Highly efficient heterogeneous acetalization of carbonyl compounds catalyzed by a titanium cation-exchanged montmorillonite

Article abstract of DOI:10.1016/S0040-4039(01)01788-9

The titanium cation-exchanged montmorillonite efficiently catalyzed the selective acetalization of various carbonyl compounds as a recyclable solid acid. This heterogeneous catalyst has an advantage of a strikingly simple workup procedure over conventional homogeneous acids.

Full text of DOI:10.1016/S0040-4039(01)01788-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8329–8332
Highly efficient heterogeneous acetalization of carbonyl
compounds catalyzed by a titanium cation-exchanged
montmorillonite
Tomonori Kawabata, Tomoo Mizugaki, Kohki Ebitani and Kiyotomi Kaneda*
Department of Chemical Science and Engineering, Graduate School of Engineering Science, Osaka University,
1
-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Received 15 August 2001; revised 17 September 2001; accepted 21 September 2001
Abstract—The titanium cation-exchanged montmorillonite efficiently catalyzed the selective acetalization of various carbonyl
compounds as a recyclable solid acid. This heterogeneous catalyst has an advantage of a strikingly simple workup procedure over
conventional homogeneous acids. © 2001 Elsevier Science Ltd. All rights reserved.
Acetalization is commonly utilized as a protecting
method for carbonyl functions because dimethyl acetals
and 1,3-dioxolanes are stable under neutral and basic
products and the production of large volumes of salt
wastes during neutralization of the acids. From the
standpoint of environmental demand for chemical pro-
1
3
conditions. It has been usually performed in the pres-
cesses, much attention has been paid to development
4,5
ence of p-toluenesulfonic acid (PTSA), BF –etherate,
of solid catalysts for the acetal formations. These
heterogeneous systems are, however, not applicable to a
wide range of carbonyl compounds. Recently, we have
reported a simple and clean synthesis of a dialkyl
3
FeCl3, or trimethylsilyl trifluoromethanesulfonate
2
(
Me SiOTf). These homogeneous acetalizations often
3
suffer from drawbacks such as troublesome isolation of
a
Table 1. Acetalization of benzophenone with diols using various solid catalysts
Entry
Catalyst
Diol
Conv. of benzophenone (%)b
Amount of adsorbed NH3 (mmol/g)c
+
Ti4 -mont
Ti -mont
1
2
3
4
5
6
7
8
9
meso-2,3-Butanediol
Propane-1,2-diol
Ethane-1,2-diol
89
53
14
11
69
25
9
0
11
8
1.89
1.89
1.89
1.89
0.95
0.75
0.17
n.m.d
0.44
n.m.d
0.36
0.24
4+
4
+
Ti -mont
4
+
Ti -mont
Catechol
3
+
Fe -mont
meso-2,3-Butanediol
meso-2,3-Butanediol
meso-2,3-Butanediol
meso-2,3-Butanediol
meso-2,3-Butanediol
meso-2,3-Butanediol
meso-2,3-Butanediol
meso-2,3-Butanediol
3
+
Al -mont
+
Na -mont
Mont K-10
2
−
SO /ZrO2
4
+
10
11
12
H
H
H
-beta
+
+
-USYe
0
0
e
-mordenite
a
All reactions were carried out under Dean–Stark conditions for 2 h: catalyst (0.15 g), benzophenone (3 mmol), diol (5 mmol), toluene (20 mL).
Conversion of benzophenone was determined by GC using an internal standard method. In all cases, the corresponding acetals were exclusively
obtained.
b
c
d
e
Acid amount was volumetrically measured. The value corresponded to the number of strongly adsorbed NH . See Ref. 6.
3
Not measured.
Si/Al=10.
Keywords: chemoselective acetalization; carbonyl compounds; montmorillonite catalyst.
*
Corresponding author. Tel./fax: +81-6-6850-6260; e-mail: kaneda@cheng.es.osaka-u.ac.jp
0
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01788-9

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T. Kawabata et al. / Tetrahedron Letters 42 (2001) 8329–8332
fluorene derivative from the aromatic alkylation of
phenoxyethanol with fluoren-9-one using a titanium
content: 3.25 wt%). Analyses using XRD, XPS and Ti
4+
K-edge XAFS showed that the Ti -mont had a chain-
like structure of titanium oxides along anionic sheets of
4+
cation-exchanged montmorillonite (Ti -mont) as a
6
6
strong solid acid catalyst. In the course of our studies
the montmorillonite.
4
+
on its catalysis, we found that this Ti -mont catalyst
was also effective for the selective acetalization of a
variety of carbonyl compounds to the corresponding
Acetalizations of benzophenone with diols using vari-
ous types of heterogeneous catalysts were summarized
7
1,3-dioxolanes. This montmorillonite catalyst system
in Table 1. Among diols used, meso-2,3-butanediol
does not require tedious neutralization during workup
procedures.
was the most suitable reagent for the acetalization of
†
benzophenone (entries 1–4). It should be noted that
n+
the metal cation-exchanged montmorillonites (M -
4
+
The Ti -mont catalyst was prepared as follows: 3.0 g
monts) exhibited higher catalytic activity than conven-
tional solid acids such as sulfate ion-treated zirconium
+
of Na -montmorillonite (Kunipia F, Kunimine Indus-
2
−
+
+
+
try Co. Ltd.) was stirred in 100 mL of 1.8 M aqueous
oxide (SO4 /ZrO ), H -beta, H -USY and H -morden-
2
n+
TiCl solution at 50°C for 24 h. The obtained slurry
ite. In the case of the M -monts (entries 1 and 5–7),
4
was filtered and washed with distilled water, then dried
at 110°C to afford 2.5 g of whitish gray powder (Ti
the catalytic activities were increased with increasing
their acid amounts. It can be said that the preeminent
4+
a
Table 2. Acetalization of various carbonyl compounds catalyzed by Ti -montmorillonite
†
Concerning a leaching of Ti4+ into the reaction mixture, we carried out the following experiment. To a reaction vessel with a Dean–Stark trap
and a reflux condenser were successively added the Ti4 -mont (0.15 g), toluene (30 mL), benzophenone (5 mmol) and meso-2,3-butanediol (15
+
4
+
mmol). After the resulting mixture was stirred under reflux for 2 h, the Ti -mont was removed by filtration (acetal yield: 54%). No acetalization
occurred while the colorless filtrate was further refluxed for 4 h. This result clearly shows that active titanium species did not leach from the
4+
Ti -mont during the acetalization.

T. Kawabata et al. / Tetrahedron Letters 42 (2001) 8329–8332
8331
4
+
catalysis of the Ti -mont arises from the strong acid
ization of 4-oxo-4H-1-benzopyran-3-carboxaldehyde 1
occurred exclusively at a formyl group to give 2-(4-oxo-
4H-1-benzopyran-3-yl)-1,3-dioxolane 2, and also ethyl
sites associated with TiO domains within an interlayer
space of the montmorillonite.
2
8
2
-oxocyclopentanecarboxylate 3 afforded an acetal 4
leaving an ester function intact in a high yield. An
unconjugated keto function of the Wieland–Miescher
ketone 5 was selectively protected to 3%,4%,8%,8%a-tetra-
hydro - 8%a - methylspiro[1,3 - dioxolane - 2,1%(2%H) - naph-
Table 2 shows the acetalization of various carbonyl
4+
compounds using the Ti -mont catalyst. Formyl func-
tions were converted into 1,3-dioxolanes in excellent
4+
yields (entries 1–6). Moreover, this Ti -mont could
10
thalen]-6%(7%H)-one 6.
smoothly transform various kinds of cyclic, aliphatic
and aromatic ketones into the cyclic acetals (entries
‡
In conclusion, the acetalization of various kinds of
7
–14). Remarkably, a bulky ketone of 1,3-diphenyl-2-
carbonyl compounds sufficiently occurs in the presence
propanone gave a quantitative yield of the correspond-
ing acetal (entry 13), whereas some zeolite catalysts do
not efficiently promote the acetalization of this ketone
because small pores obstruct an access of the bulky
compound to their acid sites.
catalysis of the Ti -mont can be ascribed to its strong
acidity and an expansion of the interlayer space under
4+
of the Ti -exchanged montmorillonite as a strong solid
acid catalyst. This heterogeneous montmorillonite cata-
lyst system has the following advantages: (1) high cata-
lytic activity and chemoselectivity, (2) wide applicability
even to bulky substrates, (3) simple workup procedures
and (4) non-polluting and recyclable catalyst.
5
d,e
The above prominent
4
+
‡
reaction conditions.
4
+
The spent Ti -mont catalyst was readily separated
Acknowledgements
from the reaction mixture by a simple filtration. The
4
+
isolated Ti -mont could be reused without an appre-
ciable loss of its high catalytic activity and selectivity;
the yield of 2-phenyl-1,3-dioxolane in the acetalization
of benzaldehyde could be kept over 99% during recy-
This work was supported by the Grant-in-Aid for
Scientific Research from Ministry of Education, Cul-
ture, Sports, Science and Technology of Japan
§
cling experiments (entries 3 and 4 in Table 2). Addi-
(
11450307). We are grateful to the Department of
tionally, hydrolysis of acetals to parent carbonyl
Chemical Science and Engineering, Graduate School of
Engineering Science, Osaka University, for scientific
supports via ‘Gas-Hydrate Analyzing System (GHAS)’
and Lend-Lease Laboratory System.
4
+
compounds was easily attained by the Ti -mont in the
9
presence of water.
Chemoselective acetalizations are exemplified in
¶
4+
Scheme 1. In the presence of the Ti -mont, the acetal-
References
O
O
O
O
HOCH2CH2OH
O
O
1. (a) Greene, T. W.; Wuts, P. G. M. Protective Groups in
Organic Synthesis; John Wiley and Sons: New York,
1999; p. 293; (b) Katritzky, A. R.; Otto, M.-C. Compre-
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mon Press: Oxford, 1995; Vol. 4, p. 176; (c) Kocienski, P.
J. Protecting Groups; George Thieme Verlag: New York,
Ti4+-mont (Ti: 3 mol%)
toluene, reflux, 1 h
O
1
2
9
1% isolated yield
O
HOCH2CH2OH
O
O
O O
1
994; p. 156.
Ti4+-mont (Ti: 3 mol%)
n-hexane, reflux, 2 h
O
O
2
. Typical examples of homogeneous 1,3-dioxolane forma-
tion, see: PTSA: (a) Ciceri, P.; Demnitz, F. W. J. Tetra-
hedron Lett. 1997, 38, 389; (b) Crimmins, M. T.;
DeLoach, J. A. J. Am. Chem. Soc. 1986, 108, 800; (c)
Dauben, W. G.; Gerdes, J. M.; Look, G. C. J. Org.
3
4
9
0% isolated yield
O
HOCH2CH2OH
O O
Chem. 1986, 51, 4964. BF –etherate: (d) Nagumo, S.;
3
Ti4+-mont (Ti: 3 mol%)
n-hexane, reflux, 1 h
O
O
Matsukuma, A.; Suemune, H.; Sakai, K. Tetrahedron
1
993, 49, 10501. FeCl : (e) Yang, H.; Li, B.; Cui, Y.
5
6
3
Synth. Commun. 1998, 28, 1233. Me SiOTf: (f) Hwu, J.
8
7% isolated yield
3
R.; Leu, L.-C.; Robl, J. A.; Anderson, D. A.; Wetzel, J.
M. J. Org. Chem. 1987, 52, 188; (g) Tsunoda, T.; Suzuki,
M.; Noyori, R. Tetrahedron Lett. 1980, 21, 1357.
Scheme 1.
3. (a) Clark, J. H. Green Chem. 1999, 1, 1; (b) Izumi, Y.;
Urabe, K.; Onaka, M. Catal. Surv. Jpn. 1997, 17.
‡
Most of the heterogeneous catalysts are inactive for the protection
4
. Examples of heterogeneous dimethyl acetal formation,
of keto functions with diols because of their low acidities: Ref. 5.
3+
§
4+
see: Ce -exchanged montmorillonite: (a) Tateiwa, J.-I.;
Horiuchi, H.; Uemura, S. J. Org. Chem. 1995, 60, 4039.
MCM-41: (b) Tanaka, Y.; Sawamura, N.; Iwamoto, M.
Tetrahedron Lett. 1998, 39, 9457.
Interlayer distance of the Ti -mont was expanded from 2.7 to 5.1 A
when soaked in toluene.
,
¶
These reactions were carried out under the same conditions as
shown in Table 2.

8
332
T. Kawabata et al. / Tetrahedron Letters 42 (2001) 8329–8332
5
. Examples of heterogeneous 1,3-dioxolane formation, see:
clay: (a) Ponde, D. E.; Deshpande, V. H.; Bulbule, V. J.;
Sudalai, A.; Gajare, A. S. J. Org. Chem. 1998, 63, 1058;
(10:1), which afforded 0.44 g of pure 2-methyl-2-phenyl-
1,3-dioxolane (90%).
4+
8. Presumably, the protonic acid sites of the Ti -mont
might be located on oxy anion species of TiꢀOꢀTi bonds,
whose acidity could be strengthened by an efficient inter-
(b) Li, T.-S.; Li, S.-H.; Li, J.-T.; Li, H.-Z. J. Chem. Res.
(
S) 1997, 27; (c) Cramarossa, M. R.; Forti, L.; Ghelfi, F.
Tetrahedron 1997, 53, 15889. Zeolite: (d) Ballini, R.;
Bosica, G.; Frullanti, B.; Maggi, R.; Sartori, G.; Schroer,
F. Tetrahedron Lett. 1998, 39, 1615; (e) Corma, A.;
Climent, M. J.; Garcia, H.; Primo, J. Appl. Catal. 1990,
4+
4+
action through the oxy anions between Ti and Si of
SiO4 tetrahedra in the layer: Tanabe, K; Misono, M;
Ono, Y; Hattori, H. New Solid Acids and Bases; Kodan-
sha-Elsevier: Tokyo-Amsterdam, 1989.
5
9, 333. Alumina: (f) Kamitori, Y.; Hojo, M.; Masuda,
9
. In a typical deacetalization procedure, a mixture of 2-(4-
R.; Yoshida, T. Tetrahedron Lett. 1985, 26, 4767.
. Ebitani, K.; Kawabata, T.; Nagashima, K.; Mizugaki, T.;
Kaneda, K. Green Chem. 2000, 2, 157.
oxo-4H-1-benzopyran-3-yl)-1,3-dioxolane 2 (1 mmol),
6
7
4+
Ti -mont (0.15 g), acetone (5 mL) and water (0.2 mL)
was stirred under reflux for 2 h. The GC analysis showed
. A typical procedure for the heterogeneous acetalization
99% yield of 4-oxo-4H-1-benzopyran-3-carboxaldehyde
of carbonyl compounds is as follows. A mixture of
4+
1.
acetophenone (3 mmol), ethane-1,2-diol (5 mmol), Ti -
1
0. In contrast, the use of a homogeneous distannoxane
catalyst can protect a conjugated carbonyl group selec-
tively to give 4%,4%a,7%,8%-tetrahydro-4%a-methylspiro[1,3-
dioxolane-2,2%(3%H)-naphthalen]-5%(6%H)-one: Otera, J.;
Dan-oh, N.; Nozaki, H. Tetrahedron 1992, 48, 1449.
mont (0.15 g) and toluene (20 mL) was refluxed under
4+
Dean–Stark conditions. After 1 h, the Ti -mont was
separated by filtration. The organic layer was concen-
trated and subjected to column chromatography on
®
Florisil with a mixture of n-hexane and ethyl acetate