Highly efficient deprotection of acetals by titanium cation-exchanged montmorillonite as a strong solid acid catalyst

Article abstract of DOI:10.1246/cl.2003.648

Deprotection of various kinds of acetals sufficiently occured in the presence of Ti⁴⁺-exchanged montmorillonite as a recyclable strong solid acid catalyst.

Full text of DOI:10.1246/cl.2003.648

6
48
Chemistry Letters Vol.32, No.7 (2003)
Highly Efficient Deprotection of Acetals by Titanium Cation-exchanged Montmorillonite
as a Strong Solid Acid Catalyst
ꢀ
Tomonori Kawabata, Masaki Kato, 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
(Received May 9, 2003;CL-030403)
Deprotection of various kinds of acetals sufficiently oc-
0
Table 1. Deprotection of 1,1 -(dimethoxymethylene)bisben-
nþ
cured in the presence of Ti4 -exchanged montmorillonite as a
þ
a
zene catalyzed by M -montmorillonites and typical solid acids
recyclable strong solid acid catalyst.
O
H CO
OCH3
3
catalyst (0.05 g)
acetone/H O, r.t., 1 h
2
In organic synthesis, carbonyl functionalities are commonly
protected as acetals, which are typically converted back to the
parent carbonyl moieties using acids or metal complexes.
Yield Amount of adsorbed
b,c
1;2
Entry
Catalyst
d
/%
NH3/mmol/g
However, these homogeneous deprotections suffer from several
drawbacks, including difficult product isolation and production
of unrecoverable salt wastes. To overcome these problems,
much effort has been devoted toward the development of het-
erogeneous deprotection systems that can be performed under
neutral conditions. Unfortunately, these heterogeneous systems
often demonstrate limited applicability for the substrate due to
low activities.
4þ
e
1
2
3
4
5
6
7
8
9
10
11
12
13
Ti -mont
>99
>99
>99
92
86
59
20
1
1.89
1.89
1.89
1.89
0.95
0.81
0.75
0.89
0.74
0.17
n.m.i
0.95
0.24
n.m.i
0.44
—
Reuse-1f
Reuse-2
g
4þ
Ti -mont
Fe -mont
3þ
3
Zr4 -mont
þ
3
þ
Al -mont
Sc -mont
3þ
Montmorillonites (monts) are structurally defined as layers
of two-dimensional silicate sheets separated by interlayer catio-
nic species. The cationic species can be readily replaced by var-
2
þ
Cu -mont
Na -mont
1
þ
10
16
7
3
1
h
nþ
mont K10
H -betaj
þ
H -mordenite
ious metal cations, giving metal cation-exchanged monts (M
monts) that can act as heterogeneous catalysts for a variety of
-
þ
þ
k,l
4
organic reactions.
Recently, we have succeeded in creating a chain-like tita-
k,m
14
15
H -ZSM-5
ꢁ-ZrO2
2
n
4þ
SO4
7
trace
nium species within the interlayers of montmorillonite (Ti
mont), which acts as an efficient solid acid catalyst for aromatic
-
1
6
—
5
a
5b
a
alkylation and acetalization of carbonyl compounds. Nota-
2
Substrate (1 mmol), catalyst (0.05 g), acetone (5 mL), H O
(0.2 mL), room temp., 1 h. Determined by GC analysis
using an internal standard technique. Yield of benzophe-
none. Acid amount was volumetrically measured. The value
corresponds to the number of strongly adsorbed NH3. See
Ref. 5(a). Ti: 0.034 mmol. The spent catalyst from entry
bly, the catalytic activities of the Ti4 -mont toward bulky sub-
þ
b
c
strates exceeded those of conventional solid acids. Herein, we
report that the Ti -mont can efficiently catalyze the deprotec-
tion of various acetal compounds in aqueous acetone solution.
d
4þ
e
f
þ
Treatment of Na -mont (Kunipia F, Kunimine Industry Co.
Ltd.) with an aqueous solution of TiCl4 afforded the Ti -mont
g
1
.
Purchased from Aldrich. Not measured. From Shokubai
Pure acetone (5.2 mL) was used as the solvent.
4þ
h
i
j
5
ꢀ
with an interlayer space of 2.7 A (Ti content: 0.68 mmol/g).
k
l
m
Kasei Kogyo. From N. E. Chemcat. Si/Al = 10. Si/Al
=
The chain-like structure of titanium oxides along the silicate
sheets was determined by Ti K-edge XAFS spectroscopy.
n
25. From Wako Pure Chemicals.
0
2ꢁ
9
Deprotections of 1,1 -(dimethoxymethylene)bisbenzene
nþ
(SO4 -ZrO2) (Entry 1 vs Entries 12–15).
0
were carried out using various M -monts, as summarized in
6
In addition to 1,1 -(dimethoxymethylene)bisbenzene, the
deprotection reaction was extended to other acetals, as shown
in Table 2. Several aromatic acetals smoothly reacted to form
the corresponding carbonyl compounds (Entries 1–6). Interest-
ingly, less reactive 2-(4-nitrophenyl)-1,3-dioxolane gave 4-ni-
trobenzaldehyde in a high yield within 6 h (Entry 5). This cata-
lyst system was also effective for the deprotection of 1,3-
dioxolanes of aliphatic ketones (Entries 7–10), including that
with a bulky steroid substrate (Entry 9). Although the deprotec-
tion of the acetals of acyclic aliphatic aldehydes using metal tri-
nþ
Table 1. The M -monts that contain metal cations with high
4
þ
3þ
4þ
valency, such as Ti , Fe , and Zr , served as effective cat-
alysts (Entries 1, 5, and 6). Among these catalysts, the Ti -
mont afforded benzophenone with the highest yield, which in-
4þ
2
c
creased in relation to the amount of strongly adsorbed NH3.
In contrast, the catalytic activities of Sc3 - and Cu -mont un-
der similar conditions were insubstantial, irrespective of the
moderate amounts of adsorbed NH3 (Entries 8 and 9). It is note-
þ
2þ
4þ
worthy that the Ti -mont exhibited significantly higher cataly-
7
2a-c
4þ
tic activities than that of commercial mont K10 (Entry 11), and
þ
flates was unsuccessful,
fective in transforming such unreactive dioxolanes to the
corresponding aldehydes within 4 h (Entry 11). In addition, fa-
the Ti -mont was surprisingly ef-
8
þ
those of conventional solid acids, such as H -beta, H -morde-
þ
nite, H -ZSM-5, and sulfate ion-treated zirconium oxide
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.7 (2003)
649
Table 2. Ti4 -mont-catalyzed deprotection of acetalsa
þ
phenone (Table 1, Entry 1 vs 4). These observations indicated
4
þ
that the Brꢁnsted acid sites of the Ti -mont play an important
role in the above deprotection reactions. Presumably, the strong
c
Productb
Time Yield
Entry
1d
Substrate
/
h
/ %
4þ
acid sites of the Ti -mont are associated with the unique TiO2
5
OCH3
domains within the interlayers of mont. One of the prominent
characteristics of mont materials is an enlargement of the inter-
CHO
1
99
OCH3
O
1
0
layer distance in solvents. Indeed, the interlayer space of the
4þ
2
3
CHO
CHO
1
>99
95
ꢀ
ꢀ
Ti -mont expanded from 2.7 A to 17.5 A when soaked in ace-
tone/H2O, as determined by XRD measurement, thus allowing
O
O
1
1
access of the substrates to the acid sites within the interlayers.
6
1
O
4þ
In conclusion, the Ti -mont acted as a heterogeneous, re-
cyclable acid catalyst for deprotection of various acetal com-
pounds. In contrast to current catalytic systems for deprotection,
our methodology has following advantages: (1) high catalytic
activity, (2) strikingly simple workup procedure, (3) recyclable
catalyst, and (4) use of a nontoxic solvent system (acetone/
H2O).
NC
NC
O
O
O
O
CHO
98
4
O
O
O
5
O N
2
O N
2
CHO
6
1
94
O
O
H3CO
OCH3
6d
References and Notes
>99 (98)
1
2
T. W. Greene and P. G. Wuts, ‘‘Protecting Groups in Organic Synthe-
sis,’’ 2nd ed., Wiley & Sons, New York (1991) and references cited
therein.
O
O
7
8
n = 1
n = 2
1
1
99
O
For recent reports on metal-catalyzed deprotection of acetals, see,
Bi(NO3)ꢂ5H2O: a) K. J. Eash, M. S. Pulia, L. C. Wieland, and R. S.
Mohan, J. Org. Chem., 65, 8399 (2000). Bi(OTf)3: b) M. D. Carrigan,
D. Sarapa, R. C. Smith, L. C. Wieland, and R. S. Mohan, J. Org.
Chem., 67, 1027 (2002). Ce(OTf)3: c) R. Dalpozzo, A. De Niro, L.
Maiuolo, A. Procopio, A. Tagarelli, G. Sindona, and G. Bartoli, J.
Org. Chem., 67, 9093 (2002). K5CoW12O40ꢂ3H2O: d) M. H. Habibi,
S. Tangestaninejad, I. M. Baltork, V. Mirkhani, and B. Yadollahi, Tet-
rahedron Lett., 42, 6771 (2001).
95 (92)
n
n
2
2
9
1
(99)
O
O
O
1
0
O
4
4
5
1
99
3
4
5
6
For solid reagents of deprotection of acetals, see, Amberlyst: a) G. M.
Coppola, Synthesis, 1984, 1021. FeCl3/SiO2: b) K. S. Kim, Y. H. Song,
B. H. Lee, and C. S. Hahn, J. Org. Chem., 51, 404 (1986).
For excellent reviews of mont-catalyzed organic syntheses, see: a) P.
Laszlo, Acc. Chem. Res., 19, 121 (1986). b) Y. Izumi and M. Onaka,
Adv. Catal., 38, 245 (1992).
O
O
O
1
1
5
4
1
95
94
CHO
O
1
2
3
O
CHO
a) K. Ebitani, T. Kawabata, K. Nagashima, T. Mizugaki, and K.
Kaneda, Green. Chem., 2, 157 (2000). b) T. Kawabata, T. Mizugaki,
K. Ebitani, and K. Kaneda, Tetrahedron Lett., 42, 8329 (2001).
O
O
O
O
O
CHO
4þ
1
O
2
99
A typical example for the deprotection by the Ti -mont is as follows:
Into a reaction vessel were placed the Ti4 -mont (0.15 g, Ti:
0
þ
O
0
.1 mmol), 1,1 -(dimethoxymethylene)bisbenzene (0.684 g, 3 mmol),
aSubstrate (1 mmol), Ti -mont (0.10 g, Ti: 0.067 mmol),
acetone (5 mL), H2O (0.2 mL), 60 C. All products were
characterized by H NMR and Mass spectra. Yield was
determined by GC using an internal standard, based on
substrates. Values in parentheses are isolated yields. In
the case of the product isolation expriments, the reaction
4þ
acetone (15 mL), and H2O (0.6 mL). After vigorous stirring of the het-
erogeneous reaction mixture at room temperature for 1 h, the catalyst
was separated by filtration. The filtrate was poured into brine and ex-
tracted with diethyl ether. The combined organic layers were dried
over MgSO4 and the crude product was recrystallized from pet ether
to afford pure benzophenone (0.53 g, 98%).
ꢃ
b
1
c
7
8
For the deprotection of acetals by montmorillonite K10, see: a) E. C. L.
Gautier, A. E. Graham, A. McKillop, S. P. Standen, and R. J. K.
Taylor, Tetrahedron Lett., 38, 1881 (1997). b) T.-S. Li and S.-H. Li,
Synth. Commun., 27, 2299 (1997). c) J.-I. Asakura, M. J. Robins, Y.
Asaka, and T. H. Kim, J. Org. Chem., 61, 9026 (1996).
For recent reports on heterogeneous reactions using zeolite beta as an
acid catalyst, see, nitration: a) K. Smith, A. Musson, and G. A.
DeBoos, J. Org. Chem., 63, 8448 (1998). acylation: b) P. Andy, J.
Garcia-Martinez, G. Lee, H. Gonzalez, C. W. Jones, and M. E. Davis,
J. Catal., 192, 215 (2000). Note that these systems are limited to the
reactions of small substrates.
scale was three times as much as that given in footnote
(
d
a). Room temp.
vorable results were obtained for conjugated 1,3-dioxolanes
Entries 12 and 13).
As shown in Table 1 (Entries 2 and 3), the spent Ti4 -mont
(
þ
catalysts could be reused with retention of its high activity and
0
selectivity. In the case of 1,1 -(dimethoxymethylene)bisben-
4þ
zene, the Ti -mont was removed by filtration after ca. 50%-
conversion of the substrate at the reaction temperature. Further
treatment of the filtrate under similar reaction conditions did not
afford any additional benzophenone. These phenomena showed
that the deprotection occurred at the chain-like Ti species on the
mont solid.
The present Ti4 -mont showed higher catalytic activities
than those of Lewis acids such as Bi(OTf)3 and Ce(OTf)3.
Moreover, the presence of water improved the yields of benzo-
9
1
M. Hino, S. Kobayashi, and K. Arata, J. Am. Chem. Soc., 110, 6439
(
1979).
Y. Lvov, K. Ariga, I. Ichinose, and T. Kunitake, Langmuir, 12, 3038
1996).
0
(
11 With respect to the solvents, a mixture of acetone and water was the
optimal solvent. A promotion effect of water might be attributable to
the larger interlayer distance than that in pure acetone (12.3
Aꢀ ). In re-
þ
lation to this, use of CH3CN, THF, or toluene resulted in lower yields
of benzophenone (26–29%);correspondingly, the interlayer spaces
using these solvents were less than 10 Aꢀ .
2
b,c
Published on the web (Advance View) June 24, 2003;DOI 10.1246/cl.2003.648