D. Baruah et al. / Catalysis Communications 77 (2016) 9–12
11
Scheme 2. Oxidation of 4-Methoxy benzyl alcohol 3a to 4-Methoxy benzaldehyde 4a. aReaction condition: 4-Methoxy benzyl alcohol 3a (1.0 mmol), Bi (NO
cellulose-Cu-NPs (20 mol%), MeCN (10 mL), 60 min, 80 °C, air. bDetermined by GC–MS. Isolated yield.
3 3 2
) ·5H O (0.75 mmol),
c
(
3
NO
3
)
3
·5H
2
O (0.75 mmol), cellulose-Cu-NPs (20 mol%), MeCN (10 mL),
·5H
system under ambient conditions. Benzylic alcohols having electron
withdrawing group (entries 5–8, Table 3) afforded slightly less product
yield as compared to electron donating counterparts (entries 1–4,
Table 3). It is noteworthy to mention that more reaction time is required
to obtain high yield of heteroaryl and cinnamyl aldehydes. Oxidation of
heteroaryl and cinnamyl alcohols using this sustainable catalyst afforded
moderate yield (55–68%) of desired aldehydes. Our catalyst was found
to be totally inert towards the oxidation of long chain aliphatic alcohols.
No over oxidized products were observed during the oxidation of prima-
ry and secondary aromatic alcohols. All the other functional groups (e.g.,
0 min, 60 °C and oxygenated atmosphere. Solubility of Bi (NO
)
3 3
2
O
was found to be varied in different solvents used. It dissolves in acetoni-
trile and water sparingly with giving a white cloudy solution in the re-
action mixture. Bi (NO ) ·5H O dissolves completely in DMF with a
3 3 2
clear solution. Preparation method and characterization data (XRD,
TEM etc.) of cellulose mediated Cu-NPs [14–17] are discussed in
supporting information section.
Having the optimized reaction conditions in hand (entry 10, Table 1),
we next tested the oxidation of a series of substituted secondary aromat-
ic alcohols. As shown in Table 2, all the secondary aromatic alcohols
2 3 2 2
–OMe, −OH, −NO , −Cl, −Br, − N (CH ) , and −NH ) were well tol-
(
entries 1–8, Table 2) were found to be successfully converted to their
respective carbonyls with almost 100% selectivity with excellent yield
95–99%). Very negligible substituent effect has been observed in the
erated in oxidation process using this versatile catalytic system.
Next we studied the oxidation of 5-hydroxymethyl furfural (HMF)
using the proposed catalytic system. HMF with two different active
functional groups has four different oxidation pathways (path a–d)
which can afford four different value added products as shown in
Scheme 3. It was found that our catalytic system selectively oxidizes
the hydroxymethyl group of HMF to formyl group with affording
diformylfuran (DFF) as a final product (path a). No over oxidized prod-
ucts like FFCA, FDCA, HMFCA were observed during the oxidation
process. The proposed catalytic system was found to be totally inert to-
wards the oxidation of formyl group of HMF. The optimized reaction
condition for oxidation of HMF to DFF was: HMF (1.0 mmol), Bi
(
oxidation process using this efficient catalytic system.
To further probe the potentiality of this new catalytic system, we
investigated the oxidation of primary aromatic alcohols under open
air atmosphere. We began to examine the proposed study by taking 4-
Methoxy benzyl alcohol 3a as a model substrate (Scheme 2). Prelimi-
nary results clearly suggested the feasibility of the proposed scheme.
From the optimization study on oxidation of 3a to 4a we finally got
the most exclusive condition for highest conversion (80%) with high
yield percentage (76%) of the desired product 4a. The optimized
reaction condition was: 4-Methoxy benzyl alcohol 3a (1.0 mmol), Bi
3 3 2
(NO ) ·5H O (0.75 mmol), cellulose-Cu-NPs (20 mol%), MeCN
(
(
NO
3
)
3
·5H
2
O (0.75 mmol), cellulose-Cu-NPs (20 mol%), MeCN
(10 mL), 80 °C, 120 min, air. Isolated yield of DFF is 82% (85%
conversion).
10 mL), 60 min, 80 °C and oxygenated atmosphere.
Utilizing the optimized reaction conditions, the scope and limita-
Studies on the reusability of the catalytic system for the oxidation of
different alcohols confirmed the non-reusability/non-recyclability of
the catalytic system. This may be due to the generation of Cu (II) in
the reaction cycle. Appeared blue colour solution in the reaction vessel
indicates the generation of Cu (II). From the literature survey [19,28],
tions in oxidation of primary aromatic alcohols were studied and the re-
sults are shown in Table 3. A variety of benzylic alcohols (entries 1–8,
Table 3), heteroaryl alcohols (entries 9&10, Table 3) and cinnamyl alco-
hols (entries 11–14, Table 3) were efficiently oxidizes to the corre-
sponding aldehydes in moderate to high yield using the catalytic
it can be summarized that Bi (NO
tion of Cu (0)/Cu (I) to Cu (II) with concomitant conversion of NO
NO . The irreversible conversion of Cu (0)/Cu (I) to Cu (II) during the
3 3 2
) ·5H O is responsible for the oxida-
3
to
2
Table 3
reaction process can be regarded as the main cause for the non-
reusability/non-recyclability. Only recollected cellulose (after work-
up) can be used further for synthesis of Cu-NPs. Further studies on
intrinsic catalytic role and synthetic application of the reactions using
this catalytic system is under investigation.
a
Oxidation of primary aromatic alcohols to aldehydes by Bi (NO
3 3 2
) ·5H O and cellulose
mediated Cu-NPs.
Entry
R
Time
Conv. (%)b
Yield (%)c
1
2
3
4
5
6
7
8
9
4-OMePh
3,4-OMePh
4-N(CH ) Ph
3 2
2-OHPh
4-ClPh
60 min
60 min
60 min
60 min
60 min
60 min
60 min
60 min
120 min
120 min
120 min
120 min
120 min
120 min
80
80
80
75
75
75
70
70
60
60
70
75
75
70
76
74
73
69
68
66
65
65
55
56
65
68
67
62
3
. Conclusion
From the overall discussion it can be concluded that our new catalyt-
ic system was found to be very effective towards the oxidation of sec-
ondary aromatic alcohols to ketones with almost 100% convertibility.
2,4-ClPh
4-NO
2-NO
2
Ph
Ph
60–80% conversion rate was found in the oxidation of primary aromatic
2
alcohols to aldehydes under this milder reaction condition. Oxidation of
5-hydroxymethyl furfural to 2,5-diformylfuran (82% yield) was one of
the great achievements by this sustainable catalytic system. Finally we
developed an economical, environmentally benign, highly efficient
and versatile catalytic system cellulose mediated Cu-NPs with bismuth
nitrate pentahydrate for oxidation of aromatic alcohols as well as HMF
to DFF. This new methodology can be applicable in industrial synthesis
of DFF and other carbonyls from aromatic alcohols with the emphasis on
economic and environmental issues.
3-Pyridyl
2-Indolyl
Cinnamyl
10
11
12
13
14
4-OMe Cinnamyl
4-N(CH Cinnamyl
4-NO Cinnamyl
3 2
)
2
a
3 3 2
Reaction condition: substrate (1.0 mmol), Bi (NO ) ·5H O (0.75 mmol), cellulose-Cu-
NPs (20 mol%), MeCN (10 mL), 80 °C, air.
b
Determined by GC–MS.
Isolated yield.
c