Recoverable polystyrene-supported catalysts for Sharpless allylic alcohols epoxidations

Article abstract of DOI:10.1016/j.reactfunctpolym.2019.01.009

In this work, new heterogeneous catalysts intended for enantioselective Sharpless epoxidation were prepared. The catalysts are based on Ti(IV) complexes of cross-linked swellable spherical copolymer beads of styrene with ethyl-(4-vinylbenzyl)-L-tartrate, or with ethyl-(2R,3R)-2,3-dihydroxy-4-oxo-5-(4-vinylphenyl)pentanoate. These catalysts were tested in epoxidation of cinnamyl alcohols. High conversion (up to 99%) and high enantioselectivity (up to 99% ee) were achieved in the case of catalysts based on copolymers of styrene with ethyl-(4-vinylbenzyl)-L-tartrate (5, 20, 50%). Unfortunately, the copolymers lost their enantioselectivity due to the leaching of L-tartrate, caused by alcoholysis of ester bond. This problem has been overcome by replacing the ester bond by a stable keto bond. The prepared catalyst based on the copolymer of styrene with ethyl-(2R,3R)-2,3-dihydroxy-4-oxo-5-(4-vinylphenyl)pentanoate (20%) achieved a similarly high conversion and enantioselectivity as in the previous case (up to 99%, up to 99% ee) and was successfully recycled.

Full text of DOI:10.1016/j.reactfunctpolym.2019.01.009

ReactiveandFunctionalPolymers137(2019)123–132
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Recoverable polystyrene-supported catalysts for Sharpless allylic alcohols
epoxidations
Jan Bartáček, Pavel Drabina⁎, Jiří Váňa, Miloš Sedlák
Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this work, new heterogeneous catalysts intended for enantioselective Sharpless epoxidation were prepared.
The catalysts are based on Ti(IV) complexes of cross-linked swellable spherical copolymer beads of styrene with
ethyl-(4-vinylbenzyl)-L-tartrate, or with ethyl-(2R,3R)-2,3-dihydroxy-4-oxo-5-(4-vinylphenyl)pentanoate. These
catalysts were tested in epoxidation of cinnamyl alcohols. High conversion (up to 99%) and high enantios-
electivity (up to 99% ee) were achieved in the case of catalysts based on copolymers of styrene with ethyl-(4-
vinylbenzyl)-L-tartrate (5, 20, 50%). Unfortunately, the copolymers lost their enantioselectivity due to the
leaching of L-tartrate, caused by alcoholysis of ester bond. This problem has been overcome by replacing the
ester bond by a stable keto bond. The prepared catalyst based on the copolymer of styrene with ethyl-(2R,3R)-
2,3-dihydroxy-4-oxo-5-(4-vinylphenyl)pentanoate (20%) achieved a similarly high conversion and enantios-
electivity as in the previous case (up to 99%, up to 99% ee) and was successfully recycled.
Sharpless epoxidation
Recyclable catalyst
Spherical copolymer beads
Enantiomeric excess
Heterogeneous catalysis
1. Introduction
antioselectivity ranged only below 86% ee. Beside this, several studies
examining the application of heterogeneous catalysts based on titanium
Sharpless epoxidation (SE) of substituted allylic alcohols by organic
peroxides catalysed by Ti(IV) complexes of esters of tartaric acid be-
longs to the earliest known and very often used asymmetric reactions
(Scheme 1) [1,2].
(IV) complexes with different polyesters of L-(+)-tartaric acid in SE
were published. However, only low enantioselectivity was found
[5,14–16]. In more recent work [17], a post-modification strategy for
preparation of a polymeric catalyst of SE was utilized. Herein, L-
(+)-tartaric acid was attached in an ester form to chloromethylated
polystyrenes (PS), cross-linked by divinylbenzene (DVB) (1% or 2%),
tetra(ethylene glycol)-diacrylate (TEGDA) (1% or 2%) or diallyl-L-tar-
trate [17] (Scheme 2). These catalysts were recycled three times
without any loss in yield and/or enantioselectivity in SE (Scheme 2)
[17].
Optically pure 2-hydroxymethyloxiranes (glycidols) prepared by SE
represent an important class of organic intermediates, which are ap-
plicable for the synthesis of many other multifunctional target mole-
cules with defined chiral centres, such as natural compounds or drugs
and their intermediates [3]. Nevertheless, experimental performance of
SE is in some cases complicated by difficult separation of a homo-
geneous catalyst from the product [4–21]. This problem can be solved
by application of esters of L-(+)-tartaric acid with poly(ethylene gly-
cols), because such a polymeric catalyst can be separated and reused by
means of precipitation with diethyl ether [18–20,22]. However, this
method of isolation of the catalyst may not be acceptable for several
reasons in industrial scale. From this point of view, easily separable
heterogeneous catalysts are much more favourable. Heterogeneous
catalysts based on Ti(IV) complexes of tartaric acid derivatives, which
were immobilized on inorganic materials, represent recyclable forms of
catalysts for SE [7–12]. Unfortunately, their maximal achieved en-
The aim of this work was the preparation of highly efficient het-
erogeneous recyclable catalysts for SE based on copolymers of styrene
with an appropriate monomer of L-(+)-tartaric acid ester or with an-
other analogical monomer (Scheme 3). The swellable funcionalized
copolymers of styrene (e.g. Merrifield resin™ [23], JandaJel resin™
[24]) belong among the most useful carriers of catalysts [25–29], be-
cause they can be easily chemically modified due to their high re-
activity. The remarkable advantage of such PS-supported catalysts lies
in their simple separation and reusability, their low price and also in the
possibility of their usage in non-hazardous continuous-flow procedures.
⁎ Corresponding author.
E-mail address: pavel.drabina@upce.cz (P. Drabina).
https://doi.org/10.1016/j.reactfunctpolym.2019.01.009
Received 2 November 2018; Received in revised form 18 January 2019; Accepted 20 January 2019
Availableonline18February2019
1381-5148/©2019ElsevierB.V.Allrightsreserved.

J. Bartáček, et al.
ReactiveandFunctionalPolymers137(2019)123–132
20
[α]D –2.7 (CHCl3, c 1 g/100 ml); 1H NMR (500 MHz, CDCl3) δ
ppm: 7.42 (d, J = 8.2 Hz, 2H, CHAr), 7.34 (d, J = 8.2 Hz, 2H, CHAr),
6.72 (dd, J = 17.6, 10.9 Hz, 1H, Ar–CH=), 5.78 (d, J = 17.6 Hz, 1H,
=CH2), 5.29 (d, J = 10.8 Hz, 1H, =CH2), 5.27 (dd, J = 19.6, 12.0 Hz,
2H, CH2-Ar), 4.59 (dd, J = 7.3, 1.6 Hz, 1H, CH), 4.56 (dd, J = 6.9,
1.7 Hz, 1H, CH), 4.31 (dq, J = 7.1, 2.0 Hz, 2H, CH2(Et)), 3.18 (d,
J = 6.8 Hz, 1H, OH), 3.16 (d, J = 7.3 Hz, 1H, OH), 1.32 (t, J = 7.1 Hz,
3H, CH3). 13C NMR (126 MHz, CDCl3) δ ppm: 2 × 171.4, 138.0, 136.1,
134.1, 128.7, 126.4, 114.7, 72.1, 71.9, 67.8, 62.5, 14.1; FT-IR (ATR)
cm−1: 3473; 3087; 3054–3006; 2982–2939; 1735; 1370; 1220; 1123;
1085; 913; 827. HRMS m/z: Calculated: [M + Na]+ 317.10011; Found:
[M + Na]+ 317.10022; Δ = 0.35 ppm.
Scheme 1. General scheme of Sharpless asymmetric epoxidation of allylic al-
cohols [1].
2. Experimental
2.1. Materials and instrumentation
The FT-IR spectra were recorded on FT-IR Nicolet iS50 using the
ATR technique. The region of diamond crystal absorption
(1900–2400 cm−1) was removed from the spectra – in the case of ab-
sence of characteristic bands in this region.
2.2.2. Ethyl-(2R,3R)-2,3-dihydroxy-4-oxo-5-(4-vinylphenyl)pentanoate
(4)
To the suspension of magnesium (1 g; 41.1 mmol) in dry diethyl
ether (50 ml; dried over 4 Å MS) containing 3 drops of CH3I, 4-vi-
nylbenzyl chloride (3.2 ml; 22.7 mmol) was added portion-wise under
ultrasonication (400 W) during 1 h. The mixture was stirred for an ad-
ditional one hour at room temperature. The prepared solution of 4-vi-
nylbenzylmagnesium chloride was cannulated within 15 min to the
solution of diethyl-2,3-O-benzylidene-L-tartrate (6 g; 20.4 mmol) in dry
DCM (40 ml) at −78 °C. In the course of 16 h, the reaction mixture was
gradually heated to r.t. and the reaction was quenched by the addition
of a saturated solution of NH4Cl (50 ml). The mixture was extracted
with diethyl ether (3 × 50 ml). The combined organic layers were
washed with brine (50 ml), water (50 ml), dried over MgSO4 and eva-
porated. While cooling (0 °C), TFA (10 ml) containing 5 drops of water
was added to the residue. The reaction progress was monitored by TLC
(~5 min). After the disappearance of the starting compound, the mix-
ture was diluted with AcOEt (50 ml) and neutralized by the step-wise
addition of solid NaHCO3. After adding water (50 ml), the organic layer
was removed and the water layer was extracted with AcOEt
(3 × 50 ml). The combined organic layers were dried over MgSO4 and
evaporated. The residue was subjected to column chromatography
(AcOEt/hexane 1:2, RF = 0.16), giving a white crystalline product 4
(1.12 g; 4.64 mmol); 23%; mp: 85–86 °C.
NMR spectra were measured at room temperature using Bruker
AVANCE III 400 or Bruker Ascend™ 500. 1H NMR spectra were cali-
brated to tetramethylsilane. 13C NMR spectra were calibrated to the
middle signal of the multiplet of used solvent. 13C NMR spectra were
measured with APT technique and with proton decoupling. 13C NMR
spectra of solid phase were measured by CP/MAS without a standard.
Mass spectra with high resolution were measured by “dried droplet”
method on the MALDI mass spectrometer LTQ Orbitrap XL (Thermo
Fisher Scientific, Bremen, Germany) equipped with a nitrogen UV laser
(337 nm, 60 Hz). The spectra were taken in the positive ion mode with
the resolution 100,000 at m/z = 400. The resulting spectrum represents
an average of all measurements. The used matrix was 2,5-dihydrox-
ybenzoic acid (DHB).
HPLC analysis were performed using the HPLC instrument with the
UV–Vis diode array (200–800 nm) SYKAM 3240 and with chiral col-
umns Daicel Chiralcel OD-H, OB-H, OJ-H. Optical rotation was mea-
sured on the Perkin Elmer Polarimeter Model 341 with a sodium-va-
pour lamp at the wavelength λ = 589 nm and the temperature 20 °C.
The samples were analysed using the scanning electron microscope
TESCAN VEGA3 SBU with the EDX probe Bruker XFlash Detector 410-
M. Accelerating voltage of the primary electron beam was 15 kV in the
low vacuum mode (~11 Pa). EDX spectra were performed using the
point analysis and the mass concentration (in %) is an average of three
measurements.
[α]D –0.68 (CHCl3, c 1 g/100 ml); 1H NMR (500 MHz, CDCl3) δ
20
ppm: 7.43–7.35 (m, 2H, CHAr), 7.22–7.15 (m, 2H, CHAr), 6.70 (dd,
J = 17.6, 10.9 Hz, 1H, Ar-CH=), 5.75 (d, J = 17.6 Hz, 1H, =CH2),
5.26 (d, J = 11.0 Hz, 1H, =CH2), 4.67 (dd, J = 7.1, 1.8 Hz, 1H, CH),
4.60 (dd, J = 6.0, 1.8 Hz, 1H, CH), 4.32 (q, J = 7.1 Hz, 2H, CH2(Et)),
3.96 (d, J = 16.3 Hz, 1H, CO-CH2-Ar), 3.88 (d, J = 16.3 Hz, 1H, CO-
CH2-Ar), 3.67 (d, J = 6.1 Hz, 1H, OH), 3.15 (d, J = 7.1 Hz, 1H, OH),
1.33 (t, J = 7.1 Hz, 3H, CH3). 13C NMR (126 MHz, CDCl3) δ ppm: 205.6,
171.5, 137.0, 136.4, 132.2, 129.9, 126.8, 114.2, 77.4, 71.4, 62.7, 44.8,
14.3; FT-IR (ATR) cm−1: 3500; 3423; 3084; 3043–3001; 2978–2873;
1717; 1691; 1370; 1220; 1123; 1085; 913; 827. HRMS m/z: Calculated:
[M + H]+ 279.12270; Found: [M + H]+ 279.12314; Δ = 1.58 ppm.
Measurement of swelling capacity of the prepared resin was done
according to the method described in literature [30].
The starting chemicals were purchased from commercial resources
and used without further purification. Column chromatography was
performed on silica gel (SiO2 60, particle size 0.040–0.063 mm, Merck)
with the use of commercially available solvents. Thin layer chromato-
graphy was performed on aluminium plates coated with silica gel SiO2
with visualization by UV light (254 or 366 nm). Flash chromatography
was performed on Reveleris® X2 on silica gel packed columns. Melting
points were determined in open capillaries on Buchi B-540 and were
not corrected.
2.3. Preparation of copolymers 1a-d and 2
Under the inflow of N2, a solution of styrene (1a: 10 g; 1b: 4 g; 1c:
4 g; 1d: 0 g; 2: 5.14 g), ethyl-(4-vinylbenzyl)-L-tartrate (1a: 0.5 g; 1b:
1 g; 1c: 4 g; 1d: 5 g) or ethyl-(2R, 3R)-2,3-dihydroxy-4-oxo-5-(4-vinyl-
phenyl)pentanoate (2: 1.17 g), tetra(ethylene glycol)-bis(4-vinylbenzyl)
ether [31] (1a-d: 1.75% w/w; 2: 4.7% w/w) and dibenzoylperoxide
(0.2 g) in chlorobenzene (1a-d; 0.5 ml·g−1 monomers) or bromo-
benzene (2; 0.5 ml·g−1 monomers) was suspended to the solution of
PVA (1.365 g) and NaCl (15.3 g) in water (400 ml) in a sulphonation
flask (500 ml) with oval magnetic stirrer (2 × 6.5 cm). Stirring was
adjusted to 250 rpm and the mixture was heated to 85 °C within 30 min
and stirred for 4 days. After cooling, the suspension was poured into
water (1 l) and the sedimented copolymer was decanted repeatedly
from the water (3 × 500 ml). The crude copolymer was gradually wa-
shed with water (3 × 200 ml), methanol (100 ml), THF (50 ml), DCM
2.2. Preparation of monomers
2.2.1. Ethyl-(4-vinylbenzyl)-L-tartrate (3)
The mixture of sodium ethyl-L-tartrate (12 g; 60 mmol), diMeOPEG
(5 g), NaI (0.5 g) and hydroquinone (0.5 g) in DMF (600 ml) was heated
to 85 °C under argon atmosphere, while 4-vinylbenzyl chloride (8 ml;
56.8 mmol) was added. After 17 days of stirring at 85 °C, the mixture
was filtered through Celite® (5 g). The filtrate was evaporated to dry-
ness and extracted several times in the system water – diethyl ether.
Combined organic layers were dried over MgSO4 and evaporated. The
crude product was purified by column chromatography (AcOEt/hexane
1/1, RF = 0.55) giving a white waxy solid 3 (10.8 g; 61%); mp:
43–44 °C.
124

J. Bartáček, et al.
ReactiveandFunctionalPolymers137(2019)123–132
Scheme 2. Preparation and use of L-(+)-tartaric acid esters immobilized on swellable copolymers of styrene [30].
Scheme 3. Newly suggested swellable copolymers of L-(+)-tartaric acid derivatives with styrene.
(50 ml) and THF (50 ml). Finally, the copolymer was extracted with the
mixture of THF–water (2: 1) in the Soxhlet extractor for 24 h. The final
spherical copolymer beads were obtained after drying in vacuum (1a:
1.5 g; 1b: 2 g; 1c: 4 g; 1d: 2.9 g; 2: 2 g).
second with brine (15 ml), then it was dried over MgSO4 and evapo-
rated. The conversion was determined by 1H NMR. Consequently, flash-
chromatography in AcOEt/hexane (0 → 30%) was performed and the
crude product was crystallized from the mixture of AcOEt/hexane.
In the cases involving recycling of the catalyst, the copolymer was
filtered, washed with DCM, THF, water, THF, DCM, dried in vacuo and
used in the next catalytic cycle.
Swelling in DCM (ml·g−1): 1a: 8.7 1b: 7 1c: 3.7 1d: 0.6 2: 3.
1c:13C NMR (126 MHz) CP/MAS, 20 kHz δ ppm: 172 (COOR), 147
(Cq,Ar), 129 (CHAr), 73 (CH–OH), 68 (Ar-CH2), 63 (CH2(Et)), 46 (CH),
41 (CH2), 15 (CH3).
1a-c (Identical frequencies of different band intensities): FT-IR
(ATR) cm−1: 3488; 3103–3024; 2980–2850; 1738; 1493; 1451; 1370;
1222; 1123; 1085; 821; 760; 697.
2.4.2. (3-Phenyl-2-oxiranyl)methanol
White solid (mp 51–52 °C); NMR in agreement with literature [32]:
1H NMR (400 MHz, CDCl3) δ ppm: 7.38–7.26 (m, 5H), 4.05 (ddd,
J = 12.8, 5.2, 2.4 Hz, 1H), 3.93 (d, J = 2.1 Hz, 1H), 3.80 (ddd,
J = 12.7, 7.7, 3.8 Hz, 1H), 3.23 (dt, J = 4.0, 2.3 Hz, 1H), 1.94 (dd,
J = 7.7, 5.3 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ ppm: 136.6, 128.5,
128.3, 125.7, 62.4, 61.2, 55.5.
1d: FT-IR (ATR) cm−1: 3475; 3053–2981; 2925; 2852; 1736; 1514;
1446; 1421; 1369; 1261; 1219; 1194; 1124; 1082; 1020; 937; 820; 733;
700; 598.
2:13C NMR (126 MHz) CP/MAS, 20 kHz δ ppm: 147 (Cq,Ar), 129
(CHAr), 79 (CO-CH-), 72 (CH–OH), 62 (CH2(Et)), 45 (CH), 41 (CH2), 14
(CH3).
The ee was determined by chiral HPLC [32]: Chiralcel OD-H
column, 90/10 hexanes/IPA, flow rate: 0.8 ml/min, λ = 230 nm;
(2S,3S)-enantiomer tr = 15.7 min.; (2R,3R)-enantiomer tr = 17.2 min;
99% ee.
2: FT-IR (ATR) cm−1: 3487; 3082–3003; 2920; 2850; 1739; 1720;
1601; 1493; 1452; 1367; 1267; 1095; 1028; 756; 696; 538
2.4.3. (2-Methyl-3-phenyloxiran-2-yl)methanol
2.4. Catalytic tests
White solid (mp 57–58 °C); NMR in agreement with literature [32]:
1H NMR (500 MHz, CDCl3) δ ppm: 7.39–7.24 (m, 5H), 4.21 (s, 1H), 3.86
(d, J = 12.4 Hz, 1H), 3.75 (dd, J = 12.0, 6.7 Hz, 1H), 2.34 (br s, 1H),
1.09 (s, 3H). 13C NMR (126 MHz, CDCl3) δ ppm: 135.7, 128.2, 127.7,
126.5, 65.12, 63.9, 60.3, 13.6.
2.4.1. Sharpless epoxidation with copolymers
Ti(OiPr)4 (7.5 mol%) and the solution of TBHP (200 mol%;
5.5 mol·1−1 in decane) were added to the stirred mixture of copolymer
(15.75 mol%, calculated to tartrate), molecular sieves 4 Å (1 g) and dry
DCM (30 ml) in a Schlenk tube at −25 °C under dry argon atmosphere.
After 1 h (−25 °C), the solution of substrate (0.9 mmol) in dry DCM
(0.2 ml) was added. After 24 h, the reaction mixture was filtered and
extracted first with a saturated solution of Na2SO3 (ca 15 ml) and
The ee was determined by chiral HPLC [32]: Chiralcel OD-H
column, 95/5 hexanes/IPA, flow rate: 0.8 ml/min, λ = 230 nm;
(2S,3S)-enantiomer tr = 12.0 min.; (2R,3R)-enantiomer tr = 16.0 min;
99% ee.
125

J. Bartáček, et al.
ReactiveandFunctionalPolymers137(2019)123–132
2.5. List of symbols
AcOEt – ethyl-acetate; APT – attached proton test; ATR – attenuated
total reflectance; CP/MAS – cross-polarization magic angle spinning;
DCM – dichloromethane; DHB – 2,5-dihydroxybenzoic acid; diMeOPEG
– α,ω-dimethoxypoly(ethylene glycol); DMF – N,N-dimethylformamide;
DVB – divinylbenzene; EDX – energy dispersive X-ray; ee – en-
antiomeric excess; FT-IR – fourier transform infrared; HPLC – high-
performance liquid chromatography; HRMS – high resolution mass
spectrometry; IPA – isopropyl alcohol; NMR – nuclear magnetic re-
sonance; PS – polystyrene; PVA – poly(vinyl alcohol); Rf – retardation
factor; SE – Sharpless epoxidation; TBHP – tert-butyl hydroperoxide;
TEA – triethylamine; TEGDA – tetra(ethylene glycol) diacrylate; TFA –
trifluoroacetic acid; THF – tetrahydrofuran; TLC – thin-layer chroma-
Scheme 4. Synthesis of monomer 3.
tography; 4-VBC
– 4-vinylbenzyl chloride; p-VBnMgCl - 4-vi-
nylbenzylmagnesium chloride.
2.4.4. (3-(4-Nitrophenyl)oxiran-2-yl)methanol
White solid (mp 97–98 °C); NMR in agreement with literature [33]:
1H NMR (400 MHz, CDCl3) δ ppm: 8.27–8.16 (m, 2H), 7.49–7.40 (m,
2H), 4.15–4.01 (m, 2H), 3.94–3.80 (m, 1H), 3.21 (dd, J = 5.3, 2.2 Hz,
1H), 2.01 (dd, J = 7.8, 5.1 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ ppm:
147.9, 144.4, 126.4, 123.8, 63.1, 60.7, 54.4.
3. 3. Results and discussion
3.1. Synthesis and characterizations of copolymers 1a-d
The key monomer – ethyl-(4-vinylbenzyl)-L-tartrate (3) – was pre-
pared from diethyl-L-tartrate [38]. The sodium salt of monoethyl-L-tar-
trate obtained by its partial hydrolysis in 45% yield was subsequently
alkylated with 4-vinylbenzyl chloride (4-VBC) in the presence of the
catalytic amount of NaI and diMeOPEG in DMF (61%) (Scheme 4).
The copolymers 1a-d were synthesized by suspension copolymer-
ization of styrene and ethyl-(4-vinylbenzyl)-L-tartrate (3) under pre-
sence of 0.45% x/x tetra(ethylene glycol)-bis(4-vinylbenzyl)ether as a
cross-linker (Scheme 5). The type and amount of cross-linker in
polymer were chosen with respect to the stability of the ether bond and
high swellability of the prepared copolymer (up to 8.7 ml·g−1 of DCM),
which is fundamental for easy diffusion of reactants to the catalytic
centre [27,30]. The mass ratio of styrene/ligand 3 was defined for the
individual copolymers as follows: 1a: 95/5; 1b: 80/20; 1c: 50/50; 1d:
0/100. The crude copolymers were purified from the residual mono-
mers by a multiple extraction in the Soxhlet extractor.
The ee was determined by chiral HPLC [34]: Chiralcel OJ-H column,
90/10 hexanes/IPA, flow rate: 1.0 ml/min, λ = 230 nm; (2R,3R)-en-
antiomer tr = 32.4 min.; (2S,3S)-enantiomer tr = 35.3 min; 98% ee.
2.4.5. (3-Methyl-3-(4-methylpent-3-en-1-yl)oxiran-2-yl)methyl-benzoate
After the epoxidation of geraniol, the crude reaction mixture was
extracted with a saturated solution of Na2SO3 and brine and conversion
was determined by 1H NMR. Consequently, 1.8 eq. TEA, 1.5 eq. benzoyl
chloride and 3 drops of pyridine were added to the extract. After
10 min, the epoxy-geraniol could already not be detected (TLC) and the
reaction mixture was evaporated and extracted with acetone
(2 × 10 ml). After purification (flash chromatography with AcOEt/
hexane as eluent), a colourless oil was obtained; NMR in agreement
with literature [35]; 1H NMR (400 MHz, CDCl3) δ ppm: 8.11–8.05 (m,
2H), 7.61–7.54 (m, 1H), 7.49–7.41 (m, 2H), 5.14–5.04 (m, 1H), 4.57
(dd, J = 12.1, 4.3 Hz, 1H), 4.30 (dd, J = 12.1, 6.8 Hz, 1H), 3.14 (dd,
J = 6.7, 4.4 Hz, 1H), 2.19–2.03 (m, 2H), 1.77–1.46 (m, 8H), 1.38 (s,
3H). 13C NMR (101 MHz, CDCl3) δ ppm: 166.5, 133.3, 132.4, 129.9,
129.8, 128.5, 123.3, 64.0, 60.7, 59.9, 38.4, 25.8, 23.8, 17.8, 17.0.
The ee was determined by chiral HPLC [35]: Chiralcel OB-H
column, 99.5/0.5 hexanes/IPA, flow rate: 1.0 ml/min, λ = 230 nm;
(2S,3S)-enantiomer tr = 9.8 min.; (2R,3R)-enantiomer tr = 18.7 min;
52% ee.
The prepared copolymers 1a-d were characterized by optical mi-
croscopy and FT-IR spectroscopy. Fig. 1 confirms the spherical char-
acter of the prepared copolymers 1a-d and particle size below ca
300 μm.
In all FT-IR spectra of copolymers 1a-d (Fig. 2), intensive bands of
valence C]O vibration of ester groups at 1738 cm−1 are observed to-
gether with bands at 697 cm−1 pertaining to phenyl out-of-plane
bending vibrations.
Moreover, the copolymer 1c was characterized by 13C CP/MAS
NMR spectroscopy (Fig. 3). The spectrum was measured at rotation
frequency 20 kHz without a standard and was mathematically filtered
by Gauss function. The signal at 172 ppm corresponds to carbonyl
carbons, at 147 ppm to quaternary aromatic carbons, at 129 ppm to
aromatic CH carbons, at 73 ppm to CH carbons in tartrate fragment, at
68 ppm to benzyl CH2 carbons and at 63 ppm to CH2 from ethoxy
group. The signal at 46 ppm can be assigned to CH carbons bound to
benzene rings and a more intensive signal at 41 ppm to the neigh-
bouring CH2 carbons. The signal at 26 ppm is probably a spinning
sideband or a signal of residual THF, whereas the signal at 15 ppm is a
signal of CH3 carbons of ethoxy group.
2.4.6. (3,3-Diphenyloxiran-2-yl)methanol
Colourless oil; NMR in agreement with literature [36]. 1H NMR
(400 MHz, CDCl3) δ ppm: 7.44–7.21 (m, 10H), 3.63 (dd, J = 14.9,
9.5 Hz, 2H), 3.45–3.34 (m, 1H), 2.12 (br s, 1H). 13C NMR (101 MHz,
CDCl3) δ ppm: 140.2, 136.8, 128.4, 128.4, 2 × 128.0, 126.8, 66.2, 65.8,
62.1.
The ee was determined by chiral HPLC [36]: Chiralcel OD-H
column, 95/5 hexanes/IPA, flow rate: 0.5 ml/min, λ = 230 nm; (S)-
enantiomer tr = 12.0 min.; (R)-enantiomer tr = 13.6 min; 40% ee.
2.4.7. (3-(4-Chlorophenyl)oxiran-2-yl)methanol
White solid (mp 63–64 °C); NMR in agreement with literature [37]:
1H NMR (500 MHz, CDCl3) δ ppm: 7.35–7.29 (m, 2H), 7.25–7.18 (m,
2H), 4.11–3.99 (m, 1H), 3.92 (d, J = 1.8 Hz, 1H), 3.85–3.75 (m, 1H),
3.23–3.14 (m, 1H), 2.11 (br s, 1H). 13C NMR (126 MHz, CDCl3) δ ppm:
135.2, 134.1, 128.8, 127.1, 62.6, 61.0, 54.9.
3.2. Study of catalytic activities of the catalysts based on the copolymers 1a-d
The catalytic activities and enantioselectivities of the catalysts
based on the copolymers 1a-d were studied in SE of (E)-cinnamyl al-
cohol with TBHP. The obtained results are summarized in Table 1.
The reaction catalysed by a homogeneous catalyst (Table 1, Entry 1)
was performed for mutual comparison with heterogeneous forms of
catalysts based on the copolymers 1a-d. From Table 1 (Entries 2–4), it is
The ee was determined by chiral HPLC [37]: Chiralcel OD-H
column, 85/15 hexanes/IPA, flow rate: 0.8 ml/min, λ = 230 nm;
(2S,3S)-enantiomer tr = 12.3 min.; (2R,3R)-enantiomer - not detected;
˃99% ee.
126

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ReactiveandFunctionalPolymers137(2019)123–132
Scheme 5. Synthesis of copolymers 1a-d.
Fig. 1. Figures from optical microscope of copolymers 1a (A), 1b (B), 1c (C), 1d
(D).
obvious that the epoxidation catalysed by the catalysts based on the
copolymers 1a-c proceeded with excellent enantioselectivity (up to
99% ee). The configuration of the obtained product was determined to
be (2S,3S)-. Generally, the configuration of the major product depends
on the stoichiometry of the Ti(IV) complex with tartrate. While
monomeric 2:1 Ti-tartrate complex gives the product with (2R,3R)-
configuration, dimeric 2:2 Ti-tartrate complex leads to the opposite
enantiomer (2S,3S)- [20]. It means that the active species involved in
reactions catalysed by the copolymers 1a-c are dimeric 2:2 Ti-tartrate
complexes. The conversions obtained by the copolymers 1a and 1b
were almost quantitative (99%), even higher than in case of the
homogeneous catalyst (81%). The application of the catalyst based on
the copolymer 1c (50% of tartrate) (Entry 4) led to lower conversion
(80%), however enantioselectivity remained unchanged (99% ee). In-
creased loading of the catalyst 1c (Entry 6) accelerated the rate of
epoxidation (99% conversion). On the other hand, lowering of en-
antioselectivity was observed (86% ee). Usage of the catalyst based on
the copolymer 1d (99% content of tartrate) (Entry 8) caused a dramatic
decrease of conversion (32%, 72 h), as well as enantioselectivity (9%
ee). From this result follows that the copolymer 1d forms a very dense
network of catalytically inactive Ti(IV) complexes [39]. In summary,
lower content of tartrate bound in polymeric matrix (5–20%) leads to
Fig. 2. Comparison of FT-IR spectra of copolymers: 1a (a), 1b (B), 1c (C), 1d
(D).
Fig. 3. 13C CP/MAS NMR spectrum of copolymer 1c.
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J. Bartáček, et al.
ReactiveandFunctionalPolymers137(2019)123–132
Table 1
An overview of performed catalytic experiments with the use of copolymers 1a-d.
d
Entry
Copolymer
Content tartrate in copolymer (mmol·g−1
)
Ratio substrate/Ti(IV)/tartrate
Cycle
Conv.a%
% eeb (2S,3S)
1
2
3
4
5
6
7
8
Homog. systemc
3.73
0.16
0.67
1.67
1.67
1.67
1.67
3.34
100:5:10
1st
1st
1st
1st
2nd
1st
2nd
1st
81
97
99
95
99
76
86
34
9
1a
1b
1c
1c
1c
1c
1d
100:7.5:15
100:7.5:15
100:9:23
99
99
80
100:9:23
36
100:23:50
100:23:50
100:7.5:15
99
70
32 (72 h)
a
b
c
The conversion was determined after 24 h by 1H NMR spectroscopy.
The enantiomeric excess determined by HPLC using Daicel Chiralcel OD-H column.
Benzyl(ethyl)-L-tartrate.
d
Estimated from ratio of monomers in starting mixture.
higher conversion and higher enantioselectivity. This effect is in
agreement with the previously described so-called dilution effect of
polymeric matrix [40].
Furthermore, the possibility of recycling of the polymer 1c (Table 1,
Entry 5, 7) was studied. Unfortunately, a dramatic decrease of both
parameters was observed after the second catalytic cycle (80 → 36%;
99 → 76% ee and 99 → 70%; 86 → 34% ee). This can be explained by
comparison of FT-IR spectra of freshly prepared copolymer 1c and the
used copolymer 1c after the first and second catalytic cycle (Fig. 4). The
spectra show significant lowering of intensity of bands at 1738 cm−1
corresponding to the valence C]O vibration of ester groups, and bands
in the range from 1086 cm−1 to 1260 cm−1 corresponding to the va-
lence CeO vibrations of ester groups as well as to the valence CeO
vibrations of alcohol groups in tartrate. This observation can be ex-
plained by leaching of tartrate from the polymeric matrix, caused by
alcoholysis of ester bond by the present alcohols catalysed by Ti(IV)
(Lewis acid). Leaching of L-tartrate leads to the formation of benzyl
Scheme 6. Alcoholysis of copolymer 1b.
alcohol moiety bonded to the polymeric matrix (Scheme 6), which can
then coordinate Ti(IV) ions originating non-selectively catalysing
complexes. These undesirable processes decrease both yield and en-
antioselectivity (Table 1).
3.3. Synthesis and characterizations of the copolymer 2
To avoid leaching and to obtain a stable recyclable catalyst, the
copolymer 2 based on ethyl-(2R,3R)-2,3-dihydroxy-4-oxo-5-(4-vinyl-
phenyl)pentanoate (4) was prepared afterwards (Scheme 3). In the case
of the copolymer 2, the tartrate moiety is bound in the polymeric
matrix via keto group, which cannot undergo alcoholysis.
The key monomer 4 for the preparation of the copolymer 2 (Scheme
3) was synthesized by the reaction of diethyl-2,3-O-benzylidene-L-tar-
trate with one equivalent of 4-vinylbenzylmagnesium chloride followed
by removing of an acetal group in 23% yield (Scheme 7).
The copolymer 2 was prepared by suspension copolymerization of
ketone 4 with styrene (Scheme 8). According to findings from pre-
viously performed experiments, the concentration of ligand in
monomer mixture was adjusted to 0.67 mmol·g−1. Moreover, the con-
tent of the cross-linker was enhanced to 1.35% x/x. Hence, the mass
Fig. 4. Illustration of leaching of L-tartrate from copolymer 1c. FT-IR spectra of
freshly prepared copolymer 1c (a), used copolymer 1c after first (b) and second
(c) cycle and reference sample of 1c after alcoholysis with MeONa 1·mol·l−1
(d).
Scheme 7. Synthesis of monomer 4.
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J. Bartáček, et al.
ReactiveandFunctionalPolymers137(2019)123–132
Scheme 8. Synthesis of copolymer 2.
ratio of styrene/ligand 4 in the copolymer 2 was 80/20 (analogically to
the polymer 1b). The crude copolymer 2 was purified by a multiple
extraction in the Soxhlet extractor similarly as in previous cases of
copolymers 1a-d.
The copolymer 2 was characterized by optical microscopy, FT-IR
spectroscopy and CP MAS spectroscopy. The Fig. 5 confirms the sphe-
rical character of the prepared copolymer 2 with particle size below ca
200 μm.
In the FT-IR spectra of the copolymer 2 (Fig. 6), characteristic bands
of carbonyl valence C]O vibrations at 1739 cm−1 (ester) and
1720 cm−1 (ketone) can be observed. The most intensive band at
696 cm−1 belongs to phenyl out-of-plane bending vibrations.
The Fig. 7 displays the 13C CP/MAS NMR spectrum of the copolymer
2 at rotation frequency 20 kHz without a standard. The measured
spectrum was mathematically filtered by the Gauss function. The signal
at 147 ppm corresponds to quaternary aromatic carbons, at 129 ppm to
aromatic CH carbons, at 79 ppm to CH carbons in tartrate fragment
neighbouring to keto group, at 72 ppm to CH carbons neighbouring to
ester group, at 62 ppm to CH2 from ethoxy group. The signal at 45 ppm
can be assigned to CH carbons bound to benzene rings and more in-
tensive signal at 41 ppm to the neighbouring CH2 carbons. The signal at
14 ppm belongs to CH3 carbons of ethoxy group. The signals corre-
sponding to quaternary carbons in COOEt and COCH2 were not de-
tected due to the low content of tartrate moiety in polymeric matrix. On
the other hand, their presence was proved in FT-IR spectrum (Fig. 6).
Fig. 6. FT-IR spectrum of copolymer 2 (a) in the comparison with copolymer 1c
(b).
3.4. Study of catalytic activities of the catalyst based on the copolymer 2
The catalytic activity, enantioselectivity and recyclability of the
catalyst based on the copolymer 2 was first tested in SE of (E)-cinnamyl
alcohol and (E)-2-methylcinnamyl alcohol with TBHP. Both epoxida-
tions were performed under the same reaction conditions used in the
case of previously studied copolymer 1b. From the results shown in
Fig. 5. Figures from optical microscope of copolymer 2.
Fig. 7. 13C CP/MAS NMR spectrum of copolymer 2.
129

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ReactiveandFunctionalPolymers137(2019)123–132
Table 2
Table 3
An overview of performed recycling experiments for epoxidation of (E)-cin-
namyl alcohol and (E)-2-methylcinnamyl alcohol with the use of copolymer 2
(Ratio substrate/Ti(IV)/tartrate: 100:7.5:15; DCM, −25 °C, 24 h).
An overview of the performed catalytic experiments for different types of allylic
alcohols with the use of copolymer
2 (Ratio substrate/Ti(IV)/tartrate:
100:7.5:15; DCM, −25 °C, 24 h).
Product
Cycle
Conversiona%
% eeb (2S,3S)
Product
Conversiona%
% eeb
1st
80
90
80
67
99
99
99
99
97
99
96
97
99
89
82
87
98
98 (2S,3S)
2nd
3rd
4th
1st
99
84
> 99 (2S,3S)
40 (R)
2nd
3rd
4th
The conversion was determined after 24 h by 1H NMR spectroscopy.
The enantiomeric excess determined by HPLC.
a
b
Table 2 follows that asymmetric SE catalysed by the Ti(IV) complex of
the polymer 2 proceeds with high conversion and enantioselectivity.
Contrary to the catalysts based on the copolymers 1a-b, the catalyst
formed from the copolymer 2 can be successfully recycled at least four
times. During the recycling of the copolymer 2 applied in the SE of (E)-
cinnamyl alcohol, good conversions (80 → 90 → 80 → 67%) with ex-
cellent values of enantioselectivity (97–99% ee) were observed. In the
case of the SE of sterically more demanding (E)-2-methylcinnamyl al-
cohol, almost quantitative conversions (99%) were achieved, even in
the fourth catalytic cycle. On the other hand, after the first and second
catalytic cycle, a slight decrease of enantioselectivity was observed,
while in the next cycle its minor increase occurred (99 → 89 → 82 →
87% ee) (Table 2).
97c
52 (2R,3R)c
The conversion was determined after 24 h by 1H NMR spectroscopy.
The enantiomeric excess determined by HPLC.
a
b
c
Product of epoxidation of geraniol was isolated in the form of benzoyl
derivative.
bands in the range around 1630 cm−1 and 1200–1000 cm−1 was ob-
served. These bands most probably belong to vibrations of TiO2, which
is in agreement with literature [41]. The content of titanium in the
copolymer 2 was independently determined by EDX. 1.5% w/w Ti was
found in the recycled polymer 2 after the first catalytic cycle and 2.3%
w/w Ti after the fourth catalytic cycle. Based on the presented findings,
it is obvious that during the recycling the hydrolysis of Ti(OiPr)4 occurs
and TiO2 formed by this way partially inhibits the catalytic centres in
the polymeric matrix. This leads to the observed slight decrease of
catalytic activity.
These experiments indirectly confirmed the assumption that an-
choring of L-tartaric acid moiety via keto group utilized in the polymer
2 is significantly more stable. Therefore, no leaching of tartrate oc-
curred in the case of the catalyst 2, in contrast with the copolymers 1a-
b. This conclusion was also supported by the comparison of FT-IR
spectra of the freshly prepared copolymer 2 and copolymer 2 after the
first and fourth cycle (Fig. 8).
Fig. 8 shows that the intensity of bands at 1739 and 1720 cm−1
corresponding to valence C]O vibrations remains unchanged during
the recycling. On the other hand, an increase of overall background and
The catalyst based on the copolymer 2 was further tested under the
same experimental conditions for the SE of other allylic alcohols: (E)-4-
nitrocinnamyl alcohol, (E)-4-chlorocinnamyl alcohol, 3,3-dipheny-
lallylalcohol and geraniol (Table 3).
From the results presented in Table 3, it is obvious that with the
application of the copolymer 2 high conversion and enantioselectivity
for both (E)-4-nitrocinnamyl alcohol (98%, 98% ee) as well as for (E)-4-
chlorocinnamyl alcohol (99%, 99% ee) were reached. On the other
hand, the SE of 3,3-diphenylallyl alcohol and geraniol proceeded with
satisfactory conversions (84% resp. 97%), but with only low enantios-
electivity (40% resp. 52% ee). This variability of enantioselectivity can
be affected by the character of the polymer network of the copolymer 2,
which possesses high substrate selectivity to (E)-cinnamyl alcohols due
to its specific microenvironment [27].
4. Conclusions
New swellable spherical copolymer beads 1a-d and 2 of styrene
with ethyl-(4-vinylbenzyl)-L-tartrate (3) or with ethyl-(2R,3R)-2,3-di-
hydroxy-4-oxo-5-(4-vinylphenyl)pentanoate (4) with the addition of ca
1–2% x/x tetra(ethylene glycol)-bis(4-vinylbenzyl)ether as a cross-
linker were prepared by suspension polymerization. These copolymers
1a-d and 2 were used in the combination with Ti(OiPr)4 as hetero-
geneous catalysts for enantioselective Sharpless epoxidation of sub-
stituted allylic alcohols. The copolymers 1a-d containing L-tartaric acid
immobilized in polymeric matrix via ester bonds provided corre-
sponding substituted hydroxymethyloxiranes with high conversion and
high enantioselectivity (up to 99% and 99% ee). Unfortunately, re-
cycling of these heterogeneous catalysts 1a-d is not possible due to
Fig. 8. Effect of recyclation on the quality of copolymer 2. FT-IR spectra of
copolymer 2: freshly prepared (a), after first (b) and fourth (c) cycle and TiO2
(d).
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ReactiveandFunctionalPolymers137(2019)123–132
gradual alcoholysis of ester bond, which leads to leaching of the chiral
ligand from polymeric matrix. A dramatic loss of enantioselectivity of
the catalysts 1a-d was caused by this way. Analogical swellable copo-
lymer 2, in which L-tartaric acid was immobilized in polymeric matrix
via keto bond, also provided corresponding substituted hydro-
xymethyloxiranes with high conversion and high enantioselectivity (up
to 99% and 99% ee). Importantly, with regards to the stability of keto
bond, the leaching of chiral ligand does not proceed. This fact leads to
only minimal loss of enantioselectivity and opens the possibility of re-
cycling of the copolymer 2. This presumption was proved in four cycles
for two substrates ((E)-cinnamyl alcohol and (E)-2-methylcinnamyl al-
cohol). The observed slight decrease of conversion resp. enantioselec-
tivity during recycling was explained by gradual minor precipitation of
TiO2 in polymeric matrix. From the presented results follows that the
copolymer 2 has a high application potential for practical performance
of SE of substituted allylic alcohols in flow reactors, where a complete
elimination of moisture action and thus formation of TiO2 will be
possible.
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Data availability
The raw data required to reproduce these findings are included in
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Appendix A. Supplementary data
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