Use of heterogenized dialdimine ligands in asymmetric transfer hydrogenation

Article abstract of DOI:10.1016/S0957-4166(98)00067-6

Heterogenized dialdimine ligands complexed to iridium were tested in the asymmetric transfer hydrogenation of acetophenone. E.e.s of up to 70% were achieved but the recycling was unsatisfactory. When such ligands were used in asymmetric epoxidation of styrene, a modest e.e. of 15% was achieved.

Full text of DOI:10.1016/S0957-4166(98)00067-6

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 897–900
Use of heterogenized dialdimine ligands in asymmetric transfer
hydrogenation
Emmanuelle Breysse, Catherine Pinel and Marc Lemaire ∗
Institut de Recherches sur la Catalyse, Laboratoire de Catalyse et Synthèse Organique (U.C.B.L.-C.P.E.), 43 bld du 11 nov.
1918, 69622 Villeurbanne Cedex, France
Received 17 December 1997; accepted 12 February 1998
Abstract
Heterogenized dialdimine ligands complexed to iridium were tested in the asymmetric transfer hydrogenation
of acetophenone. E.e.s of up to 70% were achieved but the recycling was unsatisfactory. When such ligands were
used in asymmetric epoxidation of styrene, a modest e.e. of 15% was achieved. © 1998 Elsevier Science Ltd. All
rights reserved.
Nowadays, due to their availability, lower cost and toxicity, the use of chiral dinitrogen ligands is
of increasing interest in asymmetric synthesis. As examples, chiral bisoxazolines chelated to rhodium or
iridium were efficient for hydrosilylation or reduction of ketones; chelated to copper, they have been used
for asymmetric cyclopropanation of styrene with alkyl diazoacetates. Bisoxazoline palladium complexes
have been used as enantioselective catalysts for allylic nucleophilic substitution.1 We have developed
the use of chiral diamines complexed to rhodium, iridium and even cobalt for asymmetric reduction of
C_C or C_O bonds.2 Jacobsen developed the use of chiral bis(salicylidene)ethylenediamine manganese
complexes in the asymmetric epoxidation of cis double bonds.3 Similar aldiminato–cobalt complexes
allowed borohydride reduction of ketones with high enantiomeric excess.4 Our interest in preparing
configurationally heterogenized ligands prompted us to synthesize polymerizable dialdimine ligands
from hydroxy or dihydroxy benzaldehyde. Copolymerization with styrene and divinylbenzene (DVB) and
their use in asymmetric iridium-catalyzed transfer hydrogenation or manganese-catalyzed epoxidation
will be discussed.
Polymerizable ligands were synthesized by reacting 4-chloromethylstyrene with 4-hydroxybenzalde-
hyde or 2,4-dihydroxybenzaldehyde, and then the diimine was obtained after condensation with the
enantiomerically pure (1R,2R)-cyclohexanediamine (Scheme 1).5 Modest non-optimized global yields
of 30–40% were achieved.
∗
Corresponding author. E-mail: marc.lemaire@univ-lyon1.fr
0957-4166/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00067-6

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E. Breysse et al. / Tetrahedron: Asymmetry 9 (1998) 897–900
Scheme 1. Synthesis of dialdimine ligands
Table 1
Preparation of polymerized diimine6
The monomers 3a and b were then copolymerized with various quantities of styrene and/or divinylben-
zene in order to prepare polymers with different degrees of crosslinking. These polymers were obtained
in reasonable yields (Table 1).
These ligands were first tested for the asymmetric reduction of acetophenone with iso-propanol as
the hydride source. As the iridium gave the better results in terms of conversion and enantioselectivity
we focussed only on this metal (attempts to realize the reduction with cobalt or ruthenium-based
catalysts were unsuccessful). The influence of the polymeric nature and the recycling of the catalyst
were evaluated. Amazing increases of enantioselectivity were achieved using the polymerized diimine
compared to the homogeneous ligand (entries 1 and 4, Table 2). In addition, this higher selectivity was
not observed when the complex was prepared previously before the addition of the substrate (entries
2 and 3): lower enantioselectivity, similar to that obtained in homogeneous conditions, was achieved.
Probably different active species are involved.

E. Breysse et al. / Tetrahedron: Asymmetry 9 (1998) 897–900
899
Table 2
Catalytic hydrogen transfer reduction of acetophenonea
From the influence of the nature of the polymer it is clear that there is an effect due to the degree of
crosslinking: when it increased from 15% to 71% (entries 4 and 5) a higher enantiomeric excess was
observed. However the rise in the degree of crosslinking from 71 to 100% gave no significant difference
(entries 5 and 6).
The test of the catalyst recycling was performed with a ligand to metal ratio of 10 to 1, as this ratio
affects neither the conversion nor the enantioselectivity (entries 5 and 7). The catalyst was carefully
separated from the reaction mixture and reused in a new reaction (entries 7 and 8). Unfortunately we
noticed an important loss in activity and a lower enantiomeric excess. It seems that there is partial
leaching of the metal in solution. We are not able at the moment to improve the stability of the
heterogenized complex.
As the best result in terms of conversion and enantioselectivity was obtained with the 3a-based polymer
prepared with a 71% degree of crosslinking (polymer 6, entry 5), we prepared a similar polymer starting
from ligand 3b (polymer 7). Unfortunately, lower conversion and enantioselectivity were achieved (entry
9). Therefore we decided to test this ligand 3b and its polymerized analogue 7 in manganese-catalyzed
asymmetric epoxidation of styrene as done recently with a similar compound.7 A combination of m-
chloroperbenzoic acid and N-methyl-morpholine-N-oxide was chosen as the oxidation system (Table 3).8
These preliminary results showed that lower conversion and enantioselectivity were obtained using the
ligand 3b compared to the Jacobsen’s ligand. Low improvement of the enantioselectivity was achieved
using the heterogenized ligand 7. The same value was described in a recent work using an analogous
polymeric catalyst.7 The design of the ligand has to be improved in order to obtain better results in
heterogeneous conditions and to recycle the catalyst.

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E. Breysse et al. / Tetrahedron: Asymmetry 9 (1998) 897–900
Table 3
Asymmetric epoxidation of styrene
Nevertheless, this work showed that it is possible to prepare easily polymerizable imines and to use
such polymers in asymmetric reduction or oxidation reactions even if the structures of the monomers
have to be optimized in order to increase the enantioselectivity of the reactions.
References
1. Müller, D.; Umbricht, G.; Weber, B.; Pfaltz, A. Helv. Chim. Acta 1991, 74, 232; Tanaka, H.; Ohta, T.; Noyori, R.
Asymmetric Synthesis, Ojima, I., ed.; VCH: New York, 1993, pp. 1–33; Jacobson, E. N. Asymmetric catalytic epoxydation
of unfunctionalised olefins, ibid.
2. ter Halle, R.; Bréhéret, A.; Schulz, E.; Pinel, C.; Lemaire, M. Tetrahedron: Asymmetry 1997, 8, 2101.
3. Jacobsen, E. N.; Zhang, W.; Muci, A. R.; Ecker, J. R.; Deng, L. J. Am. Chem. Soc. 1991, 113, 7063.
4. Nagata, T.; Sugi, K. D.; Yamada, T.; Mukaiyama, T. Synlett 1996, 1076.
5. Fujii, Y.; Matsutani, K.; Kikuchi, K. J. Chem. Soc., Chem. Commun. 1985, 415; Mendelson, W. L.; Holmes, M.; Dougherty,
J. Synthetic Commun. 1996, 26, 601.
6. Typical experiment (polymer 6): In a flask containing 15 mg of AIBN, a solution of 100 mg of 3a, 93 µL of styrene and 145
µL of divinylbenzene were added. The flask was flushed with argon, then it was closed and the polymerization was carried
out at 50°C for 48 h. The solvent was removed in vacuo and the polymer was crushed, washed twice with isopropanol (30
mL) and dried in vacuo (m=160 mg, 56% yield).
7. Minutolo, F.; Pini, D.; Salvadori, P. Tetrahedron Lett. 1996, 37, 3375.
8. Pakucki, M.; Pospisil, P. J.; Zhang, W.; Jacobsen, E. N. J. Am. Chem. Soc. 1994, 116, 9333.