Cationic palladium(II) complexes as catalysts for the oxidation of terminal olefins to methyl ketones using hydrogen peroxide

Article abstract of DOI:10.1039/c4gc02465f

Ligated Pd(II) complexes have been studied for the catalytic oxidation of terminal olefins to their corresponding methyl ketones. The method uses aqueous hydrogen peroxide as the terminal oxidant; a sustainable and readily accessible oxidant. The choice of ligand, counterion and solvent all have a significant effect on catalytic performance and we were able to develop systems which perform well for these challenging oxidations.

Full text of DOI:10.1039/c4gc02465f

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Cationic Palladium(II) Complexes as Catalysts for the
Oxidation of Terminal Olefins to Methyl Ketones using
Hydrogen Peroxide
Cite this: DOI: 10.1039/x0xx00000x
Received 00th XX 201x,
Accepted 00th XX 201x
Qun Cao, David S. Bailie, Runzhong Fu and Mark J. Muldoon*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Pd(0).11,12,13,14, Kaneda and co-workers have developed a system that
Ligated Pd(II) complexes have been studied for the catalytic
oxidation of terminal olefins to their corresponding methyl
ketones. The method uses aqueous hydrogen peroxide as the
utilises N,N-dimethylacetamide (DMA) as a solvent and can oxidise
both terminal and internal olefins.13 This copper-free method utilises
catalyst loadings of 5-10 mol% Pd for internal olefins13b,c and 0.5
mol% for terminal olefins (Fig. 1 A),13a under O2 atmospheres (1-6
atm of O2 pressure). A limitation of this method is that it is not
effective for the oxidation of styrenes to their corresponding
acetophenones. In terms of aerobic systems for the oxidation of
styrenes, there are two recent notable reports. The first by Reiser and
co-workers utilised 5 mol% of a Pd(II) catalyst, which was prepared
from a ch.iral pseudo C2-symmetrical bis(isonitrile) ligand (Fig 1
B).12 More recently, Wang and co-workers reported a ligand free
system, which used 10 mol% Pd(OAc)2, along with (1 equiv.) of
trifluoroacetic. acid (TFA) in a DMSO/H2O (10:1) solvent system
(Fig 1 C). 14
Based on the state-of-the-art we were interested in trying to
develop new catalysts for Wacker-type oxidation of olefins, and we
were attracted to the use of hydrogen peroxide (H2O2) as the
terminal oxidant. Aqueous H2O2 is a sustainable oxidant which can
be readily used under mild conditions and has a track record of large
scale industrial use.15 Although H2O2 has been used for Wacker
oxidation chemistry, we felt there was scope for developing more
effective methods. Indeed, the use of H2O2 for Wacker oxidations
dates back some time, with Moiseev et al. reporting its use for the
Pd(II) catalysed oxidation of ethylene in 1960.16 In 1980, both the
Tsuji17 and Mimoun18 groups reported the use of H2O2 using simple
Pd(II). salts, and in the case of the Tsuji work, this included styrene
(Fig 1 D).17, Since then, others have examined various approaches to
improve the performance of Pd(II)/H2O2 for Wacker oxidation of
styrenes, such as using phase transfer catalysts,19 micelles,20 ionic
liquids21,22 and ligands appended to solid supports.23,24 In the work
to-date, the reports have not explored a wide range of substrates or
studied a wide variety of catalyst structures. Furthermore there is a
need to improve catalyst performance (i.e. reduce Pd loading and
improve product selectivity).
terminal oxidant;
a sustainable and readily accessible
oxidant. The choice of ligand, counterion and solvent all have
a significant effect on catalytic performance and we were able
to develop systems which perform well for these challenging
oxidations.
The oxidation of a terminal olefin to a methyl ketone is a very useful
transformation in organic chemistry, one which can be employed for
the preparation of fine chemicals, agrichemicals and
pharmaceuticals. The Wacker-Tsuji oxidation is a well-known
method for this reaction and generally employs PdCl2, CuCl2 and O2
in a DMF/H2O solvent system.1 There are a number of drawbacks to
this approach however, for example, there is a need to improve both
the rate and selectivity for many types of substrates. Although the
original Wacker Process utilised copper salts for the oxidation of
ethylene2 it has been highlighted that copper salts often do more than
just reoxidise Pd(0) back to the active Pd(II) species, and their
presence can lead to unwanted side-reactions and/or reduced
selectivity when trying to prepare ketones.3 Consequently, many
groups have investigated alternative approaches, developing
improved methods for the oxidation of both terminal and internal
olefins to their corresponding ketones. In an attempt to replace the
copper co-catalyst, alternative electron transfer-mediator systems,
such
as
polyoxometalates,4
metal
macrocycle/quinone
combinations,5 or benzoquinone/NaNO2/HClO4,6 have been reported.
Some researchers have also utilised oxidants such as benzoquinone7
iron(III) sulphate,8 chromium trioxide,9 and potassium bromate.10
Using O2 as the sole oxidant is attractive, and a number of methods
have been developed which utilise O2 for the direct re-oxidation of
School of Chemistry and Chemical Engineering, Queen’s University
Belfast, David Keir Building, Stranmillis Road, Belfast, UK, BT9 5AG.
E-mail: m.j.muldoon@qub.ac.uk
†Electronic Supplementary Information (ESI) available giving details of
optimisation experiments and experimental procedures. See
DOI: 10.1039/b000000x/
An alternative peroxide to H2O2 is tert-butyl hydroperoxide
(TBHP) which has also been examined for Pd(II) Wacker-type
reactions since 1980.17,25 Recent work by Sigman and co-workers
has demonstrated the benefits of developing ligand modulated
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catalysts for improving the performance of these TBHP mediated Therefore, we used acetic acid as the solvent and screened a number
oxidations.26,27 Their quinoline-2-oxazoline (quinox) based catalyst of ligands. Pleasingly we found that the presence of a ligand could
is shown (Fig 1. E).
improve both the rate of conversation and the selectivity towards the
Unlike Pd(II)/TBHP systems, there have been very few desired acetophenone. We then discovered that if we altered the
examples of studies using ligands to improve catalyst performance counterion from acetate and prepared the cationic analogue using
for Pd(II)/H2O2 systems; therefore we wished to examine a range of weakly coordinating anions then the resultant complex delivered
ligands for Pd(II)/H2O2 mediated Wacker-type oxidations.
good catalytic performance in acetonitrile; a solvent that had
previously been poor for Pd(OAc)2 complexes (see SI†). We
screened a number of ligands in acetonitrile, forming cationic
complexes in situ using bis(acetonitrile)dichloropalladium(II), ligand
and silver triflate. The ligands screened are shown in Figure 2.
Figure 1. Literature examples of Pd(II) catalysed Wacker oxidations.
We began our studies by testing Pd(OAc)2 with acetic acid as the
solvent, as this system had previously been used by Roussel and
Mimoun.18 In their studies they had focussed on aliphatic olefins and
when we examined this system for styrene we found that the reaction
gave a significant amount of unwanted side products. The selective
oxidation to one product is a particular challenge with styrene based
substrates. Styrenes can polymerise and it is known that with Pd(II)
catalysts and H2O2 several products can be formed. For example,
products such as styrene oxide, benzaldehyde, benzoic acid, 1-
phenylethane-1,2-diol, phenylacetaldehyde and 1,3-diphenyl-1-
butene can all be produced.19,22,24,28 It is difficult to quantify every
side product by GC and so, we focussed on accurately determining
the yield of the desired acetophenone and the conversion of the
substrate using an internal standard. Acetic acid is not ideal, as H2O2
and acetic acid will lead to the formation of peracetic acid, which
can oxidise styrene to styrene oxide, and styrene oxide can then react
further in acetic acid to form a number of products.29 GC-MS and
NMR analysis confirmed that we were producing such side-products
(see SI†). Unfortunately we found that replacing acetic acid with
other organic solvents led to poor catalyst activity (see SI†).
Figure 2.
Pd(II) / H2O2 oxidation of olefins.
Structures and acronyms of the ligands that were tested for
As shown in Figure 3, ligands had a substantial impact on both the
conversion of styrene and the yield of the desired acetophenone.
Further work will be needed to understand the observed ligand
effects, as even relatively similar ligands had quite different results.
We found that the 2-(2-pyridyl)benzoxazole (PBO) ligand resulted in
the best catalytic performance for the oxidation of styrene to
acetophenone. This ligand has similarity to the “Quinox” ligand used
by Sigman and co-workers for their TBHP mediated oxidations.26 As
shown in Figure 3, the Quinox ligand did not perform as well as
PBO under our reaction conditions. Although not commercially
available, PBO is an attractive ligand as it can be prepared in high
yields from inexpensive commercially available starting materials,
using a simple procedure that was reported by Hayashi and co-
workers (Figure 4).30
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Figure 3. Evaluation of ligands for the Pd(II)/H2O2 oxidation of styrene in acetonitrile. Experimental details in SI†.
Using the silver salt method there was little difference between
[OTf]- and weaker anions such as [PF6]- or [SbF6]-. Furthermore we
found that better catalytic performance could be obtained when
complexes were prepared without using this in situ silver salt
method. In the case of triflate, the (PBO)Pd(MeCN)2(OTf)2 complex
can be readily prepared and isolated using a facile procedure with
triflic acid (see SI†).
Figure 4. Synthesis of the 2-(2-pyridyl)benzoxazole (PBO) ligand
Given the effect of switching from acetate to triflate, we tested a
number of other counter ions, as shown in Figure 5.
We then utilised the (PBO)Pd(MeCN)2(OTf)2 complex for a range of
styrene substrates (Table 1). We found that with 1 mol% catalyst we
could obtain good yields in many cases. As mentioned earlier, we
utilised an internal standard to accurately determine the substrate
conversion and yield of acetophenone. We further validated this
method by obtaining isolated yields of several of the less volatile
products and found that the isolated yields were in good agreement
with those obtained via the GC method. Both steric and electronic
effects can be seen in Table 1. Having a substituent in the 2-position
reduces the yield, for example compare entries 2, 3, 4 and 14 and
also entries 7, 8 and 9. Electron withdrawing substituents reduce the
yield, with the nitro substituted styrene the worse affected, but this is
in-line with what was previously found with Sigman’s TBHP
system.26b The lowest yield was obtained for the sterically hindered
2,4,6-trimethylstyrene (Entry 14), but tests showed the yield could
be increased with higher catalyst loadings.
We then examined the oxidation of aliphatic olefins, using
1-octene as a model substrate. Aliphatic substrates are challenging
because of the competing isomerisation of the double bond,
something that Pd(II) complexes can readily facilitate.31 We found
that the (PBO)Pd(MeCN)2(OTf)2 complex in acetonitrile was not an
effective catalyst system as it resulted in a high degree of
isomerisation and only a small amount of the desired 2-octanone was
produced. Furthermore, negligible amounts of the internal isomers
were oxidised. We investigated a number of avenues to try and
improve catalyst performance and Table 2 shows some selected
examples, with further examples detailed in the SI†.
Figure 5. Evaluation of the counter ion in the Pd(II)/H2O2 oxidation of
styrene in acetonitrile. Experimental details in SI†. Entry 1 = Pd(OAc)2
Entry 2 = Pd(CF3COO)2 Entries 3-6 = 1 mol% pre-made & isolated
(PBO)PdCl2 complex and 2.5 mol% of the corresponding silver salt.
Entry 7 = pre-made & isolated (PBO)Pd(MeCN)2(OTf)2 complex.
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Table 1 Oxidation of styrenes with (PBO)Pd(MeCN)2(OTf)2
Table 2 Selected examples of the solvent and ligand effects on
the oxidation of 1-octene
During these studies we found that acetone was the best solvent and
that other ligands gave better results than PBO. We then re-
examined our series of ligands in acetone (Figure 6) and found that
bathophenanthroline (Bphen) was the best ligand for this substrate.
Although screening studies identified acetone as the best solvent we
felt that perhaps this was not an ideal solvent (particularly if these
reactions were to be scaled up), because acetone and H2O2 can lead
to the production of dangerous peroxides such as triacetone
triperoxide and diacetone diperoxide.32 We thought that perhaps the
higher yields of desired product were due to the formation of such
peroxides and these could be responsible for increasing the rate of
olefin oxidation, allowing the catalyst to compete with the undesired
isomerisation pathway. Previous work by the Alper33 and Sigman26
groups has shown that using THF as a solvent can promote aerobic
Wacker oxidation reactions. This is thought to be due to the in situ
formation of 2-tetrahydrofuryl hydroperoxide. More recently,
Weinstein and Stahl reported that peroxide species generated in situ
from 1,4-dioxane and O2 acted as the oxidant in palladium catalysed
aryl C–H amination reactions.34
We also tested 2-butanone as a solvent and found that this also
resulted in a good yield (Entry 8 Table 2). We presumed that this
further supported the idea of in situ formation of organic peroxides
that are then acting as oxidants, as 2-butanone and H2O2 can form
2-butanone peroxide (often referred to as MEKP (methyl ethyl
ketone peroxide)). Although 2-butanone peroxide is not without its
dangers,35 it is safer than acetone peroxide and is used industrially
(e.g. in the preparation of polymers). 2-butanone peroxide is
commercially available so we tested it as an oxidant and were
surprised to find that very little 2-octanone was produced (Table 3).
So if 2-butanone peroxide is being produced in situ it would appear
that it is not responsible for the improved oxidation of 1-octene.
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Figure 6. Evaluation of ligands for the Pd(II)/H2O2 oxidation of 1-octene in acetone. Experimental details in SI†.
Table 3 Attempted use of 2-butanone peroxide as an oxidant
Table 4 Influence of solvent and ligand on isomerisation
We then looked at the influence of the solvent on the isomerisation
of 1-octene to internal olefins in the absence of H2O2 (Table 4). It
can be seen from Entries 1-3 in Table 4 that isomerisation is faster in
acetone and 2-butanone compared to in acetonitrile. Of course in the
oxidation reactions, water is present as we utilise aqueous solutions
of H2O2 and when we added water we found that it had a significant
influence. In acetonitrile (c.f. Entry 1 and 4) the addition of water
increased the amount of isomerisation. However, when water is
added to acetone (c.f. Entries 2 and 5) and 2-butanone (c.f. Entries 3
and 6) there is a dramatic reduction in the amount of isomerisation
taking place. This reduction in isomerisation helps explain why these
solvents are good for oxidation of aliphatic olefins. Given that
ketones and water have such a dramatic solvent effect we wonder if
the presence of hydrates (geminal diols) could possibly be playing a
role in reducing isomerisation. However, further studies will be
required to understand the solvent effects on these reactions as the
solvent will undoubtedly influence other steps in the catalytic cycle.
For example in catalytic studies, ethyl acetate did not promote
isomerisation but the degree of oxidation was also low (see Table 2
Entry 5 and SI† for more examples).
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13 (a) T. Mitsudome, T. Umetani, N. Nosaka, K. Mori, T. Mizugaki, K.
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Mitsudome, K. Mizumoto, T. Mizugaki, K. Jitsukawa and K. Kaneda,
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5961.
In this study we have examined a number of ligands and
reaction conditions and this has enabled us to develop catalytic
methods for the oxidation of olefins to their corresponding
methyl ketones. The catalytic performance of these systems
compares favourably to those previously reported in the
literature. Further work needs to be carried out to understand
the influence of ligand structure and solvent on the performance
of these reactions. On a small lab scale we believe that the
methods are convenient and accessible with limited hazards.
Using acetone or 2-butanone with H2O2 would be more
hazardous on a larger scale, but we believe that systems could
be safely engineered. For example, Siegel and co-workers
recently described the advantages of using a continuous flow
system when phthaloyl peroxide was used as an oxidant.36 Flow
systems would ensure that the volumes remain small and all
peroxides could be easily destroyed at the end of the reaction.
Indeed, it is also worth pointing out that on an industrial scale,
regardless of the solvent, there needs to be precautions taken
when H2O2 is used.37
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Guo and Y.-Q. Wang, Org. Lett., 2014, 16, 1610.
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Acknowledgments
For support we thank the Department of Employment and Learning,
Northern Ireland, Queen’s University Belfast Ionic Liquid
Laboratories (QUILL) and the Centre for the Theory and
Application of Catalysis (CenTACat). We also thank Prof. Paul
Davey for insightful discussions.
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