S-benzyl isothiouronium chloride as a recoverable organocatalyst for the reduction of conjugated nitroalkenes with Hantzsch ester

Article abstract of DOI:10.1016/j.tet.2012.05.104

The reduction of conjugated nitroalkenes into nitroalkanes with Hantzsch ester using S-benzyl isothiouronium chloride as a recoverable organocatalyst was successfully accomplished with high yield and excellent chemoselectivity.

Full text of DOI:10.1016/j.tet.2012.05.104

Tetrahedron 68 (2012) 6513e6516
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S-Benzyl isothiouronium chloride as a recoverable organocatalyst for the
reduction of conjugated nitroalkenes with Hantzsch ester
,
Quynh Pham Bao Nguyen a, Jae Nyoung Kim b, Taek Hyeon Kim a
*
a School of Applied Chemical Engineering and Center for Functional Nano Fine Chemicals, Chonnam National University, Gwangju 500-757, Republic of Korea
b Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju, 500-757, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 March 2012
Received in revised form 22 May 2012
Accepted 24 May 2012
Available online 31 May 2012
The reduction of conjugated nitroalkenes into nitroalkanes with Hantzsch ester using S-benzyl iso-
thiouronium chloride as a recoverable organocatalyst was successfully accomplished with high yield and
excellent chemoselectivity.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
S-Benzyl isothiouronium chloride
Recoverable organocatalyst
Hydrogen bonding
Hantzsch ester
Conjugated nitroalkenes
1. Introduction
and S-benzyl isothiouronium chloride as an organocatalyst is
herein introduced for the reduction of conjugated nitroalkenes into
Nitroalkanes have great potential for organic synthesis because
the nitro group is converted easily into the corresponding func-
tionalities, such as carbonyl, nitrile oxide, and amino groups.1
Several methods have been reported for the reduction of conju-
gated nitroalkenes to nitroalkanes. However, they have some lim-
itations in controlling chemoselectivity.2 For instance, lithium
aluminum hydride provided a mixture of products containing sat-
urated amines, nitroalkanes, oximes, and hydroxyl amines, whereas
borohydride reduction furnished primarily the corresponding
nitroalkane, which was often contaminated with the dimeric by-
product formed by Michael addition of the nitronate intermediate
to the starting nitroalkene.2f
nitroalkanes resulting in high efficiency, chemoselectivity, and easy
recovery of organocatalyst. Isothiouronium salts have been ex-
plored quite recently as a new class of hydrogen-bonding subunit
for anion recognition.5 Isothiouronium group as an anion-binding
site has several advantages: (i) it enhances the NH acidity com-
pared to the corresponding thiourea; (ii) the chemical modification
is readily varied to prepare several types of functional molecular
systems.6f Therefore we expect that some isothiouronium salts
might work as an activator for the reduction of conjugated nitro-
alkenes (Scheme 1).
2. Results and discussion
Hantzsch esters have been explored as powerful biomimetic
reductants because they overcome some limitations encountered
with traditional reductive reagents, such as hydrogen gas/metal
and metal hydrides.3 The selective reduction of nitroalkenes into
nitroalkanes using Hantzsch esters has done by the combination of
effective catalysts, such as acetic acid,2c silica gel,2d,g and thiour-
ea.2h,5c Recently, we reported S-benzyl isothiouronium chloride as
a novel organocatalyst for the direct reductive amination of alde-
hydes using Hantzsch esters.4 The combination of Hantzsch esters
Mechanically, the hydrogen bonding between the nitro group of
the substrate and the isothiouronium moiety of organocatalyst may
reduce the activation energy of the alkene (LUMO activation),
leading to acceleration of the reaction (Scheme 1).2h With this
mind, the reduction of nitrostyrene 1a was first investigated.
Pleasingly, this reaction gave the required product at 71% and 90%
yields in dichloromethane and methanol, respectively (entries 3
and 4 in Table 1). To identify the hydrogen bonding between
nitrostyrene and S-benzyl isothiouronium chloride catalyst, the 1H
NMR spectrometry method was used by monitoring the changes in
the spectra of the catalyst in the presence of the excess reagent 1a
(10 equiv) in DMSO-d6. It was obviously observed that the protons
* Corresponding author. E-mail addresses: thkim@chonnam.ac.kr, thkim@
jnu.ac.kr (T.H. Kim).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.05.104

6514
Q.P.B. Nguyen et al. / Tetrahedron 68 (2012) 6513e6516
EtO2C
CO2Et
Cl
S
N
H
H2N
NH2
NO2
NO2
R
R
EtO2C
O
O
N
1
Cl
2
H
H
NH
S
R
EtO2C
H2N
NH2
Scheme 1. Reduction of conjugated nitroalkenes using S-benzyl isothiouronium chloride catalyst.
Table 1
aromatic (entries 1e10 and 15e16), aliphatic (entry 17), and het-
erocyclic (entries 11e13) nitroalkenes in high yields. The variations
of the substituent in the aromatic ring made no effect on the out-
come of this reaction. No reductions of the nitro (entry 7), nitrile
(entry 8), and carbonyl (entries 9 and 10) moieties were observed
and the free hydroxyl group (entry 4) was tolerated. The nitro-
Optimization of the reduction of conjugated nitroalkenes
EtO2C
CO2Et
N
H
(1.1 eq)
NO2
NO2
Ph
H
C
CH2NO2
PhCH2
C
+
H
Cl
NO2
alkene with the methyl group substituted at the b-position of the
(1 eq)
1a
2a
S
3a
double bond also ran smoothly (entry 14). In the structures with
two conjugated carbonecarbon double bonds, only the one adja-
cent to the nitro group was reduced selectively, while the other one
remained intact even in an excess of the reducing reagent and
catalyst as well as under extended reaction time (entries 15 and 16).
In addition, the enone systems, such as cinnamaldehyde and 2-
cyclohenxen-1-one were not reduced at all under this reaction
condition and recovered, showing only nitroalkenes were reduced
chemoselectively.
H2N
NH2
Entry Equiv of catalyst Solvent Time (h) Temperature (ꢀC) Yield of 2a (%)
1
2
3
4
5
6
0
0
0.1
0.1
0.1
0.05
CH2Cl2 24
MeOH 10
CH2Cl2 24
Reflux
60
Reflux
60
0
50
71 (0)a
90 (41)a
55
MeOHb
5
MeOH 24
MeOH
rt
60
5
82
Finally, the recycling of the catalyst was a remarkable feature of
this protocol. As an organic salt, the S-benzyl isothiouronium
chloride catalyst was readily precipitated out by adding the excess
cooled dichloromethane. Therefore, commercially available S-ben-
zyl isothiouronium chloride organocatalyst can be easily recovered
and reused as many times as desired without loss of efficiency
(Table 3).
a
Yields in the parentheses were obtained in the presence of the excess TBAA
(1.5 equiv).
b
The insoluble solvents of isothiouronium catalyst (heterogeneous solution),
such as CH2Cl2, toluene, benzene, dioxane, and THF are less suitable than the soluble
solvent MeOH (homogeneous solution).
of eNH and eCH2Ph moieties of the S-benzyl isothiouronium
chloride catalyst were shifted from 9.37 to 9.44 ppm and from 4.55
to 4.57 ppm, respectively. In addition, the excess tetrabuty-
lammonium acetate (TBAA) was added to the reaction mixture and
their yields dramatically were reduced (entries 3 and 4 in Table 1),
which can destroy the hydrogen bonding between the S-benzyl
isothiouronium chloride and nitro group of the nitroalkene due to
the higher coordination of the acetate anion with the iso-
thiouronium ion.6e8 From this control experiments the hydrogen
bonding activation by isothiouronium ion would be a crucial factor
in this reaction. Next the effect of solvents on the reduction was
examined to optimize the reaction. The best solvent was MeOH
dissolving the catalyst, while the insoluble solvents, such as CH2Cl2,
toluene, benzene, dioxane, and THF were less suitable. This result
was quite different from that of thiourea.2h One of the most con-
siderable advantages of isothiouroniums over thioureas, as known
in anion recognition field, is that they gave higher binding con-
stants even in polar protic solvents, such as methanol or water,
proving the strong hydrogen bonding of isothiouronium de-
rivatives.6 It is also worth noting that without the organocatalyst in
the methanol (entry 2 in Table 1), there is generally loss of product
due to the formation of the dimeric by-product 3a derived from the
Michael addition of the nitronate intermediate to the starting
nitroalkene.2a,h Running the reaction at room temperature and
loading low amount of catalyst gave reduced yields of 2a (entries 5
and 6 in Table 1). In MeOH at 60 ꢀC as a best conditions S-benzyl
isothiouronium chloride provided 90% yields with only a trace of
by-product 3a (entry 4 in Table 1).
3. Conclusion
In summary, S-benzyl isothiouronium chloride has been ex-
plored successfully as a new class of hydrogen bonding organo-
catalysts for the reduction of conjugated nitroalkenes. The
isothiouronium catalyst acquired certain valuable characteristics,
such as working even on polar protic solvents, and being recycled
and reused. The asymmetric reduction of disubstituted or tri-
substituted nitroalkenes using
a novel chiral isothiouronium
organocatalysts is under consideration.
4. Experimental section
4.1. General
All reagents and solvents were obtained from commercial sup-
pliers and were used without further purification. All air- and
moisture-sensitive reactions were carried out under an argon at-
mosphere. The products were purified by using flash column
chromatography. TLC was developed on Merck silica gel 60 F254
aluminum sheets. The 1H and 13C NMR spectra were recorded at
300 and 75 MHz, respectively, using CDCl3 as solvent. HRMS were
measured on a Micromass Q-TOF instrument (ESþ ion mode).
4.2. Typical procedure for the reduction of conjugated
nitroalkenes
The various conjugated nitroalkenes (Table 2) were examined in
MeOH at 60 ꢀC. As expected, the present protocol provided the
desired products in a various conjugated nitroalkenes 1, such as
The mixture of nitroalkenes 1 (0.1 g, 0.67 mmol), Hantzsch ester
(0.19 g, 0.74 mmol) and S-benzyl isothiouronium chloride (14 mg,

Q.P.B. Nguyen et al. / Tetrahedron 68 (2012) 6513e6516
6515
Table 2
Table 3
Scope of conjugated nitroalkenes
Recovery of the catalyst
Recycle
Yield of recovered catalyst (%)
Yield of product 2a (%)
Entry
Nitroalkenes 1
Product 2
Yield (%)
First
93
90
94
92
90
85
88
87
Second
Third
Fourth
1
2
3
2a
2b
2c
90
97
71
4.2.1. Compound 2a [6125-24-2]. Yellow oil; 1H NMR (300 MHz,
CDCl3):
d
¼3.23 (t, J¼7.35 Hz, 2H), 4.53 (t, J¼7.35 Hz, 2H), 7.10e7.30
(m, 5H); 13C NMR (75 MHz, CDCl3):
135.6.
d¼33.3, 76.2, 127.3, 128.5, 128.7,
4
5
6
2d
2e
2f
96
75
80
4.2.2. Compound 2b [72538-33-1]. Yellow oil; 1H NMR (300 MHz,
CDCl3):
d
¼2.34 (s, 3H), 3.28 (t, J¼7.35 Hz, 2H), 4.59 (t, J¼7.35 Hz,
2H), 7.00e7.30 (m, 4H); 13C NMR (75 MHz, CDCl3):
d
¼21.0, 33.0,
76.4, 128.4, 129.6, 132.5, 137.1.
4.2.3. Compound 2c [123312-23-2]. Colorless oil; 1H NMR
(300 MHz, CDCl3):
d
¼3.29 (t, J¼7.41 Hz, 2H), 3.80 (s, 3H), 4.61 (t,
J¼7.41 Hz, 2H), 6.78 (m, 3H), 7.25 (m, 1H); 13C NMR (75 MHz,
7
8
9
2g
2h
2i
92
95
91
CDCl3):
d
¼33.4, 55.2, 76.1, 112.7, 114.4, 120.7, 130.0, 137.1, 160.0.
4.2.4. Compound 2d [37567-58-1]. Yellow oil; 1H NMR (300 MHz,
CDCl3):
d
¼3.22 (t, J¼7.23 Hz, 2H), 4.57 (t, J¼7.23 Hz, 2H), 5.20 (br s,
1H), 6.76 (d, J¼6.55 Hz, 2H), 7.04 (d, J¼6.55 Hz, 2H); 13C NMR
(75 MHz, CDCl3):
d¼32.6, 76.6, 115.8, 127.6, 129.8, 154.8.
10
2j
82
4.2.5. Compound 2e [137521-07-4]. Colorless oil; 1H NMR
(300 MHz, CDCl3):
d
¼3.27 (t, J¼7.20 Hz, 2H), 4.59 (t, J¼7.20 Hz, 2H),
7.08 (d, J¼6.63 Hz, 2H), 7.44 (d, J¼6.63 Hz, 2H); 13C NMR (75 MHz,
CDCl3):
d
¼32.8, 75.9, 121.5, 130.3, 132.1, 134.6.
11
12
13
2k
2l
87
82
84
4.2.6. Compound 2f [72538-34-2]. Yellow oil; 1H NMR (300 MHz,
CDCl3):
d
¼3.40 (t, J¼7.14 Hz, 2H), 4.63 (t, J¼7.14 Hz, 2H), 7.20 (s, 2H),
7.40 (s, 1H); 13C NMR (75 MHz, CDCl3):
131.9, 134.3, 134.6.
d¼30.8, 73.9, 127.6, 129.7,
2m
4.2.7. Compound 2g [155988-20-8]. Yellow oil; 1H NMR (300 MHz,
14
15
16
2n
90
65
68
CDCl3):
d
¼3.60 (t, J¼6.84 Hz, 2H), 4.77 (t, J¼6.84 Hz, 2H), 7.30e8.20
(m, 4H); 13C NMR (75 MHz, CDCl3):
132.8, 133.9.
d
¼31.2, 75.2, 125.5, 128.9, 131.4,
2oa
2pa
4.2.8. Compound 2h [126158-10-9]. White solid; mp 80 ꢀC; 1H NMR
(300 MHz, CDCl3):
d
¼3.37 (t, J¼7.02 Hz, 2H), 4.65 (t, J¼7.20 Hz, 2H),
7.34 (d, J¼8.07 Hz, 2H), 7.62 (d, J¼8.07 Hz, 2H); 13C NMR (75 MHz,
17
2q
80
CDCl3):
d
¼33.0, 75.2, 111.4, 118.3, 129.4, 132.6, 141.1.
a
Hantzsch ester (2.2 equiv), isothiouronium catalyst (0.2 equiv), 24 h.
4.2.9. Compound 2i [745059-98-7]. White solid; mp 60 ꢀC; 1H NMR
0.067 mmol) in methanol (5 ml) was stirred at 60 ꢀC for 5e12 h.
After completion of the reaction, the crude product in methanol
was concentrated and added into the excess cooled CH2Cl2. The S-
benzyl isothiouronium chloride was readily precipitated out, and
then filtered and washed many times with CH2Cl2 to be reused. The
filtrate was evaporated and the residue was purified by flash col-
umn chromatography to give the required nitroalkanes products 2.
All nitroalkanes except 2j and 2p were characterized with the
reported spectroscopic data.2,5
(300 MHz, CDCl3):
d
¼3.36 (t, J¼7.23 Hz, 2H), 3.90 (s, 3H), 4.64 (t,
J¼7.23 Hz, 2H), 7.28 (d, J¼8.22 Hz, 2H), 8.00 (d, J¼8.22 Hz, 2H); 13C
NMR (75 MHz, CDCl3):
166.6.
d
¼33.1, 52.0, 75.6, 128.5, 129.3, 130.1, 140.8,
4.2.10. Compound 2j. Colorless oil; IR (neat): nmax¼1681, 1547,
1267 cmꢁ1
;
1H NMR (300 MHz, CDCl3):
d
¼2.60 (s, 3H), 3.38 (t,
J¼7.20 Hz, 2H), 4.65 (t, J¼7.20 Hz, 2H), 7.20e7.90 (m, 4H); 13C NMR
(75 MHz, CDCl3):
d
¼26.6, 33.2, 75.9, 127.6, 128.2, 129.3, 133.2, 136.4,

6516
Q.P.B. Nguyen et al. / Tetrahedron 68 (2012) 6513e6516
137.9, 197.7; HRMS (ESI) calcd for C10H12NO3 [MþHþ]: 194.0817,
Korea Basic Science Institute Gwangju center for analysis of LCeMS/
MS Spectrometry.
found: 194.0817.
4.2.11. Compound 2k [5462-90-8]. Yellow oil; 1H NMR (300 MHz,
Supplementary data
CDCl3):
d
¼3.36 (t, J¼7.11 Hz, 2H), 4.64 (t, J¼7.11 Hz, 2H), 6.13 (d,
J¼4.70 Hz, 1H), 6.30 (m, 1H), 7.34 (d, J¼2.50 Hz, 1H); 13C NMR
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tet.2012.05.104.
(75 MHz, CDCl3):
d¼26.0, 73.3, 107.4, 110.5, 142.2, 149.3.
4.2.12. Compound 2l [30807-46-6]. Yellow oil; 1H NMR (300 MHz,
References and notes
CDCl3):
d
¼3.54 (t, J¼7.08 Hz, 2H), 4.63 (t, J¼7.08 Hz, 2H), 6.88 (d,
J¼5.40 Hz, 1H), 6.96 (m, 1H), 7.21 (d, J¼5.40 Hz, 1H); 13C NMR
1. Ono, N. The Nitro Group in Organic Synthesis; Wiley-VCH: New York, Chichester,
2001.
(75 MHz, CDCl3):
d¼27.5, 76.0, 124.9, 126.3, 127.3, 137.3.
2. (a) Meyer, A. I.; Sircar, J. C. J. Org. Chem. 1967, 32, 4134; (b) Trefouel, T.; Tintiller,
P.; Dupas, G.; Bourguignon, J.; Queguiner, G. Bull. Chem. Soc. Jpn. 1987, 60, 4492;
(c) Inoue, Y.; Imaizumi, S.; Itoh, H.; Shinya, T.; Hashimoto, H.; Miyano, S. Bull.
Chem. Soc. Jpn. 1988, 61, 3020; (d) Fujii, M. Bull. Chem. Soc. Jpn. 1988, 61, 4029; (e)
Ranu, B. C.; Chakraborty, R. Tetrahedron Lett. 1991, 32, 3579; (f) Ranu, B. C.;
Chakraborty, R. Tetrahedron Lett. 1992, 48, 5317; (g) Torchy, S.; Cordonnier, G.;
Barbry, D.; Eynde, J. J. V. Molecules 2002, 7, 528; (h) Zhang, Z.; Schreiner, P. R.
Synthesis 2007, 2559; (i) Toogood, H. S.; Fryszkowska, A.; Hare, V.; Fisher, K.;
Roujeinikova, D. L.; Gardiner, J. M.; Stephens, G. M.; Scrutton, N. S. Adv. Synth.
Catal. 2008, 350, 2789; (j) Yanto, Y.; Hall, M.; Bommarius, A. S. Org. Biomol. Chem.
2010, 8, 1826.
4.2.13. Compound 2m [115149-33-2]. Yellow oil; 1H NMR (300 MHz,
CDCl3):
d
¼3.38 (t, J¼7.02 Hz, 2H), 4.68 (t, J¼7.02 Hz, 2H), 7.37 (m,1H),
7.68 (d, J¼7.86 Hz, 1H), 8.56 (d, J¼4.11 Hz, 1H), 8.62 (s, 1H); 13C NMR
(75 MHz, CDCl3):
d
¼30.5, 75.5, 123.7, 131.3, 136.1, 149.0, 149.9.
4.2.14. Compound 2n [17322-34-8]. Colorless oil; 1H NMR
(300 MHz, CDCl3):
d
¼1.55 (d, J¼7.50 Hz, 3H), 3.01 (q, J¼7.50 Hz, 1H),
3.32 (q, J¼7.50 Hz, 1H), 4.80 (m, 1H), 7.10e7.40 (m, 5H); 13C NMR
3. Menche, D.; Hassfeld, J.; Li, J.; Menche, G.; Ritter, A.; Rudolph, S. Org. Lett. 2006, 8,
741.
4. Nguyen, Q. P. B.; Kim, T. H. Tetrahedron Lett. 2011, 52, 5004.
(75 MHz, CDCl3):
d
¼18.8, 41.2, 84.4, 127.4, 128.8, 129.0, 135.5.
5. (a) Czekelius, C.; Carreira, E. M. Org. Lett. 2004, 6, 4575; (b) Czekelius, C.; Carreira,
E. M. Org. Process Res. Dev. 2007, 11, 633; (c) Martin, N. J. A.; Ozores, L.; List, B. J.
Am. Chem. Soc. 2007, 129, 8976; (d) Soltani, O.; Martin, A. A.; Carreira, E. M. Org.
Lett. 2009, 11, 4196; (e) Tang, Y.; Xiang, J.; Cun, L.; Wang, Y.; Zhu, J.; Liao, J.; Deng,
J. Tetrahedron: Asymmetry 2010, 21, 1900; (f) Schneider, J. F.; Falk, F. C.; Frohlich,
R.; Paradies, J. Eur. J. Org. Chem. 2010, 2265.
4.2.15. Compound 2o [76024-91-4]. Colorless oil; 1H NMR
(300 MHz, CDCl3):
d
¼2.90 (m, 2H), 4.51 (t, J¼7.00 Hz, 2H), 6.13 (m,
1H), 6.56 (d, J¼13.29 Hz, 1H), 7.20e7.40 (m, 5H); 13C NMR (75 MHz,
CDCl3):
d
¼30.7, 75.0, 122.9, 126.3, 127.8, 128.6, 134.0, 136.4.
6. For the additional hydrogen bond created by the isothiouronium compound,
see: (a) Yeo, W. S.; Hong, J. I. Tetrahedron Lett. 1998, 39, 3769; (b) Yeo, W. S.;
Hong, J. I. Tetrahedron Lett. 1998, 39, 8137; (c) Kubo, Y.; Tsukahara, M.; Ish-
ihara, S.; Tokita, S. Chem. Commun. 2000, 653; (d) Nishizawa, S.; Cui, Y. Y.;
Minagawa, M.; Morita, K.; Kato, Y.; Taniguchi, S.; Kato, R.; Teramae, N. J.
Chem. Soc., Perkin Trans. 2 2002, 866; (e) Kubo, Y.; Ishihara, S.; Tsukahara, M.;
Tokita, S. J. Chem. Soc., Perkin Trans. 2 2002, 1455; (f) Kubo, Y.; Kato, M.;
Misawa, Y.; Tokita, S. Tetrahedron Lett. 2004, 45, 3769; (g) Kato, R.; Cui, Y.-Y.;
Nishizawa, S.; Yokobori, T.; Teramae, N. Tetrahedron Lett. 2004, 45, 4273; (h)
Seong, H. R.; Kim, D.-S.; Kim, S.-G.; Choi, H.-J.; Ahn, K. H. Tetrahedron Lett.
2004, 45, 723; (i) Kubo, Y.; Uchida, S.; Kemmochi, Y.; Okubo, T. Tetrahedron
Lett. 2005, 46, 4369; (j) Minami, T.; Kaneko, K.; Nagasaki, T.; Kubo, Y. Tet-
rahedron Lett. 2008, 49, 432; (k) Nguyen, Q. P. B.; Kim, J. N.; Kim, T. H. Bull.
Korean Chem. Soc. 2009, 30, 2093; (l) Nguyen, Q. P. B.; Kim, T. H. Bull. Korean
Chem. Soc. 2010, 31, 712.
4.2.16. Compound 2p. Colorless oil; IR (neat): nmax¼1547,
1377 cmꢁ1 1H NMR (300 MHz, CDCl3):
¼1.90 (s, 3H), 2.87 (t,
J¼7.26 Hz, 2H), 4.58 (t, J¼7.26 Hz, 2H), 6.36 (s, 1H), 7.20e7.40 (m,
;
d
5H); 13C NMR (75 MHz, CDCl3):
d
¼17.4, 38.1, 74.3,126.7,128.2,128.8,
128.9, 132.3, 137.3; HRMS (ESI) calcd for C11H14NO2 [MþHþ]:
192.1025, found: 192.1042.
4.2.17. Compound 2q [2216-21-9]. Colorless oil; 1H NMR (300 MHz,
CDCl3):
d
¼0.88 (t, J¼7.08 Hz, 3H), 1.30 (m, 12H), 1.99 (m, 2H), 4.37 (t,
J¼7.08 Hz, 2H); 13C NMR (75 MHz, CDCl3):
¼14.0, 22.6, 26.2, 27.4,
d
28.8, 29.1, 29.2, 31.6, 75.7.
7. For an example of destroying the hydrogen bond in the presence of TBAA, see:
Maher, D. J.; Connon, S. J. Tetrahedron Lett. 2004, 45, 1301.
8. By monitoring the 1H NMR spectra of the catalyst, it was found that the
proton of the eNH moieties of the S-benzyl isothiouronium chloride cata-
lyst were disappeared in the presence of the excess tetrabutylammonium
acetate, proving the strong interaction of the catalyst with the acetate
anion.
Acknowledgements
This research was supported by the Sabbatical Research Pro-
gram of Chonnam National University, 2010. We wish to thank the