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with air. The catalyst can be easily prepared under air using commercially
available reagents and is effective for a wide range of primary and secondary
alcohols. Reactions proceed under mild conditions (air, 40-80 1C) and in a
variety of solvents.
d
n
4
r
4
n
g
|
1
1.2.2.2 Pyridine Ligands
Neutral and cationic pyridine-based palladium compounds have been de-
veloped by Oberhauser and co-workers as ecient catalysts for the atom-
economic aerobic oxidation of unprotected diols to yield chemoselectively
the corresponding hydroxy ketone.
23
A comparative catalytic study showed
that the bis-cationic precursor (Figure 1.1), in combination with an external
base (i.e.,K
2
CO
3
) in a 19 : 1 v/v toluene-DMSO solvent mixture, outperforms
neutral complexes. The eciency of bis-cationic catalyst precursors has been
found to depend on (i) the coordination properties of the diol employed with
respect to Pd(
II
)(e.g., 1,2-diols show higher conversion than 1,3-diols) and
(ii) the bulkiness and coordination properties of the counterion (e.g., the
OTs precursor showed the highest catalytic activity and the BAr
4
counterpart
the lowest).
1.2.2.3 Palladium Pincer Complexes
The use of palladium NCN and CNC pincer complexes have been found to be
effective in the aerobic oxidation of secondary benzyl alcohols at atmos-
pheric pressure in PEG-400, a sustainable reaction medium with excellent
yields and functional group tolerability
24
(Scheme 1.5). The recycling of ac-
tive catalytic species is performed up to the fifth run, while catalyst loadings
decreased to 10
8
mol%, thus achieving significant turnover number (TON)
and turnover frequency (TOF) values. The same conditions proved to be
effective for the aerobic oxidation of benzyl methylene compounds, a little-
explored process, by palladium catalysts.
.
(OTf)
2
N
N
Pd
N
N
Figure 1.1 Pyridine-based palladium best precursor for the aerobic oxidation of
unprotected diols.
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