Chemistry Letters Vol.35, No.2 (2006)
211
ꢀꢀ
Figure 2. The DFT (B3LYP/6-31+G level) structure of the
most stable imidazole isomerization intermediate. The regard
ꢂ
of counter-ion BF4 leads to intramolecular HB (see text)
reorganization: Hydrogen atom contacts of C–H bonds of both
0
imidazole ( CH3 and H) and fumarate ( H) with fluorine are
seen as dashed lines.
1
2
2
Figure 1. Variation in conversion (%) for 1 mmol dimethyl
maleate isomerization in 1 mmol 1c with respect to time at var-
ious reaction temperatures.
tions in the modeling process were that the most acidic 3H imi-
dazolium proton forms a hydrogen bond (HB) with the two most
basic carbonyl oxygens of maleate and that in the intermediate
the 2H proton forms the 6- or 7-membered C–H intra-molecular
HB. The last structures are more stable than the first ones of 1.4–
depended upon the length of alkyl substituent. For 1a the lowest
solubility was noted. ILs 1a–1d and 3 could be recovered and
reused several times with no decrease in the extent of the conver-
sion. The isomerization was conducted in laboratory without
necessity to ensure anhydrous conditions.
1
.5 kcal/mol for ion and 3.4–5.1 kcal/mol for ion-pair, depen-
ding on conformation and both conformation and/or counter-
ion position, respectively (Figure 2). Thus, the EI-MS spectra,
following the DFT calculations provide a molecular basis to
easily explain (Z)- to (E)-isomerization in protic imidazolium
ILs.
We have established the feasibility of a quantitative cis–
trans isomerization of dimethyl maleate to fumarate. The sug-
gested mechanism relies on the addition of the protic imidazo-
lium species to carbon–carbon double bond, followed by rotation
and final imidazolium elimination. Our protocol utilizes easily
prepared ILs and compares well with the best methodologies
currently available. The attained success relies on the use of
The formation of a reactive adduct of the 1-methylimidazo-
lium cation with the carbon–carbon double bond of dimethyl
maleate is thought to explain the cis–trans isomerization. The
presence of the 1-methylimidazolium cation results in the forma-
tion of a non-reactive complex (because of cis maleate configu-
ration, stabilized by hydrogen bonding toward two carbonyl
oxygen atoms). However, the same cation might protonate a less
nucleophilic double bond and could reversibly add its heterocy-
clic moiety to the alpha carbon atom of maleate. Boron tetra-
fluoride anion seems to be an exceedingly poor nucleophile to
form a stable, covalent product. Instead of the adduct of IL
0
2
and maleate as an ion pair undergoes rotation around the C –
3
(
i.e. not purified or dried) RTILs employing a mild heating
0
C single bond and then dissociates itself releasing the dimethyl
fumarate as the final product.
and simple water dilution in neutral and non-toxic reaction
conditions.
The mechanistic considerations advanced above require
at least spectroscopic proof. In the first experiment dimethyl
References
ꢂ
ꢁ
maleate with the BF4 protic IL were maintained at 25 C and
after 48 h extracted with ether and after ether evaporation the
EI mass spectrum was recorded. In the ES+ scan, only the
relatively intense peak at m/z 227 was displayed. Indeed, the
atomic composition of 1-methylimidazolium cation adduct
was confirmed by EI high-resolution mass spectrometry (for
C10H15O4N2 calcd. 227.1032, obsd. 227.1020).
When dimethyl maleate was subjected to the reaction in 1-
methylimidazolium lactate, the products were quite different
and revealed numerous EI-MS peaks with the lack of the signal
typical for the above mentioned adduct (m/z 227.1118 and so
cannot represent the formula of C10H15O4N2). Instead of this,
we observed the m/z 235, which could be easily interpreted as
protonated lactic acid adduct of methyl maleate (for C9H15O7
calc. 235.0818, obsd. 235.0826).
1
2
3
4
5
G. R. Clemo, S. B. Graham, J. Chem. Soc. 1930, 213.
K. Nozaki, J. Am. Chem. Soc. 1941, 63, 2681.
M. Davies, F. P. Evans, Trans. Faraday Soc. 1955, 51, 1506.
Z. Rappoport, C. Degani, S. Patai, J. Chem. Soc. 1963, 4513.
A. G. Cook, A. B. Voges, A. E. Kammrath, Tetrahedron Lett.
2
001, 42, 7349.
6
7
8
9
M. Kodomari, T. Sakamoto, S. Yoshitomi, Bull. Chem. Soc.
Jpn. 1989, 62, 4053.
M. M. Baag, A. Kar, N. P. Argade, Tetrahedron 2003, 59,
6489.
K. Anandarajah, P. M. Kiefer, Jr., B. S. Donohoe, S. D.
Copley, Biochemistry 2000, 39, 5303.
S. D. Copley, Curr. Opin. Chem. Biol. 2003, 7, 265.
10 Y. Wang, G. A. Voth, J. Am. Chem. Soc. 2005, 127, 12192.
11 J. Pernak, I. Goc, Pol. J. Chem. 2003, 77, 975.
12 J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D.
Willauer, G. A. Broker, R. D. Rogers, Green Chem. 2001,
3, 156.
We have modeled both dimethyl maleate complexes: first as
a hydrogen-bonded species and the second as an ion-pair adduct
of imidazolium to the carbon–carbon double bond. The assump-