Monomeric metal aqua complexes in the interlayer space of montmorillonites as strong Lewis acid catalysts for heterogeneous carbon-carbon bond-forming reactions

Article abstract of DOI:10.1002/chem.200400672

Montmorillonite-enwrapped copper and scandium catalysts (Cu^2-- and Sc³⁺-monts) were easily prepared by treating Na^--mont with the aqueous solution of the copper nitrate and scandium triflate, respectively. The resulting Cu²⁺- and Sc³⁺-monts showed outstanding catalytic activities for a variety of carbon-carbon bond-forming reactions, such as the Michael reaction, the Sakurai-Hosomi allylation, and the Diels-Alder reaction, under solvent-free or aqueous conditions. The remarkable activity of the mont catalysts is attributable to the negatively charged silicate layers that are capable of stabilizing metal cations. Furthermore, these catalysts were reusable without any appreciable loss in activity and selectivity. The Cu²⁺-mont-catalyzed Michael reaction proceeds via a ternary complex in which both the 1,3-dicarbonyl compound and the enone are coordinated to a Lewis acid Cu²⁺ center.

Full text of DOI:10.1002/chem.200400672

Monomeric Metal Aqua Complexes in the Interlayer Space of
Montmorillonites as Strong Lewis Acid Catalysts for Heterogeneous
Carbon–Carbon Bond-Forming Reactions
Tomonori Kawabata, Masaki Kato, Tomoo Mizugaki, Kohki Ebitani, and
Kiyotomi Kaneda*[a]
Abstract: Montmorillonite-enwrapped
the Sakurai–Hosomi allylation, and the
Diels–Alder reaction, under solvent-
free or aqueous conditions. The re-
markable activity of the mont catalysts
is attributable to the negatively charg-
ed silicate layers that are capable of
stabilizing metal cations. Furthermore,
these catalysts were reusable without
any appreciable loss in activity and se-
lectivity. The Cu2+-mont-catalyzed Mi-
chael reaction proceeds via a ternary
complex in which both the 1,3-dicar-
bonyl compound and the enone are co-
ordinated to a Lewis acid Cu2+ center.
copper and scandium catalysts (Cu2+
-
and Sc3+-monts) were easily prepared
by treating Na+-mont with the aqueous
solution of the copper nitrate and scan-
dium triflate, respectively. The resulting
Cu2+
-
and Sc3+-monts showed out-
ꢀ
Keywords: C C bond formation ·
standing catalytic activities for a varie-
ty of carbon–carbon bond-forming re-
actions, such as the Michael reaction,
heterogeneous catalysis
·
Lewis
acids · montmorillonite
Introduction
Another promising synthetic approach to environmentally
friendly chemistry is to minimize or eliminate the use of
harmful organic solvents. A paradigm shift from using sol-
vents toward solvent-free reactions can improve outcomes
and simplify organic syntheses.[5] The use of water as a sol-
vent is also recognized as a significant challenge in green
chemistry, and additionally reveals unique catalytic activities
not observed under dry conditions.[6]
Lewis acids are of great interest because of their ability to
catalyze a wide variety of carbon–carbon bond-forming re-
actions.[1] They can replace traditional Brønsted acids and
bases to achieve green organic transformations.[2] Many
manufacturing reactions for fine chemicals and pharmaceuti-
cals rely on homogeneous Lewis acid catalysts. However,
these reactions often require the tedious isolation of prod-
ucts or the removal of large volumes of salt wastes during
neutralization of homogeneous acids.[1] To overcome these
disadvantages, much effort has been devoted to the develop-
ment of heterogeneous catalytic systems.[3] In this context,
numerous supported Lewis acid catalysts have been devel-
oped;[4] however, their activities are often low because
strong interactions between the Lewis acid centers and their
supports result in decreased Lewis acidities.
Montmorillonites, referred to as monts, are hydrophilic
clays with a layered structure. They are of considerable in-
terest as environmentally benign and reusable catalysts.
Owing to their ion-exchange properties, various types of
metal cations can be introduced readily into their expansible
interlayer spaces.[7,8] In the course of our studies on the
metal-cation-exchanged montmorillonite catalysts (Mn+
-
mont), we have defined two types of metal ion species with
unique structures within the interlayers of the mont: chain-
like metal species for Ti and Fe (Scheme 1a), and a mono-
meric aqua complex for Sc (Scheme 1b). These Mn+-monts
exhibited excellent catalytic performance in various organic
transformations.[9]
[a] Dr. T. Kawabata, M. Kato, Dr. T. Mizugaki, Dr. K. Ebitani,
Prof. Dr. K. Kaneda
Department of Materials Engineering Science
Graduate School of Engineering Science
Osaka University, 1–3 Machikaneyama, Toyonaka
Osaka 560–8531 (Japan)
Fax : (+81)6-6850-6260
E-mail: kaneda@cheng.es.osaka-u.ac.jp
We present novel montmorillonite-enwrapped monomeric
complexes of Cu and Sc, which can be prepared readily and
can act as highly active Lewis acid catalysts for a variety of
important carbon–carbon bond-forming reactions, such as
the Michael reaction, the Sakurai–Hosomi allylation, and
288
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/chem.200400672
Chem. Eur. J. 2005, 11, 288 – 297

FULL PAPER
Scheme 1. Schematic structures of the Mn+-monts: (a) a chain-like metal
species and (b) a monomeric aqua complex.
the Diels–Alder reaction, under solvent-free or aqueous
conditions. These heterogeneous reaction systems have the
advantages of eliminating waste production and simplifying
the workup procedure. Furthermore, the mont catalysts are
reusable without an appreciable loss of activity and selectivi-
ty. We believe the application of catalytic systems based on
monts as a macroanion goes beyond mere immobilization
and will lead to the development of high-performance heter-
ogeneous Lewis acid catalysts for ’green’ organic syntheses.
Figure 1. Normalized Cu K-edge XANES spectra of (a) Cu foil, (b)
Cu2O, (c) CuO, and (d) Cu2+-mont.
Results and Discussion
and Cu2O in shape and position of the edge, supporting for-
mation of a divalent Cu ion. Figure 2a depicts the Fourier
transform (FT) of the k3-weighted Cu K-edge extended X-
ray absorption fine structure (EXAFS) of the Cu2+-mont.
Preparation and characterization of montmorillonite-en-
wrapped metal cations: The mont-enwrapped metal cation
species, Cu, Sc, Y, Yb, La, and Zn, were prepared by treat-
ing Na+-mont with the aqueous solution of the appropriate
metal nitrates or triflates. Among the products, the catalytic
abilities of Cu- and Sc-monts were particularly attractive for
carbon–carbon bond-forming reactions.
Cu-mont: The Cu-mont was prepared by treatment of Na+-
mont with aqueous Cu(NO3)2·3H2O [Eq. (1)]. On the basis
of the X-ray diffraction (XRD) analysis, the layered struc-
ture was identified and the basal spacing for the Cu-mont
was estimated to be 2.9 Š, which is comparable to that of
the parent Na+-mont. Elemental analysis confirmed that
two Na+ ions are replaced by one Cu2+ ion during ion-ex-
change (Cu 3.21; Na 0.09%).
in water
ƒƒƒƒ!
2ꢀ
2ꢀ
CuðNO3Þ2 þ 2 Naþ-ðmontÞ
½CuðH OÞ ꢁ2þ-ðmontÞ
2
6
þ2 NaNO3
ð1Þ
UV/Vis spectra of the Cu-mont exhibited an absorption
peak at 1.7 eV, which is similar to that of Cu2+-exchanged
zeolites with CuIIO6 octahedral species within their
pores.[10,11] Figure 1 shows Cu K-edge X-ray absorption near-
edge structure (XANES) spectra of the Cu-mont and Cu
reference compounds. The absorption edge varies with re-
spect to the oxidation state of copper, and the XANES spec-
trum of Cu2O has a strong peak due to the 1s–4pp* transi-
tion near 8980 eV.[12] The XANES spectrum of the Cu-mont
resembles that of the CuO with a distorted octahedral coor-
dination environment,[13] but differs from those of Cu foil
Figure 2. (a) FT magnitude of k3-weighted EXAFS of the Cu2+-mont,
and (b) inverse FT of the peaks with the 1.1<RŠꢀ1 <1.9 range in (a).
Phase shift was not corrected in (a). The dotted line in (b) showed the
ꢀ
result of a curve-fitting analysis using two Cu O shell parameter in the
ꢀ1
range 4–12 Š
.
Chem. Eur. J. 2005, 11, 288 – 297
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
289

K. Kaneda et al.
ꢀ
The peak at 1.5 Š was assignable to a Cu O moiety (Fig-
ure 2b).[14]
The results of the curve-fitting analysis (Table 1) show a
slightly distorted CuO6 octahedron with four short (1.90 Š)
ꢀ
and two long (2.03 Š) Cu O shells. Because of the restrict-
ꢀ
ꢀ
Table 1. Results of curve-fitting analysis of the Cu O and Sc O shells
for mont-enwrapped Cu2+ and Sc3+ catalysts
Sample
Shell
CN[a]
R[b] [Š]
Ds[c] [Š2]
Cu2+-mont
fresh
ꢀ
Cu O (1)
4.0
2.0
4.0
2.0
2.1
1.0
2.1
1.0
1.90
2.03
1.92
2.03
1.93
2.39
1.86
2.60
0.0005
0.0004
0.0025
0.0028
0.0006
0.0065
0.0009
0.0063
ꢀ
Cu O (2)
recovered[d]
Cu O (1)
ꢀ
ꢀ
Cu O (2)
in substrates[e]
Cu O (1)
ꢀ
ꢀ
Cu O (2)
Figure 3. Fourier transforms of k3-weighted Sc K-edge EXAFS of (a)
Sc2O3, (b) fresh Sc3+-mont, and (c) recovered Sc3+-mont after the Mi-
chael reaction of 1a with 2b (see text). Phase shift was not corrected.
ꢀ
Cu O (3)
ꢀ
Cu O (4)
Sc3+-mont
fresh
ꢀ
Sc O
6.0
6.1
2.13
2.13
0.0019
0.0014
recovered[f]
Sc O
ꢀ
[a] Coordination number. [b] Interatomic distance. [c] Ds is the differ-
ence between the Debye–Waller factor of the sample and that of the ref-
erence material. [d] The recovered catalyst by filtration after the Michael
reaction of ethyl 2-oxocyclopentanecarboxylate (1a) with 2-cyclohexen-1-
one (2a) under solvent-free conditions (see text). [e] The catalyst treated
with both 2a and acetylacetone (1c). [f] The recovered catalyst after the
Michael reaction of 1a with 3-buten-2-one (2b) in water (see text).
ꢀ
species. The peak at 1.7 Š was assignable to a Sc O shell
and curve-fitting results revealed that the interatomic dis-
ꢀ
tance (R) and the coordination number (CN) of the Sc O
bond were 2.13 Š and 6.0, respectively (Table 1).
A monomeric aqua Sc ion is clearly formed within the in-
terlayer of the mont. The distances between the scandium
and oxygen atoms in the Sc3+-mont are comparable to those
observed in hexacoordinate scandium aqua complexes, such
as [Sc(H2O)4(picrate)2]+ (2.109 Š),[17] [Sc(H2O)4(tosylate)2]+
(2.117 Š),[18] and [ScCl3(H2O)3] [H2L]·3H2O (2.122 Š).[19]
The situations involving Cu2+ and Sc3+ ions contrast
sharply with that of the Ti4+ ion in the mont interlayer,
where the Ti cations form a chainlike structure linked by
ꢀ
ed interlayer space of the mont, the Cu O distance of
ꢀ
2.03 Š is shorter than the axial Cu O bond of 2.43 Š ob-
served in a [Cu(H2O)6]2+ complex in perchlorate solution.[15]
In the FT EXAFS, the peaks above 2 Š, attributed to the
presence of contiguous Cu sites, were not detectable for the
Cu2+-mont. Based on the basal spacing and the Cu O dis-
ꢀ
tances of the Cu2+-mont, it is reasonable to assume that the
monomeric Cu aqua complex in the interlayer is inclined at
about 458 with respect to the c axis of the mont. An analo-
gous [Cu(H2O)6]2+ species has been reported within the in-
terlayer space of hectrites[7b,16] and the pores of zeolite-X.[10]
We think that this difference can be
[9a–d]
ꢀ ꢀ
Ti O Ti moieties.
explained by the hydrolysis constant (Kh) of the metal cat-
ions.[20] Metal cations with small pKh values (2.3 for Ti4+
)
generally are easy to hydrolyze to the corresponding
[M(OH)n] cation species that are precursors to the M O M
ꢀ ꢀ
moiety. However, Cu and Sc cations have large pKh values
(7.53 for Cu2+ and 4.3 for Sc3+), and thus are stable against
hydrolysis and remain monomeric.
The present preparation method, which utilizes the
cation-exchange ability of monts, is a powerful protocol for
the stabilization of monomeric aqua metal species of Cu and
Sc within the interlayer of monts for use as heterogeneous
acid catalysts.
Sc-mont: Treatment of Na+-mont with aqueous Sc(OTf)3 af-
forded Sc-mont [Eq. (2)]. Elemental analysis (Sc 1.78, Na
0.04%) of the Sc-mont showed that three Na+ ions are re-
placed by one Sc ion.
in water
ƒƒƒƒ!
3ꢀ
3ꢀ
ScðOTfÞ3 þ 3 Naþ-ðmontÞ
½ScðH OÞ ꢁ3þ-ðmontÞ
2
6
þ3 NaOTf
Michael reaction using the Cu2+-mont catalyst: The Michael
reaction of 1,3-dicarbonyl compounds with enones provides
access to 1,5-dioxo synthons, which can be transformed
easily into cyclohexenone derivatives for use as important
intermediates in steroid and terpenoid synthesis.[21] The Mi-
chael reactions of ethyl 2-oxocyclopentanecarboxylate (1a)
with 2-cyclohexen-1-one (2a) in various solvents were con-
ducted in the presence of the Cu2+-mont (Table 2). Nitro-
methane was an optimal solvent to give 2-oxo-1-(3-oxocyclo-
hexyl)-cyclopentanecarboxylate (3a) in 82% yield (Table 2,
ð2Þ
XRD studies verified that the layered structure of the Sc-
mont is retained with a basal spacing of 3.6 Š. X-ray photo-
electron spectroscopy (XPS) revealed formation of a triva-
lent Sc species. Fourier transform of k3-weighted Sc K-edge
EXAFS of the Sc2O3 had a broad peak near 3.0 Š due to
Sc Sc and Sc O shells in the second coordination sphere
(Figure 3). In contrast, the lack of this peak for the Sc3+
mont samples supported the presence of a monomeric Sc
ꢀ
ꢀ
-
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Chem. Eur. J. 2005, 11, 288 – 297

Montmorillonite-Enwrapped Metal Catalysts
FULL PAPER
Table 2. Michael reaction of 1a with 2a using various Mn+-mont cata-
Table 3. Solvent-free Michael reaction catalyzed by Cu2+-mont.[a]
lysts.[a]
Entry
Donor
Acceptor
Product
Temp. Time Yield[b]
Entry
Catalyst
Solvent
Yield [%][b,c]
[8C]
[h]
[%]
1
2
3
4
5
6
7
8
Cu2+-mont
Cu2+-mont
Cu2+-mont
Cu2+-mont
Cu2+-mont
Cu2+-mont
Cu2+-mont
Cu2+-mont
Cu2+-mont
Cu2+-mont
Cu2+-mont
Cu2+-mont
Sc3+-mont
Y3+-mont
Yb3+-mont
La3+-mont
Zn2+-mont
Na+-mont
Cu(NO3)2·3H2O
no
neat
nitromethane
EtOH
n-hexane
toluene
ethyl acetate
1,2-dichloroethane
acetonitrile
water
DMA
DMF
DMSO
neat
neat
neat
neat
neat
neat
neat
neat
96
82
76
69
46
43
32
6
6
0
0
0
63
25
20
16
11
trace
59
0
1
2
3
20
1
99
20
20
1
1
97
97
9
10
11
12
13
14
15
16
17
18
19
20
4
5
20
20
1
2
97
1a
96
6
1a
1a
1a
2a
2a
2a
3a
3a
3a
70
70
70
2
2
2
92[c]
92[c]
92[c]
7[d]
8[e]
[a] Reaction conditions; active metal species (0.05 mmol), 1a (4 mmol),
2a (6 mmol), solvent (5 mL) (entries 2–12), 708C, 2 h. [b] Yields of prod-
ucts were determined by GC based on donor. [c] A 1:1 ratio of diaster-
eoisomers.
9
1a
70
5
99[c]
[a] Reaction conditions; Cu2+-mont (Cu: 0.05 mmol), donor (4 mmol),
acceptor (4.4 mmol). [b] Yields of products were determined by GC
based on donor. [c] A 1:1 ratio of diastereoisomers. [d] Reuse-1. [e]
Reuse-2.
entry 2). Ethanol and n-hexane were also effective solvents,
affording 76 and 69% yields of 3a, respectively (Table 2, en-
tries 3 and 4). In marked contrast to the Sc3+-mont-cata-
lyzed Michael reaction,[9f] vide infra, water was a poor sol-
vent (Table 2, entry 9). Interestingly, the Cu2+-mont exhibit-
ed the greatest catalytic activity under neat conditions
(Table 2, entry 1).[22]
desired 1,4-addition products were obtained exclusively
without formation of 1,2-addition compounds.
It is noteworthy that 100 mmol of diethyl malonate (1e)
readily reacted with 2-cyclopenten-1-one (2d) under sol-
vent-free conditions with only 0.25 mol% of the Cu2+-mont
catalyst to afford diethyl 3-oxocyclopentylmalonate (3h) in
a high yield (Scheme 2).
The catalytic activity for the Michael reaction without sol-
vents was compared by using various Mn+-monts. The Cu2+
-
mont gave the highest yield of 3a (Table 2, entry 1 versus
entries 13–17). The Michael reaction did not occur in the
presence of the parent Na+-mont (Table 2, entry 18). Nota-
bly, catalytic activity of the Cu2+-mont was greater than that
of a homogeneous Cu(NO)3·3H2O (Table 2, entry 19). The
yield of 3a increased with an increase in Lewis acid strength
of the metal cation in the mont.[23]
The scope of substrates for the Michael reaction catalyzed
by the Cu2+-mont under solvent-free conditions was exam-
ined (Table 3). The Michael reactions of various b-keto
esters and 1,3-diketones with 3-buten-2-one (2b) occurred
efficiently to afford the corresponding 1,5-dioxo compounds,
even at room temperature (Table 3, entries 1–4). In addition,
1-penten-3-one (2c) also acted as a good acceptor (Table 3,
entry 5). It is said that the solvent-free Michael reaction
with cyclic enones using FeCl3·6H2O[24] and Cu(OAc)2·
H2O[25] is unsuccessful. However, the Cu2+-mont was effec-
tive for such enones to give the corresponding Michael
adducts within 5 h (Table 3, entries 6 and 9). In all cases, the
Scheme 2. Cu2+-mont-catalyzed 100 mmol-scale Michael reaction under
solvent-free conditions.
Generally, the Michael reaction of dialkyl malonates does
not proceed easily under traditional Lewis acid catalyzed
conditions,[21d] but some base catalysts such as sodium ethox-
ide show high catalytic activity toward these substrates.[21c]
Chem. Eur. J. 2005, 11, 288 – 297
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291

K. Kaneda et al.
Shibasaki et al. recently reported that the La-linked-BINOL
catalysts were highly efficient for the asymmetric Michael
reaction of dialkyl malonates with enones, in which the
metal center and the naphthoxide moiety act as a Lewis
acid and Brønsted base, respectively.[26] In a similar fashion,
the Michael reaction by the Cu2+-mont likely proceeds by
bifunctional catalysis between the Lewis acid Cu complex
and the silicate layer of the mont; the latter efficiently ab-
stracts a proton from 1,3-dicarbonyl compounds.
of ethyl 2-oxo-1-(3-oxobutyl)-cyclopentanecarboxylate (3b)
(Table 4, entry 1) and exhibited significantly higher catalytic
activity than that of the homogeneous Sc(OTf)3 (Table 4,
entry 7).[30]
In contrast, the catalytic activities of Cu2+- and Zn2+
-
monts under similar conditions were extremely low (Table 4,
entries 5 and 6). A similar phenomenon is also observed for
Mukaiyama–aldol reactions using various metal salts under
aqueous conditions.[20]
Upon completion of the Michael reaction, the Cu2+-mont
catalyst was readily recovered from the reaction mixture by
simple filtration, and could be reused without any discern-
ible loss in activity and selectivity. As shown in Table 3,
yields of 92% were obtained in two recycling experiments
(entries 7 and 8). Elemental analysis of the used Cu2+-mont
confirmed no leaching of Cu species from the catalyst. The
retention of monomeric Cu2+ species in the recovered Cu-
mont catalyst was supported by EXAFS data (Table 1). In
the reaction of 1a with 2a, the Cu2+-mont was removed by
filtration after about 50% conversion of 1a at 708C; further
treatment of the filtrate under similar reaction conditions
did not afford any additional 3a. These observations clearly
demonstrate that the Michael reaction occurred at the Cu
species within the mont layers.
Generally, treatment of RE3+ metal triflates with water
gives RE3+ aqua complexes surrounded by low nucleophilic
OTf counteranions in the second coordination sphere, which
act as Lewis acids.[31] Yb salts with low nucleophilic counter-
ꢀ
anions such as OTfꢀ and ClO4 catalyze the aldol reaction
of silyl enol ethers with aldehydes in aqueous media, where-
as combining Yb salts with Clꢀ, OAcꢀ, NO3ꢀ, and SO4
2ꢀ
ions results in low catalytic activity.[33] Elemental analysis of
the Sc3+-mont revealed an absence of OTf groups in the
mont catalyst. Additionally, Sc/Al2O3 and Sc/SiO2 were com-
pletely inactive for Michael reactions. Presumably, Sc3+
aqua complexes partnered with anionic silicate layers of
monts, whose negative charge is delocalized along the
layer,[7,33] would provide an outstanding catalytic activity.
Under aqueous conditions, Sc3+-mont-catalyzed Michael
reactions were extended to other 1,3-dicarbonyl substrates
(Table 5). In all cases, the reaction proceeded smoothly in
Catalysis of the Sc3+-mont for Michael reactions in water:
Performing carbon–carbon bond-forming reactions in water
as the solvent is of great interest because of practical and
environmental considerations.[6,27] While many Lewis acids
such as Ti, Al, Sn, and B complexes are hydrolyzed to form
inactive hydroxides and oxides in the presence of water,[28]
rare-earth (RE) metal complexes can act as Lewis acids.
[RE(OTf)3] (OTf: trifluoromethanesulfonyl) compounds are
known as water-compatible catalysts.[29] However, these cat-
alysts exhibit low activity or require long reaction times.
The Michael reaction of 1a with 2b was carried out under
aqueous conditions using various Mn+-monts and the results
are summarized in Table 4. No reaction occurred in the ab-
Table 5. Michael reaction of various 1,3-dicarbonyl compounds with
enones catalyzed by Sc3+-mont in water.[a]
Entry
Donor
Acceptor
Product
Temp. Time Yield[b]
[8C]
[h]
[%]
1
1a
1a
1a
1a
1b
1a
2b
2b
2b
2b
2b
2c
3b
3b
3b
3b
3c
3 f
30
30
30
30
50
30
0.5
0.5
0.5
0.5
2
99
99
99
99
90
98
2[c]
3[d]
4[e]
5
6
1
7
1a
50
50
45
60
50
50
1
2
3
2
2
1
90
99
99
80
97
96
sence of the catalyst (Table 4, entry 8). As expected, Mn+
-
monts having RE3+ metal cations were effective within 0.5 h
(Table 4, entries 1–4); the Sc3+-mont gave the highest yield
8
2b
2b
2b
2b
2b
Table 4. Michael reaction of ethyl 2-oxo-cyclopentanecarboxylate (1a)
with 3-buten-2-one (2b) using various catalysts.[a]
9
10[f]
11
12
Entry
Catalyst
Yield[b] [%]
1
2
3
4
5
6
7
8
Sc3+-mont
Y3+-mont
Yb3+-mont
La3+-mont
Cu2+-mont
Zn2+-mont
[Sc(OTf)3]
no
99
83
62
62
17
16
7
13
14
1c
1d
2b
2b
3d
3e
45
50
3
1
98
97
0
[a] Reaction conditions; Sc3+-mont (Sc: 0.04 mmol), donor (2 mmol), ac-
ceptor (2.2 mmol), H2O (3 mL). [b] Yields of products were determined
[a] Reaction conditions; active metal species (0.04 mmol), 1a (2 mmol),
2b (2.2 mmol), water (3 mL), 308C, 0.5 h. [b] Yields of 3b were deter-
mined by GC based on donor.
by GC based on donor. [c] Reuse-1. [d] Reuse-2. [e] Reuse-3. [f] Sc3+
-
mont (Sc: 0.02 mmol), 1h (0.5 mmol), 2b (0.55 mmol), H2O (1 mL).
292
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Chem. Eur. J. 2005, 11, 288 – 297

Montmorillonite-Enwrapped Metal Catalysts
FULL PAPER
the presence of 2–4 mol% Sc catalyst within 3 h to afford
the corresponding Michael adducts in excellent yields. Hy-
drolysis of the ester moieties did not occur (Table 5, en-
tries 1–11). Recently, heterogeneous Michael reactions in
water using organic polymer-supported[4j,m] and surfactant-
combined[34] Sc catalysts as Lewis acids have been reported.
These systems have the drawback of waste production be-
cause activated donors such as silyl enol ethers or 1.5–
3 equivalents of acceptor are required. The Sc3+-mont over-
comes these limitations and is the most active Lewis acid
catalyst for the Michael reaction of 1,3-dicarbonyl com-
pounds with enones using water as a solvent. Importantly,
even after three recycling experiments, no appreciable loss
of catalytic activity and selectivity occurred. For reaction of
1a with 2b, 99% yields for 3b were obtained during three
recycling experiments (Table 5, entries 2–4). The used cata-
lyst retained its original Sc content as confirmed by elemen-
tal analysis. The stability of the [Sc(H2O)6]3+ species in the
recovered Sc3+-mont catalyst was established by XAFS
measurement, as shown in Figure 3 and Table 1.
silyl ether 6a, was detected. We explored the simple and ef-
ficient proton-mediated desilylation to homoallylic alcohols.
After complete conversion of 4a, ethanol was added and
the reaction mixture was stirred at 808C for 1 h to afford 7a
in 99% yield.[36]
One-pot syntheses of various homoallylic alcohols were
performed by using the Cu2+-mont catalyst, as exemplified
in Table 7. A variety of aldehydes and ketones reacted
Table 7. Allylation of carbonyl compounds with allyltrimethylsilane (5).[a]
Entry Carbonyl
compound
5 [equiv]
Product
Temp.[b] Time[b] Yield[c]
[8C]
[h]
[%]
1
4a
1.1
1.5
7a
45
1
99
2
45
1
6
97
95
3[d]
1.5
RT
Application of the Cu2+-mont catalyst to Sakurai–Hosomi
and Diels–Alder reactions: To explore the catalytic activity
of Cu2+-mont as a Lewis acid, it was applied to the Sakurai–
Hosomi reaction, that is the allylation of carbonyl com-
pounds with allylalkylsilanes, which is of great interest in or-
ganic chemistry because of the synthetic utility of homoallyl-
ic alcohols.[35] The catalytic activity in the reaction of benzal-
dehyde (4a) with allyltrimethylsilane (5) under solvent-free
conditions was compared with that of many other copper
catalysts; typical results are displayed in Table 6. Among the
4[d]
5[d]
6[d]
1.5
2
RT
40
1
2
3
90
80
81
2
55
[a] Reaction conditions; 1) Cu2+-mont (Cu: 0.05 mmol), 4a–f (4 mmol), 5
(1.1–2 equiv), 2) EtOH (1 mL), 808C, 1 h. [b] For the allylation reaction
(1). [c] Yields of products were determined by GC based on carbonyl
compound. [d] Nitromethane (3 mL) was used as the solvent.
Table 6. Allylation reaction of benzaldehyde with allyltrimethylsilane
using various Cu catalysts.[a]
smoothly with 5 to give the corresponding homoallylic alco-
hols. Cyclic and linear aliphatic aldehydes, as well as ke-
tones, required nitromethane as a solvent to obtain the de-
sired products in high yields. The recovered catalysts
showed comparable activity and selectivity with fresh ones.
Yields of the homoallylic alcohol remained at 99%, while
the catalysts were recycled three times. A 100-mmol-scale
reaction of 4a with 5 in the presence of the Cu2+-mont cata-
lyst afforded 7a in 86% yield. In contrast to previously re-
ported homogeneous aluminum bis(trifluoromethylsulfo-
nyl)amides,[37] ytterbium trichlorides,[38] trimethylsilylme-
thane sulfonates,[39] indium trichloride/chlorotrimethylsi-
lane,[40] and heterogeneous Al- and K-10 montmorillon-
ites,[41] the Cu2+-mont catalyst system is nonpolluting and re-
cyclable, and eliminates halogenated reagents and solvents.
The Cu2+-mont catalyst also promoted Diels–Alder reac-
tions under solvent-free conditions (Table 8). For example,
the reaction between 1,3-cyclohexadiene (8b) and 2b pro-
duced 2-acetylbicyclo[2.2.2]oct-5-ene (10b) in 99% yield
(Table 8, entry 2). A control experiment confirmed that a
trace of 10b formed without catalyst under identical reac-
tion conditions. The imino Diels–Alder reaction of benzyl-
ideneaniline (8d) with 2,3-dihydrofuran (9c) also proceeded
Entry
Catalyst
Cu2+-mont
Yield of (6a+7a)[b] [%]
1
2
3
4
5
6
99
0
0
0
0
Cu2+-hydrotalcite
Cu2+-zeolite-(X)
Cu/Al2O3
Cu/SiO2
Cu(NO3)2·3H2O
0
[a] Reaction conditions; catalyst (Cu: 0.05 mmol), 4a (4 mmol),
(6 mmol), 458C, 1 h. [b] Yields of products were determined by GC
based on 4a.
5
Cu catalysts tested, the Cu2+-mont possessed the greatest
activity (99% yield after 1 h, Table 6, entry 1) to afford tri-
methyl[(1-phenyl-3-butenyl)oxy]silane (6a) and 4-phenyl-1-
buten-4-ol (7a) in 98 and 1% yields, respectively. In con-
trast, the allylation reaction hardly occurred in the presence
of Cu2+-hydrotalcite, Cu2+-zeolite-(X), Cu/Al2O3, Cu/SiO2,
and Cu(NO)3·3H2O (Table 6, entries 2–6).
In the above reaction, a small amount of the homoallylic
alcohol 7a, which is formed by hydrolysis of the homoallyl
Chem. Eur. J. 2005, 11, 288 – 297
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293

K. Kaneda et al.
Table 8. Cu2+-mont-catalyzed Diels–Alder reaction.[a]
Entry Diene
Dienophile Product
Temp. Time Yield[b,c]
[8C]
[h]
[%]
1
2
2b
2b
40
1
99(90/10)
40
40
1
1
97(99/1)
3
8b
95(90/10)
4[d]
40
50
0.5
4
90
5[e]
85(54/46)[f]
Figure 4. IR spectra of a) fresh Cu2+-mont, b) the Cu2+-mont after treat-
[a] Reaction conditions; Cu2+-mont (Cu: 0.05 mmol), 8a and 8b (4.4 mmol),
2b and 9a (4 mmol). [b] Yields of products were determined by GC based on
dienophile. [c] Values in parentheses were endo/exo ratio determined by
1H NMR. [d] 8c (2 mmol), 9b (3 mmol). [e] 8d (1 mmol), 9c (3 mmol). [f] cis/
trans ratio.
ment with 2a, and c) the Cu2+-mont after treatment with both 2a and
1c. D: coordinated 2a (1658 cmꢀ1); : acetylacetonato-Cu species (1580,
*
1558, and 1539 cmꢀ1, ref. [43]); &: free 1c (1747 cmꢀ1). The fresh Cu2+
-
mont (0.2 g) was treated with 2a (1.0 equiv Cu) in nitromethane (1 mL).
The 2a-covered Cu2+-mont was further reacted with one equivalent of
1c.
to give the tetrahydroquinoline derivative (10e) in a high
yield (Table 8, entry 5). Dimerization of dienes was not ob-
served.
*
signed to the acetylacetonato–copper species ( in Fig-
ure 4c).[43] EXAFS analysis of the same sample supported
the generation of two Cu O bonds 1.86 Š in length and one
Cu O bond 2.60 Š in length, along with the loss of three
Reaction mechanism for the Cu2+-mont-catalyzed Michael
reactions: Transition-metal and rare-earth metal catalysts
have been extensively used for the Michael reaction of 1,3-
ꢀ
ꢀ
H2O ligands from the original Cu2+-mont (Table 1). On the
basis of these results, it is reasonable that the Cu2+-mont-
catalyzed Michael reactions involve the ternary copper com-
plex II, in which both the 1,3-dicarbonyl compound and the
enone coordinate to the Cu2+ center (Scheme 3).[44] The
carbon–carbon bond formation produces a Cu-alcoholate in-
termediate III, followed by protolysis to afford the Michael
adduct together with regeneration of the original Cu species
I. The same reaction mechanism has been proposed for the
Sc3+-mont-catalyzed Michael reactions in water.[9f] Notably,
Michael reactions of nitriles such as ethyl cyanoacetate and
malononitrile as donors instead of 1,3-dicarbonyl com-
pounds barely proceeded under these conditions. It is likely
that such nitrile compounds strongly coordinate to copper
and prevent the interaction of the enones with the Cu com-
plexes. It seems that the Lewis acid site originating from the
Cu2+ aqua species induces both Sakurai–Hosomi and Diels–
Alder reactions by coordination of carbonyl compounds.
One of the important characteristics of montmorillonites
is the enlargement of the interlayer distance that occurs in
solvents. Indeed, the interlayer space of the Cu2+-mont was
expanded from 2.9 to 11.5 Š when soaked in a mixture of
1c and 2a under solvent-free conditions, as confirmed by
XRD.[45] The efficient catalysis by the Cu2+-mont under sol-
vent-free conditions could be related to the expansion of the
interlayer space as well as the strong Lewis acid nature of
the aqua Cu2+ ion enwrapped in the mont interlayer.
dicarbonyl compounds with enones.[21d] For Ni2+, Cu2+
,
Co2+, Fe3+, and Sc3+, the reaction mechanism is considered
to involve a ternary complex, in which both the 1,3-dicar-
bonyl compound and the enone are coordinated to a Lewis
acid metal center.[21d]
A shift in the IR spectrum of the coordinated ketone is a
measure of the Lewis acid strength of metal cations.[42] The
n(CO) band of cyclopentanone adsorbed onto the Cu2+
-
mont appeared in the IR spectrum at 1685 cmꢀ1, which is
lower than that of the free cyclopentanone (1751 cmꢀ1). This
shift of 66 cmꢀ1 was larger than those found for the Sc3+-,
Y3+-, Yb3+-, and La3+-monts (34 cmꢀ1), demonstrating that
the Cu2+-mont catalyst possesses significantly stronger
Lewis acid strength than the other Mn+-monts. Yields of the
Michael adduct increased with increasing Lewis acid
strength of the metal cation (Table 2). Thus, it is reasonable
that the Lewis acid sites of Mn+-monts play an important
role in these carbon–carbon bond-forming reactions. Pre-
sumably, the silicate layers of the monts may act as a macro-
anion with low nucleophilicity, leading to the formation of a
cationic copper center with extremely strong Lewis acidity.
As seen in Figure 4b, upon treatment of the Cu2+-mont
with 2-cyclohexen-1-one (2a), the IR spectrum contained a
n(CO) band at 1658 cmꢀ1, ascribed to 2a coordinated to the
copper Lewis acid site. Addition of acetylacetone (1c) as a
donor gave new peaks at 1580, 1558, and 1539 cmꢀ1, as-
294
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Chem. Eur. J. 2005, 11, 288 – 297

Montmorillonite-Enwrapped Metal Catalysts
FULL PAPER
deionized water and dried at 1108C to
yield a light blue powder (2.5 g; Cu
3.21 wt%, 0.505 mmolgꢀ1).
Preparation of Sc3+-mont catalyst:
The Na+-mont (3.0 g) was stirred in
aqueous
Sc(OTf)3
(5.0”10ꢀ3 m,
200 mL) at 508C for 24 h. The result-
ing suspension was filtered, and the
solid was washed repeatedly with de-
ionized water, then dried at 1108C to
afford
powder
a
Sc3+-mont as
(2.7 g; Sc
a
light gray
1.78 wt%,
0.396 mmolgꢀ1). XPS: Sc 2p3/2
=
402.6 eV. The XPS peak position is re-
ferred to C 1 s at 284.6 eV.
Preparation of mont-enwrapped Y,
Yb, La, and Zn cation species: Na+-
mont (3.0 g) was added to 200 mL of
the aqueous solution containing 0.70 g
of the corresponding M(OTf)3, fol-
lowed by stirring at 508C for 24 h.
After filtration and washing with de-
ionized water, the powder was dried at
1108C to give Y3+-mont (Y, 3.1 wt%),
Yb3+-mont (Yb, 4.3 wt%), or La3+
-
mont (La, 4.5 wt%). Treatment of
3.0 g of the Na+-mont with aqueous
Zn(OTf)2 (1.0”10ꢀ2 m, 200 mL) fol-
lowed by filtration, washing, and drying
afforded Zn2+-mont (Zn, 6.3 wt%).
Scheme 3. Proposed mechanism for Michael reaction using the Cu2+-mont catalyst.
Conclusion
Preparation of Cu/SiO2 and Cu/Al2O3: SiO2 (3.0 g) was added to aqueous
CuCl2·2H2O (1.5”10ꢀ2 m, 100 mL) at 808C for 24 h. The solid obtained
was then dried at 1108C to give Cu/SiO2 (Cu, 3.2 wt%). The same treat-
ment using Al2O3 afforded Cu/Al2O3 (Cu, 3.2 wt%), which may involve
the formation of small clusters of CuO on the Al2O3 surface.[12a]
Montmorillonite-enwrapped copper and scandium aqua
complexes have been developed as extremely active and
versatile Lewis acid catalysts that can be applied to a variety
of carbon–carbon bond-forming reactions under solvent-free
or aqueous conditions. The remarkable activity of the mont
catalysts is attributable to the negatively charged silicate
layers that are capable of stabilizing metal cations. These
catalysts are also reusable without any appreciable loss in
activity and selectivity.
Preparation of Cu2+-hydrotalcite (Cu-Mg-Al-CO3):
A mixture of
CuCl2·2H2O (12.9 mmol), MgCl2·6H2O (129.6 mmol), and AlCl3·H2O
(43.2 mmol) was dissolved in deionized water (120 mL). Then aqueous
Na2CO3 (120 mL, 0.24 mol) and NaOH (0.39 mol) were slowly added,
and the resulting mixture heated at 658C for 18 h with vigorous stirring.
The resulting slurry was filtered, washed with deionized water, and dried
at 1008C for 15 h to give 12.0 g of Cu-Mg-Al-CO3 (Mg 23.4, Al 5.37, Cu
6.4 wt%). The hydrotalcite structure of the gray powder was confirmed
by XRD, and had a basal spacing of 7.9 Š, similar to that of the parent
Mg-Al-CO3 hydrotalcite (7.9 Š). The Cu2+ species were located within
the Brucite-like sheet by isomorphic substitution of Mg2+ cation at the
octahedral sites.[47]
Experimental Section
Preparation of Cu2+-X zeolite: Copper-ion-exchanged X zeolite (Cu-X)
was prepared by treating Na-X zeolite (Wako Pure Chemical), Na86(Al86-
Si106O384)·264H2O (Si/Al=1.23), with a 1.0”10ꢀ2 m aqueous solution of
Cu(NO3)2·3H2O. The resulting powder was washed with deionized water,
dried, and calcined at 3008C to give Cu–X[10] with a Cu content of
12.0 wt%, which corresponds to 83% ion exchange of Na+. Retention of
the crystal structure of the X zeolite was confirmed by XRD. In situ
EPR, UV/Vis, and Cu K-edge XANES spectra for the Cu–X indicated
dispersed divalent Cu species with a centrosymmetric octahedral coordi-
nation environment. Curve-fitting analysis of the k3-weighted EXAFS
showed that the Cu2+ ions displayed Jahn–Teller distorted CuO6 octahe-
General: The following instruments were used in this study: 1H and 13C
NMR (JNM-AL400), IR (JASCO FTIR-410), XRD (Philips X’Pert-
MPD), XPS (ESCA-2000), and GLC (Shimadzu GC-8A PF equipped
with a flame ionization detector and silicone UC W-98 and OV-1 col-
umns). The interlayer distance was determined by subtracting the c di-
mension of the silicate sheet (9.6 Š) from the observed d001 values in a
XRD spectrum.[16]
The sodium montmorillonite (Na+-mont) was supplied by Kunimine In-
dustry Co. Ltd. as Kunipia F (Na 2.69, Al 11.8, Fe 1.46, Mg 1.97%).
Cu(NO3)2·3H2O and Sc(OTf)3 were purchased from Wako Pure Chemi-
cal and Tokyo Kasei, respectively, and used as received. SiO2 (JRC-SIO-
8) and c-Al2O3 (JRC-ALO-4) were supplied by the Catalyst Society of
Japan as reference catalysts. All organic compounds used as substrates
and solvents were obtained from Wako Pure Chemical, Tokyo Kasei, Al-
drich, or Acros, and purified by standard procedures before use.[46] The
identities of products were confirmed by comparison with reported IR,
ꢀ
ꢀ
dra with four short Cu O distances (1.91 Š) and two long Cu O distan-
ces (2.28 Š).
Pretreatment of Cu2+-mont catalyst: Prior to use in C C bond-forming
ꢀ
reactions, the Cu2+-mont was heated under vacuum at 1008C for 2 h in a
reaction vessel. Before addition of substrate, the sample was cooled to
room temperature under vacuum. This pretreatment is necessary to
achieve high catalytic activity and reproducibility of the Michael reaction
results. EXAFS studies showed that the local structure of Cu2+ ions in
the Cu2+-mont did not change appreciably after this treatment. Removal
of adsorbed water within the interlayer of mont by the pretreatment
1
GC-MS, and H and 13C NMR data.
Preparation of Cu2+-mont catalyst: The Na+-mont (3.0 g) was stirred in
aqueous Cu(NO3)2·3H2O (8.3”10ꢀ3 m, 200 mL) at 508C for 24 h. The re-
sulting suspension was filtered and the solid was washed repeatedly with
Chem. Eur. J. 2005, 11, 288 – 297
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
295

K. Kaneda et al.
allows access of the substrates to the active Cu2+ species enwrapped in
the mont.
(16206078). We thank Prof. Masaharu Nomura (KEK-PF) for XAFS
measurements. The authors are also grateful to the center of excellence
(21COE) program “Creation of Integrated EcoChemistry” of Osaka Uni-
versity and the Department of Chemical Science and Engineering, Grad-
uate School of Engineering Science, Osaka University, for scientific sup-
port with the gas-hydrate analyzing system (GHAS).
A typical procedure for the Michael reaction with a Cu2+-mont catalyst
under solvent-free conditions: 1a (4 mmol) and 2a (4.4 mmol) were
added to the pretreated Cu2+-mont (0.09 g, Cu: 0.05 mmol). The hetero-
geneous reaction mixture was stirred at 708C for 2 h, and the Cu2+-mont
was removed by filtration and washed with acetone and deionized water,
followed by drying at 1108C. GC analysis of the filtrate showed 92%
yield of 3a.
Procedure for 100-mmol-scale Michael reaction: 1e (16.0 g, 100 mmol)
was poured onto the pretreated Cu2+-mont catalyst (0.5 g, Cu:
0.253 mmol), followed by addition of 2d (12.3 g, 150 mmol). After the
mixture had been stirred at 1008C for 14 h, the Cu2+-mont was removed
by filtration and washed with acetone. The acetone washings were com-
bined and distilled to give 3h (22.0 g; 91% yield) as a colorless oil.
A typical procedure for the Michael reaction with a Sc3+-mont catalyst in
water: A mixture of 1a (2 mmol), 2b (2.2 mmol), the Sc3+-mont (0.1 g,
Sc: 0.039 mmol), and water (3 mL) was stirred at 308C under argon.
After 0.5 h, the Sc3+-mont was separated by centrifugation and GC anal-
ysis of the supernatant showed 99% yield of 3b. The residual solid cata-
lyst was washed with acetone and reused under identical conditions.
A typical procedure for the Cu2+-mont-catalyzed Sakurai–Hosomi reac-
tion under solvent-free conditions: 4a (4 mmol) and 5 (4.4 mmol) were
added to the Cu2+-mont (0.09 g, Cu: 0.05 mmol). After the mixture had
been stirred at 458C for 1 h, EtOH (1 mL) was added and the mixture al-
lowed to react further at 808C. The Cu2+-mont was removed by filtration
and GC analysis of the filtrate showed 99% yield of 7a. The isolated cat-
alyst was washed with acetone and deionized water, followed by drying
at 1108C. Product yields remained at 99% during three cycles of the re-
action using the same catalyst.
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Procedure for 100-mmol-scale Sakurai–Hosomi reaction: Compounds 4a
(10.6 g, 100 mmol) and 5 (12.6 g, 110 mmol) were added onto pretreated
Cu2+-mont (0.5 g, Cu: 0.253 mmol). After the mixture had been stirred at
508C for 2 h, EtOH (20 mL) was added to the reaction mixture and it
was allowed to react further at 808C. The Cu2+-mont was removed by fil-
tration and washed with acetone. The combined acetone washings were
distilled to afford pure 7a as a colorless oil (12.7 g, 86%).
Diels–Alder reaction between cyclohexadiene and 3-buten-2-one: 2b
(4 mmol) and 8b (4.4 mmol) were added to the pretreated Cu2+-mont
catalyst (0.09 g, Cu: 0.05 mmol) and the resulting mixture was stirred at
408C for 1 h. A 99% yield of 10b (endo:exo=99:1) was confirmed by
1
GC analysis. The endo:exo ratio was determined by H NMR spectrosco-
py.
IR measurements for determination of the ternary complex: The IR
spectra of the Mn+-monts were obtained at room temperature in trans-
mission mode. The Mn+-monts (0.2 g) were treated with one equivalent
of a donor relative to the amount of metal cation in nitromethane (1 mL)
at room temperature. After removal of the solvent by evaporation at
308C, the Mn+-monts were subsequently treated with an acceptor
[5] For reviews on solvent-free organic synthesis, see: a) G. W. V. Cave,
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(1.0 equiv) in the same manner as the donor. In each step, the Mn+
monts were pressed into a disk and subjected to IR measurement.
-
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220, 365.
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X-ray absorption fine structure (XAFS) measurements: The Cu K-edge
X-ray absorption spectra were measured in transmission mode at the
EXAFS facilities installed at the BL-7C line of KEK-PF, Tsukuba, Japan
(prop. No.2001G143). For details of the data analysis, refer to the report-
ed procedure.[48] The curve-fitting analysis of reverse FT was conducted
ꢀ
ꢀ
by using Cu O and Sc O shells with empirical values of back scattering
amplitude and phase shift extracted from CuO and Sc2O3, respectively,
assuming the peaks originated from scattering by the neighboring
oxygen.
Acknowledgement
This work is supported by the Grant-in-Aid for Scientific Research from
Ministry of Education, Culture, Sports, Science, and Technology of Japan
[10] K. Ebitani, K. Nagashima, T. Mizugaki, K. Kaneda, Chem.
Commun. 2000, 869.
296
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Chem. Eur. J. 2005, 11, 288 – 297

Montmorillonite-Enwrapped Metal Catalysts
FULL PAPER
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Received: July 2, 2004
Published online: November 18, 2004
Chem. Eur. J. 2005, 11, 288 – 297
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