Chemistry Reference
In-Depth Information
Nevertheless, the heterogeneous approach is highly desirable for com-
mercial applications and recent reviews have dealt with the subject of liquid-
phase oxidation of alcohols over supported transition metals.
65,66
This sec-
tion summarizes the latest research regarding heterogeneously catalyzed
processes for alcohol oxidation in the liquid phase; however, the reader
interested in this wide field is referred to the authoritative papers cited above
for further details.
d
n
4
r
4
n
g
|
1
1.3.1 Oxidation of Alcohols Using Single-Metal and Alloy
Catalysts
Heterogeneous, late transition metal catalysts - notably ruthenium, gold,
platinum and palladium supported catalysts - are particularly active for the
liquid-phase aerobic oxidation of alcohols, but other metals and metal
compounds have also been studied recently, including Ag
67
and Co
3
O
4
.
68
Cinnamyl alcohol (CA) and benzyl alcohol (BA) oxidation are often used as
model reactions. As with other alcoholic substrates, CA and BA oxidations
generally proceed through an aldehyde intermediate to the final acid
products. These two molecules are both highly reactive and the C
ΒΌ
C
bond present in the CA molecule may undergo side reactions, forming
3-phenylpropan-1-ol, 3-phenylpropionaldehyde and 3-phenylpropanoic acid
(Scheme 1.16). In fact, apart from the oxidation pathway, side reactions due
to hydrogen transfer and hydrogenolysis, depending on the reaction
conditions and the catalyst used, can be observed. As an example, the side
reaction forming 3-phenyl-1-propanol is typical for allylic alcohols and may
be considered a transfer dehydrogenation reaction in which the reactant
plays the role of hydrogen acceptor.
69
Benzyl alcohol was also reported to be subjected to different reactions
depending on the catalyst used and the reaction conditions (Scheme 1.17).
Indeed, in addition to the oxidation of BA to form benzaldehyde and benzoic
acid and benzoate, reactions such as (i) disproportionation to form toluene,
benzaldehyde and H
2
O and (ii) dehydration to form dibenzyl ether have been
reported.
70
.
OH
O
O
cinnamic acid
cinnamaldehyde
OH
OH
cinnamyl alcohol
O
O
3-phenylpropanoic acid
3-phenylpropionaldehyde
OH
3-phenylpropan-1-ol
Scheme 1.16 Reaction scheme for cinnamyl alcohol oxidation.
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