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complexes, may activate dioxygen for aerobic oxidations.
34
The structure of
the majority of these complexes includes cyclic N
4
or open-chain N
2
O
2
donor
equatorial ligands. Within this field, the simple and inexpensive ligand
dimethylglyoxime (DH
2
), in combination with cobalt nitrate and TEMPO,
eciently accomplished the aerobic oxidation of various alcohols (including
primary and secondary benzylic, allylic and aliphatic alcohols), which were
quantitatively converted to the corresponding aldehydes or ketones at 70 1C
under 0.4 MPa dioxygen pressure.
35
Several other metal salts (based on Co,
Cu, Fe, Mn and Ni) and reaction conditions were screened, but led to
poorer results. Mechanistic investigations shed light on the role of the three-
component Co(NO
3
)
2
/DH
2
/TEMPO catalyst. During oxidation, the in situ-
generated cobaloxime and NO
x
played crucial roles in the activation of
dioxygen, thus resulting in two concerted catalytic pathways: cobaloxime-
activating-dioxygen TEMPO-catalyzed and NO-activating-dioxygen TEMPO-
catalyzed aerobic oxidation of alcohols. Therefore, the present system could
eciently catalyze the aerobic oxidation of aliphatic and secondary alcohols
without the need for an additional base.
d
n
4
r
4
n
g
|
1
1.2.2.7 Role of Naphthoxide in Iron-Catalyzed OKR
Within the scope of the oxidative kinetic resolution of secondary alcohols
(OKR),
36
a naphthoxide-bound iron(salan) complex has been recently re-
ported as a good example of an ecient iron-catalyzed aerobic oxidative
kinetic resolution of secondary alcohols.
37
The substrate scope was studied
with secondary alcohols in the presence of several phenol derivatives, which
are reluctant to undergo oxidative coupling, with good to high enantiomeric
differentiation using molecular oxygen. The ligand plays the main role [the
iron(salan) complex itself is inactive] and the modified complex represents a
step forward in the iron-catalyzed aerobic kinetic resolution of secondary
alcohols (Sekar and co-workers reported iron-catalyzed OKR using O
2
as the
terminal oxygen, but the substrate was limited to benzoins and a catalytic
amount of TEMPO was necessary
38
).
.
1.2.2.8 Phenanthroline Ligands
Various N-N-ligands such as phenanthroline-based ligands have been in-
vestigated. For copper catalysts, the electronic properties of the supporting
ligand have been shown to affect the catalytic eciency, identifying electron-
rich 1,10-phenanthroline derivatives as better catalysts for the aerobic
oxidation of alcohols.
39
On the other hand, for palladium catalysts, the oxi-
dative degradation of the ligand has been confirmed as an 'Achilles heel' due
to the rapid deactivation of the catalysts.
40
The longer catalyst lifetimes of Pd
complexes bearing the 2-CF
3
-substituted ligand 4-methyl-2-(trifluoromethyl)-
1,10-phenanthroline (tfmm-phen) reveal that the inhibition of ligand
oxidation can lead to stronger catalysts for aerobic alcohol oxidation.
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