Recycling asymmetric hydrogenation catalysts by their immobilisation onto ion-exchange resins

Article abstract of DOI:10.1039/b406179a

New systems based on cationic chiral phosphine-rhodium complexes anchored to a commercial cation-exchange gel-type resin showed high efficiency and easy recycling in the asymmetric hydrogenation of prochiral olefins. The Royal Society of Chemistry 2004.

Full text of DOI:10.1039/b406179a

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C O M M U N I C A T I O N
Recycling asymmetric hydrogenation catalysts by their
immobilisation onto ion-exchange resins†
a
a
a
a
Pierluigi Barbaro,* Claudio Bianchini, Giuliano Giambastiani,* Werner Oberhauser,
b
c
d
Laura Morassi Bonzi, Filippo Rossi and Vladimiro Dal Santo
a
Istituto di Chimica dei Composti Organo Metallici - CNR, Area di Ricerca di Firenze,
Via Madonna del Piano, 50019 Sesto Fiorentino, Firenze, Italy.
E-mail: pierluigi.barbaro@iccom.cnr.it; E-mail: giuliano.giambastiani@iccom.cnr.it;
Fax: +39 055 5225203
Università di Firenze, Dipartimento di Biologia Vegetale, Via La Pira 4, 50121 Firenze, Italy
Istituto di Genetica Vegetale - CNR, Area di Ricerca di Firenze, Via Madonna del Piano,
b
c
5
0019 Sesto Fiorentino, Firenze, Italy
d
Istituto di Scienze e Tecnologie Molecolari - CNR, Via Golgi 19, 20133 Milano, Italy
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Table 1 Immobilisation of rhodium complexes on lithiated-DOWEX
0WX2 resina
New systems based on cationic chiral phosphine–rhodium
complexes anchored to a commercial cation-exchange gel-
type resin showed high efficiency and easy recycling in the
asymmetric hydrogenation of prochiral olefinsꢀ
5
b
Tethered catalyst
Rh loading (w/w) (%)
Rh immobilised (%)
Li-D 50WX2-1
Li-D 50WX2-2
1.04(1)
0.93(1)
68.7
68.2
Catalyst recovery is of paramount importance in sustainable fine
chemicals production, especially in large-scale asymmetric pro-
cesses where the cost of the chiral ligands often exceeds that of
a
Experimental conditions: resin 300 mg (1.44 meq. ion exchange capacity),
metal complex 37 mg (0.044 mmol), methanol 7.5 mL, 16 h, room tempera-
1
the noble metal employed. In order to easily recover and recycle
b
ture. ICP-AES, average value over three samples.
asymmetric catalysts, various techniques and materials have been
2
developed over the last twenty years. The preparation of single-site
recyclable catalysts can be achieved through the “heterogenisation
of homogeneous catalysts”, which involves the immobilisation of
preformed molecular precursors onto various support materials.3
This strategy offers the advantage to combine the high activity and
selectivity of the homogeneous catalysts with both a facile product
separation and use of environmentally friendly experimental condi-
tions. Successful examples in asymmetric hydrogenations include
the immobilisation of transition metal complexes via anchoring to
Scheme 1
4
5
6
insoluble supports such as silica (SHB), clays, zeolites, heteropoly
7
8–10
acids or ion-exchange resins. Herein, we report the preparation
and characterization of new catalytic systems based on chiral Rh(I)
complexes immobilized onto commercially available ion-exchange
resins as well as a preliminary study of their catalytic potential for
the stereoselective hydrogenation of prochiral substrates.
The [(PP*)Rh(NBD)]PF
6
complexes containing the chiral
diphosphine (+)-DIOP and (−)-TMBTP (1 and 2, respectively)
were used as molecular precursors (Scheme 1).‡ The strong cat-
ion-exchange, sulfonated gel-type resin DOWEX 50WX2-100
was used as anchoring agent.§ After smooth lithiation of the com-
mercial support, the immobilized catalysts were readily obtained
by stirring the resin with a methanol solution of the appropriate
Rh(I) complex (Scheme 2). Due to the cost of the chiral ligands, a
catalyst (mmol)/resin (meq. ion exchange capacity) ratio of ca. 3%
was used in each preparation. The amount of rhodium found in the
tethered catalyst indicated an effective anchoring of the complex
cation onto the resin. Table 1 summarises the metal uptake proper-
ties of the lithiated resin (Li-D 50WX2) as determined by ICP-AES
Scheme 2
performed by removing the reaction solution after the first catalysis
cycle, washing of the solid catalyst with pure methanol, followed
by addition of a second amount of substrate in methanol under hy-
drogen. Selected results are reported in Table 2. Results obtained
in the homogeneous phase under analogous reaction conditions are
provided in the same table for comparison.
Scheme 3
11
spectroscopy.
The efficiency of the supported catalysts (Li-D 50WX2-1 and
Li-D 50WX2-2) in the asymmetric hydrogenation of prochiral
olefins was tested using MAA as a probe substrate (Scheme 3).¶ All
Both catalysts gave conversions and enantiomeric excesses (ee’s)
in each catalytic cycle equal or comparable to those obtained in the
homogeneous phase. Particularly, the immobilised Rh–TMBTP
complex gave a complete conversion and stereoselectivity after
2, 10 and 12 h in the first, second and third hydrogenation cycle,
respectively (Table 2, entry 2), with almost constant activity after
the second recycle. A significant decrease of the reaction rates in
the second and following cycles is commonly observed for ion-
exchange resin-supported catalysts.8 In the present case, this
reactions were performed in methanol under 5 bar H
2
at room tem-
perature. Catalyst recovery and recycling were straightforwardly
†
Electronic supplementary information (ESI) available: Experimental
31
1
section, P{ H} HP NMR spectra, typical EDS surface area spectrum and
ESEM images. See http://www.rsc.org/suppdata/dt/b4/b406179a/
,10
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 1 7 8 3 – 1 7 8 4
1 7 8 3

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Table 2 Asymmetric catalytic hydrogenation of MAA using Rh(I) complexes anchored on lithiated-DOWEX 50WX2 resina
Heterogeneousb
First cycle
Second cycle
Homogeneousc
Tethered
catalyst
t/h
Yieldd
(%)
eed
(%)
Rh leachinge
(%)
t/h
Yieldd
(%)
eed
(%)
Rh leachinge Catalyst
(%)
t/h
Yieldd
(%)
eed
(%)
Entry
1
2
Li-D 50WX2-1
Li-D 50WX2-2
2.5
2.0
99.9 [38] 54.6
99.9 [52] 99.9
1.8(1)
2.0(1)
12
10
98.2 [8]
99.9 [11]
49.6
99.9
1.5(1)
2.0(1)
1
2
2.5
2.0
99.9 [40]
99.9 [50]
57.3
99.9
a
b
Reaction conditions: solvent MeOH, room temperature, H
2
5 bar. Tethered catalyst (50 mg), ca. 1% Rh w/w, Rh (ca. 0.0048 mmol), solvent (6.5 ml), 500 rpm,
c
d
−1
substrate/Rh mol ratio 100:1. Rh (0.0034 mmol), solvent (4 ml), 400 rpm, substrate/Rh mol ratio 100:1. Reaction mixture, GC, turnover frequency h (mol
product) (mol Rh h) in square brackets. Reaction solution, GF-AAS, average value over three samples.
−
1
e
effect may be attributed to a stronger binding interaction between
the sulfonate groups from the resin and the rhodium centres, after
NBD has been reduced and replaced by weaker ligands in the first
hydrogenation step. Indeed, in situ high-pressure (HP) NMR
spectroscopy showed that the hydrogenation of both 1 and Li-D
thenium) confirm the validity and versatility of the heterogenisation
protocol described in this work. Preliminary results show that the
ruthenium(II) complex [((−)-TMBTP)Ru(p-cymene)I]PF supported
on Li-D 50WX2 resin catalyses the hydrogenation of methyl aceto-
acetate to methyl (R)-3-hydroxybutyrate in methanol with yields and
ee’s (68 and 73%, respectively) comparable to those obtained in the
homogeneous phase and with very low Ru leaching.
6
1
2
5
0WX2-1 in MeOH resulted in the formation of a species that does
not contain hydride ligands and exhibits the same chemical shift of
+
12
the bis-solvento complex [((+)-DIOP)Rh(MeOH)
A
2
] (see ESI†).
P HP NMR study under catalytic conditions was carried out
in both homogeneous (using 1) and heterogeneous phase (Li-D
0WX2-1). In either case, resonances were detected at −40 °C con-
Thanks are due to the COST D24 action and to Prof. S. Recchia
for the GF-AAS analysis.
3
1
Notes and references
5
sistent with the formation of the catalyst–substrate complex [((+)-
‡
TMBTP = 4,4′-Bis(diphenylphosphino)-2,2′,5,5′-tetramethyl-3,3′-bithio-
+
DIOP)Rh(MAA)] together with a minor amount (2%) of phosphine
phene (P. Antognazza, T. Benincori, E. Brenna, E. Cesarotti, L. Trimarco
and F. Sannicolò, Eur. Pat., EP 0770085, 1996 (to Chemi SpA); DIOP =
1
2
oxide. Taking into consideration this small loss of catalyst due to
undesired oxidation, the tethered catalyst was quantitatively recov-
ered after each run. Indeed, the ESEM images of the catalyst beads,
before and after catalysis, showed that the support material was not
affected by signs of breakage or cracking (Fig. 1).
2
,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane;
NBD = bicyclo[2.2.1]hepta-2,5-diene.
DOWEX 50WX2-100 strong cation-exchange resin, H form, gel-type is
available from Aldrich (product n. 21,744-1).
+
§
¶
MAA = Methyl 2-acetamidoacrylate.
1
2
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(
left) and after (right) use in the catalytic cycles (backscattered electrons,
6
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4
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a truly heterogeneous reaction, no catalytic activity was shown by the
solutions recovered after the first and second catalytic cycle. There-
fore, the observed rhodium loss in each experiment can be safely
ascribed to phosphine oxidation. Surface X-ray EDS microanalysis
data supported the above results showing that the rhodium content in
the tethered catalyst to be essentially the same before and after catal-
ysis. Moreover, EDS maps recorded on sections of the catalyst beads
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heterogenised asymmetric catalysts are featured by: (i) use of inex-
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modification of the chiral ligand; (iii) easy preparation and handling
of the tethered catalyst; (iv) catalyst efficiency comparable to that
observed in the homogeneous phase; (v) very mild hydrogenation
conditions; (vi) facile and effective recovery and recycling; (vii) no
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6
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2
11 The rhodium content in the tethered catalysts estimated by surface area
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7 8 4
D a l t o n T r a n s . , 2 0 0 4 , 1 7 8 3 – 1 7 8 4