Phosphorus pentoxide-montmorillonite K-10 as catalyst for the preparation of 1,1-diacetates under solvent-free conditions

Article abstract of DOI:10.1080/10426500490468146

A facile and efficient method for the preparation of 1,1-diacetates of aldehydes is improved. P₂O₅/montmorillonite K10 catalyzed 1,1-diacetates formation from aldehydes in dry media. Both aromatic and aliphatic aldehydes gave high yields (70-95%) of the corresponding 1,1-diacetates. Advantages of this method are the use of an inexpensive and selective catalyst, with high yields in simple operation and short reaction time under solvent-free conditions.

Full text of DOI:10.1080/10426500490468146

Phosphorus, Sulfur, and Silicon, 179:1341–1346, 2004
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Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490468146
PHOSPHORUS PENTOXIDE-MONTMORILLONITE
K-10 AS CATALYST FOR THE PREPARATION
OF 1,1-DIACETATES UNDER SOLVENT-FREE
CONDITIONS
Hossein Eshghi and Zinat Gordi
Sistan and Baluchestan University, Zahedan, Iran
(Received September 11, 2003; in final form October 30, 2003)
A facile and efficient method for the preparation of 1,1-diacetates
of aldehydes is improved. P2O5/montmorillonite K10 catalyzed 1,1-
diacetates formation from aldehydes in dry media. Both aromatic and
aliphatic aldehydes gave high yields (70–95%) of the corresponding
1,1-diacetates. Advantages of this method are the use of an inexpensive
and selective catalyst, with high yields in simple operation and short
reaction time under solvent-free conditions.
Keywords: Aldehydes; 1,1-diacetates; montmorillonite; phosphorus
pentoxide; protection
Many organic reactions have been devised in which the reagents are
deposited on various inorganic solid supports such as silica gels, zeo-
lites and layered clay minerals. The variety and the versatility of these
heterogeneous methods are evident from recent reviews.1−3 Since the
early work of Toda,4 application of solid state organic chemistry has
been under intensive investigation and recently has been reviewed.5
The observations obtained were: decreasing reaction time, increasing
yield and/or selectivity, cleaner reaction and easier work-up.4−6
1,1-Diacetates are a useful protective group for aldehydes.7 Relative
acid stability of 1,1-diacetates compared to the corresponding acetal,8
is an interesting feature of such 1,1-diacetates as carbonyl protect-
ing groups. These are also important building blocks for the synthe-
sis of dienes for Diels-Alder cycloaddition reaction.9 Usually, they are
synthesized10 from aldehydes and acetic anhydride using strong pro-
ton acids and Lewis acids as catalyst, typically such as sulfuric acid,11
We are thankful to the Sistan and Baluchestan University Research Council for partial
support of this work.
Address correspondence to Hossein Eshghi, Department of Chemistry, Sistan and
Baluchestan University, Zahedan, 98135-674, Iran. E-mail: heshghi@hamoon.usb.ac.ir
1341

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H. Eshghi and Z. Gordi
phosphoric acid,12 methanesulfonic acid,12 Nafion-H,13 zinc chloride,14
ferric chloride,8 iodine,15 or phosphorous trichloride.16 These methods
have not been entirely satisfactory, owing to such drawbacks as low
yields (4% in the case of 4-nitrobenzaldehyde16), long reaction time (up
to 120 h in the case of 2-furaldehyde16) and tedious work-up. In recent
years, β-zeolite,17 sulfated zirconia,18 montmorillonite clays,19 expan-
sive graphite,20 trimethylchlorosilane and sodium iodide,21 scandium
triflate,22 N-bromo succinimide,23 PVC-FeCl3 or TiO2/SO−2 solid su-
24
4
per acid25 were employed as catalyst for this purpose to obtain relatively
better results. Although some of these methods present a convenient
procedure with good to high product yields, most of the currently avail-
able methods suffer from strong acidic conditions, require high temper-
ature or long reaction time and expensive catalyst loading. Therefore
the discovery of a novel mild and catalytic protocol for the efficient con-
version of aldehydes into 1,1-diacetates is of general interest. We report
here a procedure using P2O5/montmorillonite K10 (Clayphos, for short),
as a catalyst for the preparation of 1,1-diacetates from aldehydes in the
absent of solvent.
RESULTS AND DISCUSSION
As shown in Table I, a series of 1,1-diacetates were synthesized using
Clayphos as a catalyst at room temperature in solvent free media. Both
aromatic and aliphatic aldehydes gave high yields of the corresponding
acylals except 4-(dimethylamino) benzaldehyde that remained intact in
the reaction mixture after prolonged reaction time (20 h).
R CHO + Ac2O P O /Montmorillonite −K-→10 R CH(OAc)2
(1)
2
5
RT, 10 min. Grinding
We were pleased to find that aromatic aldehydes containing electron
withdrawing or donor groups gave the corresponding 1,1-diacetates in
good to excellent yields in shorter time using P2O5/montmorillonite K10
than some of the reported methods. The nature of the substituents
on the aromatic ring seems to have no effect on the reaction sys-
tems. Moreover, the compounds with sensitive rings (furfural) or func-
tional groups (cinnamaldehyde) successfully were converted to the
corresponding 1,1-diacetates. However, phenol groups were also pro-
tected as acetates in hydroxyl containing aromatic aldehydes under
these conditions.27 Whereas literature showed that protection of 4-HO-
benzaldehyde failed19a in the presence of montmorillonite K-10 after
48 h, or take place19b in the presence of Fe+3-montmorillonite after 8 h,
our catalyst improved this reaction by 95% yield in 10 min.

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H. Eshghi and Z. Gordi
Aliphatic as well as αβ-unsaturated aldehydes also produced their
corresponding 1,1-diacetates in good yields under the same reaction
conditions. It is also worth noting that ketones, such as acetophenone
and benzophenone did not give any 1,1-diacetates under the same reac-
tion conditions and this suggested that chemoselective protection of an
aldehyde in the presence of a ketone could be achieved. Therefore we
have provided an alternative preparation of 1,1-diacetates from alde-
hydes with the advantages of the use of an inexpensive and selective
catalyst with high yields in simple operation, and short reaction times
in dry media.
Mention must be made here that the procedure of preparation of
Clayphos (P2O5/montmorrilonite K-10) and synthesis of 1,1-diacetates
were done in a septum vessels. Thus, the probability of absorption of wa-
ter with P2O5 to form H3PO4 was ruled out. The identity of the Clayphos
for this transformation is not clear.
EXPERIMENTAL
All melting points recorded are uncorrected open capillary measure-
ments. IR spectra were recorded on a Shimadzu -IR 470 spectropho-
1
tometer. H-NMR spectra were recorded on a Bruker-80 MHz instru-
ment using tetramethylsilane (TMS) as an internal standard.
Preparation of Clayphos
A mixture of phosphorous pentoxide (3 g) and montmorrilonite K-10
(7 g) was placed in a flask and stirred for 4 h. This homogeneous, free
flowing powder reagent is sensitive towards moisture and should be
stored in a desiccator.
Preparation of 1,1-Diacetates
Typical Procedure
In a typical reaction, a mixture of benzaldehyde (1.06 g, 10 mmol),
acetic anhydride (2.04 g, 20 mmol), and Clayphos (1 g, 20 mol% of
P2O5/benzaldehyde) was ground throughly in a mortar for 10 min.
The progress of the reaction was monitored by TLC and/or IR spec-
troscopy (νCO acylal; 1740–1760 cm−1). After completion of the reac-
tion, dichloromethane (50 ml) was added to the mixture and filtered.
The solid material was washed with dichloromethane (2 × 10 ml).
The filtrates were combined together and were washed with a sat-
urated solution of NaHCO3 and brine and then dried over MgSO4.

1,1-Diacetates
1345
Removal of the solvent gave the crude product which was subse-
quently purified by column chromatography (silica gel; eluted with n-
hexane/dichloromethane) to afford pure benzaldehyde 1,1-diacetate in
95% yield. m.p.: 45–46◦C (lit.,16 m.p. 44–46◦C); IR (KBr); 1750 cm−1
1H-NMR (80 MHz, CDCl3): 7.65(s, 1H), 7.4–7.8(m, 5H), 2.2(s, 6H).
.
4-Acetoxybenzaldehyde-1,1-diacetate: m.p.: 89–90◦C (lit.,19b m.p.
89–90◦C); IR (KBr); 1760 cm−1. 1H-NMR (80 MHz, CDCl3): 10.5(s, 1H),
6–6.8(complex AAꢁBBꢁ, 4H), 2.2(s, 3H), 2.0 (s, 6H).
4-Acetoxy-3-methoxybenzaldehyde-1,1-diacetate: m.p.: 90–91◦C
(lit.,19b m.p. 90–91◦C); IR (KBr); 1760 cm−1. 1H-NMR (80 MHz, CDCl3):
8.0(s, 1H), 7.3–7.7(m, 3H), 4.2(s, 3H), 2.5(s, 3H), 2.3(s, 6H).
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