Synthesis of a dialuminum-substituted silicotungstate and the diastereoselective cyclization of citronellal derivatives

Article abstract of DOI:10.1021/ja8014154

A novel dialuminum-substituted silicotungstate TBA₃H[γ- SiW₁₀O₃₆{Al(OH₂)}₂(μ-OH) ₂]·4H₂O (1, TBA = tetra-n-butylammonium) was synthesized by the reaction of the potassium salt of [γ-SiW ₁₀O₃₆]^8- (SiW10) with 2 equiv of Al(NO ₃)₃ in an acidic aqueous medium. It was confirmed by the X-ray crystallographic analysis that compound 1 was a monomer of the γ-Keggin dialuminum-substituted silicotungstate with the {Al ₂(μ-OH)₂} diamond core. The cluster framework of 1 maintained the γ-Keggin structure in the solution states. The reaction of 1 with pyridine yielded TBA₃[(C₅H₅N)H][γ- SiW₁₀O₃₆{Al(C₅H₅N)} ₂(μ-OH)₂]·2H₂O (2), and the molecular structure was successfully determined by the X-ray crystallographic analysis. In compound 2, two of three pyridine molecules coordinated to the axial positions of aluminum centers and one of them existed as a pyridinium cation, showing that compound 1 has two Lewis acid sites and one Bronsted acid site. Compound 1 showed high catalytic activity for the intramolecular cyclization of citronellal derivatives such as (+)-citronellal (3) and 3-methylcitronellal (4) without formation of byproduct resulting from etherification and dehydration. For the 1-catalyzed cyclization of 3, the diastereoselectivity toward (-)-isopulegol (3a) reached ca. 90% and the value was the highest level among those with reported systems so far. The reaction rate for the 1-catalyzed cyclization of 3 decreased by the addition of pyridine, and the cyclization hardly proceeded in the presence of 2 equiv of pyridine with respect to 1. On the other hand, the reaction rate and diastereoselectivity to 3a in the presence of 2,6-lutidine were almost the same as those in the absence. Therefore, the present cyclization is mainly promoted by the Lewis acid sites (aluminum centers) in 1. DFT calculations showed that the formation of the transition state to produce 3a is sterically and electronically more favorable than the other three transition states for the present 1-catalyzed cyclization of 3.

Full text of DOI:10.1021/ja8014154

Published on Web 11/01/2008
Synthesis of a Dialuminum-Substituted Silicotungstate and
the Diastereoselective Cyclization of Citronellal Derivatives
Yuji Kikukawa, Syuhei Yamaguchi, Yoshinao Nakagawa, Kazuhiro Uehara,
Sayaka Uchida, Kazuya Yamaguchi, and Noritaka Mizuno*
Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Received February 26, 2008; Revised Manuscript Received July 31, 2008; E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
Abstract: A novel dialuminum-substituted silicotungstate TBA3H[γ-SiW10O36{Al(OH2)}2(µ-OH)2]·4H2O (1,
TBA ) tetra-n-butylammonium) was synthesized by the reaction of the potassium salt of [γ-SiW10O36]8-
(SiW10) with 2 equiv of Al(NO3)3 in an acidic aqueous medium. It was confirmed by the X-ray crystallographic
analysis that compound 1 was a monomer of the γ-Keggin dialuminum-substituted silicotungstate with the
{Al2(µ-OH)2} diamond core. The cluster framework of 1 maintained the γ-Keggin structure in the solution
states. The reaction of 1 with pyridine yielded TBA3[(C5H5N)H][γ-SiW10O36{Al(C5H5N)}2(µ-OH)2]·2H2O (2),
and the molecular structure was successfully determined by the X-ray crystallographic analysis. In compound
2, two of three pyridine molecules coordinated to the axial positions of aluminum centers and one of them
existed as a pyridinium cation, showing that compound 1 has two Lewis acid sites and one Brønsted acid
site. Compound 1 showed high catalytic activity for the intramolecular cyclization of citronellal derivatives
such as (+)-citronellal (3) and 3-methylcitronellal (4) without formation of byproduct resulting from
etherification and dehydration. For the 1-catalyzed cyclization of 3, the diastereoselectivity toward (-)-
isopulegol (3a) reached ca. 90% and the value was the highest level among those with reported systems
so far. The reaction rate for the 1-catalyzed cyclization of 3 decreased by the addition of pyridine, and the
cyclization hardly proceeded in the presence of 2 equiv of pyridine with respect to 1. On the other hand,
the reaction rate and diastereoselectivity to 3a in the presence of 2,6-lutidine were almost the same as
those in the absence. Therefore, the present cyclization is mainly promoted by the Lewis acid sites (aluminum
centers) in 1. DFT calculations showed that the formation of the transition state to produce 3a is sterically
and electronically more favorable than the other three transition states for the present 1-catalyzed cyclization
of 3.
Introduction
avenue for the polyoxometalate chemistry and lanthanoid-
substituted polyoxometalates [R1-Ln(H2O)4P2W17O61]7- (Ln )
Polyoxometalates are a large family of anionic metal-oxygen
clusters of early transition metals and have stimulated many
current research activities in broad fields of science such as
catalysis, materials, and medicine, because their chemical
properties such as redox potentials, acidities, and solubilities
in various media can be finely tuned by choosing constituent
elements and counter cations.1 Particularly, the interest in the
catalysis of metal-substituted polyoxometalates, which are
synthesized by the substitution of metal cations into the vacant
site(s) of lacunary polyoxometalates as “structural motifs”, has
been growing because of the rich diversity of lacunary poly-
oxometalates, ranging from monovacant to hexavacant anions.1
Although polyoxometalates have been traditionally used as
Brønsted acid and oxidation catalysts, there are only a few
reports on the Lewis acid catalysis by polyoxometalates.2 The
Lewis acid catalysis of polyoxometalates is a promising new
La, Sm, Eu, and Yb), and hafnium-substituted polyoxometalates
such as [R1-Hf(H2O)nP2W17O61]6- and [R-Hf(OH)PW11O39]4-
have been reported to act as Lewis acid catalysts for imine aldol
and imino Diels-Alder reactions, for example.2
Aluminum cations are naturally electron-deficient, and their
derivatives with the weakly coordinating ligands behave as
coordinatively unsaturated compounds.3 In particular, aluminum
compounds are very useful Lewis acid catalysts.3 Since the
properties and reactivities are strongly dependent on their
structures, the syntheses of aluminum compounds with structur-
ally well-defined sites are very important. While some kinds of
aluminum-substituted polyoxometalates such as mono- and
trisubstituted Keggin heteropolytungstates (not γ-type), mono-
(2) (a) Boglio, C.; Lemie´re, G.; Hasenknopf, B.; Thorimbert, S.; Lacoˆte,
E.; Malacria, M. Angew. Chem., Int. Ed. 2006, 45, 3324. (b) Boglio,
C.; Micoine, K.; Re´my, P.; Hasenknopf, B.; Thorimbert, S.; Lacoˆte,
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41, 113. (c) Neumann, R. Prog. Inorg. Chem. 1998, 47, 317. (d)
Thematic issue on polyoxometalates. Hill, C. L. Chem. ReV. 1998,
98, 1-390. (e) Kozhevnikov, I. V. Catalysis by Polyoxometalates;
John Wiley & Sons, Ltd.: Chichester, 2002. (f) Mizuno, N; Yamaguchi,
K.; Kamata, K. Coord. Chem. ReV. 2005, 249, 1944.
(3) (a) Ooi, T.; Maruoka, K. In Lewis Acids in Organic Synthesis;
Yamamoto, H., Eds.; Wiley-VCH: Weinheim, 2000; pp 191-281. (b)
Wulff, W. D. In Lewis Acids in Organic Synthesis; Yamamoto, H.,
Eds.; Wiley-VCH: Weinheim, 2000; pp 283-354. (c) Saito, S. In Main
Group Metals in Organic Systhesis; Yamamoto, H., Oshima, K., Eds.;
Wiley-VCH: Weinheim, 2004, pp 189-306.
9
15872 J. AM. CHEM. SOC. 2008, 130, 15872–15878
10.1021/ja8014154 CCC: $40.75
2008 American Chemical Society

Synthesis of a Dialuminum-Substituted Silicotungstate
A R T I C L E S
substituted Wells-Dawson heteropolytungstates, and a trisub-
stituted ꢀ-Keggin silicotungstate dimer have been synthesized
by using lacunary polyoxometalates as “structural motifs”,4
structurally characterized aluminum-substituted polyoxometa-
lates (by X-ray crystallographic analysis) are still one of the
least reported compounds.4 In this paper, we report the synthesis,
structural characterization, and Lewis acid catalysis of a novel
dialuminum-substituted γ-Keggin-type silicotungstate TBA3H[γ-
SiW10O36{Al(OH2)}2(µ-OH)2]·4H2O (1, TBA ) tetra-n-butyl-
ammonium).
Results and Discussion
Synthesis and Structural Characterization of
a
Dialuminum-Substituted Silicotungstate. The TBA salt of a
dialuminum-substituted silicotungstate 1 was obtained by the
reaction of the potassium salt of [γ-SiW10O36]8- (SiW10)5 with
2 equiv of Al(NO3)3 with respect to SiW10 in an acidic medium
(see Experimental Section). The elemental analysis data showed
that the Si:Al:W ratio in 1 was 1:2:10, respectively. Further, it
was confirmed by the elemental analysis that no alkali metal
cations of Na+ and K+ were found in 1 (<0.1 wt%). Compound
1 was soluble in various polar organic solvents such as CH3CN,
DMF, and DMSO, and the single crystals suitable for X-ray
crystallographic analysis were successfully obtained by recrys-
tallization in a mixed solvent of CH3CN and water (see
Experimental Section). The molecular structure is represented
in Figure 1a and the selected bond lengths and angles are
summarized in Table 1. The crystallographic data are sum-
marized in Table S1, Supporting Information.
Three TBA cations per the anion were found in 1. The bond
valence sum (BVS) values of aluminum (3.14-3.19), tungsten
(5.93-6.72), and silicon (3.65) indicate that the respective
valences in 1 are +3, +6, and +4. Further, it was confirmed
by the bond lengths that the axial positions of aluminum centers
in 1 were occupied by water molecules and each aluminum
center was linked by two µ-hydroxo ligands to form the {Al2(µ-
OH)2} diamond core. The results of the BVS calculations,
elemental analysis, and TG-DTA show that the formula of 1 is
TBA3H[γ-SiW10O36{Al(OH2)}2(µ-OH)2]·4H2O. The formation
of 1 anion is expressed by the following equation:
Figure 1. Molecular structures (ORTEP drawing) of (a) TBA3H[γ-
SiW10O36{Al(OH2)}2(µ-OH)2] · 4H2O (1) and (b) TBA3[(C5H5N)H][γ-
SiW10O36{Al(C5H5N)}2(µ-OH)2]·2H2O (2). TBA and water molecules
(except for aquo ligands) were omitted for clarity.
[γ-SiW10O36]8- + 2Al3+ + 2H2O f
OH)2} diamond core was placed in the building pocket of
SiW10.6 The Al(1)-O(1), Al(1)-O(2), Al(2)-O(1), and
Al(2)-O(2) bonds (average 1.85 Å) were almost equidistant.
The average Al-O-Al and O-Al-O angles were 101.1° and
78.7°, respectively. The Al(1) · · ·Al(2) distance in 1 was 2.863(1)
Å. These bond lengths and angles were similar to that of the
previously reported Salan complex with a similar dialuminum
core.7
[γ-SiW10O36Al2(µ-OH)2]4- + 2H+ (1)
As shown in Figure 1a, the anion part of 1 was a monomer
of the dialuminum-substituted γ-Keggin silicotungstate. The
axial positions of aluminum centers were occupied by aquo
ligands, and the opposite axial positions were occupied by the
oxygen atoms of the central {SiO4} unit with the average
Al-O(Si) distance of 2.13 Å. Thus, the aluminum centers were
in axially distorted octahedral environments, and the {Al2(µ-
As shown in Figure 2a, the 29Si NMR spectrum of 1 showed
a signal at -83.9 ppm, suggesting that 1 is a single species in
the solvent. The 183W NMR spectrum of 1 in CD3CN showed
three signals at -113.4, -119.2, and -139.4 ppm with the
intensity ratio of 2:1:2, respectively (Figure 2b). The 183W NMR
spectrum of 1 in a mixed solvent of CD3CN and DMSO-d6
showed three signals at -72.1, -124.8, and -132.8 ppm with
the intensity ratio of 2:1:2, respectively (Figure 2c). These results
suggest that the cluster framework of 1 maintains the γ-Keggin
(4) (a) Zonnevijlle, F.; Tourne´, C. M.; Tourne´, G. F. Inorg. Chem. 1982,
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Hardcastle, K. I.; Hill, C. L.; Weinstock, I. A. Inorg. Chem. 2001,
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2001, 46, 327. (g) Casey, W. H. Chem. ReV. 2006, 106, 1. (h) Wang,
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(6) (a) Zhang, X.-Y.; O’Conner, C. J.; Jameson, G. B.; Pope, M. T. Inorg.
Chem. 1996, 35, 30. (b) Cadot, E.; Be´reau, V.; Marg, B.; Halut, S.;
Se´cheresse, F. Inorg. Chem. 1996, 35, 3099. (c) Nakagawa, Y.; Uehara,
K.; Mizuno, N. Inorg. Chem. 2005, 44, 9068.
(5) (a) Canny, J.; Te´ze´, A.; Thouvenot, R.; Herve´, G. Inorg. Chem. 1986,
25, 2114. (b) Te´ze´, A.; Herve´, G. Inorg. Synth. 1990, 27, 85.
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9
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A R T I C L E S
Kikukawa et al.
Table 1. Selected Bond Lengths (Å) and Angles (deg) in 1 and 2
1
2
Lengths
2.863(1) Al(1) · · ·Al(2)
1.841(19) Al(1)-O(1)
1.88(2) Al(1)-O(2)
1.837(19) Al(2)-O(1)
1.85(2) Al(2)-O(2)
Al(1)· · ·Al(2)
Al(1)-O(1)
Al(1)-O(2)
Al(2)-O(1)
Al(2)-O(2)
Al(1)-O(9)
Al(2)-O(10)
Al-OW av
2.832(8)
1.819(19)
1.87(2)
1.906(19)
1.84(2)
2.033(17)
2.061(18)
1.83
2.02
2.35
1.71
1.83
1.92
1.63
3.47(2)
3.64
2.84(3)
1.869(18) Al(1)-N(1)
1.859(19) Al(2)-N(2)
1.850
2.125
2.326
1.710
1.864
1.919
1.659
3.54(2)
Al-OW av
Al-OSi av
W-OSi av
WdOW av
W-OAl av
W-OW av
Si-Oav
N(1)-N(2)
Py· · ·Pyav
N(3)-O(25)
Al-OSi av
W-OSi av
WdOW av
W-OAl av
W-OW av
Si-Oav
O(9)-O(10)
Angles
Al(1)-O(1)-Al(2)
Al(1)-O(2)-Al(2)
O(1)-Al(1)-O(2)
O(1)-Al(2)-O(2)
O(1)-Al(1)-O(9)
O(2)-Al(1)-O(9)
O(1)-Al(2)-O(10)
O(2)-Al(2)-O(10)
O(5)-Al(1)-O(9)
O(6)-Al(1)-O(9)
O(7)-Al(2)-O(10)
O(8)-Al(2)-O(10)
O(5)-Al(1)-O(6)
O(7)-Al(2)-O(8)
O(1)-Al(1)-O(5)
O(2)-Al(1)-O(6)
O(1)-Al(2)-O(7)
O(2)-Al(2)-O(8)
O(3)-Al(1)-O(9)
102.2(9)
100.0(9)
78.2(8)
79.1(8)
95.7(8)
94.7(8)
96.3(8)
95.3(8)
94.3(8)
97.9(8)
93.2(8)
92.9(8)
90.3(8)
94.1(8)
94.9(8)
94.9(9)
93.7(9)
91.9(9)
172.4(8)
Al(1)-O(1)-Al(2)
Al(1)-O(2)-Al(2)
O(1)-Al(1)-O(2)
O(1)-Al(2)-O(2)
O(1)-Al(1)-N(1)
O(2)-Al(1)-N(1)
O(1)-Al(2)-N(2)
O(2)-Al(2)-N(2)
O(5)-Al(1)-N(1)
O(6)-Al(1)-N(1)
O(7)-Al(2)-N(2)
O(8)-Al(2)-N(2)
O(5)-Al(1)-O(6)
O(7)-Al(2)-O(8)
O(1)-Al(1)-O(5)
O(2)-Al(1)-O(6)
O(1)-Al(2)-O(7)
O(2)-Al(2)-O(8)
O(3)-Al(1)-N(1)
O(4)-Al(2)-N(2)
99.0(9)
99.6(9)
80.5(8)
79.1(8)
92.7(8)
92.8(9)
94.2(8)
87.8(8)
91.9(8)
95.0(9)
92.9(8)
91.4(7)
92.8(8)
92.3(8)
92.3(9)
93.8(9)
95.2(9)
93.3(9)
173.0(7)
172.2(8)
Figure 2. (a) 29Si NMR spectrum of 1 in CD3CN/DMSO-d6 (2:1 v/v); (b)
183W NMR spectrum in CD3CN; (c) 183W NMR spectrum in CD3CN/
DMSO-d6 (2:1 v/v).
O(4)-Al(2)-O(10) 169.9(8)
structure with the C2V symmetry in these solution. The downfield
shift and broadening of one set of tungsten signals in a mixed
solvent of CD3CN and DMSO-d6 (observed at -72.1 ppm) are
likely due to the coordination of DMSO-d6 to the aluminum
(Lewis acid) centers.
Reaction of Compound 1 with Bases (OH- and Pyridine).
As shown in Figure 3, a potentiometric titration of 1 with tetra-
n-butylammonium hydroxide (TBAOH)8 showed an inflection
point at 1.0 ( 0.2 equiv of OH- with respect to 1, showing
that complex 1 possesses one titratable proton ([H+]/[1] ) 1).
Therefore, the hydroxyl groups of the {Al2(µ-OH)2} core in 1
are not acidic enough to react with TBAOH.
Figure 3. Profile for the potentiometric titration of 1 (0.05 mmol) in a
mixed solvent of DMF and water (50 mL, 9:1 v/v) with 40% aqueous
TBAOH as a titrant. The potentials are relative to a standard Ag/AgCl
electrode.
was calculated to be 2 by using the reported integrated molar
extinction coefficients.9
Compound 1 was dissolved in CH3CN, and the successive
addition of pyridine yielded the single crystals (2, see Experi-
mental Section). The elemental analysis suggests that three
pyridine molecules are included in compound 2. In the IR
spectrum of 2, the characteristic absorption bands of pyridine
molecules coordinated to acidic sites were observed, and the
bands at 1452 and 1540 cm-1 are assignable to pyridine
molecules coordinated to Lewis acid and Brønsted acid (pyri-
dinium cation) sites, respectively (Figure 4).9 The molar ratio
of (pyridine coordinated to Lewis acid sites)/(pyridinium cation)
The solid-state 15N CPMAS NMR spectrum of 2 with 15N-
enriched pyridine showed signals assignable to pyridine coor-
dinated to the aluminum centers (236.5 and 241.8 ppm) and
pyridinium cation (183.2 ppm) (Figure 5).10 Also, the 15N NMR
spectrum of the CD3CN solution of 1 and 15N-enriched pyridine
(3 equiv with respect to 1, 32 °C) showed signals assignable to
pyridine coordinated to the aluminum centers (258.3 and 260.8
ppm).10 These signal positions were very close to those of the
(8) Weiner, H.; Aiken, J. D., III; Finke, R. G. Inorg. Chem. 1996, 35,
(10) (a) Maciel, G. E.; Haw, J. F.; Chuang, I.-S.; Hawkins, B. L.; Early,
T. A.; McKay, D. R.; Petrakis, L. J. Am. Chem. Soc. 1983, 105, 17.
(b) Ripmeester, J. A. J. Am. Chem. Soc. 1983, 105, 2925.
7905.
(9) Emeis, C. A. J. Catal. 1993, 141, 347.
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Synthesis of a Dialuminum-Substituted Silicotungstate
A R T I C L E S
centers and one of them existed as a pyridinium cation. The
distance between the two pyridine molecules on the aluminum
centers was 3.64 Å, and they are likely stabilized by the π-π
interaction. The strong hydrogen-bonding interaction between
the pyridinium cation near the SiW10 framework and O(25)
atom was observed (dN(3)-O(25) ) 2.84 Å). All these results
indicate that complex 1 possesses two Lewis acid sites
(aluminum centers) and one Brønsted acid (H+) site.
Diastereoselective Cyclization of Citronellal Derivatives. Next,
the cyclization of (+)-citronellal (3)12 was carried out with the
dialuminum-substituted silicotungstate 1. Generally, the cy-
clization shows a complex selectivity pattern since 3 can be
converted into four different diastereoisomers. The important
feature of the cyclization is the diastereoselectivity toward (-)-
isopulegol (3a), which is readily hydrogenated to the industrially
important (-)-menthol.12 Thus, the development of efficient
catalysts that can diastereoselectively produce 3a is very
attractive. Typically, high diastereoselectivities to 3a (ca.
60-80%) have been observed in the presence of Lewis acids.13
With Brønsted acids, the diastereoselectivities are lower (ca.
50-70%) than those with Lewis acids.14 There are only a few
reports on the catalytic cyclization with g90% diastereoselec-
tivity to 3a.13b,n-p
Figure 4. IR spectrum of pyridine molecules on 2. The spectrum was
obtained by subtracting the spectrum of 1 from that of 2. L ) pyridine
molecules on Lewis acidic sites (Al centers), B ) pyridine molecules on
Brønsted acidic sites (pyridinium cations).
Compound 1 showed high catalytic activity and selectivity
for the cyclization of (+)-citronellal (3) and gave the corre-
sponding isopulegol isomers (3a-3d) in high yields (Table 2).
In this case, no etherification and dehydration, which are
frequently observed in the Brønsted acid catalyzed reactions,14
was observed. Under the same reaction conditions, no cycliza-
tion proceeded in the absence of the catalysts or in the presence
(12) (a) Pybus, D. H.; Sell, C. S. The Chemistry of Fragrances; RSC
Paperbook: Cambridge, 1999. (b) da Silva Rocha, K. A.; Robles-
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Lenarda˜o, E. J.; Botteselle, G. V.; de Azambuja, F.; Perin, G.; Jacob,
R. G. Tetrahedron 2007, 63, 6671.
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V. K.; Vennall, G. P.; Davey, P. N.; Newman, C. Tetrahedron Lett.
Figure 5. Solid-state 15N CPMAS NMR spectrum of compound 2 obtained
with 15N-enriched pyridine. The triangles and squares indicate the spinning
side bands of the signals due to pyridine coordinated to the aluminum centers
(236.5 and 241.8 ppm) and pyridinium cation (183.2 ppm), respectively.
The asterisk indicates the signal due to tetra-n-butylammonium.
ˇ
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Yamaguchi, S.; Tsuchida, K.; Nakagawa, Y.; Uehara, K.; Yamaguchi,
K.; Mizuno, N. J. Am. Chem. Soc. 2008, 130, 5472. (n) Tateiwa, J.;
Kimura, A.; Takasuka, M.; Uemura, S. J. Chem. Soc., Perkin Trans.
1 1997, 2169. (o) Iwata, T.; Okeda, Y.; Hori, Y. European Patent
Application, EP 1 225 163 A2, 2002. (p) Yongzhong, Z.; Yuntong,
N.; Jaenicke, S.; Chuah, G. K. J. Catal. 2005, 229, 404.
solid-state NMR spectrum of 2. In addition to these signals, a
signal assignable to pyridine interacting with proton was
observed at 300.2 ppm. Judging from the small upperfield shift
from free pyridine (∆δ ) 16.8 ppm), compound 1 likely acts
as a weak proton donor in CD3CN.11
We successfully obtained the high quality single crystals of
2 and the molecular structure could be determined by the X-ray
crystallographic analysis. The molecular structure is represented
in Figure 1b and the selected bond lengths and angles are
summarized in Table 1. The crystallographic data are sum-
marized in Table S1. As shown in Figure 1b, complex 2 was a
monomer of the dialuminum-substituted γ-Keggin silicotung-
state with three pyridine molecules. The BVS values of oxygen
atoms O(1) (1.14) and O(2) (1.15) in 2 indicate that two
aluminum atoms are also linked by two µ-hydroxo ligands to
form the {Al2(µ-OH)2} diamond core. As expected from the
IR and solid-state 15N CPMAS NMR spectra of 2, two of three
pyridine molecules existed on the axial positions of aluminum
(14) (a) Fuentes, M.; Magraner, J.; De Las Pozas, C.; Roque-Malherbe,
R.; Pariente, J. P.; Corma, A. Appl. Catal. 1989, 47, 367. (b) Laurent,
R.; Laporterie, A.; Dubac, J.; Berlan, J.; Lefeuvre, S.; Audhuy, M. J.
Org. Chem. 1992, 57, 7099. (c) Kropp, P. J.; Breton, G. W.; Craig,
S. L.; Crawford, S. D.; Durland, W. F., Jr; Jones, J. E.; Raleigh, J. S.
J. Org. Chem. 1995, 60, 4146. (d) Chuah, G. K.; Liu, S. H.; Jaenicke,
S.; Harrison, L. J. J. Catal. 2001, 200, 352. (e) da Silva, K. A.; Robles-
Dutenhefner, P. A.; Sousa, E. M. B.; Kozhevnikova, E. F.; Kozhevni-
kov, I. V.; Gusevskaya, E. V. Catal. Commun. 2004, 5, 425. (f) Ma¨ki-
Arvela, P.; Kumar, N.; Nieminen, V.; Sjo¨holm, R.; Salmi, T.; Murzin,
D. Y. J. Catal. 2004, 225, 155. (g) Williams, J. T.; Bahia, P. S.;
Kariuki, B. M.; Spencer, N.; Philp, D.; Snaith, J. S. J. Org. Chem.
2006, 71, 2460.
(11) Haslinger, E.; Schlederer, M.; Robien, W.; Wolschann, P. Monatsh.
Chem. 1984, 115, 1345.
9
J. AM. CHEM. SOC. VOL. 130, NO. 47, 2008 15875

A R T I C L E S
Kikukawa et al.
Table 2. Intramolecular Cyclization of 3a
isomer ratio (%)
catalyst
temp (°C)
time (h)
yieldb (%)
3a
3b
3c
3d
1
75
32
32
32
75
75
75
75
10
24
24
24
10
10
10
10
93
74
58
nr
87
88
91
nd
nd
75
80
nd
9
9
6
nd
nd
19
16
nd
nd
nd
nd
nd
nd
1
4
3
3
nd
nd
5
1
1c
2
SiW10d
Al(NO)3
Hf4g
none
nr
30f
17
nr
e
1
nd
3
nd
a Reaction conditions: 3 (1 mmol), catalyst (2.5 mol %), acetonitrile (3 mL), Ar atmosphere. The yield and diastereoselectivity were determined by
GC analysis using an internal standard. b Total yield of isopulegol isomers 3a-3d. In the case of 1-catalyzed cyclization, the selectivity toward isomers
was >99% in all cases. c In the presence of 3 equiv of 2,6-lutidine. d TBA salt. e 5 mol %. f 70% selectivity. nr ) no reaction. nd ) not detected.
g Cesium crown ether clathrate salts of [(γ-SiW10O36)2{Hf(H2O)}4(µ4-O)(µ-OH)6]8- (see ref 13m).
of SiW10. The 1-catalyzed cyclization of 3 proceeded with high
diastereoselectivity toward 3a (87-91%), and these Values were
the highest leVel among those with previously reported systems
(33-99%) (Table S2, Supporting Information).13,14 The reaction
rate for the 1-catalyzed cyclization of 3 at 75 °C was 134 mM
h-1 and 24 times higher than that with the tetra-nuclear hafnium
containing polyoxometalate [(γ-SiW10O36)2{Hf(H2O)}4(µ4-O)(µ-
OH)6]8- (Hf4) under the same reaction conditions (5.6 mM
h-1).13m In addition, the diastereoselectivity toward 3a in the
presence of 1 (87%) was higher than that in the presence of
Hf4 (80%).13m The reaction profile for the 1-catalyzed cycliza-
tion of 3 shows that the diastereoselectivities toward 3a were
always around 90% at 0-99% conversions of 3a, showing that
no interconversion of these isomers (e.g., retro-ene reaction)
proceeds under the present conditions. In addition, the reaction
of 3-methylcitronellal (4) proceeded diastereoselectively to give
trans isomer as a major product (eq 2).
Figure 6. Rates for cyclization of 3 as a function of the amount of bases
added: (a) pyridine and (b) 2,6-lutidine.
(22.2 mM h-1) and diastereoselectivity (91%) to 3a even in
the presence of 3 equiv of 2,6-lutidine were almost the same as
those in the absence (23.7 mM h-1, 88%).
The above-mentioned results show that the present cyclization
is mainly promoted by the Lewis acid sites (aluminum centers)
in 1. The electrostatic potential (ESP) around 1 also supports
the idea.16 Thus, the cyclization proceeds via the Lewis acid
catalyzed carbonyl-ene-type mechanism.13 A possible reaction
mechanism for the present cyclization is given in Scheme S1,
Supporting Information. The coordination of 3 to the Lewis acid
center leads to four possible transition states shown in Figure
7a to produce the corresponding isopulegol isomers. It is likely
that the energies at the transition states are much dependent on
the kinds of the Lewis acids (steric and electronic effects).13 In
order to clarify the origin of the high diastereoselectivity (isomer
ratio) for the 1-catalyzed cyclization of 3, the energies at the
To clarify the active sites for the cyclization, the 1-catalyzed
cyclization of 3 in the presence of pyridine or 2,6-lutidine was
carried out (Figure 6). Pyridine can interact with both Brønsted
and Lewis acid sites via protonation and coordination, respec-
tively.15 In contrast, it is well-known that 2,6-lutidine can
selectively interact with Brønsted acid sites and can not interact
with Lewis acid sites due to the steric hindrance.15 It was also
confirmed by the molecular modeling that 2,6-lutidine cannot
approach to the Lewis acid sites (aluminum centers in 1), but
pyridine can approach to the sites (Figure S1, Supporting
Information). As shown in Figure 6, the reaction rate decreased
with an increase in the amount of pyridine added and the
cyclization hardly proceeded in the presence of 2 equiv of
pyridine with respect to 1. On the other hand, the reaction rate
(16) (a) We calculated the ESP around 1 from electron densities with
B3LYP level theory. The topology of ESPs has proved to be very
useful for detecting the most basic regions of polyoxometalates. Lo´pez,
X.; Bo, C.; Poblet, J. M. J. Am. Chem. Soc. 2002, 124, 12574. (b)
Nucleophilic species (citronellal derivatives) would approach to the
polyoxometalate molecule along a path where the ESP is as high as
possible. For compound 1, the regions close to the axial sites of the
aluminum centers (the ESP value at the saddle point )-0.28 hartree/
e) were calculated to be more electrophilic (acidic) than the other
sites including bridged µ-OH groups and the polyoxotungstate
regions (<-0.30 hartree/e).
(15) (a) Brown, H. C.; Johannesen, R. B. J. Am. Chem. Soc. 1953, 75, 16.
(b) Murell, L. L.; Dispenziere, N. C., Jr J. Catal. 1989, 117, 275. (c)
Corma, A. Chem. ReV. 1995, 95, 559.
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15876 J. AM. CHEM. SOC. VOL. 130, NO. 47, 2008

Synthesis of a Dialuminum-Substituted Silicotungstate
A R T I C L E S
cm µmol-1 for Lewis (1452 cm-1) and Brønsted acid sites (1540
cm-1), respectively.9 Liquid-state NMR spectra were recorded on
JEOL JNM-EX-270. 1H and 13C{1H} NMR spectra (internal
standard: TMS, 0 ppm) were measured at 270 and 67.8 MHz,
respectively. 15N (external standard CH3NO2, 379.8 ppm18), 27Al
(external standard Al(NO3)3, 0 ppm), 29Si (external standard TMS,
0 ppm), and 183W NMR spectra (external standard Na2WO4, 0 ppm)
were measured at 27.2, 70.0, 53.5, and 11.2 MHz, respectively.
Solid-state 15N CPMAS NMR spectrum of 2 obtained with 15N-
enriched pyridine (MAS rate ) 4 kHz) was recorded on Chemag-
netics CMX-300 Infinity spectrometer operating at 30.5 MHz. The
repetition time was 3 s, and the cross-polarization contact time was
5 ms. NH4NO3 (nitrate at 353.7 ppm19) was used as an external
standard for the calibration of chemical shifts. The potentiometric
titration of 1 (0.05 mmol) was carried out in a mixed solvent of
DMF and water (50 mL, 9:1 v/v) with 40% aqueous tetra-n-
butylammonium hydroxide (TBAOH) as a titrant.8 Diffraction
measurements were made on a Rigaku AFC-10 Saturn 70 CCD
detector with graphite monochromated Mo KR radiation (λ )
0.71069 Å) at 153 K. Data were collected and processed using
CrystalClear20 for Windows software and HKL200021 for Linux
software. Neutral scattering factors were obtained from the standard
source. In the reduction of data, Lorentz and polarization corrections
were made. The structural analysis was performed using Crystal-
Structure22 and Win-GX for Windows software.23 All structures
were solved by SHELXS-97 (direct methods) and refined by
SHELXH-97.24
Na2WO4 ·2H2O, Na2SiO3 ·9H2O, and aluminum salts were ob-
tained from Wako, Kanto, or Nacalai (reagent grade) and used as
received. The dilacunary precursor of K8[γ-SiW10O36]·12H2O was
synthesized according to ref 5b. (+)-Citronellal (3a) and solvents
were obtained from TCI (reagent grade) and were carefully purified
before the use.25 15N-Enriched pyridine was obtained from Aldrich
(15N > 98%) and was used as received. 3-Methylcitronellal (4a)
was synthesized by the reaction of citral with cuprate.26
Figure 7. (a) Calculated structures of transitions states to produce isopulegol
isomers for the 1-catalyzed cyclization of 3; (b) energy diagram.
Synthesis of Compound 1. Compound 1 was synthesized as
follows. K8[γ-SiW10O36]·12H2O (10.0 g, 3.4 mmol) was dissolved
in deionized water (100 mL). Then, Al(NO3)3 ·9H2O (2.6 g, 6.9
mmol) was added in a single step, and the pH of the solution was
adjusted to 3.8 with an aqueous K2CO3 solution (2.0 M). After 15
min, [(CH3)4N]Cl (4.8 g, 44 mmol) was added to the solution in a
single step followed by stirring for 30 min at room temperature
(ca. 20 °C). The white precipitate of the tetramethylammonium salt
was collected by filtration and washed with a small amount of water
and diethylether (7.0 g). The tetramethylammonium salt was
dissolved in deionized water (250 mL). Then, [(C4H9)4N]Br (28.0
g, 87.3 mmol) was added to the solution in a single step, and the
pH of the solution was adjusted to 2.6 with an aqueous HNO3
solution (1.0 M). The white precipitate of TBA salt (1) was collected
by filtration and washed with a small amount of water and
diethylether. The crude product was purified with the precipitation
method (addition of 1.0 L of water into 50 mL of a CH3CN solution
transition states were estimated by DFT calculations at the
B3LYP level theory (see Experimental Section). The transition
state to produce 3a (TS-3a) was at least by 3.6 kJ mol-1 more
stable than the other three transition states (Figure 7b), showing
that the formation of the transition state TS-3a is sterically and
electronically more favorable than the others in the 1-catalyzed
cyclization of 3.17 Therefore, the diastereoselectivity toward
different isopulegol isomers for the present 1-catalyzed cycliza-
tion of 3 can be explained by the stability of the corresponding
transition states.
Conclusion
The novel dialuminum-substituted silicotungstate 1 has been
synthesized and the molecular structure was successfully
determined. Compound 1 could act as an efficient homogeneous
Lewis acid catalyst for the cyclization of citronellal derivatives
with the high chemo- and diastereoselectivity.
(18) Wishart, D. S.; Bigam, C. G.; Yao, J.; Abildgaard, F.; Dyson, H. J.;
Oldfield, E.; Markley, J. L.; Sykes, B. D. J. Biomol. NMR 1995, 6,
135.
Experimental Section
(19) Ripmeester, J. A. J. Am. Chem. Soc. 1983, 105, 2925.
(20) (a) CrystalClear 1.3.6; Rigaku and Rigaku/MSC: The Woodlands, TX.
(b) Pflugrath, J. W. Acta Crystallogr. 1999, D55, 1718.
(21) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction Data
Collected in Oscillation Mode. In Methods in Enzymology; Carter,
C. W., Jr.; Sweet, R. M., Eds.; Macromolecular Crystallography, Part
A; Academic Press: New York, 1997; Vol. 276, pp 307-326.
(22) CrystalStructure 3.8, Rigaku and Rigaku/MSC: The Woodlands, TX.
(23) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(24) Sheldrick, G. M. SHELX97, Programs for Crystal Structure Analysis,
Release 97-2; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(25) Purification of Laboratory Chemicals, 3rd ed.; Perrin, D. D., Armarego,
W. L. F., Eds.; Pergamon Press: Oxford, 1988.
General. IR spectra were measured on Jasco FT/IR-460 Plus.
A mixture of the aluminum-substituted polyoxometalate (1 or 2)
and KBr was pressed into the disk with a radius of 10 mm, and the
spectrum was recorded at room temperature (ca. 20 °C). The
difference spectrum (Figure 4) was obtained by subtracting the
spectrum of 1 from that of 2. The ratio of pyridine adsorbed on
Lewis and Brønsted acid sites were determined by using the
following integrated molar extinction coefficients: 2.22 and 1.67
(17) The Boltzmann distribution of these transition states (TS-3a:TS-3b:
TS-3c:TS-3d ) 78:19:2:<1 (at 32 °C)) was fairly agreed with the
isomer ratio for the present cyclization (Table S3, Supporting
Information).
(26) (a) Sakane, S.; Maruoka, K.; Yamamoto, H. Tetrahedron 1986, 42,
2203. (b) Sakane, S.; Maruoka, K.; Yamamoto, H. Tetrahedron Lett.
1985, 26, 5535.
9
J. AM. CHEM. SOC. VOL. 130, NO. 47, 2008 15877

A R T I C L E S
Kikukawa et al.
of the crude product), giving 4.9 g of the purified 1 (43% yield
based on K8[γ-SiW10O36]·12H2O). The needle-like single crystals
suitable for X-ray analysis were obtained by recrystallization in a
mixed solvent of CH3CN and water. Anal. Calcd for [(n-
C4H9)4N]3H[γ-SiW10O36{Al(OH2)}2(µ-OH)2]·4H2O: C, 17.12; H,
3.68; N, 1.25; Si, 0.83; W, 54.60; Al, 1.60. Found: C, 17.67; H,
3.60; N, 1.12; Si, 0.83; W, 54.90; Al, 1.60. IR (KBr pellet;
4000-400 cm-1): 3433, 3221, 2962, 2935, 2974, 2734, 1628, 1483,
1463, 1381, 1364, 1347, 1308, 1282, 1252, 1152, 1106, 1058, 1028,
1001, 966, 905, 880, 865, 802, 627, 539, 488, 466, 411. 27Al NMR
(70.0 MHz, CD3CN/DMSO-d6 (2:1 v/v), 25 °C): δ 57.0. 29Si NMR
(53.5 MHz, CD3CN/DMSO-d6 (2:1 v/v), 25 °C): δ -83.9. 183W
NMR (11.2 MHz, CD3CN/DMSO-d6 (2:1 v/v), 25 °C) δ -72.1,
-124.8, -132.8.
Synthesis of Compound 2. Compound 2 was synthesized as
follows. Compound 1 (0.20 g, 59.4 µmol) was dissolved in a mixed
solvent of CH3CN and pyridine (0.4 mL/4.8 mL). The solution was
kept at room temperature (ca. 20 °C) for 1 week, and colorless
single crystals suitable for X-ray analysis appeared. The crystals
were collected by filtration and washed with a small amount of
CH3CN (0.11 g, 52% yield based on 1). Anal. Calcd for [(n-
C4H9)4N]3[(C5H5N)H][γ-SiW10O36{Al(C5H5N)}2(µ-OH)2]·2H2O: C,
21.42; H, 3.71; N, 2.38; Si, 0.80; W, 52.10; Al, 1.53. Found: C,
21.58; H, 3.48; N, 2.50; Si, 0.84; W, 54.29; Al, 1.62. IR (KBr pellet;
4000-400 cm-1): 3617, 3446, 3225, 3164, 3133, 3066, 2961, 2935,
2873, 2734, 1636, 1614, 1544, 1486, 1452, 1381, 1346, 1251, 1219,
1203, 1171, 1153, 1106, 1075, 1052, 997, 961, 907, 883, 821, 792,
710, 648, 621, 607, 545, 528, 508, 489, 474, 467, 453, 439, 414.
DFT Calculation. The calculations were carried out at the
B3LYP level theory27 with 6-31G* basis sets for H, C, O, Al, and
Si atoms, and the double-ꢁ quality basis sets with effective core
potentials proposed by Hay and Wadt28 for W atoms. Transition
state structures were searched by numerically estimating the matrix
of second-order energy derivatives at every optimization step and
by requiring exactly one eigenvalue of this matrix to be negative.
The optimized geometries are shown in Figure 7a. The zero-point
vibrational energies were not included. All calculations were
performed with the Gaussian03 program package.29
Cyclization of Citronellal Derivatives. The catalytic cyclization
was carried out with a glass tube reactor. All operations were carried
out in a glovebox under Ar. Compound 1, substrate, CH3CN, and
naphthalene (internal standard) were successively placed into a glass
tube reactor. A Teflon-coated magnetic stir bar was added, and the
reaction mixture was stirred (800 rpm) at 75 °C (or 32 °C) under
Ar atmosphere. The yield, chemoselectivity, and diastereoselectivity
were periodically determined by GC analysis. The products were
confirmed by the comparison of GC retention times, mass spectra,
1
and H and 13C NMR spectra with those of authentic samples.
Acknowledgment. This work was supported in part by the Core
Research for Evolutional Science and Technology (CREST)
program of the Japan Science and Technology Agency (JST),
Global COE Program (Chemistry Innovation through Cooperation
of Science and Engineering), and the Grants-in-Aid for Scientific
Researches from Ministry of Education, Culture, Sports, Science
and Technology.
Supporting Information Available: CIF files of 1 and 2,
Tables S1-S7, Figure S1 and Schemes S1 and S2, and complete
ref 29. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA8014154
(27) Becke, A. D. J. Chem. Phys. 1993, 98, 1372.
(28) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
(29) Frisch, M. J.; et al. Gaussian 03 (Revision D.02); Gaussian, Inc.:
Wallingford, CT, 2004.
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15878 J. AM. CHEM. SOC. VOL. 130, NO. 47, 2008