A highly efficient palladium(ii)/polyoxometalate catalyst system for aerobic oxidation of alcohols

Article abstract of DOI:10.1039/c4cy01632g

A simple catalyst system composed of Pd(OAc)₂, phosphomolybdic acid and tetrabutylammonium acetate oxidises a range of alcohols efficiently, with turnover numbers (TONs) of up to 10000.

Full text of DOI:10.1039/c4cy01632g

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A Highly Efficient Palladium(II)/Polyoxometalate
Catalyst System for Aerobic Oxidation of Alcohols
Cite this: DOI: 10.1039/x0xx00000x
Laura M. Dornan and Mark J. Muldoon*
Received XXXX
Accepted XXXX
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
simple catalyst system composed of Pd(OAc)2, using Cu(II) is not suitable for all reactions as it can have a negative
impact on the rate and selectivity. Copperꢀfree systems have been
employed which use mediators such as benzoquinone and iron
phosphomolybdic acid and tetrabutylammonium acetate
oxidises a range of alcohols efficiently, with turnover
numbers (TONs) of up to 10,000.
9
phthalocyanine (displayed in Figure 1(b)). Although these elegant
multiꢀredox processes can be very effective it would be desirable to
have a less expensive and simpler approach. An approach that we
feel is attractive is the use of polyoxometalates (POMs) or
heteropoly acids (HPAs) as ETMs (displayed in Figure 1 (c)).
Developing efficient catalysts for the aerobic oxidation of alcohols is
an important challenge as this transformation is often a problem in
1
the production of fine chemicals and pharmaceuticals. In a survey
of the transformations carried out by the pharmaceutical industry, it
was stated that “adjustment of the oxidation state from alcohol to
carbonyl is rarely performed even though it is a common feature of
1
many published targetꢀorientated syntheses”. This is because many
traditional methods produce significant amounts of toxic waste
which can be difficult to handle and to remove from the product.
Consequently there is a need to develop sustainable, catalytic
protocols for the oxidation of alcohols. Ideally these catalysts should
2
employ O as the terminal oxidant and palladium(II) based catalysts
2
3
show considerable promise for this challenge. In recent times there
has been significant developments in Pd(II) catalysts which employ
3
ligands to enable direct O coupled turnover. A limitation of such
2
systems is that they exhibit relatively low turnover numbers (TONs).
The best catalyst systems require between 0.1 and 0.5 mol% of
4,5,6
catalyst,
however most reported catalysts require higher loadings.
Such loadings may be suitable for the production of expensive
pharmaceuticals but the production of less expensive, high volume,
fine chemicals will require lower loadings. Furthermore, from a
sustainability point of view, we should be aiming to develop systems
that allow this precious metal to be used at very low loadings.
An alternative to using ligands is to employ electron transfer
mediators (ETMs), an approach which has a long history with Pd(II)
7,8
oxidation reactions, in particular Wackerꢀtype oxidations. The
ETMs oxidise Pd(0) to Pd(II) within the catalytic cycle and this
Figure 1: Schematic representation of the catalytic cycle of Pd(II) oxidation
reactions in the presence of electron transfer meditators. (a) Pd(II)/CuCl
system (b) Pd(II)/benzoquinone/iron phthalocyanine
(c) Pd(II)/polyoxometalates (POMs)
2
leads to an increased TON in these systems by slowing down
8
aggregation of Pd(0). The traditional Wacker process utilises CuCl
2
7
as an electron transfer mediator (displayed in Figure 1(a)), however
POMs and HPAs are metalꢀoxo clusters which are known to be
beneficial for Pd(II) oxidation systems, assisting in the oxidation of
a
School of Chemistry and Chemical Engineering, Queen’s University
10,11
Belfast, David Keir Building, Stranmillis Road, Belfast, UK, BT9 5AG.
E-mail: m.j.muldoon@qub.ac.uk
†
experimental details and optimisation experiments. See
DOI: 10.1039/b000000x/
Pd(0) to Pd(II) species.
There have previously been a number of
reports of Pd(II)/POM systems for the aerobic oxidation of
Electronic Supplementary Information (ESI) available giving
12,13
alcohols;
however the performance of these systems (in terms of
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DOI: 10.1039/C4CY016 G
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4,5,6,16
rate and TON) still requires significant improvement. For example, a reactions.
The organic cation is probably improving the
recent report from Huang et al. employed a catalyst composed of solubility of the H PMo O , an approach which is commonly
3
12 40
2
+
4ꢀ
10,11a
two [Pd(dpa)(DMSO)2] cation units and one [HPMo V O ]
exploited for POM catalysed reactions.
polyanion unit (where dpa= 2,2’ꢀdipyridylamine). This system mentioning that it is unusual that our system works well in organic
demonstrated the best TONs toꢀdate and employed 0.625 mol% of solvents such as toluene and ethyl acetate, as POM systems typically
It is also worth
10
2
40
1
3
this catalyst at 130 °C in DMSO with 1 atm of O . For activated require more polar solvents. We have employed ethyl acetate for the
2
substrates high yields could be obtained with reaction times of 6ꢀ12 majority of our studies. Although α,α,αꢀtrifluorotoluene (TFT)
†
hours, but the catalyst was not efficient for unactivated aliphatic delivered slightly higher rates in initial screening (see SI ) we
substrates (e.g. cyclohexanol only gave 18.4% conversion in 12 decided to utilise ethyl acetate as it regarded as a more desirable,
17
hours).
Herein we report the use of a simple catalyst system comprised
of commercially available components: Pd(OAc) , phosphomolybdic reaction showed better performance with a lower loading of 0.1
sustainable, organic solvent by industry.
We also optimised the loading of H
PMo12O40 and found that the
3
2
acid,
(H PMo O )
and
tetrabutylammonium
acetate mol%. It is likely that the polyoxometalate is acting as a ligand
3
12 40
(
[Bu N][OAc]). We chose to use H PMo O as it is commercially towards the Pd, as previous reports have crystal structures of Pdꢀ
4
3
12 40
12,13
available and inexpensive and it has previously been employed in POM structures.
Pd(II) oxidation systems. We have found that this catalyst system can be detrimental to the performance of Pd(II) catalysed alcohol
can deliver excellent performance using low loadings of Pd(OAc)2 oxidation, as excess ligand hinders key parts of the catalytic cycle,
It is known that high concentrations of ligand
1
4
1
6
even for unactivated aliphatic substrates. The simple catalyst such as Pdꢀalkoxide formation and βꢀhydride elimination.
requires no preꢀformation and the components can simply be added
to the flask or reactor. Selected examples of reaction optimisation Table 2: Oxidation of primary alcohols to aldehydes
†
are shown in Table 1 and full studies can be found in the SI.
Table 1: Selected examples of optimisation reactions.
0
.05 mol% Pd(OAc)2
OH
O
ETM
5
mol% additive
o
EtOAc, 100 C, 1 h
0 bar air
3
a
Ebntry
1
ETM
0.5 mol%
Additive
[Bu N][OAc]
% Yield
0
4
H PMo12O
3 40
2
3
none
0.5 mol%
[Bu
4
N][OAc]
none
24
0
3 40
H PMo12O
4
5
6
7
8
9
1.2 mol%
Cu(OAc)2
1.2 mol%
benzoquinone
0.5 mol%
H PMo12O
3 40
0.5 mol%
3 40
H PMo12O
0.5 mol%
H PMo12O
3 40
0.5 mol%
3 40
H PMo12O
0.5 mol%
3 40
H PMo12O
0.1 mol%
H PW12O
3 40
0.1 mol%
[Bu
4
N][OAc]
N][OAc]
9
0
[Bu
4
N-methylimidazole
0
NaOAc
0
[Bu
4
N][Br]
0
4
[Et N][OAc]
55
73
46
90
1
0
1
2
[Bu
[Bu
[Bu
4
N][OAc]
N][OAc]
N][OAc]
1
1
4
4
3 40
H PMo12O
a
determined by GC using biphenyl as an internal standard.
no Pd(OAc) added.
2
b
An important feature of the catalyst system worth highlighting is that
Bu N][OAc] is a crucial component in determining the excellent
[
4
performance. We tested a number of bases and salts and found that
we required an acetate anion with lipophilic cation
tetrabutylammonium was superior to tetraethylammonium while
sodium was inactive). It is known that organic salts / phase transfer
a
(
15
agents can have a dramatic effect in catalytic oxidation reactions.
In our system it is necessary to have an organic acetate salt, and it is
a
conversion and yield determined by GC using biphenyl as an internal
likely that [Bu N][OAc] is likely playing a number of roles. The
b
4
2
standard from an average of a minimum of two reactions; 60 bar 8% O /N2;
acetate is acting as a base and labile ligand; something which has
been well documented for aerobic Pd(II) catalysed alcohol oxidation
c
d
isolated yield; 1 mol% TEMPO was added
2
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Table 2 shows the oxidation of a number of benzylic alcohols
Copper/TEMPO systems are not effective for the oxidation
to their corresponding aldehyde. Previous work by Sheldon and of secondary alcohols. Replacing TEMPO with unhindered
19
coꢀworkers found that electron withdrawing groups lead to a nitroxyl radicals enables the oxidation of secondary alcohols,
reduction in reaction rate, and the poor yield obtained for but these are significantly more expensive than TEMPO at this
4
c
4
ꢀnitrobenzyl alcohol is in agreement with these studies. In time. Table 3 shows that the Pd/POM/[Bu N][OAc], system can
4
terms of halide substituents it would seem that those with a oxidise secondary alcohols efficiently, including aliphatic
greater propensity to undergo oxidative addition (Table 2, substrates. In the case of more reactive activated secondary
Entries 8 ꢀ 11) lead to catalyst deactivation. In the case of 1ꢀ alcohols we found that good performance could be obtained
octanol, such aliphatic alcohols readily undergo autoꢀoxidation with just 0.01 mol% Pd(OAc) , for example in the case of Entry
2
under the reaction conditions, producing the carboxylic acid 5 in Table 3, this corresponds to a TON of 10,000 in just 1
which can also lead to the ester (octyl octoanoate). In 1 hour we hour.
obtained 38% octanal at 62% conversion, with 16% octanoic
In the majority of reactions we utilised 30 bar of air
acid and 2 % octyl octanoate. The addition of the stable free pressure, but we are aware that on a larger scale the use of safer
20
radical
TEMPO
((2,2,6,6ꢀtetramethylpiperidinꢀ1ꢀyl)oxy) limiting oxygen concentrations (LOC) would be required or
reduces autoꢀoxidation and in 1 hour octanal can be obtained in engineering solutions such as microreactors may offer a safer,
2
1
5
2
7 % yield, at 64 % conversion with just 1% octanoic acid and more scalable approach. We explored the use of LOC, using
% octyl octanoate. Arguably the selective oxidation of O :N (8:92) and found it was necessary to employ higher
2
2
†
primary alcohols to aldehydes may be better carried out using a pressures to obtain good yields in this timeframe (see SI for
copper/TEMPO type system which can readily oxidise primary pressure studies)
We also found that these reactions gave
.
18
alcohols to the corresponding aldehyde with high selectivity.
similar performance on a larger scale (~2g substrate) in a 100
mL reactor (see SI ). In the majority of cases the reactions were
†
Table 3: Oxidation of secondary alcohols to ketones
highly selective towards the desired carbonyl. Conversion and
product yield were determined by GC using an internal
standard and excellent mass balance was observed in most
cases. We also obtained isolated yields for selection of the less
volatile products, and these were inꢀline with the GC data.
We are conscious that many research labs do not have access to
high pressure reactors and we also examined these reactions using
0
.05 mol% Pd(OAc)2
0
.1 mol% H3PMo12O40
O
OH
R2
R1
5 mol% [Bu N][OAc] R1 R2
4
o
EtOAc, 100 C, 1 h
3
0 bar air
Product
O
Conversiona Yielda
Entry
1
Starting Material
OH
90
4b
90
84b
standard glassware with an O balloon (Table 4). With a loading of
.1 mol% Pd(OAc) , excellent yields were observed in 16 hours. In
2
8
0
2
1
00
100
93b
79d
this case we used toluene as the reaction solvent as it is less volatile.
93b
2
OH
OH
O
7
9d
Table 4: Examples of oxidation in glassware with O balloon.
2
O
3
4
100
96
100
96
OH
O
OH
O
O
8
9b
00d
88b
99d
5
1
9
3c
OH
100
1
00
4c
6
7
9
OH
O
100
34
100
6c
9
OH
O
8
9
32
OH
O
a
determined by GC using biphenyl as an internal standard
isolated yield.
100
69
100
69
b
1
0
1
In terms of the role of the POM, it has previously been suggested
that H PMo O breaks down into phosphoric acid and Mo salts in
OH
O
3
12 40
4b
1
dilute solution. It is possible that under the reactions conditions,
the Keggin structure of the polyoxometalate is not maintained. To
explore this possibility we carried out some tests where we replaced
the H PMo O polyoxometalate with MoO (using an equivalent
1
OH
O
86
86
OH
O
1
2
38
34
3
12 40
3
mol% of Mo) (Figure 2). It was found that MoO also improved Pd
TON compared to when there was no Mo source present. The
3
a
conversion and yield determined by GC using biphenyl as an internal
b
standard from an average of a minimum of two reactions; 60 bar 8% O
2
/N
2
addition of H PO4 aids the reaction and similar effect was also
3
c
d
isolated yield; 0.01 mol% Pd(OAc)
2
observed with the addition of acetic acid. A full screen of acid
†
concentrations is presented in the SI. It has previously been found
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that the addition of small amounts of acid can aid ligand modulated Arends, G. –J. ten Brink and R. A. Sheldon, J. Mol. Catal. A: Chem., 2006,
6
b,22
2
51, 246. (f) M. Mifsud, K. V. Parkhomenko, I. W. C. E. Arends and R. A.
Pd(II) catalysed oxidation reaction,
reoxidation of Pd(0) to Pd(II), which goes via a Pd peroxo species.
In this study molybdenum is involved in the reoxidation of the Pd(0)
as protons are required in the
23
Sheldon, Tetrahedron¸ 2010, 66, 1040.
5
(a) D. R. Jenson, M. J. Schultz, J. A. Mueller and M. S. Sigman, Angew.
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1
4b
but it is known that acid also benefits this system.
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(a) D. S. Bailie, G. M. A. Clendenning, L. McNamee and M. J. Muldoon,
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0
.05 mol% Pd(OAc)2
OH
O
Mo source
2
526.
7
J. Smidt, W. Hafner, R. Jira, J. Sedlmeier, R. Sieber, R. Rüttinger and H.
5
mol% [Bu N][OAc]
4
o
Kojer, Angew. Chem. 1959, 71, 176.
EtOAc, 100 C, 1 h
8
Review on oxidations using ETMs: J. Piera and J. –E. Bäckvall, Angew.
30 bar air
Chem. Int. Ed., 2008, 47, 19, 3506.
9
J. E. Bäckvall, R. B. Hopkins, H. Grennberg, M. Mader, A. K. Awasthi, J.
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10
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(
Polyoxometalates, Chapter 9 in J. –E. Bäckvall (Ed.) Modern Oxidation
nd
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11
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Figure 2: Graph showing the effect of conditions and Mo source on the
15
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4
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Conclusions
5
16
The Pd(OAc) /H PMo O /[Bu N][OAc] catalyst system is
2
3
12 40
4
3
able to oxidise alcohols with very low loadings of Pd, with
TONs up to 10,000 demonstrated. H PMo O and
Bu N][OAc] are commercially available and are inexpensive
17
(a) K. Alfonsi, J. Colberg, P. J. Dunn, T. Fevig, S. Jennings, T. A. Johnson,
3
12 40
H. P. Kleine, C. Knight, M. A. Nagy, D. A. Perry and M. Stefaniak, Green
Chem., 2008, 10, 31. (b) D. Prat , O Pardigon , H.ꢀW. Flemming, S. Letestu,
V. Ducandas, P. Isnard, E. Guntrum, T. Senac, S. Ruisseau, P. Cruciani and
P. Hosek, Org. Process Res. Dev. 2013, 17, 1517. (c) D. Prat, J. Hayler and
[
4
additives. The catalyst system requires no preꢀformation with
the components simply added to the reaction. The catalyst is
also attractive as it allows ethyl acetate to be used as an organic A. Wells, Green Chem., 2014,16, 4546.
18
solvent. [Bu N][OAc] is crucial in delivering the excellent
(a) Recent reviews: (a) Q. Cao, L. M. Dornan, L. Rogan, N. L. Hughes and
4
catalytic performance.
M. J. Muldoon, Chem. Commun, 2014, 50, 4524. (b) Y. Seki, K. Oisaki and
M. Kanai, Tetrahedron Lett., 2014, 55, 3738. (c) B. L. Ryland and S. S.
Stahl, Angew. Chem. Int. Ed., 2014, 53, 8824.
19
Acknowledgments
(a) J. E. Steves and S. S. Stahl, J. Am. Chem. Soc., 2013, 135, 15742. (b)
L. Rogan, N. L. Hughes, Q. Cao, L. M. Dornan and M. J. Muldoon, Catal.
For support we thank the Department of Employment and
Learning, Northern Ireland, Queen’s University Belfast Ionic
Sci. Technol., 2014, 14, 1720.
20
(a) M. Molname, P. Mizsey and V. Schröder, J. Hazard. Mater., 2005, 1,
Liquid Laboratories (QUILL) and the Centre for the Theory 45. (b) C. –C. Chiang, J. –C. Lee, Y. –M. Change, C. –F. Chuang and C. –M.
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rd
Ivan Kozhevnikov (University of Liverpool) for insightful Introduction to Fire Dynamics, 3 edition,, Wiley, 2008. (d) E. Brandes and
W. Möller, Safety Characteristic Data: Flammable Liquids and Gases,
discussions.
Wirtschaftsverlag NW, Bremerhaven, 2008, vol. 1.
2
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K. Jähnisch, V. Hessel, H. Löwe and M. Baerns, Angew. Chem. Int. Ed.,
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