systems similarly catalyze autoxidations of

saturated hydrocarbons; see Section 3.4 for further details on radical
processes, where the presence of the porphyrin ligand is not always essential.
Reports have appeared on the rates of decomposition of cyclohexyl
hydroperoxide (an intermediate in the industrial oxidation of
cyclohexane 126,127 ) to cyclohexanol and cyclohexanone catalyzed by
Ru(porp)CO and

l

, TDCPP,
TMCPP, TMP, TPP) either in solution or anchored to polystyrene or
silica 128-130 . The systems were studied in 20 : 1 at 25°C,
when decompositions in the 28-66% range were observed after 2 h, and
close to 100% after 48 h 129,130 . Several, plausible reaction pathways were
presented for decomposition of the alkyl hydroperoxides 130 .
Introduction of bulky and chiral substituents at the 5,10,15,20 (meso)-
positions of the porphyrin ring allows for aerobic, enantioselective
epoxidation of olefins 131 . Use of chiral Fe(III)- and especially Mn(III)-

systems (porp = t CPP ,m C t PP