CS), 1.10 (s, 3H, CS-CH3), 0.95 (t, J ) 7.3 Hz, 3H, CH2CH3),
0.79(s, 3H, CS-CH3). 13C NMR (75.46 MHz, DMSO-d6-TMS):
δ 216.4 (s), 144.4 (s), 123.1-120.5 (m, 2C), 58.4 (s), 47.4-46.4
(m, 3C), 43.3-41.8 (m, 2C), 34.8 (m), 31.4 (m), 27.3-23.3 (m,
2C), 21.6-18.3 (m, 3C), 13.5 (q, J ) 65.7 Hz), 9.3 (m, J ) 105.3
Hz). Anal. Calcd for C19H32N2O4S: C, 59.35; H, 8.39; N, 7.29.
Found: C, 59.07; H, 8.47; N, 7.29.
Diels-Alder Reaction in ILs. Cyclopentadiene (0.37 mL,
4.5 mmol) was dissolved in [bmim][BF4] (2.3 mL) or in a mixture
of [bmim][CS], [bmim][MS], [bmim][OTf] or [bm2im][CS], and
[bmim][BF4] (15:100 mol/mol, 2.3 mL). Ethyl acrylate (0.30 mL,
3.0 mmol) was added into the diene solution,which was then
stirred for 20 h at room temperature or -10 °C. The reaction
mixture maintained a single phase, not biphase. Extraction with
diethyl ether followed by purification column chromatography
(silicagel, BW-80S, Fuji Silysia, ethyl acetate/ hexane 1/10 (v/
v)) provided the isolate Diels-Alder products. The endo/exo
ratios were calculated by means of 1H NMR spectra of Diels-
Alder products. The CHCO2C2H5 proton in DMSO-d6 appears
at 3.20 and 3.03 ppm as endo and exo isomers, respectively.
These signals were used for the determination of the endo/exo
ratios. All ionic liquids were vacuum dried (0.08 mmHg, 60 °C,
5days) prior to use.
Positive Ion Mass spectra. To obtain information on the
microscopic structures of ions, we have used the mass spectros-
copy designed specially for observing clusters isolated through
fragmentation of liquid droplets. For the analysis of electrolyte
solution, an electrospray interface was used to form the liquid
droplets. The details of this procedure have been reported
previously.8 As is shown schematically in the Supporting
Information, the mass spectrometer is composed of a five-stage
differentially pumped vacuum system, a homemade electrospray
interface and a quadrupole mass filter (Extral C50). For the
electrospray, electric voltages were supplied to the nozzle and
three skimmers (E1-E4), and nitrogen gas was made to flow to
maintain an appropriate pressure balance (P1-P5). Positive ion
mass spectra of clusters generated from methanolic ILs at room
temperature include the following: (a) [bmim][MS] (electric
voltage: E1 ) +2.84 kV, E2 +208 V, E3 ) +211 V, E4 ) +178 V,
pressure: P1 ) 647.8 Torr, P2 ) 9.58 Torr, P3 ) 0.007 Torr, P4
) 1.13 × 10-5 Torr, P5 ) 6.30 × 10-7 Torr); (b) [bmim][OTf]
(electric voltage: E1 ) +2.87 kV, E2 ) +207 V, E3 ) +211 V, E4
) +177 V; pressure: P1 ) 646.1 Torr, P2 ) 9.63 Torr, P3 ) 0.007
Torr, P4 ) 1.14 × 10-5 Torr, P5 ) 8.90 × 10-7 Torr); (c) [bmim]-
[CS] (electric voltage: E1 ) +3.06 kV, E2 ) +206 V, E3 ) +218
V, E4 ) +178 V; pressure: P1 ) 646.1 Torr, P2 ) 9.81 Torr, P3 )
0.008 Torr, P4 ) 1.23 × 10-5 Torr, P5 ) 8.30 × 10-7 Torr); and
(d) [bm2im][CS] (electric voltage: E1 ) +4.61 kV, E2 ) +207 V,
E3 ) +218 V, E4 ) +177 V; pressure: P1 ) 645.7 Torr, P2 ) 9.83
Torr, P3 ) 0.008 Torr, P4 ) 1.01 × 10-5 Torr, P5 ) 3.34 × 10-7
Torr).
of free cations. This facilitates the formation of the
hydrogen bond between the C2-H of the cation and the
carbonyl of the dienophile. Like a Lewis acid catalyst for
the dienophile,12 the hydrogen bond can also stabilize the
LUMO energy of the dienophile, providing the endo
isomer as the preferred species.3d,13
In conclusion, we have attempted to demonstrate that
utilizing the CS anion makes the imidazolium cation free
enough to expedite the stereoselectivity compared to MS
and OTf anions. It can be deduced that the reactivity is
relevant to the cation-anion association, i.e., ion states
of the imidazolium ILs at least for CS, MS, and OTf
anions. To determine whether devising the size of the
anions is the effective methodology for regulating the ion
states of ILs, various sulfonates, which are different in
size, are currently coupled with bmim.
Experimental Section
Preparation of 3-Butyl-1-methylimidazolium Camphor-
sulfonate ([bmim][CS]). [Bmim][Br] prepared by a general
method14 was transformed into the aimed camphorsulfonate salt
via [bmim]2[SO4] by means of a strong base anion exchange
column chromatography. Into a 400 mL aqueous solution of
[bmim][Br] (39 g, 0.18 mol) was added concentrated H2SO4 (5
mL) with stirring for 3 h at room temperature. After removal of
the solvent, the residual liquid was further dried in vacuo (under
0.1 mbar) at 60 °C for 24 h to remove water and HBr as much
as possible. [Bmim]2[SO4] (37 g) was obtained as a pale yellow
viscous oil. The obtained oil was subjected to the column packed
with the resin, which was replaced by camphorsulfonate to give
pale yellow viscous oil. The oil was dried in vacuo (under 0.1
mbar) at 60 °C for 5 days, producing a pale yellow viscous liquid
(yield: 64.8 g, 97%). The ion exchanged ratio was calculated by
the 1H NMR spectrum in Figure S1, i.e., the ratio of the average
of the integrated values of two CH3s of [CS] at 1.03 and 0.73
ppm (3.10) against the integrated values of CH2CH3 of [bmim]
at 0.89 ppm (3.15) was used. 1H NMR (300 MHz, DMSO-d6-
TMS): δ 9.12 (s, 1H, C(2)H), 7.76 (t, J ) 1.7 Hz, 1H, C(4)H),
7.69 (t, J ) 1.7 Hz, 1H, C(5)H), 4.15 (t, J ) 7.2 Hz, 2H, N-CH2),
3.84 (s, 3H, N-CH3), 2.89 (d, J ) 14.7 Hz, 1H, CS), 2.65 (m,
1H, CS), 2.39 (d, J ) 14.7 Hz, 1H, CS), 2.22 (tt, J ) 18.1 Hz, J
) 3.8 Hz, 1H, CS), 1.94 (t, J ) 4.4 Hz, 1H, CS), 1.93-1.72 (m,
4H, N-CH2CH2 and CS), 1.30-1.21 (m, 4H, CH2CH3 and CS),
1.03 (s, 3H, CS-CH3), 0.89 (t, J ) 7.3 Hz, 3H, CH2CH3), 0.73 (s,
3H, CS-CH3). 13C NMR (75.46 MHz, DMSO-d6-TMS): δ 216.4
(s), 137.1 (t), 124.4-122.0 (m, 2C), 58.4 (s), 48.7-46.8 (m, 3C),
43.4-41.8 (m, 2C), 35.8 (m), 32.9-30.1 (m), 27.3-23.4 (m, 2C),
20.6-18.1 (m, 3C), 13.4 (q, J ) 65.7). UV-vis (H2O) λmax/nm
(log ꢀ) 265 (4.04). Anal. Calcd for C18H30N2O4S: C, 58.35; H, 8.16;
N, 7.56. Found: C, 58.17; H, 8.43; N, 7.74.
Acknowledgment. This work was supported by the
Ministry of Education, Science, Sports and Culture,
Grant-in-Aid for Incentive Research (14919115, 2002
and 16919148, 2004), Exploratory Research (15655014,
2003-2004) and Scientific Research (16350025, 2004-
2005).
Preparation of 3-Butyl-1,2-dimethylimidazolium Cam-
phorsulfonate ([bm2im][CS]). [Bm2im][CS] was prepared by
using the same procedure as described above in the preparation
of [bm2im][CS] from [bm2im][Br] and camphorsulfonic acid
(yield: 4.3 g, 85%). [Bmim][CS] is a viscous liquid, whereas
[bm2im][CS] is a white solid. 1H NMR (300 MHz, DMSO-d6-
TMS): δ 7.73 (d, J ) 2.1 Hz, 1H, C(4)H), 7.70 (d, J ) 2.1 Hz,
1H, C(5)H), 4.17 (t, J ) 7.3 Hz, 2H, N-CH2), 3.81 (s, 3H,
N-CH3), 2.91 (d, J ) 14.7 Hz, 1H, CS), 2.75 (m, H, CS), 2.65
(S,3H, C-CH3), 2.41 (d, J ) 14.7 Hz, 1H, CS), 2.28 (tt, J ) 18.1
Hz, J ) 3.6 Hz, 1H, CS), 1.99 (t, J ) 4.5 Hz, 1H, CS), 1.96-1.72
(m, 4H, N-CH2CH2 and CS), 1.40-1.25 (m, 4H, CH2CH3 and
Supporting Information Available: General methods,
1H NMR spectra of [bmim][CS] and [bm2im][CS], UV-vis and
CD spectra of [bmim][CS], a schematic illustration of the mass
spectrometry for clusters isolated from electrolyte solutions
with ES-MS, and a set of positive ion mass spectra of clusters
generated from methanolic ILs. This material is available free
(12) Houk, K. N.; Strozier, R. W. J. Am. Chem. Soc. 1973, 95, 4094.
(13) Schreiner, P. R.; Wittkopp, A. Org. Lett. 2002, 4, 217-220.
(14) Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Phys.
Chem. Chem. Phys. 2001, 3, 5192.
JO051669X
10108 J. Org. Chem., Vol. 70, No. 24, 2005