Preyssler-type heteropoly acid: A new, mild and efficient catalyst for protection of carbonyl compounds

Article abstract of DOI:10.1016/j.cclet.2010.10.052

Preyssler-type heteropoly acid is introduced as a new, mild and efficient catalyst for protection of a variety of carbonyl compounds with 1,3-propane dithiol.

Full text of DOI:10.1016/j.cclet.2010.10.052

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 435–438
www.elsevier.com/locate/cclet
Preyssler-type heteropoly acid: A new, mild and efficient catalyst
for protection of carbonyl compounds
a
a
Mohammad Rahimizadeh a, , Tahmineh Bazazan , Ali Shiri ,
*
Mehdi Bakavoli a, Hassan Hassani b
a Department of Chemistry, School of Sciences, Ferdowsi University of Mashhad, 91775-1436 Mashhad, Iran
b Department of Chemistry, Payam Noor University, Birjand, Iran
Received 24 August 2010
Available online 15 January 2011
Abstract
Preyssler-type heteropoly acid is introduced as a new, mild and efficient catalyst for protection of a variety of carbonyl
compounds with 1,3-propane dithiol.
# 2010 Mohammad Rahimizadeh. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Preyssler-type heteropoly acid; Protection; Carbonyl groups; 1,3-Propanedithiol
The protection of carbonyl compounds as dithioacetals is an important sequence in multi-step preparation of many
importantorganiccompoundsincludingmultifunctionalcomplex molecules[1,2]. Moreover, the electrophilicproperties
of carbonyl group in aldehydes and ketones can be a good choice to be attacked by a nucleophile at the site. Thus, one of
the most popular methods for protection of carbonyl groups is to convert them into their corresponding dithioacetals [3].
There are many methods in the literature for the protection of carbonyl compounds as dithioacetals using various
catalysts or stoichiometric reagents [3]. However, most of these procedures have some restrictions such as low yields
of the products, long reaction times, harsh reaction conditions, difficulties in work-up and the requirement for an inert
atmosphere. Therefore, the search for alternative methods that can overcome these drawbacks is desirable.
In the past three decades, the broad utility of heteropoly acids (HPAs) as solid acids and multi-electron oxidation
catalysts in solution as well as in the solid state for a wide variety of synthetic and industrial organic useful
transformations like alcohol dehydration, alkylation, and esterification has been demonstrated [4–10].
On the other hand, precise control of the acidity in a small-scale reaction with usual corrosive any strong acid is
extremely difficult. Considering the reversible nature of some processes in organic syntheses, low yield in reaction
with pervious usual strong liquid acid is expected. So Preyssler-type heteropoly acid (H14NaP5W30O110) has been
introduced as an efficient alternative. The advantages of Preyssler acid as a solid acid catalyst may include large
number of balanced protons, strong acidity, high hydrothermal stability, and wide pH range stability in solution
[10,11]. A few of researchers have investigated the catalytic performances of Preyssler-type heteropoly acid in a
number of organic synthesis processes [12–15].
* Corresponding author.
E-mail address: rahimizh@yahoo.com (M. Rahimizadeh).
1001-8417/$ – see front matter # 2010 Mohammad Rahimizadeh. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.10.052

[()TD$FIG]
436
M. Rahimizadeh et al. / Chinese Chemical Letters 22 (2011) 435–438
O
Preyssler-type heteropoly acid
S
S
+
HS
SH
R
R'
R
R'
CHCl , rt
3
(3a-p)
(1a-p)
(2)
Scheme 1.
Within our ongoing research program to develop new synthetic methodologies [16–20], we proposed that
Preyssler-type heteropoly acid might be a useful catalyst for thioacetalization of carbonyl compounds. In this paper,
we wish to report a simple and easy method for chemoselective thioacetalization of various aromatic and aliphatic
aldehydes and ketones using Preyssler-type heteropoly acid as a new and efficient catalyst, depicted in Scheme 1.
The Preyssler-type heteropoly acid was prepared and characterized according to the previously published method
[21] and then the reaction of 1,3-propanedithiol (2) with benzaldehyde (1a) as a model reaction was performed in
different solvents and different catalytic amount of Preyssler acid. On the basis of the reaction times and yields, CHCl3
and 0.1 mol% of the catalyst was selected as the most suitable reaction conditions.
Various aldehydes and ketones (1a–1p) used as substrates to react with 1,3-propanedithiol (2). The results in
Table 1 clearly demonstrate that protecting of carbonyl compounds using 1,3-propanedithiol as protection reagent in
Table 1
Protection of different aldehydes and ketones as their corresponding 1,3-dithianes by reaction with 1,3-propanedithiol.a
Entry Substrate
Time (min) Product
Yieldb (%) Entry Substrate
Time (min) Product
5
Yieldb (%)
90
O
O
1a
5
96
1i
S
S
[DT$INLE]
]
H
[TD$INLE]
[TD$INLINE]
H
S
S
O
O
1b
10
92
1j
15
87
S
S
[DT$INLE]
S
H
]
H
[DT$INLE]
S
[TD$INLE]
H
Cl
Cl
O
O
O
1c
1d
1e
1f
10
5
90
95
96
95
1k
1l
15
25
40
45
80
92
83
87
S
S
S
H
[DT$INLE]
H
H
[TD$INLINE]
H
[
[TD$INLE]
S
Br
Br
O
S
S
S
[DT$INLE]
CH3
[TD$INLINE]
[DT$INLE]
[DT$INLE]
S
CH
3
Me
Me
O
O
2
1m
1n
S
S
S
[DT$INLE]
H
[TD$INLINE]
[DT$INLE]
[DT$INLE]
S
MeO
MeO
O
O
2
S
S
S
S
[TD$INLE]
[DT$INLE]
H
[TD$INLINE]
[DT$INLE]
H
HO
O
O
O
S
1g
1h
10
20
85
80
1o
1p
30
20
90
90
S
[DT$INLE]
O N
2
[TD$INLINE]
[DT$INLE]
O2N
[TD$INLE]
H
S
S
O
S
S
[DT$INLE]
[DT$INLE]
H
[TD$INLINE]
[DT$INLE]
S
S
O
N
2
O
N
2
a
Reaction conditions as explained in the experimental procedure and the products are characterized from their spectral data.
Isolated yields.
b

[()TD$FIG]
M. Rahimizadeh et al. / Chinese Chemical Letters 22 (2011) 435–438
437
S
S
O
O
H
H
Preyssler-type heteropoly acid
(0.1 %mol)
100%
HS
SH
+
S
S
CHCl , rt
3
CH
3
CH
3
0%
Scheme 2.
the presence of catalytic amounts of Preyssler-type heteropoly acid is efficient, with almost quantitative conversion
and exclusive selectivity in most cases.
Aromatic aldehydes, with electron donating or electron withdrawing substituents such as 4-chlorobenzaldehyde
(1b), 4-bromobenzaldehyde (1c), 4-methoxybenzaldehyde (1e), 4-hydroxybenzaldehyde (1f) and 4-nitrobenzal-
dehye (1h) could be protected to afford the corresponding 1,3-dithianes with high conversions in the same conditions.
This method is also applicable for aldehydes bearing double bond like cinnamaldehyde. It was converted to the
corresponding protected product without any reaction on the double bond (entry 1i).
The thioacetalization and thioketalization reactions were also examined over the Preyssler acid system. The linear
chain aldehydes such as hexanal and octanal can also be converted to the corresponding thioacetals in relatively good
yields (entries 1j and 1k). Cyclopentanone and cyclohexanone are also worked well, but the reactivity of
cyclopentanone seems to be slightly lower than that of cyclohexanone (entries 1o and 1p). This method is also
applicable for protecting of relatively unreactive and steric-hindered ketones such as benzophenone and
diisopropylketone (entries 1m and 1n).
It is noteworthy that the method can also be extended for the chemoselective protection of an aldehyde in the
presence of a ketone. For instance, when an equimolar mixture of benzaldehyde and acetophenone was allowed to
react with 1,3-propanedithiol in the presence of a catalytic amount of Preyssler acid, only the corresponding aldehyde
was protected (Scheme 2). This indicates that the presented protocol is potentially applicable for the chemoselective
protection of aldehydes to the corresponding thioacetals in the presence of ketones in multi-functional compounds. In
this condition, the aldehyde functionality of a keto-aldehyde compound was protected chemoselectively under
identical condition.
These reactions which proceeded in CHCl3 were performed heterogeneously and the isolation of the catalyst from
the reaction mixture was easy and not a time consuming process. Thus, the reusability of the catalyst was checked.
After each run, Preyssler acid was filtered and washed with CHCl3 and dried at 70 8C for 1 h. This catalyst was reused
for the protection reaction four times without significant loss of activity.
The comparison of Preyssler-type heteropoly acid with previous published catalysts for the dithioacetalization of a
typical carbonyl compound like benzaldehyde with 1,3-propanedithiol was studied. As it is shown in Table 2, the
Preyssler-type heteropoly acid is a good catalyst for the dithioacetalization of carbonyl compounds.
Table 2
The comparison of some other catalyst with Preyssler acid.
Entry
Catalyst
mol% catalyst
Time (min)
Yield (%)
Literature
1
2
3
4
5
6
Y(OTf)3
a
5
10
10
10
10
0.1
45
170
35
89
94
90
94
95
96
[22]
[23]
[24]
[25]
[26]
–
NiCl2
DBHb
c
MoO2(acac)2
d
180
10
I2
Preyssler acide
5
a
The reactions were conducted in a solution of CH2Cl2:MeOH (5:1) and also the ketones could not be protected.
1,3-Dibromo-5,5-dimethylhydantoin.
b
c
d
e
The reactions were conducted under an atmosphere of argon.
The reactions eventually were quenched with aqueous solutions of Na2S2O3 and the catalyst could not be recovered.
Reaction conditions as exemplified in the experimental procedure.

438
M. Rahimizadeh et al. / Chinese Chemical Letters 22 (2011) 435–438
In conclusion, a very simple and benign method for the protection of various aldehydes and ketones as dithioacetals
using a catalytic amount of Preyssler-type heteropoly acid has been demonstrated. Further, the catalyst can be readily
recovered and reused, thus making this method profitable and environmentally acceptable. A high degree of
chemoselectivity, easy work-up, low reaction times and high product yields are the main advantages of this new
method, which will make it a useful and important addition to the present methodologies.
1. Experimental
1.1. General procedure for thioacetalization of aldehydes and ketones (1a–1p)
To a magnetically stirred solution of aldehyde or ketone (1a–1p) (2 mmol) and 1,3-propane dithiol (2) (2.1 mmol,
0.23 g) in CHCl3 (10 mL), Preyssler acid (0.1 mol%) was added. The mixture was stirred at room temperature and the
progress of the reaction was monitored by TLC using petroleum ether:EtOAc (7:3). After the completion of the
reaction, Preyssler acid was filtered and the mother liquor was washed with 10% NaOH solution (10 mL) and water
(10 mL) and dried over anhydrous Na2SO4, respectively. The solvent was removed under reduced pressure to obtain
the corresponding product (3a–3p).
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