Amino-polystyrene supported hexaethylene glycol-bridged ionic liquid as an efficient heterogeneous catalyst for water-mediated nucleophilic hydroxylation

Article abstract of DOI:10.1039/C9RA00590K

We report a simple and eco-friendly method for producing an amino-polystyrene supported hexaethylene glycol-bridged ionic liquid (APS-HEGBIL) based on the copolymerization of amino-styrene with 1-vinyl imidazolium ionic liquid bearing hexaethylene glycol moieties, and its characterization by several analytical techniques. The resulting APS-HEGBIL catalyst was found to be remarkably efficient at catalyzing the selective nucleophilic hydroxylation of alkyl halides to produce the corresponding alcohols in water, which acted as a solvent and a nucleophilic hydroxide source. The catalyst was easily recycled and maintained its catalytic activity and stability after ten cycles with excellent yields. The main attributes of the catalyst were that it significantly enhanced the nucleophilicity of water during reactions and promoted the rapid conversions of polar and base-sensitive alkyl halide reactants to alcohols in excellent yields. The combination of ionic liquids and polymeric materials afforded quasi-homogeneous catalysts that were recycled by simple filtration and provided environmentally benign means for conducted catalytic procedures.

Full text of DOI:10.1039/C9RA00590K

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Amino-polystyrene supported hexaethylene
glycol-bridged ionic liquid as an efficient
heterogeneous catalyst for water-mediated
nucleophilic hydroxylation†
Cite this: RSC Adv., 2019, 9, 9435
Mudumala Veeranarayana Reddy, Seok Min Kang, Suah Yoo, Sang Sik Woo
*
and Dong Wook Kim
We report a simple and eco-friendly method for producing an amino-polystyrene supported hexaethylene
glycol-bridged ionic liquid (APS-HEGBIL) based on the copolymerization of amino-styrene with 1-vinyl
imidazolium ionic liquid bearing hexaethylene glycol moieties, and its characterization by several analytical
techniques. The resulting APS-HEGBIL catalyst was found to be remarkably efficient at catalyzing the
selective nucleophilic hydroxylation of alkyl halides to produce the corresponding alcohols in water, which
acted as a solvent and a nucleophilic hydroxide source. The catalyst was easily recycled and maintained its
catalytic activity and stability after ten cycles with excellent yields. The main attributes of the catalyst were
that it significantly enhanced the nucleophilicity of water during reactions and promoted the rapid
conversions of polar and base-sensitive alkyl halide reactants to alcohols in excellent yields. The
combination of ionic liquids and polymeric materials afforded quasi-homogeneous catalysts that were
recycled by simple filtration and provided environmentally benign means for conducted catalytic procedures.
Received 23rd January 2019
Accepted 15th March 2019
DOI: 10.1039/c9ra00590k
rsc.li/rsc-advances
separation of catalysts has resulted in many attempts to utilize
PSILs in various organic transformations over the last two
decades.10–13
Introduction
Currently, the synthesis of organic compounds by utilizing
improved environmentally benign catalytic systems and condi-
tions is an aspiration of organic chemists.1 The sustainability of
green chemistry depends on; (i) the use of multicomponent
reactions, solvent-free conditions, or benign solvents, (ii) the
discovery of promising, less toxic reagents and catalytic
systems, (iii) the design of reliable and cost-effective method-
ologies, and (iv) the use of renewable resources, energy efficient
catalytic processes, and the further development of separation
procedures.2,3 With these objectives in mind, considerable
interest has been focused on ionic liquids (ILs) during the past
decade because they can provide efficient media and catalysts
for organic reactions (Fig. 1).4,5 The attractive properties of ILs
include ease of separation, recyclability, and immobilization on
heterogenous solid supports,6–9 and the combined advantages
of environmentally friendly heterogeneous catalysis and
sustainable catalytic properties of polymer supported ionic
liquids (PSILs) have resulted in their being recognized as
ranking among the best green catalytic systems. Furthermore,
the heterogenization of active molecules to aid the isolation and
On the other hand, environmental issues have steered
research in the catalytic eld towards environmentally friendly
reaction media, such as water as a solvent/co-solvent.14,15 In
addition to the low cost of water and its availability and non-
toxic nature, its high polarity, ability to form hydrogen
bonding networks, high specic heat capacity, high cohesive
energy, and large surface tension signicantly inuence organic
conversions.16–18
A diverse range of biologically relevant molecules contain the
hydroxyl group, and thus, alcohols constitute important
Department of Chemistry and Chemical Engineering, World Class Smart Laboratory
(WCSL), Inha University, 100 Inha-ro, Nam-gu, Incheon 402-751, Korea. E-mail:
kimdw@inha.ac.kr; Fax: +82-32-867-5604; Tel: +82-32-860-7679
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra00590k
Fig. 1 Conventional and custom-made ionic liquids. bmim ¼ 1-n-
butyl-3-methylimidazolium; hexaEGmim ¼ 1-hexaethylene glycol 3-
methylimidazolium; hexaEG-DHIM ¼ hexaethylene glycol-bridged
dicationic imidazolium dimesylate.
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Scheme 1 Preparation of APS-HEGBIL.
synthetic targets in the pharmaceutical eld. While the
synthesis of alcohols from alkyl halides by nucleophilic
substitution has a long history,19 simple replacement of halo-
gens by water is difficult due to its poor nucleophilicity under
mild alkaline conditions.20–22 Therefore, various methods have
been devised to transform halo compounds into alcohols under
Results and discussion
APS-HEGBIL was synthesized as shown in Scheme 1. First,
HEGBDVIM was prepared by N-alkylation of vinyl imidazole 2
with hexaethylene glycol dimesylate 1. Cross-linking polymeri-
zation between HEGBDVIM and amino styrene 3 generated APS-
HEGBIL, which was characterized by elemental analysis (EA),
FT-IR, solid state NMR, XPS, and TGA. Based on EA analysis of
sulfur, APS-HEGBIL was found to contain approximately
1.2 mmol per gram of the imidazolium mesylate moiety.
A comparison of the FT-IR spectra of APS-HEGBIL and
HEGBDVIM (Fig. 2) revealed the disappearance of terminal
alkene (]C–H) stretch and of bending vibrations at 3137–3084,
1577, and 1547 cmꢀ1, which conrmed APS-HEGBIL formation.
All remaining absorption peaks, including those of imidazole
ring C]N and mesylate S]O and S–O stretching vibrations
were unchanged.
2
homogeneous catalytic conditions via SN type reactions in the
presence or absence of strong bases.23–30 However, these
procedures suffer from the need for large amounts of chemical
catalysts, requirements for high boiling solvents and long
reaction times, the formation of by-products, and difficulties
associated with separating homogeneous catalysts from reac-
tion media. The eco-friendly, nucleophilic hydroxylation of
halide compounds using aqueous ILs (Fig. 1) was recently re-
ported by our group.30,31 However, this methodology also suffers
from long reaction times, the requirement for excessive
amounts of ILs, and difficulties separating ILs from reaction
media. Therefore, the development of a truly recyclable
heterogeneous catalytic process that uses green reaction
conditions would represent a signicant advancement.
Chemical bonding in APS-HEGBIL and HEGBDVIM were
investigated by X-ray photoelectron spectroscopy (XPS) (Fig. 3).
In this work, by taking advantage of the above described
merits of PSILs, we developed an efficient and simple protocol
for preparing an amino-polystyrene supported hexaethylene
glycol-bridged ionic liquid (APS-HEGBIL) by using a cross-
linking polymerization reaction between hexaethylene glycol-
bridged dicationic 1-vinyl imidazole salt (HEGBDVIM) and
amino-styrene. The as-synthesized APS-HEGBIL material was
used as a heterogeneous catalyst for the nucleophilic hydrox-
ylation of alkyl halides in water and resulting in corresponding
alcohol products. As a result of polymer-support and the use of
exible imidazolium linkers, the catalyst produced acted as an
efficient “quasi-homogeneous” catalyst with high activity and
selectivity, but was easily isolated and reused like heteroge-
neous catalysts. As these goals were accomplished on the
surface of a polymer, this novel catalyst represents a signi-
cant bridge between homogeneous and heterogeneous
catalysis.
Fig. 2 FT-IR spectra of APS-HEGBIL and HEGBDVIM.
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Fig. 3 XPS patterns of (a–c) HEGBDVIM and (d–f) APS-HEGBIL.
Elemental peaks were dominated by O, C, S, and N of
HEGBDVIM and APS-HEGBIL (Fig. 3a and d). Noticeably, the
peak intensity of C was almost double that of N in HEGBDVIM
(Fig. 3a). However, aer polymerization, C peak intensity was
three times greater than that of N. Moreover, both HEGBDVIM
and APS-HEGBIL contained the same C 1s peaks at 284.7–
284.8 eV (C–C bonding) and at 286–286.3 eV (C–O bonding)
(Fig. 3b and e). Successful polymerization was also indicated by
the peak intensity ratios of bonding types. Specically,
HEGBDVIM had a peak intensity at 286 eV (C–O bonding) that
was twice as high as that at 284.8 (C–C bonding), and the peak
intensity of p–p bonding was low. Aer APS-HEGBIL formation,
peak intensities changed according to the number of bonds in
the circuit. In particular, the C–C peak was signicantly larger
than the C–O peak, and the height of the p–p bonding peak also
increased due to the aromatic groups. Furthermore, N 1s base
peak was observed at 401 eV (C–N bonding) in the XPS pattern
of HEGBDVIM (Fig. 3c and f), whereas a new peak was observed
at 399.4 eV (indicating N–H bond formation) was observed aer
reaction (Fig. 3f), showing that APS-HEGBIL had been covalently
linked to HEGBDVIM.
Solid state 13C NMR spectroscopy conrmed the formation
of APS-HEGBIL (Fig. 4a). All aromatic and imidazolium ring
carbon signals appeared between 131–126 ppm. A higher
intensity signal appeared at 71.49 ppm, indicating methylene
carbons were attached to the imidazolium and aromatic rings at
side chains. Quaternary and tertiary carbons in the side chains
of the alkyl polymer appeared between 44.9 and 41.3 ppm.
Finally, the remaining methylene and methyl carbons appeared
at 25.6 and 21.9 ppm, respectively.
Fig. 4 (a) Solid state 13C NMR spectrum of APS-HEGBIL. (b) TGA
patterns of APS-HEGBIL and HEGBDVIM.
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Table 1 Hydroxylation of alkyl bromide 4 in water under various reaction conditionsa
Yieldb
Acid scavenger
Entry
Catalyst or other OHꢀsource
(3 equiv.)
Time (min)
4
5a
5b
1
2
3
4
5
6
7
8
—
K2CO3
K2CO3
—
NaHCO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
—
180
25
65
55
60
25
75
80
45
65
65
35
35
30
—
—
—
20
—
—
10
—
—
10
—
—
30
98
72
84
80
98
90
55
85
50
55
90
98
40
Trace
27
APS-HEGBIL (100 mg)
APS-HEGBIL (100 mg)
APS-HEGBIL (100 mg)
APS-HEGBIL (50 mg)
APS-HEGBIL (150 mg)
[hexaEGmim][OMs] (1 equiv.)
[bmim][OMs] (1 equiv.)
HEGBDVIM (1 equiv.)
TBAOH (3 equiv.)
15
—
10
35
15
25
35
10
—
9
10
11
12
13c
KOH/18-crown-6 (3 equiv.)
APS-HEGBIL (100 mg) in PBS
APS-HEGBIL (100 mg)
—
—
K2CO3
a
All reactions were carried out on a 1.0 mmol scale of 4 using 3 equiv. of acid scavenger or OHꢀ source in 3.0 mL of water at 90 ꢁC. b Yields were
determined by 1H NMR spectroscopy. This reaction was performed in a solution of 1,4-dioxane (2.0 mL) and H2O (2.0 mL).
c
Thermogravimetric analysis (TGA) was used to investigate used as a hydroxide source, under different reaction conditions.
the thermal properties of APS-HEGBIL and of HEGBDVIM Initially, we performed the hydroxylation reaction using hexa-
(Fig. 4b). The results obtained showed APS-HEGBIL was stable ethylene glycol (hexaEG) bromide 4 as a model substrate (Table
ꢁ
up to 300 C (a high decomposition temperature for an ionic 1). When K2CO3 was used in the absence of APS-HEGBIL in
catalyst). On the other hand, HEGBDVIM lost 10% of its mass water at 90 ꢁC, the hydroxylation of 4 was incomplete at
between 65 ꢁC and 275 ꢁC due to mesylate anion loss, and 180 min, and the alkene by-product 5b (40%) was produced by
a further substantial weight loss (accountiꢁng for 80% of weight b-elimination (entry 1). The same reaction in the presence of
loss) was observed between 275 and 410 C, due to decompo- 100 mg of APS-HEGBIL (ꢂ0.12 equiv. of the IL portion) under
sition of imidazole groups. According to the thermograms ob- the same reaction conditions affording the desired alcohol 5a in
tained, thermal decomposition of APS-HEGBIL occurred in four 98% yield within 30 min (entry 2). On the other hand, in the
stages. Between 115 ꢁC and 300 ꢁC, it slowly lost approximately presence of APS-HEGBIL by without K2CO3 5a was obtained at
5% of its weight, presumably due to loss of trapped moisture, a yield of only 72% with the alkene by-product 5b (27%, entry 3).
ꢁ
from 300 ꢁC to 412 C, it lost 25% due to mesylate decomposi- When the reaction was conducted with APS-HEGBIL in the
tion, from 412 ꢁC to 470 ꢁC, its lost 28% due to imidazole presence of NaHCO3, the rate of the reaction was slow and a 5a
ꢁ
decomposition, and at temperatures above 470 C, it lost 17% was obtained in moderate yield (Table 1, entry 4). Given that
due to high molecular weight polymer.
K2CO3 was found to be the most efficient acid scavenger for this
We investigated the catalytic efficacy of APS-HEGBIL for the catalytic system, it was used in subsequent experiments. When
nucleophilic hydroxylation of alkyl halides in water, which was the amount of APS-HEGBIL was reduced, 5a yields decreased
Scheme 2 Comparison of the activity of APS-HEGBIL as a heterogeneous promoter with homogenous custom-made IL promoters.
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Table 2 Synthesis of various alcohols from alkyl halides using APS-HEGBILa
Entry
1
R–X
R–OH
Temp (oC)
90
Time (min)
30
Yield (%)b
96
8
2
3
4
5a
5a
5a
90
90
90
42
38
30
98
97
96
5
6
9
90
40
97
94
10
110
200
7
8
11
12
110
90
250
95
96
96
9
12
13
110
100
105
35
95
98
10
11
12
14
15
110
90
60
96
95
180
13
16
110
120
92
a
All reactions were performed on a 1.0 mmol scale of the substrate in the presence of APS-HEGBIL (100 mg) and 3.0 equiv. of K2CO3 in 3.0 mL of
water. b Isolated yield.
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and reaction times increased (entry 5), whereas increasing the in excellent yields. Hydroxylations of 3-bromopropanol, hexaEG
amount of catalyst beyond did not substantially increase yield chloride, hexaEG iodide, hexaEG dibromide, and 4-
(entry 6). To evaluate the catalytic activity and other character- nitroimidazole-1-propyl bromide proceeded smoothly to
istics of APS-HEGBIL, we compared it with other ILs. Initially, provide the corresponding alcohols (8, 5a or 9) in excellent
we performed the above hydroxylation reaction in the presence yields (96, 98, 97, 96, and 97%, entries 1–5, respectively), which
of the conventional ILs (1.0 equiv.) [hexEGmim][OMs] or demonstrated the APS-HEGBIL/water system offers a useful
[bmim][OMs] in water at 90 ꢁC, but found the reaction sluggish means of synthesizing polar alcohols. Generally, the hydroxyl-
and that 5a was obtained in only moderate yield aer prolonged ations of base-sensitive halides using a basic hydroxide proceed
reaction with alkene 5b (entries 7 and 8, respectively). When with difficulty because of a tendency to undergo elimination to
hydroxylation was performed using 1 equiv. of HEGBDVIM form alkenes. Furthermore, APS-HEGBIL enabled the selective
under the same reaction conditions, the reaction was slightly hydroxylations of base sensitive substrates like sec-alkyl
faster than for [hexEGmim] [OMs] or [bmim][OMs], but the bromide and bromoethyl aromatic compounds to correspond-
elimination side reaction still occurred (entry 9). When the ing alcohols (10 and 11) at high yields (94 and 96%, entries 6
reaction was conducted in the presence of the conventional and 7, respectively). In addition, the biologically active 3-(3-
phase-transfer catalytic (PTC) reagents, TBAOH or KOH/18- bromopropoxy)-2H-chromen-2-one and 3-(3-chloropropoxy)-2H-
crown-6 complex, reaction times were prolonged and 5b was chromen-2-one gave excellent yields of 3-(3-hydroxypropoxy)-
obtained at higher yields (entries 10 and 11, respectively). 2H-chromen-2-one (12) even aer shorter reaction times
Hydroxylation without K2CO3 in phosphate-buffered saline (entries 8 and 9, respectively). Bromo-sugar and 3-chloro-pico-
(PBS) provided 5a in 90% yield (entry 12). The model reaction in line-N-oxide were also converted to the corresponding alcohols
organic/aqueous co-solvents, such as 1,4-dioxane with water in high yields with shorter reaction times (entries 11 and 12,
ꢁ
(1 : 1) and APS-HEGBIL at 90 C afforded the expected product respectively).
in excellent yield (entry 13). However, from an environmental
As
a
nal example,
a
good yield of N-Fmoc-2-
point of view, this method was clearly inferior to reaction in hydroxymethylaniline 16 was obtained from the highly base
water in the presence of K2CO3 (Table 1, entry 2). Hence, this sensitive substrate N-Fmoc-2-chloromethylaniline in water at
protocol was found to be efficient and straightforward and to 110 ꢁC for 2 h (entry 13). While the extremely base sensitive
enable catalyst recovery.
Fmoc group can easily be lost under basic conditions, such as
In addition, to the observed merits of APS-HEGBIL in KOH/18-crown-6 in CH3CN,32 this methodology avoided
comparison with reported conventional ILs, we investigated cleavage of the Fmoc group and resulted in a good yield of
hydroxylation reactions of the nonpolar 2-(3-bromopropoxy) alcohol with minimal Fmoc deprotection. These results show
naphthalene (6). The results obtained demonstrated the cata- that this protocol provides required alcohol products rapidly in
lytic superiority of APS-HEGBIL in Scheme 2. In previous studies high yields even from non-polar substrates and extremely base-
on IL such as [bmim][BF4] and hexaEG-DHIM, these homoge- sensitive raw materials, avoiding the productions of side
neous catalysts were used, which were problematic with respect products.
to catalyst purication, the use of volatile organic solvents, and
required expensive catalysts at high loadings. By contrast, APS-
HEGBIL was easily recovered by ltration. In addition, even
Conclusions
smaller amounts of APS-HEGBIL showed excellent performance In conclusion, a highly efficient, air stable, and recyclable APS-
as compared with conventional homogeneous ILs for the HEGBIL catalyst was designed and prepared that functioned
hydroxylation of 6.31,32 These results suggest the nucleophilicity well as a multifunctional heterogeneous catalyst for the nucle-
of water might be enhanced by the imidazolium moiety of APS- ophilic hydroxylation of halides in aqueous medium. This APS-
HEGBIL due to PTC effect.32 The basic nature of the aniline HEGBIL catalytic system enhanced the nucleophilicity of water
amino groups of APS-HEGBIL inhibits the reverse reaction and the catalyst was easily recovered by simple ltration and
through reduction of acidity of the reaction medium from in situ reused several times without signicant loss in catalytic activity.
generated HBr. In addition, the uniform distribution of active The described protocol provided alcohols in excellent yields
cavities along the wrinkled structure of the catalyst, provides from polar, non-polar, and even base-sensitive alkyl halide
a “site-specic effect” on the surfaces of the catalyst.3 The substrates. Furthermore, were believe the protocol encom-
recovery and reusability of a catalyst is an important factor from passes the requirements of green chemistry as it generates few
a green chemistry perspective. Therefore, we studied the reus- side products, is cost effective, requires non-volatile solvents, is
ability of APS-HEGBIL in the model reaction. Aer completion eco-benign, and reduces the environmental toxic chemical
the reaction, APS-HEGBIL was separated by simple ltration, burden. We hope the described methodology opens new routes
and we found it could be reused ten times over without appre- to the development of eco-friendly, sustainable hydroxylation.
ciable loss in catalytic activity. Aer each recovery cycle, the
reaction product 7 was obtained in excellent yield (90–98%)
(Table S1 in ESI†).
Experimental
General remarks
Having optimized reaction conditions, we evaluated the
hydroxylations of a variety of alkyl halides (Table 2). Most of the Unless otherwise noted, all reagents and solvents were obtained
reactions proceeded efficiently and provided desired products commercially. Reaction progress was followed by TLC on
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0.25 mm silica gel glass plates containing F-254 indicator by and the ltrate was evaporated under reduced pressure. Flash
visualization under UV light (254 nm). Flash chromatography column chromatography (5% methanol/dichloromethane) of
was performed using 230-400 mesh silica gel. 1H and 13C NMR the ltrate afforded 278 mg (0.98 mmol, 98%) of hexaethylene
spectra were recorded on a 400 MHz spectrometer, and chem- glycol (5a) as a colorless oil. 1H NMR (400 MHz, CDCl3) d 3.68 (d,
ical shis were reported in d units (ppm) relative to tetrame- J ¼ 11.0 Hz, 4H), 3.62 (s, 17H), 3.59–3.54 (m, 4H), 2.59 (s, 2H);
thylsilane. FT-IR spectra were obtained using a Bruker Vertex 13C-NMR (100 MHz, CDCl3) d 72.4, 70.3, 70.2, 70.0, 61.3; MS (EI)
80v FT-IR spectrometer. Low- and high-resolution electron m/z 283.1 (M+); HRMS (EI TOF) m/z calcd for C 12H26O7 (M+)
impact (EI, 70 eV) spectra were obtained using a high resolution 283.1757, found 283.1759.
mass spectrometer (Korea Basic Science Institute). X-ray
photoelectron spectroscopy (XPS) measurements were per-
formed using an angle-resolved X-ray photoelectron spectrom-
Conflicts of interest
eter (Theta Probe AR-XPS, Thermo Fisher Scientic, U.K.) There are no conicts to declare.
equipped with an MXR1 Gun 400 mm 15 keV spectrometer.
Thermogravimetric analysis (TGA) was performed using a TG
Acknowledgements
209 F3 unit (NETZSCH, Germany) at a 5 C minꢀ1 heating rate
ꢁ
between 10 and 800 ꢁC under an argon atmosphere. Elemental This work was supported by an Inha University Research Grant
analysis was performed using a FLASH EA 1112 Elemental (56909).
Analyzer (Thermo Electron Corporation).
Notes and references
Preparation of 3,30-(3,6,9,12,15-pentaoxaheptadecane-1,17-diyl)
bis(1-vinyl-1H-imidazol-3-ium) methanesulfonate (HEGBDVIM)
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1-Vinyl-1H-imidazole (376 mg, 4.00 mmol) was added dropwise
to a solution of 876 mg (2.00 mmol) of hexaethylene glycol
dimesylate in dried CH3CN (50 mL). The reaction mixture was
stirred at 90 ꢁC for 48 h. The reaction mixture was then
concentrated by rotary evaporation; the concentrated mixture
was washed several times with ethyl acetate (75 mL) and dried
under high vacuum overnight at room temperature to afford
1.04 g (1.75 mmol, 87%) of HEGBDVIM as a pale yellow solid. 1H
NMR (400 MHz, DMSO-d6) d 9.42 (t, J ¼ 1.3 Hz, 2H), 8.20 (t, J ¼
1.8 Hz, 2H), 7.88 (t, J ¼ 1.6 Hz, 2H), 7.32 (q, J ¼ 8.2 Hz, 2H), 5.98
(d, J ¼ 2.3 Hz, 1H), 5.94 (d, J ¼ 2.3 Hz, 1H), 5.44 (dd, J ¼ 8.7,
2.3 Hz, 2H), 4.38 (d, J ¼ 5.0 Hz, 4H), 3.79 (t, J ¼ 4.8 Hz, 4H), 3.56–
3.47 (m, 16H), 2.32 (s, 6H); 13C NMR (100. MHz, DMSO-d6) d:
135.7, 128.6, 123.5, 118.6, 108.8, 69.4, 67.8, 49.0, 39.0. MS (ESI)
m/z 594 (M+ + H); HRMS (ESI) m/z calcd for C24H42N4O9S2 (M+ +
H) 594.2393, found 594.2390.
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Preparation of APS-HEGBIL
To a mixture of p-aminostyrene (1.8 g), HEGBDVIM (594 mg),
and dry CH2Cl2 (20 mL) in a 50 mL round bottom ask was
added 30 mg of AIBN. The reaction mixture was then stirred for
24 h at 70 ꢁC under N2, and the product so obtained was
precipitated from diethyl ether and washed several times with
acetone and methanol. The APS-HEGBIL catalyst was then dried
under vacuum at 40 ꢁC overnight. Anal.; N 8.2, C 60.4, H 7.7, S
4.0 (1.2 mmol S gꢀ1).
Typical procedure for nucleophilic hydroxylation in Table 1
(entry 2)
15 Y. Gu, Green Chem., 2012, 14, 2091–2128.
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2358.
(conrmed by TLC), the mixture was cooled to room tempera- 18 M. B. Gawande, V. D. B. Bonifacio, R. Luque, P. S. Branco and
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