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L. Androvic et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
5
virtually proceeded quantitatively in all five catalytic cycles.
However, in the fifth cycle, the enantioselectivity decreased to
88% ee (Fig. 5). After fivefold recycling, a partial mechanical degra-
dation of polymeric matrix of catalyst took place, which compli-
cated its separation from the reaction medium and reuse.
scanning with the output image resolution 2560 Â 1920 pix is
7.2 frames per second. At the highest resolution of the output
image and using the lens with the highest magnification, the
length of one pix corresponds to 0.1 microns. High-resolution mass
spectra were measured on the Thermo Fisher Scientific MALDI LTQ
Orbitrap instrument. The used matrices were 0.2 M solutions of
2,5-dihydroxybenzoic acid (DHB) in MeCN/H2O (95:5) or 2-[(2E)-
3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile
(DCTB) in MeCN. HPLC analyses were performed on Watrex HPLC
instrument with UV–Vis DAD (200–800 nm) SYKAM 3240 and with
chiral Daicel columns: Chiralcel OD-H, Chiralpak AS-H and
Chiralpak AD-H (250 mm  4.6 mm). The microanalyses were per-
formed on an apparatus of FISONS Instruments, EA 1108 CHNS.
3. Conclusion
The (S)-1-benzoyl-3-(pyrrolidine-2-ylmethyl)thiourea cova-
lently bonded into a polymeric matrix of a swellable pearl-like
copolymer of styrene (20–600 lm) 5 was prepared by a series of
reactions. The heterogeneous organocatalyst 5 prepared in this
way was used for the catalysis of the Michael addition reaction
of acetone, butanone and cyclohexanone with b-nitrostyrene and
its derivatives. The selectivity of this addition reaction increased
with an increase in the rigidity of the ketone molecule, with the
best result being obtained with cyclohexanone. The most suitable
reaction medium found was toluene and/or cyclohexanone itself
without an added acid. Under these conditions, the enantioselec-
tivity of immobilized catalyst 5 was comparable with the enantios-
electivity of the homogeneous variant, i.e., catalyst 1.6a In the case
of the addition reaction of cyclohexanone, the effects of
substituents in the aromatic moiety of b-nitrostyrene upon the
reaction parameters were studied. The corresponding
functionalized 4-nitroketones were formed quantitatively with
high ee values (up to 98% ee). The possibility of recycling of
heterogeneous organocatalyst 5 was also studied. In the first four
catalytic cycles, the enantioselectivity of catalysts was very high
(95–98% ee); only in the fifth cycle, did a partial decrease in
enantioselectivity take place (95 ? 88% ee). The attained
chemical yields were virtually quantitative in all five cycles. After
fivefold recycling, a partial mechanical degradation of polymer
matrix of catalyst was observed, which probably could be
prevented by placing the catalyst into a continuous-flow reactor.
The prepared polymeric catalyst possesses a high application
4.2. Synthesis
4.2.1. 2-Chloroethyl 4-vinylbenzoate
A mixture of 4-vinylbenzoic acid (5 g; 34 mmol), oxalyl chloride
(15 mL; 175 mmol), DMF (2 drops) and CHCl3 (15 mL) was refluxed
for 5 h. The solvent and excess of oxalyl chloride were evaporated
off and the residue was mixed with a solution of freshly distilled 2-
chloroethanol (10 mL, 150 mmol) and Et3N (16 mL, 115 mmol) in
CH2Cl2 (15 mL). The mixture was stirred overnight at room tem-
perature and then evaporated under reduced pressure. The crude
product was purified by column chromatography (SiO2/CH2Cl2, Rf
0.56). Yield: 5.5 g (78%); yellow oil; 1H NMR (400 MHz, CDCl3): d
8.08 (m, 2H, Ar), 7.52 (m, 2H, Ar), 8.80 (dd, 1H, J = 10.8 Hz;
17.6 Hz), 5.92 (dd, 1H, J = 0.8 Hz, 17.6 Hz), 5.45 (dd, 1H, J = 0.8 Hz,
10.8 Hz), 4.62 (t, 2H, J = 5.5 Hz), 3.87 (t, 2H, J = 5.5 Hz); 13C NMR
(100 MHz, CDCl3): d 166.1, 142.5, 136.1, 130.3, 128.9, 126.4,
116.9, 64.6, 41.6; HR-MALDI-MS: calcd. for
C11H11ClO2 m/z
211.05203 ([M+H]+), found 211.05222.
4.2.2. Preparation of pearl-like copolymer 2 (suspension poly-
merization)
potential for the practical realization of
reaction, and simultaneously it fulfils a number of demands
placed by green and sustainable chemistry.
a Michael addition
The preparation of pearl-like copolymer by suspension poly-
merization was performed in accordance with the literature in a
defined apparatus of 500 mL.7 The vessel was charged with a solu-
tion of NaCl (15 g) and poly(vinyl alcohol) (88% hydrolyzed poly-
mer; M = 85–124 kDa) (1.34 g) in water (400 mL), followed by a
solution of benzoyl peroxide (410 mg; 1.64 mmol), 2-chloroethyl
4-vinylbenzoate (5.52 g; 26.2 mmol), styrene (8.2 g; 78.8 mmol),
and tetra(ethylene glycol)-bis(4-vinylbenzyl)ether7 (250 mg;
0.66 mmol) in chlorobenzene (6 mL). The stirrer was adjusted at
a rate of 500 rpm, and the temperature of the reaction mixture
was increased to 85 °C for 30 min. The obtained suspension of
the spherical particles of the organic phase was continuously stir-
red and kept at 85 °C for 18 h. After cooling, the obtained pearl-like
copolymer was collected by filtration and washed with water
(3 Â 100 mL), MeOH (3 Â 100 mL), THF (3 Â 100 mL) and CH2Cl2
(3 Â 100 mL). The polymer was then extracted in a Soxhlet extrac-
tor for 48 h with THF. The prepared pearl-like polymer (20–
4. Experimental
4.1. General
Unless otherwise stated, the starting chemicals and solvents
were obtained from Sigma–Aldrich or Acros Organics and used
without further purification. 1H NMR spectra were recorded on a
Bruker Avance 400 instrument or Bruker Avance 500 instrument.
Chemical shifts d were referenced to the residual peak of the CDCl3
at 7.26 ppm. The 13C NMR spectra were calibrated with respect to
the middle signal in the multiplet of solvent (d = 77.23). The 13C CP/
MAS NMR experiments were performed at 125.613 MHz frequency
for 13C, equipped with a MAS probe head using 4 mm ZrO2 rotors. A
sample of glycine was used for setting the Hartmann–Hahn condi-
tion, and glycine was used as a secondary chemical shift reference
(d = 176.04 ppm) from external TMS. The Raman spectra were
measured at room temperature using FT-IR spectrophotometer
IFS 55 provided with Raman FRA-106 accessory (Bruker) for back
scattering method. The YAG: Nd3+ laser line (1064 nm) was used
for excitation. The resolution was 2 cmÀ1 and the laser power
was 50 mW. The FT-Raman data are presented in cmÀ1 (w, weak;
m, medium; s, strong; sh, shoulder). The morphology and grain size
of the pearl-like polymer styrene-VBC-TEG was determined by
means of optical microscope Nikon Eclipse LV100D with detach-
able head having high resolution power Nikon DS-Fi1 with soft-
ware NIS Elements AR. The scanning head allows color scanning;
the resolution of the CCD chip is 5.07 Mpix. The top speed of
600 lm) was dried under reduced pressure at the temperature of
40 °C. The yield was 7.9 g of polymer with elemental composition:
C, 75.76; H, 6.82; Cl, 3.74; FT-Raman: 620 (m), 637 (m), 670 (w),
761 (w), 796 (w), 823 (w), 867 (w), 908 (w), 1001 (s), 1032 (m),
1068 (w), 1095 (w), 1155 (m), 1182(m), 1200 (m), 1226 (w),
1250 (sh), 1275 (m), 1323 (w), 1380 (w), 1450 (m), 1584 (m),
1609 (s), 1722 (s), 2853 (sh), 2908 (s), 2962 (s), 3001 (m), 3040
(sh), 3056 (s).
4.2.3. Preparation of pearl-like copolymer 3 (polystyrene carbo-
xylic acid)
A
mixture of polymer
3
(2.5 g), Me4N+OHÀÁ5H2O (5 g;
27.5 mmol), Bu4N+BrÀ (5 mg; 15
l
mol), DMSO (100 mL), water
(30 mL) and THF (40 mL) was heated at 90 °C for 48 h. After
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