Structure effect of imidazolium-based dicationic ionic liquids on claisen rearrangement

Article abstract of DOI:10.1080/00397911.2013.873940

A large group of imidazolium-based dicationic ionic liquids (DILs) has been prepared in good yields. Thermal stability of all DILs has been determined. The effects of the reaction time, cation, imidazolium C₂-H acidity, and anion on the Claisen rearrangement of allyl phenyl ether have been investigated. Type of anion and the presence of the acidic C₂-H bond in the imidazolium moieties have proven to be essential for this reaction. The simple procedure and the possibility of avoiding the use catalysts and volatile organic solvents make this synthetic method environmentally benign and adaptable for large-scale applications.

Full text of DOI:10.1080/00397911.2013.873940

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Structure Effect of Imidazolium-Based
Dicationic Ionic Liquids on Claisen
Rearrangement
Elina Priede a , Ilva Nakurte a & Andris Zicmanis a
a University of Latvia, Faculty of Chemistry , Riga , Latvia
Published online: 14 May 2014.
To cite this article: Elina Priede , Ilva Nakurte & Andris Zicmanis (2014) Structure Effect of
Imidazolium-Based Dicationic Ionic Liquids on Claisen Rearrangement, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 44:12, 1803-1809,
DOI: 10.1080/00397911.2013.873940
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Synthetic Communications1, 44: 1803–1809, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2013.873940
STRUCTURE EFFECT OF IMIDAZOLIUM-BASED
DICATIONIC IONIC LIQUIDS ON CLAISEN
REARRANGEMENT
Elina Priede, Ilva Nakurte, and Andris Zicmanis
University of Latvia, Faculty of Chemistry, Riga, Latvia
GRAPHICAL ABSTRACT
Abstract A large group of imidazolium-based dicationic ionic liquids (DILs) has been
prepared in good yields. Thermal stability of all DILs has been determined. The effects
of the reaction time, cation, imidazolium C2–H acidity, and anion on the Claisen rearrange-
ment of allyl phenyl ether have been investigated. Type of anion and the presence of the
acidic C2–H bond in the imidazolium moieties have proven to be essential for this reaction.
The simple procedure and the possibility of avoiding the use catalysts and volatile organic
solvents make this synthetic method environmentally benign and adaptable for large-scale
applications.
Keywords Allyl phenyl ether; Claisen rearrangement; dicationic ionic liquid; thermal
stability
INTRODUCTION
Ionic liquids (ILs) have acquired a remarkable status in organic synthesis
because of their advantageous properties.[1,2] Many task-specific ILs have been
synthesized and successfully applied in organic chemistry to improve yields and
decrease reaction times.[3] Recently, dicationic ionic liquids (DILs) have also attracted
Received December 1, 2013.
Address correspondence to Elina Priede, University of Latvia, Faculty of Chemistry, 19, Rainis
Blvd., Riga, 1586 Latvia. E-mail: priede_elina@inbox.lv
Color versions of one or more of the figures in the article can be found online at www.tandfonline.
com/lsyc.
1803

1804
E. PRIEDE, I. NAKURTE, AND A. ZICMANIS
increasing interest. The possible combinations of cations and anions in DILs provide
a broader variability of their properties than is the case for monocationic ILs.[4–9]
Various organic transformations have been performed by using monocationic and
dicationic ILs as solvents or catalysts.[2,10,11]
The Claisen rearrangement is a powerful carbon–carbon bond-forming
reaction that may be applied in the synthesis of heterocyclic compounds, condensed
aromatic structures, carboxylic acid derivatives, etc. Conventional conditions for the
Claisen rearrangement usually include the use of high temperature, high pressure,
and=or microwave irradiation, or different catalysts (ammonium chloride, transition
metal complexes, Lewis acids, Brønsted acids, bases, water, etc.). However, these
methods require the use of chemically and thermally stable solvents that provide
convenient conditions of the reaction.[12,13] Only a few groups have reported the
Claisen rearrangement in dicationic ILs. Armstrong and coworkers have performed
the thermal Claisen rearrangement of allyl aryl ethers in imidazolium- and
pyrrolidinium-based DILs C9(bim)2–NTf2, C9(mim)2–NTf2, and C9(mpy)2–
NTf2.[10] To the best of our knowledge, the results of this rearrangement in other
imidazolium-based DILs with shorter alkyl chains and different substituents on
the aromatic moieties have not been reported. In this communication, an examin-
ation of the use of imidazolium-based DILs (the same or similar to the DILs
reported in literature)[7] as solvents for the Claisen rearrangement of allyl phenyl
ether is reported. The effects of the reaction time, cation, imidazolium C2–H acidity,
and anion have been investigated. The simple procedure and the possibility of
avoiding the use of catalysts and volatile organic solvents make this approach
environmentally benign and adaptable for large-scale applications.
RESULTS AND DISCUSSION
Imidazolium-based DILs for this research were prepared by reacting substituted
ꢀ
imidazoles with a,x-dihaloalkanes. Halide anion exchange of DILs with NTf2 and
PF6 anions was performed according to the procedure reported in the literature[7]
ꢀ
with LiNTf2 and KPF6, respectively. To the best of our knowledge, the preparation,
properties, and application of DILs 5b, 5c, 9c, 9d (see the Supplementary Material),
1a, 1c (Fig. 1), 4a, 4c, and 4d (Fig. 2) have not been previously reported. Therefore, we
present here an advanced synthesis of these DILs and their application in the Claisen
rearrangement.
The application of DILs in the Claisen rearrangement has not yet been suffi-
ciently widely investigated; hence we have examined in more detail the role they might
play in facilitating this transformation. The main advantage of DILs in comparison to
their monocationic analogs is their increased thermal stability.[4] Our measurements
have shown that DILs with bis(trifluoromethylsulfonyl)imide anions are more ther-
mally stable than other DILs; the thermal stability of DILs with various anions
ꢀ
ꢀ
increases in the following sequence: Brꢀ ꢁ Clꢀ < PF6 < NTf2 (see the Sup-
plementary Material). Stability at high temperatures was one of the key advantages
considered in the selection of these DILs for use in thermal Claisen rearrangements.
Although the Claisen rearrangement in DILs has been reported previously,[10]
no attempts were made to clarify the impact of the structures of the media on the
product yields. In this communication, we report for the first time the use of the most

STRUCTURE EFFECT OF DICATIONIC IONIC LIQUIDS
1805
Figure 1. Yield of 2-allylphenol (3) obtained in DILs at different reaction times. (a) Determined by GC-
FID (internal standard method).
thermally stable DILs with the bis(trifluoromethylsulfonyl)imide anion as solvents
for the thermal Claisen rearrangement of allyl phenyl ether (2). The greatest yield
of the product, 2-allylphenol (3), was reached after 30 min (Fig. 1) and remained
at this level or slightly increased when the reaction time was extended to 60 min. Only
in some DILs was a decrease in the yield of 2-allylphenol (3) observed after 60 min,
due to side reactions of the product. Stirring the pure product 3 under the same con-
ditions as used for the Claisen rearrangement in the relevant DILs confirmed that
after 60 min 28% and 44% of the product had been converted into unidentified side
products. The results depicted in Fig. 1 also revealed that the distance (spacer length)
between the imidazolium moieties and the alkyl chain length at the imidazolium N
atom had only negligible effect on the yield of the rearrangement product 3.
Additionally, the effect of the C2–H acidity of the imidazolium cation on the out-
come of the Claisen rearrangement was investigated by comparing the products in four
DILs bearing methyl groups at their imidazolium C2 atoms with those in the analogous
DILs with C2–H bonds. This comparison [Fig. 2(I)] showed that, on the contrary, the
presence of the acidic C2–H bond in the imidazolium moieties was beneficial for the
Claisen rearrangement of allyl phenyl ether (2). This implies that, according to the
literature,[14,15] the C2–H bond of the imidazolium cation may be involved in additional
hydrogen bonding with the oxygen atom of the ether and therefore may be responsible

1806
E. PRIEDE, I. NAKURTE, AND A. ZICMANIS
Figure 2. (I) Yield of 2-allylphenol (3) after 30 min in DILs bearing methyl groups at their imidazolium C2
atoms and in the analogous DILs with C2-H bonds. (II) Impact of the anions of DILs on the yield of
2-allylphenol (3) after 30 min. (a) Determined by GC-FID (internal standard method). (b) The reaction
was performed under Ar atmosphere.
for the solute–solvent interaction in the investigated reaction systems in DILs. This
would also be in accordance with reported mechanistic insights into the Claisen
rearrangement.[16,17] Besides the diminished yields of the product 3, the lack of a
hydrogen at the imidazolium C2 atom also led to increased formation of side products.
The most significant C2–H bond effects on clean Claisen rearrangement after 30min
were observed for DILs 1e and 1h. For the selection of convenient DILs for the Claisen
rearrangement, the effect of the anions was also investigated.
A significant impact of the anions of the DILs on the Claisen rearrangement
was observed. A comparison of DILs with NTf2ꢀ; PF6ꢀ, and Brꢀ anions, having
a common 1,3-bis(3-butylimidazolium-1-yl)propane cation,ꢀrevealed that the yield
of the product decreased significantly in the DIL with PF6 (1b) as the anion and
that no product was detected in the DIL with Brꢀ (5a) as the anion [Fig. 2(II)]. This
might be attributed to the influence of the anions on the polarity of the DILs, with 1a
N
N
N
(ET ¼ 0:574) and 1b (ET ¼ 0:571) being more polar than 5a (ET ¼ 0:538). (The
N
normalized polarity parameter ET was determined for binary mixtures of DILs

STRUCTURE EFFECT OF DICATIONIC IONIC LIQUIDS
1807
with dimethylsulfoxide according to a known procedure.[18,19]) This assumption is
consistent with the reported polar solvent effect on acceleration of the Claisen
rearrangement.[20] However, some of the examined DILs with NTf2ꢀ anions are even
less polar than 5a, yet still provided reasonable yields of the product 3. This
prompted us to speculate that the cation–anion interaction in DILs could affect
the capability of DILs to interact with the transition state during the Claisen
rearrangement of allyl phenyl ether (2). To substantiate this assumption, the electro-
spray ionization (ESI) mass spectra acquired in negative-ion mode of the different
investigated DILs were analyzed. The results, based on the relative abundances
and peak distribution of naked anions (Aꢀ) and singly charged anionic aggregates
[(C þ 3A)ꢀ, (2C þ 5A)ꢀ, (3C þ 7A)ꢀ, etc.], have allowed us to propose the qualitative
order of bond strengths of the different anions to a,x-bis(3-alkylimidazolium-1-
yl)alkane cations as follows: Brꢀ >PF6ꢀ >NTf2ꢀ. This trend is in accordance with
the previously reported abilities of the anions to coordinate to the cation moiety
in monocationic ILs[21] and correlates with the results depicted in Fig. 2(II). In
addition to the aforementioned, the thermal stability of DILs 1a, 1b, and 5a was
considered as a possible reason for the observed results of the reaction. The relatively
lower thermal stability of DIL 5a than of DILs 1a and 1b (see the Supplementary
Material, Table S1), which is in accordance with the information in the literature,[7]
may also be crucial for this reaction. Partial decomposition (determined by 1H NMR;
see the Supplementary Material) of this solvent under the conditions of the thermal
Claisen rearrangement may lead to the loss of its functions and thereby reduce the
expected yield of the product 3.
Further studies on the structure effect of imidazolium-based DILs with OMs,
OTs, and OP(O)(OCH3)2 anions and their monocationic analogs on the thermal
Claisen rearrangement are in progress. Different cation and anion combinations pro-
vide the broad range of properties seen for DILs. This new information on the effects
of the cations and anions of imidazolium-based DILs on the Claisen rearrangement
should greatly expedite the design of new, advanced ILs for this reaction.
In conclusion, a large group of dicationic ionic liquids (DILs), nine of them
being new compounds, have been successfully applied as media for the thermal
Claisen rearrangement of allyl phenyl ether. The greatest yields of the product were
obtained in DILs with bis(trifluoromethylsulfonyl)imide as the anion and hydrogen
at the imidazolium C2 atom. The distance between the two imidazolium cations in
the DILs was found to have no significant impact on the yields of the Claisen
rearrangement product.
EXPERIMENTAL
Instruments and Reagents
All chemicals (except 1-butyl-2-methylimidazole, which was synthesized
according to a procedure reported in the literature[22]) were of analytical grade from
Alfa Aesar or Sigma-Aldrich and were used as received without any further
1
purification. H NMR spectra were recorded on a Varian 400 MR spectrometer.
For more information about the instruments and their parameters, see the
Supplementary Material.

1808
E. PRIEDE, I. NAKURTE, AND A. ZICMANIS
General Procedure for the Synthesis of Dicationic Ionic Liquids
The requisite substituted imidazole (30 mmol) and acetonitrile (25 mL), unless
mentioned otherwise, were placed in a 100-mL flask equipped with a reflux condenser
and a drying tube. The alkylating agent (15 mmol) was added at room temperature
and the resulting mixture was heated under reflux for 24 h. After cooling to room
temperature, the reaction mixture was filtered and the product was washed with cold
acetonitrile (2 ꢂ 5 mL) and then diethyl ether (2 ꢂ 10 mL). Any remaining solvent was
removed by means of a rotary evaporator and the product was dried under vacuum
(2 Torr) at 60 ꢃC for 2 h. For slight modifications of this procedure, see the
Supplementary Material.
DILs 1a, 1c, 1d, 1e, 1g, 1h, and 4a–d were synthesized according to the
procedure reported in the literature,[7] except that the reaction mixture was stirred
for 24 h. The reported procedure[7] for the preparation of DILs 1b and 1f was modi-
fied by using KPF6 instead of hexafluorophosphoric acid and by stirring the reaction
mixture for 24 h. DILs formed by anion-exchange reactions were tested with AgNO3
to verify that no halide impurities remained.
General Procedure for the Claisen Rearrangement in Imidazolium-
Based Dicationic Ionic Liquids
The rearrangement of allyl phenyl ether (2) was performed according to the
literature[10] and the molar ratio of substrate to DIL was 1:1. Unless mentioned other-
wise, the reaction was carried out under an air atmosphere. For kinetic studies and
quantitative analysis, 0.2 equiv. of an internal standard (biphenyl) was added to
the reaction mixture and five samples (taken after 5, 10, 20, 30, and 60 min, respect-
ively) in diethyl ether were prepared and analyzed by gas chromatography–flame ioni-
zation (GC-FID). The yield of the product 3 varied according to the reaction
conditions.
For the characterization of the product 3, the reaction mixture obtained in DIL
1a was extracted and purified according to the literature[10] by flash chromatography
(gradient CH3OH=CH2Cl2) to give 2-allylphenol (3) as a colorless liquid, 53% yield.
1H NMR (CDCl3) d 3.43 (2H, d, J ¼ 6.4 Hz, ArCH2CH), 4.95 (1H, s, ArOH),
5.21–5.14 (2H, m, CH2CHCH2), 6.10–5.97 (1H, m, ArCH2CHCH2), 6.82 (1H, dd,
J ¼ 7.9, 1.1 Hz, Ar), 6.90 (1H, td, J ¼ 7.4, 1.2 Hz, Ar), 7.17–7.09 (2H, m, Ar).
Rf(0.5% CH3OH=CH2Cl2) 0.47. m=z (EI, %) 134 (100, Mþ), 119 (41), 115 (39), 91
(49), 77 (36).
SUPPLEMENTAL MATERIAL
Supplemental data for this article can be accessed on the publisher’s website.
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