Chiral vanadyl salen complex anchored on supports as recoverable catalysts for the enantioselective cyanosilylation of aldehydes. Comparison among silica, single wall carbon nanotube, activated carbon and imidazolium ion as support

Article abstract of DOI:10.1016/j.tet.2004.08.077

The activity and enantiomeric excess (ee) (in some cases >85%) obtained for the asymmetric addition of trimethylsilyl cyanide to aldehydes using different heterogeneous chiral catalysts are compared. A library of recoverable catalysts was developed by imm

Full text of DOI:10.1016/j.tet.2004.08.077

Tetrahedron 60 (2004) 10461–10468
Chiral vanadyl salen complex anchored on supports as
recoverable catalysts for the enantioselective cyanosilylation of
aldehydes. Comparison among silica, single wall carbon nanotube,
activated carbon and imidazolium ion as support
a,b
Carlos Baleiz a˜ o, B a´ rbara Gigante, Hermenegildo Garc ´ı a and Avelino Corma
a
b,*
b
a
INETI, Departamento de Tecnologia das Ind u´ strias Qu ´ı micas, Estrada Pa c¸ o do Lumiar 22, 1649-038 Lisboa, Portugal
Instituto de Tecnolog ´ı a Qu ´ı mica/CSIC, Universidad Polit e´ cnica de Valencia, Avda. de los Naranjos s/n, 46022 Valencia, Spain
b
Received 21 May 2004; revised 26 July 2004; accepted 23 August 2004
Available online 15 September 2004
Abstract—The activity and enantiomeric excess (ee) (in some cases O85%) obtained for the asymmetric addition of trimethylsilyl cyanide
to aldehydes using different heterogeneous chiral catalysts are compared. A library of recoverable catalysts was developed by immobilization
of a chiral vanadyl salen complex having a terminal carbon–carbon double bond onto a series of scaffolds including silica, single-wall carbon
nanotubes, activated carbon and room-temperature ionic liquids. The covalent linkage has been achieved by radical initiated addition of
mercapto groups to C]C. The highest enantiomeric excesses, similar to those obtained in the homogeneous phase, were achieved using
silica as support or with the homogeneous tetra-tert-butyl salen catalyst dissolved in an imidazolium ionic liquid. The use of silica as support
permits an easier separation and reuse of the catalyst from the reaction media.
q 2004 Elsevier Ltd. All rights reserved.
1
. Introduction
if deactivation occurs, a suitable re-activation protocol can
be devised. Heterogeneous catalysts permit continuous flow
operations simplifying the process for large-scale
Among the chiral synthetic intermediates produced on an
industrial scale, cyanohydrins play an important role, since
these molecules can be used as precursor in the preparation
of a wide range of pharmaceuticals, agrochemicals and
insecticides. For example, enantiomerically pure mandelo-
nitrile is currently produced in multi-hundred ton scale, and
so represents one of the compounds that is produced with
8
operation.
Our groups have been involved in a project aimed at
developing suitable solid catalysts containing immobilized
chiral metal salen complexes. In addition to polymer
supported complexes, amorphous and structured inorganic
1
,2
9
high ee in large quantities.
oxides have also been used. More recently, we have been
interested in the use of covalently functionalised single wall
carbon nanotubes (SWNT), particularly in comparison to
Chiral vanadyl salen complexes are among the most active
and selective catalysts for the asymmetric cyanosilylation of
aromatic and aliphatic aldehydes. A general strategy to
convert a successful homogeneous catalyst into a hetero-
geneous system consists of the covalent anchoring of the
1
0
activated carbon (AC). A parallel strategy that we have
also developed based on the concept of ‘ionophilicity’, is to
append an imidazolium moiety to the complex to make the
catalyst resemble room-temperature ionic liquids and
unrecoverable from them by extraction with conventional
3
,4
5
–7
catalytically active site onto an insoluble support.
1
1,12
organic solvents.
Heterogeneous catalysts are easily separated from the
reaction mixture and can be recovered and reused provided
that they do not become deactivated during recycling. Also
In the present work, we provide a comparison of the
asymmetric induction ability of a library of solid catalysts in
which a chiral vanadyl salen complex derived from
cyclohexanediamine has been covalently anchored either
onto a solid support such as silica or carbon or function-
alised by appending an imidazolium unit to make the system
‘ionophilic’ to be used in room temperature ionic liquids.
Keywords: Cyanohydrins; Heterogeneous asymmetric catalysis; Chiral
vanadyl salen complex; Silica as support; Single-wall carbon nanotubes;
Activated carbon; Ionic liquids.
*
Corresponding author. Tel.: C34963877805; fax: C34963877809;
e-mail: hgarcia@qim.upv.es
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.08.077

10462
C. Baleiz a˜ o et al. / Tetrahedron 60 (2004) 10461–10468
2
. Results and discussion
surface and the vanadyl complex that eventually can interact
with the solvent and reagents similarly to homogeneous
catalysis. The synthetic route followed for the preparation of
this solid catalyst (VOsalenfsilica) is shown in Scheme 1.
2.1. Preparation of the library of recoverable vanadyl
salen catalysts
The series of catalysts used in the present study encom-
passes three different types of recoverable and reusable
catalysts depending on the support to which the complex has
been anchored or on the presence of an imidazolium unit.
The key point in the preparation of the precursor is the
synthesis of a statistical mixture of an asymmetrically
substituted chiral salen ligand 4b in which the predominant
component 4a cannot anchor to the solid and the desired
target ligand 4b is formed predominantly in a ratio 6:1 with
respect to the symmetrical ligand 4c. Ligand 4c is doubly
functionalised and, therefore, will also be firmly anchored to
the support. Although the complex derived from 4c will also
be catalytically active, its asymmetric induction ability is
believed to be lower than the complex derived from 4b due
to the lower steric encumbrance around the active vanadyl
center. This statistical mixture 4a, 4b and 4c was used
without purification in the following steps. The actual
stoichiometry of the starting material and the resulting
mixture of salen ligands are also indicated in the Scheme 1.
In the first catalyst, we covalently anchored an asymmetric
vanadyl salen precursor having in the para-position of one
of the two phenolic moieties a u-alkenyloxy chain to the
surface of silica particles that have been previously
functionalised with 3-mercaptopropyltrimethoxysilane. In
particular, we used a u-undecenyl chain because earlier
studies from us have shown that a long alkene chain linking
the chiral vanadyl complex to the silica surface has a
9
positive effect on the ee of the resulting cyanohydrin. A
long alkyl chain permits a larger distance between the solid
˚
Scheme 1. Reactions and conditions: (a) molecular sieves 3 A, CH
2
Cl
2
, rt, 24 h; (b) VOSO $3H
4
2
O, EtOH, reflux, 1 h; (c) HO(CH
, 24 h; (f) AIBN, CHCl , N , 80 8C, 24 h.
2 9 2
) CH]CH , xylene, reflux,
2
4 h; (d) HS(CH
2
)
3
Si(OMe)
3
, toluene, reflux, N
2
, 48 h; (e) EtOSi(CH
3
)
3
, toluene, reflux, N
2
3
2

C. Baleiz a˜ o et al. / Tetrahedron 60 (2004) 10461–10468
10463
3 2 2 2 2 2 3 2 2 2
Scheme 2. Reactions and conditions: (a) HNO (3 M), reflux, 24 h; (b) SOCl , DMF, N , 65 8C, 24 h; (c) H NCH CH SH$HCl, Et N, N , CH Cl , 48 h.
This strategy to obtain the target complex 5 in a single step
although not pure) overcomes other alternative time-
consuming, multistep synthesis.
taken advantage of the appearance of carboxylic acid groups
and after formation of the corresponding acyl chloride,
reacted them with 2-mercaptoethylamine (Scheme 2).
Activated carbon also undergoes a partial oxidation upon
treatment with hot concentrated nitric acid. In this case,
the reaction is more complex and other groups such as
quinones or sulphonic acids are also known to be formed
during the treatment in addition to the carboxylic groups.
These carboxylic groups were subsequently modified by
transformation into acyl chlorides and reaction with
(
It has to be noted that although complexes 4 and 5 were
prepared under inert atmosphere and are drawn as V(IV)
complexes, it is very likely that they undergo an
spontaneous oxidation either during manipulation or during
the catalytic reactions to V(V) as it has been previously
1
3
reported.
2-mercaptoethylamine to form the corresponding 2-mer-
captoethylamide (Scheme 2).
Concerning the modification of the silica support, we first
introduced a given amount of 3-mercaptopropyl groups and
the large proportion of the silanol groups remaining on the
silica surface were subsequently masked by reacting them
with an excess of ethoxytrimethylsilane. In previous work it
has been demonstrated that the presence of free silanol
groups plays a negative role in the catalysis by decreasing
At this point, it is interesting to remark on the large
differences in terms of structure and composition between
SWNT and activated carbons. Thus, while SWNT have a
regular geometry formed by bundles of long (!100 nm)
flexible narrow (1–3 nm) cylinders made of a single wall of
9
2
the ee.
graphite sp carbons with a large periodicity of the local
structure around the sites, the situation in activated carbons
is completely different. In the later case, significant amounts
of heteroatoms, particularly oxygen, nitrogen and sulphur,
are also present and the structure is undefined consisting of a
wide distribution of graphitic platelets of various sizes
interconnected by different types of bridges. Also the
specific surface area of SWNT is considerably smaller than
that of activated carbons (see Table 1).
The propose of this work is to compare the catalytic activity
and the asymmetric induction ability of recoverable vanadyl
salen complexes linked to different supports. Thus the
performance of VOsalenfsilica will be compared with
other type of catalysts. The second catalyst type contains the
vanadyl salen complex covalently anchored on carbon-
aceous supports. In particular, we used as supports a
conventional activated carbon (AC) with a very large
specific surface area and the recently discovered single wall
carbon nanotubes (SWNT). It is known that during the
purification of SWNT to remove catalyst particles with hot
concentrated nitric acid, SWNT becomes partly oxidized;
the average length of SWNT is shortened and carboxylic
acid groups are created at the tips of the tubes. We have
Mercapto functionalised SWNT (SHfSWNT) and acti-
vated carbons (SHfAC) were finally connected to the
chiral vanadyl salen complex using a statistical mixture
containing the unsymmetrical complex 9b substituted with a
p-styryl group in one of the phenolic rings. Preparation of
this precursor was accomplished following a synthetic route

10464
C. Baleiz a˜ o et al. / Tetrahedron 60 (2004) 10461–10468
Table 1. Structural, textural and analytical data for the library of recoverable catalysts
2
Surface area (m g
K1
K1
Catalyst
Main feature
Amorphous particles (1 mm diameter)
)
V content (mmol g
)
VOsalenfsilica
VOsalenfSWNT
VOsalenfAC
VOsalenfIL
350
300
0.04
0.082
0.06
0.039
8.34
Long tubes O100 nm of 1.3 nm diameter made of a single graphite wall
Carbon platelets
Positively charged imidazolium unit
1100
a
—
—
t
t
a
BuVOsalen
Unsupported complex with Bu substituents in o- and p- positions of the
phenolic rings
a
Soluble in ionic liquids.
analogous to that used in the case of VOsalenfsilica.
Scheme 3 outlines the synthesis of precursor 9b in which the
key step is the preparation of 3-tert-butyl-2-hydroxy-5-(4-
vinylphenyl)benzaldehyde (8), obtained by Suzuki cross-
coupling between 5-bromo-3-tert-butylsalicylaldehyde and
was recorded. The vanadium content was determined by
quantitative absorption spectroscopy from an aqueous
solution of the catalyst (in the case of VOsalenfIL) or in
the resulting liquor after dissolving the solid with concen-
trated hydrofluoric acid (in the case of VOsalenfsilica). In
the case of VOsalenfSWNT and VOsalenfAC catalysts,
the vanadium content was estimated indirectly by combus-
tion chemical analyses from the differences in the
percentage of nitrogen in the support before and after
anchoring the complex. The vanadium content data is also
indicated in Table 1. For the sake of comparison, we have
also included in our study as reference catalyst the chiral
vanadyl tetra tert-butyl salen complex that was the catalyst
initially reported by North and co-workers to give high ee in
4
tetrakis(triphenylphosphine).
-vinylphenyl boronic acid catalysed by palladium
1
5
The linkage between SWNT or AC and the catalytically
active chiral vanadyl salen complex (9b) was accomplished
through a radical chain addition of the mercapto group to the
terminal C]C initiated by AIBN (Scheme 4).
The last type of recoverable chiral vanadyl salen catalyst
prepared was VOsalenfIL in which precursor 9b was
attached through the AIBN initiated radical chain reaction
3
dichloromethane.
0
to N-(3-mercaptopropyl)-N -methyl imidazolium (SHfIL).
The actual synthetic route is indicated in Scheme 4.
2
.2. Catalytic tests
The four recoverable catalysts and the standard chiral
vanadyl tetra tert-butyl salen complex were initially tested
for enantioselective cyanosilylation of benzaldehyde using
trimethylsilyl cyanide (TMSCN) (Scheme 5). The exper-
iments were carried out at 0 8C, except for the reactions in
ionic liquids which were conducted at room temperature.
All catalysts exhibited moderate to high conversions, with
the corresponding silylated cyanohydrins being the only
detectable product. The results of the catalytic activity
obtained for benzaldehyde are indicated in Table 2.
The above sequences exemplify how from a common
precursor having a terminal C]C double bond, a library of
vanadyl salen catalyst can be easily obtained provided that
the scaffolds are suitably functionalised with a mercapto
group. The list of vanadyl salen catalysts that have been
prepared and their main analytical and physicochemical
parameters are included in Table 1.
All the catalysts prepared exhibit in UV visible spec-
troscopy an absorption at about 380 nm corresponding to the
ligand-to-metal charge transfer band characteristic of the
vanadyl salen complexes. Also in IR spectroscopy the
Concerning the ee, the best catalyst of the library was the
tetra BuVOsalen complex dissolved in 1-butyl-3-ethylimi-
t
K1
expected peak for the vanadyl salen complexes at 1540 cm
dazolium hexafluorphosphate (beim PF ) ionic liquid, but it
6
Scheme 3. Reactions and conditions: (a) Br
2
, CH
2 2 2 6 4 2 3 4 2 3
Cl , 0 8C, 30 min; (b) para-(CH ]CH)C H B(OH) , Pd(PPh ) , Na CO , THF, reflux, 6 h; (c) 2, 3,
˚
molecular sieves 3 A, CH Cl , rt, 24 h; (d) VOacac, MeOH, rt, 16 h.
2 2

C. Baleiz a˜ o et al. / Tetrahedron 60 (2004) 10461–10468
10465
Scheme 4. Pictorial representation of the covalent anchoring of chiral vanadyl salen complex on three different scaffolds.
is noteworthy that the VOsalenfsilica exhibits a very
similar ee in spite of being a solid catalyst. The activity of
VOsalenfIL was high but the ee is considerably reduced
explanation for this reduced asymmetric induction ability of
VOsalenfIL is the interaction of the vanadyl group of the
complex with the associated chloride anion present in
SHfIL. In this regard we performed an alternative
experiment in which the amount of chloride was increased,
whereby a diminution of the ee was observed. The catalytic
activity of the VOsalenfIL is, however, very high giving
the highest turnover frequency values (TOF) for the library
of vanadyl salen catalysts.
t
compared to the soluble BuVOsalen complex. One possible
In the case of activated carbon, the ill-defined structure of
the support, together with the possible presence of
Scheme 5. Catalytic cyanosilylation of benzaldehyde.
Table 2. Results of the enantioselective catalytic addition of TMSCN to benzaldehyde in the presence of different chiral vanadyl salen systems in
homogeneous and heterogeneous phases
a
K1
b
Catalyst
Conditions
Conversion (%)
TOF (h
)
ee (%)
VOsalenfsilica
VOsalenfsilica
Tetra VOsalen
CHCl , 0.24 mol%, 0 8C
3
78
75
2.7
2.6
3.5
3.5
3.1
3.75
18.3
85
85
89
89
66
48
57
3rd use
[beim]PF
5th use
tBu
b
6
, 1 mol%, rt
85
83
tBu
b
Tetra VOsalen
VOsalenfSWNT
VOsalenfAC
VOsalenfIL
CHCl
CHCl
3
, 0.3 mol%, 0 8C
, 0.3 mol%, 0 8C
67
81
3
b
88
[bmim]PF
6
, 0.2 mol%, rt
a
Determined using a GC column (TRB5, 30 m, 0.25 mm), using nitrobenzene as internal standard.
Determined using a chiral GC column (Chiraldex g-TA, 30 m, 0.25 mm).
b

10466
C. Baleiz a˜ o et al. / Tetrahedron 60 (2004) 10461–10468
functional groups derived from the sulphur and oxygen
groups could be responsible for the low ee. Also in the case
of SWNT, the presence of a residual population of free
carboxylic groups is the most likely factor responsible for
the moderate ee value. Some additional treatments to the
SWNT or activated carbons to decrease the population of
sites playing a negative role would be necessary to test this
hypothesis. It is interesting to note that the TOF for
the reaction of benzaldehyde in the presence of both
VOsalenfSWNT and VOsalenfAC is also high with
3. Conclusions
In the present work, we have developed a library of chiral
vanadyl salen complexes that are covalently attached to
solid supports or with a high ionophilicity that makes the
catalyst soluble in ionic liquids and not extractable by
conventional organic solvents. The preparation of this
library illustrates a simple methodology to obtain hetero-
geneous catalysts based on the addition of mercapto groups to
terminal C]C double bonds. From this series, we have
prepared a chiral vanadyl salen complex anchored to silica
t
comparable values to those observed for the tetra BuVO-
(
activity and asymmetric induction ability is very similar to
VOsalenfsilica) through a 14 membered chain whose
salen complex in homogeneous phase and higher than
observed for catalyst anchored to silica and, therefore, it
may be worth while to try to optimise the asymmetric
induction of these two catalysts.
t
that of chiral tetra BuVOsalen complex in ionic liquid. The
activity of the latter complex in solution can be considered
as the maximum intrinsic activity achievable for the vanadyl
salen complexes and, therefore, VOsalenfsilica is con-
sidered as an optimum recoverable catalyst since it exhibits
the activity of the homogeneous complex plus the
advantages of heterogeneous catalysts in terms of recovery
and reusability. No vanadium leaching has been observed
for this system.
One of the most important issues when studying a
heterogeneous catalyst is its performance upon reuse. In
t
the case of homogeneous phase tetra BuVOsalen
complex in [beim]PF ionic liquid, upon extraction of
6
the product with hexane, the extracted solution becomes
green indicating that some fraction of the vanadyl
catalyst present in the ionic liquid was also extracted
together with the reaction products. Although, a second
t
4. Experimental
use of the ionic liquid containing chiral tetra BuVO-
salen complex gave essentially the same conversion and
ee values as in the first use, it is clear that over the long
term a depletion of the vanadyl salen chiral complex
would eventually lead to a reduction of this activity.
Also, separation of the vanadium traces from the
reaction mixture would be problematic. In the case of
the VOsalenfsilica as catalyst, reuse of the solid after
one run was simply achieved by filtration of the solid
and subsequent washings with fresh aliquots of chloro-
form. Upon removal of the solid catalyst VOsalenfsilica,
the reaction mixture was surveyed for the presence of
catalytic active species. In no case was it observed that
the reaction progresses in a solution that has been
reacted up to 40% conversion in the presence of
VOsalenfsilica when the solid has subsequently been
removed and the supernatant allowed to react further
under normal conditions.
Silica (Aldrich), SWNT (Carbolex, HiPCO, O80% purity),
activated carbon (Norit), and [bmim]PF (Aldrich) were
commercial samples.
6
t
Chiral tetra BuVOsalen complex was prepared by dissol-
0
t
ving in methanol tetra tert-butyl ligand 4 (RZR Z Bu)
1 mmol) and VOacac (1.1 mmol) and stirring the solution
(
overnight at room temperature. The complex was purified
by a short chromatographic column using silica as stationary
phase and eluting initially with hexane to remove the ligand
followed by diethyl ether. Tetra tert-butyl salen ligand (4,
0
t
RZR Z Bu) was prepared by condensation at room
temperature of 2 equiv of 3,5-di-tert-butylsalicylaldehyde
0
and following the reported procedure.
and 1 equiv of (C)-R,R -1,2-cyclohexanediamine in ethanol
16
Precursor 5 was obtained as a mixture starting from 3-tert-
butyl-5-chloromethylsalicylaldehyde, 3,5-di-tert-butylsali-
Based on the results achieved with benzaldehyde, we
t
selected two systems, namely chiral tetra BuVOsalen
0
cylaldehyde and (C)-R,R -1,2-cyclohexanediamine in
ethanol in the proportions indicated in Scheme 1, and
adding vanadyl sulphate before reacting the complex with
undecen-1-ol in xylene at reflux temperature, following the
complex in [beim]PF and VOsalenfsilica in CHCl as
6
3
the systems with the highest asymmetric induction ability
and studied the transformation of two more aldehydes into
the corresponding chiral silylated cyanohydrins in order to
expand the scope of the heterogeneous catalysts and to show
the generality of our system. The results achieved are listed
in Table 3.
9
detailed experimental procedure previously reported by us.
Functionalised silica containing 3-mercaptopropylsilyl
groups was obtained by reacting silica (ALDRICH) with
3
-mercaptopropyl trimethoxysilane in toluene at reflux
temperature under an inert atmosphere, and subsequently
submitting the resulting solid to exhaustive silylation using
excess of ethoxytrimethylsilane under the same conditions
as for 3-mercaptopropylsilylation. The anchoring of pre-
cursor 5 to the functionalised silica was accomplished using
AIBN as radical initiator in the absence of oxygen following
As can be seen in Table 3, although as in the case of
benzaldehyde the activity and TOF for the cyanosilylation
of a substituted benzaldehyde (4-methoxybenzaldehyde)
and an aliphatic aldehyde (hexanal) using VOsalenfsilica
as solid catalyst is somewhat lower than that of the chiral
t
tetra BuVOsalen complex in [beim]PF , the ee values of
9
6
a procedure described elsewhere.
both catalytic systems were very similar with the additional
advantage that the solid catalyst can be easily separated and
reused.
Precursor 8 was synthesized starting from 5-bromo-3-
methylsalicylaldehyde that was submitted to Suzuki

C. Baleiz a˜ o et al. / Tetrahedron 60 (2004) 10461–10468
10467
t
Table 3. Results of the cyanosilylation of several aldehydes with TMSCN using as catalysts either chiral tetra BuVOsalen complex or VOsalenfsilica
Catalyst
Data
4-OMe benzaldehyde
Hexanal
a
b
b
VOsalenfsilica
Conversion (%)
K1
70
85
TOF (h
c
ee (%)
)
2.4
78
2.9
82
tBu
Tetra VOsalen
d
b
b
Conversion (%)
K1
88
97
TOF (h
c
ee (%)
)
3.7
81
4.1
83
a
3
CHCl , 0.24 mol%, 0 8C.
Determined using a GC column (TRB5, 30 m, 0.25 mm), using nitrobenzene as internal standard.
Determined using a chiral GC column (Chiraldex g-TA, 30 m, 0.25 mm).
b
c
d
6
[beim]PF , 1 mol%, rt.
cross-coupling with 4-vinylphenylboronic acid and in the
presence of Pd[(PPh ) ] followed by condensation with 3,5-
4.2. General procedure for the asymmetric addition of
trimethylsilyl cyanide to aldehydes catalysed by VOsalen
in ionic liquids
3
4
0
di-tert-butylsalicylaldehyde and (C)-R,R -1,2-cyclohexa-
nediamine and VOacac. The experimental procedure has
1
4
been reported in detail in a previous work.
The ionic liquid (1 ml) was dried by heating at 50 8C under
reduced pressure for 12 h (0.1 Torr). The vanadyl salen
t
complex either BuVOsalen or VOsalenfIL was added and
the liquid stirred at room temperature for 15 min. Aldehyde
Activated carbon and commercial HiPCO SWNT were
treated at reflux temperature with a 3 M aqueous solution of
HNO3 for 24 h. After this treatment the carbonaceous
materials were filtered, washed with distilled water and THF
and dried under vacuum. The resulting black solids once
(1.64 mmol) and TMSCN (1.1 equiv) were added to the
ionic liquid containing the previously dissolved catalyst and
the mixture stirred at room temperature under N . The
2
course of the reaction was followed by removal of a given
amount of ionic liquid, which was weighed and dissolved in
dichloromethane. To this solution, nitrobenzene (5 ml) was
added as external standard and the solution injected into GC
dried, were suspended in a solution of SOCl in DMF and
2
stirred at 65 8C for 24 h. Then, the solids were recovered by
filtration, washed with anhydrous THF and treated with a
solution of 2-mercaptoethylamine hydrochloride and
triethylamine in dry dichloromethane. The suspension was
allowed to react under nitrogen at 40 8C for 48 h.
(
TRB5, capillary column). At the end of the reaction, the
ionic melt was exhaustively extracted with hexane (2!
0 ml). The ee was determined in a chiral GC column
1
0
(Chiraldex g-TA, 30 m, 0.25 mm).
N-Methyl-N -(3-mercaptopropyl)-imidazolium chloride
(
panethiol and N-methyl-imidazolium at 80 8C for 96 h,
SHfIL) was obtained by reaction between 3-chloropro-
Acknowledgements
following the detailed experimental procedure previously
1
reported by us.
2
Financial support by the Spanish Ministry of Science and
Technology (MAT2003-1226) is gratefully acknowledged.
CB thanks the Funda c¸ a˜ o para a Ci eˆ ncia e Tecnologia,
Portugal (PRAXIS XXI/BD/21375/99) for a PhD
scholarship.
Immobilization of precursor 9 on the modified supports
SHfSWNT, SHfAC and SHfIL) was accomplished
using AIBN at 80 8C for 20 h in degassed chloroform under
(
an inert atmosphere, and submitting the resulting solids to
0,12
exhaustive solid–liquid extraction with dichloromethane.
1
4
.1. General procedure for the asymmetric addition of
References and notes
trimethylsilyl cyanide to aldehydes catalysed by
heterogeneous catalysts
1
. Collins, A. N.; Sheldrake, G. N.; Crosby, J. Chirality in
Industry, the Commercial Manufacture and Applications of
Optically Active Compounds; Wiley: West Sussex, 1992; pp
The heterogeneous catalyst was suspended in dry chloro-
form (1.9 ml) followed by the addition of the aldehyde
279–299.
. Cholod, M. S. Cyanohydrins, 4th ed.; Grant, M. H., Ed.;
Encyclopedia of Chemical Technology; Wiley: New York,
(
(
1.64 mmol) and nitrobenzene as internal standard
1.64 mmol). The suspension was stirred for 5 min and
2
then TMSCN (4.92 mmol) was added. The reaction mixture
was stirred at 0 8C and the course of the reaction followed by
analysing the organic phase by GC (TRB5, 30 m, 0.25 mm).
The physical and analytical data of the trimethylsilyl ethers
of corresponding cyanohydrins were in agreement with
1
993; Vol. 7, pp 821–834.
. Belokon, Y. N.; North, M.; Parsons, T. Org. Lett. 2000, 2,
617–1619.
3
4
5
1
. Belokon, Y. N.; Green, B.; Ikonnikov, N. S.; North, M.;
Parsons, T.; Tararov, V. I. Tetrahedron 2001, 57, 771–779.
. Leadbeater, N. E.; Marco, M. Chem. Rev. 2002, 102,
3217–3273.
1
6,17
those reported in the literature.
In all cases, the (R,R)-
isomer of the catalyst gave an excess of the (S)-enantiomer
of the cyanohydrin derivative, whose configuration was
determined by comparison with the existing literature
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