Novel bronsted acidic ionic liquids and their use as dual solvent-catalysts

Article abstract of DOI:10.1021/ja026290w

The reaction of triphenylphosphine or N-butylimidazole with cyclic sultones gives zwitterions that are subsequently converted into ionic liquids by reaction with trifluoromethane sulfonic acid or p-toluenesulfonic acid. The resulting ionic liquids have cations to which are tethered alkane sulfonic acid groups. These Bronsted acidic ionic liquids are useful solvent/catalysts for several organic reactions, including Fischer esterification, alcohol dehydrodimerization and the pinacol rearrangement. The new ionic liquids combine the low volatility and ease of separation from product normally associated with solid acid catalysts, with the higher activity and yields normally found using conventional liquid acids. Copyright

Full text of DOI:10.1021/ja026290w

Published on Web 05/07/2002
Novel Brønsted Acidic Ionic Liquids and Their Use as Dual Solvent-Catalysts
Amanda C. Cole, Jessica L. Jensen, Ioanna Ntai, Kim Loan T. Tran, Kristin J. Weaver,
David C. Forbes,* and James H. Davis, Jr.*
Department of Chemistry, UniVersity of South Alabama, Mobile, Alabama 36688
Received March 22, 2002
From undergraduate laboratories to chemical manufacturing
plants, the use of strong Brønsted acids is ubiquitous.1 In this
context, solid acids are being more widely used since, as nonvolatile
materials, they are deemed less noxious than traditional liquid acids.2
Still, solid acids have shortcomings as well. Among the more
troublesome of these are restricted accessibility of the matrix-bound
acidic sites, high molecular weight/active-site ratios, and rapid
deactivation from coking.3,4
Figure 1. Brønsted acidic IL and the precursor zwitterions.
Scheme 1. Representative Reactions Using IL 1a and 2a
Bearing in mind both the advantages and disadvantages of solid
acids, the identification of systems that are Brønsted acids with
solidlike nonvolatility but that manifest the motility, greater effective
surface area, and potential activity of a liquid phase continues to
be useful. Combining just these characteristics, ionic liquids (IL)
have been described as one of the most promising new reaction
mediums.5 Not only can these unusual materials dissolve many
organic and inorganic substrates, they are also readily recycled and
are tunable to specific chemical tasks.6-11 While IL that manifest
innately Lewis-acidic character are well-precedented and have been
thoroughly studied (especially those based upon chloroaluminate
anions12) we report here what we believe to be the first ionic liquids
that are designed to be strong Brønsted acids. In each of the new
IL, an alkane sulfonic acid group is covalently tethered to the IL
cation.
to IL 1a and 2a (Figure 1).14 Each acidification is accomplished
by stirring together the neat reagents and warming gently for
2-24 h.
The synthetic approach used to assemble the zwitterionic
precursors to these acidic IL is well-precedented. Reaction of the
neutral nucleophiles N-butyl imidazole or triphenylphosphine with
1,4-butane- or 1,3-propane sultone, respectively, produces the
requisite zwitterions in excellent yields.13 In the second step, the
simultaneous realization of the latent acidity of the zwitterions and
their conversion into ionic liquids is accomplished. The chemical
yields for both the zwitterion formation and acidification steps are
essentially quantitative. Moreover, since neither reaction produces
byproducts, the IL syntheses are 100% atom efficient.
The zwitterion acidification is accomplished by combining 1:1
molar quantities of the zwitterions with an acid possessing a pKa
sufficiently low to convert the pendant sulfonate group into an
alkane sulfonic acid, the pKa of the latter being expected to be ∼-2.
The result is the transformation of the zwitterion into an IL cation
bearing an appended sulfonic acid group, with the conjugate base
of the exogenous acid becoming the IL anion. Given that in these
systems effectively two sites of formal negative charge are present
per acidic proton, the systems may be regarded as internally self-
buffered. For the IL syntheses reported here, the donor acids were
trifluoromethane sulfonic acid and p-toluenesulfonic acid hydrate,
pTSA‚H2O. These acids were chosen largely because of the
resistance of their anions toward hydrolytic decomposition, a
common problem with some strong acid anions (e.g., PF6-). These
acids were then used to convert, respectively, zwitterions 1 and 2
The new IL 1a is a somewhat viscous liquid at room temperature,
while 2a is a stiff glass that liquefies around 80 °C. In keeping
with the behavior of other IL, neither of the new species fumes or
manifests any observable degree of vapor pressure, unlike strong
acids dissolved in conventional IL, which frequently continue to
emit noxious vapors. Further, treatment of 1a under vacuum (10
Torr) at 150 °C results in no observed loss of triflic acid (CF3-
SO3H bp ) 162 °C at 760 Torr) from the IL. Conversely, washing
2a with toluene or diethyl ether results in no extraction of free
pTSA (soluble in either liquid). Both of these behaviors are
consistent with the donor acids being fully incorporated into their
respective IL structures, rather than remaining simply mixtures of
added strong acid with dissolved zwitterion, in which case some
retention of premixing characteristics (e.g., triflic acid volatility)
would be expected.
Both new IL were screened as solvent/catalysts for several
classical acid-promoted organic reactions, although we placed an
emphasis upon probing the chemistry of 2a (vide infra). The
reaction types screened were Fischer esterification, alcohol dehy-
drodimerization and the pinacol/benzopinacole rearrangement.
Reactions and results are outlined in Scheme 1.
Both new ionic liquids proved catalytically active in these
reactions. However, we placed an emphasis at this early stage of
our studies upon more fully probing the chemistry of 2a. Our
* To whom correspondence should be addressed. E-mail: jdavis@
jaguar1.usouthal.edu; dforbes@jaguar1.usouthal.edu.
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10.1021/ja026290w CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Recycling of 2a in the Synthesis of Ethyl Acetate
activity of 2a was found to increase, in line with the degree to
which water is removed from the system.
a
cycle
ethyl acetate, %
For an equilibrium reaction in which water is a product, the initial
increase in ester yield accompanying the retention of water in cycles
1-3 was unexpected. It appears that for reasons yet to be
determined, the presence of a threshold quantity of water in the
ionic liquid contributes to higher reaction yields. To test the
plausibility of this theory, we charged a dried sample of 2a with a
bolus of water, estimated to be equivalent to the cumulative amount
retained after cycles 1 and 2. Supporting the hypothesis, we found
the yield of ethyl acetate (entry 5) to be greater than that obtained
using dried 2a (entry 1).
Overall, the new IL are versatile solvent/catalysts for the reactions
examined, and provide further examples of the capacity of ionic
liquids to be fashioned for specific chemical applications. They
provide good product selectivities as well as a balance between
the yields achievable using a homogeneous acid catalyst and the
ease of catalyst/substrate separation provided by a heterogeneous
catalyst. Even so, improvements in these systems would be
welcome, such as developing IL that bear appended superacid
functionalities. Efforts toward these ends are in progress, as are
efforts to measure the pKa of these acids and to understand their
behavior at a molecular level.
1
2
3
82
91
96
81
87
4
5b
a Isolated yield. b Using regenerated 2a plus water.
motivation for doing so originates in recent reports by Karodia and
co-workers in which tetraorganophosphonium tosylate salts (mp
> 70 °C) were used as solvents for several organic reactions.15-17
In those reports, the cooling of the solvent upon completion of the
reaction resulted in the separation of the IL as a solid. We reasoned
that 2a might behave similarly, providing direct access to a
convenient mode of separation, decantation, which parallels the
manner in which solid acids are removed from reaction media. As
expected, this proved to be the case in most of the reactions in
which 2a was used.
The reaction of alcohols with strong acids is used both for alkene
and ether synthesis, the favored product being selected by the
judicious choice of acid and reaction conditions. Depending upon
the substrate/2a stoichiometry, 1-octanol is selectively converted
to octyl ether in 16-56% isolated yield with minimal byproduct
formation. In a control experiment, pTSA‚H2O gave a better yield
of octyl ether; however, more byproducts were formed, and the
separation of the pTSA from the reaction milieu was considerably
more difficult. Using Nafion-117 as a control, we found the catalyst/
product separation to be straightforward and byproduct formation
to be minimal, but the yield of octyl ether was quite poor (3%).
The rearrangement of pinacol to pinacolone is a process of
considerable industrial importance. The latter provides a synthetic
entre´e to trimethylpyruvate and then tert-leucine, a building block
of several peptidomimetic drugs and chiral catalysts.18 Although
existing procedures use H2SO4 or H3PO4 to catalyze the reaction,
interest has been expressed in the replacement of these species by
solid acids. Using various solid acid catalysts, reported yields of
pinacolone range from 2 to 71%, but long reaction periods are
typical, and the use of a volatile organic solvent is required,
complicating isolation.19 Using 2a as catalyst/solvent, we obtained
an unoptimized yield of pinacolone of 35% during a 1-h reaction
period, and an 88% yield of benzopinacolone over a 2-h period.
Moreover, the pinacolone is readily distilled as a pure compound
straight from the reaction milieu, unreacted pinacol being retained
by the solvent/catalyst phase.
Ultimately, the ease with which these IL are recycled is central
to their utility. Consequently, we examined the formation of an
important commodity ester,20 ethyl acetate, from ethanol and acetic
acid using 2a as the solvent/catalyst in a batch-type process,
recycling the 2a. The results of a representative round of recycling
experiments are summarized in Table 1.
As shown, the yield of the ester increases from cycles 1 to 3,
only to drop off again in cycle 4. During these cycles, the mass of
the solvent/catalyst medium also increases, consistent with the
entrapment of materials by the cooled catalyst phase. Post-cycling
analysis of the IL by GC and NMR was consistent with the retention
by it of appreciable quantities of water and acetic acid. When heated
under vacuum to remove these volatile materials, the catalytic
Acknowledgment. D.C.F. thanks the Research Corporation for
partial funding of this research.
Supporting Information Available: Synthetic/experimental pro-
cedures, analytical data, and NMR spectra (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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