Vanadyl salen complexes covalently anchored to an imidazolium ion as catalysts for the cyanosilylation of aldehydes in ionic liquids

Article abstract of DOI:10.1016/S0040-4039(03)01746-5

Two vanadyl salen complexes having peripheral styryl substituents have been reacted with 1-methyl-3-(3-mercaptopropyl)-imidazolium chloride using azoisobutyronitrile as radical initiator. The resulting compounds contain at the same time a vanadyl salen co

Full text of DOI:10.1016/S0040-4039(03)01746-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6813–6816
Vanadyl salen complexes covalently anchored to an imidazolium
ion as catalysts for the cyanosilylation of aldehydes in
ionic liquids
a,b
a,
b,
b,
Carlos Baleiz a˜ o, B a´ rbara Gigante, * Hermenegildo Garcia * and Avelino Corma *
a
INETI-Departamento de Tecnologia das Industrias Qu ´ı micas, Estrada do Pa c¸ o do Lumiar, 22, 1649-038 Lisboa, Portugal
b
Instituto de Tecnolog ´ı a Qu ´ı mica/CSIC-Av. de los Naranjos, s/n, 46022 Valencia, Spain
Received 4 May 2003; revised 10 July 2003; accepted 17 July 2003
Abstract—Two vanadyl salen complexes having peripheral styryl substituents have been reacted with 1-methyl-3-(3-mercapto-
propyl)-imidazolium chloride using azoisobutyronitrile as radical initiator. The resulting compounds contain at the same time a
vanadyl salen complex and one imidazolium cation. In agreement with the expectations in view of their structure, these
compounds were insoluble in conventional organic solvents, but completely miscible in imidazolium ionic liquids. These vanadyl
salen complexes bonded to an imidazolium cation are highly active and reusable catalysts for the cyanosilylation of aldehydes.
Moderate enantiomeric excesses were obtained using the chiral version of this complex.
©
2003 Elsevier Ltd. All rights reserved.
Ionic liquids derived from 1,3-dialkylimidazolium
cation have been found to be convenient reaction media
for green chemistry, avoiding the use of volatile organic
compounds and making possible in some cases the
recovery of the catalyst. When a catalytic reaction is
carried in ionic liquid, the most frequent methodology
uses the same catalyst as in conventional organic sol-
vents. Thus, reuse of the catalyst requires that one uses
solvents immiscible with ionic liquids, such as hexane
or diethyl ether, and that at the same time these sol-
vents do not dissolve the catalyst. For example, in
related precedents to this work, chiral metal salen com-
plexes of Cr, Mn and V were dissolved in ionic liquids
and they were found to exhibit high activity and enan-
4
tioselectivity for the epoxidation of alkenes, epoxide
1–3
5
6
ring aperture and carbonyl additions. In these prece-
dents, the metal salen catalysts used in ionic liquids
were exactly the same as those employed in conven-
tional organic solvents, such as chloroform. The normal
problem encountered in these cases is, however, that
during the recovery of the products from the ionic
liquid, some catalyst is also extracted thus producing its
Scheme 1. Reagents and conditions: (a) Br , CH Cl , 0°C, 1 h; (b) 4-vinylphenylboronic acid, [Pd(PPh ) ], 2 M Na CO , THF,
2
2
2
3 4
2
3
70°C, 3 h; (c) 3-methylsalicylaldehyde, 1,2-ethanediamine, EtOH, reflux, 1 h; (d) VOacac, MeOH, rt, overnight.
Keywords: ionic liquids; vanadyl salen complex; cyanohydrins; asymmetric catalysis.
*
Corresponding authors. Tel.: +34-963877807; fax: +34-963877809; e-mail: hgarcia@qim.upv.es
0
040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01746-5

6
814
C. Baleiz a˜ o et al. / Tetrahedron Letters 44 (2003) 6813–6816
Scheme 2. Reagents and conditions: (a) 80°C, 96 h; (b) AIBN, CHCl and CH CN (degassed), 70°C, 20 h.
3
3
gradual depletion from the ionic liquid. Herein we have
gone a step forward in the use of ionic liquids as medium
for catalytic reactions by modifying adequately the metal
salen catalysts to make them more alike to ionic liquids
and less soluble in organic solvents.
We are following a strategy widely used in perfluorinated
solvents in which to increase the solubility of a certain
catalyst, a derivative is prepared containing a perfluori-
nated pony tail that enormously enhances the fluoricity
7
of the catalyst. Mimicking this methodology, herein we
have appended an imidazolium unit to the peripheral
position of the salen ligand of vanadyl complexes, the
resulting complexes exhibiting a very high activity for the
cyanosilylation of aldehydes.
Preparation of the vanadyl salen complexes appended to
an imidazolium cation (VOsalen@IL) is depicted in
Schemes 1 and 2. In the first place, a statistical mixture
of three vanadyl salen complexes (VOsalen) in which one
of the components is an unsymmetrical VOsalen having
a para styryl substituent on the salen ligand was obtained.
This synthetic approach has the advantage that unsym-
metrical salen ligands are prepared in a single pot without
requiring a more time-consuming and stepwise reaction
Figure 1. UV-Vis of SH@IL (a), VOsalen@IL (b) and com-
pound 2 (c).
8
,9
sequence. In this mixture, the predominant component
can not react with 1-methyl-3-(3-mercaptopropyl)-imida-
zolium chloride (SH@IL) and will become separated due
to its different solubility. On the other hand, the symmet-
rical minor component having two para-styryl units is six
times less abundant that the unsymmetrical one and, in
addition, this compound can also act as catalyst irrespec-
tively that one or the two styryl groups are derivatized
by imidazolium cations. After formation of the vanadyl
salen complexes, the last step of our sequence was the
covalent attachment of an imidazolium unit (Scheme 2).
This was accomplished through a radical chain mecha-
nism initiated by azoisobutyronitrile (AIBN), that pro-
duces the addition of the terminal mercapto group to the
CꢀC double bound of the styryl compound. All the
reaction intermediates, including VOsalen complexes and
VOsalen@IL, were characterized by optical, IR, FAB-
MS and NMR spectroscopy. The transmission UV–vis
spectra of the VOsalen@IL and the corresponding pre-
cursor exhibit the characteristic charge transfer band
indicating the presence of the VOsalen complex (Fig. 1).
Figure 2. IR of SH@IL (a) and VOsalen@IL (b).
Metal salen complexes exhibits in the IR a phenolate
−
1
stretching vibration band at 1540 cm that was been
proposed as characteristic of this type of complexes.
Figure 2 shows the IR spectra recorded for SH@IL and
1
for the imidazolium derivative VOsalen@IL. H NMR
of VOsalen@IL exhibits at the most characteristic peaks
the signal corresponding to the imine proton at 8.4 ppm.

C. Baleiz a˜ o et al. / Tetrahedron Letters 44 (2003) 6813–6816
6815
a
Table 1. Results of the cyanosilylation of aldehydes with TMSCN in the presence of VOsalen@IL
R
Phb
–
94
Ph
0
91
Ph
1
89
Ph
2
92
Ph
3
90
Ph
4
88
Ph
5
91
Ph
6
90
4-F-Ph
–
90
4-OMe-Ph
–
89
C H
5 11
–
92
Reused
Conv. (%)
a
Reactions were run at room temperature under N2 atmosphere for 3 h: aldehyde (1.64 mmol), TMSCN (1.5 equiv.), VOsalen@IL (64 mg, 0.2
mol%), [bmim]PF6 (1 g) and nitrobenzene as external standard. Selectivity: >98%. Mass balance: >95%.
b
Homogeneous catalyst, CHCl , 0.2% mol, 3 h.
3
As expected according to its structure, VOsalen@IL
was insoluble in hexane and diethyl ether but, com-
pletely miscible in 1-butyl-3-methylimidazolium hexa-
Table 2. Results of the asymmetric cyanosilylation of benz-
aldehyde in the presence of a chiral VOsalen*@IL as
a
catalyst
fluorophosphate ([bmim]PF ). The latter ionic liquid
6
Solvent
Time (h)
Conversion (%)
e.e. (%)
containing 2.6% in weight of VOsalen@IL was used as
a reaction medium for the addition of trimethylsilyl
[
[
[
bmim]PF6
bmim]Cl
bmim]BF4
3
24
24
88
95
16
57
41
20
1
0
cyanide (TMSCN) to aldehydes. The results and con-
ditions employed are contained in Table 1. For the sake
of comparison, the results of a control experiment in
chloroform in which the tetra-tert-butyl VOsalen com-
plex was used as catalyst is also included in Table 1.
The most remarkable facts concerning Table 1 are the
high conversion and selectivity to the cyanohydrin
obtained with the hybrid VOsalen@IL complex–ionic
liquid as catalyst. After completion of the reaction, the
ionic liquid was thoroughly extracted with diethyl ether
and the cyanohydrin product recovered with very high
mass balances as assessed using nitrobenzene as exter-
nal standard.
a
Reaction were run at room temperature under N2 atmosphere:
benzaldehyde (1.64 mmol), TMSCN (1.5 equiv.), VOsalen*@IL (0.2
mol%), [bmim]PF6 (1 g) and nitrobenzene as external standard.
4–6
reported, it is difficult to correlate the nature of the
anion and the ee at this moment. Variations of viscos-
ity, melting point and other physical parameters of IL
are important factors that depend on the counter anion,
all of them playing a role in catalysis.
After recovery of the products by extraction and out-
gassing under reduced pressure to eliminate diethyl
ether, the ionic liquid containing VOsalen@IL was
reused up to six times without noticeable decrease in
activity. Moreover, chemical analysis of vanadium of
the reused ionic liquid gives a vanadium content identi-
cal to that of the fresh ionic liquid. Reuse and vana-
dium chemical analysis both agree with the fact that the
catalyst remains intact and immobilized in the ionic
liquid upon extensive reuses. This contrasts with other
vanadyl salen complexes lacking the imidazolium unit,
which are extracted in part during the recovery of the
products. Visually the extracts were colorless using
VOsalen@IL while they are green when tetra-tert-butyl
VOsalen is used as catalyst in [bmim]PF6.
The ee shown in Table 2 are relatively low compared to
the ee previously reported to VOsalen complexes in
6
related [beim]PF ionic liquid (90%). Apparently the
6
covalent linkage between imidazolium and the vanadyl
site affects negatively to the ability of the latter to
induce asymmetry. One possibility worth to be explored
to increase ee would be to disfavor the intramolecular
association of the vanadyl group in VOsalen*@IL with
the imidazolium or to exchange chloride by a different
counter anion.
In conclusion, by modifying a vanadyl salen complex
with the covalent attachment of a imidazolium cation
we have obtained a catalyst insoluble in conventional
organic solvents but totally miscible with imidazolium
ionic liquids. The catalyst shows high activity for the
cyanosilylation of aldehydes, remains immobilized in
ionic liquid phase and can be reused in consecutive runs
avoiding the leaching of vanadium. Further improve-
ments are however necessary in order to enhance the
asymmetric induction ability of this vanadyl salen imi-
dazolium complex.
We expanded the above methodology to the prepara-
tion of a chiral vanadyl salen complex (VOsalen*@IL)
derived from (1R,2R)-(−)-diaminocyclohexane and 3,5-
di-tert-butylsalicylaldehyde. The resulting chiral com-
plex was tested for the enantioselective addition of
9
TMSCN to benzaldehyde. The conversions and enan-
tiomeric excess (ee) obtained depend on the nature of
+
the counteranion accompanying to the [bmim] cation.
Acknowledgements
The results are summarized in Table 2. Although the
+
influence of the counter anion of [bmim] in the conver-
Financial support to C. Baleiz a˜ o from Funda c¸ a˜ o para a
Ci eˆ ncia e Tecnologia, Portugal (PRAXIS XXI/BD/
sions and enantioselectivities achieved for other chiral
metal salen complexes in ionic liquids has been

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C. Baleiz a˜ o et al. / Tetrahedron Letters 44 (2003) 6813–6816
2
1375/99) is gratefully acknowledged. The Spanish
838.
DGES (MAT2000-1768-CO2-01) has financed part of
this work.
5. Song, C. E.; Oh, C. R.; Roh, E. J.; Choo, D. J. Chem.
Commun. 2000, 1743–1744.
. Baleiz a˜ o, C.; Gigante, B.; Garcia, H.; Corma, A. Green
6
Chem. 2002, 4, 272–274.
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