Polyaniline-Supported Sulfuric Acid Salt as a Powerful Catalyst for the Protection and Deprotection of Carbonyl Compounds

Article abstract of DOI:10.1055/s-2003-41412

Structurally different carbonyl compounds were converted into their corresponding cyclic acetals using polyaniline-sulfate salt as catalyst in dry toluene in excellent yield. In turn, useful deacetalization in aqueous medium was demonstrated. Chemoselective protection of carbonyl compounds was also demonstrated. The advantages of the polyaniline-sulfate salt are ease of preparation and handling, stability, reusability and activity.

Full text of DOI:10.1055/s-2003-41412

LETTER
1793
Polyaniline-Supported Sulfuric Acid Salt as a Powerful Catalyst for the
Protection and Deprotection of Carbonyl Compounds
P
olyaniline Sa
r
lt as
C
at
i
alyst fo
n
r
Protection
i
and
D
e
v
protection asan Palaniappan,*b Puli Narender,a Chandrasekaran Saravanan,a Vaidya Jayathirtha Raoa
a
Organic Chemistry Division II, Indian Institute of Chemical Technology, Hyderabad 500 007, India
b
Organic Coatings & Polymers Division, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Fax +91(40)27160757; E-mail: palaniappan@iict.ap.nic.in
Received 29 July 2003
Abstract: Structurally different carbonyl compounds were convert-
ed into their corresponding cyclic acetals using polyaniline-sulfate
salt as catalyst in dry toluene in excellent yield. In turn, useful deac-
etalization in aqueous medium was demonstrated. Chemoselective
protection of carbonyl compounds was also demonstrated. The ad-
vantages of the polyaniline-sulfate salt are ease of preparation and
handling, stability, reusability and activity.
Scheme 1
Key words: protection, deprotection, chemoselectivity, poly-
aniline-sulfate salt, carbonyl compounds
O
O O
(i), (ii) & (iii)
12h
O
O
5
2
4
2
Protection and deprotection of carbonyl compounds are
frequently encountered synthetic steps in the preparation
of complex compounds.1,2 The protective reaction may be
catalyzed by protic acids,3 Lewis acids,4 iodine,5 N-
bromosuccinamide,6 lanthanides,7 metal-catalysts,8 zeo-
lites,9 sulfated zirconia,10 montmorillonite clay,11 zeolite
HSY-36012 and graphite.13 In recent years, the develop-
ment of more economical and environmental friendly
conversion processes is gaining interest in the chemical
community. In this communication, polyaniline-sulfate
salt is used for chemoselective protection, deprotection of
carbonyl compounds. The polyaniline-sulfate salt can be
0%
100%
Scheme 2
Scheme 3
prepared easily14a and possesses useful handling proper- However, p-hydroxybenzaldehyde takes a longer time to
ties with regard to recovery, reusability, stability and ac- give the product (85% in 150 min, entry 6) and this may
tivity.14b
be due to the lower solubility of p-hydroxybenzaldehyde
in toluene. The protection reaction carried out on naphth-
aldehyde resulted in 95% yield in 90 minutes (entry 9);
slower than that of cinnamaldehyde (96% in 60 min., en-
try 8), possibly because of the bulkier nature of naph-
thaldehyde. Catalytic application of the polyaniline-
sulfate salt was extended to protection of different types
of ketones such as acetophenone and benzophenone (en-
tries 10 and 11). The time required for the completion of
the reaction was found to be longer for benzophenone
(90% yield in 240 min, entry 11) than acetophenone (95%
yield in 80 min, entry 10). Furthermore, excellent yields
were observed with alicyclic keto compounds (entries 12,
13), acyclic aliphatic aldehydes (entry 14) and b-keto es-
ters (entry 15).
We found that polyaniline-sulfate salt could be used for
protection and deprotection of carbonyl compounds with
1,2-ethane-diol in dry toluene. There was no appreciable
reaction when benzaldehyde was allowed to react with
1,2-ethane-diol in the absence of catalyst in toluene. How-
ever, the presence of polyaniline-sulfate salt (20 wt% with
respect to carbonyl compound) provided 98% yield of
protected benzaldehyde (Table 1, entry 1) in a clean reac-
tion with simple work up procedure.15a Encouraged by
this result, we carried out further investigations on several
carbonyl compounds (Table 1, entries 2–15). Aldehydes
containing electron-donating groups (90–98% yield in
45 min, entries 2–5) react faster than those containing
electron-withdrawing group (85% yield in 180 min, entry
7).
Reusability of the catalyst was checked by the protection
of p-chlorobenzaldehyde with 1,2-ethanediol with polya-
niline-sulfate salt in dry toluene medium, which results in
96% yield. The polyaniline-sulfate salt was recovered and
reused for a further seven times and resulted in 93–96%
yields. This indicates that polyaniline-sulfate salt does not
lose its activity and can be reused.
SYNLETT 2003, No. 12, pp 1793–1796
Advanced online publication: 28.08.2003
DOI: 10.1055/s-2003-41412; Art ID: ,ftx,en;D15803st.pdf
© Georg Thieme Verlag Stuttgart · New York
2
9
.0
9
.2
0
0
3

1794
S. Palaniappan et al.
LETTER
Table 2 Deprotection of Protected Carbonyl Compounds with Wa-
ter Using Polyaniline-Sulfate Salt (20% w.r.t. Protected Carbonyl
Compounds)
Table 1 Protection of Carbonyl Compounds with 1,2-Ethanediol in
Prescence of Polyaniline-Sulfate Salt (20% w.r.t. Carbonyl Com-
pounds) in Toluene
NH
O
O
R1
O
(H2SO4) x
n
R2
R1
C
R2
C
(R1 , R2 = Alkyl,
(R1 , R2 = Alkyl,
Water
Aryl or Hydrogen)
Aryl or Hydrogen)
Entry Substrate
Time (min)
45
Isolated yield (%)
98
Entry Substrate
Time (min) Isolated yield (%)
1
2
3
4
5
CHO
1
30
95
96
O
O
45
45
45
45
97
96
98
90
CHO
H3C
2
30
O
CHO
O
Cl
H3C
3
30
45
96
90
CHO
O
O
H3CO
CHO
Cl
4
O
HO
OCH3
O
6
7
8
150
180
60
85
85
96
CHO
CHO
H3CO
5
40
95
HO
O
O
HO
O2N
OCH3
CHO
6
7
45
90
90
95
O
O
HO
9
90
95
CHO
O
O
O2N
10
11
80
95
90
O
8
9
30
95
90
O
O
CH3
240
O
C
120
O
O
12
13
60
90
90
85
O
O
10
11
45
95
90
O
O
CH3
14
15
90
90
95
90
CHO
180
O
O
OC2H5
Synlett 2003, No. 12, 1793–1796 © Thieme Stuttgart · New York

LETTER
Polyaniline Salt as Catalyst for Protection and Deprotection
1795
Table 2 Deprotection of Protected Carbonyl Compounds with Wa-
ter Using Polyaniline-Sulfate Salt (20% w.r.t. Protected Carbonyl
Compounds) (continued)
takes 120 minutes, (entry 9) and benzophenone takes 180
minutes, (entry 11) and p-nitro benzaldehyde takes 90
minutes, presumably due to the electron-withdrawing
group in this latter case (entry 7).
NH
O
O
R1
O
(H2SO4) x
In summary, we have demonstrated that polyaniline-
sulfate salt acts as a highly efficient polymer supported
acid catalyst in protection and deprotection of carbonyl
compounds with 1,2-ethanediol. Furthermore, a very use-
ful chemoselectivity for aliphatic and aromatic carbonyl
compounds has also been demonstrated. This work leads
to a simple, convenient, mild and efficient protection and
deprotection of carbonyl compounds in organic synthesis.
n
R2
R1
C
R2
C
(R1 , R2 = Alkyl,
(R1 , R2 = Alkyl,
Water
Aryl or Hydrogen)
Aryl or Hydrogen)
Entry Substrate
12
Time (min) Isolated yield (%)
45
95
O
O
O
13
45
90
Reference
O
(1) (a) Greene, T. W. Protective Groups in Organic Synthesis;
Wiley: New York, 1981. (b) Loewenthal, H. J. E. In
Protective Groups in Organic Chemistry; McOmie, J. F. W.,
Ed.; Plenum: New York, 1973, 323–402.
14
15
30
45
96
90
O
O
(2) Kunz, H.; Waldmann, H. Comprehensive Organic Synthesis,
Vol. 6; Trost, B. M.; Fleming, I., Eds.; Pergamon: New
York, 1991, 677–681.
(3) Freeman, I.; Karchetski, E. M. J. Chem. Eng. Data 1977, 22,
335.
(4) (a) Scriabine, I. Bull. Soc. Chim. Fr. 1961, 1194.
(b) Michie, J. K.; Miller, J. A. Synthesis 1981, 824.
(5) Thomas, J. M. Angew. Chem., Int. Ed. Engl. 1988, 27, 1673;
Angew. Chem. 1988, 100, 1735.
(6) Karimi, B.; Seradj, H.; Ebrahimian, R. G. Synlett 2000, 623.
(7) (a) Luche, J.-L.; Gemal, A. L. J. Chem. Soc., Chem.
Commun. 1978, 976. (b) Gemal, A. L.; Luche, J.-L. J. Org.
Chem. 1979, 44, 4187.
(8) (a) Ott, J.; Ramos Tombo, G. M.; Schmid, B.; Venanzi, L.
M.; Wang, G.; Ward, T. R. Tetrahedron Lett. 1989, 30,
6151. (b) Gorla, F.; Venanzi, L. M. Helv. Chim. Acta 1990,
73, 690.
(9) (a) Kumar, P.; Hedge, V. R.; Kumar, T. P. Tetrahedron Lett.
1995, 601. (b) Pereira, C.; Gigante, B.; Marcelocurto, M. J.;
Carreyra, H.; Perot, G.; Guisnat, M. Synthesis 1995, 1077.
(10) Rajn, S. V. J. Chem. Res., Synop. 1996, 68.
(11) Joshi, M. V.; Narasimbam, C. S. J. Catal. 1993, 141, 308.
(12) Ballini, R.; Bordoni, M.; Bosica, G.; Maggi, R.; Sartori, G.
Tetrahedron Lett. 1998, 39, 7587.
We observed that the protection of aldehydes takes place
faster in the presence of polyaniline-sulfate salt when
compared to the ketones (Table 1). The difference in reac-
tivity of the polyaniline-sulfate salt towards aldehydes
and ketones gave us the impetus to study chemoselective
reactions. We carried out an initial experiment with an
equimolar (1.0 equiv) mixture of p-methylbenzaldehyde
(1) and acetophenone (2) with 1.5 equivalent of 1,2-
ethanediol and polyaniline-sulfate salt (20 wt% with re-
spect to p-methylbenzaldehyde) in dry toluene. The ratio
of the products was determined by 1H NMR analysis. The
reaction resulted in complete conversion of p-methylben-
zaldehyde (3) and no conversion of acetophenone (2).
One can infer that the reaction with the aromatic aldehyde
takes place faster than with the aromatic ketone in the
presence of polyaniline-sulfate salt (Scheme 1). Likewise
we were able to achieve selective protection of an alicy-
clic ketone (4) in the presence of an aromatic ketone (2)
by this method (Scheme 2). Furthermore we studied
chemoselective protection of an aliphatic aldehyde (6) in
the presence of an aromatic aldehyde (7), which also ex-
hibited more selectivity towards the heptaldehyde (100%
conversion) (8) than p-chlorobenzaldehyde (10%) (9)
(Scheme 3). Polyaniline-sulfate salt thus demonstrates
pronounced selectivity between these functionalities.
(13) Jin, T. S.; Ma, Y. R.; Zhang, Z. H.; Li, T. S. Synth. Commun.
1997, 3379.
(14) (a) Preparation of Polyaniline-Sulfate Salt: In a typical
experiment, benzoyl peroxide (24.2 g, 0.1 M) was dissolved
in 300 mL acetone. Then 200 mL aqueous solution
containing 10 g of sodium lauryl sulfate (0.034 M) was
added into the above solution slowly. To this solution, 465
mL aqueous solution containing 27 mL of (1.0 N) sulfuric
acid and 9.3 g aniline (0.1 M) was added dropwise over 15–
20 min and the mixture was stirred at 40 °C for 8 h. The
precipitated polyaniline salt was filtered, washed with
distilled H2O, followed by MeOH and acetone. The sample
was dried at 100 °C to a constant weight. (b) Physical Data
of Polyaniline-Sulfate Salt: Yield: 82.1 % with respect to
the amount of aniline used. Conductivity (0.04 S/cm),
Sulfuric acid group present in polyaniline salt (30 %), Pellet
density (1.15 g/cm3), Particle size (0.3–75 mm), Elemental
analysis: C, 52.5%; H, 3.3%; N, 10.7%; S, 7.4 %.
(15) General Experimental Procedures: (a) For protection: In
a 50 mL round-bottom flask was placed p-
The cyclic acetals and ketals were deprotected to their
corresponding carbonyls in excellent yields (90–96%) us-
ing polyaniline-sulfate salt in aqueous medium15b and the
results are shown in Table 2 (entries 1–15). Cyclic acetals
of aromatic aldehydes (entries1–6), conjugated aldehydes
(entry 8), aromatic ketones (entry 10), alicyclic ketones
(entries12, 13), acyclic aliphatic aldehydes (entry 14) and
b-keto esters (entry 15) are effectively deprotected in 30–
45 minutes. However, the cyclic acetal of naphthaldehyde
methylbenzaldehyde (1 g, 8 mmol), 1,2-ethanediol (0.75 g,
12 mmol), 200 mg activated polyaniline-sulfate salt (20 wt%
Synlett 2003, No. 12, 1793–1796 © Thieme Stuttgart · New York

1796
S. Palaniappan et al.
LETTER
methylbenzaldehyde (1 g, 6 mmol), 200 mg activated
with respect to p-methylbenzaldehyde) and 25 mL of
toluene. This was refluxed for 45 min with a Dean–Stark
apparatus for azeotropic removal of H2O; reaction being
monitored by TLC until the starting material disappeared.
The reaction mixture was cooled and filtered to remove the
catalyst. The filtrate was washed with H2O and the organic
phase separated, dried over Na2SO4, filtered and
concentrated in vacuum. The crude mixture was purified by
preparative column chromatography (1:99 EtOAc:hexane)
affording the cyclic acetal of p-methylbenzaldehyde
(Table 1). (b) For Deprotection: The cyclic acetal of p-
polyaniline-sulfate salt (20 wt% with respect to cyclic acetal
of p-methylbenzaldehyde) and 25 mL of water in a 50 mL
round bottom flask were refluxed for 30 min, and the
reaction was monitored by TLC. The reaction mixture was
cooled and filtered to remove the catalyst. The filtrate was
extracted with CHCl3 and the organic extract dried over
Na2SO4, filtered and concentrated in vacuum. The resultant
sample was purified by preparative column chromatography
(1:99 EtOAc:hexane) affording the parent p-methylbenz-
aldehyde (Table 2).
Synlett 2003, No. 12, 1793–1796 © Thieme Stuttgart · New York