Chemistry Reference
In-Depth Information
SO
3
H
N
SO
3
H
Au Au
Au Au
OH
N
Au
O
2
n
R
1
R
2
MeO
MeO
2 H
+
, 2 e
-
2 H
+
, 2 e
-
H
H
SO
3
H
SO
3
O
Au Au
Au Au
N
N
H
2
O
Au
R
1
R
2
n
MeO
MeO
Scheme 7.7 Redox-active PMAS in multicatalytic aerobic oxidation of alcohols.
7.3 Synergistic Bimetallic Systems
Synergistic activities with bimetallic systems have been reported for the
catalytic aerobic oxidation of alcohols. In most cases, the first transition
metal was the oxidant whereas the second one activated molecular oxygen to
regenerate the catalyst in the good oxidation state (Scheme 7.8).
Murahashi's group reported a dual ruthenium-cobalt catalytic system for
the aerobic oxidation of alcohols in presence of acetaldehyde (Table 7.2).
18
Secondary alcohols were converted to ketones in good to excellent yields,
whereas primary alcohols produced the corresponding carboxylic acid. The
proposed mechanism can be rationalized by the formation of a peracid
intermediate from acetaldehyde and the Co
III
complex in presence of mo-
lecular oxygen, via a radical pathway. The peracid intermediate oxidized the
Ru
III
complex to form the catalytically active low-valent oxoruthenium
complex that performed the oxidation of the alcohol substrate (Scheme 7.9).
Other cooperative systems, namely homogeneous Os-Cu
19
and Os-Cr
20
bimetallic catalysts, have also been developed. The Os-Cu bimetallic com-
plexes were effective for the oxidation of primary and secondary allylic and
benzylic alcohols to produce aldehydes and ketones (Equations 7.4
19a
and
7.5
19b
). The exact dual activity between both transition metals is unknown
R
1
M
A
n+2
M
B
n
OH
O
2
R
2
R
1
M
A
n
O
M
B
n+2
H
2
O
R
2
Scheme 7.8
Synergistic bimetallic systems for catalytic aerobic oxidations of
alcohols.
Search WWH ::
Custom Search