A highly negatively charged γ-Keggin germanodecatungstate efficient for Knoevenagel condensation

Article abstract of DOI:10.1039/c2cc33839d

A tetra-n-butylammonium (TBA) salt of a γ-Keggin -6-charged germanodecatungstate, [γ-H₂GeW₁₀O₃₆] ^6- (I), could act as an efficient homogeneous catalyst for Knoevenagel condensation of active methylene compounds with carbonyl compounds.

Full text of DOI:10.1039/c2cc33839d

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COMMUNICATION
A highly negatively charged c-Keggin germanodecatungstate efficient for
Knoevenagel condensationw
Kosei Sugahara, Toshihiro Kimura, Keigo Kamata, Kazuya Yamaguchi and
Noritaka Mizuno*
Received 29th May 2012, Accepted 29th June 2012
DOI: 10.1039/c2cc33839d
A tetra-n-butylammonium (TBA) salt of a c-Keggin ꢀ6-charged
germanodecatungstate, [c-H2GeW10O36]6ꢀ (I), could act as an
efficient homogeneous catalyst for Knoevenagel condensation of
active methylene compounds with carbonyl compounds.
Knoevenagel condensation of active methylene compounds
with carbonyl compounds.4 To the best of our knowledge,
efficient POM-based catalysts for Knoevenagel condensation
of a wide range of substrates including unreactive phenyl-
acetonitrile (as a donor) and ketones (as acceptors) are
unprecedented.
Polyoxometalates (POMs) are a large family of anionic metal–
oxygen clusters of early transition metals and have stimulated
many current research activities in broad fields such as cata-
lysis, material science, and medicine, because their chemical
and physical properties can be fine-tuned by choosing
constituent elements and countercations.1,2 In particular, acid
forms of fully-occupied Keggin-type H3[a-PW12O40] and
H4[a-SiW12O40] are strong acids because of the weak conju-
gate bases of the corresponding anions with small negative
charges.2 To date, the base catalysis by POMs has scarcely
been reported.3
The analytically pure I could be synthesized by the replace-
ment of two protons in [g-GeW10O34(H2O)2]4ꢀ with two
equivalents of TBA followed by reprecipitation with diethyl
ether (see ESIw). Single crystals of I suitable for the X-ray
structure analysis were successfully obtained by recrystalliza-
tion from acetonitrile solution of I with vapor diffusion of
diethyl ether. The anion I is a monomeric g-Keggin divacant
germanodecatungstate (Fig. 1). The existence of six TBA
cations per anion I implies that the charge of the anion is
ꢀ6. The elemental analysis data revealed that the molar ratio
of TBA : Ge : W was 6 : 1 : 10, respectively, in accord with
the crystallographic data. The terminal WQO bond lengths at
the vacant sites of I (1.71–1.75 A) were close to each other,
while two of eight W–O bonds at the lacunary sites of
[g-GeW10O34(H2O)2]4ꢀ were longer than the others (2.17 and
2.15 A vs. 1.70–1.74 A) (Fig. S1 and Table S1, ESIw). There-
fore, the non-protonated lacunary oxygen atoms in I possibly
work as basic sites to abstract protons from substrates.
We envisaged that highly negatively charged POMs can
likely act as strong base catalysts because the constituent
oxygen atoms become more basic with increase in the negative
charges of POMs. Lacunary polyoxotungstates are good
candidates as strong basic POM catalysts because (i) the
negative charges of POMs can be increased by introducing
lacunary sites, (ii) versatile lacunary POMs (e.g., mono-,
di-, and trivacant POMs) can precisely be synthesized, and
(iii) oxygen atoms at lacunary sites are basic enough to react
with protons. They are typically synthesized as alkali metal
salts and intrinsically insoluble in common organic solvents.
In addition, oxygen atoms at vacant sites (possible active
site(s)) are coordinated to alkali metal cations. For the
above-mentioned reasons, their use as base catalysts has been
limited. In this communication, we successfully synthesized
a novel organic-solvent-soluble TBA salt of a highly negati-
vely charged germanodecatungstate, [g-H2GeW10O36]6ꢀ (I).
POM I could act as an efficient homogeneous catalyst for
Department of Applied Chemistry, School of Engineering,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656,
Japan. E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp;
Fax: +81 3-5841-7220; Tel: +81 3-5841-7272
w Electronic supplementary information (ESI) available: Experimental
details, data of products, effect of catalysts, comparison with other
systems, crystallographic data, NMR and CSI-MS spectra. CCDC
884094 and 884095. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cc33839d
Fig. 1 Molecular structure of I. Thermal ellipsoids set at 50%
probability.
c
8422 Chem. Commun., 2012, 48, 8422–8424
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The bond valence sum (BVS) values of tungsten (5.82–6.07)
and germanium (4.02) indicate that the respective valences are
+6 and +4. The BVS values of O20 (1.19) and O25 (1.22)
were lower than those of the other oxygen atoms (1.51–2.04).
These results indicate that O20 and O25 are monoprotonated,
resulting in formation of m-OH groups.
value was the second highest among the base-mediated systems
(Table S3, ESIw).y4–6
The 183W NMR spectrum of I in dimethyl sulfoxide-d6
(DMSO-d6) showed three signals at ꢀ97.8, ꢀ114.7, and
ꢀ168.3 ppm with the respective intensity ratio of 2 : 2 : 1,
suggesting the C2v symmetry in DMSO (Fig. S2, ESIw). The
1H NMR spectrum of I in DMSO-d6 showed one signal at
5.37 ppm assignable to the hydroxo protons (Fig. S2, ESIw).
The positive-ion cold-spray ionization mass (CSI-MS) spectrum
of I in acetone exhibited the most intense parent peaks (centered
at m/z = 2214 and 4186) assignable to [TBA8H2GeW10O36]2+
and [TBA7H2GeW10O36]+, respectively (Fig. S3, ESIw). All
these NMR and CSI-MS results suggest that I is a single
species and that the solid-state structure is maintained in the
solution-state.
ð1Þ
The scope of the I-catalyzed system with regard to various
kinds of carbonyl compounds and active methylene compounds
was investigated (Table 1). Reactions of active methylene com-
pounds with different pKa values with 2a were carried out (Table 1,
entries 1–3). In the case of malononitrile (1b, pKa = 11.1), the
condensation proceeded efficiently to give benzylidenemalono-
nitrile (3b) in >99% yield for 0.5 h even with low catalyst
loading (0.5 mol%). Notably, even phenylacetonitrile (1c) with
a higher pKa value (pKa = 21.9) could be converted into
a-phenylcinnamonitrile (3c) in 96% yield, while large amounts
of strong bases such as NaOH (Z10 mol%) or higher reaction
temperatures (383–453 K) are usually required to attain
high yields (Table S4, ESIw).5,6 The reactivities of diethyl
malonate (1d) and nitromethane (1e) were much lower than
Knoevenagel condensation of active methylene compounds
with carbonyl compounds is one of the most important
carbon–carbon bond formation reactions. The reaction is
typically catalyzed by bases such as ammonia and primary
and secondary amines and their salts. While a number of
catalysts such as zeolites, surface-modified materials, ionic
liquids, hydrotalcites, mixed metal oxides, and molecular
organic frameworks have been developed, they have some
disadvantages: (i) narrow applicability to a limited number of
active methylene compounds with low pKa values, (ii) low
catalytic activity, and/or (iii) need of high reaction temperatures
and long reaction times. Especially, there have been only a few
reports on catalytic systems applicable to phenylacetonitrile
(as a donor) and ketones (as acceptors).4–6
Table 1 Knoevenagel condensation of active methylene compounds
with carbonyl compounds catalyzed by Ia
Time
(h)
Entry Donor
1
Acceptor
Product (Yield (%))
2
2b
3c
2a
2a
0.5
2
Initially, the Knoevenagel condensation of ethyl cyano-
acetate (1a) with benzaldehyde (2a) was carried out under
various conditions (Table S2, ESIw). Among the catalysts
tested, I showed the highest yield (98%) of ethyl trans-
4
5
1a
1a
3
7
a-cyanocinnamate (3a).z The significant formation of
byproducts was not confirmed (Fig. S4, ESIw), and the selec-
tivity to 3a was 99%. The catalyst precursors of Na2WO4,
GeO2, TBABr, and a potassium salt of a divacant germano-
decatungstate, K8[g-GeW10O36], were almost inactive. The
reaction rate of I was about fifty times higher than that of
[g-GeW10O34(H2O)2]4ꢀ, showing that the increase in negative
charges of POMs from ꢀ4 to ꢀ6 dramatically enhanced the
catalytic activity. The CSI-MS spectrum of I after the reaction
of 1a with 2a exhibited the intense peaks (centered at
m/z = 2214 and 4186) due to I with the new weak peaks
(centered at m/z = 2105 and 3968) assignable to [TBA7-
HGeW10O35(CH3CN)]2+ and [TBA6HGeW10O35(CH3CN)]+,
respectively (Fig. S5, ESIw). These results suggest that the
POM structure of I is retained after the condensation and that
the deprotonation/protonation steps are likely involved in the
present reaction. The I-catalyzed reaction under the stoichio-
metric conditions (1a : 2a = 1 : 1) also proceeded efficiently to
give 3a in 91% yield. The present system was also applicable to
the Knoevenagel condensation of 1a with 2a under solvent-
free conditions (eqn (1)). In this case, the turnover frequency
(TOF, determined by the initial rate) was 3000 hꢀ1, and the
1
6
1a
1a
2
7d
0.5
8
1a
1
9c,e
1a
4
2
10c,e 1a
a
Reaction conditions: I (1 mol% with respect to 1), 1 (1.0 mmol),
2 (1.5 mmol), CH3CN (1 mL), 305 K. Yield (%) = 3 (mol)/initial
b
c
1 (mol) ꢁ 100. I (0.5 mol% with respect to 1). Reaction conditions:
I (5 mol% with respect to 1), 1 (0.5 mmol), 2 (0.75 mmol), CH3CN
d
(0.5 mL), 353 K. 2e (3 mmol). 2 (1.5 mmol).
e
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8422–8424 8423

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Research from the Ministry of Education, Culture, Science,
Sports, and Technology of Japan.
Notes and references
z A divacant silicodecatungstate, [g-SiW10O34(H2O)2]4ꢀ, gave 3a in
18% yield. The yield of 3a could be increased from 18% to 35% by
replacement of Si with Ge, in accord with the acid strength order
(Si > Ge) of H4[XW12O40].7
Scheme 1 Reaction of 1b with salicylaldehyde catalyzed by I.
y Kaneda and co-workers reported that calcium vanadate apatite can
act as an efficient heterogeneous catalyst for Knoevenagel condensa-
tion and that the TOF is 7917 hꢀ1 (based on surface vanadium).5a
z The reactions of 1d, 1e, and acetone (1f) with 2a were carried out
under the same reaction conditions as those of entry 3 in Table 1. The
reaction of 1d gave the corresponding a,b-unsaturated product in 28%
yield. The reactions of 1e and 1f did not proceed.
1 (a) M. T. Pope, in Comprehensive Coordination Chemistry II,
ed. J. A. McCleverty and T. J. Meyer, Elsevier Pergamon,
Amsterdam, 2004, vol. 4, pp. 635–678; (b) P. Mialane, A. Dolbecq
Scheme 2 Cyanosilylation of (a) n-hexanal and (b) acetophenone
with trimethylsilyl cyanide catalyzed by I.
and F. Secheresse, Chem. Commun., 2006, 3477; (c) A. Proust,
´
R. Thouvenot and P. Gouzerh, Chem. Commun., 2008, 1837;
(d) D.-L. Long, R. Tsunashima and L. Cronin, Angew. Chem.,
Int. Ed., 2010, 49, 1736.
that of 1c, while the pKa values of 1d (16.4) and 1e (17.2)
are lower than that (21.9) of 1c, suggesting the specific
reactivity of I for the active methylene compounds with cyano
groups.z POM I also showed high catalytic activity for reac-
tion of 1a with various aldehydes (Table 1, entries 4–8).
Reactions of benzaldehydes with electron-donating as well as
-withdrawing para-substituents (2b and 2c) proceeded to
afford the corresponding products in high yields (Table 1,
entries 4 and 5). Not only an aromatic but also an aliphatic
aldehyde (2d) gave an excellent yield of the corresponding
alkene (Table 1, entry 6). The reaction of aldehydes containing
a double bond and a heteroatom also proceeded selectively to
give the corresponding a,b-unsaturated compounds (Table 1,
entries 7 and 8). Notably, ketones 2g and 2h worked well
as reaction partners of 1a (Table 1, entries 9 and 10), while
the reaction of 1c with 2g did not proceed. A substituted
4H-chromene derivative could be obtained in 82% yield by the
reaction of 1b with salicylaldehyde (Scheme 1).
2 (a) T. Okuhara, N. Mizuno and M. Misono, Adv. Catal., 1996,
41, 113; (b) I. V. Kozhevnikov, Catalysts for Fine Chemical
Synthesis, Catalysis by Polyoxometalates, John Wiley & Sons,
Chichester, 2002, vol. 2; (c) C. L. Hill, in Comprehensive Coordina-
tion Chemistry II, ed. J. A. McCleverty and T. J. Meyer, Elsevier
Science, New York, 2004, vol. 4, pp. 679–759; (d) R. Neumann and
A. M. Khenkin, Chem. Commun., 2006, 2529; (e) N. Mizuno,
K. Kamata, S. Uchida and K. Yamaguchi, in Modern Hetero-
geneous Oxidation Catalysis, ed. N. Mizuno, Wiley-VCH,
Weinheim, 2009, pp. 185–216.
3 (a) A. Yoshida, S. Hikichi and N. Mizuno, J. Organomet. Chem.,
2007, 692, 455; (b) T. Kimura, K. Kamata and N. Mizuno, Angew.
Chem., Int. Ed., 2012, 51, 6700.
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POM I also efficiently catalyzed cyanosilylation of carbonyl
compounds with trimethylsilyl cyanide (TMSCN) (Scheme 2).
The reaction of an aliphatic aldehyde efficiently proceeded to
give the corresponding cyanohydrin trimethylsilyl ether in
95% yield even with 0.01 mol% of I. In this case, the TOF
was 572 000 h–1, and the turnover number (TON) reached up
to 9530. The reaction of unreactive acetophenone also quanti-
tatively proceeded. The catalytic activity of I was much higher
than that of [(g-SiYW10O36)2]10–, which is reported to be active
for cyanosilylation of carbonyl compounds.8
M. J. Climent, A. Corma, S. Iborra, R. Monton and
´
M. J. Sabater, Chem.–Eur. J., 2010, 16, 1221; (h) K. Motokura,
N. Fujita, K. Mori, T. Mizugaki, K. Ebitani, K. Jitsukawa and
K. Kaneda, Chem.–Eur. J., 2006, 12, 8228; (i) J. Gascon, U. Aktay,
M. D. Hernadez-Alonso, G. P. M. van Klink and F. Kapteijn,
J. Catal., 2009, 261, 75.
In conclusion, POM I could be synthesized by controlling
the negative charge in an organic medium. POM I could act as
an efficient homogeneous catalyst for Knoevenagel conden-
sation and cyanosilylation, and various combinations of sub-
strates were efficiently converted to the desired products in
high yields.
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8 Y. Kikukawa, K. Suzuki, M. Sugawa, T. Hirano, K. Kamata,
K. Yamaguchi and N. Mizuno, Angew. Chem., Int. Ed., 2012,
51, 3686.
This work was supported in part by the Japan Society
for the Promotion of Science (JSPS) through its ‘‘Funding
Program for World-Leading Innovative R&D on Science and
Technology (FIRST Program)’’ and a Grant-in-Aid for Scientific
c
8424 Chem. Commun., 2012, 48, 8422–8424
This journal is The Royal Society of Chemistry 2012