Highly efficient triphenyl(3-sulfopropyl)phosphonium functionalized phosphotungstic acid on silica as a solid acid catalyst for selective mono-allylation of acetals

Article abstract of DOI:10.1039/c5cy00531k

Silica supported phosphotungstic acid functionalized with triphenyl(3-sulfopropyl)phosphonium (PW-Si/TPSP) was developed as a solid acid catalyst for C-C bond formation via Hosomi-Sakurai allylation of acetals. Functionalization of PW as well as its bindi

Full text of DOI:10.1039/c5cy00531k

Catalysis
Science &
Technology
View Article Online
View Journal
PAPER
Highly efficient triphenylIJ3-sulfopropyl)phosphonium
functionalized phosphotungstic acid on silica as a
solid acid catalyst for selective mono-allylation of
acetals†
Cite this: DOI: 10.1039/c5cy00531k
*
Sumit B. Kamble, Suhas H. Shinde and Chandrashekhar V. Rode
Silica supported phosphotungstic acid functionalized with triphenylIJ3-sulfopropyl)phosphonium (PW-Si/
TPSP) was developed as a solid acid catalyst for C–C bond formation via Hosomi–Sakurai allylation of ace-
tals. Functionalization of PW as well as its binding to silica was confirmed by solid state 31P-NMR and 29Si-
NMR, respectively. Among the various catalysts prepared, the 30% PW loaded (30PW-Si/TPSP) catalyst gave
an excellent yield of homoallyl ethers (HAEs) via selective mono-allylation of acetals with allyl-
trimethylsilane. A plausible reaction pathway has also been proposed in which the strong Brønsted acid
sites of 30PW-Si/TPSP play an important role in activating the acetals to form the corresponding oxonium
cations. The versatility of our catalyst was demonstrated for the allylation of a wide variety of acetals while
its stability was established in five successful recycling runs.
Received 11th April 2015,
Accepted 28th May 2015
DOI: 10.1039/c5cy00531k
www.rsc.org/catalysis
gave excellent activity but again compromising the recovery
of the catalyst.23 Silicomolybdic acid supported on silica facil-
Introduction
Acid catalyzed Hosomi–Sakurai allylation is one of the most
important carbon–carbon bond forming reactions which is a
key step in natural product synthesis, e.g. concise synthesis of
nematocidal oxylipid1 and heliespirone A and C having inhib-
itory activity in the coleoptile bioassay (Fig. 1).2 Homoallyl
ethers (HAEs) synthesized by the reaction of acetals with allyl-
trimethylsilane are also used as starting materials for many
organic transformations.3a–e
Initially, Hosomi and Sakurai reported the allylation of
acetals with titanium chloride as a catalyst.4 Thereafter, con-
ventional Lewis acids in stoichiometric or catalytic quantities
were studied which included TMSNTf2,5 ScIJOTf)3,6
diphenylboryl triflate,7 TMSOTf,8a–c BiIJOTf)3,9 AlBr3/CuBr,10
TMSI,11 NbCl5/AgClO4,12 CuBr/microwave,13 liquid SO2,14
trityl perchlorate,15 FeCl3,16 and BiBr3,17 AlCl3, BF3·Et2O,18,19
TiCp2IJCF3SO3)2,20 and TMSNIJSO2F)2.21 All these catalysts suf-
fered from cumbersome work-up procedures due to
unwanted salt formation. Exceptionally, a few Brønsted acids
have been studied for the allylation of acetals such as sulfo-
nimides with good yields but about 50% loss of the catalyst
was observed up to 3rd recycling run and simultaneously
needed aqueous work-up.22 List et al. screened different sul-
fonic acids among which 2,4-dinitrobenzenesulfonic acid
itated the allylation of acetals and aldehydes to HAEs but the
activity diminished to 15% in the 3rd recycling run.24 It is
very important for a greener approach that the catalyst should
be stable for subsequent reuse for several times without
compromising the activity under the reaction conditions. Thus,
there exists a scope to design a catalyst for allylation of acetals
which does not undergo any leaching of the active component
and can be recycled several times without losing its activity.
Heteropolyanion based 1-methyl-3-IJ3-sulfopropyl)imidazolium,
1-IJ3-sulfopropyl)pyridinium and triphenylIJ3-sulfopropyl)phosphonium
(TPSP) salts were well reported for acid-catalyzed esterification
due to their self-induced phase separation properties.25–27
Despite their self-separation nature, synthetic utility of these
catalysts for new organic transformations is restricted due to
Chemical Engineering & Process Development Division, CSIR-National Chemical
Laboratory, Pune 411008, India. E-mail: cv.rode@ncl.res.in
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy00531k
Fig. 1 Nematocidal oxylipid and heliespirones.
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
their high solubility in polar solvents like alcohols, acetoni-
trile, etc. Therefore, more efforts are desired to develop new
strategies for these catalytic systems to render them truly
heterogeneous for their easy and wide range of applications
in transformations of practical significance.
In the continuation of our previous work on the heteroge-
neous povidone (PVP) based phosphotungstic acid catalyst
system,28 we report here the silica supported triphenylIJ3-
sulfopropyl)phosphonium salt of phosphotungstic acid (PW-
Si/TPSP) as a novel heterogeneous and recyclable acid catalyst
for the selective mono-allylation of acetals by the Hosomi–
Sakurai reaction (Scheme 1).
These catalysts were prepared by sequential loading of
phosphotungstic acid IJPW/H3PW12O40) in the range of 10–
30% on silica IJw/w) followed by abstraction of protons by
TPSP (Scheme 2).
The catalysts were well characterized and their activities
were tested for allylation of acetals with allyltrimethylsilane
to synthesize homoallyl ethers. New motifs were synthesized
after the selective mono-allylation of o-allylated phenolic ace-
tals which can be further extended to polymer synthesis.
Again, due to the well-defined solid acid nature, these cata-
lysts will be useful for important acid-catalyzed organic trans-
formations to resolve the problems of conventional acid sys-
tems like recovery, solubility, recyclability etc.
Scheme
(30PW-Si/TPSP).
2
Synthesis of silica-supported PW-functionalized TPSP
merged), respectively.27 Thus, the IR study provided clear evi-
dence of structural characterization of the phosphonium cat-
ion and sulfonic acid and retention of the Keggin unit pres-
ent in the prepared catalysts.
The Keggin structure of the catalysts was also confirmed
by XRD as shown in Fig. 3. The broad peak at 2θ = 21.71° was
attributed to silica showing its amorphous nature (Fig. 3, A)
while the sharp peaks at 2θ = 20.3°, 26.3°, 28.38° and 36.27°
were assigned to the Keggin structure of PW (Fig. 3, B).31,32
These characteristic reflection peaks were also observed
for 30PW-Si at 2θ = 20.8°, 26.1°, 28°, and 35° confirming the
presence of well-dispersed PW on silica (Fig. 3, C). Among
the four peaks, two small peaks at 2θ = 21.38° and 26.1° for
the 30PW-Si/TPSP catalyst sample also indicated that the
Keggin structure was retained but intensities of the peaks
decreased due to the exchange of protons with TPSP cations
as evidenced by the decrease in crystallite size of the cubic
Results and discussion
Catalyst characterization
Fig. 2 shows the FT-IR spectra of PW-TPSP (A), 30PW-Si/TPSP
(B) and recovered 30PW-Si/TPSPR (C) catalyst samples. Four
peaks observed at 1075 cm−1 (P–O), 972 cm−1 (WO) (termi-
nal), 892 cm−1 (W–Ob–W) (corner-sharing) and 807 cm−1 (W–
Oc–W) (edge-sharing) exactly matched with those of parent
PW confirming the intact Keggin unit of phosphotungstate in
PW/TPSP (Fig. 2, A).29,30 However, a slight blue shift was
observed for the peaks of 30PW-Si/TPSP and recovered 30PW-
Si/TPSPR samples (Fig. 2, B and C) which confirmed the
interaction between silica and the TPSP salt. The poorly visi-
ble peaks at 1640 cm−1, 1587 cm−1 and 1485 cm−1 could be
due to the stretching vibrations of CC, in accordance with
the aromatic C–H bending vibrations as evidenced by the
presence of peaks at 725 cm−1 and 687 cm−1. The peaks due
to asymmetric and symmetric stretching of OSO
appeared at 1193 cm−1 and 1037 cm−1, respectively. P–Ar and
P–C bond formation with the phosphonium cation was con-
firmed by the peaks at 1437 cm−1 and 742 cm−1 (broad,
Scheme 1 Hosomi–Sakurai mono-allylation of acetals using 30PW-Si/
TPSP.
Fig. 2 FT-IR of (A) PW-TPSP, (B) 30PW-Si/TPSP and (C) 30PW-Si/TPSPR.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
Fig. 3 X-ray diffraction analysis of (A) silica, (B) PW, (C) 30PW-Si and
(D) 30PW-Si/TPSP.
heteropolyanion resulting in shrinking of lattice cells
(Fig. 3, D).33a,b,34
Fig. 4 (a) Solid-state MAS CP-29Si NMR of (A) 30PW-Si and (B) 30PW-
Si/TPSP samples. (b) Solid-state MAS CP-31P NMR of (A) TPSP, (B) Si/
TPSP, (C) 30PW-Si/TPSP and (D) 30PW-Si samples.
The solid-state magic-angle spinning CP-29Si-NMR study
revealed the incorporation of PW onto the silica surface
(Fig. 4a) while CP-31P-NMR confirmed the presence of phos-
phonium cations in the catalyst and was also useful to under-
stand the surrounding environment of ‘P’ (Fig. 4b). 29Si-MAS
NMR of 30PW-Si and 30PW-Si/TPSP showed the peaks at
−112.54 and −112.27 ppm for Q4-ijSiIJOSi4)] SiO2 with 63.15%
and 64.70% and at −104.01 and −105.96 ppm for Q3-ijSiIJOSi3)OH]
with 36.15% and 35.9%, respectively (Fig. 4a, A and B). The
amount of the dominant Q4 in silica decreased while that of
Q3 increased in the 30PW-Si/TPSP catalyst due to the incorpo-
ration of PW into silica matrix. Thus, the ratio of Q4/Q3 was
lowered up to 1.764 in 30PW-Si/TPSP as compared to that in
the parent silica. This lowering of the Q4/Q3 ratio was clearly
due to bonding of heteropolyacid molecules to silicon atoms
near the surface, thus decreasing the number of silicon
atoms having 4 Si neighbours.34,35
At 100–105 °C, the PW-TPSP and 30PW-Si/TPSP samples
showed 3% weight loss while 8% weight loss was observed
for the 30PW-Si sample due to physically adsorbed water
(Fig. 5, B, C and D). Beyond 104 °C, sequential thermal
decomposition was observed in the range of 280–300 °C, as
evidenced by 7% weight loss for PW-TPSP (Fig. 5, B) while
only 2% weight loss for the 30PW-Si/TPSP sample (Fig. 5, C)
was detected. Further decomposition of PW to form metal
oxides and water was also observed beyond 460–500 °C with
a maximum weight loss of 9% for PW-TPSP (Fig. 5, B) while
only 5% and 5.5% weight losses were observed for 30PW-Si/
TPSP and 30PW-Si samples (Fig. 5, C and D), respectively.40,41
In summary, high thermal stability was attained due to
strong ionic interactions of PW with silica and TPSP.
31P-NMR of TPSP showed a sharp peak at 22.70 ppm for
‘P’ (Fig. 4b, A) which was slightly shifted to 23.46 ppm in Si/
TPSP, indicating the existence of some interactions of TPSP
with the silica surface (Fig. 4b, B). However, the presence of a
broad peak at a still further downfield position at 24.03 ppm
for the 30PW-Si/TPSP sample (Fig. 4b, A and C) evidenced the
interaction of TPSP with PW.36 Similarly, a sharp peak at
−14.91 ppm for the 30PW-Si sample was due to the tetrahe-
dral ‘P’ atom of the Keggin structure while shifting of this
peak to −15.28 ppm for the 30PW-Si/TPSP sample proved the
interactions of heteropolyanions with triphenylphosphine
and silica (Fig. 4b, B and C).37–39
The thermal stability and characteristic decomposition pat-
terns of silica (A), PW-TPSP (B), 30PW-Si/TPSP (C) and 30PW-Si
(D) samples were determined using thermogravimetric analysis
(TGA) (Fig. 5). The silica support was thermally stable with a
total weight loss of only about 3% up to 700 °C (Fig. 5, A).
Fig. 5 TGA profiles of (A) Si, (B) PW-TPSP, (C) 30PW-Si/TPSP and (D)
30PW-Si samples.
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
samples was found to be PW (0.3983 mmoles NH3 per g) >
PW-TPSP (0.2381 mmoles NH3 per g) > 30PW-Si/TPSP (0.176
mmoles NH3 per g) > silica (0.011191 mmoles NH3 per g)
(ESI† Table S1). Although the 30PW-Si/TPSP sample showed
lower acidity as compared to PW-TPSP, the distribution of
acid sites on silica played an important role in enhancing the
product yields. The BET surface area of the 30PW-Si/TPSP
(110.69 m2 g−1) and 30PW-Si (116.70 m2 g−1) catalysts and that
of the silica support (232.57 m2 g−1) were also evidence for the
increase in active sites due to the smooth distribution of PW
which has only a surface area of 5 m2 g−1 (ESI† Fig. S1).
Fig. 7 shows the pyridine-FTIR characterization of various
catalyst samples carried out at 50 °C in order to distinguish
between Lewis and Brønsted acid sites. Bare PW showed
strong peaks at 1542 cm−1, 1636 cm−1 and 1611 cm−1 due to
the pyridinium ion bonded to hydrogen confirming its strong
Brønsted acidity and a peak at 1490 cm−1 attributed to
unspecifically adsorbed pyridine.
Interestingly, no peak was observed at 1450 cm−1, indicat-
ing the absence of any Lewis acid sites (Fig. 7).44 In the case
of PW-TPSP and 30PW-Si/TPSP samples, the peaks at 1544
cm−1 and 1590 cm−1 could be attributed to Brønsted acid
sites and the peak at 1490 cm−1 was assigned to
unspecifically adsorbed pyridine but a new peak appearing at
1440 cm−1 was due to pyridine bonded to phosphonium cat-
ions (Fig. 7, B and C).45 This again clearly confirmed the
presence of phosphonium cations and thus the exchange of
protons from PW to the sulfate, imparting the Brønsted
acidic nature to the 30PW-Si/TPSP catalyst. The concentration
of the acid sites determined from the area under the curve
showed the order of acidity as follows: PW > PW-TPSP >
30PW-Si/TPSP (ESI† Table S2, Fig. S2, A–C).
Fig. 6 NH3-TPD of (A) silica, (B) PW, (C) PW-TPSP and (D) 30PW-Si/
TPSP samples.
Since the acidity of the catalysts play an important role in
the allylation of acetals, the acid strengths of various samples
were compared by measuring the temperature programmed
desorption of ammonia (NH3-TPD) and the results are shown
in Fig. 6.
NH3-TPD of the silica, PW and 30PW-Si/TPSP samples
showed the first peak in the region of 100–200 °C
(Fig. 6, A, B and D) due to weak acidity. Peaks in the region
of 550–650 °C in the PW sample indicated the presence of
strong acid sites (Fig. 6, B),42,43 while the peaks in the region
of 250–550 °C for the PW-TPSP and 30PW-Si/TPSP samples
(Fig. 6, C and D) indicated the presence of strong acid sites
at lower temperature. These peaks could be due to the newly
created acid sites by the total exchange of protons from silica
supported PW to TPSP in the 30PW-Si/TPSP sample forming
sulfonic acid. The order of the acid amount of various
Fig. 8 Plausible reaction mechanism for the H–S reaction of acetals
Fig. 7 Pyridine-FTIR of (A) PW, (B) PW-TPSP and (C) 30PW-Si/TPSP.
with allyl-TMS over 30PW-Si/TPSP catalyst.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
Reaction optimization study
the above catalysts, the silica supported hydrophobic
triphenylIJ3-sulfopropyl)phosphonium salt of phosphotungstic
acid (PW-Si/TPSP) with different loadings of PW was used.
This proved to be a successful strategy and the yield of the
product obtained was in the ascending order of PW loading
ranging from 10–50%, giving 53% and 86% yields for 10%
(10PW-Si/TPSP) and 20% (20PW-Si/TPSP) loadings, respec-
tively (Table 1, entries 9(A) and (B)). For both 30% (30PW-Si/
TPSP) and 50% (50PW-Si/TPSP) loadings, more than 99%
yields of the products were obtained (Table 1, entries 9(C)
and (D)); hence, 30% PW loading was considered to be the
optimum for complete conversion of acetals. The quantity of
the 30PW-Si/TPSP catalyst also dramatically influenced the
yield of the product. With 0.025 g of the catalyst, 61% prod-
uct yield was obtained while with 0.050 g and 0.1 g of the cat-
alyst, more than 99% yields were achieved (Table 1, entries
10IJA)–IJC)).
Table 1 shows the studies on catalyst screening and the effect
of solvents to achieve the optimum reaction conditions for
allylation of acetals with allyl-TMS.
Initially, benzaldehyde dimethyl acetal (BDMA) was
reacted without a catalyst and also in the presence of the
triphenylIJ3-sulfopropyl)phosphonium salt or zwitterion
(TPSP) alone, showing no formation of any products which
indicates that acid sites are needed to catalyze the reaction
(Table 1, entries 1 and 2). A commercially available acid cata-
lyst, Amberlyst 15, was found to give only 40% of homoallyl
ethers (HAEs) but mainly favoured the deprotection of BDMA
to benzaldehyde (Table 1, entry 3). Heteropolyacid/PW
(H3PW12O40·xH2O) alone and that supported on silica gave
higher product yields of 74% and 80%, respectively, than
Amberlyst 15. However, the remaining product formed was
again benzaldehyde (Table 1, entries 4 and 5).
The solvent studies showed very interesting trends. The
reaction without any solvent yielded only 60% of the product,
however, it was accompanied with the formation of some
tarry material (Table 1, entry 11). When water was used as a
solvent, only 30% product yield was obtained with benzalde-
hyde (70%) formed as the major product as the deprotection
of acetals is well known to occur in the presence of water and
an acid catalyst (Table 1, entry 12). In the presence of metha-
nol, the product yield was almost completely suppressed to
the extent of only 5% (Table 1, entry 13). This was due to the
rapid degradation of allyl-TMS by methanol to form alkoxy-
TMS, thus, allyl-TMS is unavailable to attack on the oxonium
cation which is a driving force to facilitate the reaction. Sol-
vents like toluene and DCM gave moderate to good yields of
about 50% and 83%, respectively (Table 1, entries 14 and 15).
Acetonitrile served as the best solvent giving more than 99%
yield in a very short time of 0.5 h (Table 1, entry 16). There-
fore, acetonitrile was used as the solvent in further studies.
Silica, when replaced with zirconia (ZrO2) as the support,
adversely affected the product yield up to 81% with benzalde-
hyde as the by-product (Table 1, entry 17).
TriphenylIJ3-sulfopropyl)phosphonium hydrogen sulfate
showed still lower activity (Table 1, entry 6) but when PW was
used as the anion with zwitterions of 1-methyl-3-IJ3-
sulfopropyl)imidazolium (PW-MIMBS) and triphenylIJ3-
sulfopropyl)phosphonium (PW-TPSP), the yield of the prod-
uct increased up to 81% and 84%, respectively (Table 1,
entries 7 and 8). This is because the heteropolyanion plays a
vital role in stabilizing the intermediate carbocation.46
Hence, to obtain an excellent product yield using PW and at
the same time to overcome the drawback of the solubility of
Table 1 Reaction optimization for allylation of benzaldehyde via the
Hosomi–Sakurai reactiona
Entry
Catalysts
Solvent
Conversionc (%)
1
2
3
4
5
6
7
8
9
Without catalyst
TPSP
Amberlyst 15
H3PW12O40 (PW)
30PW-Si
H2SO4-TPSP
PW-MIMBS
PW-TPSP
A. 10PW-Si/TPSP
B. 20PW-Si/TPSP
C. 30PW-Si/TPSP
D. 50PW-Si/TPSP
30PW-Si/TPSP
A. 0.025 g
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
—
—
40b
74b
80b
24
81
84
53
86
>99
>99
61
Substrate scope
After having achieved the best catalyst and optimized reac-
tion conditions, the scope of our catalyst was further
extended by screening a variety of acetals for the Hosomi–
Sakurai allylation reaction and the results are presented in
Table 2.
BDMA reacted with allyl-TMS to give 99% product yield
(Table 2, entry 1). Mono- and di-methoxy substituted com-
pounds having electron donating effects also reacted rapidly
to give 96% and 98% product yields, respectively (Table 2,
entries 2 and 3). Ortho- and para-substitutions in chloro-
dimethyl acetals did not affect the allylation reaction
resulting in 96% and 97% product yields, respectively (Table
2, entries 4 and 5). Piperonaldehyde dimethyl acetal also gave
99% yield in a very short time (Table 2, entry 6). Not only
1-naphthaldehyde dimethyl acetal but also the corresponding
10
Acetonitrile
B. 0.050 g
C. 0.100 g
>99
>99
60e
30b
<5
11
12
13
14
15
16
17
30PW-Si/TPSP
30PW-Si/TPSP
30PW-Si/TPSP
30PW-Si/TPSP
30PW-Si/TPSP
30PW-Si/TPSP
30PW-Zr/TPSP
NEAT
Water
Methanol
Toluene
Dichloromethane
Acetonitrile
Acetonitrile
50
83
>99d
81b
a
Reaction conditions: benzaldehyde dimethyl acetal (1 mmol ) : allyl-
TMS (1.5 mmol), solvent (2 mL), catalyst (0.050 g), temperature (RT),
b
N2, reaction time (1 h). The remaining product is benzaldehyde.
c
Yields on GC. d Reaction time: 0.5 h. e Tarry product formed.
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
diethyl acetal reacted to give 99% product yield (Table 2,
entries 7 and 8). The presence of an electron withdrawing
substituent on dimethyl and diethyl acetals retarded the rate
of reaction as indicated by four-fold longer reaction times to
achieve the same level of product yield (Table 2, entries 9–
13).
Aliphatic acetals like 3,3-dimethoxypropylbenzene also
reacted with allyl-TMS to give 97% product yield in 3 h (Table
2, entry 14). α,β-Unsaturated cinnamaldehyde dimethyl and
diethyl acetals smoothly reacted to achieve product yields of
95% and 98%, respectively (Table 2, entries 15 and 16). The
30PW-Si/TPSP catalyst was also successfully applied to the
allylation of o-allylated phenolic acetals showing 93–99%
product yield (Table 2, entries 17–21). This demonstrates very
well the versatility and efficiency of the new catalyst devel-
oped for the Hosomi–Sakurai reaction of different functional
groups. These new double allylated moieties will be further
extended to polymerization reactions.
Reaction pathway
A plausible reaction mechanism is proposed to explain the
pathway for allylation of acetals by the H–S reaction over the
PW-Si/TPSP catalyst. The catalytic cycle was initiated by
strong Brønsted acid sites of 30PW-Si/TPSP, activating the
acetal to form the corresponding oxonium cation A, which
further reacted with allyl-TMS to form the silica-stabilized
carbocation B. This was followed by the removal of volatile
alkoxy-TMS by the attack of methanol, generated in the initial
step, to give the desired homoallyl ether C and the catalyst
was regenerated for the subsequent cycle (Fig. 8).
Catalyst recycling study
In order to establish the stability for reuse of our 30PW-Si/
TPSP catalyst, a recycling study was also carried out for the
Hosomi–Sakurai reaction of benzaldehyde dimethyl acetal and
allyl-TMS in acetonitrile and the results are shown in Fig. 9.
Table 2 Substrate scope of 30PW-Si/TPSP catalyst for the Hosomi–Sakurai allylation of acetalsa,b
1, 99%, 0.5 h
2, 96%, 0.5 h
4, 96%, 0.5 h
8, 99%, 1 h
3, 98%, 1 h
5, 97%, 0.5 h
9, 98%, 2 h
7, 99%, 1 h
6, 99%, 1 h
10, 95%, 2 h
12, 95%, 2 h
11, 97%, 2 h
14, 95%, 1 h
17, 95%, 1 h
15, 95%, 1.5 h
13, 98%, 1.5 h
16, 98%, 1.5 h
18, 99%, 1 h
19, 93%, 1 h
20, 96%, 1 h
21, 98%, 1 h
a
b
Reaction conditions: acetals (1 mmol) : allyl-TMS (1.5 mmol), acetonitrile (2 mL), 30PW-Si/TPSP (0.050 g), temperature (RT), N2. Isolated
yields after column chromatography. This is because of the fact that the −I effect restricted the protonation of oxygen by acetals which slowed
down the process of formation of oxonium cations and thus the rate decreased drastically.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
variety of substrates such as mono- and di-methoxy
substituted compounds, o- and p-substituted chloro-dimethyl
acetals, piperonaldehyde dimethyl acetal, and 1-naphth-
aldehyde dimethyl/diethyl acetals. The stability of this new
heterogeneous acid catalyst was established by its successful
recycling for five times.
Experimental section
General methods
All the reactions were carried out in a flame-dried glass appa-
ratus under inert nitrogen atmosphere using specially pro-
cured chemicals and AR-grade dried solvents from authentic
suppliers like Aldrich, Alfa Aesar, Thomas Baker, etc. Fumed
silica and heteropolyacids were purchased from Aldrich. TLC
plates were purchased from Loba. 100–200 mesh silica was
utilized for column chromatography using 5% ethyl acetate
in pet ether as the mobile phase.
Fig. 9 Recycling of 30PW-Si/TPSP catalyst for the Hosomi–Sakurai
allylation of acetal. Reaction conditions: benzaldehyde dimethyl acetal
(1 mmol) : allyl-TMS (1.5 mmol), acetonitrile (2 mL), 30PW-Si/TPSP
(0.050 g), temperature (RT), N2. Yields on GC.
Some of the acetals and o-allylated acetals were prepared
by the reported procedures.28 Conversion of the substrates
was calculated by liquid analysis using a Hewlett Packard
6890 GC (HP-5 column) and the products were characterised
For this purpose, after the first run, the catalyst was fil-
tered, recovered and charged for the subsequent run. The
fresh catalyst showed a 99% yield of HAE which then margin-
ally decreased to 94% after the fourth consecutive recycling
run and remained the same for the fifth run. Thus, the devel-
oped catalyst can be reused at least up to five times under
the standard reaction conditions.
The filtrates were analyzed by ICP-OES for ‘W’ after recov-
ery of the 30PW-Si/TPSP catalyst which didn't show even
traces of ‘W’ which means that the slow decrease of activities
during the catalyst reusability test was due to handling.
1
by H-NMR and 13C-NMR using CDCl3 (0.01% TMS) and D6-
DMSO (0.01% TMS) as solvents on a 200 MHz frequency
Bruker instrument. Solid-state MAS CP-NMR of the catalyst
samples were carried out using a JEOL-400 MHz instrument.
The products were confirmed using a QP-2010 Ultra GC-MS
Shimadzu instrument with an RTX-5 column and helium as a
carrier gas in EI mode at an ionization source temperature
about 200 °C. A Micromeritics 2120 chemisorption instru-
ment was used for determination of BET surface area and
also NH3-TPD using helium as a carrier gas and 30% NH3 in
helium as an adsorbent. Functional group characterization
by FT-IR of various compounds and catalysts was performed
on a PerkinElmer Frontier instrument in ATR (PIKE Tech)
mode at room temperature. Pyridine-FTIR was conducted
using a Harrick diffuse reflectance praying mantis assembly
with a temperature controller (30–850 °C) under nitrogen as
a carrier gas at a flow rate of 150 mL min−1. Wide-angle X-ray
diffraction (WAXRD) spectra were recorded on a PANalytical
X-Pert PRO-1712 PXRD instrument using Ni filtered Cu-Kα
radiation (λ = 0.154 nm) as a source (current intensity, 30
mA; voltage, 40 kV) and an X'Celerator detector. TGA was car-
ried out on a TGA-7 PerkinElmer instrument under N2 up to
800 °C with a heating rate of 10 °C min−1. ICP-OES and EDAX
were carried out on Thermo Fisher and AMETEK instru-
ments, respectively.
Conclusion
In summary, we have prepared
a
new triphenylIJ3-
sulfopropyl)phosphonium (TPSP) functionalized phospho-
tungstic acid on a silica support as an efficient solid Brønsted
acid catalyst for the selective mono-allylation of acetals via
the Hosomi–Sakurai reaction. The acidity of the catalyst was
systematically varied by varying the PW loading in a range of
10–30% silica. FT-IR and XRD characterization confirmed the
retention of the Keggin structure of heteropolyanions. The
upfield shift from 22.70 ppm to 24.03 ppm for ‘P’ in 31P-
NMR of the 30PW-Si/TPSP sample confirmed the binding of
triphenylIJ3-sulfopropyl)phosphonium cations to PW, while
the appearance of distinct peaks at −112.54 and −104.01 ppm
for Q4-ijSiIJOSi4)] SiO2 tetrahedral sites and Q3-ijSiIJOSi3)OH],
respectively, in 29Si-NMR of 30PW-Si confirmed the incorpo-
ration of PW into the fumed silica. The acidic strength and
Brønsted acidic sites of the catalyst were determined by
ammonia-TPD and pyridine-FTIR and the order of acidity
was observed as follows: PW > PW-TPSP > 30PW-Si/TPSP.
Interestingly, the distribution of acid sites on silica of the
30PW-Si/TPSP sample played an important role in enhancing
its activity for selective mono-allylation, giving 99% product
yield. The versatility of our catalyst was demonstrated by
achieving quantitative yields in the H–S reaction using a wide
General procedure for catalyst preparation
Synthesis of triphenylIJ3-sulfopropyl)phosphonium and
related zwitterions (Scheme 2, B). To
a solution of
triphenylphosphine (2.6229 g, 0.1 mole) in 30 mL of toluene,
1,3-propane sultone (1.22 g, 0.1 mole) was added. Then, the
mixture was refluxed at 130 °C for 48 hours, and the resul-
tant white solid was filtered and washed with 30 mL of
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
toluene. The crude product residue was dried under vacuum
at 120 °C and characterized using 1H-NMR.
TMS): δ 2.42–2.50 (m, 2H), 3.26 (s, 3H), 4.53–4.57 (CH2–O, dt,
J = 1.64, 4.93 Hz, 2H), 4.72–4.78 (t, 1H), 4.98–5.11 (m, 2H),
5.23–5.31 (dq, J = 1.52, 10.61 Hz, 1H) 5.36–5.47 (dq, J = 1.64,
17.2 Hz, 1H), 5.76–6.15 (m, 2H), 6.83–6.87 (dd, J = 0.88, 8.08
Hz, 1H), 6.95–7.02 (td, J = 1.01, 7.33, 7.45 Hz, 2H), 7.17–7.26
(td, J = 1.77, 7.45, 8.08 Hz, 1H), 7.35–7.40 (dd, J = 1.77, 7.58
Hz, 1H). 13C NMR (150.92 MHz, CDCl3): δ 41.09, 56.94, 68.55,
76.90, 111.50, 116.87, 120.82, 126.49, 127.99, 130.20, 133.29,
135.38, 155.85. GC-MS: m/z 218.13.
1-IJAllyloxy)-2-methoxy-4-IJ1-methoxybut-3-en-1-yl)benzene
(Table 2, entry 19). Colorless liquid, 1H NMR (200.17 MHz,
CDCl3, TMS): δ 2.38–2.56 (m, 2H), 3.21 (s, 3H), 3.89 (s, 3H),
4.07–4.14 (m, 1H), 4.60–4.64 (m, 2H), 5.00–5.12 (m, 2H),
5.26–5.47 (m, 2H), 5.70–5.83 (m, 1H), 6.01–6.17 (m, 1H),
6.75–6.87 (m, 3H). 13C NMR (150.92 MHz, CDCl3): δ 42.51,
55.99, 56.51, 69.93, 83.45, 109.74, 113.00, 116.78, 117.89,
119.23, 133.47, 134.66, 134.93, 147.51, 149.62. GC-MS: m/z
248.14.
1H-NMR. 1H NMR (200.17 MHz, CDCl3, TMS): δ 2.2–2.18
(m, 2H), 2.97–3.04 (t, J = 7.07 Hz, 2H), 3.37–3.52 (m, 2H),
7.64–7.82 (m, 15H).
Typical procedure for the preparation of 30PW-Si/TPSP
catalyst (Scheme 2, C). TriphenylIJ3-sulfopropyl)phosphonium
(0.0798 g, 0.000208 mole) was dissolved in 10 mL of distilled
water to which 1.0 g of 30PW/Si (amount of PW: 0.3 g,
0.000104 mole) was slowly added. Then, the mixture was
stirred at room temperature for 12 hours. Water was evapo-
rated from the residue at 90 °C for 1 hour on a rotavapour
and dried at 150 °C for 12 hours.
All the other catalysts (10PW-Si/TPSP, 20PW-Si/TPSP and
50PW-Si/TPSP) were prepared by using the same procedures
with appropriate changes in the amount of zwitterions.
General procedure for the allylation of acetals. The 30PW-
Si/TPSP catalyst (0.05 g) was added to the solution of benzal-
dehyde dimethyl acetal (1 mmol, 0.152 g) and allyl-
trimethylsilane (1.5 mmol, 0.237 mL) in acetonitrile (2 mL).
The mixture was stirred at room temperature under nitrogen
till the reaction was completed as monitored by TLC. Then,
the catalyst was filtered and washed with 10 mL of acetoni-
trile. The solvent was removed from the filtrate under
reduced pressure and the residue was purified by chromatog-
raphy on silica gel (petroleum ether/ethyl acetate = 9.5 : 0.5)
to give the desired product in 99% yield.
2-IJAllyloxy)-1,3-dimethoxy-5-IJ1-methoxybut-3-en-1-yl)-
1
benzene (Table 2, entry 20). Colorless liquid, H NMR (200.17
MHz, CDCl3, TMS): δ 2.36–2.57 (m, 2H), 3.24 (s, 3H), 3.85 (s,
6H), 4.07–4.10 (q, 1H) 4.50–4.52 (dt, J = 1.37, 6.41 Hz, 2H),
5.02–5.10 (m, 2H), 5.16–5.20 (dq, J = 1.37, 10.53 Hz, 1H), 5.28–
5.34 (dq, J = 1.37, 16.94 Hz, 1H), 5.73–5.83 (m, 1H), 6.07–6.17
(m, 1H), 6.51 (s, 2H). 13C NMR (150.92 MHz, CDCl3): δ 42.68,
56.17, 56.84, 74.22, 83.92, 103.39, 116.95, 117.68, 134.65,
134.93, 135.89, 137.59, 153.51. GC-MS: m/z 278.15.
Procedure for catalyst recycling. The 30PW-Si/TPSP catalyst
recovered from the above mentioned procedure was dried at
150 °C and reused for the same reaction. This procedure was
repeated 5 times with the same catalyst.
The amount of S and W was calculated using EDAX for
the fresh and recovered 30PW-Si/TPSP catalyst which showed
similar weight% of W and S which means that no leaching of
the active components occurred after reaction (ESI† Table S3
and S4, Fig. S3 and S4).
IJE)-1-IJAllyloxy)-2-methoxy-4-IJ3-methoxyhexa-1,5-dien-1-yl)-
benzene (Table 2, entry 21). Colorless liquid, H NMR (200.17
1
MHz, CDCl3, TMS): δ 2.36–2.47 (m, 2H), 3.33 (s, 3H), 3.71–
3.81 (m, 1H), 3.91 (s, 3H), 4.61–4.65 (dt, J = 1.52, 5.31 Hz,
2H), 5.04–5.18 (m, 2H), 5.26–5.46 (m, 2H), 5.75–6.19 (m, 3H),
6.45–6.52 (d, 1H), 6.81–6.97 (m, 3H). 13C NMR (150.92 MHz,
CDCl3): δ 40.40, 55.97, 56.34, 69.95, 82.22, 109.35, 113.35,
117.11, 118.14, 119.63, 127.81, 130.00, 132.39, 133.29, 134.60,
141.97, 149.58. GC-MS: m/z 274.16.
Abbreviations
Spectroscopic data of selected compounds
TPSP, triphenylIJ3-sulfopropyl)phosphonium zwitterion; PW,
phosphotungstic acid; Si, silica; TMS, trimethylsilane; TLC,
thin layer chromatography.
IJ1-Methoxybut-3-en-1-yl)benzene (Table 2, entry 1). Color-
less liquid, 1H NMR (200.17 MHz, CDCl3, TMS): δ 2.42–2.60
(m, 2H), 3.24 (s, 3H), 4.15–4.22 (m, 1H), 2.64–2.58 (m, 1H),
2.53–2.48 (m, 1H), 2.38–2.32 (m, 1H), 1.76–1.70 (m, 1H), 1.17
(d, J = 6.18 Hz, 3H). 13C NMR (150.92 MHz, CDCl3): δ 42.56,
56.67, 83.69, 116.89, 126.75, 127.9, 128.40, 134.84, 141.71.
GC-MS: m/z 162.10.
1-IJAllyloxy)-4-IJ1-methoxybut-3-en-1-yl)benzene (Table 2,
entry 17). Colorless liquid, 1H NMR (200.17 MHz, CDCl3,
TMS): δ 2.39–2.57 (m, 2H), 3.19 (s, 3H), 4.11 (t, 1H), 4.51–4.56
(d, 2H), 4.99–5.08 (m, 2H), 5.26–5.31 (m, 1H), 5.46–5.47 (m,
1H), 5.73–5.77 (m, 1H), 6.00–6.11 (m, 1H), 6.91 (d, J = 8.72
Hz, 2H), 7.17 (d, J = 8.59 Hz, 2H). 13C NMR (150.92 MHz,
CDCl3): δ 42.46, 56.42, 68.85, 83.19, 114.57, 116.77, 117.61,
127.93, 133.38, 133.89, 134.96, 158.17. GC-MS: m/z 218.13.
1-IJAllyloxy)-2-IJ1-methoxybut-3-en-1-yl)benzene (Table 2,
entry 18). Colorless liquid, 1H NMR (200.17 MHz, CDCl3,
Acknowledgements
Sumit B. Kamble is grateful to the University Grants Commis-
sion, Delhi for a senior research fellowship (SRF) under the
Ph.D. programme of the Academy of Scientific and Innovative
Research, CSIR-National Chemical Laboratory, Pune, India.
Notes and references
1 L. Li, X. Ye, Y. Wu, L. Gao, Z. Song, Z. Yin and Y. Xu, Org.
Lett., 2013, 15(5), 1068–1071.
2 A. Miyawaki, D. Kikuchi, M. Niki, Y. Manabe, M. Kanematsu,
H. Yokoe, M. Yoshida and K. Shishido, J. Org. Chem.,
2012, 77, 8231–8243.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
3 (a) X. Fan, L. Fu, N. Li, H. Lv, X. Cui and Y. Qi, Org. Biomol.
Chem., 2013, 11, 2147–2153; (b) B. Lu and J. Falck, J. Org.
Chem., 2010, 75, 1701–1705; (c) E. Thiery, C. Aouf, J. Belloy,
D. Harakat, J. Bras and J. Muzart, J. Org. Chem., 2010, 75,
1771–1774; (d) Y. Cui, Q. Huang, J. Xu, L. Chen, J. Li, Q. Ye,
J. Li and F. Nan, Bioorg. Med. Chem. Lett., 2005, 15,
4130–4132; (e) M. Dai, C. Wang, G. Dong, J. Xiang, T. Luo, B.
Liang, J. Chen and Z. Yang, Eur. J. Org. Chem.,
2003, 4346–4348.
4 A. Hosomi, M. Endo and H. Sakurai, Chem. Lett.,
1976, 941–942.
5 A. Ishii, O. Kotera, T. Saeki and K. Mikami, Synlett,
1997, 1145–1146.
24 K. Murugan, S. Srimurugan and C. Chen, Tetrahedron,
2011, 67, 5621–5627.
25 (a) Y. Leng, J. Wang, D. Zhu, X. Ren, H. Ge and L. Shen,
Angew. Chem., Int. Ed., 2009, 48, 168–171; (b) Y. Leng, J.
Wang, D. Zhu, Y. Wu and P. Zhao, J. Mol. Catal. A: Chem.,
2009, 313, 1–6.
26 K. Li, L. Chen, H. Wang, W. Lin and Z. Yan, Appl. Catal., A,
2011, 39, 2233–2238.
27 W. Zhang, Y. Leng, D. Zhu, Y. Wu and J. Wang, Catal.
Commun., 2009, 11, 151–154.
28 S. Kamble, R. Swami, S. Sakate and C. V. Rode,
ChemPlusChem, 2013, 78, 1393–1399.
29 T. Vu, H. Au, T. Nguyen, M. Pham, T. Bach and H. Nong,
Catal. Sci. Technol., 2013, 3, 699–705.
30 M. Rao, A. Wolfson, S. Kababya, S. Vega and M. Landaua,
J. Catal., 2005, 232, 210–225.
6 J. S. Yadav, S. Reddy and B. Srihari, Synlett, 2001, 673–675.
7 T. Mukaiyama, H. Nagaoka, M. Murakami and M. Ohshima,
Chem. Lett., 1985, 7, 977–980.
8 (a) H. Zerth, N. Leonard and R. Mohan, Org. Lett., 2003, 5,
55–57; (b) R. Noyori, S. Murata and M. Suzuki, Tetrahedron,
1981, 37, 3899–3910; (c) T. Tsunoda, M. Suzuki and R.
Noyori, Tetrahedron Lett., 1980, 21, 71–74.
9 M. Kawai, M. Onaka and Y. Izumi, Chem. Lett., 1986, 3,
381–384.
10 M. E. Jung and A. Maderna, Tetrahedron Lett., 2004, 45,
5301–5303.
11 H. Sakurai, K. Sasaki and A. Hosomi, Tetrahedron Lett.,
1981, 22, 745–748.
31 B. Bardin and J. Davis, Appl. Catal., A, 2000, 200, 219–231.
32 Y. Zhang and S. Yang, J. Wuhan Univ. Technol., Mater. Sci.
Ed., 2008, 23, 346–349.
33 (a) J. Zhang, M. Sun, C. Cao, Q. Zhang, Y. Wang and H.
Wan, Appl. Catal., A, 2010, 380, 87–94; (b) G. Brown, M. Noe-
Spirlet, T. Busing and H. Levy, Acta Crystallogr., 1977, 33,
1038–1046.
34 Á. Kukovecz, Zs. Balogi, Z. Kónya, M. Tobab, P. Lentz, S.
Niwa, F. Mizukamib, Á. Molnár, J. Nagy and I. Kiricsi, Appl.
Catal., A, 2002, 228, 83–94.
12 S. Arai, Y. Sudo and A. Nishida, Tetrahedron, 2005, 61,
4639–4642.
13 M. Barbero, S. Bazzi, S. Cadamuro, S. Dughera and C.
Piccinini, Synthesis, 2010, 2, 315–319.
14 M. E. Jung and A. Maderna, J. Org. Chem., 2004, 69,
7755–7757.
15 L. C. Wieland, H. Zerth and R. Mohan, Tetrahedron Lett.,
2002, 43, 4597–4600.
35 L. Aany, A. Krishnan, M. Sankar, R. Kala, P. Manikandan, P.
Rajamohanan and T. Ajithkumar, J. Phys. Chem. C,
2009, 113, 21114–21122.
36 C. Glaubitza, A. Groger, K. Gottschalk, P.. Spooner, A. Watts,
S. Schuldiner and H. Kessler, FEBS Lett., 2000, 480, 131–135.
37 (a) R. Gao, Q. Zhu, W. Dai and K. Fan, Green Chem.,
2011, 13, 702–708; (b) I. Kozhevnikov, K. Kloetstra, A.
Sinnema, H. Zandbergen and H. Bekkum, J. Mol. Catal. A:
Chem., 1996, 114, 287–298.
38 J. Edwards, C. Thiel, B. Benac and J. Knifton, Catal. Lett.,
1998, 51, 77–83.
39 I. Kozhevnikov, J. Mol. Catal. A: Chem., 2007, 262, 86–92.
40 M. Misono, Catal. Rev.: Sci. Eng., 1987, 29, 269–321.
41 J. Zhang, Z. Zhu, C. Li, L. Wen and M. Enze, J. Mol. Catal. A:
Chem., 2003, 198, 359–364.
16 T. Watahiki, Y. Akabane, S. Mori and T. Oriyama, Org. Lett.,
2003, 5, 3045–3048.
17 N. Komatsu, M. Uda, H. Suzuki, T. Takahashi, T. Domae and
M. Wada, Tetrahedron Lett., 1997, 38, 7215–7218.
18 (a) C. Lucero and K. Woerpel, J. Org. Chem., 2006, 71,
2641–2647; (b) S. Hathaway and L. Paquette, J. Org. Chem.,
1983, 48, 3351–3353.
19 H. Mayr, G. Gorath and B. Bauer, Angew. Chem., Int. Ed.
Engl., 1994, 33, 788–789.
42 Y. Liu, E. Lotero and J. Goodwin, J. Catal., 2006, 242,
278–286.
20 T. Hollis, N. Robinson, J. Whelan and B. Bosnich,
Tetrahedron Lett., 1993, 34, 4309–4312.
43 K. Rajasekar, A. Balkis and A. Pandurangan, Catal.
Commun., 2008, 10, 150–155.
21 A. Trehan, A. Vij, M. Walia, G. Kaur, R. Verma and S.
Trehan, Tetrahedron Lett., 1993, 34, 7335–7338.
22 H. Barbero, S. Bazzi, S. Cadamuro, S. Dughera and C.
Piccinini, Synthesis, 2010, 2, 315–319.
44 E. Parry, J. Catal., 1963, 2, 371–379.
45 C. Rode, A. Garade and R. Chikate, Catal. Surv. Asia,
2009, 13, 205–220.
46 A. Garade, V. Kshirsagar, R. Mane, A. Ghalwadkar, U. Joshi
and C. Rode, Appl. Clay Sci., 2010, 48, 164–170.
23 D. Kampen and B. List, Synthesis, 2006, 16, 2589–2592.
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol.