Ionic liquids as recyclable and separable reaction media in Rh-catalyzed decarbonylation of aromatic and aliphatic aldehydes

Article abstract of DOI:10.1039/c4ra07103d

Ionic liquids (ILs) have been applied as recyclable reaction media in the decarbonylation of aldehydes in the presence of a rhodium-phosphine complex catalyst. The performance of several new catalytic systems based on imidazolium-based ILs and [Rh(dppp)₂]Cl (dppp: 1,3-diphenylphosphinopropane) were excellent in the decarbonylation of both aromatic and aliphatic aldehydes providing >99 yield of benzenes and alkanes, respectively. The catalytic performance depended, however, strongly on the employed IL and its thermal stability. In addition, the ILs afforded good catalyst immobilization as well as a biphasic system with the product allowing recovery and reuse of the employed catalyst.

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DOI: 10.1039/C4RA07103D
Ionic liquids as recyclable and separable reaction media in Rh-catalyzed
decarbonylation of aromatic and aliphatic aldehydes
Phillip Malcho, Eduardo J. GarcíaꢀSuárez, and Anders Riisager*
Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of
Denmark, DKꢀ2800 Kgs. Lyngby, Denmark
*Corresponding author: Eꢀmail address: ar@kemi.dtu.dk (A. Riisager); Fax: (+45) 45883136
Abstract
Ionic liquids (ILs) have been applied as recyclable reaction media in the
decarbonylation of aldehydes in the presence of a rhodiumꢀphosphine complex
catalyst. The performance of several new catalytic systems based on imidazoliumꢀ
based ILs and [Rh(dppp)2]Cl (dppp: 1,3ꢀdiphenylphosphinopropane) were excellent in
the decarbonylation of both aromatic and aliphatic aldehydes providing >99 yield of
benzenes and alkanes, respectively. The catalytic performance depended, however,
strongly on the employed IL and its thermal stability. In addition, the ILs afforded
good catalyst immobilization as well as a biphasic system with the product allowing
recovery and reuse of the employed catalyst.
Keywords: Aldehydes; decarbonylation; rhodium; ionic liquids; homogeneous
catalysis.
1. Introduction
Selective catalytic decarbonylation is an important transformation in both synthetic
chemistry and biology enabling oxygen removal from aldehydes as carbon monoxide
(CO) [1ꢀ4]. Such catalytically obtained CO may find use in organic tandem reactions
as in-situ source of CO, thus avoiding hazardous CO gas as an external reagent [5ꢀ
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11]. Additionally, selective decarbonylation can be a viable method for upgrading
carbohydrates and furfurals (e.g., 5ꢀhydroxymethylfurfural) into bioꢀfuels as well as
intermediate structures and chemical building blocks in bioꢀrefineries [12].
The decarbonylation reaction was conducted successfully for the first time in the
early 1960’s by cleaving organic molecules with stoichiometric amounts of
Wilkinson’s rhodium complex catalyst, [RhCl(PPh3)3], under mild conditions [13,14].
This approach required large amounts of reactants and the catalyst could not be
reused. Later significantly improved catalytic performance was shown to be obtained
with the catalyst system at elevated temperatures in high boiling solvents, where CO
abstraction from the rhodium center is facilitated [15]. Recently, the development of
more proficient homogeneous Rh decarbonylation catalysts has been thoroughly
investigated by Madsen and coꢀworkers [17ꢀ19]. Here it was found that that a
superior ꢀ but not recoverable ꢀ catalytic system could be obtained with two
equivalents of dppp (1,3ꢀdiphenylphosphinopropane) phosphine ligand and one
equivalent of [Rh(COD)Cl]2 (COD: 1,5ꢀcyclooctadiene) in diglyme as reaction media,
when the reaction was completed at temperatures above 165 °C. Subsequently,
good results with alternative catalyst systems based on Ir complexes and other
ligands have also been reported [20ꢀ23].
The recovery and recycling of the catalysts is still a challenge in homogeneous
catalysis [24]. This is also the case for Rhꢀcatalyzed decarbonylation reaction where ꢀ
to the best of our knowledge – only Blum et al. have reported so far a solꢀgel
entrapped catalyst that could be recycle up to seven times without any loss of activity
in aldehydes decarbonylation [25]. The use of ILs has emerged as a promising
alternative in order to overcome the problems associated to homogeneous catalysis.
Ionic liquids have in several reactions been demonstrated to be viable reaction media
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where catalyst immobilization is facilitated by phaseꢀseparation of the ILꢀcatalyst
phase and the product phase, including Rhꢀcatalyzed alkene hydroformylation [26]
and methoxycarbonylation [27]. Beside catalyst separation, such IL reactions are in
many cases also more benign and green compared to traditional processes using
organic solvents due to inherent IL properties such as, e.g. negligible vapor pressure,
good thermal stability and tunable coordination properties [28]. Furthermore, ILs can
also relatively easy be designed to accommodate functional groups which can
provide the ILs with auxiliary reactivity [29].
In this study, we present for the first time the use of ILs as reaction media in the Rhꢀ
phosphine complex catalyzed decarbonylation of aromatic and aliphatic aldehydes to
make benzene derivatives and alkanes, respectively (Scheme 1). Furthermore, it is
demonstrated that the use of an IL yields a biphasic system which can be recovered
and reused in successive reaction cycles.
Scheme 1. Decarbonylation reaction with IL catalyst system.
2. Experimental
2.1 Materials
All chemicals were purchased from SigmaꢀAldrich and used without purification
unless otherwise noted, including the ILs. The precursor metal compound
[Rh(dppp)2]Cl used for catalyst preparation were prepared under an inert atmosphere
of Argon (grade 5.0, Air Liquide) as described in literature [28].
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2.2 Catalytic reactions
A 10 ml flask was charged with 1.0 ml of IL (or solvent) and the desired amount of
[Rh(dppp)2]Cl was subsequently added to obtain a chosen metal loading. The
solution was then stirred for initially 30 min to ensure complete solvation, where after
55 ꢁl mesitylene (internal standard) was added and the mixture stirred for another 1
h at 165 °C with a water cooling unit preventing evaporation of the mesitylene. The
aldehyde substrate was then added to the reaction mixture and left for a desired time
period under continuous stirring. After reaction, 3 ml of deionized water and 3 ml of
diethylether were added to the IL mixture, the ethereal phase decanted off and dried
with magnesium sulfate and finally analyzed by GCꢀFID (HP 5890 Series II
chromatograph, SGE BP1 nonꢀpolar 100% dimethyl polysiloxane capillary column 50
m x 0.32 mm x 0.25 mm). Standard curves with authentic products and mesitylene
(internal standard) were used to quantify product yields.
2.2 Recycling experiments
The recycling experiments where performed as the regular catalytic reactions, but at
the end of each reaction run the product was separated and the remaining substrate
and/or products in the IL phase were extracted with diethylether. The yield and
conversion were calculated by means of GCꢀFID analysis. The ILꢀcatalyst solution
was subsequently applied directly for the next reaction run.
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3. Results and Discussion
The decarbonylation of aldehydes was conducted in batch reactions at 165 °C in
runs of 8 h or 24 h using catalyst systems composed of imidazoliumꢀbased ILs and
[Rh(dppp)2]Cl as preꢀcatalyst. This preꢀcatalyst was chosen because it has been
reported to facilitate decarbonylation of aldehydes in a wide variety of solvents and
temperatures [12ꢀ15,17ꢀ19]. The product yield was found to depend both on the IL
composition, catalyst concentration and substrate as reported below.
3.1Influence of the ionic liquid composition
Five ILs (Scheme 2) were tested in the decarbonylation of pꢀtoulaldehyde to produce
toluene from pꢀtolualdehyde using 3 mol% of the prepared [Rh(dppp)2]Cl complex as
preꢀcatalyst. The results are depicted in Figure 1.
Scheme 2. ILs tested in the studied decarbonylation reaction ([BMIm]: 1ꢀbutylꢀ3ꢀmethylimidazolium,
[EMIm]: 1ꢀethylꢀ3ꢀmethylimidazolium, [OMIm]: 1ꢀmethylꢀ3ꢀoctylimidazolium).
The highest reaction rate and product yield (48%) was achieved when [BMIm]Cl was
used as reaction media and the lowest yield (16%) when [BMIm]OAc was applied. By
changing the IL cation from [BMIm]+ to [EMIm]+ a similar trend in rate and yield was
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observed with Clꢀ as anion (30% yield) compared to using OAcꢀ as anion (23% yield).
The better performance of the chlorideꢀbased ILs can most be attributed to the higher
thermal stability of these ILs (see supporting information, S1), which will ensure that
the active complex remains solubilized. Concerning the alkyl chain length in the
imidazolium moiety, the best result (48%) was obtained for the chlorides with the
[BMIm]+ cation having an intermediate chain length. For the acetates the [EMIm]+ꢀ
based IL gave higher yield than the [BMIm]+ꢀbased (23 and 16 %, respectively).
50
40
30
20
10
0
48
36
30
23
16
Figure 1. Decarbonylation of pꢀtolualdehyde in different ILs. Reaction conditions: IL (1 ml), aldehyde
(8.5 mmol), mesitylene (0.4 mmol), 3 mol% [Rh(dppp)2]Cl, 165 °C, 8 h.
3.2 Influence of the catalyst loading and the substrate
Decarbonylation of pꢀtolualdehyde was next performed with different amounts of
[Rh(dppp)2]Cl preꢀcatalyst in [BMIm]Cl to examine the effect of catalyst loading. The
[BMIm]Cl was selected for further use as it proved superior in the initial IL screening
(vide infra). The obtained results are showed in Table 1. As expected, the yield of
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toluene increased gradually from 20 to >99 % when the amount of added catalyst
was increased from 2 to 5 mol% in 8 h reactions (Table 1, entries 1ꢀ4). Furthermore,
the IL system proved more efficient than the analogous system based on diglyme,
which is the best performing decarbonylation system reported so far [18]. Hence,
after 24 h of reaction 80 % toluene yield was obtained with the ILs system, whereas
only 71 % was achieved with pure diglyme (Table 1, entries 5 and 7). A mixture of
[BMIm]Cl and dyglyme (1:1 volume) was further selected as reaction media in order
to facilitate generation of a biphasic mixture that could allow ease separation of the
product. Here a comparable product yield of only 56 % was obtained after 24 h of
reaction (Table 1, entry 6), suggesting the interphase reaction to be significantly
slower than both the reaction with IL solvent and the one with diglyme. Importantly,
the reaction mixtures where IL were present seemed to provide more stable Rhꢀ
complex catalyst systems than the system with diglyme alone, where darkening of
the solution, due to Rh catalyst decomposition, was clearly observed after reaction
(Figure 2).
Table 1. Decarbonylation of aldehydes in different reaction mediaa
Entry Catalyst (mol%)
Solvent
[BMIm]Cl
[BMIm]Cl
[BMIm]Cl
[BMIm]Cl
Diglyme
Substrate
Yield (%)
1
2
2
3
4
5
1
1
1
pꢀtolualdehyde
pꢀtolualdehyde
pꢀtolualdehyde
pꢀtolualdehyde
pꢀtolualdehyde
20
48
3
97
4
>99
71
5b
6b,c
7b
Diglyme:[BMIm]Cl pꢀtolualdehyde
[BMIm]Cl pꢀtolualdehyde
56
80
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8
9
3
3
3
3
[BMIm]Cl
oꢀtolualdehyde
mꢀtolualdehyde
pꢀanisaldehyde
1ꢀnonanal
>99
>99
62
[BMIm]Cl
[BMIm]Cl
[BMIm]Cl
10
11
>99
a
Reaction conditions: Solvent (1 ml), aldehyde (8.5 mmol), mesitylene (0.4 mmol), catalyst
[Rh(dppp)2]Cl, 165 °C, 8 h. b 24 h reaction as in ref. 18; c [BMIm]Cl:diglyme = 1:1.
Figure 2. Pictures of the postꢀreaction mixtures with only [BMIm]Cl (left), 1:1 mixture of
diglyme:[BMIm]Cl (middle) and pure diglyme (right).
In order to evaluate the steric and electronic influence of the substrates in the
decarbonylation reaction, alternative aromatic aldehydes were also tested with the
[BMIm]Clꢀcatalyst system (Table 1,entries 8ꢀ10). To enable direct comparison of the
catalyst activity with the different substrates the catalyst loading was selected to be 3
mol% to provide moderate yields like obtained in the case of pꢀtolualdehyde (48 %,
Table 1, entry 2). The best results were obtained with oꢀtolualdehyde and mꢀ
tolualdehyde, where almost quantitative yield was achieved (Table 1, entries 8 and
9). The comparable yield obtained with a more electronꢀdonating methoxyꢀgroup
substituted in the para position was 62 % (Table 1, entry 10). These results suggest
that substitution in the orthoꢀ and the metaꢀpositions induce steric hindrance, which
eases the migratory extrusion step in the decarbonylation mechanism (as well as the
reductive elimination step) that has been proposed to be the rate determining with
the applied catalyst [17].
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The Rhꢀbased catalyst system could also be applied in decarbonylation of aliphatic
aldehydes, as exemplified for 1ꢀnonanal where an excellent yield of >99 % 1ꢀoctane
was obtained under the same reaction conditions used to obtain toluene from orthoꢀ
and metaꢀtolualdehyde (Table 1, entry 11).
3.3 Catalyst recyclability
One of the major issues in homogeneous catalysis is the recovery and reutilization of
the normally expensive precious metal catalyst. With this in mind, we examined the
recyclability of the [BMIm]Cl ILꢀcatalyst system in three consecutive experiments with
pꢀtolualdehyde and 5 mol% of the Rh catalyst. The results are shown in Figure 3.
99
100
90
85
80
60
40
20
0
Figure 3. Recyclability of the IL catalytic system in the decarbonylation of pꢀtolualdehyde. Reaction
conditions: [BMIm]Cl (1 ml), pꢀtolualdehyde (8.5 mmol), mesitylene (0.4 mmol), 5 mol% [Rh(dppp)2]Cl,
165 °C, 8 h.
As it is shown, the catalytic system could be recovered and reused for the three
consecutive reactions. The gradual observed drop in conversion upon reuse could be
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attributed to loss of IL and therefore catalyst during the IL phase recovery. The use of
the IL provided a biphasic system due to immiscibility of the product in the IL phase
and efficient catalyst immobilization. At this point, it is worthy to point out that no
1
signs of decomposition of the ionic liquid (BMimCl) were evidenced by H NMR
analysis just comparing a pure sample of the employed ionic liquid and the reꢀused
ionic liquid phase (see supporting information S2 and S4). In addition, no ionic liquid
was found in the ether phase after extraction, thus ensuring a good recyclability and
immobilization of the catalyst in the ionic liquid phase (see supporting information S2
and S3). In Figure 4, a picture of the orange colored IL phase after the three reaction
cycles is shown. Since, no sign of catalyst decomposition or Rh black are observed it
can be assumed that the IL stabilizes the catalytic species involved in the reaction.
Figure 4. [BMIM]Clꢀcatalyst phase after three reaction cycles in the decarbonylation of pꢀtolualdehyde.
4. Conclusions
Ionic liquids have been applied as reaction media for the first time in the
homogeneous decarbonylation of aldehydes with [Rh(dppp)2]Cl as catalyst. The
performance of the catalytic system with [BMIm]Cl was excellent in the
decarbonylation of both aromatic and aliphatic aldehydes allowing to obtain >99 %
product yield. Five different ILs were tested at similar reaction conditions and their
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catalytic performance correlated with thermal stability and ion composition
demonstrating both the anion and the cation to play a role. Furthermore, the ILs
provided a biphasic system with excellent catalyst immobilization allowing recovery
and successful reuse of the ILꢀcatalyst system at least three times. Notably, during
these consecutive runs the product was easily obtained by decantation without
impurities. In perspective, selective decarbonyaltion with ILs as reaction and
separable media could potentially be implemented as a key methodology to remove
oxygen in biomass valorization.
Acknowledgements
The Danish Council for Independent ResearchꢀTechnology and Production Sciences
(project no. 10ꢀ081991) is thanked for financial support of the work.
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