(NH4)3PW12O40 as an efficient and reusable catalyst for the synthesis and deprotection of 1,1-diacetates

Article abstract of DOI:10.1080/00397910701798002

An efficient method for the synthesis of 1,1-diacetates (acylals) from different aldehydes in the presence of (NH4)3PW12O40 and acetic anhydride under solvent-free conditions at ambient temperature is achieved. Selective conversion of aldehydes was observed in the presence of ketones. The deprotection of acylals has also been achieved using (NH4)3PW12O40 in methanol. The catalyst was found to possess good activity and considerable level of reusability. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701798002

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(NH4)3PW12O40 as an Efficient and Reusable
Catalyst for the Synthesis and Deprotection
of 1,1‐Diacetates
Jitendra R. Satam a & Radha V. Jayaram a
a Department of Chemistry, Institute of Chemical Technology, Matunga,
Mumbai, India
Version of record first published: 27 Feb 2008.
To cite this article: Jitendra R. Satam & Radha V. Jayaram (2008): (NH4)3PW12O40 as an Efficient and Reusable
Catalyst for the Synthesis and Deprotection of 1,1‐Diacetates, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 38:4, 595-602
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Synthetic Communicationsw, 38: 595–602, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701798002
(NH4)3PW12O40 as an Efficient and Reusable
Catalyst for the Synthesis and Deprotection
of 1,1-Diacetates
Jitendra R. Satam and Radha V. Jayaram
Department of Chemistry, Institute of Chemical Technology, Matunga,
Mumbai, India
Abstract: An efficient method for the synthesis of 1,1-diacetates (acylals) from
different aldehydes in the presence of (NH4)3PW12O40 and acetic anhydride under
solvent-free conditions at ambient temperature is achieved. Selective conversion of
aldehydes was observed in the presence of ketones. The deprotection of acylals has
also been achieved using (NH4)3PW12O40 in methanol. The catalyst was found to
possess good activity and considerable level of reusability.
Keywords: 1,1-diacetates, (NH4)3PW12O40, protection and deprotection, room
temperature
INTRODUCTION
Selective protection and deprotection of carbonyl groups are essential steps in
synthetic organic chemistry.[1] 1,1-Diacetates are one of the most useful
carbonyl protecting groups because of their stability under both neutral and
basic media as well as under mild conditions.[2] The diacetates of a,b-unsaturated
aldehydes are important starting materials for Diels–Alder reactions, and they are
also useful intermediates.[3,4] Moreover, acylal functionality can be converted into
other functional groups by reaction with appropriate nucleophiles.[5,6] Some of the
reported reagents for the preparation of 1,1-diacetates from aldehydes and acetic
anhydride include sulphuric acid,[7] triflic acid,[8] PCl3,[9] iodine,[10] FeCl3,[11]
Received in India August 13, 2007
Address correspondence to Radha V. Jayaram, Department of Chemistry, Institute
of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai 400 019, India.
E-mail: rvjayaram@hotmail.com; jiten_uict@yahoo.co.in
595

596
J. R. Satam and R. V. Jayaram
N-bromosuccinimide,[12] Sc(OTf)3,[13] Cu(OTf)3,[14] Bi(OTf)3,[15] Ceric
ammonium nitrate (CAN),[16] AlPW12O40,[17] b-zeolite,[18] LiBF4,[19] ZrCl4,[20]
In(OTf)3,[21] H2NSO3H,[22] Zn-montmorillonite,[23] bismuth nitrate,[24]
(H6P2W18O62 24H2O),[25] and silica sulfuric acid.[26] Very few reports are appli-
.
cable to both the synthesis and deprotection of acylals.[13,20,26,27] Although some
of these methods have convenient protocols with good to high yields, the majority
of these methods suffer at least from one of the following disadvantages: reactions
under oxidizing conditions, use of halogenated solvents, high temperature, long
reaction times, moisture sensitivity of the catalyst, high cost, and high toxicity.
(NH4)3PW12O40 is an excellent heterogeneous catalyst used for synthetic
organic reactions such as cracking of long-chain hydrocarbons,[28] methanol-
to-hydrocarbon conversion, cracking of alkenes,[29] and esterification.[30]
Moreover, this catalyst is safe, easy to handle, reusable, and environmentally
benign. (NH4)3PW12O40 was prepared by standard procedure.[28] For optimiz-
ation of the reaction conditions, we have tried the conversion of 4-methylben-
zaldehyde (2 mmol) to the corresponding acylal with acetic anhydride
(4 mmol) in the presence of various solvents and also under solvent-free
conditions. In comparison to conventional methods the yield, of the product
under solvent-free conditions is higher and reaction time is shorter
(Table 1). Therefore, we have employed solvent-free conditions for the
conversion of various aldehydes to the corresponding acylals (Table 2).
Excellent yield was obtained with 0.5 mol% of the catalyst with two equiva-
lents of Ac2O. No further increase in the yield was observed on increasing the
amount of catalyst and molar equivalents of Ac2O.
Herein we report an efficient solvent-free method for the preparation of
acylals from different aldehydes in the presence of (NH4)3PW12O40 and Ac2O
(Scheme 1). Various aldehydes were converted to their corresponding diacetates
in good yields and in short reaction times (Table 2). Aliphatic and a,b-unsaturated
Table 1. Conversion of 4-methylbenzaldehyde to its corresponding
diacetate in different solvents and under solvent-free conditions in
the presence of (NH4)3PW12O40 at 308Ca
Entry
Solvent
Yield (%)b
Time (h)
1
2
3
4
5
6
Cyclohexane
Et2O
EtOAc
70
62
52
84
78
99
4
4
2
CH2Cl2
Acetone
Solvent freec
1.5
1.5
1
aReaction conditions: 4-methylbenzaldehyde (2 mmol), Ac2O
(4 mmol), and catalyst (0.5 mol%) at room temperature for specified
time.
bYields refer to isolated pure product.
cReaction was conducted by stirring with 4 mmol of Ac2O.

Synthesis of 1,1-Diacetates
597
Table 2. (NH4)3PW12O40-catalyzed protection of aldehydes under solvent-free
conditions and their deprotection in methanol
Protection
Time (h) Yield (%)a,b,c Time (h) Yield (%)a,b,c
Deprotectiond
Entry
a
Substrate
1.5
2
98
94
88
2
3
3
96
92
86
b
c
2
d
e
2
92
90
2.5
3
89
92
2.5
f
2.5
1
90
99
3
1
92
93
g
h
i
1
2
99
97
1
2
98
92
j
1.5
1
94
94
2
92
90
k
1.5
l
1
2
95
2
94
m
n
85
3
90
—
10
NRe
—
o
10
NR
—
—
aReaction conditions: substrate (2 mmol), Ac2O (4 mmol), and catalyst (0.5 mol%)
at room temperature for specified time.
bThe yields refer to isolated pure product.
1
cThe products are characterized from their spectra (IR, H NMR) and comparison
with authentic samples.
dThe reaction is performed using methanol (2 mL) as a solvent.
eNR: no reaction.

598
J. R. Satam and R. V. Jayaram
Scheme 1.
aldehydes produced acylals in good yields (Table 2, entries j and k). Several
aldehydes with electron-withdrawing as well as electron-donating groups on the
aromatic ring were studied. To explore the activity of the catalyst, we have regen-
erated the parent aldehyde through deprotection of acylal. Deprotection was done
using methanol as a solvent at room temperature (Table 2).
To explore the selectivity of this method, we investigated competitive
reactions for acylal formation in the mixtures of aldehydes (Table 3, entries
1–3) and in the mixture of aldehyde and ketone (Table 3, entries 4 and 5).
These reactions proceeded with high selectivity in the presence of this
catalyst, showing the electronic effects upon these reactions. Ketones are
not affected under similar conditions. (NH4)3PW12O40 has the added
advantage that it is reusable for five cycles and could be used for the
synthesis as well as deprotection of acylals.
EXPERIMENTAL
General Procedure for the Synthesis of 1,1-Diacetates
To a solution of aldehyde (2 mmol) in freshly distilled Ac2O (4 mmol),
(NH4)3PW12O40 (0.5 mol%) was added, and the mixture was stirred magneti-
cally for the specified time at room temperature (Table 2). The course of the
reaction was monitored by thin-layer chromatography (TLC, n-hexane–
EtOAc, 9:1) and GC. The reaction mixture was diluted with Et2O (20 mL)
and filtered to separate the catalyst. The filtrate was washed successively
with 10% aq. NaHCO3 to remove excess of Ac2O and then with aq. sat.
NaHSO3 to remove unreacted aldehyde and dried over Na2SO4. The solvent
was evaporated under reduced pressure to give the corresponding pure
1,1-diacetates, whose data were in full agreement with the melting points
1
and spectral data (IR, H NMR) of the authentic samples. The catalyst was
found to be reusable for five cycles without significantly affecting the yield.
General Procedure for the Deprotection of 1,1-Diacetates
A mixture of 1,1-diacetates (2 mmol) and (NH4)3PW12O40 (0.5 mol%,
0.09 g) in methanol (2 mL) was stirred vigorously for the specified time

Synthesis of 1,1-Diacetates
599
Table 3. Competitive acylal formation reactions using (NH4)3PW12O40 under
solvent-free conditions
Entry
1
Substrate
Yield (%)a,,b
2
3
4
5
aReaction conditions: Ac2O (4 mmol) and catalyst (0.5 mol%) at room temperature
for 1 h.
bYields refer to isolated pure product. The isolation was done by column chromato-
graphy using (n-hexane–EtOAc, 9:1).
(Table 2). The reaction was monitored by TLC. After completion of the
reaction, the reaction mixture was diluted by Et2O (20 mL) and filtered to
separate the catalyst. The filtrate was washed successively with 10% aq.
NaHCO3 to remove excess Ac2O and dried over Na2SO4. The solvent was
evaporated under reduced pressure. The resultant product was passed
through a short column of silica gel (n-hexane–EtOAc, 9:1) to afford pure
aldehyde.

600
J. R. Satam and R. V. Jayaram
Data
Compound 2a. Mp: 43–458C (lit.[2] 44–468C); IR (neat): 3060, 1755, 1605,
1
1470, 1375, 1245, 1205, 1065, 1010, 760 cm21; H NMR (CDCl3): d 7.62
(s, 1H), 7.42–7.24 (m, 5H), and 1.95 (s, 6H).
Compound 2b. Mp: 81–828C (lit.[26a] 82–838C); IR (neat): 3060, 1760, 1605,
1470, 1210, 1065, 1010, 780 cm21; 1H NMR (CDCl3): d 7.56 (s, 1H), 7.32–
7.20 (m, 4H), and 2.12 (s, 6H).
Compound 2c. Mp: 88–908C (lit.[26a] 88.58C); IR (neat): 3075, 1765, 1605,
1
1490, 1380, 1250, 1020, 930, 820 cm21; H NMR (CDCl3): d 8.3 (s, 1H),
7.37–7.12 (m, 3H), and 2.10 (s, 6H).
Compound 2d. Mp: 100–1028C (lit.[2] 98–1028C); IR (neat): 3070, 2190,
1
1750, 1450, 1230, 1010, 830 cm21; H NMR (CDCl3): d 7.68 (s, 4H), 7.35
(s, 1H), and 2.12 (s, 6H).
Compound 2e. Mp: 64–668C (lit.[26a] 65–668C); IR (neat): 3090, 1755, 1535,
1350, 1235, 1200, 1055, 1010, 855 cm21; 1H NMR (CDCl3): d 8.30 (s, 1H),
8.22 (d, J ¼ 7 Hz, 1H), 7.8–7.5 (m, 3H), and 2.0 (s, 6H).
Compound 2f. Mp: 124–1268C (lit.[26a] 124–1268C); IR (neat): 3095, 1765,
1610, 1535, 1345, 1205, 1010, 850 cm21 1H NMR (CDCl3): d 8.10
;
(d, J ¼ 9 Hz, 2H), 7.55 (d, J ¼ 9 Hz, 2H), 7.5 (s, 1H), and 2.0 (s, 6H).
Compound 2g. Mp: oil (lit.[26a] oil); IR (neat): 3030, 2950, 2800, 1760, 1500,
1225, 1015, 790 cm21; 1H NMR (CDCl3): d 7.55 (s, 1H), 7.25 (t, J ¼ 7.5 Hz,
1H), 7.15–7.05 (m, 2H), 6.85 (d, J ¼ 7.5 Hz, 1H), 3.70 (s, 3H), and 2.0 (s, 6H).
Compound 2h. Mp: 68–698C (lit.[9] 68–708C); IR (neat): 3040, 2935, 2810, 1765,
1750, 1230, 1230, 1070, 960, 815 cm21; 1H NMR (CDCl3): d 7.63 (s, 1H), 7.30
(t, J ¼ 8.0 Hz, 1H), 6.90 (d, J ¼ 7.5 Hz, 4H), 2.35 (s, 3H), and 2.10 (s, 6H).
Compound 2i. Mp: 100–1028C (lit.[23] 101–1038C); IR (neat): 3050, 2985, 1760,
1
1750, 1610, 1530, 1250, 1070, 950, 810, 750 cm21; H NMR (CDCl3): d 7.75
(s, 1H), 7.30–7.20 (m, 2H), 7.1–7.0 (m, 2H), 2.18 (s, 3H), and 1.94 (s, 6H).
Compound 2j. Mp: oil (lit.[2]); IR (neat): 2970, 1755, 1460, 1375, 1205, 1090,
1020, 990, 650 cm21; 1H NMR (CDCl3): d 6.6 (t, J ¼ 5.5 Hz, 1H), 2.0 (s, 6H),
1.8–1.2 (m, 8H), and 0.9 (t, J ¼ 5.5 Hz, 3H).
Compound 2k. Mp: 84–868C (lit.[2] 84–868C); IR (neat): 3080, 2930, 1750,
1
1605, 1610, 1495, 1250, 1020, 750 cm21; H NMR (CDCl3): d 7.60–7.10
(m, 6H), 6.8 (d, J ¼ 16 Hz, 1H), 5.60 (dd, J ¼ 16, 7H, 1H), and 2.10 (s, 6H).

Synthesis of 1,1-Diacetates
601
Compound 2l. Mp: oil (lit.[2]); IR (neat): 3020, 2935, 1750, 1500, 1460, 1240,
1105, 935, 830, 750 cm21 1H NMR (CDCl3): d 7.20 (s, 5H), 6.80
;
(t, J ¼ 6 Hz, 1H), 2.72 (m, 2H), and 2.10 (s, 6H).
Compound 2m. Mp: 51–538C (lit.[22] 52–538C); IR (neat): 3095, 1760, 1655,
1500, 1375, 1240, 1060, 940, 750 cm21; 1H NMR (CDCl3): d 7.70 (s, 1H), 7.4
(s, 1H), 6.44–6.30 (m, 2H), 2.12 (s, 6H).
CONCLUSION
In conclusion, herein we report a mild and efficient catalyst for the preparation
of 1,1-diacetates from aldehydes in the presence of acetic anhydride under
solvent-free conditions at room temperature. This method is selective for
the preparation of 1,1-diacetates from aldehydes in presence of ketones.
1,1-Diacetates can be conveniently deprotected by using (NH4)3PW12O40 in
methanol. Good yields were obtained within short reaction times. Reusability
of the catalyst, nontoxic nature, and mild conditions are some of the notable
features of this protocol.
ACKNOWLEDGMENT
The authors are thankful to Regional Sophisticated Instrumentation Centre (RSIC),
Indian Institute of Technology Mumbai, for providing the 1H NMR facility.
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