Imidazolium Based Fluorous N-Heterocyclic Carbenes as Effective and Recyclable Organocatalysts for Redox Esterification

Article abstract of DOI:10.1002/ejoc.202000273

A series of new highly fluorophilic ionic liquids (f > 110) was synthetized from 3-iodopropyltris(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane and N-alkyl imidazoles, followed by anion exchange. N-heterocyclic carbenes generated in situ from obtained imidazolium salts were employed to catalyze redox esterification (umpolung) of cinnamaldehyde with alcohols. The most effective N-methyl derivative with iodide as a counter anion was studied in detail with respect to the optimization of reaction conditions, substrate scope and recyclability. Recovery of the precatalyst was achieved using either fluorous extraction or performing the reaction in suitable fluorous biphase system with direct recycling of the fluorinated precatalyst phase. For both tested options, the catalytic activity did not significantly decrease within 5 subsequent cycles. The redox esterification was shown to proceed also in supercritical carbon dioxide (scCO₂) as an alternative solvent where the activity of the fluorinated catalyst was also superior to the nonfluorinated model, while retaining the benefit of easy recycling.

Full text of DOI:10.1002/ejoc.202000273

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Imidazolium Based Fluorous N-Heterocyclic Carbenes as
Effective and Recyclable Organocatalysts for Redox Esterification
Authors: Lucie Červenková Šťastná, Veronika Bílková, Tereza
Cézová, Petra Cuřínová, Jindřich Karban, Jan Čermák, Alena
Krupková, and Tomáš Strašák
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content of this Accepted Article.
To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202000273
Link to VoR: https://doi.org/10.1002/ejoc.202000273
01/2020

10.1002/ejoc.202000273
European Journal of Organic Chemistry
FULL PAPER
Imidazolium Based Fluorous N-Heterocyclic Carbenes as Effective and
Recyclable Organocatalysts for Redox Esterification
Lucie Červenková Šťastná,[a. b] Veronika Bílková,[a] Tereza Cézová,[b] Petra Cuřínová,[a. b] Jindřich
Karban,[a] Jan Čermák,[a. b] Alena Krupková,*[a. b] and Tomáš Strašák*[a. b]
[a]
[b]
Dr. L. Červenková Šťastná, V. Bílková, Dr. P. Cuřínová, Dr. J. Karban, Doc. J. Čermák, Dr. A. Krupková, Dr. T. Strašák
The Czech Academy of Sciences, Institute of Chemical Process Fundamentals
Rozvojová 135, 165 02 Prague, Czech Republic
E-mail: krupkova@icpf.cas.cz; strasak@icpf.cas.cz
Dr. L. Červenková Šťastná, T. Cézová, Dr. P. Cuřínová, Doc. J. Čermák, Dr. A. Krupková, Dr. T. Strašák
J. E. Purkyně University
České mládeže 8, 400 96 Ústí nad Labem, Czech Republic
Supporting information for this article is given via a link at the end of the document
Abstract: A series of new highly fluorophilic ionic liquids (f > 110)
was synthetized from 3-iodopropyltris(3,3,4,4,5,5,6,6,7,7,8,8,8-
tridecafluorooctyl)silane and N-alkyl imidazoles, followed by anion
exchange. N-heterocyclic carbenes generated in situ from obtained
imidazolium salts were employed to catalyze redox esterification
(umpolung) of cinnamaldehyde with alcohols. The most effective N-
methyl derivative with iodide as a counter anion was studied in detail
with respect to the optimization of reaction conditions, substrate
scope and recyclability. Recovery of the precatalyst was achieved
using either fluorous extraction or performing the reaction in suitable
fluorous biphase system with direct recycling of the fluorinated
precatalyst phase. For both tested options, the catalytic activity did
not significantly decrease within 5 subsequent cycles. The redox
esterification was shown to proceed also in supercritical carbon
dioxide (scCO2) as an alternative solvent where the activity of the
fluorinated catalyst was also superior to the nonfluorinated model,
while retaining the benefit of easy recycling.
Although many aspects of NHC catalyzed umpolung
reactions were studied and optimized, the methodology of
catalyst separation stayed somewhat aside. Increasing the
sustainability of organocatalytic processes and overcoming the
major drawback of organocatalysis, i. e. high catalyst loading,
by means of catalyst separation and recycling is still a challenge.
Inesi et al. applied electrogenerated NHCs as catalysts for the
redox esterification of enals, thus avoiding the use of any base
and lowering the number of reaction components.[13] Up to now,
the effort focused on the application of separable (pre)catalysts
was limited to benzoin condensation or Stetter reaction. Chen et
al. published benzoin self-coupling of furfural catalyzed by
polystyrene[14] or inorganic silica[15] supported azolium salts
paired with an acetate counterion. Poly(ionic liquid)s based on
imidazolium hydrogen carbonate were studied as precatalysts
for benzoin condensation of benzaldehyde.[16] Active NHCs
released from Ag(I)-crosslinked single-chain nanoparticles were
tested in the same benchmark reaction.[17] Products of parallel
Stetter reactions catalyzed by gel-supported thiazolium iodide
prepared via ROMPolymerization were obtained in high yields
and excellent purities after minimal purification.[18]
Introduction
The use of supercritical CO2 (scCO2) as an alternative green
solvent in different reactions is receiving considerable attention
due to its numerous advantageous properties.[19] Surprisingly, in
contrast to many reports on transition metal catalysis in scCO2,
studies dealing with organocatalysis in this environmentally
friendly medium are extremely rare.[20] This is probably due to
low solubility of polar organocatalysts in nonpolar scCO2.
In the course of our ongoing studies focused on tagging of
organic and organometallic compounds by fluorous chains[21] we
have recently synthesized a new type of fluorinated imidazolium
ionic liquid.[22] Here we report the synthesis, properties and
catalytic activity of several new organocatalysts bearing large
fluorous substituent in the redox esterification of
cinnamaldehyde, together with a suitable recycling procedure,
which is, in contrast to recyclable polymer supported catalyst
systems, based on a unique behavior of highly fluorinated
substances.[23] As a second benefit, the large fluorophilic domain
N-heterocyclic carbenes (NHCs) have rapidly gained interest in
many areas of chemical research,[1] particularly as excellent
ligands in metal-based catalytic reactions[2] but also as efficient
organocatalysts for a wide range of chemical transformations
and processes.[3] The ability of NHCs to induce the inversion of
polarity (umpolung) in the structure of aldehydes, described first
by Breslow,[4] was later utilized in several transformations e.g.
benzoin condensation[5] or Stetter reaction.[6] In the case of α,β-
unsaturated aldehydes, homoenolates are formed on the
reaction with NHCs via transfer of the nucleophilic character to
β-position, which was initially exploited for the synthesis of
γ-butyrolactones[7] and redox esterification of enals producing
saturated esters.[8] Further research was focused on the
reactivity of homoenolates towards different nucleophiles[9] and
on targeting the observed byproducts such as unsaturates
esters[10] or γ-lactones[11] as main products by modification of
reaction conditions and NHC type. Steric demands of the
substituent on nitrogen were identified as a key factor controlling
the reversibility of formation of the reactive species (Breslow
intermediate) and thus affecting the NHC performance.[12]
should also increase the solubility of the tagged organocatalysts
[24]
in scCO2,
so their performance was tested also in this
nonconventional solvent. To our best knowledge, this is the first
use of flourous NHCs as catalysts for umpolung reaction.
1
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10.1002/ejoc.202000273
European Journal of Organic Chemistry
FULL PAPER
Results and Discussion
As the precatalyst 1a showed the highest activity in preliminary
tests, it was used for further optimization, recycling tests and
substrate screening. We found that decreasing the precatalyst
loading from 20 to 10 mol % had a negligible effect on the
reaction rate while the yield of 3a increased; further decrease of
the precatalyst loading to 5 mol % led to slight decrease in the
reaction rate (Figure 1c). Presumably, at higher concentrations
NHCs are stabilized due to intermolecular interactions and so
the concentration of catalytically active species is lower than it
would correspond to precatalyst loading.[27]
As depicted in scheme in Table 1, the saturated ester 3a is
not the sole product of the studied reaction. The formation of
lactone 4, a product of aldehyde dimerization, was associated
with the onset of the reaction. Precatalysts of similar structure
were recently shown to yield predominantly γ-lactones.[8b,10a]
The second type of side product was identified as an
unsaturated ester 5a. Its emergence was ascribed to the
presence of oxygen in the reaction mixture and this hypothesis
was later confirmed by an experiment in oxygen atmosphere,
where nearly equal amount of both 3a and 5a was observed,
together with lower reaction rate (Table 1, Entry 13). This
particularly interesting behavior is likely caused by high affinity
of fluorous chains to oxygen[28] and it will be the subject of further
experiments as this type of reaction was described to proceed
with unsaturated aldehydes only in the presence of cocatalysts
that are easily oxidized by molecular oxygen.[29]
Unsymmetrically substituted imidazolium salts containing
fluorous moiety and methyl or isopropyl substituent were
prepared analogously to recently described butyl derivative.[22]
As shown in Scheme 1, this procedure involved quarternization
of imidazoles with silicon-branched fluorous tag containing
iodopropyl group. Compounds 1a and 2a were obtained as waxy
solids soluble in fluorinated as well as in polar protic (MeOH,
EtOH) and aprotic (DMSO, MeCN) solvents. Compounds 1b, c
and 2b, c were prepared by anion exchange. The reaction of 1a
with NaBF4 did not proceed to completion so AgBF4 was used
to obtain tetrafluoroborates. Hexafluorophosphate anion was
introduced by the reaction of 1a or 2a with NH4PF6. The new
ionic liquids 1a-c and 2a-c show the highest fluorophilicity
compared to literature data[25] even though they contain only cca
60 % of fluorine. This fact makes them ideal candidates for
application in fluorous biphasic catalysis.
To explore the scope of the catalyst, we employed several
types of alcohols as substrates in the redox esterification of
cinnamaldehyde with 1a as the precatalyst (Figure 2; Table 2).
All reactions were let to proceed for 5 h with 10 mol % of 1a and
20 mol % of DBU, the products were identified by GC-MS and
their amount and the conversion of cinnamaldehyde were
determined by CG-FID with respect to the internal standard.
Quantitative conversions and yields comparable to benzyl
alcohol were obtained in case of all tested primary
monofunctional alcohols, allyl alcohol proved to be an excellent
substrate giving 91 % yield. The only detected side products
were the lactone 4 (4 – 7 %) and unsaturated esters (1 – 5 %);
however, the difference between the conversion and overall
yield points to the presence of higher molecular weight
impurities (presumably oligomers of cinnamaldehyde) which
could not be isolated and identified. Secondary alcohols were
slightly less reactive while tertiary alcohol did not react at all;
cyclic alcohols gave better yields than acyclic ones. The amount
of detected side products was lower than in case of primary
alcohols, however the discrepancy between the conversion and
overall yield increased which points to greater extent of
aldehyde oligomerization. The sensitivity of the reaction to steric
characteristics of the substrates was well demonstrated in the
reaction with borneol; whereas the starting endo-1,7,7-
Scheme 1. Synthesis, fluorous partition coefficient P and fluorophilicity f of
fluorous imidazolium salts.
Catalytic activity of newly prepared compounds was tested
in redox esterification of cinnamaldehyde (Table 1). Preliminary
experiments were performed in inert atmosphere with toluene
as a solvent and benzyl alcohol as a nucleophile. First, we
examined the effect of anion on catalytic activity using rather
high loading (20 mol %). After 1 hour the conversion of aldehyde
was 77, 28 and 36 % for 1a, 1b and 1c, respectively (Table 1,
Entries 1-3), which corresponds quite well to the trend of
carbene stability as a function of the anion observed by Feroci
et al. for electrogenerated carbenes.[26] When comparing the
iodides, methyl substituted fluorinated carbene proved to be
more active than carbene generated from a nonfluorinated
model 1-ethyl-3-methylimidazolium iodide ([EMIM]+I-; Table 1,
Entry 7; Figure 1a), which in turn was much more efficient than
isopropyl derivative 2a. The addition of excess phenol as a
source of protons for the catalytic cycle, previously shown to
improve the outcome of this reaction[8a] led in our case only to a
slight increase in the reaction rate when sterically hindered 2,6-
di-tert-butyl-4-methylphenol (BHT) was used, whereas the
addition of 2 equiv of PhOH actually resulted in its decrease
(Figure 1b). Since the presence of phenols did not improve the
final yield of the target ester, next experiments were done
without any additive unless otherwise stated.
trimethylbicyclo[2.2.1]heptan-2-ol contained 11
% of exo-
isomer according to GC-FID, the amount of exo- ester in the
product mixture was only 3 %. Concerning halogenated alcohols,
their use as substrates is limited by their reactivity towards DBU,
which rules out iodides and bromides. 4-chloro-1-butanol gave
no product and aldehyde was recovered almost quantitatively.
The absence of any reaction implies the quarternization of the
base which in turn cannot deprotonate the imidazolium salt and
form the active catalyst. On the other hand, dichloroethanol and
tetrafluoropropanol provided the target esters in moderate yield,
in spite of lower reaction rate; the selectivity was comparable to
the reaction with primary alcohols.
2
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10.1002/ejoc.202000273
European Journal of Organic Chemistry
FULL PAPER
Table 1. Screening of precatalysts and reaction conditions for model redox esterification of cinnamaldehyde with benzyl alcohol.
Entry
Precatalyst
Loading [mol %]
20
Solvent
toluene
Time [h]
1
Conversion [%][a]
77
Yield 3a / 4 / 5a [%][a]
1a
54 / 7 / 3
1
1b
20
20
20
20
20
20
20
20
10
5
toluene
1
1
1
1
1
1
1
1
1
1
5
5
5
5
5
5
28
36
12
6
20 / 3 / 3
27 / 3 / 1
9 / 3 / 0.5
4 / 2 / 0.3
8 / 2 / 0.5
27 / 9 / 4
41 / 6 / 2
64 / 9 / 2
54 / 8 / 2
41 / 5 / 1
77 / 9 / 10
28 / 0 / 34
0 / 0 / 0
2
1c
toluene
3
2a
toluene
4
2b
toluene
5
2c
toluene
12
44
53
87
77
54
100
62
0
6
[EMIM]+I-
1a[b]
1a[c]
1a
toluene
7
toluene
8
toluene
9
toluene
10
11
12
13
14
15
16
17
1a
toluene
1a
1a[d]
20
20
20
20
20
20
toluene
toluene
1a
toluene/PFMC (4:1)
toluene/(C4F9)3N (4:1)
toluene/FC-770 (4:1)
toluene/FC-770 (4:1)
1a
53
80
88
25 / 5 / 4
39 / 10 / 2
53 / 10 / 2
1a
1a[e]
[a] determined by GC; [b] 2 equiv of PhOH were added; [c] 2 equiv of BHT were added; [d] 0.11 MPa O2; [e] recycled precatalyst from Entry 16.
Figure 1. Time-course of redox esterification of cinnamaldehyde with benzyl alcohol; (a) influence of catalyst structure: comparison of catalysts 1a, 2a and
[EMIM]+I; (b) impact of additional proton source on performance of catalyst 1a (no additive, BHT, PhOH); (c) influence of catalyst loading (20, 10 and 5 mol %
of 1a); (d) progress of reaction catalyzed by 1a in scCO2 (data points from separate experiments).
3
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10.1002/ejoc.202000273
European Journal of Organic Chemistry
FULL PAPER
aliphatic secondary alcohols (Table 2, Entry 20) and formation
of a mixture of products can be expected. In fact, it seems that
by contrast the secondary alcohol promotes the reactivity of
phenol when both of them are present in the reaction mixture,
as apparent from Entry 12. The addition of 2 equiv of phenol to
the reaction of cinnamaldehyde and borneol in 1:1 ratio did
increase the conversion and yield only with respect to the
reaction with phenol, while the target ester 3i was obtained in
much lower yield (cf. Entry 11, 12 and 20 in Table 2).
We were particularly interested in the proposal of suitable
protocol for catalyst recycling and its reuse in next reaction
cycles under the conditions of fluorous biphase catalysis (FBC).
Commonly used solvent system consisting of toluene and
perfluoromethylcyclohexane (PFMC) proved to be unsuitable in
our case because of low boiling point of PFMC (76 °C). On the
other hand, PFMC can be used to quantitatively separate the
precatalyst from the reaction mixture. By extraction with PFMC,
1a was recycled and reused five times without the loss of activity.
From other tested FBC systems with suitable boiling point
and miscibility properties, the smallest decrease of catalytic
activity was found for the solvent system toluene:FC-770 (Table
1, Entry 16). Although the reaction proceeds at slightly lower
temperature (declared b.p. of FC-770 is 90-98 °C) and the two
phases are not fully miscible at the reflux temperature of the
mixture, the conversion of the benchmark reaction was 80 %
after 5 h. The fluorinated phase containing precatalyst was
separated and reused in the subsequent cycle with even better
outcome (Table 1, Entry 17). The slightly lower yield compared
to the experiments in pure toluene can be due to lower
availability of alcohol reagent in the fluorinated phase and thus
higher propensity for formation of aldehyde oligomers. Lowering
of the reaction temperature to 75 °C while extending the
reaction time led to decrease of both yield and selectivity. The
following set of recycling experiments was therefore conducted
at FC-770 reflux temperature and the reaction time was set so
as not to reach the full conversion to allow a straightforward
comparison of the catalyst performance in particular runs. The
yield of 3a in the second and following cycles was again higher
than in the first cycle and the conversion over 90 % was reached
in 6 h with overall outcome 70 – 80 % according to GC (Table
3). No substrates or products are retained in the fluorinated
catalytic phase after its separation, as it was demonstrated by
changing the substrate from benzyl alcohol to cycloheptanol in
the 6th catalytic cycle (Table 3, Entry 6). No contamination of the
product mixture with benzyl alcohol or benzyl ester was
observed; moreover the yield of cycloheptyl ester was similar to
that obtained in pure toluene (cf. Table 2, Entry 8).
Figure 2. Target products of redox esterification of cinnamaldehyde with
different alcohols.
Table 2. Redox esterification of cinnamaldehyde with different alcohols.[a]
Entry
Substrate
Conversio
n [%][b]
Satur.
ester
Yield 3 / 4 / 5
[%][b]
1
1-butanol
100
100
99
3b
78 / 5 / 3
2
2-butanol
3c
60 / 0 / 0
3
3-pentanol
3d
55 / 0 / 1
4
t-butanol
0
-
0 / 0 / 0
5
allyl alcohol
propargyl alcohol
cyclohexanol
cycloheptanol
borneol
borneol[c,d]
borneol[d]
100
100
100
89
3e
91 / 5 / 1
6
3f
78 / 7 / 6
7
3g
64 / 3 / 2
8
3h
68 / 3 / 3
9
100
79
3i
71 / 2 / 1
10
11
12
13
14
15
16
17
3i
26 / 1 / 1
91
3i
41 / 0 / 0
borneol[d,e]
92
3i + 3p
3j + 3k
3j + 3k
3l + 3m
3l + 3m
3l + 3m
4 + 72 / 7 / 0
26 + 11 / 5 / 0
27 + 12 / 6 / 0
16 + 50 / 3 / 3[g]
39 + 2 / 17 / 0
37 + 1 / 11 / 0
1,2-propandiol
1,2-propandiol[c]
1,5-pentandiol[c,f]
1,5-pentandiol
1,5-pentandiol[c]
69
78
100
92
93
2,2,3,3-tetrafluoro-
1-propanol
18
65
3n
44 / 8 / 4
2,2-dichloro-1-
ethanol
19
20
70
66
3o
3p
49 / 6 / 8
56 / 5 / 1
phenol
[a] Conditions: 10 mol % of 1a, 20 mol % of DBU, 4 equiv of alcohol, 100 °C,
5h. [b] Determined by GC. [c] 2 equiv of BHT were added. [d] 1 equiv of
alcohol. [e] 2 equiv of PhOH were added. [f] 0.5 equiv of alcohol. [g] Yield of
5m (monounsaturated diester).
Table 3. Recycling of 1a in the redox esterification of cinnamaldehyde in FBC
The reaction with excess diols gives predominantly the
monoesters while the decrease of amount of 1,5-pentandiol to
0.5 equiv relative to the aldehyde in the presence of BHT leads
to the preferential formation of diester 3m. In case of
unsymmetrical 1,2-propandiol we were not able to distinguish
between the regioisomers 3j and 3k from the mass spectra but
we assume on the basis of general reactivity of primary and
secondary alcohols that the primary isomer 3j would be the
more abundant one. The attempt to add BHT to the reaction
mixture so as to promote the reaction with less reactive
substrates was unsuccessful, the yields were at best the same
as without this additive. Phenol itself cannot be used as such
additive because in our system its reactivity is similar to that of
system.[a]
Run
1
2[c]
Substrate
Conversion [%][b]
Yield of 3a / 4 / 5a [%][b]
53 / 13 / 3
benzyl alcohol
benzyl alcohol
benzyl alcohol
benzyl alcohol
benzyl alcohol
cycloheptanol
94
100
90
64 / 9 / 7
3
61 / 8 / 4
4
92
62 / 9 / 3
5
95
64 / 9 / 3
65[d]/ 4 / 2[e]
6
100
[a] Conditions: 20 mol % of 1a, 20 mol % of DBU, 4 equiv of alcohol, 95 °C,
toluene:FC-770 4:1, 6 h. [b] Determined by GC. [c] reaction time 7 h. [d] Yield
of 3h. [e] Yield of 5h.
4
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10.1002/ejoc.202000273
European Journal of Organic Chemistry
FULL PAPER
Table 5. Redox esterification of cinnamaldehyde with different alcohols in
scCO2, catalyzed by 1a.[a]
Since high fluorine content is known to increase the solubility of
a substance in scCO2, the prepared imidazolium salts should be
good candidates for the application in this medium. The catalytic
activity of 1a in the redox esterification of cinnamaldehyde with
benzyl alcohol was tested under various conditions and
compared to that of nonfluorinated analog, [BMIM]+I- (Table 4).
As preliminary experiments showed lower reaction rate in scCO2
than in conventional solvent, the catalyst loading was increased
to 15 mol %. The effect of temperature on the reaction outcome
was more pronounced in the case of [BMIM]+I- than with 1a,
which is likely due to poor solubility of this nonfluorinated ionic
liquid in scCO2. Thus, both Eyring-type and solubility
contributions can participate. At higher temperature, the ratio
between the performance of [BMIM]+I- and 1a and also the purity
of the obtained product is similar to the results obtained in
toluene; relative activity of 2a is low but somewhat higher than
in organic solvents. The pressure also influences the reaction
rate through changes in the solvation power (Entry 7).
Interestingly, the reaction in scCO2 is very sensitive to the type
and structure of the base; except DBU the desired product was
in much lower yield obtained only with potassium tert-butoxide
(Entry 8), whereas in the presence of triethylamine, DIPEA or
sodium ethanolate the reaction did not proceed. The progress
of the reaction under optimized conditions (15 mol % of 1a,
130 °C, 17 – 18 MPa) is shown in Figure 1d. The supercritical
reaction setup did not allow for taking samples in the course of
experiment, so each point represents a separate run under
identical conditions, let to proceed for a specified time.
Entry
Substrate
Product
3b
Yield [%][b]
1
2
3
4
5
1-butanol
30
40
46
19
21
1-butanol[c]
1-butanol[d]
cyclohexanol
cyclohexanol[c]
3b
3b
3g
3g
[a] Conditions: 15 mol % of 1a, 15 mol % of DBU, 4 equiv of alcohol, 130 °C,
17 – 18 MPa, 5 h. [b] Determined by GC. [c] 1.5 equiv of BHT was added. [d]
reaction time 12 h.
Table 6. Recycling of 1a in the redox esterification of cinnamaldehyde with
benzyl alcohol in scCO2.[a]
Entry
Conversion [%][b]
Yield 3a / 4 / 5a [%][b]
60 / 5 / 8
1
2
3
4
5
6
76
83
92
96
97
86
71 / 3 / 5
78 / 4 / 7
51 / 3 / 7
65 / 3 / 6
72 / 3 / 14
[a] Conditions: 15 mol % of 1a, 15 mol % of DBU, 4 equiv of alcohol, 130 °C,
17 – 18 MPa, 5 h. [b] Determined by GC.
Conclusion
We have shown a great potential of our fluorous tag for the
synthesis of highly fluorophilic catalytically active ionic liquids.
The introduced fluorinated domain increased the activity of
NHCs generated from the corresponding imidazolium salts in
the redox esterification of cinnamaldehyde with respect to
nonfluorinated analog while at the same time it enabled facile
separation of the precatalyst from the reaction mixture by means
of fluorous extraction and its recycling without the loss of activity.
The redox esterification employing the prepared fluorinated
catalysts can also be carried out in fluorous biphase system or
in scCO2 with only a slight decrease of the reaction rate. Also in
these systems the precatalyst is readily recovered and when
reused, it preserves its activity for at least 5 cycles. The
presented concept can be easily applied to the synthesis of
other classes of ionic liquids to make them fluorophilic and
recyclable. Beside this, the unexpected formation of
considerable quantity of unsaturated ester under aerobic
conditions opens up a question, whether it would be possible to
exploit the known high affinity of the fluorinated materials to
oxygen for the purpose of catalytic oxidative esterification of ,-
unsaturated aldehydes with molecular oxygen as the sole
oxidant.
Table 4. Redox esterification of cinnamaldehyde in scCO2.[a]
Entry
Precatalyst
Loading
[mol %]
Temperature
[°C]
Yield of 3a [%][b]
1
1a
8
130
130
130
100
110
130
130
130
100
110
130
130
54
60
79
19
37
72
61
14
5
2
1a
10
20
15
15
15
15
15
15
15
15
15
3
1a
4
1a
5
1a
6
1a
1ac)
1ad)
[BMIM]+I-
[BMIM]+I-
[BMIM]+I-
2a
7
8
9
10
11
12
30
52
21
[a] Conditions: DBU (equimolar to precatalyst), 4 equiv of BnOH, 17 – 18 MPa,
5 h. [b] Determined by GC. [c] 13 MPa. [d] t-BuOK instead of DBU.
As apparent from Table 5, the redox esterification in scCO2
is more substrate sensitive than when carried out in toluene (cf.
Table 2); the addition of BHT leads to a slight increase of the
reaction rate and yield. Clearly, physical properties, especially
the polarity of substrates, play an important role in the
supercritical reaction medium.
Experimental Section
The stability of 1a under supercritical conditions and its
General. All experiments were carried out in anhydrous conditions under
an inert atmosphere of argon or nitrogen, using standard Schlenk or
glove box techniques, unless otherwise stated. Solvents were dried by
usual procedures, then distilled and kept under argon. CDCl3,
CF2ClCFCl2 and FC-84 (commercially available fluorocarbon liquid with
average molecular weight 388 g.mol-1 and b.p. 80 °C) were dried over
molecular sieves and stored under argon. FC-770 (commercially
recyclability were evaluated in
a series of consecutive
experiments (Table 6). Similarly to the results in FBC system,
slightly lower activity was observed in the first run. Although
some fluctuations in conversion and yield were observed, there
is no decreasing trend which would point to catalyst loss or
degradation.
5
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202000273
European Journal of Organic Chemistry
FULL PAPER
available fluorocarbon liquid with average molecular weight 399 g.mol-1
1193.1080, found 1193.1082, (ESI-): Calcd. for [BF4]- 87.0024 (100%),
found 87.0018. NMR (CD3OD): 1H (400.1 MHz): 0.74 – 0.78 (m, 2H,
SiCH2CH2CH2); 0.96 – 1.00 (m, 6H, CH2CH2CF2); 1.89 – 1.93 (m, 2H,
CH2CH2CH2); 2.12 – 2.25 (m, 6H, CH2CF2); 3.93 (s, 3H, CH3); 4.22 (t, 3J
= 7.1 Hz, 2H, CH2N); 7.58, 7.64 (2 × t, 3J = 1.8 Hz, 2 × 1H, CH); 8.88 (br
s, 1H, CH). 13C (100.6 MHz): 1.7 (s, CH2CH2CF2); 8.6 (s, SiCH2CH2CH2);
and b.p. 90 – 98 °C) was obtained as a kindly gift from 3M and used as
received.
(3-Iodopropyl)tris(3,3,4,4,5,5,6,6,7,7,8,8,8-
tridecafluorooctyl)silane was prepared as previously described.[21] 1,8-
Diazabicyclo[5.4.0]undec-7-ene (DBU) was purchased from Acros
Organics. Dioctyl phthalate, propargyl alcohol and 2,2-dichloro-1-
ethanol were purchased from Sigma-Aldrich. Fluorinated chemicals
were purchased from Apollo Scientific. Other chemicals were from
laboratory stock and their purity was checked before use by GC-MS.
2
25.5 (s, CH2CH2CH2); 26.3 (t, JCF = 23.6 Hz, CH2CF2); 36.4 (s, CH3);
53.3 (s, CH2N); 109.0 – 122.6 (m, CF3, CF2); 123.5, 125.0, 137.8 (3 × s,
3 × CH). 29Si (79.5 MHz): 7.25. 19F (376.5 MHz): -154.38 (s, 10BF4); -
154.33 (s, 11BF4); -127.41 (m, 6F); -124.32 (bs, 6F); -123.95 (m, 6F); -
122.99 (m, 6F); -117.00 (m, 6F); -82.49 (t, 9F, J = 10.1 Hz).
NMR spectroscopy. NMR spectra were measured on Bruker Avance
400 or Varian Inova 500 (1H at 400.1/499.9 MHz; 13C at 100.6/125.7
MHz; 29Si {1H} (inept technique) at 79.5 MHz; 19F at 376.4 MHz) and 31
P
1-Methyl-3-(3-(tris(3,3,4,4,5,5,6,6,7,7,8,8,8-
at 361.95 MHz at 25°C. 1H and 13C chemical shifts (δ/ppm) are given
relative to solvent signals (δH/δC: CDCl3 7.26/77.16; CD3OD 3.31/49.00).
29Si and 19F spectra were referenced to external standards.
tridecafluorooctyl)silyl)propyl)imidazolium hexafluorophosphate
(1c). Iodide 1a (411 mg, 0.31 mmol) and NH4PF6 (56 mg, 0.34 mmol)
were dissolved in acetonitrile (2 mL) and the mixture was stirred at room
temperature for 2 days. The reaction mixture was extracted between
water and ethyl acetate and filtered through a pad of silica gel. The
solvent was evaporated and the product was dried in vacuum. Yield 300
mg (0.22 mmol, 72 %) of orange solid. HRMS (ESI+): Calcd. for
[C31H24F39N2Si]+ 1193.1080, found 1193.1081, (ESI-): Calcd. for [PF6]-
144.9647, found 144.9644. NMR (CD3OD): 1H (499.9 MHz): 0.74 – 0.77
(m, 2H, SiCH2CH2CH2); 0.97 – 1.00 (m, 6H, CH2CH2CF2); 1.88 – 1.95
(m, 2H, CH2CH2CH2); 2.13 – 2.24 (m, 6H, CH2CF2); 3.93 (s, 3H, CH3);
4.22 (t, 2H, J = 7.1 Hz, CH2N); 7.58, 7.64 (2 × t, J = 1.9 Hz, 2 × 1H,
CH); 8.90 (br s, 1H, CH). 13C (125.7 MHz): 1.7 (s, CH2CH2CF2); 8.7 (s,
SiCH2CH2CH2); 25.5 (s, CH2CH2CH2); 26.3 (t, 2JCF = 23.5 Hz, CH2CF2);
36.4 (s, CH3); 53.3 (s, CH2N); 106 – 121 (m, CF3, CF2); 123.6, 125.1,
138.0 (3 × s, 3 × CH). 29Si (79.5 MHz): 7.26. 19F (376.5 MHz): -127.40
(m, 6F), -124.32 (bs, 6F), -123.94 (br s, 6F), -122.96 (br s, 6F), -116.98
Mass spectrometry. HRMS spectra were measured on Bruker
MicrOTOF-QIII instrument in ESI+/ESI- (80 – 2000/30 – 1000 m/z,
calibrated by sodium formate) or APCI+ mode (50 – 2000 m/z, calibrated
by Tuning mix APCITOF (Fluka)).
Gas chromatography. GC/FID and GC/MS analyses were performed
on Agilent 6890 gas chromatograph equipped with DB-5MS column
(30 m_0.2 mm_0.25 mm). Injector and detector temperatures were set
at 260 °C and 300 °C, respectively. Helium was used as a carrier gas at
a flow rate 1.61 mL/min. Oven temperature was set at 50 °C for 3 min
and then programmed to 290 °C at a rate 10 °C/min. Quantitative data
were calculated using Agilent MSD Chemstation D.02 from the FID peak
areas assuming a linear response for the analytes and the internal
standard dioctyl phthalate and applying relative response factors
calculated from the molecular formula according to previously published
method.[30] For cinnamaldehyde, benzyl 3-phenylpropanoate and benzyl
cinnamate the results were corrected with respect to experimentally
determined calibration curve relative to dioctyl phthalate.
3
3
2
1
(m, 6F), -82.47 (t, 9F, JFF = 10.1 Hz), -74.71 (d, 6F, JPF = 707.8 Hz,
PF6). 31P (161.95 MHz): -144.62 (hept, 1JFP = 707.7 Hz, PF6).
1-Isopropyl-3-(3-(tris(3,3,4,4,5,5,6,6,7,7,8,8,8-
tridecafluorooctyl)silyl)propyl)imidazolium iodide
compound was synthetized according to a protocol for the synthesis of
1a, starting from (3-iodopropyl)tris(3,3,4,4,5,5,6,6,7,7,8,8,8-
(2a). This
1-Methyl-3-(3-(tris(3,3,4,4,5,5,6,6,7,7,8,8,8-
tridecafluorooctyl)silane and N-isopropylimidazole. HRMS (ESI+): Calcd.
for [C33H28F39N2Si]+ 1221.1393, found 1221.1389, (ESI-): Calcd. for [I]-
126.9036, found 126.9042. NMR (CD3OD): 1H (400.1MHz): 0.75 – 0.78
(m, 2H, SiCH2CH2CH2); 0.97 – 1.01 (m, 6H, CH2CH2CF2); 1.59 (d, 3J =
6.7 Hz, 6H, CHMe2), 1.92 – 1.98 (m, 2H, CH2CH2CH2); 2.13 – 2.26 (m,
tridecafluorooctyl)silyl)propyl)imidazolium
iodide
(1a).
(3-iodopropyl)tris(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane
(2.00 g, 1.62 mmol) was dissolved in toluene (12 mL), then N-
methylimidazole (0.14 g, 1.70 mmol) was added and the mixture was
stirred at 100 °C for 2 days. The reaction mixture was then cooled to 4 °C,
organic layer was separated and the product layer was washed with
toluene. Residual solvent was evaporated in vacuum. Yield 1.90 g (1.44
mmol, 89 %) of a honey-like compound. Compound 1a was also
3
3
6H, CH2CF2); 4.24 (t, J = 7.2 Hz, 2H, CH2N); 4.68 (hept, J = 6.7 Hz,
1H, CHMe2); 7.69, 7.77, 9.12 (3 × t, J = 1.7 Hz, 3 × 1H, 3 × CH). 13
C
3
(100.6 MHz): 1.8 (s, CH2CH2CF2); 8.8 (s, SiCH2CH2CH2); 23.0 (s,
CHMe2); 25.5 (s, CH2CH2CH2); 26.3 (t, 2JCF = 23.6 Hz, CH2CF2); 53.4 (s,
CH2N); 54.6 (s, CHMe2); 107 – 122 (m, CF3, CF2); 122.0, 123.8 (2 × s, 2
× CH); 136.3 (from HMBC, CH). 29Si (79.5 MHz): 7.25. 19F (376.5 MHz):
-127.37 (m, 6F); -124.29 (bs, 6F); -123.94 (m, 6F); -122.94 (m, 6F); -
116.91 (m, 6F); -82.47 (m, 9F).
prepared in
a similar manner without a solvent; unreacted N-
methylimidazole was evaporated in vacuum at 90 °C. Yield 2.10 g (1.59
mmol, 98 %). HRMS (ESI+): Calcd. for [C31H24F39N2Si]+ 1193.1080,
found 1193.1082, (ESI-): Calcd. for [I]- 126.9050, found 126.9054. NMR
1
(CD3OD): H (499.9 MHz): 0.75 – 0.79 (m, 2H, SiCH2CH2CH2); 0.97 –
1.01 (m, 6H, CH2CH2CF2); 1.89 – 1.99 (m, 2H, CH2CH2CH2); 2.14 – 2.25
(m, 6H, CH2CF2); 3.94 (s, 3H, CH3); 4.24 (t, 2H, 3J = 7.1 Hz, CH2N); 7.59,
7.66 (2 × t, 3J = 1.9 Hz, 2 × 1H, CH); 8.96 (br s, 1H, CH). 13C (125.7
MHz): 1.8 (s, CH2CH2CF2); 8.8 (s, SiCH2CH2CH2); 25.5 (s, CH2CH2CH2);
1-Isopropyl-3-(3-(tris(3,3,4,4,5,5,6,6,7,7,8,8,8-
tridecafluorooctyl)silyl)propyl)imidazolium tetrafluoroborate (2b).
This compound was synthetized according to a protocol for the synthesis
of 1b, starting from 2a. HRMS (ESI+): Calcd. for [C33H28F39N2Si]+
1221.1393, found 1221.1398, (ESI-): Calcd. for [BF4]- 87.0024 (100%),
found 87.0020. NMR (CD3OD): 1H (400.1 MHz): 0.74 – 0.78 (m, 2H,
SiCH2CH2CH2); 0.96 – 1.01 (m, 6H, CH2CH2CF2); 1.58 (d, 3J = 6.7 Hz,
6H, CHMe2), 1.89 – 1.97 (m, 2H, CH2CH2CH2); 2.12 – 2.25 (m, 6H,
2
26.3 (t, JCF = 23.5 Hz, CH2CF2); 36.5 (s, CH3); 53.3 (s, CH2N); 106 –
121 (m, CF3, CF2); 123.5, 125.1 (2 × s, 2 × CH); not detected (s, CH).
29Si (79.5 MHz): 7.23. 19F (376.5 MHz): -127.44 – -127.36 (m, 6F); -
124.30 – -124.27 (m, 6F); -123.95 (br s, 6F); -122.96 (br s, 6F); -117.02
– -116.88 (m, 6F); -82.50 (t, 9F, J = 10.2 Hz).
3
3
CH2CF2); 4.23 (t, J = 7.2 Hz, 2H, CH2N); 4.67 (hept, J = 6.7 Hz, 1H,
CHMe2); 7.66, 7.75, 9.03 (3 × t, 3J = 1.7 Hz, 3 × 1H, 3 × CH). 13C (100.6
MHz): 1.7 (s, CH2CH2CF2); 8.7 (s, SiCH2CH2CH2); 22.9 (s, CHMe2); 25.5
(s, CH2CH2CH2); 26.3 (t, 2JCF = 23.7 Hz, CH2CF2); 53.4 (s, CH2N); 54.6
(s, CHMe2); 107 – 122 (m, CF3, CF2); 122.0, 123.8, 136.0 (3 × s, 3 × CH).
29Si (79.5 MHz): 7.23. 19F (376.5 MHz): -154.28 (s, 10BF4); -154.33 (s,
11BF4); -127.40 (m, 6F); -124.33 (bs, 6F); -123.95 (m, 6F); -122.96 (m,
6F); -116.97 (m, 6F); -82.50 (t, 9F, J = 10.2 Hz).
1-Methyl-3-(3-(tris(3,3,4,4,5,5,6,6,7,7,8,8,8-
tridecafluorooctyl)silyl)propyl)imidazolium tetrafluoroborate (1b).
Iodide 1a (200 mg, 0.15 mmol) and AgBF4 (33 mg, 0.17 mmol) were
dissolved in acetonitrile (1 mL) and the mixture was stirred at room
temperature for 18 hours. The reaction mixture was washed with
aqueous solution of Na2S2O3 (1 M), extracted into ethyl acetate and
filtered through a pad of silica gel. The solvent was evaporated and the
product was dried in vacuum. Yield 163 mg (0.11 mmol, 71 %) of a
caramel-like compound. HRMS (ESI+): Calcd. for [C31H24F39N2Si]+
6
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202000273
European Journal of Organic Chemistry
FULL PAPER
1-Isopropyl-3-(3-(tris(3,3,4,4,5,5,6,6,7,7,8,8,8-
Keywords: Fluorous ionic liquids • N-heterocyclic carbenes •
Organocatalysis • Redox esterification • Umpolung
tridecafluorooctyl)silyl)propyl)imidazolium hexafluorophosphate
(2c). This compound was synthetized according to a protocol for the
synthesis of 1c, starting from 2a. HRMS (ESI+): Calcd. for
[C33H28F39N2Si]+ 1221.1393, found 1221.1408, (ESI-): Calcd. for [PF6]-
144.9647, found 144.9641. NMR (CD3OD): 1H (400.1MHz): 0.73 – 0.78
(m, 2H, SiCH2CH2CH2); 0.96 – 1.00 (m, 6H, CH2CH2CF2); 1.58 (d, 3J =
6.7 Hz, 6H, CHMe2), 1.88 – 1.96 (m, 2H, CH2CH2CH2); 2.11 – 2.24 (m,
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3
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6H, CH2CF2); 4.22 (t, J = 7.2 Hz, 2H, CH2N); 4.65 (hept, J = 6.7 Hz,
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3
(100.6 MHz): 1.7 (s, CH2CH2CF2); 8.7 (s, SiCH2CH2CH2); 22.9 (s,
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-124.33 (bs, 6F); -123.95 (m, 6F); -122.98 (m, 6F); -116.96 (m, 6F); -
82.51 (t, 9F, J = 10.2 Hz); -74.40 (d, 1JFP = 708.0 Hz, PF6). 31P (161.95
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This work was supported from ERDF/ESF project "UniQSurf -
Centre of biointerfaces and hybrid functional materials" (No.
CZ.02.1.01/0.0/0.0/17_048/0007411).
7
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202000273
European Journal of Organic Chemistry
FULL PAPER
B. M. Berven, G. A. Koutsantonis, B. W. Skelton, R. D. Trengove, A. H.
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10.1002/ejoc.202000273
European Journal of Organic Chemistry
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Let’s get fluorophilic! Tagging the imidazolium ionic liquids catalytically active in redox esterification with large fluorous substituent
enables their facile recovery from reaction mixtures and multiple reuse without loss of activity. It also allows for their application as
catalysts in fluorous biphase catalysis and supercritical CO2.
9
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