Solid-supported Pd(0): An efficient heterogeneous catalyst for aerobic oxidation of benzyl alcohols into aldehydes and ketones

Article abstract of DOI:10.1016/j.tetlet.2011.07.073

Solid-supported nano- and microparticles of Pd(0) (SS-Pd) were used as heterogeneous catalysts for aerobic oxidation of benzyl alcohols. Primary and secondary benzyl alcohols gave the corresponding products in good yields. In addition, the catalysts could be reused up to five runs without significant loss of activities.

Full text of DOI:10.1016/j.tetlet.2011.07.073

Tetrahedron Letters 52 (2011) 4954–4956
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Solid-supported Pd(0): an efficient heterogeneous catalyst for aerobic
oxidation of benzyl alcohols into aldehydes and ketones q
Bandna 1, Nidhi Aggarwal 1, Pralay Das
⇑
Natural Plant Products Division, Institute of Himalayan Bioresource Technology (Council of Scientific and Industrial Research), Palampur 176061, H.P., India
a r t i c l e i n f o
a b s t r a c t
Article history:
Solid-supported nano- and microparticles of Pd(0) (SS-Pd) were used as heterogeneous catalysts for aer-
obic oxidation of benzyl alcohols. Primary and secondary benzyl alcohols gave the corresponding prod-
ucts in good yields. In addition, the catalysts could be reused up to five runs without significant loss of
activities.
Received 8 June 2011
Revised 13 July 2011
Accepted 17 July 2011
Available online 22 July 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Aerobic oxidation
Heterogeneous catalyst
Nano- and microparticles
Solid supported palladium
The oxidation of alcohols into the corresponding carbonyls is a
very important and useful organic transformation1 since aldehydes
and ketones of industrial significance are used as intermediates in
the manufacture of dyes and pharmaceuticals.2 The use of transi-
tion metals in selective catalytic oxidation with environmentally
benign oxidants has emerged as a very interesting alternative to
the conventional waste-producing oxidation procedures which
require stoichiometric amounts of toxic inorganic salts, in the past
years.3,4 Recently, aerobic oxidation processes have received
increasing attention. The use of oxygen has great benefits from
both the economic and green chemistry view points, because oxy-
gen is relatively cheap and produces water as the only byproduct.
Thus due to the wide scope of the development of catalysts, suit-
able active metals for both homo- and heterogeneous catalyzed
oxidations with molecular oxygen have been explored which
include Fe,5 Ru,6 Co,7 Cu,8 Mn,9 Os,10 and Pd.11 Solid supported
transition metal nanoparticles have found immense interest in
recent years for the development of new synthetic methodologies
due to its high catalytic activity and reusability.12 Few recent
reports showed that heterogeneous nanoparticles of palladium
are capable of aerobic oxidation of aromatic alcohols.13
alcohols to the corresponding aldehydes and ketones using molec-
ular oxygen as the oxidant. Primary and secondary benzyl alcohols
yielded the corresponding products in good yields. Moreover, the
catalyst system could be recycled up to five runs without signifi-
cant loss of activity.
We investigated the oxidation of 2-ethoxybenzylalcohol 1a as
the model substrate. The progress of reaction was monitored by
Table 1
Effect of catalysts and solvents for the oxidation of 2-ethoxybenzylalcohol
Catalyst,O2
Solvent, 110 0C
OH
O
OC2H5
OC2H5
5h
1a
2a
Entry
Catalyst (mol % Pd)
Solvent
Yield (%)a
TON
TOF hÀ1
1
2
3
SS-Pd(1)
SS-Pd(3)
SS-Pd(5)
Toluene
Toluene
(i) Toluene
(ii) THF
(iii) Dioxane
(iv) C6H6
(v) CH3CN
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
11
68
80
9
1100
2266
1600
180
60
1100
460
1333
1380
40
220
453
320
36
12
220
92
267
276
8
4
80
3
Recently our group described the synthesis of solid supported
nano/microparticles of Pd(0) (SS-Pd(0)) and its application in
Suzuki–Miyaura cross coupling reaction.14 Herein, we have inves-
tigated the scopes of SS-Pd(0) catalyst in the oxidation of benzyl
55
23
80
69
2
1
20
39
4
5
6
7
8
9
SS-Pd(6)
Pd(OAc)2(5)
Pd/C(5)
Pd2C6H10Cl2(10)
Pd2(dba)3(10)
Pd[P(C6H5)3]4(5)
20
400
780
q
156
IHBT Communication No. 2200
⇑
Corresponding author. Fax: +91 1894 230433.
E-mail address: pdas@ihbt.res.in (P. Das).
These authors contributed equally to this work.
TON = % conversion  mmole of substrate/mmole of catalyst, TOF = TON/time.
a
Determined by means of GC, based on the 2-ethoxy benzyl alcohol.
1
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.07.073

Bandna et al. / Tetrahedron Letters 52 (2011) 4954–4956
4955
Table 2
Oxidation of primary and secondary benzyl alcohols to aldehydes and ketones
R2
R2
SS-Pd, O2
OH
O
Toulene, 110 0C
3-5 h
R1
R1
2a-2m
1a-1m
Entry
Substrate
R1
R2
Time (h)
Product
Yield (%)a
TON /TOFÀh
1
2
3
4
5
6
7
8
9
10
11
12
13
1a
1b
1c
1d
1e
1f
1g
1h
1i
2-OC2H5
H
4-NO2
3-OMe
4-Br
H
CH3
H
H
H
5
5
3
3
3
3
3
5
5
5
5
5
5
2a
2b
2c
2d
2e
2f
2g
2h
2i
78
96
70
90
80
83
85
90
93
90
90
74
90
1560/312
1963/393
1400/467
1800/600
1600/533
1660/553
1700/567
1800/360
1860/372
1800/360
1800/360
1480/296
1800/360
4-Cl
2-Br
H
H
4-Me
2,4-OMe
2,5-OMe
4-OMe
4-Cl
CH3
CH3
CH3
CH3
C6H5
CH3
1j
1k
1l
2j
2k
2l
1m
4-Br
2m
a
Isolated yield.
TLC and GC analysis. Initially optimization of the solvent and cata-
lytic system was carried out (Table 1). Different loadings of SS-Pd
with 1, 3, 5, and 6 mol % Pd were also tested for optimization of
the reaction condition. Highest turnover number (TON, 2266)
and turnover frequencies (TOF, 453 hÀ1) were calculated when
SS-Pd (3 mol % Pd) was used for the oxidation reaction of 1a (Table
1, entry 2). However, only 68% yield of 2a was observed by GC anal-
ysis. SS-Pd with 5 mol % Pd in toluene showed 80% yield of 2a and
TON/TOF was calculated as 1600/320 hÀ1(Table 1, entry 3). No fur-
ther improvement of yield was observed by enhancing the catalyst
loading up to 6 mol % Pd (Table 1, entry 4). Hence SS-Pd (5 mol %
Pd) and toluene solvent were found to be optimum under the reac-
tion conditions. Comparative studies of different catalytic systems
were also performed (Table 1, entries 5–9) and the best result was
obtained using SS-Pd under the same reaction condition.
Further investigations were performed to enhance the scopes of
oxidation reactions in different substituted primary and secondary
benzyl alcohols. Both the primary and secondary benzyl alcohols
on oxidation gave satisfactory to good yields of the corresponding
aldehydes and ketones. Secondary benzyl alcohol was also found to
be reactive toward oxidation under the same reaction condition
and the product 2b was obtained in excellent yield15 (Table 2,
entry 2). Primary benzyl alcohols with electron donating and with-
drawing groups afforded products 2c and d in good yields (Table 2,
entries 3 and 4).
Primary benzyl alcohols bearing halogen groups were also suc-
cessfully oxidized to benzaldehydes 2e–g with satisfactory yields
(Table 2, entries 5–7). Secondary benzyl alcohols with electron
donating groups afforded products 2h–k in good yields (Table 2,
entries 8–11) and halogen substituted secondary benzyl alcohols
also gave products 2l and m in satisfactory yields without any
dehalogenation products (Table 2, entries 12 and 13). Evaluation
of recyclability was done on the same test substrate 1a. After com-
pletion of reaction, the product was extracted with ethyl acetate
and the recovered catalyst was washed with acetone, dried, and
used for further reactions. The catalytic activity of SS-Pd remained
almost the same upto five runs as indicated by the yields of the
corresponding aldehyde 2a (Table 3). Gradual decrease in yields
was encountered after six to seven runs of SS-Pd catalyst. As shown
in Table 3, on recycling, from the first to the seventh run the TONs
drop from 1600 to 1000 and total turnover number over the seven
cycles was 10400.
Table 3
Recyclability experiments of 2a using SS-Pd
Run
1st
80
2nd
80
3rd
79
4th
79
5th
78
6th
74b
7th
50b
Yield
(%)a
TON
1600
TTON over seven
cycles
1600 1580 1580 1560 1480 1000
10400
TTON = total turnover number; procedure followed as described for 2a.
a
Determined by means of GC.
Isolated yield.
b
In conclusion, aerobic oxidation of benzyl alcohols using SS-Pd
has been developed. Different primary and secondary benzyl alco-
hols afforded the corresponding products in good yields. Due to the
stability of the SS-Pd catalyst in moisture and air, it is very easy to
handle under reaction conditions. This methodology will find
interest in both academic as well as industry due to its atom-econ-
omy and cost-effective process by using heterogeneous nano and
microparticles as. ligand free catalysts.
Acknowledgments
Authors are grateful to Dr. P. S. Ahuja, Director IHBT for provid-
ing necessary facilities during the course of the work. We gratefully
acknowledge financial assistance from the Department of Science
& Technology (Nano Mission), New Delhi (Grant No. SR/NM/NS-
95/2009). Bandna (SRF) thanks CSIR, New Delhi for awarding
senior research fellowships.
Supplementary data
Supplementary data (general experimental procedures and
characterization data for all compounds) associated with this arti-
cle can be found, in the online version, at doi:10.1016/
j.tetlet.2011.07.073.
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