Methylated imidazolinium-dithiocarboxylates: Two representatives of a new class of ionic liquids

Article abstract of DOI:10.1055/s-0028-1083317

The work describes the methylation of chiral and achiral imidazolinium-dithiocarboxylates. The resulting salts qualify as a novel class of ionic liquids after an anion metathesis was performed. Their NMR and UV/Vis spectra were studied. Georg Thieme Verla

Full text of DOI:10.1055/s-0028-1083317

PAPER
583
Methylated Imidazolinium-Dithiocarboxylates: Two Representatives of a New
Class of Ionic Liquids
A
N
ew Class of Io
mnic Liquids élie Blanrue, René Wilhelm*
Institute of Organic Chemistry, Clausthal University of Technology, Leibnizstr. 6, 38678 Clausthal-Zellerfeld, Germany
Fax +49(5323)722834; E-mail: rene.wilhelm@tu-clausthal.de
Received 7 August 2008; revised 9 October 2008
First, the simple zwitterion 411 was prepared according to
Abstract: The work describes the methylation of chiral and achiral
imidazolinium-dithiocarboxylates. The resulting salts qualify as a
novel class of ionic liquids after an anion metathesis was performed.
Their NMR and UV/Vis spectra were studied.
the literature. The zwitterion was then transferred into salt
5 with methyl iodide in 94% yield. The salt, as the starting
zwitterion, was red and had a melting point of 145 °C.
Through an anion metathesis, NTf2 was introduced as an
anion and the resulting red salt 6 was obtained in 96%
yield with a water content of 0.069% (Scheme 1). The salt
is liquid at room temperature and remains a liquid even at
storage in a freezer at –28 °C for several months.
Key words: ionic liquids, chirality, chiral cations, imidazolinium
salts, zwitterions
Ionic liquids have emerged as a new promising class of or-
ganic material due to their potential as novel solvents for
reactions and electrochemical processes.1 Some of these
liquids are expected to be ‘green solvents’, for example,
due to their negligible vapor pressure.2 The organic salts
have a melting point below 100 °C. An additional advan-
tage is the efficient recovery of some of these salts. Nev-
ertheless, in a few examples it is known that the ionic
liquids are not inert and react with some reagents,3 which
could be a disadvantage in some applications. The collec-
tion of ionic liquids based on the combinations of cations
and anions has dramatically increased, and constantly new
salts4 and solvent mixtures5 are prepared. In case the ionic
liquids are chiral, they have an additional potential as
chiral solvents, shift reagents, and catalysts.6
Bn
N+
Bn
N+
S
S
MeI, MeCN
94%
I–
S–
SMe
N
N
Bn
Bn
5
4
LiNTf2
CH2Cl2/H2O
96%
Bn
N+
S
–
NTf2
N
SMe
Bn
Due to our efforts in ionic liquids7 and imidazolinium-
dithiocarboxylates8 we were interested to explore the pos-
sibility to prepare a new interesting class of ionic liquids
from imidazolinium-dithiocarboxylates. The latter belong
to the extraordinary class of carbene complexes of
nonmetals9 and can be formally prepared by the addition
of an imidazolinium carbene to CS2. The CS2 group is
nearly perpendicular to the imidazolinium ring. It is
known that the CS2 group can be methylated to give salts
as shown in Figure 1. However, the examples in the liter-
ature are very limited and only simple salts with very high
melting points were reported.10
6
Scheme 1
Next, an enantiopure analogue was prepared. Starting
from diamine 7 the zwitterion 8 was prepared in analogy
to zwitterion 4. Treating diamine 7 with of DMF dimeth-
ylacetal at 100 °C resulted in the carbene dimer, which
was directly dissolved in xylene and reacted with CS2 at
140 °C for 45 minutes to give the desired zwitterion 8 in
43% yield.12 Treatment of 8 with methyl iodide gave the
desired salt 9 in 92% yield. The salt has a melting point of
87 °C. In addition it was possible to change the counter
anion by stirring salt 9 with KPF6 or LiNTf2 in a mixture
of CH2Cl2 and water, which gave the desired salts 10 and
11 in 88 and 98% yield, respectively (Scheme 2). Interest-
ingly, salt 10 with a PF6 counter anion had a melting point
of 74 °C, while salt 11 with the more lipophilic NTf2 an-
ion had a higher melting point of 109 °C. The salts were
as stable as their so far reported analogues in the literature
and no decomposition was observed at their melting
points.
I–
Pr2N+
Pr2N
N+
N
Et2N+
Et2N
S
S
S
I–
SMe
I–
SMe
SMe
3
1
2
Figure 1 Salts 1–3
SYNTHESIS 2009, No. 4, pp 0583–0586
x
x
.
x
x
.2
0
0
9
Advanced online publication: 09.01.2009
DOI: 10.1055/s-0028-1083317; Art ID: T13608SS
© Georg Thieme Verlag Stuttgart · New York

584
A. Blanrue, R. Wilhelm
PAPER
1
22 FT-IR spectrophotometer from Bruker. H NMR spectra were
taken on an AMX 400 (400 MHz) or on an AC 250 P (200 MHz)
spectrometer from Bruker in CDCl3 and calibrated using the peak at
7.26 ppm as an internal reference. 13C NMR spectra were taken on
an AMX 400 (100 MHz) or on an AC 250 P (50 MHz) spectrometer
from Bruker in CDCl3 and calibrated using the peak 77.2 ppm as an
internal reference. Mass spectra were recorded on MS 5889 B mass
spectrometer from Hewlett Packard. Electron spray mass spectrom-
etry was performed directly on a MS LC/MSD 1100 MSD mass
spectrometer from Hewlett Packard. Elemental analysis were car-
ried out on a ‘Elementar Analyzer’, model 1106 from Carlo Erba In-
strumentazione at the Institute of Pharmaceutical Chemistry of the
Technical University of Braunschweig and are reported as the aver-
age of two runs. Optical rotations were measured using a 1 dm path
length (c is given as g/100 mL) on a PerkinElmer 243 B polarimeter
in the reported solvent. UV/Vis spectra were recorded on a Hewlett
Packard 8452A Diode Array Spectrophotometer. Melting points
were taken on a Dr. Tottoli apparatus from Büchi and are uncorrect-
ed. H2O content was determined via the Karl Fischer method. 1,3-
Dibenzylimidazolinium-2-dithiocarboxylate (4)11 and (1R,2R)-
N,N-dibenzylcyclohexane-1,2-diamine (7)15were prepared accord-
ing to the literature. All other chemicals were purchased from
Aldrich, Fluka, Merck, or Lancaster.
Bn
N+
H
N
i) Me2NC(OMe)2
S
Bn
100 °C
Bn
S–
N
H
N
ii) CS2, xylene
140 °C
Bn
7
43%
8
MeI
MeCN
92%
Bn
N+
Bn
N+
S
S
KPF6 or LiNTf2
CH2Cl2/H2O
SMe
N
SMe
I–
N
Bn
Bn
X–
9
10 X = PF6 88%
11 X = NTf2 98%
Scheme 2
1
The salts displayed some interesting behavior in the H
NMR spectra. While the diastereotopic benzylic protons
came in salt 5 as a singlet at d = 4.69, they showed in salt
6 a typical AB system, one doublet at d = 4.65 and one at
d = 4.51. Although achiral, it is obvious that through de-
symmetrization a new chiral center would be formed at
the benzylic position and the salt would possess an axial
chirality along the axis of the imidazolinium ring and the
CS2Me group. In addition it was possible to observe in
salts 9, 10, and 11 only very broad singlets for the benzylic
protons, which is due to the slow rotation of the CS2Me
group at room temperature. However, this rotation is sup-
pressed at 0 °C. A 1H NMR spectrum at this temperature
showed for salt 10 the first AB system with two doublets
at d = 5.00 and 4.88 and the second AB system with two
doublets at d = 4.84 and 4.70.
1,3-Dibenzyl-2-(methylthiocarbonothioyl)imidazolinium
Iodide (5)
MeI (0.5 mL, 8 mmol) was added to 1,3-dibenzylimidazolinium-2-
dithiocarboxylate (4; 200 mg, 0.673 mmol) in MeCN (4 mL) to
give, after 16 h, a deep red solution. The solvent was removed and
the crude product obtained was washed with Et2O (10 mL) to give
the expected product as a red solid (269 mg, 0.63 mmol, 94%); mp
145 °C.
IR (KBr): 3424, 2897, 1612, 1264, 1122, 1059, 740 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.45–7.31 (m, 10 H, C6H5), 4.68
(s, 4 H, NCH2Ph), 4.52–4.43 (m, 2 H, NCH2CH2N), 3.74–3.59 (m,
2 H, NCH2CH2N), 2.93 (s, 3 H, CH3).
13C NMR (50 MHz, CDCl3): d = 208.0 (CS2), 164.4 (CN2), 131.7
(C6H5), 129.4 (C6H5), 129.3 (C6H5), 128.6 (C6H5), 52.0 (NCH2Ph),
48.4 (NCH2CH2N), 20.6 (CH3).
MS (ESI, 0 V): m/z = 341 (M+, 100%).
Because of the red color of the cation, due to charge trans-
fer, it is also possible to observe different shifts in the UV/
Vis spectra depending on the lipophilicity of the counter
anions. In chloroform, salt 9 had a maximum at l = 422
nm, salt 10 at 498 nm, and salt 11 at 504 nm. Changing the
solvent to methanol, salt 9 had a maximum at l = 500 nm.
Anal. Calcd for C19H21IN2S2: C, 48.72; H, 4.52; N, 5.98. Found: C,
48.52; H, 4.57; N, 6.31.
1,3-Dibenzyl-2-(methylthiocarbonothioyl)imidazolinium
Bis(trifluoromethylsulfonyl)amide (6)
An anion metathesis was performed with 5 (100 mg, 0.213 mmol)
in CH2Cl2 (5 mL) with a solution of LiNTf2 (92 mg, 0.319 mmol) in
H2O (5 mL). After stirring vigorously for 30 min, the organic phase
was washed with H2O (3 × 5 mL) and dried (MS 3 Å) to give the
desired product as a red liquid (127 mg, 0.21 mmol, 96%).
In conclusion, we have demonstrated that methylated im-
idazolinium salts qualify as new chiral and room temper-
ature ionic liquids. Because of their straightforward
synthesis, it will be possible to prepare various analogues
and to incorporate an axial chirality. Use of other alkyla-
tion agent instead of methyl iodide is also possible.13 Due
to their color it is possible to use these salts to determine
the polarity of solvents and the lipophilic character of an-
ions.14
IR (NaCl): 1599, 1350, 1190, 1133, 1054 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.44–7.36 (m, 6 H, C6H5), 7.31–
7.26 (m, 4 H, C6H5), 4.65 (d, J = 14.0 Hz, 2 H, NCH2Ph), 4.51 (d,
J = 14.0 Hz, 2 H, NCH2Ph), 4.21–4.01 (m, 2 H, NCH2CH2N), 3.71–
3.62 (m, 2 H, NCH2CH2N), 2.96 (s, 3 H, CH3).
13C NMR (50 MHz, CDCl3): d = 207.6 (CS2), 163.5 (CN2), 131.4
(C6H5), 129.6 (C6H5), 129.5 (C6H5), 128.6 (C6H5), 120.1 (q,
J = 319.0 Hz, CF3), 51.6 (NCH2Ph), 47.7 (NCH2CH2N), 20.3
(CH3).
All reactions were conducted under a protective atmosphere of dry
N2. MeCN and CH2Cl2 were distilled from CaH2. Anhyd xylene was
purchased from Sigma-Aldrich and used without further purifica-
tion. Reactions were monitored by TLC with Merck silica gel 60
MS (ESI, 0 V): m/z = 341 (M+, 100%).
Anal. Calcd for C21H21F6N3O4S4: C, 40.57; H, 3.40; N, 6.76. Found:
C, 40.93; H, 3.44; N, 6.81.
F
254 plates. Flash column chromatography was performed on silica
gel 60 (70–230 mesh ASTM). IR spectra were recorded on a Vector
Synthesis 2009, No. 4, 583–586 © Thieme Stuttgart · New York

PAPER
A New Class of Ionic Liquids
585
(3R,7R)-1,3-Dibenzyl-3,4,5,6,7,7-hexahydrobenzoimidazol-1-
ium-2-dithiocarboxylate (8)
50.4 (NCH), 28.6 (NCHCH2CH2), 27.5 (NCHCH2CH2), 23.7
(NCHCH2CH2), 20.2 (SCH3).
(1R,2R)-N,N-Dibenzylcyclohexane-1,2-diamine (7;
3 g, 10.2
MS (ESI, 0 V): m/z = 395 (M+, 100%).
mmol) was heated in the presence of DMF dimethylacetal (8.5 mL,
61.1 mmol) at 100 °C in a distillation set in order to remove MeOH
and Me2NH formed during the reaction. After 2 d, the temperature
was raised to 115 °C and after slow evaporation, the mixture was
dried overnight under vacuum at r.t. Thereafter, xylene (7.5 mL)
and CS2 (6 mL, 99.3 mmol) were added. After 45 min heating at
140 °C, the mixture was evaporated and the red solid was washed
with hexane (20 mL) (1.45 g, 3.82 mmol, 38%); mp 209 °C;
[a]D22 +288 (c = 1.09, CHCl3).
Anal. Calcd for C23H27F6N2PS2: C, 51.10; H, 5.03; N, 5.18. Found:
C, 51.21; H, 5.35; N, 5.11.
(3R,7R)-1,3-Dibenzyl-2-(methylthiocarbonothioyl)-3,4,5,6,7,7-
hexahydrobenzoimidazol-1-ium Bis(trifluoromethylsulfo-
nyl)amide (11)
An anion metathesis was performed with 9 (100 mg, 0.191 mmol)
in CH2Cl2 (5 mL) with a solution of LiNTf2 (113 mg, 0.393 mmol)
in H2O (5 mL). After stirring vigorously for 30 min, the organic
phase was washed with H2O (3 × 5 mL) and dried (MS 3 Å) to give
the desired product as a red solid (126 mg, 0.187 mmol, 98%); mp
109 °C; [a]D22 +50 (c = 1.185, CHCl3).
IR (KBr): 3423, 2945, 1521, 1454, 1365, 1261, 1065, 1047, 755,
713, 696 cm–1.
1H NMR (400 MHz, CDCl3): d = 7.55–7.52 (m, 4 H, C6H5), 7.42–
7.33 (m, 6 H, C6H5), 4.82 (d, J = 15.5 Hz, 2 H, NCH2Ph), 4.65 (d,
J = 15.5 Hz, 2 H, NCH2Ph), 3.29–3.18 (m, 2 H, 2 NCH), 1.94–1.73
(m, 4 H, 2 NCHCH2), 1.32–1.11 (m, 4 H, 2 NCHCH2CH2).
13C NMR (100 MHz, CDCl3): d = 225.9 (CS2), 169.9 (CN2), 134.0
(C6H5), 128.9 (C6H5), 128.5 (C6H5), 66.9 (NCH2Ph), 50.2 (NCH),
28.3 (NCHCH2), 23.9 (NCHCH2CH2).
IR (KBr): 3441, 2950, 1556, 1348, 1191, 1134, 1057 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.41–7.29 (m, 10 H, C6H5), 4.67
(br s, 4 H, NCH2Ph), 4.09 (br s, 1 H, NCH), 3.48 (br s, 1 H, NCH),
2.88 (s, 3 H, SCH3), 1.97–1.61 (m, 4 H, 2 NCHCH2CH2), 1.35–1.05
(m, 4 H, NCHCH2CH2).
13C NMR (50 MHz, CDCl3): d = 208.1 (CS2), 166.8 (CN2), 132.2
(C6H5), 129.3 (C6H5), 129.1 (C6H5), 127.7 (C6H5), 120.2 (q, J = 319
Hz, CF3), 68.6 (NCH2Ph), 50.5 (NCH), 28.5 (NCHCH2CH2), 27.7
(NCHCH2CH2), 23.6 (NCHCH2CH2), 20.2 (SCH3).
MS (EI): m/z (%) = 379 (M+, 18), 289 (34), 106 (33), 91 (100).
Anal. Calcd for C22H24N2S2: C, 69.43; H, 6.36; N, 7.36. Found: C,
69.06; H, 6.39; N, 7.56.
MS (ESI, 0 V): m/z = 395 (M+, 100%).
(3R,7R)-1,3-Dibenzyl-2-(methylthiocarbonothioyl)-3,4,5,6,7,7-
hexahydrobenzoimidazol-1-ium Iodide (9)
Anal. Calcd for C25H27F6N3O4S4: C, 44.43; H, 4.03; N, 6.22. Found:
C, 44.59; H, 4.19; N, 6.40.
MeI (0.5 mL, 8 mmol) was added to 8 (200 mg, 0.525 mmol) in
MeCN (4 mL) to give after 16 h, a very deep red solution. The sol-
vent was removed and the obtained crude product was washed with
Et2O (10 mL) to give the expected product as a red solid (252 mg,
0.483 mmol, 92%); mp 87 °C; [a]D22 –103 (c = 1.06, CHCl3).
Acknowledgment
Financial support by Fonds der Chemischen Industrie and DFG
are gratefully acknowledged. The authors would like to thank Dr.
J. Namyslo for NMR measurements.
IR (KBr): 3423, 2938, 2859, 1547, 1453, 1264, 1098, 1030, 695
cm–1.
1H NMR (200 MHz, CDCl3): d = 7.47–7.39 (m, 10 H, C6H5), 4.83
(br s, 5 H, NCH2Ph, NCH), 3.55 (br s, 1 H, NCH), 2.86 (s, 3 H,
SCH3), 1.96–1.64 (m, 4 H, 2 NCHCH2CH2), 1.40–1.05 (m, 4 H,
NCHCH2CH2).
13C NMR (50 MHz, CDCl3): d = 208.7 (CS2), 167.2 (CN2), 130.7
(C6H5), 129.2 (C6H5), 128.9 (C6H5), 127.8 (C6H5), 68.7 (NCH2Ph),
51.0 (NCH), 28.7 (NCHCH2CH2), 27.3 (NCHCH2CH2), 23.8
(NCHCH2CH2), 20.8 (SCH3).
References
(1) (a) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis;
Wiley-VCH: Weinheim, 2002, Vol. 1. (b)Wasserscheid,P.;
Welton, T. Ionic Liquids in Synthesis, 2nd ed., Vol. 2;
Wiley-VCH: Weinheim, 2008.
(2) Rogers, R. D.; Seddon, K. R. Ionic Liquids as Green
Solvents: Progress and Prospects; American Chemical
Society: Washington DC, 2003, 2–12.
MS (ESI, 0 V): m/z = 395 (M+, 100%).
(3) (a) Chowdhury, S.; Mohan, R. S.; Scott, J. L. Tetrahedron
2007, 63, 2363. (b) Jurčík, V.; Wilhelm, R. Green Chem.
2005, 7, 844. (c) Dupont, J.; Spencer, J. Angew. Chem. Int.
Ed. 2004, 43, 5296.
Anal. Calcd for C23H27N2S2: C, 52.87; H, 5.21; N, 5.36. Found: C,
52.33; H, 5.23; N, 4.95.
(3R,7R)-1,3-Dibenzyl-2-(methylthiocarbonothioyl)-3,4,5,6,7,7-
hexahydrobenzoimidazol-1-ium Hexafluorophosphate (10)
An anion metathesis was carried out on 9 (274 mg, 0.525 mmol) in
CH2Cl2 (5 mL) with a solution of KPF6 (126 mg, 0.68 mmol) in H2O
(5 mL). After stirring vigorously for 30 min, the organic phase was
washed with H2O (3 × 5 mL) and dried (MS 3 Å) to give the desired
product as a red solid (251 mg, 0.46 mmol, 88%); mp 74 °C;
[a]D22 +62 (c = 1.05, CHCl3).
(4) (a) Katritzky, A. R.; Singh, S.; Kirichenko, K.; Holbrey, J.
D.; Smiglak, M.; Reichert, W. M.; Rogers, R. D. Chem.
Commun. 2005, 868. (b) Fukumoto, K.; Yoshizawa, M.;
Ohno, H. J. Am. Chem. Soc. 2005, 127, 2398. (c) Carter, E.
B.; Culver, S. L.; Fox, P. A.; Goode, R. D.; Ntai, I.; Tickell,
M. D.; Traylor, R. K.; Hoffman, N. W.; Davis, J. H. Chem.
Commun. 2004, 630. (d) Keglevich, G.; Baan, Z.; Hermecz,
I.; Novak, T.; Odinets, I. L. Curr. Org. Chem. 2007, 11, 107.
(5) (a) Imperato, G.; Eibler, E.; Niedermaier, J.; König, B.
Chem. Commun. 2005, 1170. (b) Imperato, G.; König, B.;
Chiappe, C. Eur. J. Org. Chem. 2007, 1049.
IR (KBr): 3431, 2945, 1550, 1455, 1267, 838 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.41–7.29 (m, 10 H, C6H5), 4.71
(br s, 4 H, NCH2Ph), 4.24 (br s, 1 H, NCH), 3.45 (br s, 1 H, NCH),
2.87 (s, 3 H, SCH3), 1.93–1.67 (m, 4 H, 2 NCHCH2CH2), 1.32–1.04
(m, 4 H, NCHCH2CH2).
(6) For reviews, see: (a) Baudequin, C.; Baudoux, J.; Levillain,
J.; Cahard, D.; Gaumont, A. C.; Plaquevent, J. C.
Tetrahedron: Asymmetry 2003, 14, 3081. (b) Ding, J.;
Armstrong, D. W. Chirality 2005, 17, 281. (c) Baudequin,
C.; Bregeon, D.; Levillain, J.; Guillen, F.; Plaquevent, J. C.;
13C NMR (50 MHz, CDCl3): d = 208.3 (CS2), 166.7 (CN2), 132.1
(C6H5), 129.3 (C6H5), 129.0 (C6H5), 127.7 (C6H5), 68.4 (NCH2Ph),
Synthesis 2009, No. 4, 583–586 © Thieme Stuttgart · New York

586
A. Blanrue, R. Wilhelm
PAPER
Akiyma, I. J. Chem. Soc., Chem. Commun. 1992, 1522.
(c) Nagasawa, A.; Akiyama, I.; Mashima, S.; Nakayama, J.
Heteroat. Chem. 1995, 6, 45. (d) Clemens, D. H.; Bell, A. J.;
O’Brien, J. L. Tetrahedron Lett. 1965, 6, 3257.
(e) Winberg, H. E.; Coffman, D. D. J. Am. Chem. Soc. 1965,
87, 2776. (f) Hansen, J.; Heinemann, F. W. Phosphorus
Sulfur Silicon 1996, 118, 155. (g) Schössler, W.; Regitz, M.
Chem. Ber. 1974, 107, 1931.
Gaumont, A. C. Tetrahedron: Asymmetry 2005, 16, 3921.
(d) Winkel, A.; Reddy, P. V. G.; Wilhelm, R. Synthesis
2008, 999. (e) Bica, K.; Gaertner, P. Eur. J. Org. Chem.
2008, 3235. (f) Headley, A. D.; Ni, B. Aldrichimica Acta
2007, 40, 107. (g) Ohno, H.; Fukumoto, K. Acc. Chem. Res.
2007, 40, 1122.
(7) (a) Jurčík, V.; Gilani, M.; Wilhelm, R. Eur. J. Org. Chem.
2006, 5103. (b) Jurčík, V.; Wilhelm, R. Org. Biomol. Chem.
2005, 3, 239. (c) Jurčík, V.; Wilhelm, R. Tetrahedron:
Asymmetry 2006, 17, 801. (d) Clemens, N.; Sereda, O.;
Wilhelm, R. Org. Biomol. Chem. 2006, 4, 2285.
(11) Krasuski, W.; Nikolaus, D.; Regitz, M. Liebigs Ann. Chem.
1982, 1451.
(12) For zwitterion 4, a yield of 80% was obtained. The
procedure was not optimized for zwitterion 8.
(8) Blanrue, A.; Wilhelm, R. Synlett 2004, 2621.
(9) For reviews, see: (a) Nakayama, J. J. Synth. Org. Chem. Jpn.
2002, 60, 106. (b) Kirmse, W. Eur. J. Org. Chem. 2005, 237.
(10) (a) Nakayama, J.; Otani, T.; Sugihara, Y.; Ishii, A.
Tetrahedron Lett. 1997, 38, 5013. (b) Nakayma, J.;
(13) Coady, D. J.; Norris, B. C.; Lynch, V. M.; Bielawski, W.
Macromolecules 2008, 41, 3775.
(14) Reichardt, C. Solvents and Solvent Effects in Organic
Chemistry, 3rd ed.; Wiley-VCH: Weinheim, 2003.
(15) Bennani, Y.; Hanessian, S. Tetrahedron 1996, 52, 13837.
Synthesis 2009, No. 4, 583–586 © Thieme Stuttgart · New York