Chemistry Reference
In-Depth Information
Recently, higher activities were reached using RuCl
2
(PPh
3
)
3
as a catalyst
with ionic liquids as solvents. These compounds have been tested as en-
vironmentally friendly solvents for a wide variety of catalytic organic re-
actions.
27
In this particular case, tetramethylammonium hydroxide and
Aliquat 336 (tricaprylmethylammonium chloride) were used as solvents and
rapid conversion of 1-phenylethanol and benzyl alcohol was observed.
28
Moreover, the tetramethylammonium hydroxide/RuCl
2
(PPh
3
)
3
could be
reused after extraction of the product.
Many ruthenium complexes have been widely used as catalysts for
hydrogen-transfer reactions. These systems can be readily adapted to the
aerobic oxidation of alcohols by employing molecular O
2
, in combination
with a hydrogen acceptor as a co-catalyst, in a multistep process. For
example, B¨ckvall and co-workers used low-valent ruthenium complexes,
such as RuCl(OAc)(PPh
3
)
3
, in combination with hydroquinone and a cobalt-
Schiff's base complex, cobalt(salophen)(PPh
3
)[H
2
salophen
¼
N,N-bis(salicy-
lidene)-o-phenylenediamine] (Figure 3.1).
29
The low-valent ruthenium com-
plex reacts with alcohols to give aldehyde and ketone products together with
the formation of ruthenium hydride species. The latter undergoes hydrogen-
transfer to benzoquinone to give hydroquinone with concomitant regener-
ation of the low-valent active ruthenium species. The cobalt-Schiff's base
complex catalyzes the subsequent aerobic oxidation of hydroquinone to
benzoquinone to complete the catalytic cycle. Optimization of the electron-
rich quinone, combined with so-called 'Shvo' ruthenium catalyst
(Figure 3.2), led to one of the fastest catalytic systems reported for the oxi-
dation of secondary alcohols.
30,31
The regeneration of benzoquinone can also be achieved with molecular O
2
in the absence of the cobalt co-catalyst. Ishii and co-workers
32
showed that a
combination of a catalytic amount of RuCl
2
(PPh
3
)
3
, hydroquinone and
d
n
4
r
4
n
g
|
0
.
N
N
Co
O
O
PPh
3
Figure 3.1
Structure of Co(salophen)(PPh
3
).
O
O
Ph
Ph
H
Ph
Ph
Ph
Ph
H
Ph
Ph
Ru
Ru
CO
OC
CO
CO
Figure 3.2
Structure of 'Shvo' ruthenium catalyst.
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