Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 40

biphasic system ((hexane and (9:1)) : water = 10 : 1), preference for

the cleavage of -double bonds yields a mixture of aldehydes (15'- , 12'-

and l0'-apocarotenals (1:0.95:0.5)) in 30% yield. The “simpler”
system also can mediate oxidative cleavage of
to give a mixture of

E

and

Isomerization of trans- to cis -isomers of first takes place,

followed by fast epoxidation of the -isomers. Simultaneously oxidation

of the initially present trans- C=C double bonds yields the corresponding 5,6-
and 5,8-epoxides, with subsequent cleavage leading to the mixture of
products (Fig. 18) 158 .

trans

3.4 Oxidation of saturated hydrocarbons

A marginally catalytic oxidation of cyclohexane using a
system was mentioned above (Section 3.3.). The
species is thermally unreactive toward cyclooctane at 1 atm
at 70°C, but some O-atom insertion into saturated C-H bonds has been
achieved under photolytic conditions and with the electrochemically
generated species 40 ; few details are available, but the latter

system showed for adamantane oxidation the usual radical selectivity

(tertiary > secondary carbon).
Work in this laboratory has shown also that the complexes
(porp = TMP, TDCPP, and are “practically” inactive for
thermal of saturated hydrocarbons 65 . Some activity data for
0.2 mM Ru solutions in benzene under air at ~25°C for “optimum” substrates
such as adamantane and triphenylmethane at 6 mM did show selective
formation of 1-adamantol and trityl alcohol, respectively, but with turnover
numbers of only ~0.2 per day; the maximum turnover realized was ~15 after
40 days for the TDCPP system! Nevertheless, this was a non-radical
catalytic processes; there was < 10% decomposition of the
and a genuine O-atom transfer process was envisaged 65 . Quite remarkably
(and as mentioned briefly in Section 3.3), at the much lower concentration of
0.05 mM, in neat cyclooctene gave effective oxidation.
For example, at 90°C under 1 atm an essentially linear oxidation rate
over 55 h gave about ~70% conversion of the olefin with ~ 80% selectivity
to the epoxide; however, the system was completely bleached after ~ 20 h
and, as the activity was completely inhibited by addition of the radical
inhibitor BHT, the catalysis is operating by a radical process, but in any case
the conversion corresponds to a turnover of 110,000 65 ! As in related
Fe(porp) systems (Section 3.3, ref. 121), the Ru(porp) species are considered
to be very effective catalysts for the decomposition of hydroperoxides (eqs.
6, 7) 65,124,128-1 30 . The radical nature was more obvious in a corresponding