Magnetic Fe3O4@silica sulfuric acid nanoparticles promoted regioselective protection/deprotection of alcohols with dihydropyran under solvent-free conditions

Article abstract of DOI:10.1039/c7ra12458a

Protection (and deprotection) of hydroxyl groups via tetrahydropyranylation was carried out effectively using a catalytic amount of Fe₃O₄ supported silica sulphuric acid nanoparticles (Fe₃O₄@SiO₂@SO₃H) under solvent-free conditions. The synthesized nanocatalyst was characterized by XRD, TEM, FT-IR etc. A wide range of tetrahydropyranylated alcohol derivatives were synthesized using this heterogeneous magnetic nanocatalyst within 10-20 min with high yields. In addition, tetrahydropyranyl ethers could also be deprotected to the parent alcoholic compounds in the presence of MeOH using the same catalyst. After completion of the reactions, the catalyst was easily separated from the reaction medium using an external magnet, which ameliorated the overall synthetic process. The catalyst was recovered and reused for five successive reactions without any appreciable loss in its activity. Mild reactions conditions, operational simplicity, solvent free conditions, high selectivity, easy recyclability of the magnetic nanocatalyst, and high yields can be considered as the advantageous features of our procedure.

Full text of DOI:10.1039/c7ra12458a

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Magnetic Fe3O4@silica sulfuric acid nanoparticles
promoted regioselective protection/deprotection
of alcohols with dihydropyran under solvent-free
conditions†
Cite this: RSC Adv., 2017, 7, 56559
*
Kalyani Rajkumari, Juri Kalita, Diparjun Das and Lalthazuala Rokhum
Protection (and deprotection) of hydroxyl groups via tetrahydropyranylation was carried out effectively
using a catalytic amount of Fe3O4 supported silica sulphuric acid nanoparticles (Fe3O4@SiO2@SO3H)
under solvent-free conditions. The synthesized nanocatalyst was characterized by XRD, TEM, FT-IR etc.
A wide range of tetrahydropyranylated alcohol derivatives were synthesized using this heterogeneous
magnetic nanocatalyst within 10–20 min with high yields. In addition, tetrahydropyranyl ethers could
also be deprotected to the parent alcoholic compounds in the presence of MeOH using the same
catalyst. After completion of the reactions, the catalyst was easily separated from the reaction medium
using an external magnet, which ameliorated the overall synthetic process. The catalyst was recovered
and reused for five successive reactions without any appreciable loss in its activity. Mild reactions
conditions, operational simplicity, solvent free conditions, high selectivity, easy recyclability of the
magnetic nanocatalyst, and high yields can be considered as the advantageous features of our procedure.
Received 15th November 2017
Accepted 1st December 2017
DOI: 10.1039/c7ra12458a
rsc.li/rsc-advances
separation and recoverability, several effective heterogeneous
catalysts have been reported in recent years.9,10 However
1. Introduction
a serious disadvantage of heterogeneous catalysts is that their
efficiency gets restricted due to the limited surface area.11 This
drawback can be overcome by a nanoporous catalyst support
due to its “plenty of room at the bottom” nature, which can also
be considered as a bridge between heterogeneous and homog-
enous catalysts.12
During the synthesis of an organic compound, the protection
(and deprotection) of the free hydroxyl group in a multifunc-
tional alcohol substrate or intermediate is one of the most
frequently used strategies.1,2 For this type of functional
manipulation, etherication of the hydroxyl group using 3,4-
dihydro-2H-pyran (DHP) is recognized as the most popular and
easy protocol. This is because of its several attractive advantages
over other protecting reagents, which include ease of its prep-
aration, stability of the corresponding tetrahydropyranyl ethers
towards various reaction conditions and use of reagents such as
strong bases, hydrides, Grignard reagents and other organo-
metallic reagents.3–5 Moreover, THP ethers can easily be con-
verted to various derivatives, such as suldes, halides,6 esters,7
cyanides and carbonyl compounds8 employing specic
methods, which makes them ideal protecting reagents for
tetrahydropyranylation.
In recent decades, magnetic nanoparticles (MNPs) have
gained enormous attention since they can be easily separated
from the reaction medium by an external magnet, preventing
the use of column chromatography, avoiding loss of product,
enhancing recyclability and thus ameliorating the overall
synthesis process.13 Magnetic Fe3O4 has been investigated the
most due to its potential magnetic nature although bare iron
oxide nanoparticles have several limitations, which include
immediate aggregation into large clusters and rapid exothermic
reactions with oxygen.14,15 However surface modication by
forming a silica coating over the MNP surface can make it
stable, inert, non-toxic, resistant under catalytic conditions as
well as improve the surface functionality due to the availability
of abundant silanol groups (–SiOH), which act as a support for
immobilization of the SO3H function. Owing to the unique
properties of magnetic nanoparticles, sulphuric acid adsorbed
on silica coating MNPs core–shell structures (Fe3O4@SiO2@-
SO3H) have been utilized as a catalyst in various organic
transformations, such as Biginelli reaction,16 synthesis of ami-
doalkyl naphthols17 and protection–deprotection of hydroxyl
Enumerable catalysts including both homogeneous and
heterogeneous catalysts have been reported to date, which have
been proposed to catalyze tetrahydropyranylation of free
hydroxyl groups. Taking into account the advantages of
heterogeneous catalysts over homogeneous catalysts in terms of
Department of Chemistry, National Institute of Technology, Silchar-788010, Assam,
India. E-mail: lalthazualarokhum@gmail.com; rokhum@che.nits.ac.in; Fax: +91
3842 224797; Tel: +91 3842 242915
† Electronic supplementary information (ESI) available: 1H and 13C NMR spectra
of synthesized compounds. See DOI: 10.1039/c7ra12458a
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groups using hexamethyldisilazane (HMDS).18 In this commu- several times with deionized water and ethanol and dried at
nication, we report tetrahydropyranylation/depyranylation of 80 ꢀC for 10 h under vacuum (24 mmHg). The thickness of the
alcohols with sulphuric acid adsorbed on a silica coating MNPs coated silica can be controlled by tuning the experimental
core–shell structure under solvent-free conditions, which parameters.
makes it free from the hazardous effects of organic toxic
solvents, offering a green and eco-friendly approach.
2.2.3 Preparation of SO3H functionalized silica-coated iron
oxide magnetic nanoparticles; Fe3O4 supported silica sulfonic
acid (Fe3O4@SiO2@SO3H). To immobilize the SO3H functional
group onto the surface of Fe3O4@SiO2 nanoparticles, a suction
ask equipped with a constant pressure dropping funnel was
used. The gas outlet was connected to a vacuum system through
an absorbing solution of alkali trap for conducting HCl gas.
Fe3O4@SiO2 was dispersed into the ask containing dry DCM.
Chlorosulfonic acid (ClSO3H) was then added dropwise to the
mixture of Fe3O4@SiO2 and DCM in a cooled ice-bath over
a period of 30 min at room temperature. HCl gas evolved from
the reaction vessel immediately. Aer completion of the addi-
tion, the mixture was stirred and occasionally shaken for 3 h,
while the residual HCl was eliminated by suction. Then, Fe3-
O4@silica sulfonic acid was separated from the reaction
mixture by an external magnet and washed several times with
DCM. Finally, Fe3O4@silica sulfonic acid was dried under
2. Experimental methodology
2.1 Materials and methods
All alcohols, DHP (3,4-dihydro-2H-pyran), silica gel for TLC
(Thin Layer Chromatography) and column chromatography
were of analytical grade and purchased from SpectroChem.
TEOS (tetraethyl orthosilicate) was purchased from Sigma
Aldrich, India. The chemicals were used without further puri-
cation. The solvent used were of extra pure grade, purchased
from Merck India. Double distilled deionized water was used for
the synthesis of magnetic nanoparticles.
For characterization of the catalyst, high resolution trans-
mission electron microscopy (HRTEM) was conducted using an
electron microscope JEM-2100, 200 kV, JEOL (SAIF-NEHU,
Shillong). The samples were dispersed in ethanol, DCM
(dichloromethane) and then treated ultrasonically in order to
disperse individual particles over a copper grid. Powder X-Ray
Diffraction (XRD) patterns were obtained on an X'Pert Pro
PANalytical diffractometer (SAIF-GU, Guwahati) under the
ꢀ
vacuum at 60 C for 24 h (Scheme 1).
2.3 General procedure for the tetrahydropyranylation of
alcohols using Fe3O4@silica sulfonic acid
(Fe3O4@SiO2@SO3H)
ꢀ
following conditions: K-Alpha 1 wavelength (l ¼ 1.54056 A), K-
A mixture of alcohol (1 mmol), DHP (1.2 mmol) and Fe3O4@-
silica sulfonic acid catalyst (0.15 g, 20 mol% of substrate) was
stirred in a small reaction vessel at room temperature. The
reaction progress was monitored by thin layer chromatography.
Aer completion of the reaction, ethyl acetate (10 mL) was
added to the solution and the catalyst was separated by an
external magnet. The solution was concentrated under reduced
pressure and the product was passed through a column of silica
gel eluting with hexane/ethyl acetate (9 : 1) to obtain pure THP
ether with high purity. The catalyst was washed several times
with ethyl acetate and dried under vacuum (24 mmHg) at 60 ꢀC
for 24 h.
ꢀ
Alpha 2 wavelength (l ¼ 1.54439 A), generator voltage of 40 kV,
a tube current of 35 mA and the count time of 0.5 s per 0.02ꢀ in
the range of 5ꢀ–90ꢀ with a copper anode. FTIR spectra were
recorded on a Nicolet 6700, Nicolet Continuum FTIR Micro-
scope (IASST, Guwahati). NMR spectra were recorded on Bruker
Avance II, 400 MHz (SAIF NEHU, Shillong).
2.2 Preparation of catalysts
2.2.1 Preparation of iron oxide magnetic nanoparticles,
Fe3O4 MNPs. Magnetic nanoparticles were synthesized accord-
ing to a reported procedure.19–21 Co-precipitation of Fe(II) and
Fe(III) was carried out by rst dissolving FeCl2$4H2O (1 mmol)
and FeCl3$6H2O (2 mmol) in deionized water under a nitrogen
atmosphere at 90 ꢀC. Ammonium hydroxide solution was added
to the above solution with vigorous mechanical stirring. The
reaction was carried out for 60 min in under N2 atmosphere
until the color of the bulk solution turned black. An external
magnet was used to separate out the resulting black MNPs,
washed 3 times with deionized water and then dried under
2.4 General procedure for the deprotection of alcohols
THP ether (1 mmol) and Fe3O4@silica sulfonic acid catalyst
(20 mol% of substrate) were stirred in methanol (0.5 mL) at
ꢀ
vacuum at 60 C for 12 h.
2.2.2 Preparation of the silica coating of iron oxide
magnetic nanoparticles, Fe3O4@SiO2. Silica coating of Fe3O4
MNPs was carried out using a sol–gel method (Stober method)
that involves hydrolysis of TEOS on the surface of iron oxide
nanospheres. For this, Fe3O4 particles were dispersed in
a mixture of ethanol, deionized water and TEOS, followed by the
addition of 10% NaOH solution. The solution was stirred using
a magnetic stirrer for about 1 h at room temperature. Then, the
Scheme 1 Schematic diagram for synthesis of Fe3O4@silica sulfonic
product was isolated by an external magnet and was washed acid nanocatalyst.
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room temperature for 30 min. The cleavage of THP ethers was
monitored by TLC. Aer completion of the reaction, the catalyst
was separated with the help of an external magnet and washed
with ethyl acetate. The solution was concentrated under
reduced pressure to obtain the corresponding pure hydroxyl
compound in a quantitative yield.
3. Result and discussion
3.1 Catalyst characterization
The synthesized Fe3O4@silica sulfonic acid nanoparticles were
characterized by Fourier Transform Infrared (FT-IR) spectros-
copy, X-Ray Diffraction and Transmission Electron Microscopy
(TEM).
3.1.1 Fourier transform infrared (FT-IR) spectroscopy. The
FT-IR spectra of pure Fe3O4, Fe3O4@SiO2 core–shell, and Fe3-
O4@silica sulfonic acid nanoparticles are shown in Fig. 1(a)–(c),
respectively.
Fig. 2 XRD spectra of (a) Fe3O4, (b) Fe3O4@silica sulfonic acid.
The absorption peak at approximately 579 cmꢁ1 (Fig. 1(a))
corresponds to the stretching vibration of the Fe–O bond17 and
the adsorption of the silica coating on the magnetite surface
was indicated by the band near 1031 and 893 cmꢁ1, which are
assigned to the Si–O stretching vibration (Fig. 1(b)). In addition,
successful sulfonic acid functionalization of the silica layer on
Fe3O4 surface was evidenced by the absorption bands at
1076 cmꢁ1 and 1136 cmꢁ1 related to the stretching of the S–O
bonds. A peak appeared at about 3402 cmꢁ1 due to the
stretching of the OH groups in the SO3H moiety (Fig. 1(c)).
These FT-IR spectra provided evidence of the formation of
a silica shell onto the surface of Fe3O4 and the acid function-
alization of the silica shell.17,18
3.1.2 X-ray diffraction (XRD). The structure of (a) Fe3O4 and
(b) Fe3O4@SiO2 was analyzed by XRD spectroscopy (Fig. 2). The
XRD pattern of the bare Fe3O4 NPs shows diffraction peaks at 2q
values 30.2ꢀ, 35.6ꢀ, 43.4ꢀ, 54ꢀ, 57.3ꢀ and 63.1ꢀ corresponding to
the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) planes. This
suggests an inverse spinal structure for the magnetic crystal and
matched well with the library patterns (JCPDS no. 19-0629). The
peaks for Fe3O4@silica were same as that of Fe3O4 MNPs, which
rmly indicates retention of the crystalline inverse cubic spinel
structure during the silica-coating process. The broad peak
appearing in the range of 2q ¼ 15–30 (Fig. 2) is the characteristic
peak of amorphous silica.
3.1.3 Transmission electron microscopy. The size and
shape of Fe3O4 nanoparticles and the prepared catalyst were
examined by TEM. Fig. 3(a) and (b) show the TEM images of the
Fe3O4 nanoparticles at different magnications. It can be
observed that formation of ‘quasi-spherical’ shaped nano-
particles have taken place (Fig. 3(a)). Magnetite particles of
diameter 17.68 nm can be observed.
Fig. 3 (a) HRTEM images of Fe3O4 nanoparticles, (b) HRTEM images of
Fig. 1 FT IR spectra of (a) Fe3O4, (b) Fe3O4@SiO2 and (c) Fe3O4@silica Fe3O4@SiO2, (c) HRTEM images of Fe3O4@SiO2@SO3H, (d) ED pattern
sulfonic acid.
of Fe3O4@SiO2@SO3H.
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The ne coating of silica over the Fe3O4 nanoparticles can be giving the corresponding protected alcohols in excellent yields
observed from the TEM images shown in Fig. 3(b) and (c), which (Table 2, entries 2 and 3). However, reactions of benzyl alcohols
indicates the retention of the silica coating even aer the substituted with electron-withdrawing groups were, to some
sulfonic acid functionalization on the silica surface.17,19
extent, slower as compared to their electron-donating counter-
Further, the SAED (Selected Area Electron Diffraction) parts (Table 2, entry 5). Although the reaction rates of aliphatic
pattern (Fig. 3(d)) of the Fe3O4 nanoparticles conrms that the alcohols were lower as compared to the benzylic ones, they were
sample is crystalline and also conrms our conclusions from easily converted into their corresponding products in high
the X-ray diffraction pattern.
yields. It was observed that aliphatic alcohols with larger chain
lengths take more time for completion than those with smaller
chain lengths (Table 2, entries 7, 8, and 9). However, secondary
and tertiary alcohols took longer time for complete conversion
(Table 2, entries 11 and 12). Allyl alcohols (Table 2, entries 13
and 14) were converted into the corresponding protected alco-
hols leaving the olenic bonds intact.
3.2 Protection and deprotection of alcohols
In continuation of our ongoing research interest, we applied the
synthesized catalyst in tetrahydropyranylation of various
alcohol substituents. Initially, we tried it with benzyl alcohol
and, to our delight; protection of alcohol was successful under
solvent-free conditions within a very short time of 1 h only.
First, we took an equimolar mixture of benzyl alcohol (1 mmol)
and DHP (1 mmol) and 10 mol% of the nanocatalyst and stirred
the mixture for 1 h. Aer stirring for 1 h, we observed only slight
formation of the product as indicated by TLC. Surprisingly, this
did not increase even aer stirring for another 1 h. Then, we
added additional DHP (0.2 mmol) and found that the reaction
was completed within the next 1 h. Inspired by these ndings,
our next evident target was to optimize the amount of catalyst.
To our satisfaction, we found that 20 mol% of the nanocatalyst
was optimum for the protection of alcohols (Table 1, entry 4).
Encouraged by the result and the advantage of using
magnetic nanocatalysts over soluble homogeneous catalysts, we
carried out the conversion of various alcohols into their pro-
tected products as described in Scheme 2.
Table 2 Tetrahydropyranylation of alcoholsa
Entry Alcohols
Product
Time (min) Yieldb (%)
1
2
45
40
95
95
3
4
35
50
96
92
With the optimum reaction condition in hand, we carried
5
6
7
55
45
45
90
88
87
out the tetrahydropyranylation of
a variety of primary,
secondary, tertiary, benzylic and allylic alcohols which were
smoothly converted to the corresponding desired products in
good to excellent yields at room temperature. It was observed
that benzylic alcohols substituted with electron-donating
groups are highly reactive under our reaction condition,
8
9
50
55
85
83
Table 1 Optimization of the catalysta
Entry
Catalyst amount (mol%)
Time (min)
Yield (%)
10
11
65
75
84
82
1
2
3
4
5
5
10
15
20
25
150
120
55
45
45
65
85
90
95
95
a
Reaction conditions: benzyl alcohol (1 mmol), DHP (1.2 mmol), room
temperature, solvent-free condition.
12
13
90
50
80
84
14
50
86
a
Reaction condition: alcohol (1 mmol), DHP (1.2 mmol), catalyst
(20 mol%), room temperature, solvent-free conditions. b Isolated yield.
Scheme 2 Tetrahydropyranylation of alcohols using 20 mol% of
nanocatalyst.
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Table 3 Competitive reactions of different binary mixturesa
Entry
Alcohols
Products
Time (min)
% Conversion
95
5
Fig. 4 Recyclability test of nanocatalyst for protection/deprotection
of alcohols.
1
45
The mechanism proposed for the tetrahydropyranylation of
alcohols catalyzed by Fe3O4@SiO2@SO3H is shown in Scheme 3.
100
0
2
3
45
60
60
3.3 Recycling of the catalyst
The recyclability of the magnetic nanocatalyst was investigated
with consecutive tetrahydropyranylation reactions using
different substrates. Fig. 4 depicts the results of consecutive
runs performed by reusing the catalyst under our optimal
reaction conditions. Aer each catalytic run, the catalyst was
94
6
100
Table 4 Reusability test of the catalyst under optimized reaction
4
conditionsa
0
Entry
1
Products
Run/yield %
a
Reaction condition: alcohol (1 mmol), DHP (1.2 mmol), catalyst
(20 mol%), room temperature, solvent-free conditions.
Run
Yield (%)
1
96
2
96
3
94
4
94
5
93
Run
1
2
3
4
5
Yield (%)
90
89
89
89
88
2
3
In order to gain more insights into the selectivity of our
method, we studied various competitive reactions between
structurally different alcohols in binary mixtures as shown in
Table 3. This study reveals that benzylic alcohols can be con-
verted into their protected products with excellent selectivity in
the presence of aliphatic alcohols (Table 3, entry 1). Interest-
ingly, benzylic alcohols were converted quantitatively into their
protected alcohols, while the secondary alcohol remained intact
(Table 3, entry 2).
Run
Yield (%)
1
88
2
88
3
86
4
86
5
84
a
Reaction condition: alcohol (1 mmol), DHP (1.2 mmol), catalyst
(20 mol%), room temperature.
Table 5 Deprotection of THP ethers
Allyl alcohols also reacted faster as compared to their
aliphatic counterparts, affording the allyl products in high yield
(Table 3, entry 3). Similarly, primary alcohols can also be con-
verted to their corresponding THP ether in the presence of
tertiary ones with complete selectivity (Table 3, entry 4).
Entry
1
Alcohols
Product
Time (min)
25
Yielda (%)
95
2
30
92
3
4
25
20
95
90
5
30
90
a
Scheme 3 Proposed mechanism for the tetrahydropyranylation of
alcohols using Fe3O4@SiO2@SO3H nanocatalyst.
Reaction condition: THP ether (1 mmol), methanol (0.5 ml), catalyst
(20 mol%), room temperature.
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5.2 2-(4-Methoxybenzyloxy)-tetrahydro-2H-pyran (Table 2,
entry 3)
1H-NMR (400 MHz, CDCl3, TMS): d 7.26–7.28 (d, J ¼ 8 Hz, 2H),
6.84–6.86 (d, J ¼ 8 Hz, 2H), 4.66–4.71 (m, 2H), 4.43 (d, J ¼ 4 Hz,
1H), 3.90 (t, J ¼ 4 Hz, 1H), 3.74 (s, 3H), 3.53 (t, J ¼ 4 Hz, 1H),
1.49–1.83 (m, 6H); 13C-NMR (100 MHz, CDCl3, TMS): d 159.24,
130.37, 129.56, 113.81, 97.49, 68.53, 62.11, 55.21, 30.70, 25.61,
25.57, 19.50.
5.3 Tetrahydro-2-(phenethyloxy)-2H-pyran (Table 2, entry 6)
1H-NMR (400 MHz, CDCl3, TMS): d 7.13–7.25 (m, 4H), 4.56 (t, J ¼
3.6 Hz, 1H), 3.38–3.95 (m, 4H), 2.88 (t, J ¼ 7.2 Hz, 2H), 1.42–1.82
(m, 7H); 13C-NMR (100 MHz, CDCl3, TMS): d 139.15, 128.76,
126.25, 98.614, 68.291, 62.8, 39.38, 36.446, 30.712, 25.546,
19.63.
Scheme 4 Plausible mechanism for the deprotection/dehydropyr-
anylation of THP ether.
recovered by an external magnet, washed with ethyl acetate and
ꢀ
then dried at 100 C in Abderhalden apparatus under reduced
pressure overnight before being used again in a new reaction.
We demonstrated that no depreciations in the catalytic perfor-
mance were observed in all the test reactions even aer ve
catalytic cycles. In addition, no difference in the IR spectrum of
the catalyst was observed aer these repeated cycles (Table 4).
5.4 2-(2-Ethylhexyloxy)-tetrahydro-2H-pyran (Table 2,
entry 7)
1H-NMR (400 MHz, CDCl3, TMS): d 4.51–4.52 (t, J ¼ 4 Hz, 1H),
3.58–3.81 (m, 2H), 3.18–3.56 (m, 2H), 1.48–1.77 (m, 7H), 1.24–
1.29 (m, 8H), 0.82–0.86 (m, 6H); 13C-NMR (100 MHz, CDCl3,
TMS): d 99.04, 98.93, 70.4, 62.08, 39.73, 39.66, 30.79, 30.61,
29.17, 29.13, 25.62, 23.94, 23.14, 19.57, 14.13, 11.17, 11.06.
3.4 Deprotection of THP ether
The deprotection of THP ethers was investigated by changing
the solvent system. We found that the addition of methanol
serves as an efficient deprotecting reagent for THP ethers in the
presence of Fe3O4@silica sulfonic acid as a catalyst at room
temperature to provide the corresponding free alcohols in an
excellent yield of 90–95% (Table 5) irrespective of the structural
variations. It is interesting to note that the deprotection of all
the compounds reported in the present study was completed
within 30 min. The proposed mechanism for the deprotection
of THP ether is shown in Scheme 4.
5.5 2-(8-Methylnonyloxy)-tetrahydro-2H-pyran (Table 2,
entry 8)
1H-NMR (400 MHz, CDCl3, TMS): d 4.572 (s, 1H), 3.72–3.88 (m,
2H), 3.49 (d, J ¼ 12 Hz, 2H), 1.40–1.84 (m, 19H), 1.07–1.33 (m,
10H); 13C-NMR (100 MHz, CDCl3, TMS): d 19.45, 22.74, 25.26,
26.68, 27.14, 27.80, 29.24, 29.81, 30.62, 41.04, 62.13, 67.38, 98.47.
5.6 2-(Octyloxy)-tetrahydro-2H-pyran (Table 2, entry 9)
1H-NMR (400 MHz, CDCl3, TMS): d 4.54 (t, J ¼ 4 Hz, 1H), 3.83–
3.70 (m, 1H), 3.69–3.66 (m, 1H), 3.51–3.47 (m, 1H), 3.35–3.32
(m, 1H), 1.78–1.55 (m, 1H), 1.53–1.51 (m, 1H), 1.51–1.48 (m,
6H), 1.32–1.23 (m, 11H), 0.85 (t, J ¼ 8 Hz, 3H); 13C-NMR (100
MHz, CDCl3, TMS): d 98.87, 67.74, 62.34, 31.91, 30.84, 29.82,
29.53, 26.32, 25.58, 22.73, 19.74, 14.16.
4. Conclusions
Protection/deprotection is one of the most frequently applied
synthetic strategies by organic chemists; discovery of more
green and efficient heterogeneous catalysts for these reactions
has been a fundamental necessity. The tetrahydropyranylation
of alcohols using our reported magnetic Fe3O4@silica sulfonic
acid nanocatalyst offers a green, mild, less toxic, stable, and
solvent-free process of protection/deprotection reaction with
easy recoverability of the catalyst. In short, our method is highly
economical and environmentally benign.
Conflicts of interest
There are no conicts of interest to declare.
Acknowledgements
The authors gratefully acknowledge SERB, New Delhi for
nancial support (Grant No. SB/FT/CS-103/2013 and SB/EMEQ-
076/2014).
5. Spectral data
5.1 2-((4-Methylbenzyl)oxy)-tetrahydro-2H-pyran (Table 2,
entry 2)
1H-NMR (400 MHz, CDCl3, TMS): d 7.27 (d, J ¼ 8 Hz, 2H), 7.18 (d,
J ¼ 8 Hz, 2H), 4.79–4.71 (m, 2H), 4.50 (d, J ¼ 12 Hz, 1H), 3.95–
3.92 (m, 1H), 3.58–3.55 (m, 1H), 2.36 (s, 3H), 1.91–1.67 (m, 1H),
1.58–1.54 (m, 5H); 13C-NMR (100 MHz, CDCl3, TMS): d 137.28,
135.34, 129.16, 128.09, 97.64, 68.27, 62.17, 30.71, 25.64, 21.29,
19.49.
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