MCM-41-supported phosphotungstic acid-catalyzed cleavage of C-O bond in allyl aryl ethers

Article abstract of DOI:10.1039/c7nj00375g

Removal of the protecting allyl group from allyl aryl ethers in the presence of other oxygen protecting groups was successfully achieved using a solid acid supported on the high surface area material MCM-41. The catalyst showed excellent activity in the presence of various electron withdrawing, electron donating, and oxidizable functional groups. The methodology is also very useful for the removal of protecting allyl groups of various natural products such as vanillin, isovanillin, and other oxygen functionalized aldehydes and ketones.

Full text of DOI:10.1039/c7nj00375g

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MCM-41-supported phosphotungstic acid-catalyzed
cleavage of C–O bond in allyl aryl ethers†
Cite this:DOI: 10.1039/c7nj00375g
c
Sachin S. Sakate,ab Sumit B. Kamble,a Rajeev C. Chikate
and
a
Chandrashekhar V. Rode
*
Removal of the protecting allyl group from allyl aryl ethers in the presence of other oxygen protecting
groups was successfully achieved using a solid acid supported on the high surface area material
MCM-41. The catalyst showed excellent activity in the presence of various electron withdrawing,
electron donating, and oxidizable functional groups. The methodology is also very useful for the
removal of protecting allyl groups of various natural products such as vanillin, isovanillin, and other
oxygen functionalized aldehydes and ketones.
Received 31st January 2017,
Accepted 10th May 2017
DOI: 10.1039/c7nj00375g
rsc.li/njc
in the presence of H+ or Hg2+ with strong bases such as KOtBu-
DMSO21 or transition metal catalysts such as the Wilkinson
Introduction
Reactive oxygen in phenols is commonly protected to form catalyst.22 The conventional methods of deprotection of allyl
stable allyl aryl ethers in order to carry out various transformations aryl ethers combine a transition-metal-catalyzed isomerization
in the presence of the allyloxy group in organic syntheses which (Pd, Rh or Ir) of the double bond to the enol ether and
include synthesis of glycopeptides,1a,b three-component Hosomi– subsequent hydrolysis of the latter to furnish the corresponding
Sakurai reaction,2 isatin and isatin ketoimine allylation by alcohol.23 Removal of a protecting allyl group is also reported
Sakurai–Hosomi reaction,3 dendrimer synthesis etc.4 The allyl using palladium catalysts under reductive conditions.24
group has been frequently used in organic synthesis as a
Phosphotunstic acid (PWA) with unique oxygen anion
protecting group for alcohols and amines, owing to its stability clusters and well-defined structure (Keggin ion) are regarded as
under basic and acidic conditions.5 Some biologically important environmentally benign and economically feasible alternatives to
molecules, the synthesis of which involves selective allylic
protection and deprotection strategies, are shown in Fig. 1.
Therefore, it is necessary to develop methodologies that afford
high yielding deprotection under mild reaction conditions by
using catalytic as well as non-catalytic methods. A broad spectrum
of reagents has been employed for the deprotection of allyl
ethers in the literature. These include N-bromosuccinimide
(NBS),6 Cp2Zr,7 SmCl3,8 SmI2/H2O/Et3N,9 TiCl3,10 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ),11 NaBH4/I2,12 CeCl3Á7H2O/
NaI,13 SmCl3,14 NaI/Me3SiCl,15 Pd(PPh3)4 with NaBH4,16 Bu3SnH17
and NiCl2(dppp)/DIBALH.18 The single electron-transfer agent
SmI2 has received much attention for its reducing properties both
in selective reductions and in reductive couplings.19 Removal of a
protecting allyl group typically involves a two-step sequence in
which isomerization of the double bond to the corresponding
prop-1-enyl ether is followed by hydrolysis or oxidative cleavage20
a CSIR-National Chemical Laboratory, Dr Homi Bhabha road, Pashan,
Pune 411008, India. E-mail: cv.rode@ncl.res.in
Fig. 1 Some selectively deallylated and allylated biologically important
compounds (a) (tert-butoxycarbonyl)-D-tyrosine, (b) 2-(7-hydroxy-2,2-
diphenylbenzo[d][1,3]dioxol-5-yl)acetonitrile, (c) (3aR,5R,6S,6aR)-6-(allyloxy)-
5-((R)-2,2-dimethyl-1,3-dioxolan-4-yl)-2,2-dimethyltetrahydrofuro[2,3-d][1,3]-
dioxole, (d) (1S,2R,5S)-2-isopropyl-5-methylcyclohexan-1-ol.
b P. E. Society’s Modern College of Arts, Science and Commerce, Shivajinagar,
Pune 411005, India
c M. E. Society’s Abasaheb Garware College, Karve road, Pune 411004, India
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj00375g
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the conventional acid catalysts, owing to their Brønsted acidity, high
proton mobility, non-toxicity and stability.25–28 They have excellent
catalytic behavior in a wide variety of acid-catalyzed reactions
including alkylation and acylation of hydrocarbons, hydration of
alkenes and polymerization of THF.29–31 PWA-catalyzed reactions
require extra steps such as extraction or distillation to separate PWA
from the reaction medium for the next catalytic cycle. Additionally,
the specific surface area of bulk PWA is extremely small (o5 m2 gÀ1),
which limits their inherent catalytic activity owing to the low number
of available acidic sites. To resolve these issues, effective strategies
can be easily applied by dispersing HPAs on a suitable support
having a very high surface area.32–34 In continuation of our efforts in
this direction, we report here MCM-41-supported phosphotungstic
acid (PWA) as a heterogeneous catalyst for the deallylation of
allyl aryl ethers.
Results and discussion
Catalyst characterization
Fig. 3 Wide angle XRD patterns of (a) PWA, (b) MCM-41, (c) 10% PWA/
MCM-41, (d) 20% PWA/MCM-41.
The prepared MCM-41 was well characterized with various
techniques. Surface areas of PWA, MCM-41, 10% PWA/MCM-41
and 20% PWA/MCM-41 were found to be 5 m2 gÀ1, 1127 m2 gÀ1
,
946 m2 gÀ1 and 712 m2 gÀ1 respectively, which were in con-
currence with the loading of the PWA on the support MCM-41.
The low angle X-ray diffraction (XRD) patterns of pure MCM-41
and 20% PWA/MCM-41 samples in Fig. 2 showed that the pure
MCM-41 sample (Fig. 2(a)) gave three ((100), (110) and (200))
diffraction peaks below 101 (2y) which indicated the formation
of an ordered hexagonal mesoporous structure.35,36 Interestingly,
20% PWA/MCM-41 Fig. 2(b) samples showed XRD peaks similar
to those of parent MCM-41; however, the intensity of the (100)
reflection peak of the MCM-41 diminished with an increase in
PWA loading, which could be due to beginning of disorder in the
hexagonal structure of MCM-41.37
Wide angle X-ray diffraction patterns of bulk PWA, MCM-41
and PWA/MCM-41 are shown in Fig. 3. The parent MCM-41 in
Fig. 3(b) shows a broad band in the range of 2y = 15–401, which
is a characteristic of siliceous material.38 PWA/MCM-41 samples
(10–20% PWA loading on MCM-41) showed XRD patterns as in
Fig. 3(c and d) similar to that of the parent MCM-41. No separate
crystal phase characteristic of bulk PWA (Fig. 3(a)) existed in
10% and 20% PWA/MCM-41 samples owing to the larger surface
area of MCM-41. This also confirmed the high dispersion of
PWA on MCM-41.
The type of acidic sites in the samples containing PWA in a
range from 10% to 20% was identified by pyridine Fourier
transform infrared spectroscopy (FTIR) (Fig. 4). The band in the
region of 1540–1545 cmÀ1 (pyridinium ion) was assigned to
Brønsted acid sites, while the band at 1450 cmÀ1 was assigned
to the Lewis acid sites. The band at about 1490 cmÀ1 (hydrogen
bonded pyridine) was common to both types of acidic sites.39,40
The presence of PWA supported on MCM-41 was also con-
firmed by 31P-NMR spectroscopy. Pure PWA showed a peak at
À14.27d, which was in very good concurrence with the literature
Fig. 2 Low angle XRD patterns of (a) MCM-41, (b) 20% PWA/MCM-41.
Fig. 4 Pyridine IR spectra of (a) PWA, (b) MCM-41, (c) 20% PWA/MCM-41.
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acetonitrile (Table 1, entry 5) and 65% yield in dioxane at
110 1C with 10 h reaction time (Table 1, entry 6). This clearly
indicates that a Brønsted acidic catalyst was necessary for
deallylation of the allyl aryl ether. Since PWA alone has sig-
nificant solubility in the reaction medium, PWA was dispersed
on MCM-41 and then the deallylation reaction was explored. To
start with, PWA with minimum loading of 10% on MCM-41
was evaluated, which gave 42% yield of phenol at 110 1C in
1,4-dioxane (Table 1, entry 9). This was somewhat lower than
that obtained for PWA alone (Table 1, entry 6), because of the
very low (10%) PWA concentration, as in both the cases the
amount of catalyst charged was the same (100 mg). When PWA
loading was increased from 10 to 20% on MCM-41, yield of the
product was significantly enhanced to 78% (Table 1, entry 13),
which was higher than that with neat PWA. Thus, as compared
with bulk PWA, the well dispersed PWA on MCM-41 allowed
better exposure of active sites available for the reaction. Further
increase in PWA loading to 30% did not affect the product yield
(Table 1, entry 16).
Fig. 5 31P CP-MAS NMR spectra of PWA (phosphotungstic acid) and 20%
PWA/MCM-41.
value.41 Similarly, 31P cross-polarization magic angle spinning
(CP-MAS) NMR spectra of 20% PWA/MCM-41 showed a peak at
À14.19d value which confirmed the presence of the PWA on the
MCM-41. A very low shielding effect was evidenced by the shift of
31P from À14.27d to À14.19d (Fig. 5).
Substrate scope
After arriving at the standard reaction conditions for the
deprotection of allyl aryl ethers using PWA supported on
MCM-41, a variety of aryl allyl ethers having varying electronic
features were screened for the deallylation reaction and the
results are shown in Table 2. Initially, allyl aryl ethers with electron
donating substituents like monomethyl, dimethyl and tertiary butyl
were deallylated to yield the corresponding phenols in 70–79% yield
(Table 2, entries 1–5). Similarly, allyl aryl ethers substituted with
various acid sensitive functional groups like aldehydes and ketones
also underwent deallylation but with lower yields of the corres-
ponding phenols (61–67%) owing to the electron withdrawing effect
of the carbonyl groups (Table 2, entries 6–8). Chloro substituted aryl
allyl ethers were also deprotected to their respective phenols with
75–78% yields (Table 2, entries 9 and 10).
Catalyst and solvent screening for deallylation of allyl aryl
ethers
Initial runs for the deprotection reaction with parent MCM-41
under reflux conditions in various solvents (60–110 1C) even
after 24 h did not show any results (Table 1, entries 1–3). PWA
alone when used for the deprotection gave 45% yield in
Table 1 Optimization of reaction conditions for deallylation of allyl aryl
ethers
Allyl aryl ethers with strong electron withdrawing groups like
nitro (–NO2) did not undergo the deallylation because of the
restriction of the protonation of the allyloxy oxygen of the ethers
(Table 2, entries 11 and 12). Highly important prenyl aryl ethers
underwent smooth deprenylation to give 70–72% yields of phenols
under our standard reaction conditions (Table 2, entries 13 and 14).
A plausible reaction pathway for the cleavage of allyl aryl
ethers has been proposed (Scheme 1), in which proton transfer
from PWA to allyl aryl ethers is a key step (I). The protonated
ether is then further attacked by the PWA anion (II) yielding
C–O cleavage and converting the ether into the parent phenol
(III). The allyl group is further converted into propene or the
allyl alcohol (IV) and the catalyst is regenerated for further
catalytic cycles.
Entry
Catalyst
MCM-41
Solvent
Temp. (1C)
% Yielda
1
2
3
Ethanol
Acetonitrile
1,4-Dioxane
60
84
110
NR
NR
NR
4
5
6
PWAb
Ethanol
Acetonitrile
1,4-Dioxane
60
84
110
NR
45
65
7
8
9
10% PWA/MCM-41
20% PWA/MCM-41
Ethanol
Acetonitrile
1,4-Dioxane
60
84
110
NR
20
42
10
11
12
13
Ethanol
Acetonitrile
Toluene
60
84
110
110
NR
22
NR
78
1,4-Dioxane
Recycling study for the deallylation of allyl aryl ether
14
15
16
30% PWA/MCM-41
Ethanol
Acetonitrile
1,4-Dioxane
60
84
110
NR
22
80
The efficiency of 20% PWA/MCM-41 catalyst was established by
recycling studies (Fig. 6). After completion of the first run,
catalyst was separated by filtration, washed several times with
1,4-dioxane dried at 120 1C and then used for three subsequent
reactions. Although, the conversion decreased from 78 to 67%
a
b
All the reported yields are GC % yields. Catalyst (20 mg). Reaction
conditions: 1-allyloxy-4-(tert-butyl)benzene (1 mmol, 0.190 g), catalyst
(100 mg), solvent (5 mL), reflux, 10–12 h. NR, no reaction.
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Table 2 Deallylation of allyl aryl ethers
Table 2 (continued)
Entry
1
Reactant
Product
% Yielda
Entry
10
Reactant
Product
% Yielda
74
75
2
3
72
70
11
12
NR
NR
13
14
72
4
5
79
75
70
72
15
6
7
65
67
a
Isolated yields are reported. Reaction conditions: allyl aryl ether
(1 mmol), 20% PWA/MCM-41 (0.100 g), 1,4-dioxane (5 mL), temp.
110 1C.
for the third use, the reaction crude did not show any leaching
of PWA as confirmed by the hot filtration test (Fig. 7). Thus the
catalyst was quite stable under the reaction conditions; however, the
decrease in activity could be due to handling losses of the catalyst
after each run, as the initial catalyst charges was only 100 mg.
8
9
61
78
Experimental section
Materials and methods
Cetyltrimethylammonium bromide (CTAB) and tetraethyl ortho-
silicate (TEOS) were purchased from Aldrich Chemicals, Bangalore,
India. Phosphotungstic acid and H2SO4 were purchased from
Thomas Baker, India. Solvents 1,4-dioxane and acetonitrile were
purchased from Chemlabs, Bangalore, India. All chemicals were
used as received without any further purification.
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Ni-filtered Cu Ka radiation (l = 0.154 nm) over the range
0.5–101 (2y), with a scan rate of 21 per minute. Wide angle X-ray
diffraction (WAXRD) patterns were recorded on a PANalytical
PXRD model X-Pert PRO-1712, using Ni filtered Cu Ka radiation
(l = 0.154 nm) as source (current intensity, 30 mA; voltage, 40 kV)
and an Xcelerator detector. Nitrogen adsorption/desorption iso-
therms were measured on a Quantachrome Autosorb-1C sorption
unit. The Brunauer–Emmett–Teller (BET) method was used to
calculate the multipoint BET surface area. IR spectra of the PWA
impregnated MCM-41 molecular sieves were collected with an
FTIR (Spectrum 2000 PerkinElmer) instrument using KBr as
background. The IR spectrum was recorded in the range 4000–
400 cmÀ1. Solid state NMR was recorded on a JEOL 400 instrument
as appropriate.
Synthesis of MCM-41
2.4 g CTAB, 100 g distilled water and 6.2 mL EAl (70 wt%) were
mixed together at ambient temperature. After a homogeneous
mixture was formed, 10 mL tetraethyl orthosilicate (TEOS) was
added dropwise under stirring. The mixture was stirred for a
further 4 h and was then placed in a PTFE-lined stainless steel
autoclave which was subsequently heated to 100 1C for 48 h.
The product was recovered by filtration, washed with distilled
water and air dried. The template was removed by calcination
of the as-synthesized sample at 550 1C in air for 6 h, with a
heating rate of 11 minÀ1 from room temperature to 550 1C.
Scheme 1 Plausible mechanism for the deallylation of the allyl aryl ethers.
Preparation of PWA/MCM-41 catalysts
A series of PWA (10–30 wt%) impregnated MCM-41 catalysts
were prepared by a wet impregnation method. In a typical
procedure, the calculated quantity of PWA was dissolved in
40 mL of methanol, which was stirred for 5 min, followed by the
slow addition of MCM-41 to the solution and kept for 6 h at
room temperature under stirring. Then, solvent was evaporated on a
Rotavapor and finally the catalysts were calcined at 300 1C for 3 h.
Fig. 6 Catalyst recycling study. Reaction conditions: allyl aryl ether
(1 mmol), 20% PWA/MCM-41 (0.100 g), 1,4-dioxane (5 mL), temp. 110 1C.
General experimental procedure for deallylation/deprenylation
In a typical experimental procedure, allyl aryl ether/aryl prenyl
ether (1 mmol) was dissolved in 1,4-dioxane (5 mL) and catalyst
(100 mg) was added into the solution. The reaction mixture was
then refluxed at 110 1C for 8–10 h. After completion of reaction,
monitored using thin layer chromatography (TLC), the reaction
mixture was filtered and the catalyst was washed with 10 mL of
1,4-dioxane. After evaporation of solvent, the product was
purified by using column chromatography, and isolated yields
are reported.
Conclusions
In conclusion, the deprotection of allyl/prenyl aryl ethers was
successfully achieved over a heterogeneous PWA/MCM-41 catalyst
under mild reaction conditions. Disordered hexagonal mesoporous
structure of MCM-41 in 20% PWA/MCM-41 was observed by low
Fig. 7 Hot filtration test (leaching test).
Low angle powder X-ray diffraction (LAXRD) patterns were angle XRD (LAXRD), which also confirmed the homogeneous
recorded on a Rigaku D MAX III VC diffraction system using dispersion of PWA, and later characterized by wide angle XRD
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(WAXRD). Retention of the PWA on the MCM-41 was also
confirmed by 31P CP-MAS solid state NMR study. The bands at
1486 and 1536 cmÀ1 in pyridine FTIR analysis evidenced the
Brønsted acid sites of the catalyst responsible for its high activity
in the deallylation reaction. Allyl aryl ethers with different
substituents having various electronic effects were selectively
deprotected over 20% PWA/MCM-41 catalyst. Deprenylation of
various substrates was also studied using similar reaction
conditions. Our catalyst (20% PWA/MCM-41) could be efficiently
recycled at least up to three times with marginal decrease in the
yields while leaching of the PWA from MCM-41 was not
observed by the hot filtration test.
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Acknowledgements
One of the authors, Sachin S. Sakate, working at the P. E. Society’s
Modern College of Arts, Science and Commerce, Shivajinagar,
Pune-411005, is grateful to UGC-Delhi for the award of an FIP
(Faculty Improvement Programme) fellowship to carry out this
research work at CSIR-National Chemical Laboratory, Pune, India.
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