Lanthanide complexes of the monovacant Dawson polyoxotungstate [α1-P2W17O61]10- as selective and recoverable Lewis acid catalysts

Article abstract of DOI:10.1002/anie.200600364

(Chemical Equation Presented) Catalytic cornerstone: Lanthanide(III) complexes of a lacunary Dawson-type polyoxometalate catalyze Lewis acid mediated reactions (see figure, TMS = trimethylsilyl). The compounds (NBu ₄)₅H₂[α

Full text of DOI:10.1002/anie.200600364

Communications
Polyoxometalate Catalysts
NaH [SiW CeO ]as the active site for the aerobic oxidation
3 11 39
[
22]
of formaldehyde.
Because lanthanide(iii) salts are
DOI: 10.1002/anie.200600364
extremely efficient, water-tolerant Lewis acids, their use in
organic reactions has soared over the past ten years.
However, the quest for more selective and recoverable
catalysts remains a challenging task.
[
23–26]
Lanthanide Complexes of the Monovacant
1
0À
Dawson Polyoxotungstate [a -P W O ] as
1
2
17 61
Selective and Recoverable Lewis Acid Catalysts**
The reactivity of lanthanide ions is still present when
anionic ligands, such as binaphthyl derivatives, are used to
[
27]
tune the catalytic activity. The use of inorganic polyanionic
POMs as ligands should thus also be possible. Indeed,
lanthanide complexes with monovacant POMs can be pre-
pared with free coordination sites on the lanthanide ion,
which are required to maintain Lewis acid activity. POMs
would also bring an additional property: their solubility
depends on the counterions. Group 1 metal ions lead to water-
soluble complexes that can be used in biphasic catalyst
CØcile Boglio, Gilles Lemi›re, Bernold Hasenknopf,
Serge Thorimbert,* Emmanuel Lacôte,* and
Max Malacria*
Polyoxometalates (POMs) are a large family of metal–oxygen
clusters of the early transition metals in high oxidation states,
V
VI
VI [1]
most commonly V , Mo , and W . Their intrinsic chemical
properties as electron acceptors or as very strong Brønsted
acids (in their protonated form) makes them catalysts in
[
28]
systems, whereas lipophilic ammonium ions solubilize the
polyanions in organic solvents. Thus, POMs can be separated
easily from small organic molecules by precipitation with
[
2]
organic transformations, and their ability to form peroxo
complexes or to serve as a support for catalytically active
metal ions in a high oxidation state is particularly useful in
[
29]
diethyl ether or by nanofiltration. Both methods have been
applied for the recycling of POM-based oxidation catalysts,
and could be extended to the Lewis acidic POM–Ln
complexes.
[
3]
oxidation reactions. Organometallic derivatives of POMs
have proven to be effective catalysts in hydrogenation
[
4]
reactions.
We have chosen to evaluate the lanthanide complexes of
To the best of our knowledge, however, no Lewis acid
catalysis involving POMs has been reported so far, despite the
wide array of transformations that could be addressed. We
imagined that lanthanide complexes of monovacant POMs—
the
monolacunary
Dawson
polyoxotungstate
[a1-
1
0À
P W O ]
.
The complexes (TBA) H [a -Ln(H O) -
2
17 61
5
2
1
2
4
P W O ](Ln = La, Sm, Eu, Yb; TBA = tetrabutylammo-
2
17 61
nium; Figure 1) are soluble in organic solvents, and the water
molecules on the lanthanide ions are labile, thus providing the
metal centers with available coordination sites for other
ligands.
and late lanthanides.
[
5,6]
often called Peacock–Weakley type —and in particular of
1
0À [7–18]
[
a -P2W17O61]
1
would be good candidates for such a
[
11,20,30–34]
task. Some of these compounds have already been studied for
their luminescence and magnetic properties provided by the
We surveyed catalysts including early, mid,
[
19–21]
lanthanide ions.
Hill and co-workers have used the
oxidant properties of cerium(iv) in the Keggin derivative
[*] C. Boglio, G. Lemi›re, Dr. S. Thorimbert, Dr. E. Lacôte,
Prof. M. Malacria
UniversitØ Pierre et Marie Curie-Paris 6
Institut de chimie molØculaire (FR 2769)
Laboratoire de chimie organique (UMR CNRS 7611)
case 229, 4 place Jussieu, 75252 Paris cedex 05 (France)
Fax: (+33)1-4427-7360
E-mail: serge.thorimbert@upmc.fr
lacote@ccr.jussieu.fr
max.malacria@upmc.fr
C. Boglio, Dr. B. Hasenknopf
UniversitØ Pierre et Marie Curie-Paris 6
Institut de chimie molØculaire (FR 2769)
Laboratoire de chimie inorganique et matØriaux molØculaires (UMR
CNRS 7071)
case 42, 4 place Jussieu, 75252 Paris cedex 05 (France)
1
0À
3+
Figure 1. Lanthanide complexes of [a -P W O ] : the Ln ions are
1
2
17 61
[
**] We thank the CNRS and UPMC for financial support. This work was
coordinated to the vacant site in the lacunary Dawson structure and
have four or five open coordination sites.
supported with a UPMC Presidential Young Investigator award
(
B.H., E.L., S.T.). C.B. thanks the Minist›re de l’Enseignement
SupØrieur et de la Recherche and IUF (of whom M.M. is a senior
member) for a grant. Technical support from ICSN (Gif sur Yvette,
France) is gratefully acknowledged (HRMS and elemental analyses).
We are also indebted to Prof. Lynn C. Francesconi (Hunter College,
CUNY (USA)) for fruitful discussions.
We first examined Mannich-type reactions of silyl enol
[
35,36]
ethers with imines
and found that no reaction occurred
with the potassium salts of the POM–Ln complexes, possibly
due to solubility issues. We thus switched to complexes that
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
[
30]
are soluble in organic solvents, especially acetonitrile, and
3
324
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Angewandte
Chemie
were able to isolate b-amino ketones 1a–e, thus proving the
catalytic activity of the POM–Ln complexes, although the
reaction times were generally longer than those observed with
the corresponding lanthanide triflates (two days vs. a few
hours). This was expected considering the decreased Lewis
acidity of the lanthanides when complexed to the monovacant
POM ligand. Nonetheless, the products 1a–e were isolated in
good to excellent yields (Table 1). No reaction occurred when
the substrates were mixed in the presence of lacunary
(
TBA) H [a -P W O ], which allowed us to exclude a
Scheme 1. Comparison of the chemoselectivity for POM–Ln catalysts
and Ln triflates.
6
4
1
2
17 61
Brønsted acid catalysis for the reaction. Formation of 1a
worked best with the Yb complex as catalyst (Table 1,
particularly
noticeable
with
Table 1: POM–Ln-catalyzed aldol reactions of imines.
(
TBA) H [a -YbP W O ](selec-
5
2
1
2
17 61
tivity changed from around 3:1 to
00:0), and, most interestingly,
1
with
and
(TBA) H [a -SmP W O ]
5 2 1 2 17 61
(TBA) H [a -EuP W O ],
5
2
1
2
17 61
Entry
R
Ar
Catalyst
Product
Yield [%]
d.r.
which actually brought about an
inversion of selectivity. Improve-
ment was still observed with
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
Ph
Ph
Ph
o-HOC H4
CO Et
CO Et
Ph
Ph
Ph
Ph
(TBA) H [a -YbP W O ]
1a
1a
1a
1a
1b
1c
96
91
50:50
50:50
50:50
50:50
50:50
60:40
–
5
2
1
2
17 61
(TBA) H [a -SmP W O ]
5
2
1
2
17 61
[
a]
(TBA)
5
H
2
[a
1
-LaP
2
W
17 61
O
]
81
(TBA) H [a -LaP W O ]
,
a
5
2
1
2
17 61
[
b]
(TBA) H [a -EuP W O ]
71
84
5
2
1
2
17 61
derivative with
a
lanthanide,
o-HOC H4
p-MeOC H4
Ph
Ph
(TBA) H [a -YbP W O ]
5 2 1 2 17 61
6
[
c]
whose triflate already greatly
favors additions to imines. Overall,
the POM–Ln compounds are
extremely selective catalysts that
are slightly to much better than
their parent triflates.
We also carried out a three-
component reaction starting from
aniline, benzaldehyde, and the
(TBA) H [a -YbP W O ]
81
6
5
2
1
2
17 61
[
d]
(TBA) H [a -YbP W O ]
–
–
70
77
6
5
2
1
2
17 61
(TBA) H [a -YbP W O ]
1d
1e
68:32
65:35
2
5
2
1
2
17 61
p-MeOC H4
(TBA) H [a -YbP W O ]
5 2 1 2 17 61
2
6
[a] Yield of isolated product. Conversion was approx. 90%. See Supporting Information for experimental
details. [b] Yield of isolated product. Conversion was approx. 80%. [c] Reaction took six days. [d] No
conversion was observed.
entry 1), followed by the Sm complex (Table 1, entry 2). La
and Eu derivatives were also acceptable (Table 1, entries 3
and 4), although the reactions were slower and the conversion
was not optimal. The aldolization is not stereoselective, but
neither is it stereoselective with the corresponding triflates.
Except where noted, we decided to proceed with the Yb
complex to evaluate the influence of substituents on the
imine. Introduction of a hydroxy substituent in the ortho
position of the aromatic ring attached to the imine carbon
totally shut down the reaction, and only the hydrolyzed
products were isolated (Table 1, entry 7). We suppose that
such a substitution diminishes the electrophilicity of the
carbon atom to such an extent that our compounds, which
have lower Lewis acidity, are now inactive. On the contrary,
the glyoxaldehyde-derived imines reacted much faster, and
the reactions were finished in less than an hour (Table 1,
entries 8 and 9).
same silyl enol ether as before (Scheme 2). Amine 1a was
isolated almost quantitatively, and the reaction proceeded
without molecular sieves.
Scheme 2. The three-component reaction.
We next turned our attention to the imino Diels–Alder
reaction of various imines with the Danishefsky diene in the
[
38]
presence of (TBA) H [a -YbP W O ]. Cyclic adducts 2a–
5
2
1
2
17 61
d were isolated in good yields and the reactions proceeded
smoothly (Table 2, entries 1–3). Introduction of an ortho-
[
39a]
The decreased Lewis acidity of our compounds made
them good candidates for chemoselective reactions. Lantha-
nides favor activation of aldimines over aldehydes, but
selectivity is usually achieved only at À238C and is not
hydroxy substituent on the aniline
resulted in a lower yield
(57%; Table 2, entry 4), and the use of imines derived from
ethyl glyoxalate also proved detrimental (20–30% yield). In
this latter case oligomers of the starting materials were also
present.
[
37]
always excellent, particularly with samarium derivatives.
We thus ran competition reactions between the benzimine of
aniline and benzaldehyde (Scheme 1): only the aldimine
reacted at room temperature in the presence of our POM–Ln
Encouraged by the preceding results, we turned our
attention to a variation of the imino Diels–Alder reaction that
uses an imine as the aza-diene and an enol ether as the
[
39]
complexes. The improved selectivities over Ln(OTf) were
dienophile.
Reaction of diphenylimine with ethyl vinyl
3
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3325

Communications
Table 2: POM–Yb-catalyzed imino Diels–Alder reaction with the Dani-
shefsky diene.
Table 3: POM–Yb-catalyzed imino Diels-Alder using imines as aza-
dienes.
[
a]
Entry
Ar
Product
Yield [%]
[
a]
1
2
3
4
Ph
2a
2b
2c
2d
89
98
76
57
Entry
Imine
n
1
2
Product
Yield
[%]
d.r.
[
a]
p-MeOC H4
p-ClC H4
o-HOC H4
6
6
6
67
60:40
(50:50)
1
2
3a
3b
(80)
[a] See Supporting Information for experimental details.
ether in the presence of our POM–Yb catalyst was hampered
by aromatization of the desired compound into the corre-
sponding quinoline; therefore, in order to limit this decom-
position path, we switched to cyclic enol ethers. We selected
the same two classes of imines we had used previously, namely
the bis-aromatic compounds (Table 3, entries 1–4) and the
derivatives of ethyl glyoxalate (Table 3, entries 5–8). The
former reacted smoothly, although the yields were slightly
79
80:20
(50:50)
(89)
63
90:10
(55:45)
3
4
1
2
3c
(74)
[
b]
39
80:20
(50:50)
3d
3e
(64)
lower than those we obtained with Yb(OTf) . The essential
3
advantage of the POM–Ln catalysts over lanthanide triflates
lies in the stereochemical outcome of the reactions: in all
examples the diastereoselectivity of the reaction increased
noticeably. Given that this increase is most pronounced with
ortho-hydroxy substrates, we assume that H-bonding inter-
actions between the reactants and the polyanionic ligand
[
[
b,c]
b,c]
5
6
7
8
1
2
1
2
39
50:50
50:50
3 f <40
[
11,12,40]
exist.
This change in selectivity when the POM–Yb
complex is used further indicates that the catalytically active
species is not free Yb .
67
50:50
(50:50)
3
+
3g
3h
(48)
In our hands, no significant yields of the tricyclic products
could be attained with Yb(OTf)3 and ethyl glyoxalate
derivatives (Table 3, entries 5–8) even though we observed
rapid consumption of the starting material. We thus thought it
might be interesting to use weaker Lewis acids. If our
assumption that decomposition is due to an acceleration of
62
50:50
<20 ) (50:50)
[
b]
(
[
a] The value in brackets refers to the same reaction using Yb(OTf) . See
3
Supporting Information for experimental details. [b] Aromatic degrada-
tion products were also observed. [c] The starting imine reacted with
itself.
the divergent reaction pathways by Yb(OTf) was correct,
3
then using the weaker POM–Yb Lewis acid should help. This
turned out to be partially the case (Table 3, entries 7 and 8).
The stereoselectivity did not vary in these cases, despite
possible interactions of the substrates with the polyoxotungs-
tic framework. These intriguing findings show that we need to
know more about the POM–Ln–substrate interactions in
order to rationalize and fully master their reactivity.
3
+
that there had been no leaching of Yb : 1) our initial work
bears out the decomplexation of Yb from the POM upon
3
+
[
30]
3+
precipitation; 2) Yb is paramagnetic and would influence
the NMR spectra of the filtrate, which was not the case (no
paramagnetic shift or line broadening); 3) after removal of
the POM–Ln complex, the supernatant showed no catalytic
activity.
In conclusion, we have discovered a new class of reactions
catalyzed by functionalized POMs. The complexation of
lanthanide ions by a POM ligand confers essential features,
among which a sharply increased chemoselectivity and an
increase of the diastereoselectivity in some cases stand out.
Furthermore, the catalysts can be easily recovered and reused
with no drop in reactivity. Work aimed at further extending
the array of possible reactions, understanding the POM–Ln–
substrate interactions, which we think provide the basis of
diastereoselectivity, as well as examining the enantioselective
version of these reactions with additional chiral ligands is
It remained to be shown that the catalysts can be
[
29,41]
recovered and reused.
As stated before, we wished to
take advantage of the solubility properties of POMs to devise
an operationally simple recovery procedure. The complexes
we used are soluble in acetonitrile, but not in diethyl ether, in
which the vast majority of the adducts are soluble. Indeed,
addition of a diethyl ether/ethanol/acetone solution (20:1:1 by
volume) to the reaction mixture leading to 1a (Table 1,
entry 1) caused precipitation of a white solid, which was
isolated by centrifugation. High-field NMR spectroscopic
analysis showed that the solid was pure (TBA) H [a -
[
36]
5
2
1
YbP W O ], which was isolated quantitatively. We were
able to reuse the same catalyst up to ten times with no loss in
yield. The following experimental facts made us confident
2
17 61
3
326
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Chemie
1
0À
underway. The POM [a -P W O ] itself is chiral and has
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1
2
17 61
6
been used as a racemate throughout this study. Provided an
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catalysts without the need for an additional ligand. Progress
along these lines will be presented in due course.
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Keywords: aldol reaction · cycloaddition · lanthanides ·
Lewis acids · polyoxometalates
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