Chiral mesoporous templated silicas as heterogeneous inorganic-organic catalysts in the enantioselective alkylation of benzaldehyde

Article abstract of DOI:10.1016/S0957-4166(98)00294-8

New mesoporous templated silicas of the MCM-41 family covalently linked to chiral ephedrine are used as heterogeneous chiral auxiliaries in the enantioselective alkylation of benzaldehyde by diethylzinc. Promising results are obtained with these inorganic

Full text of DOI:10.1016/S0957-4166(98)00294-8

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 3053–3064
Chiral mesoporous templated silicas as heterogeneous
inorganic–organic catalysts in the enantioselective alkylation of
benzaldehyde
∗
M. Laspéras, N. Bellocq, D. Brunel and P. Moreau
Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, CNRS UMR 5618, Ecole Nationale Supérieure de
Chimie de Montpellier, 8, rue de l’Ecole Normale, 34296 Montpellier Cédex 5, France
Received 2 July 1998; accepted 24 July 1998
Abstract
New mesoporous templated silicas of the MCM-41 family covalently linked to chiral ephedrine are used as
heterogeneous chiral auxiliaries in the enantioselective alkylation of benzaldehyde by diethylzinc. Promising
results are obtained with these inorganic–organiccatalysts. Various factors which may affect the activity, selectivity
and enantioselectivity are studied. Comparison with homogeneous catalysis is carried out. © 1998 Elsevier Science
Ltd. All rights reserved.
1. Introduction
In the domain of asymmetric carbon–carbon bond forming reactions, the enantioselective addition of
organometallic reagents to aldehydes has attracted much attention owing to its application in the synthesis
of chiral secondary alcohols.1–4 Among the various organometallic compounds, diorganozincs act as
ideal alkyl donors for catalytic asymmetric alkylation. β-Aminoalcohols are the chiral auxiliaries most
commonly used in the catalytic process. Although the mechanism of the reaction has been elucidated
in homogeneous catalysis, studies are still in progress in order to generalize the applicability of a chiral
auxiliary in the reaction with carbonyl compounds of various reactivities.
Taking into account environmental and economical considerations, the handling of immobilized chiral
auxiliaries presents many advantages such as easier separation and recovery from the reaction mixture
and thus enhanced recycling possibilities, which are now well established in fine organic synthesis.
Some studies deal with the heterogenization of chiral auxiliaries on different polymer supports.5
Beside organic polymers, inorganic materials are a new class of support for the heterogenization of
–7
∗
Corresponding author: Tel: +33-4-67147223; fax: +33-4-67144349; e-mail: lasperas@cit.enscm.fr
0957-4166/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00294-8
tetasy 2443 Article

3054
M. Laspéras et al. / Tetrahedron: Asymmetry 9 (1998) 3053–3064
8
homogeneous catalysts. These inorganic supports have many advantages over most polymers because of
9
their superior mechanical and thermal stability. However, no report has appeared to our knowledge since
the pioneering work of Soai et al. concerning the immobilization of β-aminoalcohols on alumina and
silica gel and their use in the enantioselective addition of dialkylzinc to aldehydes, leading to moderate
10
activities and ees.
The increasing interest focused on supported catalysts led us to study the new mesoporous micelle
templated silicas (MTS) as supports. These silicas are different from amorphous silicas due to their
regular porosity, consisting of uniformly sized channels with pore diameters within a mesoporous range
11
of 2 to 10 nm. Taking into account their characteristic structure, the insertion and grafting of functional
molecules was investigated in our laboratory,1
2,13
with the aim of enhancing the turnover frequency
through better accessibility to the catalytic sites.
The present work is concerned with the synthesis and characterization of new supported chiral auxil-
iaries prepared by immobilization of a chiral β-aminoalcohol, the (−)-(1R,2S)- or (+)-(1S,2R)-ephedrine,
on MTS surfaces and their use in the enantioselective addition of diethylzinc to benzaldehyde. The latter
study deals with the influence of the ligand amount, diethylzinc and benzaldehyde concentrations, which
are known to affect both the reaction rate and the enantioselectivity, and with the effect of the stirring
rate and of the density of chiral modifier at the mineral surface, which are characteristic of heterogeneous
catalysis. The results thus obtained using heterogeneous catalysis are compared with those described
under homogeneous conditions.
2. Results and discussion
The synthesis and the characterization of the solid chiral auxiliaries containing MTS and the results
concerning asymmetric alkylation of benzaldehyde with these solids are described below.
2.1. Immobilization of ephedrine over MTS: synthesis and characterization of the chiral materials
2.1.1. Synthesis of the chiral inorganic–organic materials
The immobilization of ephedrine over mesoporous MTS type silicas 1 was carried out in two steps
(Scheme 1).
Scheme 1. Immobilization of (−)- or (+)-ephedrine over mesoporous MTS type silicas
The first step involved the grafting of 3-chloro- or 3-iodopropyltrialkoxysilane (XPAS) onto the surface
through Si–O–Si bonds, leading to the solids 2a–c. Solids 2d and 2e were obtained by a basic treatment
performed after the grafting of the halogeno functions in order to increase the number of links between
the entering silicon atom and the inorganic surface.14 In the second step, nucleophilic substitution of the

M. Laspéras et al. / Tetrahedron: Asymmetry 9 (1998) 3053–3064
3055
Table 1
Characterization of the chiral inorganic–organic MTS silicas
0
halogen by the basic amino group of (−)- or (+)-ephedrine led to the catalysts 3a–e or 3a , respectively.
The excess ephedrine or ephedrine chlorohydrate was removed by accurate washing with methanol.
2.1.2. Characterization of the solid chiral auxiliaries
The solids thus obtained have been characterized by FTIR spectroscopy, thermogravimetry and
elemental analyses.14
The infrared spectra of solids 3 show aromatic CH stretching vibrations characteristic of immobilized
ephedrine.
−1
Loadings (mol g ) of grafted moieties, N or N , for solids bearing either a halogeno (solids 2a–e)
X
0
E
or an ephedrine function (solids 3a–e and 3a ) were calculated from elemental and thermogravimetric
data. Both results are in good agreement; the average number obtained is given in Table 1, together with
the specific area and mesoporous volume values. Whatever the solid, the substitution of the halogen
atom by ephedrine is never quantitative; indeed, halogen moieties remain on the final chiral supported
catalysts (Table 1). It is worth noting that conservation of the total number of grafted species is generally
observed. Therefore, after modification by ephedrine, the sum of the number of grafted ephedrine N and
E
of remaining halogeno moieties N is nearly equal to the initial number of grafted coupling halogeno
X
functions. Taking into account this result, we can assume that ephedrine substitutes the halogen atom.
The textural characteristics of these new inorganic MTS supports were determined by nitrogen
volumetry; typical adsorption and desorption isotherms (type IV) are obtained.15 After grafting of the
organic functions on the surface, the conservation of the same type of isotherms indicates that the
structure of the inorganic surface was maintained after the modification. However, surface areas S
2
−1 −1
m g ) and mesoporous volume V (mL g ) decrease with the grafting of halogeno functions as
mp
(
shown by the comparison between the naked support (solid 1) and any of the inorganic–organic XPAS
materials (Table 1). Moreover, the substitution of the halogen by (−)- or (+)-ephedrine leads to another
decrease of both surface areas and available mesoporous volumes, particularly for 3c synthesized by
iodine substitution.

3056
M. Laspéras et al. / Tetrahedron: Asymmetry 9 (1998) 3053–3064
2.2. Enantioselective addition of diethylzinc to benzaldehyde
2.2.1. General features
Enantioselective alkylation of benzaldehyde by diethylzinc (Scheme 2) performed using (−)- and (+)-
ephedrine and their supported MTS analogues as chiral auxiliaries yields (R)- or (S)-1-phenylpropan-1-ol
via their alcoholates.
Scheme 2. Enantioselective addition of diethylzinc to benzaldehyde
The mechanism (Scheme 3) of this reaction in homogeneous conditions has been established by
Noyori et al. according to the results obtained with (−)-3-exo-(dimethylamino)isoborneol (DAIB) as
3,16
17,18
the chiral auxiliary.
This mechanism generalized to other chiral auxiliaries
implies the addition of
dialkylzinc to the chiral auxiliary which leads to the coordinatively unsaturated catalyst 1. This complex
is in equilibrium with the corresponding dimer. An intramolecular alkyl transfer is proposed to occur
via a 4/4-bicyclic transition structure 2 from another Et Zn molecule complexed on the Lewis base to
2
benzaldehyde activated by the tricoordinate zinc atom of 1. The coordination geometry of the zinc atom
in the coordinated Et Zn molecule serves to increase the nucleophilicity of the alkyl group. The obtained
2
enantiomeric phenylpropanol zinc alcoholates afford, after hydrolysis, the enantiomeric alcohols together
with the regenerated catalyst.
This mechanism is in fair agreement with the one proposed by Frechet et al. using DAIB immobilized
on polymeric supports as the chiral auxiliary.19
Table 2 shows our results using (−)- or (+)-ephedrine immobilized on inorganic MTS. Under standard
conditions (8.5 mol%), (R)- or (S)-1-phenylpropan-1-ol (entry 6 or 7) is obtained with an enantio-
selectivity equal to 35 or 26%, respectively. Enantioselectivities are moderate but these supported ligands
control the sense of the asymmetric induction, (−)-ephedrine leading to (R)-phenylpropanol (entry 6) and
(+)-ephedrine to the (S)-enantiomer (entry 7). Such an effect was previously described in homogeneous
catalysis, the configuration of the six-center transition state 2 (Scheme 3) depending on the configuration
2
of the α and β substituents of the chiral auxiliary. The α S and β R configuration of the chiral auxiliary
led to the (S)-enantiomer whereas the (R)-enantiomer was obtained from the α R and β S configuration.
A typical reaction obtained with supported (−)-ephedrine is shown in Fig. 1.
Disappearance of benzaldehyde (BA) leads to the formation of the enantiomeric alcohols together
with benzylalcohol (PhCH OH) as a by-product, as in homogeneous conditions. The initial rate of
2
benzaldehyde disappearance agrees with a competitive mechanism, according to:
d[BA]
dt
d([R] + [S]) d[PhCH OH]
2
−
=
+
dt
dt
The initial rate (r , Table 2) depicts the rate of formation of the enantiomeric alcohols in each
0
experiment: r =d([R]+[S])/dt.
0
In most cases, the selectivity in enantiomeric alcohols, expressed as:
[
R] + [S]
%
selectivity = 100 × [
R] + [S] + [PhCH OH]
2

M. Laspéras et al. / Tetrahedron: Asymmetry 9 (1998) 3053–3064
3057
Scheme 3. Mechanism of the asymmetric alkylation of aldehydes by diethylzinc in homogeneous conditions
Table 2
Effect of the immobilization of (+)- or (−)-ephedrine on the reaction and sense of the asymmetric
induction

3058
M. Laspéras et al. / Tetrahedron: Asymmetry 9 (1998) 3053–3064
Fig. 1. Evolution of the alkylation of benzaldehyde with time using solid chiral auxiliaries
does not depend on the conversion. This result is explained, as suggested by the rate law, by the
formation of benzylalcohol in a competitive way from benzaldehyde reduction by Et Zn (Scheme 4).
2
Scheme 4. Mechanisms of the formation of benzylalcohol
20
The formation of benzylalcohol in a consecutive way (Meerwein–Pondorf–Verley reduction ) would
result in a decrease of selectivity with conversion, especially at high conversions (vide infra results
concerning the effects of the stirring rate and of Et Zn concentration) accompanied by the appearance of
2
1-phenylpropan-1-one (Scheme 4).
At the same time, enantioselectivities are considered as constant all along the reaction. Toluene or
hexane have been used as solvents. As previously reported,21 higher rates and selectivities are obtained
in hexane (Table 2, entries 1 and 2 or 5 and 6). Nevertheless, ees are slightly higher in toluene (entries 5
and 6).
Without catalyst and whatever the solvent, the reaction is characterized by low rates r and selectivities
0
(entries 1 and 2). Addition of 8.5 mol% of supported chiral auxiliary (entries 5 and 6) serves to increase
the rates and selectivities which indicate that alkyl transfer is favored. However, rates and ees are notably
lower than under homogeneous conditions (entry 3). On the other hand, it can be noted that the naked
surface (entry 4) enhances the racemic alkyl transfer.

M. Laspéras et al. / Tetrahedron: Asymmetry 9 (1998) 3053–3064
3059
Table 3
Effect of the stirring rate on the reaction
Table 4
Effect of the amount of chiral auxiliary on the reaction
In order to understand the reasons which affect the catalyst activity, various parameters have been
studied affecting both the kinetic and the mechanism of the reaction. They include the effect of the
stirring rate, of the amount of chiral auxiliary, of the alkylzinc and benzaldehyde concentrations and of
the density of the chiral modifier at the surface of the inorganic support. Results are compared to those
described using homogeneous catalysis. Reactions are carried out in toluene as the solvent.
2.2.2. Effect of the stirring rate
The effect of the stirring rate has been studied to ensure the absence of external diffusional
22
limitations. The heterogeneous medium was thus stirred at 400, 750 and 1000 rpm, under the standard
conditions using solid 3e as the chiral auxiliary.
The results (Table 3) show that the stirring rate has no effect either on r or on the enantiomeric excess.
0
Thus, in the enantioselective alkylation of BA by Et Zn, external diffusion of reactants to the external
2
surface of the solid chiral auxiliary does not seem to be the limiting factor. The decrease of the selectivity
observed for the reaction stirred at 1000 rpm can be accounted for by a decrease of Et Zn concentration
2
due to the fast stirring rate under a nitrogen flow. Effectively, appearance of phenylpropanone is noticed
indicating that formation of benzylalcohol occurs, in this case, in a consecutive way (Scheme 4). Hence,
catalytic experiments were performed at a stirring rate of 750 rpm.
2.2.3. Effect of the amount of chiral auxiliary
The absence of external diffusional constraints is corroborated by studying the effect of the weight of
solid 3d on r (Table 4). In this case, the amount of chiral auxiliary (percentage n ) is directly related to
0
E
the weight of solid.

3060
M. Laspéras et al. / Tetrahedron: Asymmetry 9 (1998) 3053–3064
Fig. 2. Effect of the initial concentration of diethylzinc on r
0
Effectively, r increases linearly with the weight of solid and, therefore, with the amount of chiral
0
23
auxiliary as in homogeneous catalysis. With regards to the selectivity or to the enantioselectivity, it
is worth noting that increasing the amount of ephedrine leads to better selectivities and enantiomeric
excesses (Table 4). For low weights, the participation of the uncatalyzed reaction is not negligible and
selectivities are close to that obtained without catalyst. The latter increase with the ratio of the observed
rate to the uncatalyzed rate. Selectivities of the same order of magnitude as in homogeneous conditions
3
,23
are obtained using 17.0 mol% of chiral auxiliary. Under standard catalytic homogeneous conditions,
−
1
the rate of the reaction was shown to be first order in chiral auxiliary concentration (2–25 mmol L )
−1
−1
using [BA] =0.3–1.3 mol L and [Et Zn] =0.3–1.3 mol L in toluene at 0°C. However, an increase in
the concentration of the catalyst enhanced the rate but the enantioselectivity remained virtually constant.
0
2
0
2
2
.2.4. Effect of the concentration of diethylzinc
While keeping the initial BA concentration constant ([BA] =0.33 mol L ) and using the same amount
−1
0
(
8.5 mol%) of the grafted auxiliary, the initial Et Zn concentration was varied leading to [Et Zn] /[BA]
2 2 0 0
ratios between 1.5 and 6.4 (Fig. 2). Under the conditions used, r does not vary with [Et Zn] , which is
0
2
0
characteristic of a zero order.
Fitting of our results with the general first order Eq. 1:
d[BA]
−
= kobs[BA]
dt
BA] = [BA]0e−
kobst
(1)
[
leads to good correlations between BA concentration and time of reaction indicating global first order
kinetics. Taking into account the observed zero order with initial Et Zn concentration, we can assume a
2
first order with aldehyde concentration.
23
Under homogeneous conditions, Noyori et al. reported that when using (−)-DAIB as the chiral auxil-
iary, the reaction was zero order with aldehyde concentration, in default of Et Zn ([Et Zn] =cte=0.33 mol
2
2
0

M. Laspéras et al. / Tetrahedron: Asymmetry 9 (1998) 3053–3064
3061
Table 5
Effect on the reaction of the organic loading on the silica surface
−
1
−1
L
and [BA] >0.33 mol L ). Nevertheless, a first order was obtained when the reaction was performed
0
−1
−1
in an excess of Et Zn ([Et Zn] =cte=0.33 mol L and [BA] <0.33 mol L ). Concerning the effect of
2
2
0
0
the initial Et Zn concentration, the initial rate did not depend on [Et Zn] , in excess of alkylzinc and
2
2
0
became first order in default.
Hence, kinetic results for heterogeneous alkyl transfer catalyzed by immobilized ephedrine are in
good agreement with those of Noyori et al. showing a first or zero order versus aldehyde or alkylzinc
concentration respectively, when excess of Et Zn is used.
2
Decreasing of the [Et Zn] /[BA] ratio leads to a decrease of selectivity at high conversions. When
2
0
0
the diethylzinc concentration diminishes, as noticed above at high stirring rate, the formation of benzyl-
alcohol and of phenylpropanone in a consecutive way lowers the phenylpropanol concentration and the
selectivity (Fig. 2). Similarly, in homogeneous catalysis, benzylalcohol was observed as the major by-
product whose yield increases with a decreasing alkylzinc/benzaldehyde ratio.2
The variation of the [Et Zn] /[BA] ratio results in no change in the enantiomeric excess (ee=31ꢀ3%)
2
0
0
2
in agreement with results reported in homogeneous media. However, an increase of the enantioselectiv-
ity was simultaneously mentioned.
24
Our results related to kinetics and mechanism are in good agreement with those obtained in homo-
geneous catalysis. Nevertheless, rates and enantiomeric excesses remain moderate whatever the condi-
tions used. A key point in the comprehensive study of enantioselective heterogeneous catalysis may be the
understanding of the role of the various parameters linked to heterogenization of the chiral auxiliaries.
With this in mind, it is important to consider the accessibility of the reactants to the chiral molecules
grafted on the internal surface of the support.
2.2.5. Effect of the density of organic ligands on the inorganic surface
Results obtained with the various chiral inorganic–organic auxiliaries (Table 1) are summarized in
Table 5.
The density of grafted ligands on the mineral surface can be expressed by the ratio of their number
− −1 −2
mol g ) to the surface area (m g ) of the parent MTS (N /S mol m ). Table 5 shows that densities
E
1
2
(
−2
are included between 0.9 and 1.4 mol m . A regular decrease of r with such a density is observed. For
0
example, the more loaded solid 3c presents the lower activity and enantioselectivity; in this case, r is
0
only five times higher than the rate of the reaction without catalyst. Such a solid is characterized by the
lowest available mesoporous volume (Table 1). It can then be concluded that the functionalization of the
same support leads to heterogeneous chiral auxiliaries whose activity may be related to the accessibility
of the catalytic sites.

3062
M. Laspéras et al. / Tetrahedron: Asymmetry 9 (1998) 3053–3064
With regard to the enantioselectivity, except for 3c, enantiomeric excess does not seem to be cor-
2
5
related with r . In heterogeneous enantioselective hydrogenation as in homogeneous asymmetric
0
26
dihydroxylation, a ligand-accelerated catalysis was noticed; for example, rate accelerations by a factor
of 10–100 were observed. In these cases, the ligand efficiency affected both initial rate and enantiomeric
purity. In our case, such a lack of correlation can be explained taking into account that the enantioselective
alkylation seems to be limited by the accessibility to the chiral catalytic sites. Another assumption could
be the participation of the mineral surface to the racemic alkyl transfer. On the one hand, internal diffusion
limitation may reduce the accessibility to the catalytic sites; moreover, the proximity of these sites may
be responsible for the formation of dimers (Scheme 3) which are relatively stable in homogeneous
catalysis. On the other hand, the activity of the inorganic surface can not be discarded compared to
the activities of the chiral solids (Table 2). However, the reactions were performed with similar weights
of catalysts and therefore, the participation of the uncovered surface would be of the same order of
magnitude for all the catalysts. Consequently, the effect of the accessibility and of the proximity of the
catalytic sites appears to be of prime importance.
16
The results we obtained, using a support which is characterized by a regular mesoporosity, are
comparable to those previously reported with silica gel as a support.10 Work is in progress to provide
a better internal diffusion of the reactants inside the pores and to study the effect of the inorganic surface
on the rates, selectivities and enantioselectivities of the alkylation by diethylzinc.
3. Conclusion
Immobilization of chiral auxiliaries on inorganic mesoporous templated silicas was carried out. New
solid auxiliaries were prepared and used in the enantioselective alkylation of benzaldehyde by diethyl-
zinc; as in homogeneous catalysis, the configuration of the chiral auxiliary controls the configuration
of the product. In order to increase the efficiency of these new solid auxiliaries and to compare
heterogeneous with homogeneous catalysis, various parameters were studied. They include the effect
of the stirring rate, of the amount of chiral auxiliary, of diethylzinc and benzaldehyde concentrations
and of the density of catalytic sites at the surface. Comparison with homogeneous catalysis shows that
various effects are identical (amount of chiral auxiliary, concentrations). The lower rates, selectivities and
enantioselectivities obtained in heterogeneous than in homogeneous catalysis are explained either by the
participation of the uncovered surface to the racemic alkyl transfer or by a restricted accessibility to the
catalytic sites in heterogeneous catalysis, the latter being certainly the most important.
4
4
4
. Experimental
.1. Immobilization of ephedrine over MTS
.1.1. Grafting of XPAS
MTS 1, prepared following a previously described procedure,11 was first calcined at 550°C and then
activated at 140°C for 16 h. Then, a suspension of freshly activated MTS 1 (1 g) in toluene (10 mL) was
refluxed and stirred for 2 h with XPAS (4.2 mmol) under dry nitrogen. After distillation in a Dean–Stark
collector of a fraction of toluene containing methanol (or ethanol), the mixture was again heated at
toluene refluxing temperature for 2 h and the distillation sequence was repeated. The modified solid was
filtered, extracted in a Soxhlet apparatus for 24 h with methanol, then dried at 100°C (solids 2a–e).

M. Laspéras et al. / Tetrahedron: Asymmetry 9 (1998) 3053–3064
3063
Elemental analysis: 2a: C 6.75, Cl 5.23%; 2b: C 7.58, Cl 4.78%; 2c: C 6.66, I 19.93%; 2d: C 6.74, Cl
.45%; 2e: C 6.22, Cl 4.57%.
5
4.1.2. Modification of the halogeno function by ephedrine
A suspension of the freshly activated modified solid (solids 2a–e) (1 g) in xylene (10 mL) was refluxed
and stirred with an excess of (−)- or (+)-ephedrine (9.6 mmol) for 6 h. The functionalized solid was
0
filtered, extracted according to the previous procedure, then dried at 100°C (solids 3a–e or 3a ).
0
Elemental analysis: 3a: C 14.93, Cl 2.43, N 1.15%; 3a : C 14.06, Cl 1.96, N 1.06%; 3b: C 12.98, Cl
0.92, N 1.21%; 3c: C 17.76, I 4.64, N 1.55%; 3d: C 13.91, Cl 2.68, N 0.99%; 3e: C 14.37, Cl 2.06, N
1.16%.
4.2. General procedure for the enantioselective addition of diethylzinc to benzaldehyde by using these
supported catalysts
The catalyst (8.5 mol%) was activated at 130°C under vacuum for 16 h. Then, 8 mL of 1.1 M solution
of diethylzinc in toluene (or 8.8 mL of 1 M solution of diethylzinc in hexane (2.3 mmol) was added under
a nitrogen atmosphere. The resulting mixture was stirred at 0°C for 15 min. Benzaldehyde (1 mmol) was
added dropwise in 2 mL of toluene. The reaction mixture was stirred at 0°C. The progress of the reaction
was monitored by periodically withdrawing samples which were analyzed by gas chromatography on a
chiral capillary column (LIPODEX E) after acidic treatment (HCl 0.5 M, CH Cl ). The final product was
2
2
analyzed after the same acidic treatment.
In order to study the effect of the diethylzinc concentration, the 1 M solution of diethylzinc in hexane
was used. This solution was first diluted and then concentrated by distillation of hexane under a nitrogen
atmosphere.
References
1
2
. Solladie, G. Asymmetric Synthesis, Morrison, J. D., ed.; Academic Press: New York, 1983; Vol. 2A, chapter 6.
. Noyori, R.; Suga, S.; Kawai, K.; Okada, S.; Kitamura, N.; Oguni, N.; Hayashi, M.; Kaneko, T.; Matsuda, M. J. Organomet.
Chem. 1990, 382, 19–37.
3
4
5
6
7
8
9
. Noyori, R.; Kitamura, N. Angew. Chem., Int. Ed. Engl. 1991, 30, 49–69.
. Soai, K.; Niwa, S. Chem. Rev. 1992, 92, 833–856.
. Soai, K.; Niwa, S.; Watanabe, M. J. Org. Chem. 1988, 53, 927–928.
. Watanabe, M.; Soai, K. J. Chem. Soc., Perkin Trans. 1 1994, 3125–3128.
. El Moualij, N.; Caze, C. Eur. Polym. J. 1995, 31, 193–198.
. Pugin, B.; Müller, M. Stud. Surf. Sci. Catal. 1993, 78, 107–114.
. Jorna, A. M. J.; Boelrijk, A. E. M.; Hoorn, H. J.; Reedijk, J. Reactive and Functional Polym. 1996, 29, 101–114.
10. Soai, K.; Watanabe, M.; Yamamoto, A. J. Org. Chem. 1990, 55, 4832–4835.
1
1. Beck, J. S.; Vartuli, J. C.; Toth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.;
Sheppard, E. W.; Mc Cullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834–10843.
2. Brunel, D.; Cauvel, A.; Fajula, F.; Di Renzo, F. Stud. Surf. Sci. Catal. 1995, 97, 173–180.
3. Cauvel, A.; Renard, G.; Brunel, D. J. Org. Chem. 1997, 62, 749–751.
1
1
1
1
1
1
1
1
4. Bellocq, N.; Brunel, D.; Laspéras, M.; Moreau, P. Stud. Surf. Sci. Catal. 1997, 108, 485–492.
5. Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723–1732.
6. Yamakawa, M.; Noyori, R. J. Am. Chem. Soc. 1995, 117, 6327–6335.
7. Soai, K.; Yokoyama, S.; Hayasaka, T. J. Org. Chem. 1991, 56, 4264–4268.
8. Conti, S.; Falorni, M.; Giacomelli, G.; Soccolini, F. Tetrahedron 1992, 48, 8993–9000.
9. Itsuno, S.; Fréchet, J. M. J. J. Org. Chem. 1987, 52, 4140–4142.

3064
M. Laspéras et al. / Tetrahedron: Asymmetry 9 (1998) 3053–3064
20. March, J. Advanced Organic Chemistry: Reactions, Mechanisms and Structure, 4th edn; John Wiley & Sons: New York,
1992, pp. 913–917.
2
2
2
2
2
1. Soai, K.; Niwa, S.; Watanabe, M. J. Chem. Soc., Perkin Trans 1 1989, 109–113.
2. Bond, G. C. Heterogeneous Catalysis: Principle and Applications, Clarendon Press: Oxford, 1974.
3. Kitamura, M.; Okada, S.; Suga, S.; Noyori, R. J. Am. Chem. Soc. 1989, 111, 4028–4036.
4. Chalonner, P. A.; Langadianou, E. Tetrahedron Lett. 1990, 31, 5185–5188.
5. Blaser, H.-U.; Jalett, H.-P.; Monti, D. M.; Reber, J. F.; Wehrli, J. T. Stud. Surf. Sci. Catal. 1988, 41, 153–163. Blaser, H.-U.;
Garland, M.; Jalett, H.-P. J. Catal. 1993, 144, 569–578.
26. Jacobsen, E. N.; Markö, I.; Mungall, W. S.; Schröder, G.; Sharpless, B. J. Am. Chem. Soc. 1988, 110, 1968–1970.