Deuterium NMR spectroscopy is a versatile and economical tool for monitoring reaction kinetics in ionic liquids

Article abstract of DOI:10.1039/b108864e

Time-resolved ²H NMR spectroscopy is used to monitor the progress of and gain kinetic information for a variety of reactions in different ionic media.

Full text of DOI:10.1039/b108864e

Deuterium NMR spectroscopy is a versatile and economical tool for
monitoring reaction kinetics in ionic liquids†
Armando Durazo and Mahdi M. Abu-Omar*
University of California, Los Angeles, Department of Chemistry and Biochemistry, 607 Charles E. Young
Drive East, Los Angeles CA 90095-1569, USA. E-mail: mao@chem.ucla.edu
Received (in Purdue, IN, USA) 29th September 2001, Accepted 7th November 2001
First published as an Advance Article on the web 10th December 2001
Time-resolved 2H NMR spectroscopy is used to monitor the
progress of and gain kinetic information for a variety of
reactions in different ionic media.
Py]BF4). In the first experiment, a tenfold molar excess of H2O2
is used, and its concentration (0.5 M) is such that under steady-
state conditions the major rhenium peroxo species is
2
(CH3)Re(O)2(h -O2).11 A smooth conversion of reactant (R) to
Ionic liquids are used as reaction media for various types of
organic and organometallic transformations. These media hold
many advantages, such as high thermal stability and negligible
volatility, over conventional molecular solvents.1 These liquids
are easily prepared, are recyclable, and allow for the easy
separation of products from a reaction mixture. There is much
interest in using ionic liquids as solvents for homogeneous
transition metal-catalyzed industrial processes, since the in-
efficient separation of costly catalysts from products leads to
increased costs and the risk of introducing reactive species into
the surroundings.2 Spectrophotometric kinetic studies have
been conducted on reactions in these media;3,4 however, due to
the low availability and high cost of perdeuteriated ionic liquids
there are currently no reports of kinetic studies that utilize NMR
spectroscopy. An alternative method to conduct NMR kinetic
studies in these media is to use proteated ionic liquids and
employ 2H NMR to monitor the conversion of deuteriated
product (P) over the course of 1 h at room temperature is
observed, with the rapid formation of a deuteriated intermediate
(I) species at d = 4.3 ppm that remains in nearly constant
concentration over the course of the reaction (Fig. 1). 2H
resonances for the alkyl portion of the product appear at d = 5.0
ppm (a) and 4.1 ppm (b); the ratio of integrations (a+b) is 1+2
throughout the reaction.
When this reaction is conducted with a higher concentration
of H2O2 (5.0 M), the conversion proceeds at nearly the same
rate; however, the presence of another steady-state intermediate
with a different chemical shift (d = 5.3 ppm) is observed via 2H
NMR (Fig. S1, ESI†). The aliphatic 2H resonances of the
product, [D8]styrene-1,2-diol, are present at the same d values
as in the previous reaction. At the latter concentration of H2O2,
2
the major rhenium species is (CH3)Re(O)(h -O2)2.11 After 24 h
at ambient temperature, neither of these intermediate species is
present, as their 2H signals vanish.
2
reactants over time. We report herein on the use of H NMR
In the epoxidation of [D8]styrene‡ with urea hydrogen
peroxide (UHP), an anhydrous source of hydrogen peroxide,
these intermediates are absent and a quantitative yield of
racemic [D8]styrene oxide is obtained (Fig. S2, ESI†). Sim-
ilarly, no such intermediates are present when [D10]cyclohex-
ene is epoxidized with UHP (Fig. S3, ESI†). Furthermore, when
[D10]cyclohexene is dihydroxylated with aqueous H2O2/MTO
in ionic media, a steady-state intermediate is again observed at
spectroscopy, a versatile and economical method, in monitoring
reaction progress in ionic media.
NMR spectroscopy has been used to gain insight into the
nature of species present in room temperature chloroaluminate
melts.5–7 For example, Zawodzinski and Osteryoung employed
1H, 2H, and 17O NMR to study the equilibria between HCl, Cl2,
and HCl22, as well as the water content in these media.5,6
McMath and co-workers reported one facile ring deuteriation
of imidazole and mono- and dialkylimidazolium cations with
D2O and catalytic amounts of Pd/C.8 In order to prepare
dialkylimidazolium cations with perdeuteriated N-alkyl groups
1
suitable for use as H NMR solvents, the use of expensive
perdeuteriated alkylating agents is required. Furthermore, in
order to obtain high ( > 97%) levels of incorporation of
deuterium into the ring positions, two successive treatments
with D2O and Pd/C are necessary.
2
In our work, H NMR is used to monitor the progress of
several fundamental organic reactions in different proteated
ionic liquids.‡Using deuteriated reactants, one is able to directly
and quantitatively monitor the conversion of reactants to
products in real time. The initial concentration of the deuteriated
starting materials is approximately 50 mM in each case and an
external standard is present as a reference for chemical shifts
and integrations.
We illustrate the high utility of this technique in several
important chemical transformations. The first reaction is the
methyltrioxorhenium (MTO)-catalyzed dihydroxylation of
[D8]styrene by aqueous hydrogen peroxide,9 which is an
environmentally friendly oxidant since its only reaction by-
product is water.10 [D8]Styrene is a viable reactant and is
commercially available. This transformation was carried out in
the ionic liquid N-ethylpyridinium tetrafluoroborate ([Et-
† Electronic supplementary information (ESI) available: Figs. S1–6: NMR
stack plots and kinetic traces. See http://www.rsc.org/suppdata/cc/b1/
b108864e/
Fig. 1 [D8]Styrene dihydroxylation, 0.5 M H2O2.
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CHEM. COMMUN., 2002, 66–67
This journal is © The Royal Society of Chemistry 2002

2
d = 4.3 ppm throughout the course of the reaction (Fig. S4,
ESI†). Based on these observations, we conclude that these
no residual solvent peaks are observed in the H spectra. It is
therefore possible to easily conduct NMR kinetic experiments
in a variety of easily prepared ionic liquids. Using this method
it is possible to detect reaction intermediates that are undetect-
able using other forms of spectroscopy or are not present in
reactions carried out in molecular solvents.
2
steady-state intermediates are the h -diolato complexes of the
predominant Re(VII) peroxo species in the reaction mixtures.
Since the major rhenium-containing species is dependent on the
concentration of H2O2,12 different intermediates are observed at
different concentrations of H2O2.
This technique allows a variety of chemical transformations
to be studied in ionic media, since many types of organic
molecules can be regioselectively deuteriated using conven-
tional, economical H/D exchange processes. Examples include
carbonyl compounds, alkenes,13,14 terminal alkynes,15 and
cyclopentadiene.16 Only one reactant need be deuteriated, and
deuteriation is required only in positions that experience a
change during the course of a reaction. Given that certain
reactions exhibit significant kinetic (H/D) isotope effects,
caution must be employed in applying quantitative kinetic data
to reactions involving the analogous proteated substrates.
Nonetheless, by conducting reactions in structurally different
ionic liquids, the effects of solvent–solute interactions, such as
hydrogen bonding, Lewis acid–base, ionic, and ion–dipole, on
reaction rates, yields, and selectivities can be investigated.
These studies are currently in progress.
Using commercially available deuteriated substrates, we also
examined several other fundamental organic transformations in
ionic media (Table 1). These reactions are a testimony to the
various types of reactions and substrates that can be monitored
in ionic media using 2H NMR. Although each of these reactions
proceeds to completion using stoichiometric amounts of
reactants, we used a large excess of one reactant in order to
conduct these reactions under pseudo-first order conditions.
Two reactions were conducted in different ionic liquids to probe
the dependency of rate on the nature of the solvent (entries 3, 4
and 7, 8)—in these cases there is no appreciable difference in
rate when the reactions are conducted in two structurally
different ionic liquids. Here we also show how 2H NMR can be
used to easily monitor reactions in various ionic liquids.
There are many advantages to using this technique. This
method is highly cost efficient, since only a small amount of
deuteriated starting material is required for any given experi-
ment. There are many types of commercially available
perdeuteriated compounds that can be used as reactants (1H
NMR solvents are good examples). Most importantly, there is
no need to prepare relatively large amounts of various costly
perdeuteriated ionic liquids. Since the extent of protium
incorporation in proteated ionic liquids is naturally high (99%),
This contribution is dedicated to Professor Robert G.
Bergman. This material is based upon work supported by the
Arnold and Mabel Beckman Foundation (BYI award to M. M.
A.-O.) and by the National Science Foundation under grant
CHE-9874857-CAREER and equipment grant number CHE-
9974928.
Notes and references
Table 1 Reactions monitored via 2H NMR in ionic media
kobs/[Re]T/
‡ A typical experimental procedure: ([D8]Styrene epoxidation). An NMR
tube was charged with [Emim]BF4 (0.5 mL), UHP (47 mg), and 125 mL of
a 0.040 M MTO stock solution in CH3CN. After 10 min, an external
standard of CD3CN in C6H6 (30% v/v, d = 1.55 ppm) was added, along
with [D8]styrene (6 mL). The contents of the tube were mixed well, and 2H
NMR spectra were subsequently collected every 5 min on a Brüker Avance
500 MHz (1H) spectrometer.
Entry
Reactiona
Solventb
M21 s21
1
[EtPy]BF4
0.20 0.02
2
3
4
5
6
[EtPy]BF4
0.090 0.02
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[Emim]BF4 0.034 0.004
[BuPy]BF4 0.040 0.008
[Emim]BF4 2.600 0.003
[Emim]BF4 1.750 0.006
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8
9
[Emim]BF4 0.20 0.01
[BuPy]BF4 0.23 0.02
[Emim]BF4 3.000 0.002c
3969–3976.
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a Although these reactions proceed to completion using stoichiometric
amounts of reactants, a large excess of one reactant is used here to ensure
pseudo-first order conditions. UHP = Urea hydrogen peroxide complex,
MTO = methyltrioxorhenium(VII). b [Emim]BF4 = 1-ethyl-3-methylimi-
dazolium tetrafluoroborate, [BuPy]BF4
fluoroborate. c Since this is not a catalytic reaction, the reported rate
= N-(n-butyl)pyridinium tetra-
16 J. E. Baldwin and K. D. Belfield, J. Org. Chem., 1987, 52,
constant is given by kobs/[CH3I].
4772–4776.
CHEM. COMMUN., 2002, 66–67
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