Supported palladium nanoparticles as switchable catalyst for aldehyde conjugate/s and acetate ester syntheses from alcohols

Article abstract of DOI:10.1039/c6nj03769k

Polymer-supported Pd(0) (Pd@PS) nanoparticles (NPs) were explored as a switchable catalyst for oxidative aldehyde conjugate/s (AC/s) and acetate esters (AEs) syntheses from alcohols. Using the same substrates, the catalyst in the presence of oxygen and K₂CO₃ participated in AC/s synthesis, and in the presence of traces of air and NaO^tBu, unusual AEs products were obtained.

Full text of DOI:10.1039/c6nj03769k

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Supported Palladium Nanoparticles as Switchable Catalyst for
Aldehyde Conjugate/s and Acetate Ester Synthesis from Alcohols
Sandeep Kumar,a,b Abha Chaudhary,a Bandna,a Dhananjay Bhattacherjee,a,bVandna Thakura,b and
Pralay Das*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Polymer supported Pd(0) (Pd@PS) nanoparticles (NPs) have been low temperature is a challenging task4d and rarely explored.
explored as switchable catalyst for oxidative aldehyde Electro chemical oxidation of ethanol under palladium
a
conjugate/s (AC/s) and acetate esters (AEs) synthesis from catalyzed condition is a developing area for fuel cell research
alcohols. Using the same substrates, the catalyst in the presence compared to thermo-chemical oxidation of ethanol to
of oxygen and K2CO3 participates in AC/s synthesis and in traces of acetaldehyde.6 Palladium alloys (Pd-Zn, Pd-Ga and Pd-In) have
air and NaOtBu condition give unusual AEs products.
been reported for selective lower level conversion of
acetaldehyde from ethanol while total decomposition of
ethanol to CO, CH4 and H2 has been observed under metallic
Pd condition.7
The oxidative transformations using metal catalysts have
evolved in the present era due to environmental benignity
replacing traditional methodologies.1 The aldehyde
conjugate/s (AC/s) such as cinnamaldehydes are valuable
molecules present in several natural products, bioactive
molecules and fine chemicals while acetate esters (AEs) are
useful as fragrant molecules, perfumes, drug candidates, and
precursor for polymer synthesis.2 The synthesis of α,β-
unsaturated aldehydes such as cinnamaldehydes can be
performed by following different traditional condensation and
dehydration apporaches.2a But there is no single report yet
available for the synthesis of AC/s from alcohols such as
benzyl alcohols and ethanol following oxidation-condensation
approaches. The AEs can be synthesized from alcohols by
Fischer esterification reaction, acylation with activated acid
derivatives, ketene addition, enzyme catalyzed acetylation and
trans esterification.3 In recent reports, oxidative cross-
esterification reactions of benzyl and aliphatic alcohols by
using molecular O2 in presence of palladium4 or other
transition metal catalysts5 have been reported accounting for
the fact that ethanol was not oxidized under the aerobic
reaction conditions. Beller et al. have also reported that
ethanol remains unoxidized under palladium catalyzed
oxidative cross-esterification reactions with benzyl alcohols
(Scheme 1).4b From the studies so far it is clear that palladium
catalyzed oxidation of ethanol under milder basic condition at
The condition controlled synthesis of such AC/s and AEs by
oxidation of ethanol for its use as intermediate has not yet
been attempted. In our recent reports, first time we have
applied Rh@PS as an efficient catalyst for facile oxidative
esterification of methanol and ethanol for acetate ester
synthesis.8 As a part of our investigations on the applications of
supported transition metal NPs,9 herein we explore the
application of Pd@PS catalyst that promotes selective
oxidation-condensation to yield AC/s and unconventional
oxidative esterification for AEs synthesis from benzyl/alkyl
alcohols and ethanol by control over reaction conditions
(Scheme 1).
O
Previous work
Pd(OAc)2, ligand
OC2H5
OH
R
R
AgPF6, K2CO3
C2H5OH, O2, 50-60 o
C
K2CO3, O2, 60 o
C
CHO
)
n
(
R
Our work
n = 1 & 2
O
Pd@PS
OH
O
R
C2H5OH
NaOtBu, 125 o
C
CH3
R
Scheme 1. Comparative study of Pd-catalyzed oxidative reactions of alcohols
The Pd@PS catalyst was synthesized using previously reported
reduction deposition approach using Amberlite IRA 900 Cl-
form resin as polymer support (PS).9a The Pd@PS was
characterized by SEM, SEM-EDS, TEM, SAED and FFT studies
(Figure S1, details in Supporting information, ESI). The catalyst
Pd@PS after characterization was used for an
unprecedentedreaction of alcohols to AC/s synthesis.
a.Natural Product Chemistry & Process Development,CSIR-Institute of Himalayan
Bioresource Technology, Palampur -176061, H.P., India. Fax: (+91)-1894-230-433;
e-mail: pdas@ihbt.res.in; pdas_nbu@yahoo.com.
b.Academy of Scientific & Innovative Research (AcSIR), New Delhi, India.
Electronic Supplementary Information (ESI) available: [Preparation of Pd@PS
catalyst, recyclability experiment, Typical experimental procedures, spectral data
1H, 13C NMR spectra of products]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Table 1. Optimization studies of ethanol and benzyl alcohol for AC/s synthesisa
Table 2. Pd@PS catalyzed AC/s synthesis from ethanol and benzyl alcoholsa
Catalyst,
CHO
CHO
)
2
OH
+
CHO
CHO
CHO
2
(
OH
+
Base
Pd@PS, K2CO3
R
C2H5OH
2
R
R
C2H5OH
2
+
+
O2, 60 o
C
+
O2, 60 o
Solvent
C
1
3
4
1a
4a
5a
3a
1,4-dioxane
Yield (%)
Yield (%)
Product (4)
Entry
Product (3)
R
Yield (%)b
Entry
1
Catalyst (mol% Pd) Base (equiv.)
Solvent
5a
3a
4a
MeO
CHO
MeO
CHO
10c
n.d
40b
38b
3-OMe
(1b)
2
1
2
Pd@PS (2)
Pd@PS (2)
Pd@PS (2)
Pd@PS (2)
KF (0.5)
95
2
n.d.
n.d.
n.d.
C2H5OH
C2H5OH
C2H5OH
4b
3b
Et3N (0.5)
55
97
2
3
17
Na2CO3 (0.25)
MeO
CHO
MeO
CHO
n.d.
6
3,4-OMe
(1c)
2
K2CO3 (0.25)
C2H5OH
C2H5OH
4
n.d.
n.d.
n.d.
88
90
65
7
4c
MeO
Me
MeO
Me
3c
3d
3e
Pd@PS (2)
Cs2CO3 (0.25)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (2)
5
6
8
CHO
2
Pd@PS (2)
Pd@PS (2)
Pd@PS (2)
Pd@PS (2)
Pd@PS (2)
Pd@PS (3)
29
CHO
CHO
DMF
20b
30b
3,4-Me
(1d)
3
Toluene
Dioxane
Dioxane
Dioxane
Dioxane
46
7
8
27
9
Me
4d
4e
Me
52
39
43
54
49
CHO
2
K2CO3 (3)
K2CO3 (4)
K2CO3 (4)
16
25
12
30
16
31
9
40b
30b
tracesc
n.d
10c
11
12
4-Cl
(1e)
4
Cl
Cl
Cl
Cl
Pd@PS (2)
Pd/C (2)
Na2CO3 (4)
K2CO3 (4)
K2CO3 (4)
no base
n.d.
5
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
n.d.
6
83
n.d.
17
CHO
2
CHO
5d
3-Cl
(1f)
13
14
15
4f
Pd(OAc)2 (2)
Pd@PS (2)
Pd@PS (2)
3f
23
19
CHO
2
n.d.
36
n.d.
20
n.d.
19
CHO
CHO
tracesb
20b
K2CO3 (4)
33b
25b
16d
6d
7
4-F
(1g)
F
4g
3g
aReaction conditions: 1a (0.92 mmol), 2 (18.5 equiv.), Pd@PS (2.0 mol%), solvent
(1 mL), 24 h. bGCMS yield. c3a (50%, isolated yield). dMolecular Sieves 3Å used,
n.d. = not detected.
F
CHO
2
naphthyl
(1h)
4h
3h
To optimize the reaction conditions, the effect of base, solvent
and temperature was critically investigated (Table 1). Among
different organic/inorganic bases, K2CO3 was screened as
effective base (Table 1, entry 1-10) with Pd@PS (2 mol% Pd),
aReaction conditions: 1 (1 equiv), 2 (20 equiv), Pd@PS (2 mol%), dioxane (1 mL),
60 oC, 48 h, corresponding aldehydes 5 (28-51%, Supporting information,
ESI).bIsolated yield. cGC-MS yield. dFor 24 h.
o
aldehydes 3b, 3c and 3d in 40, 38 and 20% yields respectively
K2CO3 (4 equiv.) in 1,4-dioxane as solvent at 60 C giving the
and α,β,γ,δ-unsaturated aldehydes 4b and 4d in moderate
yields (Table 2, entries 1- 3). The reaction in the presence of
halide substituent, Cl and F on the benzene ring also gave α,β-
best results of AC/s products 3a and 4a in 54 and 25% yields
(Table 1, entry 10) with approximate 100% conversion.
Notably, the addition of base as well as O2 was necessary for
the reaction. The marked effect of temperature variation on
the reactivity was observed. With lowering the temperature to
42 oC resulted into the increase in benzaldehyde percentage
(32%) with desired cinnamaldehyde 3a in 28% (Table S1, entry
1, Supporting information, ESI). Further decrease in the
temperature up to 30 oC led to the formation of benzaldehyde
5a (41%) as the major product with only 6% of 3a (Table S1,
entry 2, Supporting information, ESI). The results indicated
that the rate of oxidation of ethanol and benzyl alcohol slowed
unsaturated aldehydes 3e, 3f and 3g in low yields (Table 2,
entries 4, 5 and 6). The naphthylmethanol also afforded the
mono- and bis-condensation products 3h and 4h in
considerable yields (Table 2, entry 7). In all the reactions,
benzaldehydes were detected as un-reacted product. The
moderate yields were due to the probability that highly
volatile acetaldehyde as intermediate (low boiling 20 oC)
participated in the condensation reaction with in situ
generated aldehydes at high temperature 60 oC and self
condensation of acetaldehyde cannot be ignored (GC-MS,
Supporting information, ESI). But yet now there is no single
report available for Pd catalyzed thermal oxidation of ethanol
and benzyl alcohols to AC/s synthesis under such a milder
condition. Therefore, as a first report and complex nature of
reactivity, the yields may be considered as reasonable.
Moreover, the reaction of benzyl alcohol with other aliphatic
alcohols such as methanol, isopropanol and 2-phenylethanol
was also carried out under standard reaction conditions and
results were summarized in Table S4 (Supporting information,
ESI).
o
down with decrease in temperature and 60 C is the optimum
temperature for the high performance of oxidative
unsaturation reaction. But under this study, acetaldehyde was
not detected in traces and the reason may be low boiling
point, unstable and high reactivity of benzaldehyde to give
AC/s products. Moreover, to identify reaction intermediate, a
separate reaction was designed with a low temperature
distillation setup to collect in situ produced volatile
acetaldehyde distillates in a separate flask. The collected
acetaldehyde mixture with ethanol was characterized by NMR
spectroscopy (Supporting information, ESI).
A very interesting shift in the reactivity of benzyl alcohols and
ethanol for AEs synthesis was observed under the same
catalytic condition by changing the oxidant O2 to air, base
K2CO3 to NaOtBu and temperature 60 to 125 oC. Such an
To demonstrate the general applicability of this method,
various benzyl alcohols were studied in carbon unsaturation
reaction with ethanol for AC/s synthesis (Table 2). Different -
OCH3 and -CH3 substituted benzyl alcohols and ethanol under
oxidation condition gave AC/s such as α,β-unsaturated
2 | J. Name., 2012, 00, 1-3
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Table 3. Optimization of reaction conditions for synthesis of benzylacetatea
Table 4. Pd@PS catalyzed AEs synthesis from alcoholsa
O
OH
O
CH3
Pd-catalyst
+
CH3CH2OH
2
Base, solvent
Temp., 60 h
OCH3
1b
6b
Temp (oC) Yield (%)b
OCH3
Entry Catalyst (mol% Pd)
Base
Solvent
Pd@PS (2)
K2CO3
K2CO3
NaOtBu
1
1,4-Dioxane
110
n.d
Pd@PS (2)
Pd@PS (2)
2
3
1,4-Dioxane
1,4-Dioxane
125
60
n.d
n.d
NaOtBu
NaOtBu
NaOtBu
KOtBu
NaOtBu
Pd@PS (2)
Pd@PS (2)
Pd@PS (2)
Pd@PS (2)
Pd@PS (2)
4
5
6
7
8
1,4-Dioxane
1,4-Dioxane
Toluene
110
125
125
125
125
n.d
62
57
54
62
1,4-Dioxane
1,4-Dioxane
NaOtBu
NaOtBu
NaOtBu
Pd@PS (1)
Pd@PS (2)
Pd@PS (2)
9
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
125
125
125
45
55
60
10c
11d
12e
NaOtBu
Pd@PS (2)
1,4-Dioxane
1,4-Dioxane
125
n.d
Pd@PS (2)
Pd@PS (2)
Pd@PS (2)
Pd@PS (2)
Pd@PS (2)
Pd/C(2)
125
125
125
125
125
125
125
40
n.d
n.d
n.d
n.d
n.d
n.d
13
14
15
16
17
18
19
NaOH
DABCO 1,4-Dioxane
Na2CO3
1,4-Dioxane
Et3N
1,4-Dioxane
NaOtBu
DMF
NaOtBu
1,4-Dioxane
NaOtBu
Pd(OAc)2 (2)
1,4-Dioxane
aReaction conditions: 1a (1.45 mmol), 2: (2 mL), NaOtBu (4 equiv.), solvent (1.2
mL). bisolated yield.cNaOtBu (3 equiv.).dNaOtBu (5 equiv.).eN2 atmosphere.
interesting shift of their reactivity for the synthesis of AC/s and
AEs using the same substrates and catalyst are very
[a]Reaction conditions: 1 (1 equiv), 2 (2 mL), Pd@PS (2 mol%), 1,4-dioxane (1.2
mL).[b]Isolated yield.
uncommon approach. After screening
a
number of
temperature variations, bases, and solvents (Table 3): NaOtBu
(4 equiv.), Pd@PS (2 mol% Pd), 1,4-dioxane at 125 oC was
selected as optimized conditions giving maximum yield of the
benzylacetate (Table 3, entry 5). The reaction is also following
the same pathway as described in our earlier reports under Rh
catalytic condition.8a
The developed methodology was extended for its use by
varying different substituted benzyl and alkyl alcohols (Table
4). The OCH3, CH3, unsubstiuted and Cl substituted benzyl
alcohols (1i
,
1j
,
1a and 1e) were found to be active and gave
6d 6a and 6e in considerably good
desired AEs products 6c
,
,
yields (58-61%). The bicyclic system such as napthyl-1-
methanol 1h also participated in the same reaction to give AE,
6f in good yield (Table 3, entry 6). Interestingly the aliphatic
alcohols were also subjected to oxidative esterification,
delightfully the 2-phenylethanol 1k and cyclohexylmethanol 1l
gave the corresponding AE products 6g and 6h in 70 and 68%
yields.
Scheme 2. Control experiments for mechanistic understanding
cyclohexylcarboxaldehyde 5l which indicates ethanol
promoted reduction of 5l and subsequent esterification
(Scheme 2d).
To understand the mechanism of the reaction, control
experiments for the determination of intermediates and in situ
redox reactions were performed. The confirmation of
acetaldehyde as intermediate was done by NMR
spectroscopyof crude mixture of acetaldehyde in ethanol by
using differentexperimental setup (Scheme 2, details in
Supporting information, ESI). It was also observed that in
A plausible mechanism will be involving the coordination of
oxygen molecule to the palladium surface giving the palladium
4c
species (II
)
that coordinates with the oxygen atom of the
benzyl alcohol and ethanol to give intermediate (III) (Scheme
3). We propose that in the presence of O2 as hydrogen
acceptor,
acetaldehyde which further participated in condensation
reaction to give AC/s products ( and 4) (route 1). In route-2,
the intermediate (III) in absence of additional O2 and under
NaOtBu condition gave oxidized products benzaldehyde (
(
III)
get converted to benzaldehyde and
absence of ethanol, benzyl alcohol
(1a) is oxidized to
3
benzaldehyde (5a) (Scheme 2). But in presence of ethanol, 5a
was reduced to benzyl alcohol (1a) which clearly indicated that
5
)
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Year Plan programme under title ORIGIN (CSC-0108). SK, AC, B,
DB, VT thank UGC and CSIR, New Delhi for awarding
fellowships.
IHBT communication No. 4079.
Notes and references
1
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(a) P. R. C. E., F. Mackie, Aldehydes: α,β- Unsaturated
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3;(b) P. S. Babu, S. Prabuseenivasan and S. Ignacimuthu,
Phytomedicine.,2007, 14, 15; (c) P. Maity, S. Yamazoe and T.
Tsukuda, ACS Catal. 2013, 3, 182; (d) J. Otera, Esterification:
methods, reactions, and applications, Wiley-VCH, Weinheim,
2003.
Scheme 3. Possible mechanistic pathways for AC/s and AE synthesis
3
4
(a) R. C. Larock, Comprehensive Organic Transformations: A
Guide to Functional Group Preparations, 2nd ed, Wiley, New
York, 1999.
(a) M. Caporaso, G. Cravotto, S.Georgakopoulos,
G.Heropoulos, K. Martina and S.Tagliapietra, Beilstein J. Org.
Chem., 2014, 10, 1454; (b) S. Gowrisankar, H. Neumann and
M. Beller, Angew. Chem. Int. Ed., 2011, 50, 5139; (c) X. F. Bai,
L. S. Ye, G. Q. Zheng, C. G. Lai, L. Xia and W. Xu, Chem.
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S. L. Mowbray, A. Karlen and M. Larhed, J. Org. Chem. 2011,
76,8986; (b) L. Wang, J. Li, W. Dai, Y. Lv, Y. Zhanga and S.
Gao, Green Chem., 2014, 16, 2164. (c) N. Yamamoto, Y.
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Maki, A. Chan, E. M. Phillips, K. A. Scheidt, Org. Lett., 2007, 9,
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and acetaldehyde (2a). As an unusual approach, H-Pd@PS
again participated in reduction reaction of benzaldehyde (5) to
benzyl alcohol in a selective manner. The reaction of benzyl
alcohol with acetaldehyde produce the hemiacetal which
further coordinated with Pd@PS to give intermediate (IV).
Through β-hydride elimination, the intermediate
converted to corresponding benzylacetate (6) product 8a
(
IV
)
.
The
catalyst, Pd@PS used under this study contains palladium in
different nano-cluster form as supported by TEM
characterization (Fig. S1, S2 in ESI). Therefore, supported
palladium (Pd@PS) catalyst may be involving in the mechanism
preferably in the form of multiple type of active centers rather
than single active centers.10
5
The recyclability experiment of Pd@PS was carried out in
oxidative esterification reaction using 3-methoxy benzyl
alcohol (1b) as model substrate. The catalyst was used in the
same reaction for five times without significant catalytic
deactivation. The TEM characterization after five runs revealed
that the Pd NPs with a large size are still impregnated in the
Pd@PS catalyst (ESI). The heterogeneity of the active Pd
species in Pd@PS was confirmed by Hg(0) poisoning test that
yielded traces of product. The Inductively coupled plasma
atomic emission spectra (ICP-AES) analysis of the reaction
mixture during the recyclability experiment showed <2 ppm
leaching of palladium metal into the reaction mixture
(Supporting information, ESI).
6
7
(a) A. B. Sanchez, N. Homs, S. Miachon, J. A. Dalmon,J. L.
G.Fierroand and P. R. de la Piscina, Green Chem. 2011, 13
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,
Piscina, Catalysis Today, 2005, 107, 431.
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194; (b) S. K.Aksu, M. N.Hasan,W. R.Hagen, F.Hollmann,
D.Sordi, R. A. Sheldon and I. W. C. E.Arends, Chem.
Commun., 2012, 48, 5745; (c) K. Mori, T. Hara, T. Mizugaki, K.
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(a) N. R. Guha, S. Sharma, D. Bhattacherjee, V. Thakur, R.
Bharti, C. B. Reddy and P. Das, Green Chem., 2016, 18, 1206;
(b) N. R. Guha, D. Bhattacherjee and P. Das, Catal. Sci.
8
9
Technol, 2015, 5, 2575.
In summary, Pd@PS has been applied as an efficient catalyst
for oxidative condensation and esterification reaction of
benzyl alcohols and ethanol. A wide range of aryl/alkyl alcohol
has been established as substrates for efficient synthesis of
AC/s and AEs. The Pd@PS assisted control over the redox
nature of ethanol by variation in the reaction conditions is the
essence of current methodology that enables to obtain
different products.
(a) P. Das, D. Sharma, A. K. Shil and A. Kumari, Tetrahedron
Lett. 2011,52, 1176; (b) A. K. Shil and P. Das, Green Chem.
2013, 15, 3421; (c) A. K. Shil, S. Kumar, C. B. Reddy, V.
Thakur, S. Dadwal and P. Das, Org. Lett, 2015, 17, 5352. (d)
S. Kumar, S. Sharma and P. Das, Adv. Synth. Catal, 2016, 358
2889; (e) N. R. Guha, C. B. Reddy, N. Aggarwal, D. Sharma, A.
,
K. Shil, Bandna and P. Das, Adv. Synth. Catal., 2012, 354
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Acknowledgement
Authors are grateful to Director CSIR-IHBT for providing
necessary facilities during the course of the work. We thank
Biotechnology Division, CSIR-IHBT for SEM, EDS and Dr G. Saini
(AIRF), JNU-New Delhi, India for the TEM analysis. The authors
thank CSIR, New Delhi for financial support as part of XII Five
4 | J. Name., 2012, 00, 1-3
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New Journal of Chemistry
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DOI: 10.1039/C6NJ03769K
78x32mm (150 x 150 DPI)