Borenium ionic liquids as catalysts for Diels-Alder reaction: Tuneable Lewis superacids for catalytic applications

Article abstract of DOI:10.1039/c7cy00106a

Ionic liquids based on the tricoordinate borenium cation were used for the first time as Lewis acid catalysts for a model Diels-Alder reaction. The conversion of the dienophile was successfully correlated with the Gutmann acceptor number values of the ionic liquids. Borenium ionic liquids exceeded the performance of catalysts reported in the literature.

Full text of DOI:10.1039/c7cy00106a

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Borenium ionic liquids as catalysts for Diels-Alder reaction:
tuneable Lewis superacids for catalytic applications
K. Matuszek,a† S. Coffie,b† A. Chrobok,a* and M. Swadźba-Kwaśnyb*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Ionic liquids based on tricoordinate borenium cation were used
for the first time as Lewis acidic catalysts for a model Diels-Alder
was attributed to the positive charge on the boron centre,
combined with lack of solvation from a molecular solvent in
reaction. The conversion of dienophile was successfully correlated neat ionic liquid.
AN values have been reported to correlate with catalytic
with the Gutmann Acceptor Numbers values of ionic liquids.
Borenium ionic liquids exceeded the performance of catalysts
reported in the literature.
activity of strong boron Lewis acids, such as BBr3 or BI3 (AN <
120).6 Nevertheless, in claiming practical importance of Lewis
superacidity of borenium ionic liquid, it was crucial to establish
that such correlation holds also for extremely high AN values.
In order to compare the new borenium systems to known
catalysts, a well-established model reaction was required.
Diels-Alder cycloaddition was selected, being a popular model
reaction to test the catalytic performance of Lewis acids.7
Diels–Alder reaction is a powerful tool in organic chemistry,
allowing for a one-step synthesis of six-membered carbocycles
from a diene and a dienophile (Scheme 2). Lewis acids catalyse
Diels-Alder reactions by coordination to the dienophile,
influencing both the reaction rate and the selectivity to
endo:exo or ortho/para isomers.8 A range of Lewis acids have
been studied to date, from simple Lewis acidic halides (ZnCl2,
BF3, SnCl4 or AlCl3), through chiral borenium cations,9 to ionic
Borenium cations are tricoordinate, strongly Lewis acidic
species, with acidity of the free p orbital of boron enhanced by
the positive charge.1,2,3 They have been used as homogenous
Lewis acidic catalysts in a range of reactions, including
borylations, transformations involving carbonyl groups, or as
components of frustrated Lewis pairs.1,3,4 Nearly all of these
studies were carried out in aprotic organic solvents.
Scheme 1 Synthesis of borenium ionic liquids, L = 1-methylimidazole (mim), 4-
(P888),
picoline (4pic), dimethylacetamide (dma), trioctylphosphine
trioctylphosphine oxide (P888O), M = Al or Ga, x = 1 or 2.
liquids.
Among
Lewis
acidic
ionic
liquids,10,11,12
chloroaluminate(III) systems were the most active catalysts
reported to date.12
Recently, ionic liquids based on borenium cations have been
reported. They were prepared in a two-step process: the
formation of
a tetracoordinate trichloroborane adduct,
followed by the chloride abstraction under solventless
conditions (Scheme 1). For a range of ligands, homogenous
ionic liquids were obtained as products.5 The ionic liquid
environment enabled the measurement of Lewis acidity of
borenium cations under solventless conditions. Gutmann
Acceptor Numbers were extremely high, up to AN = 182: this
Scheme 2 Diels-Alder cycloaddition of cyclopentadiene to ethyl acrylate.
Borenium ionic liquids, of a general structure shown in Scheme
1, were prepared according to the literature.5 A range of
ligands on the boron centre was tested: 4-picoline (4pic), 1-
methylimidazol
(mim),
trioctylphosphine
(P888),
trioctylphosphine oxide (P888O) and dimethylacetamide (dma).
Chlorometallate anions, formed through halide abstraction,
were either neutral: [AlCl4]- or [GaCl4]-, or Lewis acidic: [Al2Cl7]-
or [Ga2Cl7]-. Gutmann Acceptor Number values (unless
reported in the literature)5 were measured following the
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approach adapted for ionic liquids,13 with triethylphosphine anion. It also showcases that Diels-Alder cycloaddition is very
oxide as the 31P NMR spectroscopic probe. For ionic liquids sensitive to Lewis acid strength, making it an excellent model
based on chloroaluminate(III) anions, there two 31P NMR system.7 However, by altering the anion and/or the ligand on
signals were recorded, one within the range corresponding to the boron centre - Lewis acidity and thus catalytic activity can
chloroaluminate(III) anions (ANan = 90-98), and another to the be fine-tuned. The endo:exo selectivities remained constant
borenium cation (ANcat
> 120). For ionic liquids with (94:6 - 96:4), irrespective of the ionic liquid used.
chlorogallate(III) anions, only one strong signal was detected, The influence of catalyst loading (0.10 to 1.00 mol%) on
in the region corresponding to the borenium cation, ANcat. This conversion and selectivity was studied using one of the less
was in agreement with the literature findings.5 As a general Lewis acidic ionic liquid, [BCl2(4pic)][GaCl4] (ANcat = 121). Even
rule, Lewis acidity of borenium cations associated with this system proved to be very active, with ≥0.50 mol% loading
monomeric anions, [MCl4]-, was lower than that of identical sufficient to achieve full conversion within 5 min (Figure 2).
cations, but associated with dimeric anions, [M2Cl7]-. Endo:exo selectivities remained unaffected by the catalyst
Monomeric cations are stronger coordinating, which resulted concentration (94:6).
in stronger cation-anion interactions, and consequently
100
weaker Lewis acidity - this is also in agreement with prior
observations.5
80
The reaction of cyclopentadiene with ethyl acrylate (Scheme 2)
was used as a model Diels-Alder transformation. The reaction
60
was carried out solventless, with very small amount of ionic
liquid catalyst dissolved in neat reactants (with diene in
excess).14 Anticipating high activity of borenium ionic liquids,
they were firstly tested at 0 °C, at extremely low
concentrations of 0.1 mol%. For comparison, using a mildly
acidic chlorozincate(II) system, [choline]Cl-ZnCl2 (χZnCl2 = 0.67),
10 mol% of ionic liquid was required to achieve ca. 90%
conversion in 8 min, at ambient temperature.10 Even highly
acidic chloroaluminate(III) and chlorogallate(III) systems (χMCl3
= 0.67), required 5 mol% to reach 99% conversion in 5 min,
also at ambient temperature.15
40
20
0
0
0.10
0.2
0.25
0.50
1.00
Catalyst amount, mol %/EA
Figure 2. The influence of catalyst loading on the conversion of ethyl acrylate.
Reaction conditions: cyclopentadiene (1.586 g, 24 mmol), ethyl acrylate (1.602 g,
16 mmol), catalyst [BCl2(4pic)][GaCl4]; ice bath, 5 min, 1500 rpm, conversion
determined by GC.
Table
1 Conversions and endo:exo selectivities in Diels-Alder cycloaddition of
cyclopentadiene and ethyl acrylate, catalysed with borenium ionic liquids, AlCl3 and
chloroaluminate(III) ionic liquids, along with corresponding AN values.
Adj. R-Square = 0.9978
12
10
100
11
8
9
13
No
1
Catalyst
AlCl3
Amount, mol%
0.10
AN
α
EA, %
80
60
40
20
6
96 18
0.25
100
3
2
3
[C2mim]Cl
0.50
-
-
[C2mim][AlCl4]
0.50
3
0.10
3
4
5
6
7
[C2mim][Al2Cl7]
[BCl2(P888)][Al2Cl7]
[BCl2(4pic)][GaCl4]
[BCl2(4pic)][AlCl4]
9611
0.25
99
21
100
18
97
23
100
0.10
985
7
5
0.50
6
0.10
1215
1245
0.50
90
100
110
120
130
140
150
160
170
180
0.10
AN
0.50
Figure 1 Conversions in Diels-Alder cycloaddition of cyclopentadiene to ethyl
acrylate, catalysed with 0.10 mol% of Lewis acidic ionic liquids vs. AN values of
the catalysts; numbers on the figure are identical as catalysts number in table 1.
Reaction conditions: cyclopentadiene (1.586 g, 24 mmol), ethyl acrylate (1.602 g,
16 mmol), ice bath, 5 min, 1500 rpm, conversion determined by GC.
0.10
0.20
90
8
[BCl2(4pic)][Ga2Cl7]
1405
100
9
[BCl2(dma)][Al2Cl7]
[BCl2(dma)][Ga2Cl7]
[BCl2(4pic)][Al2Cl7]
[BCl2(mim)][Ga2Cl7]
[BCl2(mim)][Al2Cl7]
0.10
0.10
0.10
0.10
0.10
1635
1635
1705
1725
1745
98
97
Conversion of the dienophile at 0.10% loading of borenium
ionic liquid was plotted against ANcat values (Figure 1). It
appears that there is a Boltzmann sigmoidal correlation
between the AN values and the dienophile conversion. A very
good correlation was found with the experimental data (R2 =
0.9978), despite using extremely small catalyst amount, which
could be expected to bear significant measurement error. This
suggests that the main catalytic process involves the borenium
cation, irrespective of the Lewis acidic or neutral nature of the
10
11
12
13
100
100
98
Reaction conditions: cyclopentadiene (1.586 g, 24 mmol), ethyl acrylate (1.602 g,
16 mmol), ice bath, 5 min, 1500 rpm, conversion and endo:exo ratio determined
by GC.
All borenium ionic liquids tested in Diels-Alder cycloaddition,
the corresponding ANcat values, conversions at 0.10% catalyst
2 | J. Name., 2012, 00, 1-3
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loadings, catalyst loadings required to achieve full conversions must be carried out to reveal all possible catalytic species,
within 5 min, and corresponding endo:exo selectivities are especially when acidic [M2Cl7]- anions would be considered.
listed in Table 1.
a)
[BCl2(4pic)]+
For comparison, four experiments were carried out with
benchmark catalysts (Table 1): AlCl3, [C2mim]Cl, [C2mim][AlCl4]
and [C2mim][Al2Cl7], where [C2mim]+ is 1-ethyl-3-
methylimidazolium cation. AlCl3 is a Lewis acid (AN = 96),
commonly used in Lewis acid catalysis. The [C2mim][Al2Cl7]
ionic liquid is also a strong Lewis acid (AN = 96), with acidity
arising from the dimeric [Al2Cl7]- anion, and has been used in
b)
numerous
catalytic
applications.16,17,18
In
contrast,
[C2mim][AlCl4] is considered to be neutral, and thus
catalytically inactive. Finally, [C2mim]Cl is a Lewis base due to
the presence of chloride anion.
[BCl2(4pic)(EA)]+
Both benchmark Lewis acids: AlCl3 and [C2mim][Al2Cl7], had
negligible catalytic activity at 0.10% loading, but were
catalytically active at a higher concentration (0.25%). Neither
the neutral chloroaluminate(III) ionic liquid, [C2mim][AlCl4], nor
the basic halide salt, [C2mim]Cl, were catalytically active,
irrespective of concentration. In comparison, all borenium
ionic liquids were catalytically active at 0.10% loading, which
explicitly confirms that the catalytic activity originates from the
borenium cation.
ppm
Figure
3
11B NMR spectra of a) neat [BCl2(4pic)][AlCl4], and b) solution of
[BCl2(4pic)][AlCl4] in ethyl acrylate (EA). Both recorded with external d6-DMSO lock.
a)
Identification of the actual catalytic species is a very complex
task. Ionic liquid anions are known for their complex
speciation, with various chlorometallate anions remaining in
complex equilibria with each other (Eqn 1).17 Furthermore,
solution studies on borocations with chlorometallate
counterions reveal further complexity in speciation, due to
ligand scrambling (Eqn 2).3 The same scrambling was indeed
reported for borenium ionic liquids.5
b)
2[Ga2Cl7]- ⇄ [GaCl4]- + [Ga3Cl10]-
[BCl2L][AlCl4] ⇄ BCl3 + [AlCl3L]
(Eqn 1)
(Eqn 2)
ppm
Figure
4
27Al NMR spectra of a) neat [BCl2(4pic)][AlCl4], and b) solution of
[BCl2(4pic)][AlCl4] in ethyl acrylate (EA). Both recorded with external d6-DMSO lock.
In order to gain some insight into the identity of the main
catalytic species, an ionic liquid, [BCl2(4pic)][AlCl4] was
dissolved in excess of dienophile, ethyl acrylate (EA), and
studied by 11B and 27Al NMR spectroscopy. The dominant 11B
NMR signal shifted from 34 ppm in neat ionic liquid (Figure 3a)
- corresponding to tricoordinate borenium cation, [BCl2(4pic)]+,
To test the robustness of borenium ionic liquid catalysts, a
substrate scope study was carried out (Table 2). A range of
Diels-Alder cycloadditions, utilizing various dienes and
dienophiles (Figure 5), were carried out in the presence of 0.20
mol% of [BCl2(mim)][Al2Cl7] per dienophile. Full conversions
were obtained after 5-15 min reaction times at 0 °C, with
isolated yields ranging from 86 to 93%. Cyclopentadiene
reacted with dienophiles: diethyl maleate, methyl acrylate,
methacrolein and methyl vinyl ketone, to give full dienophile
conversion in 5 min at 0 °C, with very high endo selectivities,
to 6.4 ppm in ethyl acrylate solution (Figure 3b)
-
corresponding to
a
tetracoordinate boronium cation,
[BCl2(4pic)(EA)]+. The 27Al NMR spectrum of neat
[BCl2(4pic)][AlCl4] features a broad signal at 102 ppm with an
upfield shoulder (Figure 4a). Broadening arises both from the which is typical for Lewis acid catalysed reactions. Dienophiles
reacting with isoprene and cyclohexadiene reached full
conversion in 15 min at 0 °C, which again points to a very high
activity of the catalyst. Moreover, adducts of isoprene and
methyl acrylate or methyl vinyl ketone formed a nearly pure
endo isomer (endo:exo ratio of 95:5). Only in reaction of
cyclohexadiene with methyl vinyl ketone, slightly lower endo
selectivities was recorded (83:17). No other products (e.g.
viscosity of the neat ionic liquid and from dynamic equilibria,
and the shoulder corresponds to the ligand scrambling
product. The 27Al NMR spectrum recorded in ethyl acrylate
solution (Figure 4b) features a dominant sharp signal at 102
ppm - characteristic of [AlCl4]-, and a very weak and broader
signal at 92 ppm, corresponding to AlCl3 with an O-donor,
namely [AlCl3(EA)]-. This demonstrates that the dienophile
binds to the borenium cation and, despite its excess, to a very
small extent it exchanges with chlorides in [AlCl4]-. It must be
pointed out that this is just an insight, and detailed studies
haloborated side-products) have been detected
.
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Table 2 Diels-Alder reaction of various dienes with dienophiles in the presence of
[BCl2(mim)][Al2Cl7] as catalyst.
catalytic activity was found to correlate to ANcat values,
irrespective of the anion, (ii) borenium ionic liquids were active
at 0.10% loading, at which Lewis chloroaluminate(III) ionic
liquid, [C2mim][Al2Cl7], was inactive, and (iii) borenium ionic
liquids with [MCl4]- anions were catalytically active, whereas
[C2mim][AlCl4] was inactive, irrespective of the loading.
αdienophile
,
Isolated
yield, %
92
Isomer
ratio, %
98:2
Diene
Dienophile
Product
%
1
1
1
1
2
2
3
a
b
c
1a
1b
1c
1d
2b
2d
3d
100
98
93
95:5
100
99
91
98:2
d
b
d
d
92
94:6
Acknowledgements
100
100
99
91
95:5
AC and KM would like to acknowledge the National Science
Centre of Poland for grant no. UMO-2012/06/M/ST8/00030.
SC would like to thank the COST Action (CM1206 EXIL -
Exchange on Ionic Liquids) for supporting STSM in Silesian
University of Technology, Gliwice, Poland.
90
95:5
90
83:17
Reaction conditions: diene (24 mmol), diepnophile (16 mmol), catalyst loading
0.20 mol% of [BCl2(mim)][Al2Cl7] per diepnophile; ice bath, 5-15 min, 1500 rpm.
Notes and references
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8
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Figure 5 Dienes and dienophiles used in Diels-Alder reaction.
14 Diels-Alder reaction: The dienophile (16 mmol) and the
borenium ionic liquid (0.1 mol% per dienophile) were placed
In conclusion, Lewis superacidity of borenium ionic liquids,
previously postulated from high AN values, was confirmed in
exceptionally high catalytic activity in Diels-Alder reaction. Due
to their ionic liquid form, it the use of solvent was avoided;
small amounts of ionic liquid were dissolved into the reactant
mixture, thereby avoiding ‘levelling effect’ from the solvent
and lowering potential environmental impact. At very low
catalyst concentration (0.10 mol%), good correlation between
the Lewis acidity strength (AN values) and the conversion of
the dienophile was observed. Upon increasing the catalyst
loading to 0.20-0.50 mol%, full conversions were reached in 5
min at 0 °C. Endo:exo selectivities were consistently high,
in
a two-necked round-bottomed flask equipped with
septum, argon balloon, ice-bath and stirring bar. The diene
(24 mmol) was added dropwise to the vigorously stirred
(1500 rpm) reaction mixture. The reaction was carried out
for 5-15 min, and then quenched with a few drops of water.
Dichloromethane (2 ml) and decane (internal standard) were
added. Conversion of ethyl acrylate and selectivity were
measured by GC. Products were purified by column
chromatography (silica gel, 100% of chloroform as eluent).
15 C. W. Lee, Tetrahedron Lett., 1999, 40, 2461.
16 H.-P. Steinrück and P. Wasserscheid, Catal. Lett., 2014, 145
380.
,
17 J. Estager, J. D. Holbrey and M. Swadźba-Kwaśny, Chem. Soc.
Rev., 2014, 43, 847.
unaffected by the catalyst AN value or loading. Borenium ionic 18 K. Matuszek, A. Chrobok, P. Latos, M. Markiton, K.
Szymanska, A. Jarzebski and M. Swadzba-Kwasny, Catal. Sci.
Technol., 2016, , 8129.
liquids were catalytically active against a wide substrate scope,
demonstrating robustness of these systems.
6
Anions in borenium ionic liquids were either monomeric and
thus neutral, [MCl4]-, or dimeric and thus Lewis acidic, [M2Cl7]-.
This could potentially raise concern as to the nature of the
catalytic species. This issue was addressed on several levels: (i)
4 | J. Name., 2012, 00, 1-3
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Ionic liquids with Lewis superacidic borenium cations were used as catalysts in solventless Diels-Alder cycloaddition.
Extremely high catalytic activity correlated with Lewis acidity, expressed as Gutmann acceptor number.