Highly efficient povidone-phosphotungstic acid catalyst for the tandem acetalization of aldehydes to bis- and tris(indolyl)methanes

Article abstract of DOI:10.1002/cplu.201300248

A novel, nonleachable hybrid of heteropoly acid and polyvinylpyrrolidone (or povidone) catalyzes the acetalization of aldehydes in methanol at room temperature followed by reaction with indole to give bis(indolyl)methanes (BIMs) and tris- (indolyl)methanes (TIMs) in quantitative yields (90-97 %). The catalyst was shown by pyridine FTIR spectroscopy to possess Bronsted acidity, and the hybrid formation was confirmed by XRD and 31P NMR studies. Friedel-Crafts alkylation of indole as well as the tandem synthesis of BIMs and TIMs were established with several types of carbonyl and indole substrates to give the corresponding products quantitatively. The catalyst was recycled efficiently for three successive runs without losing its original activity. Copyright

Full text of DOI:10.1002/cplu.201300248

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DOI: 10.1002/cplu.201300248
Highly Efficient Povidone–Phosphotungstic Acid Catalyst
for the Tandem Acetalization of Aldehydes to Bis- and
Tris(indolyl)methanes
Sumit B. Kamble, Rameshwar K. Swami, Sachin S. Sakate, and Chandrashekhar V. Rode*[a]
A novel, nonleachable hybrid of heteropoly acid and polyvinyl-
pyrrolidone (or povidone) catalyzes the acetalization of alde-
hydes in methanol at room temperature followed by reaction
with indole to give bis(indolyl)methanes (BIMs) and tris-
(indolyl)methanes (TIMs) in quantitative yields (90–97%). The
catalyst was shown by pyridine FTIR spectroscopy to possess
Brønsted acidity, and the hybrid formation was confirmed by
XRD and 31P NMR studies. Friedel–Crafts alkylation of indole as
well as the tandem synthesis of BIMs and TIMs were estab-
lished with several types of carbonyl and indole substrates to
give the corresponding products quantitatively. The catalyst
was recycled efficiently for three successive runs without
losing its original activity.
Introduction
Bis(indolyl)methanes (BIMs) and tris(indolyl)methanes (TIMs)
are considered an important class of biologically active com-
pounds owing to their significant applications in cancer che-
motherapy and also as dietary supplements.[1a–c] BIMs have
been extracted extensively from various terrestrial and marine
natural resources;[1d] however, they can also be prepared syn-
thetically by Friedel–Crafts alkylation of indoles with carbonyl
compounds in the presence of HCl, acetic acid, Ln triflates, and
so forth.[2a,b] Despite the usefulness of these acidic reagents in
the syntheses, some serious drawbacks to be overcome are
their corrosive nature, cumbersome workup procedure, non-re-
usability, and their use in stoichiometric amounts. Yadav et al.
reported the use of ionic liquids, such as 1-butyl-3-methylimi-
dazolium tetrafluoroborate ([bmim]BF4) or 1-butyl-3-methylimi-
dazolium hexafluorophosphate ([bmim]PF6), to promote the re-
actions along with reasonable recovery of the catalysts;[3a] nev-
ertheless, total recovery was not possible owing to loss of
ionic liquids during product separation. Hence, the use of solid
acid catalysts, such as amberlyst, silica-supported NaHSO4, and
zeolites, was attempted for the synthesis of BIMs in good
yields[3b,c] mainly using the aldehydes as substrates. Surprising-
ly, acetals remained largely unexplored for Friedel–Crafts reac-
tions except for a recent report on nanoporous aluminosilicate
as a catalyst for the synthesis of BIMs through in situ acetaliza-
tion of aldehydes.[4a] In spite of its seminal impact, synthesis of
the catalyst is tedious and higher loading restricts its exploita-
tion for large-scale production of BIMs and TIMs. Another
recent report appeared on the use of triphenylphosphine-m-
sulfonate in combination with carbon tetrabromide as a cata-
lyst for acetals, ketones, and aldehydes to synthesize bis-
(indolyl)alkanes in good yields with recyclability.[4b]
The development of more versatile, ecofriendly, heterogene-
ous acid catalyst systems for the efficient synthesis of BIMs and
TIMs is still an ongoing research effort. Heteropoly acids (HPAs)
have proven their ability as highly active and selective catalysts
in various organic and biomass transformations,[5a–d] including
the synthesis of BIMs and TIMs, which involve different HPAs in
homogeneous medium as well as on various supports, such as
SiO2, Al2O3, TiO2, and ZrO2.[1a,6a–c] Nevertheless, the key chal-
lenge of these supported HPA catalysts is the prevention of
leaching of the active component under reaction conditions.
This can be successfully achieved by the counter anion ex-
change of HPAs with metal cations such as Cs, Na, and K and
also with ammonia, which showed high surface area possess-
ing both Brønsted acidity and Lewis acidity.[7a,b] Zhang et al.
prepared Brønsted HPA salts with N-methyl-2-pyrollidone as an
organo cation, which showed comparable acidity to H-type
zeolite and successful application in the Prins reaction with
aqueous formaldehyde, in which the catalyst acts as a pseudo-
liquid.[8]
In continuation of our prior results on supported HPAs for
the synthesis of bisphenol and phenolphthalein,[9a,b] herein we
report the synthesis of new hybrid HPA salts with nonhazar-
dous polyvinylpyrrolidone (PVP or povidone) as a counter
cation. This unique combination was found to be truly hetero-
geneous, with Brønsted acid sites, was easily reusable, and was
highly active for the Friedel–Crafts alkylation of indole to syn-
thesize BIMs and TIMs. Our route involves tandem acetalization
of aldehydes in methanol at room temperature followed by
acetal and indole reaction to give BIMs, which eliminates the
extra effort needed to synthesize acetals separately
(Scheme 1). The same catalyst is also useful for the synthesis of
[a] S. B. Kamble, R. K. Swami, S. S. Sakate, Dr. C. V. Rode
Chemical Engineering and Process Development Division
CSIR-National Chemical Laboratory
Dr. Homi Bhabha Road, Pune 411008 (India)
Fax: (+91)2590-2620
E-mail: cv.rode@ncl.res.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201300248.
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(Figure 1c) clearly indicate the
amorphous carbonic or semicrys-
talline nature of povidone.
Figure 2 shows the adsorbed
pyridine FTIR spectra of both
PWA and PVP-PWA samples. The
four bands at 1080, 978, 895,
and 812 cmÀ1 of PWA corre-
spond to the stretching absorp-
tion modes of the oxygen atom
with the terminal oxygen of PÀ
O, W=O, corner-sharing oxygen
of WÀObÀW, and edge-sharing
oxygen of WÀOcÀW, respectively,
which represent the Keggin
structure (PW12O40)3À.[11] All these
bands were also clearly observed
in the case of PVP-PWA (Fig-
ure 2b), thus confirming the
conservation
of
heteropoly
anion in the catalyst. The band
at 1634 cmÀ1 resulted from
Scheme 1. Different approaches for the synthesis BIMs. PVP-PWA=polyvinylpyrrolidone–phosphotungstic acid.
acetals from aldehydes in good yields, which is again a unique
property shown by our catalyst.
Results and Discussion
The X-ray diffraction (XRD) pattern of phosphotungstic acid
(PWA) in Figure 1 exhibits characteristic peaks at 2q=10.34,
25.44, 34.67, 53.37, and 59.978, which were also observed for
PVP-PWA (3:1) catalyst. This confirmed the retention of the
well-ordered structure of PWA even after its hybrid formation
with povidone (Figure 1a,b).[10] The intensity of these peaks de-
creased drastically in PVP-PWA (3:1) as a result of strong inter-
actions of PWA with povidone, which strongly supports the
total exchange of protons of PWA with povidone. Very broad
peaks at 2q=16.35 and 25.448 in the XRD pattern of povidone
Figure 2. Pyridine-IR analysis of PWA and PVP-PWA.
amide C=O stretching in povidone.[7] The strong bands at 1488
and 1540 cmÀ1 were assigned to the chemisorbed pyridium
ion, which confirmed the true Brønsted acid sites of the cata-
lyst.
Thermogravimetric analysis (TGA)–differential thermal analy-
sis of PWA exhibited weight loss of physically adsorbed and
coordinated water at 100–2008C and the peak beyond 5008C
indicated the decomposition of PWA to the corresponding
oxides.[12] In the case of PVA-PWA (3:1) catalyst, minimal
weight loss (3–4%) was observed at 70 and 1608C owing to
physically adsorbed and coordinated water molecules, respec-
tively, whereas major weight loss (5–6%) at 3008C resulted
from degradation of the povidone. The total weight loss of the
catalyst was only about 15%, thus indicating the high thermal
Figure 1. XRD patterns of PWA, PVP-PWA (3:1), and povidone.
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stability of the catalyst even on heating to 9008C (Figure S1 in
the Supporting Information).
Solid-state 31P NMR spectroscopy of hydrated PWA
(H3PW12O40·xH2O) gave only a single sharp peak at about
À15 ppm as no protons were available in the environment
of P (Figure 3a,b).[13] Similarly, PVP-PWA (3:1) catalyst also
screening studies showed that the highest yield of BIMs (95%)
was achieved in methanol. The reaction in methanol was com-
pleted within 2 h with 93 and 95% yield (Table 1, entries 4 and
5) at room temperature and 508C, respectively, which indicat-
ed that the reaction proceeded smoothly even at room tem-
perature. On the other hand, with acetonitrile and dichlorome-
thane as solvents (Table 1, entries 6 and 7), products were ob-
tained with 50 and 90% yields, respectively, after 5 h at room
temperature. Interestingly, in methanol, benzaldehyde-dime-
thylacetal with indole gave only 70% BIMs (Table 1, entry 8)
but with dichloromethane as solvent, the rate of reaction of
benzaldehyde-dimethylacetal with indole increased dramatical-
ly to twice that of benzaldehyde alone, thus proving that the
catalyst is highly active for acetals (Table 1, entry 9). Catalyst
prepared with different molar ratios of povidone to PWA (1:1,
2:1, and 3:1) gave yields of isolated products of 90, 92, and
93%, respectively (Table 1, entries 10, 11, and 4). Although
there was no appreciable difference in the yields, the hetero-
geneous nature of the catalysts was in the order 3:1>2:1>
1:1, which affected the recyclability of the catalyst. So, PVP-
PWA (3:1) was used for further studies on the effects of reac-
tion parameters on the yield of product.
Figure 3. Solid-state 31P NMR spectra of a) PWA and b) PVP-PWA (3:1).
The effect of catalyst concentration on yield showed that
with double the amount of catalyst, the yield increased by
about 33%; however, with a further increase in catalyst con-
centration, the yield remained almost constant (95%; Table 2).
Thus, 0.01 mmol catalyst was the optimum concentration for
further reactions.
showed a single peak at À15.45 ppm, with slight downfield
shift owing to replacement of the PWA proton by the povi-
done cation. The nature of protons in HPA and the catalyst is
shown in Figure 4.
The versatility of our PVP-PWA catalyst was demonstrated
for the acetylation of indole with various acetals in dichlorome-
thane at room temperature (Scheme 2), and the results are
Figure 4. Nature of the protons in hydrated HPA (H3PW12O40·xH2O) and cata-
lyst.
Scheme 2. Acetylation of indoles with acetals.
Table 1. Catalyst optimization for the Friedel–Crafts reaction of indole.[a]
The activity results of PVP-
PWA (3:1) catalyst for the Frie-
del–Crafts alkylation of benzalde-
hyde and benzaldehyde-dime-
thylacetal with indole are shown
in Table 1. A control experiment
with only povidone did not
show any reaction, which con-
firmed that acid sites were
needed for this reaction (Table 1,
entry 1). PWA alone was less
active giving only 60% yield
with benzaldehyde in methanol
and that with the acetal was
Entry
Catalyst
Reactant
Solvent
Time [h]
Yield [%]
TON[e]
1
2
3
4
5
6
7
8
PVP
PWA
PWA
benzaldehyde
benzaldehyde
benzaldehyde-dimethylacetal
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde-dimethylacetal
benzaldehyde-dimethylacetal
benzaldehyde
MeOH
MeOH
CH2Cl2
MeOH
MeOH
acetonitrile
CH2Cl2
MeOH
CH2Cl2
MeOH
MeOH
2
2
2
2
2
5
5
2
1
2
2
–
–
60[c]
40[c]
93[b]
95[b][d]
50[c]
90[c]
70[b]
95[c]
90[b]
92[b]
200
133
310
317
167
300
233
317
300
307
PVP-PWA (3:1)
PVP-PWA (3:1)
PVP-PWA (3:1)
PVP-PWA (3:1)
PVP-PWA (3:1)
PVP-PWA (3:1)
PVP-PWA (1:1)
PVP-PWA (2:1)
9
10
11
benzaldehyde
[a] Reaction conditions: aldehydes/acetals (1 mmol), indole (2 mmol), catalyst (0.01 mmol), solvent (2.5 mL), N2,
258C. [b] Yield of isolated product after filtration and recrystallization in acetone. [c] Yield of isolated product
after column chromatography. [d] Reaction temperature 508C. [e] Turnover number (TON) as the ratio of moles
of substrate converted per mole of PVP present in the catalyst.
40%
in
dichloromethane
(Table 1, entries 2 and 3). Solvent
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and 7), and thus acetals served as the best substrates for syn-
thesis of BIMs.
Table 2. Effect of catalyst concentration on the reaction.[a]
Entry
PVP-PWA [mmol]
t [h]
Yield [%][b]
Encouraged by these results, we attempted Friedel–Crafts re-
actions of indoles with aldehydes as substrates. Table 4 shows
the results for the tandem acetalization approach to the syn-
thesis of BIMs and TIMs under mild reaction conditions
(Scheme 3). Aromatic aldehydes with electron-withdrawing
1
2
3
0.005
0.01
0.02
2
2
2
60
93
95
[a] Reaction conditions: benzaldehyde (1 mmol), indole (2 mmol), MeOH
(2.5 mL), N2, 258C. [b] Yield of isolated product after filtration and recrys-
tallization in acetone.
shown in Table 3. Dimethoxymethane and 2-dimethoxypro-
pane reacted smoothly with indoles to give 92 and 90% yields
of the corresponding BIMs, respectively (Table 3, entries 1 and
2). Electron-withdrawing and -donating substituents of the aro-
matic dimethylacetals decreased and increased the rate of re-
action, respectively (Table 3, entries 3–5, 8, and 9). This was
owing to a contrast effect on the protonation of the acetals
during substitution of the methoxy group by indoles. Diethyla-
cetals also showed excellent product yields (Table 3, entries 6
Scheme 3. Friedel–Crafts alkylation of indoles by tandem acetalization of al-
dehydes.
and -donating substituents gave excellent yields of the corre-
sponding products on treatment with indole (Table 4, en-
tries 1–5, 7–9, and 11–13), as was observed if treated with ace-
tals (Table 3, entries 3–5, 8, and 9). The typical syntheses of
TIMs were successfully achieved by using 3-formylin-
dole as a carbonyl compound reacting with indole, 1-
Table 3. Acetylation of indoles with acetals.[a]
methylindole, and 5-nitroindole in excellent yields
(Table 4, entries 6, 10, and 14). To ensure the reaction
path, GC–MS analysis (Figure S2) of the reaction mix-
ture after a certain time interval confirmed the pres-
ence of acetals; thus, the reaction proceeds through
the acetal as an intermediate in methanol and not
through azafulvene as in the conventional route
(Scheme 1). Our proposed reaction pathway
(Scheme 4) involves the role of the heteropoly anion
mainly for the stabilization of the oxonium ion after
protonation of acetals and during subsequent substi-
tution of methoxy groups with indoles.
Entry
1
Indole
indole
Substrates
t [h]
Yield [%][b]
92
1
2
3
indole
indole
1
90
95
0.75
4
5
indole
indole
0.5
1.5
96
90
Three consecutive recycles of PVP-PWA catalyst
were performed for the reactions of benzaldehyde
and benzaldehyde-dimethylacetal in methanol and
dichloromethane, which showed no appreciable de-
crease in the yield of products (Figure 5). Thus, the
catalyst can be easily recycled for both substrates
without losing its acid sites property and without de-
activation, at least up to three times.
6
7
indole
1
2
94
93
Conclusion
1-methylindole
By using polyvinylpyrrolidone (povidone, PVP) as
a counter cation for heteropoly acid (phosphotungs-
tic acid, PWA), a new type of solid catalyst that is
stable in polar medium under reaction conditions
was developed. The formation of a hybrid was con-
firmed by the retention of a well-ordered structure of
PWA in XRD; however, the peak intensity was drasti-
cally diminished in PVP-PWA, which suggests the
total exchange of protons of PWA with povidone.
The strong bands at 1488 and 1540 cmÀ1 in pyridine
FTIR analysis evidenced the Brønsted acid sites of the
8
9
5-nitroindole
5-nitroindole
1
1
93
96
[a] Reaction conditions: acetal (1 mmol), indole (2 mmol), catalyst (0.01 mmol), CH2Cl2
(2.5 mL), N2, 258C. [b] Yield of isolated product determined by column chromatogra-
phy.
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Experimental Section
Table 4. Friedel–Crafts alkylation through tandem acetalization of aldehydes to BIMs
and TIMs.[a]
All the reactions were performed in flame-dried glass ap-
paratus, under an inert atmosphere of N2 unless other-
wise stated. Solvents and chemicals were purchased
from Merck, Rankem, Aldrich, and Thomas Baker and
used without further purification. Prepared catalysts
were characterized by XRD using powder XRD expert-
1712 with nickel-filtered Cu Ka radiation source, and
FTIR spectra were recorded on a Spectrum One Perki-
nElmer spectrophotometer in DRS mode (KBr plates).
Solid-state 31P NMR analysis was performed on
a 300 MHz Bruker instrument with 85% phosphoric acid
as standard. TGA was performed with a TGA-7 Perki-
nElmer instrument under N2 up to 9008C at a heating
rate of 108CminÀ1. The products were confirmed with
authentic samples and characterized by 1H NMR spec-
troscopy at 200 MHz on Brukers instruments. GC–MS
analysis was performed with a GCMS-QP2010 ultra Shi-
madzu instrument using an FFAP column with a flow of
helium as carrier gas, electrospray ionization mode, and
an ionization source temperature of about 2008C. TLC
plates were obtained from Merck and silica 60–120 mesh
was used for column chromatography with 10% ethyl
acetate in petroleum ether polar systems.
Entry
1
Indole
indole
Substrates
t [h]
Yield [%][b]
93
2
2
3
4
5
indole
indole
indole
indole
1.5
3
97
90
95
95
2
2
6
7
indole
2
2
93
96
1-methylindole
8
9
1-methylindole
1-methylindole
3
2
92
95
Experimental procedure
10
11
1-methylindole
1-methylindole
2
2
92
95
Different catalysts were prepared by varying the mole
ratios of povidone/PWA as 1:1, 2:1, and 3:1. These cata-
lysts were designated as PVP-PWA (1:1), PVP-PWA (2:1),
and PVP-PWA (3:1).
12
13
5-nitroindole
5-nitroindole
2
2
90
93
Synthesis of PVP-PWA catalyst
Povidone (0.111 g, 1 mmol or 0.222 g, 2 mmol or 0.333 g,
3 mmol) was placed in a 250 mL round-bottom flask
with stirrer. A solution of PWA (2.880 g, 1 mmol) in dis-
tilled water (30 mL) was added dropwise using an addi-
tion funnel for 15 min and the mixture was further
stirred for 12 h at room temperature. A white precipitate
was obtained and the water was evaporated on a Rotava-
por apparatus for 2 h at 808C. The white powdered cata-
lyst obtained in 95% yield was then dried at 1008C for
6 h.
14
5-nitroindole
2
92
[a] Reaction conditions: benzaldehyde (1 mmol), indole (2 mmol), catalyst (0.01 mmol),
MeOH (2.5 mL), N2, 258C. [b] Yield of isolated product after filtration and recrystalliza-
tion in acetone.
catalyst responsible for its high activity in the Friedel–Crafts al-
kylation of benzaldehyde and benzaldehyde-dimethylacetal
with indole. Among acetonitrile, methanol, and dichlorome-
thane solvents, a BIM yield as high as 95% was achieved in
General procedure for the synthesis of BIMs using acetals
methanol for the reaction of indole and benzaldehyde. The
novelty of our approach is the tandem acetalization of alde-
hydes followed by acetal and indole reaction to give BIMs,
which eliminates the extra effort needed to synthesize acetals
separately. The typical syntheses of TIMs were also successfully
achieved by using 3-formylindole as a carbonyl compound
with other indoles in quantitative yields. The proposed mecha-
nism suggests the role of heteropoly anion is mainly for the
stabilization of the oxonium ion after protonation of acetals
and during subsequent substitution of methoxy groups with
indoles to give BIMs.
Benzaldehyde-dimethylacetal (0.152 g, 1 mmol) was placed in di-
chloromethane (2.5 mL), then the catalyst PVP-PWA (0.03213 g,
0.01 mmol) and indole (0.234 g, 2 mmol) were added and the mix-
ture was stirred for 1 h at room temperature under an inert atmos-
phere. Progress of the reaction was monitored by using TLC. After
total consumption of acetals, the catalyst was isolated by filtration
and washed with dichloromethane (3 mL), and then the reaction
crude was adsorbed on silica for separation of the product by
column chromatography.
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Spectroscopic data of selected
compounds
3,3’-(Phenylmethyl)bis(1H-indole):
Table 3, entry 3; Table 4, entry 1;
white solid. 1H NMR (200 MHz,
CDCl3, 258C, TMS): d=5.88 (s, 1H;
CH), 6.66 (s, 2H; CH), 7.03–7.16 (m,
4H; ArÀH), 7.33 (m, 8H; ArÀH),
7.93 ppm (brs, 2H; NH); 13C NMR
(100 MHz, CDCl3, 258C): d=40.80,
111.64, 119.82, 120.33, 120.54,
122.52, 124.23, 126.74, 127.67,
128.83,
129.33,
137.28,
144.61 ppm; MS (70 eV): m/z (%):
322.03 (100) [C23H18N2]+ [M]+.
3,3’-[(4-Methoxyphenyl)methyle-
ne]bis(1-methyl-1H-indole): Table 3,
entry 4; Table 4, entry 2; white
solid. 1H NMR (200 MHz, CDCl3,
258C, TMS): d=3.60 (s, 6H; CH3),
3.72 (s, 3H; O-CH3), 5.78 (s, 1H;
CH), 6.46 (s, 2H; 2CH), 6.75 (m, 2H;
ArÀH), 6.97 (m, 2H; ArÀH), 7.14–
Scheme 4. Plausible mechanism for the tandem acetalization of aldehydes.
7.31 ppm (m, 8H; ArÀH); 13C NMR
(100 MHz, CDCl3, 258C): d=32.58,
39.17, 55.11, 109.00, 113.47, 118.54,
121.33, 127.38, 128.13, 129.50, 136.61, 137.35, 157.77 ppm; IR (ATR
mode): n˜ =3043, 2943, 1608, 1506, 1237, 1022, 730.71 cmÀ1; HRMS:
m/z calcd for [C26H24N2ONa]+: 423.1832; found: 423.1833.
3,3’-[(4-Chlorophenyl)methylene]bis(5-nitro-1H-indole):
Table 3,
entry 9; Table 4, entry 13; yellow solid. 1H NMR (200 MHz,
[D6]DMSO, 258C, TMS): d=6.26 (s, 1H; CÀH), 7.15 (s, 2H; CÀH),
7.39 (m, 4H; ArÀH), 7.57 (m, 2H; ArÀH), 8.00 (m, 2H; ArÀH), 8.34
(m, 2H; ArÀH), 11.72 ppm (brs, 2H; In-NH); 13C NMR (100 MHz,
[D6]DMSO, 258C): d=37.71, 112.22, 116.18, 116.75, 120.14, 125.71,
127.74, 128.50, 130.08, 130.98, 139.84, 140.29, 142.88 ppm; IR (ATR
mode): n˜ =3308 (NH), 2959, 1316, 1086, 1039, 1012, 828, 794, 735,
665 cmÀ1; HRMS: m/z calcd for [C23H15N4O4ClNa]+: 469.0674; found:
469.0670.
1H NMR spectra of the all products agreed with those of com-
pounds reported in the literature.[4a,b]
Acknowledgements
Figure 5. Recycle study of PVP-PWA catalyst. Reaction conditions: benzalde-
hyde (1 mmol), indole (2 mmol), catalyst (0.01 mmol), MeOH (2.5 mL), N2,
258C. A) Benzaldehyde with indole to BIMs (yield of isolated product after fil-
tration). B) Benzaldehyde-dimethylacetal with indole to BIMs (yield of isolat-
ed product after column chromatography).
S.B.K. is grateful to CSIR-National Chemical Laboratory, Pune,
India for providing the research facilities under the AcSIR scheme.
He also thanks the University Grants Commission, New Delhi,
India for providing the research fellowship.
General procedure for the synthesis of BIMs and TIMs by
tandem acetalization of aldehydes
Keywords: acetals
· acidity · aldehydes · heterogeneous
catalysis · heteropoly acids
Indole (0.234 g, 2 mmol) was added to a mixture of aldehyde
(0.106 g, 1 mmol) in methanol (2.5 mL) and PVP-PWA catalyst
(0.03213 g, 0.01 mmol) at room temperature under an inert N2 at-
mosphere and the mixture was stirred for 2 h. After completion of
the reaction, which was monitored by TLC, the precipitated prod-
uct was separated by filtration, washed with MeOH (5 mL), and dis-
solved in acetone to remove the catalyst. The acetone from the fil-
trate was evaporated at reduced pressure on a Rotavapor appara-
tus to obtain the solid product in 90–97% yield.
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