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Mendeleev Commun., 2002, 12(2), 45–46
Oxidative and anaerobic reactions of benzyl alcohol catalysed by a Pd-561
giant cluster
Serhiy S. Hladyi,a Mykhailo K. Starchevsky,*a Yuriy A. Pazdersky,a Michael N. Vargaftikb and Ilya I. Moiseevb
a ‘Sintez’ Research Institute, 82300 Borislav, Ukraine. Fax: +380 3248 41369; e-mail: main@insyntez.com.ua
b N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russian Federation.
Fax: +7 095 954 1279; e-mail: mvar@igic.ras.ru
10.1070/MC2002v012n02ABEH001568
The unusual redox disproportionation of benzyl alcohol to benzaldehyde and toluene catalysed by the Pd561phen60(OAc)180 giant
cluster under anaerobic conditions was found, whereas in an O2 atmosphere the Pd giant cluster catalyses benzyl alcohol oxidation
to benzaldehyde and inhibits its further oxidation.
Giant cluster Pd561phen60(OAc)180 1 is known to catalyse the
4.0
polar oxidation of alcohols by dioxygen to the corresponding
aldehydes under mild conditions (1 atm O2, 20–60 °C).1,2 In the
case of lower aliphatic alcohols, the aldehyde formed is further
10.0
oxidised into the corresponding carboxylic acid, anhydride and
1
ester. In a parallel route, the aldehyde and starting alcohol pro-
duce acetal.3–5 In this work, we found that, unlike aliphatic
2
alcohols, benzyl alcohol is transformed in a solution of cluster 1
under O2 into benzaldehyde and toluene as the major reaction
products, and the minor reaction products are benzoic acid,
benzene and CO2 (Table 1).†
2.0
8.0
The initial rate of PhCH2OH consumption at 60 °C and [1] =
= 4.55×10–5 mol dm–3 was 6.3×10–2 mol dm–3 min–1. After 120 min
of the reaction, the conversion of benzyl alcohol reached 40%.
As can be seen in Figure 1, the curves for both PhCH2OH
consumption and product accumulation have no induction period.
Our experiments showed that benzaldehyde undergoes autoxida-
tion by dioxygen with a noticeable rate already at 20 °C when
neither initiators nor catalysts are present. Meanwhile, in the
presence of cluster 1, benzaldehyde is rather stable in air and
even under O2.
3
5
4
40
80
t/min
Figure 1 Curves for benzyl alcohol consumption and product accumulation
in the presence of cluster 1 (4.55×10–5 mol dm–3) under O2 (1 atm) in neat
benzyl alcohol at 60 °C: (1) benzyl alcohol, (2) benzaldehyde, (3) toluene,
(4) benzene and (5) benzoic acid.
For instance, benzaldehyde (0.77 mol dm–3 solution in MeCN)
was completely oxidised to benzoic acid upon stirring under O2
(1 atm) for 1 h at 60 °C (initial autoxidation rate was 9.0×10–4 s–1).
Unlike this, under the same conditions but in the presence
of the Pd-561 cluster (7.0×10–5 mol dm–3), the concentration of
benzaldehyde remained almost unchanged and O2 was not ab-
sorbed during 600 min.
In a solution of isopropanol, the autoxidation of benzaldehyde
is much slower (initial rate is 1.7×10–5 s–1 at 60 °C), and iso-
propanol does not undergo oxidation. Meanwhile, when cluster
1 was introduced into the solution after 200 min, the O2 con-
sumption rate increased and PriOH oxidation to acetone and
water started. Since that moment, the concentration of benzalde-
hyde became constant (Figure 2).
clarify the reaction mechanism, we studied the behaviour of
benzyl alcohol in the presence of cluster 1 in an Ar atmosphere.
When a solution of cluster 1 (4.60×10–5 mol dm–3) in thoroughly
degassed benzyl alcohol was stored under Ar for 120 min at
60 °C, ~8% benzyl alcohol was converted into benzaldehyde
and toluene. As shown in Figure 3, both of the reaction pro-
ducts were formed in equal amounts.
Hence, the Pd-561 cluster causes the redox disproportionation
of benzyl alcohol to benzaldehyde and toluene under anaerobic
conditions.
2PhCH2OH ® PhCHO + PhMe + H2O
(1)
All these facts point to a parallel rather than consecutive
formation of the products of PhCH2OH conversion. In order to
and at least a fraction of benzaldehyde and toluene that formed
during the oxidation of benzyl alcohol can be due to the anaero-
bic conversion of the alcohol.
All these findings point to the fact that cluster 1 not only
provides the catalytic polar oxidation and redox disproportiona-
tion of benzaldehyde but also retards its free-radical oxidation.
It is most likely that the metal core of the cluster can terminate
free-radical oxidation chains similarly to other metal complex
inhibitors.6
The found synchronism in the accumulation of benzaldehyde
and toluene during the contact of benzyl alcohol with the Pd-561
giant cluster implies two parallel reaction pathways, which are
originated from two different modes of PhCH2OH coordination
by the Pd atoms of the metal core of cluster 1. The first
coordination mode is the oxidative addition of the PhCH2OH
molecule via C–H bond dissociation [Scheme 1, route (a)], and
the second mode occurs via C–OH bond cleavage [Scheme 1,
route (b)]. As a result, four coordinated (adsorbed) species occur
at the surface layer of the metal core of cluster 1: [PhCH2],
[PhCHOH], [H] and [OH]. The formation of benzaldehyde and
Table 1 The products of cluster 1-catalysed benzyl alcohol reactions (neat
benzyl alcohol as a solvent, 1 atm O2, 60 °C, [1] = 4.55×10–5 mol dm–3 and
reaction time 120 min).
Concentration/
mol dm–3
Yield based on PhCH2OH
consumed (%)
Reaction product
PhCHO
PhMe
PhCOOH
PhH
3.09
0.96
0.27
0.084
0.08
70.2
21.8
6.1
1.9
1.8
CO2
†
Experiments were carried out according to published procedures1,3 in
a 20 cm3 glass reactor equipped with a sampler, a thermostat, a vibration
stirrer (frequency of 200–450 min–1) and a gas burette for the measure-
ment of gas volumes to within 0.1 cm3. Cluster 1 (0.020 g) and the
working solution (5.0 cm3) were loaded in the reactor. The reaction solu-
tion was rigorously shaken under O2 at 60 °C. The reactants and reaction
products were analysed by GLC. The reaction rates were determined by
O2 absorption and the GLC analysis of liquid reaction products.
– 45 –
Hladyi, Serhiy S.
