First heterogenisation of Rh–MeDuPHOS by occlusion in PDMS
(polydimethylsiloxane) membranes
Ivo Vankelecom,*a Adi Wolfson,b Shimona Geresh,b Miron Landau,b Moshe Gottliebb and Moti Hershkovitzb
a
Centre for Surface Chemistry and Catalysis, Faculty of Agricultural and Applied Biological Sciences, Katholieke
Universiteit Leuven, Kardinaal Mercierlaan 92, 3001 Leuven, Belgium.
E-mail: ivo.vankelecom@agr.kuleuven.ac.be
Blechner Center, Chemical Engineering Department, Ben-Gurion University of the Negev, PO Box 653,
b
Beer-Sheva, Israel
Received (in Cambridge, UK) 6th September 1999, Accepted 26th October 1999
The first heterogeneous system of Rh–MeDuPHOS, ob-
tained by occlusion of the complex in a PDMS membrane, is
reported and tested in the hydrogenation of methylacetoace-
tate (MAA).
Asymmetric synthesis with homogeneous chiral complexes is
one of the best methods to prepare enantiopure compounds.1 In
1991, Burk developed a set of new chiral biphosphine ligands
referred to as DuPHOS [1,2-bis(phospholano)benzene] and
BPE [1,2-bis(phospholano)ethane],2 both commercially availa-
ble nowadays. The Rh and Ru complexes of these ligands were
found to be extremely effective in the enantioselective hydro-
genation of a vast array of unsaturated substrates.3–8 All
Fig. 1 PDMS constituents.
reactions are characterized by high convenience, selectivity,
catalytic productivity and activity, adaptability and durability.
A major class of these substrates form the b-ketoesters. Their
reduction was recently reviewed by Klabunovskii and Sheldon.9
The authors stated that chiral metal complexes constitute the
most elegant and efficient way to asymmetric hydrogenation
with the broadest scope, but that facilitation of their recycling by
immobilizing them, still remained an important future chal-
lenge.
Fig. 2 Rh salt and ligand for Rh–MeDUPHOS synthesis.
We report here the first heterogenisation of Rh–MeDuPHOS,
chosen as a typical representative of the class of catalysts
reported by Burk et al.2–8 Immobilisation was realized by
occluding the catalyst in a polydimethylsiloxane membrane.
This easy approach to heterogenisation was earlier applied
already for several transition metal complexes.10–12 The
hydrogenation of MAA was selected as a test reaction (Scheme
1). A PDMS solution was prepared by mixing 0.075 g of
crosslinker [tetrakis-(dimethylsiloxy)silane, United Chemical
Technologies], 22 ml of Pt catalyst [cis-dichlorobis(diethyl
sulfide)platinum(ii), Aldrich, applied as a 2 wt% solution in
toluene], 2.25 g of the vinyldimethyl terminated silicone
polymer (Mn = 5350 with a polydispersity = 2.27, obtained
after stripping the polymer under vacuum till constant weight,
United Chemical Technologies), and 0.465 g of the silica (Hi-sil
233, pretreated at 120 °C, PPG Industries) in 11.5 g of
dichloromethane (Fig. 1). The catalyst was prepared by
dissolving the Rh salt [bis(cycloocta-1,5-diene)rhodium(i)
trifluoromethanesulfonate, 99%, Strem] and the ligand
{(2)-1,2-bis[(2R,5R)-2,5-dimethylphospholano]benzene, 99%,
Strem} separately in methanol (Fig. 2). An amount of 0.0397 g
of Rh salt and 0.0264 g of ligand were dissolved in 2.1 and 0.7
g of methanol, respectively. The ligand solution was then added
dropwise to the Rh salt solution under constant stirring. Further
stirring for 15 min turned the yellow solution into dark orange.
A portion of 0.174 g of this solution, corresponding to 5 mmol
of the Rh complex, was poured in a Petri dish and after removal
of the methanol, 1.5 g of the PDMS solution was added. After
complete dissolution of the complex in this mixture, the solvent
was allowed to evaporate. Approximately 3 h later, the curing of
the PDMS was finished and the resulting membrane was stored
under nitrogen prior to use in the reactor. A typical TEM image
of such a composite membrane is shown in Fig. 3, proving the
good dispersion of the silica particles in the membrane matrix.
A 20 ml reactor with magnetic stirring was used in which the
membrane was clamped between two porous stainless steel
disks. A volume of 18 ml of methanol was added to the reactor,
together with 2 g of MAA. Reaction took place at 60 °C at a
pressure of 40 bar during 24 h for the membrane reaction and 2
h for the homogeneous reference reaction. The reaction mixture
was analysed with gas chromatography (Chiraldex G-TA
column, Astec).
A homogeneous reaction (Table 1) was performed in order to
test the activity of the prepared catalyst and its ability to reduce
MAA. Even though attempts to utilize Rh–MeDuPHOS in
enantioselective ketone hydrogenations were reported to be
unavailing5 and in spite of the fact that Burk et al. found other
DuPHOS and BPE related catalysts performing better than Ru-
BINAP,3,4 we found for Rh–MeDuPHOS an activity in the
MAA hydrogenation at 60 °C that was even superior to that of
Ru–BINAP, a catalyst commonly used for this reaction.9,13,14
The membrane occluded catalyst (membrane thickness 452 mm)
was first tested in ethylene glycol (EG), the solvent preferen-
tially used for the analogous Ru–BINAP/PDMS system.15 The
activity of the catalyst was lower than in the homogeneous
reaction but remained constant in a second run, in which the
same enantioselectivity was maintained. No reactivity was
Scheme 1 Asymmetric hydrogenation of MAA.
Chem. Commun., 1999, 2407–2408
This journal is © The Royal Society of Chemistry 1999
2407
Vankelecom, Ivo
