Highly selective tetrahydropyranylation/dehydropyranylation of alcohols and phenols using porous phenolsulfonic acid-formaldehyde resin catalyst under solvent-free condition

Article abstract of DOI:10.1016/j.reactfunctpolym.2020.104519

An efficient protocol for solvent-free chemoselective tetrahydropyranylation/depyranylation of alcohols and phenols is reported herein using mesoporous Phenolsulfonic Acid Formaldehyde Resins as a heterogeneous acid catalyst. The catalyst successfully performed chemoselective protection and deprotection reactions of a wide range of substrates ranging from primary to secondary and tertiary alcohols and also phenols. The reactions were carried out at ambient temperature under solvent-free condition (SolFC) which resulted in high yields within a very short time. FT-IR, TEM, SEM, EDS and TG-DSC analysis techniques were employed to characterize the synthesized polymeric catalyst. The chemoselective nature of our method was confirmed using ¹³C DEPT-135 NMR studies. The polymer catalyst was found to be recoverable even after 10th catalytic cycle without much depreciation in its activity. The heterogeneity of the catalyst was verified by hot filtration method. Good yield, energy and cost- effective method, solvent-free protocol, mild reaction conditions, no inert atmosphere, metal-free heterogeneous polymer catalyst and excellent recoverability of the catalyst are notable milestones of the reported protocol.

Full text of DOI:10.1016/j.reactfunctpolym.2020.104519

ReactiveandFunctionalPolymers149(2020)104519
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Highly selective tetrahydropyranylation/dehydropyranylation of alcohols
and phenols using porous phenolsulfonic acid-formaldehyde resin catalyst
under solvent-free condition
Kalyani Rajkumaria, Ikbal Bahar Laskarb, Anupama Kumaria, Bandita Kalitac,
⁎
Lalthazuala Rokhuma,d,
a Department of Chemistry, National Institute of Technology Silchar, Assam 788010, India
b Department of Mechanical Engineering, National Institute of Technology, Silchar, Assam, India
c Department of Chemistry, Institute of Advanced Study in Science & Technology, Guwahati, Assam, India
d Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB21EW, UK
A R T I C L E I N F O
A B S T R A C T
Keywords:
An efficient protocol for solvent-free chemoselective tetrahydropyranylation/depyranylation of alcohols and
phenols is reported herein using mesoporous Phenolsulfonic Acid Formaldehyde Resins as a heterogeneous acid
catalyst. The catalyst successfully performed chemoselective protection and deprotection reactions of a wide
range of substrates ranging from primary to secondary and tertiary alcohols and also phenols. The reactions were
carried out at ambient temperature under solvent-free condition (SolFC) which resulted in high yields within a
very short time. FT-IR, TEM, SEM, EDS and TG-DSC analysis techniques were employed to characterize the
synthesized polymeric catalyst. The chemoselective nature of our method was confirmed using 13C DEPT-135
NMR studies. The polymer catalyst was found to be recoverable even after 10th catalytic cycle without much
depreciation in its activity. The heterogeneity of the catalyst was verified by hot filtration method. Good yield,
energy and cost- effective method, solvent-free protocol, mild reaction conditions, no inert atmosphere, metal-
free heterogeneous polymer catalyst and excellent recoverability of the catalyst are notable milestones of the
reported protocol.
Protection/deprotection
Tetrahydropyranylation/depyranylation
Phenol-sulfonic acid formaldehyde resin, ion-
exchange resin catalyst, heterogeneous
catalysts
1. Introduction
wider and appreciable because they are mostly synthesized from re-
newable resources and therefore problems raised due to waste disposal,
The hunt for an alternative green and efficient catalyst with en-
vironment-friendly technologies has been a perceived attention in
current scientific research. Heterogenization of homogeneous catalysts
onto solid supports is undergoing well-investigation with an objective
to enhance recyclability of catalyst and thereby minimize disposal
problems [1,2]. Although polymer-supported catalysts and reagents are
being utilized in organic synthesis for decades, but the high-cost issue
involved in the post-immobilization process of such catalysts and low
catalyst loading constrain their applicabilities [3]. As a powerful al-
ternative to this, porous organic polymers synthesized via bottom-up
approach has emerged to be simple, low-cost, easily separable and re-
coverable heterogeneous catalysts for organic transformations. More-
over, the bottom-up synthesis strategy offers prospects to design with
different functionalities to be used as catalysts [4–6]. The framework of
applications of carbon-based, metal-free polymeric resins are much
leaching, metal-contamination in desired products etc. can be avoided
easily [7].
We report herein a mesoporous Phenolsulfonic Acid Formaldehyde
Resin (PAFR) catalyst that exhibit efficient catalytic activity towards
chemoselective protection and deprotection of alcohols and phenols.
The history of resin based on phenol and formaldehyde dates back to
1872 when it was discovered by A. Von. Bayer [8]. Since the in-
troduction of ion- exchange property of phenol-formaldehyde resins by
Adams and Holmes in 1935, they have been extensively studied and
applied as ion exchangers both in academia and industries [9]. Func-
tionalized ion exchange resins are regarded as a flag bearer of solid acid
catalyst [10–12].
Recently, Yamada et al. [13,14] reported an in-water direct dehy-
drative esterification process of alcohol and carboxylic acid utilizing
Phenolsulfonic Acid Formaldehyde Resin as catalyst without removal of
⁎ Corresponding author at: Department of Chemistry, National Institute of Technology Silchar, Assam 788010, India.
E-mail addresses: lalthazualarokhum@gmail.com, rokhum@che.nits.ac.in (L. Rokhum).
https://doi.org/10.1016/j.reactfunctpolym.2020.104519
Received 19 October 2019; Received in revised form 25 January 2020; Accepted 3 February 2020
Availableonline04February2020
1381-5148/©2020ElsevierB.V.Allrightsreserved.

K. Rajkumari, et al.
ReactiveandFunctionalPolymers149(2020)104519
byproduct. In addition, our group reported the synthesis of solketal, a
potent fuel additives [15] using PAFR resin as a recyclable solid cata-
lyst. In the present study, PAFR was found to effectively catalyze the
tetrahydropyranylation and depyranylation of alcohols and phenols. In
a valuable multi-step organic synthetic route, protection/deprotection
of reactive functional groups plays a fundamental yet immense useful
step for chemoselective attack on certain specific reactive sites [16,17].
3,4-dihydro-2H-pyran (DHP) is recognized as an ideal protecting group
owing to its simple preparation, low cost, ease of handling, strong
tolerability of the protected 2-tetrahydropyranyl (THP) ethers towards
different reagents and easy deprotection [18–21]. Thesaurus of syn-
thetic methodologies can be found in literature [22–32], each method
having their own pros and cons. Many of the protocols lack sustain-
ability in their methods due to the use of corrosive acid catalysts like
silica sulfuric acid, PTSA [23,24], non- recoverable homogeneous cat-
alysts [25,26], hygroscopic, costly and non-recyclable catalysts like
AlCl3, molecular iodine etc. [21,27], heterogeneous metal catalysts
[28–30] which may create problems like waste- disposable, con-
tamination of the metal to desired product etc. Moreover most of the
reported protocol use of toxic chlorinated solvents [15,31], high tem-
perature [21,25] etc. and also need vigorous conditions to carry out
protection of bulky substrates having secondary, tertiary carbons and
phenols specifically [31,32].
Scheme 1. Preparation of PAFR catalyst by condensative method.
on SEM (SEM-EDX) were recorded on Zeiss Sigma 500VP FESEM in-
strument. Thermogravimetric (TG) and Differential Scanning
Calorimetry (DSC) analysis were recorded for temperature range
0–500 °C at PERKIN ELMER, USA. Diamond TG/DTA model. Fourier
Transform Infrared (FT-IR) Spectra were recorded on a Perkin-Elmer
Spectrum One FTIR spectrometer. Inductively Coupled Plasma-Optical
Emission Spectrometry (ICP-OES) was done on iCAP 7600 ICP-OES Duo
model. Nuclear Magnetic Resonance (NMR) spectra were recorded in
Brukar Advance II, 400 MHz. The value of Chemical shifts were re-
ported in ppm (δ-scale) relative to the internal standard TMS
(0.00 ppm) using CDCl3 as solvent.
2.2. Catalyst preparation
Our continuous interest in the application of polymeric reagents and
catalysts [33,34] and establishing a more general method for synthesis
of THP ethers [35] prompted us to exploit the hydrophilic mesoporous
PAFR catalyst for this conversion. This polymeric acid catalyst mimics
the chemistry of enzymes through a phase separated reaction condition.
The hydrophilic active site of the catalyst first attracts the fairly hy-
drophilic alcohol or phenol substrate and then after its conversion into
the relatively hydrophobic ether product, it is apparently kicked out
from the catalyst site. With this working hypothesis, we tried protection
(deprotection) of alcohol and phenol substrate to their corresponding
tetrahydropyranylated products. Since organic solvents are the major
contributor of E-factors in the fine chemical and pharmaceutical in-
dustries, solvent-free reaction condition is a preferable solution to the
waste problem [36]. Interestingly, our protocol showed excellent re-
sults in solvent-free condition and selectively protected a large variety
of alcohol and phenol molecules affording high yield within a very
short time at ambient temperature without any requirement to remove
byproduct. The catalyst seems to be advantageous over several other
industrially used catalyst for the tetrahydropyranylation and presents
major benefits that comprises of easy recovery by filtration, high purity
of products compared to homogeneous catalytic systems, elimination of
waste disposal problem, high selectivity, lack of side products and note
worthily, environmentally benign solvent-free reaction condition. The
reaction protocol developed is feasible at very mild reaction conditions
viz., at room temperature, with very low catalyst loading and the cat-
alyst employed is ‘metal free’ polymeric material.
We prepared the PAFR solid acid polymer resin according to re-
ported procedure in literature [12] where 4-hydroxybenzenesulphonic
acid (phenolsulfonic acid) undergoes condensation polymerization with
5 mol equivalent of formaldehyde in H2O at 120 °C for 6–7 h (Scheme
1).
The reaction mixture was then gradually cooled to 25 °C over 12 h
which resulted a pale brownish gel. After being dried under reduced
pressure in vacuum oven at 60 °C for 12 h, the gel became a reddish
brown, hardly soluble solid giving 70% yield. The C to S ratio was
obtained as 17.2:1 from EDX analysis which deduced that the phe-
nolsulfonic acid moiety/phenol moiety ratio is about 1:3.
2.3. Typical procedure for tetrahydropyranylation of alcohol using PAFR
catalyst
As a pilot protocol, a mixture of 1-octanol (0.130 g, 1 mmol), DHP
(0.084 g, 2 mmol) and PAFR catalyst (0.025 g, 7.48 mol%) was me-
chanically stirred in a small reaction vessel at room temperature. The
progress of the reaction was monitored by thin layer chromatography
(TLC). After completion of the reaction as indicated by TLC, the catalyst
was separated by simple filtration and the reaction mixture was ex-
tracted with ethyl acetate, the extracted compound thus obtained was
then charged into a short silica gel chromatography column using
hexane/ethyl acetate (9:1 ratio) as eluent to afford 98% of isolated
yield of desired product.
2.4. Depyranylation (deprotection) of alcohols using PAFR catalyst
2. Experimental section
2-(octyloxy)tetrahydro-2H-pyran, the THP ether of 1-Octanol,
(0.198 g, 1 mmol) and PAFR catalyst (0.025 g, 7.48 mol%) were stirred
in methanol (0.5 mL) in a mechanical stirrer for 1 h. The cleavage of
ether and regeneration of the corresponding alcohol was monitored by
checking TLC. After complete generation of the alcohol, the catalyst
was separated by filtration and the residue was eluted through a short
column of silica gel to afford 97% of the isolated alcohol as product.
2.1. Materials and methods
The chemicals required, different alcohols, 3,4-Dihydropyran
(DHP), silica gel for thin layer chromatography (TLC) and column
chromatography were of analytical grade and purchased from
SpectroChem and were used without further purification.
Formaldehyde and p-hydroxybenzenesulphonic acid used in prepara-
tion of the polymeric resin were purchased from Sigma Aldrich and
used as such. Solvent used were of extra pure grade purchased from
Merk India. Double distilled deionized water was used for the synthesis
of the polymer catalyst.
2.5. Catalyst recyclability test
The recyclability of mesoporous catalyst was investigated with
consecutive tetrahydropyranylation reactions reusing the catalyst for
multiple times. After each catalytic run, the catalyst was separated by
simple filtration and washed it with methanol followed by hexane and
High Resolution Transmission Electron Microscopy (HR-TEM) was
recorded on an electron microscope JEM-2100, 200 kV, JEOL. Scanning
Electron Microscopy (SEM) and Energy dispersive X-ray spectrometry
2

K. Rajkumari, et al.
ReactiveandFunctionalPolymers149(2020)104519
decanted and was continued to stir for another 6 h. The further progress
of the reaction was monitored with TLC and finally filtrate was ana-
lyzed by ICP-OES test.
3. Results and discussion
3.1. Catalyst characterization
At the onset, FT-IR spectroscopic analysis of the PAFR catalyst was
carried out to identify the different functional groups present in the
catalyst. The IR spectra in Fig.
1 shows distinguished peak at
1034 cm−1 which is the characteristic absorption peak for symmetric
S]O stretching due to the introduction of eSO3H group in the polymer.
The two peaks near 594 cm−1 and 3438 cm−1 can be attributed to the
bending and stretching vibrations of eOH group respectively while
absorption at 1224 cm−1 suggests aromatic eOH stretching. Another
two absorption bands at 1474 cm−1 and 1650 cm−1 can be assigned to
the C]C stretching of the benzene ring of the polymer [37].
To have a distinct knowledge about the morphology of the polymer
catalyst, HR-TEM and SEM analysis were performed. HR-TEM images
(Fig. 2a–b) revealed botryoidal structure of the catalyst and clearly
showed its mesoporosity in the range 20–100 nm. SEM images
(Fig. 2c–d) also exposed the mesoporous surface of the PAFR polymer
catalyst. Results from SEM-EDX in Fig. 3 displayed the presence of
sulfur in its structure confirming anchoring of eSO3H moiety to the
polymer. The wt% of C, O and S were obtained as 68.98%, 27.01% and
4.01%.
Fig. 1. IR spectra of PAFR catalyst.
finally with chloroform. The mentioned process was repeated twice or
thrice followed by subsequent drying of the catalyst at 60 °C for 12 h in
oven and was then used for the next catalytic cycle. The recovered
catalyst was further investigated by IR, SEM-EDX analysis.
2.6. Hot-filtration method
The mesoporosity of the catalyst was confirmed by N2 adsorption-
desorption analysis by BET model which was reported in our previous
work [15]. The adsorption-desorption isotherm showed characteristic
features of type IV hysteresis loop referring capillary condensation in
mesopores. The surface area was obtained as 59.78 m2/g with average
pore size of 10.57 nm (ranging from 2.5 nm to 26 nm).
To verify the heterogeneity of the polymer catalyst, hot filtration
method was employed. For this, after stirring the reaction mixture of
protection of alcohol for 15 min (yield was 30% at that time), the
catalyst was separated by centrifugation. The reaction mixture was then
Fig. 2. a-b) HR-TEM images c) SEM image and d) EDX spectra of PAFR catalyst.
3

K. Rajkumari, et al.
ReactiveandFunctionalPolymers149(2020)104519
The thermal stability of the catalyst was enquired by TGA and DSC
analysis. The representative curve for TGA in Fig. 4 shows total three
humps at 60–150 °C, 200–300 °C and 450–500 °C. The weight loss near
100 °C can be associated with the release of physically adsorbed
moisture on the surface of the polymer. The mass loss in the range
200–300 °C may be due to the decomposition of sulfonic acid group
while in the third step, mass loss above 400 °C is possibly attributable to
the breakdown of polymeric backbone. The endothermic peaks ap-
peared in the DSC plot also compliments the three steps of mass loss of
the polymer as suggested by the TGA plot [38].
3.2. Tetrahydropyranylation of alcohols
After preparation and characterization of the catalyst, we proceeded
for testing its catalytic activity against tetrahydropyranylation of alco-
hols and phenols as well. We, at the outset, took an equimolar mixture
of 1-octanol and DHP and stirred it with 20 mg of PAFR catalyst in
solvent-free condition at room temperature as a pilot protocol for the
reaction. The activity and feasibility of the reaction was monitored by
TLC. To our pleasure, the reaction was observed to progress indicating
formation of tetrahydropyranylated ether in TLC and completion of the
reaction was obtained after 3 h of the reaction.
Fig. 3. SEM-EDX spectra of PAFR catalyst.
With this rough idea and in an attempt to find an optimum amount
of catalyst for achieving maximum yield in minimum time, we checked
feasibility of the reaction at the same conditions but with different
amount of catalyst loading i.e. 0.015 g, 0.025 g, 0.030 and 0.035 g. It
was found that the reaction resulted best yield in shortest time with
0.025 g (7.48 mol%) of catalyst (Fig. 5). Further increase in the catalyst
amount had no any significant effect on the yield. The product yield and
conversion time were developed by optimizing the appropriate amount
of DHP which was obtained to be 2 equivalent of the alcohol (Fig. 6).
We also compared the efficiency of the present catalyst with other
reported methodologies for tetrahydropyranylation of alcohols. The
results are displayed in the Table 1. It was observed that most of the
Results of NH3-TPD (Temperature programmed desorption) test of
PAFR catalyst15 exhibited a single ammonia desorption peak at 385 °C
suggesting only Brǿnsted acid sites in the catalyst surface offering
moderate acidity having acid sites of 648 μmol g−1. Strong Brǿnsted acid
sites surrounded by hydrophobic organic moieties are found to be pre-
ferable condition for conducting in-water organic reactions in forward
direction exclusively. It can be anticipated that the hydrophobic ‘tail’ of
the substrate alcohol lies parallel along the hydrophobic surface of the
catalyst preventing adsorption of water over the active sites (which re-
stricts its access with the substrate molecules) and thereby helps to retain
its catalytic activity without interacting with by-product water.
Fig. 4. TGA-DSC graph of PAFR catalyst.
4

K. Rajkumari, et al.
ReactiveandFunctionalPolymers149(2020)104519
finds difficulty in separating the catalyst from reaction mixture which
needs tedious workup. Several heterogeneous polymer and zeolite
catalyst were found to be employed in this protection reaction (Table 1,
Entry 6–8) but use of dichloromethane (CH2Cl2) and toluene as solvents
in their method contradict their benign nature. Imidazole-based
Brønsted–Lewis acidic Ionic Liquid (Table 1, Entry 9) was also reported
as a recyclable catalyst under solvent-free condition for this conversion
but although the protocol selectively protected aliphatic and benzylic
alcohols but failed in case of phenols. Apparently the present protocol
for tetrahydropyranylation of alcohols and phenols using hydrophobic
PAFR resin catalyst was appeared to be beneficial over most of the
reported protocols in literature.
With the optimized conditions in hand, we investigated the scope of
the polymeric PAFR acid catalyst for a large number of primary, sec-
ondary, tertiary, allylic and benzylic alcohols and phenols bearing
various substituents ranging from electron-donating to electron-with-
drawing groups at different position. The outcome is depicted in
Table 2 where it can be observed that the alcohols were smoothly
converted to the desired products in moderate to excellent yield. To our
surprise, the catalytic system is equally effective in phenolic compounds
(Table 2, Entry 14–16) also, which make the protocol extremely gen-
eralized. Some differences were encountered with different substrates
in the time taken to achieve completion of reaction. For instance, tet-
rahydropyranylation of aliphatic alcohol (Table 2, Entry 8–11) and
those of benzylic alcohol with electron-donating substituent (Table 2,
Entry 1–3) at one of its position in benzene ring was reported to be
highly reactive giving corresponding ether in excellent yield. However,
benzylic group with two electron-donating substituent at the same time
(Table 2, Entry 4) was found to be quite slow as compared to afore-
mentioned reactions. In contrast presence of electron-withdrawing
substituent (Table 2, Entry 5–7) was observed to slow down the reac-
tion. Further, the secondary hydroxyl group of Cholesterol (Table 2,
Entry 12) and tertiary hydroxyl group of adamantanol (Table 2, Entry
13) were smoothly converted to corresponding desired products under
the stated reaction condition without affecting the olefinic group of the
substrate, which was observed in case of citronellol also (Table 2, Entry
11). This not only shows mild nature of the catalyst, but also exposes
supreme chemoselectivity of the protocol.
Fig. 5. Optimization of catalyst loading using 1-octanol (1 mmol), DHP
(2 mmol), Sol FC, room temperature.
To study the selectivity between primary and secondary alcohols,
we tried competitive reaction with 1,3-butandiol. Interestingly we ob-
served that the terminal (primary) hydroxyl groups were protected
selectively leading to exclusive formation of the corresponding ether
(Table 2, entry 17). The structure of the product was confirmed by 13C
DEPT-135 NMR study (Supporting Information S23), wherein the pre-
sence of the tertiary carbon at δ 101.88 ppm and shifting of the adjacent
methylene carbon to δ 66.56 ppm indicated the attachment of the DHP
moiety to the primary hydroxyl group.
Fig. 6. Optimization of DHP equivalent (mmol) using octanol (1 mmol), cata-
lyst 7.48 mol% (0.025 g), Sol FC, room temperature.
protocol uses some environmentally unfriendly solvents, while yield of
the solvent-free protocols (Table 1, Entry 5 and 9) are not up to the
mark, also need vigorous condition for the reaction (Table 1, Entry 5).
Homogeneous catalysts (Table 1, Entry 1 and 5), on the other hand,
To have a more comprehensive idea about the selectivity of the
stated method, various competitive reactions were carried out between
structurally different alcohols in their binary mixtures. The results are
Table 1
Catalytic activity of PAFR catalyst with that of several reported catalysts.
Sl no
Catalyst
Conversion
Reaction condition
Reference
1
Fe(ClO4)3
75–98%
Et2O, RT, 60–180 min
CH2Cl2, RT, 15–100 min
MeOH, 30–80 °C, 30–180 min
CH2Cl2, RT, 45 min- 24 h
SolFc, 50 °C, 8 h
[28]
2
SO3H-SiO2
40–99%
[24]
3
AlCl3.6H2O
74–98%
[21]
4
Al(OTf)3
65–100%
38–75%
[18]
5
Polyaniline salts
Dowex 50WX4–100
Polystyrene-supported AlCl3
Ersorb-4 (Zeolite)
Brønsted–Lewis acidic Ionic Liquid
PAFR
[25]
6
75–98%
CH2Cl2, RT, 7 min- 24 h
CH2Cl2, RT, 7 min- 24 h
Toluene, RT, 30–240 min
SolFc, RT, 40–90 min
[31]
7
89–97%
[19]
8
44–58%
[32]
9
70–97% (Phenol 0%)
86–98%
[39]
10
SolFC, RT, 60–180 min
Present work
5

K. Rajkumari, et al.
ReactiveandFunctionalPolymers149(2020)104519
Table 2
Scope of tetrahydropyranylation of alcohols and phenolsa.
Yieldb
Entry
Alcohol
Product
Time (hrs)
(%)
1
2
3
4
5
1
95
1.10
1
97
98
94
97
2
1.15
6
7
1.15
2.5
97
95
8
9
1.30
1.10
98
96
10
11
1.20
1.30
97
97
12
3
97
13
14
1.45
1.45
96
95
15
16
17
18
1.35
1.30
1.30
1.35
98
98
86
87
6

K. Rajkumari, et al.
ReactiveandFunctionalPolymers149(2020)104519
Table 3
Competitive reactions of different binary mixturesa.
Entry
1
Alcohols
Products
Time (min)
60
% Conversion
95
5
2
92
8
90
90
3
4
82
18
100
0
60
aAlcohol (1 mmol), DHP (2 mmol), catalyst (0.025 g, 7.48 mol%), SolFC, room temperature; bIsolated yield.
shown in Table 3. As can be observed in the table, in the competitive
reaction between benzylic alcohol and aliphatic alcohols, the former
shows dominance in terms of selectivity and yield (Table 3, Entry 1).
Similarly, allylic alcohols can be converted to its corresponding ether
with excellent selectivity in presence of benzylic alcohols (Table 3,
Entry 2). There were moderate selectivity of straight chain alcohols
over branched chain compounds. (Table 3, Entry 3). Benzylic alcohol
was converted quantitatively to the respective THP ether while the
tertiary alcohol counterpart observed almost no progress (Table 3,
Entry 4).
3.4. Recyclability test of catalyst
The reusability of the catalyst was also investigated by carrying out
successive protection reaction of alcohol substrates. After each run, the
catalyst was filtered and washed with hexane and chloroform and dried
for the next catalytic run. The results of five catalytic recycling runs are
depicted in Fig. 9. It clearly shows that the efficiency of the catalyst has
not deteriorated much even after the tenth cycle which undoubtedly
reveals the excellent recoverability of the catalyst. IR and SEM-EDX
analysis of the recovered catalyst after 10th catalytic cycles were also
investigated to check any possible depreciation or change in its com-
position. To our delight, the IR spectrum of the recovered catalyst gave
similar peaks as in the spectrum of fresh catalyst (SI, Fig. S1). Never-
theless, the morphology of the recovered catalyst as observed in its SEM
image (SI, Fig. S2a) is almost similar with the fresh one. The EDX
analysis revealed wt% of C, O and S as 67.71%, 28.70%, 3.59% re-
spectively. A slight decrease in the weight % of S from 4.01% to 3.59%
in the recycled catalyst can be attributed to its little leaching of SO3H
group from the catalyst surface during washing. The surface area ana-
lysis of the recovered catalyst was also explored by BET method which
measured the surface area as 53.3 m2/g.
The probable mechanism of tetrahydropyranylation of alcohols
using PAFR catalyst is depicted in Fig. 6.
3.3. Deprotection of THP ether using PAFR catalyst
In order to establish the regenerative potential of the protocol, the
capability of PAFR catalyst to cleave the THP group was also in-
vestigated. For this, few drops of methanol was introduced to the re-
action. We observed that addition of methanol served as a deprotecting
agent to obtain the alcohols from THP ethers. Interestingly variety of
THP ethers were easily deprotected by stirring with 0.025 g (7.48 mol
%) PAFR catalyst in 1 mL methanol within only 60 min, resulting their
respective free alcohol products with excellent yield. (Table 4).
The mechanism proposed for protection (tetrahydropyranylation)
and deprotection (depyranylation) of alcohols using PAFR catalyst are
shown in Fig. 7 and Fig. 8 respectively.
3.5. Hot filtration method
The heterogeneous nature of the PAFR catalyst was examined by
Sheldon's Hot filtration method to investigate possible leaching of ac-
tive species of the catalyst to the reaction mixture. For this, the catalyst
7

K. Rajkumari, et al.
ReactiveandFunctionalPolymers149(2020)104519
Table 4
Deprotection of THP ethers using PAFR catalysta.
Deprotected
product
Entry
THP ether
Time (min)
Yieldb (%)
1
2
55
60
97
95
3
4
5
50
45
55
96
97
94
6
7
40
40
92
96
aTHP ether (1 mmol), catalyst (0.025 g, 7.48 mol%), methanol (1 mL), room temperature; bIsolated yield.
Fig. 8. Proposed mechanism for depyranylation of THP ethers using PAFR
catalyst.
Fig. 7. Probable mechanism of tetrahydropyranylation using PAFR catalyst.
during this time and also no trace of S was obtained in ICP analysis of
the filtrate. This confirmed that no leaching of acid sites from the cat-
alyst surface took place to the reaction medium and established the
excellent heterogeneity of PAFR catalyst as well.
was separated from the reaction mixture after 15 min of the reaction
and then the decanted reaction mixture was continued to stir for an-
other 6 h. But it was observed that the reaction did not progress at all
8

K. Rajkumari, et al.
ReactiveandFunctionalPolymers149(2020)104519
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.reactfunctpolym.2020.104519.
References
[1] Y. Zhang, S.N. Riduan, Chem. Soc. Rev. 41 (2012) 2083–2094.
[2] M.X. Tan, L. Gu, N. Li, J.Y. Ying, Y. Zhang, Green Chem. 15 (2013) 1127–1132.
[3] K.C. Nicolaou, S.A. Snyder, Classics in Total Synthesis II, Weinheim, Wiley-VCH,
2003, p. 594.
[4] P. Kaur, J.T. Hupp, S.B.T. Nguyen, ACS Catal. 1 (2011) 819–835.
[5] J. Lu, P.H. Toy, Chem. Rev. 109 (2009) 815–838.
[6] M. Benaglia, A. Puglisi, F. Cozzi, Chem. Rev. 103 (2003) 3401–3427.
[7] D.R. Dreyer, C.W. Bielawski, Chem. Sci. 2 (2011) 1233–1240.
[8] A. Gardziella, L. Pilato, A. Knop, Phenolic Resins, Chemistry, Applications,
Standardization, Safety and Ecology, Springer, Heidelberg, NY, 2000, pp. 1–560.
[9] R.S. Myers, J.W. Eastes, F.J. Myers, Ind. Eng. Chem. 33 (1941) 697–706.
[10] A. Chakrabarti, M.M. Sharma, React. Polym. 20 (1993) 1–45.
[11] G. Gelbard, Ind. Eng. Chem. Res. 44 (2005) 8468–8498.
[12] S.H. Ali, A. Tarakmah, S.Q. Merchant, T. Al-Sahhaf, Chem. Eng. Sci. 62 (2007)
3197–3217.
Fig. 9. Recyclability test for PAFR catalyst.
4. Conclusions
[13] M. Minakawa, H. Baek, Y.M.A. Yamada, J.W. Han, Y. Uozumi, Org. Lett. 15 (2013)
5798–5801.
Protection (and deprotection) of the free hydroxyl group in a mul-
tifunctional alcohol substrate is one of the most frequently confronted
synthetic strategies, in which ecofriendly and efficient catalytic ap-
proaches are highly desirable. We reported here a facile protocol to
synthesize THP ethers by protection of variety of alcohols and phenols
using catalytic amount of mesoporous Phenolsulfonic Acid-
Formaldehyde Resin (PAFR) catalyst which resulted moderate to ex-
cellent yield at room temperature. The same catalyst was also effective
for cleavage of THP ethers to obtain the free alcohols by easy depro-
tection method. PAFR was found to be superior to many commercially
available solid acid catalyst in terms of yield and rate of reaction.
Excellent heterogeneity as verified by hot filtration method is another
notable feature of the catalyst. Our protocols showed excellent che-
moselectivity in favor of the primary alcohols. In addition, phenols
were also successfully protected using our method, which otherwise are
difficult to achieve. The reaction was carried out at room temperature
under solvent-free condition and the spent catalyst was reused at least
10 times which are of great significance from green chemistry per-
spective.
[14] H. Baek, M. Minakawa, Y.M.A. Yamada, J.W. Han, Y. Uozumi, Sci. Rep. 6 (2016)
25925–25934.
[15] I.B. Laskar, K. Rajkumari, R. Gupta, L. Rokhum, Energy Fuel 32 (2018)
12567–12576.
[16] P.P. Bora, K. Vanlaldinpuia, L. Rokhum, G. Bez, Synth. Commun. 41 (2011)
2674–2683.
[17] K. Vanlaldinpuia, H.A. Sema, L. Rokhum, G. Bez, Chem. Lett. 39 (2010) 228–229.
[18] D.B.G. Williams, S.B. Simelane, M. Lawton, H. Kinfe, Tetrahedron 66 (2010)
4573–4576.
[19] B. Tamami, K.P. Borujeny, Tetrahedron Lett. 45 (2004) 715–718.
[20] B. Kumar, M.A. Aga, D. Mukherjee, S.S. Chimni, S.C. Taneja, Tetrahedron Lett. 50
(2009) 6236–6240.
[21] V.V. Namboodiri, R.S. Varma, Tetrahedron Lett. 43 (2002) 1143–1146.
[22] B. Kumar, M.A. Aga, A. Rouf, B.A. Shah, S.C. Taneja, RSC Adv. 4 (2014)
21121–21130.
[23] J.H.V. Boom, J.D.M. Herscheid, Synthesis 3 (1973) 169–170.
[24] K.I. Shimizu, E.T. Hayashi, T. Hatamachi, T. Kodamab, Y. Kitayama, Tetrahedron
Lett. 45 (2004) 5135–5138.
[25] S. Palaniappan, M.S. Ram, C.A. Amarnath, Green Chem. 4 (2002) 369–371.
[26] C.E. Yeom, Y.J. Shin, B.M. Kim, Bull. Kor. Chem. Soc. 28 (2007) 103–107.
[27] H.M.S. Kumar, B.V.S. Reddy, E.J. Reddy, J.S. Yadav, Chem. Lett. 28 (1999)
857–858.
[28] M.M. Heravi, F.K. Behbahani, H.A. Oskooie, R.H. Shoar, Tetrahedron Lett. 46
(2005) 2543–2545.
[29] S.A. Taghavi, M. Moghadam, I.M. Baltork, S. Tangestaninejad, V. Mirkhani,
A.R. Khosropour, C. R. Chimie 14 (2011) 1095–1102.
Declaration of Competing Interest
[30] M. Moghadama, S. Tangestaninejada, V. Mirkhania, I.M. Baltorka, S. Gharaati, J.
Mol. Catal. A Chem. 337 (2011) 95–101.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
[31] P.S. Poon, A.K. Banerjee, L. Bedoya, M.S. Laya, E.V. Cabrera, K.M. Albornoz, Synth.
Commun. 39 (2009) 3369–3377.
[32] A. Hegedüs, I. Vígh, Z. Hell, Synth. Commun. 34 (2004) 4145–4152.
[33] G. Pathak, L. Rokhum, ACS Comb. Sci. 17 (2015) 483–487.
[34] D. Das, G. Pathak, L. Rokhum, RSC Adv. 6 (2016) 104154–104163.
[35] K. Rajkumari, J. Kalita, D. Das, L. Rokhum, RSC Adv. 7 (2017) 56559–56565.
[36] R.A. Sheldon, Green Chem. 19 (2017) 18–43.
Acknowledgements
[37] W. Zhou, M. Yoshino, H. Kita, K.I. Okamoto, Ind. Eng. Chem. Res. 40 (2001)
4801–4807.
Science and Engineering Research Board (SERB), New Delhi is
thankfully acknowledged for financial support (Grant No. SB/FT/CS-
103/2013 and SB/EMEQ-076/2014). Also we are thankful to SAIF
NEHU; STIC, Cochin, IASST, Guwahati and NEIST Jorhat for various
analysis.
[38] G. Fan, C. Liao, T. Fang, L. Shanshan, S. Guangsen, Carbohydr. Polym. 112 (2014)
203–209.
[39] N. Azizi, M.A. Senejani, F. Abbasi, Tetrahedron Lett. 57 (2016) 5009–5011.
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