J. Bartáček, et al.
ReactiveandFunctionalPolymers153(2020)104615
extracted with hot DCM (20 ml). The combined organic portions were
extracted with 20% NaOH (50 ml). The water layer was washed with
DCM (3× 15 ml) and combined organics layer were washed with brine,
dried over MgSO4 and subjected to flash chromatography
(hexane→AcOEt +5% TEA). The product was obtained as yellow solid
(3.17 g; 10.6 mmol; 95%).
2: 13C CP/MAS NMR (20 kHz rotation, 126 MHz): δ = 164, 147,
128, 76, 68, 41, 34, 27, 26 ppm; FT-IR (ATR) cm−1: 3379; 3107; 3082;
3059; 3026; 3003; 2922; 2852; 1720; 1672; 1601; 1514; 1493; 1452;
1365; 1066; 1028; 906; 829; 756; 696; 540; elemental analysis (%)
found C 86.36, H 7.70, N 2.05; swellability (ml·g−1) 1,2-DCE: 5.3,
AcOEt: 2.
20
[α]D –56.6 (CHCl3, c 1 g/100 ml); 1H NMR (400 MHz, CDCl3):
3: FT-IR (ATR) cm−1: 3379; 3103; 3082; 3059; 3026; 3001; 2922;
2850; 1720; 1676; 1601; 1514; 1493; 1452; 1365; 1068; 1028; 906;
829; 756; 696; 538; elemental analysis (%) found C 88.51, H 7.77, N
1.30; swellability (ml·g−1) 1,2-DCE: 8.8, AcOEt: 2.7.
δ = 8.52 (d, J = 5.2 Hz, 1H), 8.27 (d, J = 0.9 Hz, 1H), 7.56 (dd,
J = 5.1, 1.3 Hz, 1H), 4.46 (t, J = 9.5 Hz, 1H), 4.32 (t, J = 8.6 Hz, 1H),
4.20–4.07 (m, 1H), 0.98 ppm (s, 9H); 13C NMR (101 MHz, CDCl3):
δ = 161.4, 150.4, 148.1, 133.3, 128.7, 127.3, 76.6, 69.5, 34.0,
26.0 ppm; FT-IR (ATR) cm−1: 3053; 2957; 2904; 2868; 1643; 1568;
1550; 1477; 1464; 1410; 1363; 1331; 1306; 1255; 1115; 1084; 964;
2.3.2. Copolymeric complexes 1–3*Pd
Copolymers 1–3 and Pd(TFA)2 were placed into flask. Next, a sol-
vent (1,2-DCE or AcOEt; 20 ml per 1 g of polymer) was added and after
ultrasound homogenization (400 W; 1 min) the mixture was heated up
to 60 °C and stirred at this temperature. After 3 days, the copolymer was
filtered, washed with DCM, dried and precisely weighted. The weight
difference of the copolymer before and after complexation, was iden-
tified with amount of complexed Pd and its content was estimated
(mmol·g−1). 13 mol% of this complex was used in catalytic experi-
ments.
914; 833; 744; 677; HRMS (MALDI): m/z calcd for C12H15BrN2O + H+
:
283.04405 [M + H]+; found: 283.04417.
2.2.3. (S)-4-(tert-butyl)-2-(4-(4-vinylphenyl)pyridin-2-yl)-4,5-
dihydrooxazole
In 250 ml flask (S)-2-(4-bromopyridin-2-yl)-4-(tert-butyl)-4,5-dihy-
drooxazole (3.4 g; 12 mmol), 4-vinylphenylboronic acid (2.5 g;
16.9 mmol) and Na2CO3 (2.6 g; 24.5 mmol) were mixed in a THF/water
mixture (4:1; 70 ml) and degassed with argon. Pd(PPh3)2Cl2 (420 mg;
0.6 mmol) was added and the mixture was heated up to 65 °C and
stirred at this temperature. After 2 days no more starting material was
detectable via TLC. The reaction mixture was filtered through silica pad
with AcOEt +5% TEA and evaporated. The residue was subjected to
flash chromatography (hexane→AcOEt +5% TEA). The product was
obtained as a yellow oil (3.8 g; 11.7 mmol; 98%).
1*Pd: FT-IR (ATR) cm−1: 3352; 3103; 3082; 3059; 3026; 3001;
2920; 2848; 1714; 1676; 1601; 1493; 1452; 1265; 1194; 1138; 1066;
1028; 906; 829; 756; 735; 696; 536; 2*Pd(a-c) (same bands of different
intensity): FT-IR (ATR) cm−1: 3103; 3082; 3060; 3026; 3003; 2922;
2850; 1716; 1684; 1601; 1514; 1493; 1452; 1369; 1192; 1142; 1028;
906; 827; 756; 696; 534; 3*Pd: FT-IR (ATR) cm−1: 3103; 3082; 3059;
3026; 3003; 2922; 2850; 1714; 1693; 1601; 1493; 1452; 1369; 1192;
1142; 1028; 906; 829; 756; 696; 538.
20
[α]D –19.8 (CHCl3, c 1 g/100 ml); 1H NMR (400 MHz, CDCl3):
δ = 8.73 (d, J = 5.1 Hz, 1H), 8.32 (d, J = 1.0 Hz, 1H), 7.73–7.62 (m,
2H), 7.59 (dd, J = 5.1, 1.7 Hz, 1H), 7.55–7.47 (m, 2H), 6.76 (dd,
J = 17.6, 10.9 Hz, 1H), 5.84 (d, J = 17.6 Hz, 1H), 5.34 (d, J = 10.9 Hz,
1H), 4.47 (dd, J = 10.1, 8.9 Hz, 1H), 4.34 (t, J = 8.5 Hz, 1H), 4.15 (dd,
J = 10.1, 8.4 Hz, 1H), 1.00 ppm (s, 9H); 13C NMR (101 MHz, CDCl3):
δ = 162.6, 150.2, 148.6, 147.6, 138.7, 136.7, 136.1, 127.3, 127.0,
2.4. Catalytic experiments
2.4.1. General procedure for catalytic experiments
Arylboronic acid (0.91 mmol), enone substrate (0.45 mmol), cata-
lyst (0.06 mmol of Pd) and HFIP (2.27 mmol) were mixed in 1,2-DCE
(6 ml) and heated to 60 °C in a flask. The catalyst turned from a yel-
lowish color to dark black immediately after the heating was initiated.
After 24 h, catalyst was filtered off, washed with DCM and EtOH,
conversion was determined by 1H NMR and the product was directly
isolated by flash chromatography (hexane→THF). Enantiomeric excess
was determined by chiral-phase HPLC.
123.0, 121.6, 115.3, 76.7, 69.4, 34.1, 26.0 ppm; FT-IR (ATR) cm−1
:
3305; 3059; 3037; 2985; 2951; 2904; 2868; 1658; 1587; 1475; 1358;
1240; 1119; 1084; 993; 955; 922; 914; 829; 746; 690; HRMS (MALDI):
m/z calcd for
C : ; found:
20H22N2O + H+ 307.18049 [M + H]+
307.17899.
2.3. Preparation of polymeric materials
2.4.2. Catalyst reactivation
2.3.1. Copolymers 1–3
Previously used catalyst (0.06 mmol Pd) was mixed with p-chloranil
(37 mg; 0.15 mmol) and 10 drops of TFA in 1,2-DCE and stirred for
4 days at 60 °C. Next, the catalyst was filtered, washed with DCM and
after drying used in the next cycle.
To the Ar degassed solution of PVA (1.36 g) and NaCl (15 g) in water
(400 ml) in a conical-shaped flask (500 ml) with oval magnetic stirrer
(2 × 6.5 cm) was, under the inflow of N2, suspended a solution of
styrene (1: 2.63 g; 2: 6.03 g; 3: 4.03 g), (S)-4-(tert-butyl)-2-(4-(4-vinyl-
phenyl)pyridin-2-yl)-4,5-dihydrooxazole (1: 0.6 g; 2: 1.7 g; 3: 0.82 g),
crosslinker (0.9 mol% - 1: 65.7 mg of DVB; 2: 135 mg of BVPE; 3:
142 mg of BVPC12), dibenzoylperoxide (1: 0,16 g; 2: 0.29 g; 3: 0.16 g)
and bromobenzene (0.6 mL·g−1 monomers). Stirring was adjusted to
250 min−1 and the mixture was heated to 95 °C within 30 min and
stirred for 4 days. After cooling the suspension was poured into water
(0.5 l) and the sedimented copolymer was decanted repeatedly from
water (3 × 500 ml). The crude pearl-like copolymer was gradually
washed with water (3 × 200 ml), methanol (100 ml), THF (50 ml) and
DCM (50 mL) and extracted with THF-water mixture (2:1) in Soxhlet
extractor for 24 h. The final pearl-like copolymer was obtained after
drying in vacuo (1: 2.35 g – 81% yield; 2: 4.5 g – 70% yield; 3: 3.3 g –
68% yield). Ligand content was calculated from microanalysis (1:0.571;
2:0.732; 3:0.464 mmol·g−1).
3. Results and discussion
3.1. Synthesis and characterizations of copolymers 1–3
The PyOx monomer (S)-4-(tert-butyl)-2-[4-(4-vinylphenyl)pyridin-
2-yl]-4,5-dihydrooxazole was synthetized from the commercially
available 4-bromopyridine-2‑carbonitrile in two steps with the overall
yield of 93%. The used methodology consists in the formation of a
chiral oxazoline ring from cyano-group and L-tert-leucinol catalyzed by
ZnCl2 [41], followed by the introduction of a 4-vinylphenyl group via
We gradually prepared copolymers 1–3 by suspension copolymer-
ization of a PyOx monomer, i.e. (S)-4-(tert-butyl)-2-[4-(4-vinylphenyl)
pyridin-2-yl]-4,5-dihydrooxazole, with styrene and one of the three
different cross-linkers (1%) (Scheme 4). The type and amount of cross-
linker used for production of polymers 1–3 were selected in order to
achieve samples with various swellability. It represents an important
parameter for easy diffusion of reactants to the catalytic centers
1: FT-IR (ATR) cm−1: 3373; 3103; 3082; 3059; 3026; 2991; 2924;
2850; 1720; 1670; 1601; 1514; 1493; 1452; 1265; 1066; 1028; 906;
829; 758; 737; 696; 540; elemental analysis (%) found C 85.98, H 7.68,
N 1.60; swellability (ml·g−1) 1,2-DCE: 3.8, AcOEt: 2.3.
3