An efficient and chemoselective method for the preparation of

An efficient and chemoselective
method for the preparation of acylals from different aldehydes using kaolin
supported catalyst (P2O5) under solvent-free conditions is described herein. The present
protocol offers several advantages including use of inexpensive and non-toxic
catalyst support i.e., natural kaolin. Preparation of the
supported catalyst is easy, the process is simple in operation, maintaining
solvent free conditions, with short reaction times, high yields and affording
selective protection of aldehyde in presence of ketone.

Keywords: Kaolin,P2O5, solvent-free, chemoselective, support

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Protection and deprotection of carbonyl groups are extremely
important steps in multistep syntheses due to acylal stability in neutral or
basic media as well as their easy formation and deprotection1-3.
Acylals have been used as cross-linking agents for cellulose in cotton,
substrates in nucleophilic substitution reactions, intermediates in industries
and starting materials for the synthesis of valuable vinyl acetates and acetoxy
dienes for Diels Alder cycloaddition reaction 4-5.

The conventional
methods employed for the preparation of 1,1-diacetates from aldehydes and
acetic anhydride have been using protic or Lewis acids as catalysts for a long
time. Numerous catalysts have been employed for this reaction such as triflic
acid6, b-zeolite7,
Sc(OTf)3 (Ref 8), Iodine9, FeCl3 (Ref 10),
NBS (Ref 11), Cu(OTf)2 (Ref 12), Bi(OTf)3 (Ref 13),
CAN (Ref 14), AlPW12O40 (Ref 15),
LiBF4 (Ref 16), Zn–montmorillonite17, In(OTf)3 (Ref 18),H2NSO3H
(Ref 19), ZrCl4 (Ref 20), Bi(NO3)35H2O (Ref 21),
Wells–Dawson acid (H6P2W18O6 24H2O) (Ref 22),
silica sulfuric acid23, silica-bonded sulfonic acid24,
BEA-SO3H (Ref 25), silica-bonded
propyl-diethylene-triamine-N-sulfamic acid26, [email protected]/Schiff
base complex of Cr(III) (Ref 27) are reported.

Catalyst supported on
natural material is in demand in various industrial applications due to their
ecofriendly nature28,29. One such natural support is kaolin (clay)
which is widespread, easily available and low-cost chemical substances and
shows good potential as support material for catalyst. Both in their native
state and in numerous modified forms, kaolin is versatile materials that
catalyze a variety of chemical reactions. Transition metal chlorides supported
on natural support such as kaolin, montmorillonite K10 and KSF were employed
for the esterification of tert-butanol to tert-butyl acetate30.
Thermally activated Nigerian Ukpor kaolinite clay and Udi clay were shown to be
good catalysts for the esterification of propanol31. Brazilian
kaolinite intercalated with a porphyrin derivative catalyzed Baeyer–Villiger
oxidation of cyclohexanone to ?-caprolactam by hydrogen peroxide. Bizaiaet al.,
2009 (Ref 32) used the same catalyst for the epoxidation of
cyclooctene and oxidation of cylcohexane to cyclohexanone. Acid-treated clay
(K10, bentonite, or kaolin) catalyzed the triazenes formation by diazotization
of aryl amines followed by addition of a cyclic secondary amine33. Gordia Z et al., 2012 (Ref 34) used natural
kaolin supported sulfuric acid for the synthesis of bis(indolyl)methanes.
1,3-Dibromo-5,5-dimethylhy-dantoin (DBH)/kaolin was reported the synthesis of
14-aryl-14H-dibenzoa,jxanthenes under solvent-free conditions.  Phosphoric acid modified kaolin
supported ferric-oxalate catalyst was used for the degradation of phenol35. Use of heterogeneous
catalysts and solvent-free conditions simplify the purification processes. In
this paper natural kaolin supported phosphorus pentoxide (P2O5)
was employed as an efficient catalyst for the preparation of 1,1-diacetates
from aldehydes in solvent-free condition (Scheme I).

 

 

Scheme I — Synthesis of 1,1-diacetates

 

We started our study with preparation of kaolin supported P2O5.
After this, the supported catalyst was tested for its catalytic potential in
different solvents. For this we performed the reaction of benzaldehyde (2 mmol)
with acetic anhydride (4 mmol) in the presence of 50 mg of P2O5/kaolin.
Among the tested solvents, acetonitrile was found best in terms of yield and
time (Table I, entry 4). Then we carried out the reaction without solvent,
which gave 83% yield in 30 min. The result of screening of solvents for the
1,1-diacetate synthesis is given in Table I.

After we came to know
that reaction performed without solvent is the best reaction condition, we had
to further determine the optimum amount of catalyst required for the reaction.
For this we started from 10 mg of supported catalyst, when reaction could be
completed in 2 h, 10 min with 65% yield. In this way catalyst amount
was increased upto 70 mg with 10 mg increment at each reaction. It has been
observed that as amount of P2O5/kaolin was increased,
rate also increased. The yield of the product under solvent-free conditions is
higher and reaction time is shorter than the reactions carried out in solvent.
The optimum amount of catalyst was found to be 0.050 g P2O5/kaolin
at room temperature. Table II summarizes the result of this study. We also
tried kaolin and P2O5 both separately for the conversion
of aldehyde to diacetate. However, no reaction could be observed in kaolin
alone, but in P2O5 reaction was observed at a slower rate
than using P2O5 supported in kaolin, which may be due to
a more uniform distribution of P2O5 on kaolin.

Then the optimized
reaction conditions were applied for the preparation of different acylals.
Aliphatic as well as aromatic aldehydes produced their corresponding
1,1-diacetates in good yields (Table III). Aromatic aldehyde containing electron donating and
withdrawing groups gave 1,1-diacetate in excellent yield with shorter reaction
time. 4-Diethylaminobenzaldehyde failed to give the corresponding diacetate
under the same reaction conditions. The prepared catalyst was also tested for
some unreported diacetates synthesis as per our literature search. These are 2-bromo-5-fluoro benzylidene diacetate and
2-chloro-5-fluoro benzylidene diacetate, characterized by 1H
and 13C NMR, and IR.

To evaluate the
chemoselectivity of this method, competitive reactions for acylation of
aldehydes in the presence of ketones using P2O5/kaolin as
catalyst were examined. Ketones, such as acetophenone, trimethoxyacetophenone
and benzophenone in presence of aldehyde, did not give 1,1-diacetates under the
same reaction conditions and this indicated that chemoselective protection of
an aldehyde in the presence of a ketone can be achieved (Scheme II).

A mild and efficient
catalyst P2O5/kaolin was prepared and used for the
protection of aldehydes under solvent free conditions. Some advantages of this
protocol are use of inexpensive and non-toxic catalyst support i.e.,
natural kaolin, simple operation, solvent free conditions, short reaction
times, high yields, selectively protect aldehyde in presence of ketone, easy
preparation and handling of the catalyst.

 

Experimental Section

All materials and
solvents were commercial products with the highest purity available (>98%)
and used for the reactions without further purification. 1H NMR
spectra were recorded on a Bruker 300 MHz instrument using
tetramethylsilane (TMS) as an internal standard. All of the products are known
products and all of the isolated products gave satisfactory NMR spectra.

 

Preparation
of the phosphorous pentoxide supported on kaolin

The heterogeneous
catalyst was prepared by the mixing of P2O5 (3 g) and
kaolin (3 g) for 10 min in a sealed round-bottomed flask until a fine and
homogenous powder was obtained. This reagent was heated in an oven at 120°C for
1 h. The homogeneous, free flowing, white powder reagent is sensitive
towards moisture and should be stored in a desiccator.

 

General
procedure for the preparation of gem-diacetates

To a stirred solution
of respective aldehyde (2 mmol) in freshly distilled acetic anhydride (0.48 mL)
was added P2O5/kaolin (0.050 g) and the reaction mixture
was stirred at RT for the time specified in Table I. The reaction was
followed by TLC (n-hexane–EtOAc, 9:1). After completion of the reaction,
the mixture was diluted with ethyl acetate and filtered. The organic layer was
washed with 10% NaHCO3 solution and saturated solution of NaHSO3
and then dried over anhydrous Na2SO4. The solvent was
evaporated under reduced pressure to give the corresponding pure products.