4
A. Rostami et al. / C. R. Chimie xxx (2016) 1e5
Ph
after 3 h (Table 1, entry 5). To optimize the amounts of
catalyst, different amounts of VO(ephedrine)2@MNPs (0, 30
and 40 mg) were used in the oxidation of benzyl alcohol
using TBHP (1 mmol) in PEG at 80 ꢀC. In the absence of a
catalyst, the reaction was incomplete even after 24 h (Table
1, entry 6). When 30 and 40 mg of catalyst were used, the
reaction times were prolonged to 8 and 7 h under same
reaction conditions respectively (Table 1, entries 7 and 8). It
should be noted that the use of Fe3O4 MNPs instead of
VO(ephedrine)2@MNPs gives a lower yield of benzaldehyde
even after prolonging the reaction time (Table 1, entry 9).
As a result, the optimized reaction conditions were iden-
tified as using 50 mg of VO(ephedrine)2@MNPs in PEG as a
solvent at 80 ꢀC.
MNPs
O
V
N
O
N
O
MNPs
Ph
VO(ephedrine)2@MNPs
tBuOOH
Ph
O
O
N
V
O
OH
N
O
tBu
O
R1
R2
tBuOOH
Ph
In order to generalize the scope of the reaction, a series
of alcohols was subjected to oxidation under the optimized
reaction conditions, the results are presented in Table 2. As
shown in Table 2, various types of primary benzylic alco-
hols, including those with both electron-withdrawing and
electron-donating groups, were selectively converted to
the corresponding carbonyl compounds in excellent yields
under optimal reaction conditions (Table 2, entries 1e6).
Interestingly, 2-methyl benzyl alcohol as a model for hin-
dered primary benzylic alcohols was also successfully
oxidized to its corresponding carbonyl compound,
although a longer reaction time was required (Table 2,
entry 3). The primary benzylic alcohols with electron-
withdrawing groups show less reactivity and their oxida-
tion requires some more of TBHP (4 mmol) (Table 2, entries
4e6). The oxidation of 2-phenyl ethanol as a model for
primary aliphatic alcohols was satisfactorily subjected as
well (Table 2, entry 8). In general, alcohols containing
heterocyclic moiety are highly challenging substrates for
oxidation in most transition-metal catalyst systems
because they have a tendency to bind to transition metals
and can act as catalyst deactivators. Another important
aspect of this method is the successful oxidation of furfuryl
alcohol and 3-pyridinemethanol to give the expected al-
dehydes (Table 2, entries 9 and 10). Furthermore, benzylic
and cyclic secondary alcohols could be effectively oxidized
using the present method to give excellent yields of the
corresponding carbonyl products (Table 2, entries 11e14).
On the basis of a previously reported mechanism for the
oxidation of alcohols using peroxides (H2O2 or TBHP) in the
presence of vanadium based catalysts, one explanation for
this process is that the mechanism involves radical in-
termediates [30]. Another explanation is that the oxidation
process involves the reaction of the TBHP with VO(ephe-
drine)2@MNPs to form an active peroxidovanadium (V) in-
termediate, which subsequently oxidizes the substrate
molecule and returns to the original state [31,32]. In order to
conduct mechanistic investigation, the oxidation reaction of
Ph
Ph
O
O
O
O
tBu
O
O
N
N
V
N
N
V
O
H
O
O
O
R1
R2
Ph
Ph
O
tBuOH
+
R1
R2
Scheme 2. Proposed mechanism.
exact mechanism of the reaction is not clear and the actual
role of this catalyst should be further studied in detail.
For practical purposes, the ability to easily recover and
recycle the catalyst is highly desirable. The reusability of
this magnetic nanocatalyst was examined using benzyl
alcohol as a model substrate. After the first use of the
catalyst in the oxidation of benzyl alcohol to give benzal-
dehyde, the catalyst was rapidly separated by an external
magnet and was washed thoroughly with EtOAC. It was
reused for subsequent experiments under similar reaction
conditions. As shown in Fig. 1, the catalyst can be recycled
up to six runs without significant loss of activity.
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
t
benzyl alcohol using BuOOH as an oxidant and VO(ephe-
drine)2@MNPs asa catalystwascarried out inthepresenceof
2,20-azobis(isobutyronitrile)asa radicaltrap. Itwasobserved
that addition of 2,20-azobis(isobutyronitrile) has no signifi-
cant effect on the yield of the product and reaction time. This
observation suggests that the mechanism probably involves
the TBHP activation by vanadium (V) centers forming a
peroxy intermediate (Scheme 2). However, at this time the
Run Number
Fig. 1. The recycling experiment of VO(ephedrine)2@MNPs (50 mg) in
oxidation of benzyl alcohol using TBHP in PEG at 80 ꢀC for 3 h.
Please cite this article in press as: A. Rostami, et al., Efficient and green oxidation of alcohols with tert-butyl hydrogenperoxide
catalyzed by a recyclable magnetic core-shell nanoparticle-supported oxo-vanadium ephedrine complex, Comptes Rendus
Rostami, Amin
