Programmed assembly of two different ligands with metallic ions: Generation of self-supported Noyori-type catalysts for heterogeneous asymmetric hydrogenation of ketones

Article abstract of DOI:10.1021/ja050737u

Programmed assembly strategy has been first applied to the generation of self-supported Noyori-type catalysts for asymmetric hydrogenation of ketones by spontaneous heterocoordination of bridged diphosphine and diamine ligands with Ru(II) metallic ions. The immobilized catalyst demonstrates excellent enantioselectivity and activity in the heterogeneous catalysis of the hydrogenation of aromatic ketones and can be recovered and recycled at least seven times without obvious loss of selectivity and activity. Copyright

Full text of DOI:10.1021/ja050737u

Published on Web 05/04/2005
Programmed Assembly of Two Different Ligands with Metallic Ions:
Generation of Self-Supported Noyori-type Catalysts for Heterogeneous
Asymmetric Hydrogenation of Ketones
Yuxue Liang, Qing Jing, Xin Li, Lei Shi, and Kuiling Ding*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 354 Fenglin Road, Shanghai 200032, P. R. China
Received February 4, 2005; E-mail: kding@mail.sioc.ac.cn
Scheme 1. Schematic Representation of Strategies for the
Generation of Self-Supported Enantioselective Heterogeneous
Catalysts
The immobilization of chiral catalysts for asymmetric reactions
as one of the most promising solutions to the problems associated
with the difficulties in the recovery and reuse of expensive
homogeneous catalysts, as well as the product contamination caused
by metal leaching, has attracted a great deal of interest recently.1
Among various strategies for homogeneous catalyst immobilization,
“self-supported” chiral catalysts exhibited some salient features,
including facile preparation, remarkable stability, high density of
catalytically active units, and excellent stereocontrol performance,
as well as simple recovery and reuse.2 With this strategy, chiral
catalysts could be heterogenized by a homocombination of multi-
topic chiral ligands with metallic ions (Scheme 1a) without using
any supports.
Scheme 2
Although assembly of a chiral catalyst by heterocombination of
different multitopic chiral ligands with metal ions (Scheme 1b)
could, in principle, provide a viable alternative to the route in
Scheme 1a, no work has been done to demonstrate the validity of
this strategy. The major challenge to this goal is that a complex
multispecies system would be formed when the three reacting
components were mixed together. Thus, selective formation of a
heteroligand complex would require the structural and coordination
information stored in the ligands and metallic ion, respectively, to
be sufficiently strong to dictate their coordination organization and
thus direct the assembly process in a programmed way.3 In this
respect, the structural feature of Noyori’s catalyst,4 [RuCl2{(R)-
BINAP}{(R,R)-DPEN}] (BINAP ) 2,2′-bis(diphenylphosphino)-
1,1′-binaphthyl; DPEN ) 1,2-diphenylethylenediamine), may pro-
vide an excellent opportunity for generation of self-supported
catalysts by the approach shown in Scheme 1b, due to the selective
incorporation of a chiral diphosphine and a chiral diamine in a Ru-
(II) complex. In the present communication, we report our
preliminary results on the generation of a self-supported Noyori-
type catalyst for asymmetric hydrogenation of ketones by pro-
grammed assembly of bridged diphosphine (1) and diamine (2)
ligands with Ru(II) metallic ions (Scheme 2).
As shown in Scheme 2, both bridged BINAP 1 and diamine 2
were designed to possess 1,4-phenylene and a 1,4-phenylenebis-
methoxy linkers, which were assembled at the 6-position of the
corresponding 1,1′-binaphthyl backbone or at the 4′-position of the
(S,S)-DPEN derivative, respectively, to avoid the intramolecular
interaction of two chiral units. The self-supported catalysts (3a-
b) were prepared by reacting bridged BINAP ligands (1a,b) with
[(C6H6)RuCl2]2 in DMF at 100 °C, followed by the treatment of
the resulting reddish brown solution with 1 equiv of bridged DPEN
2 at room temperature. After removal of the solvent under vacuum,
the resulting solids were washed with 2-propanol three times to
afford catalysts 3a,b as pale brown solids in nearly quantitative
yields. Elemental C/H/N/Cl/P and Ru analyses, FT-IR, and 31P CP/
MAS showed that the composition of the resulting solids was
consistent with the structure of 3 as expected. SEM images showed
that the solids were composed of micrometer particles (ca. 1 µm),
and the powder X-ray diffraction (PXD) indicated they were
noncrystalline solids.
The self-supported catalyst 3 was then submitted to the catalysis
of the hydrogenation of acetophenone (4a). As shown in Table 1,
catalyst 3a promoted the hydrogenation of acetophenone (4a) to
afford 5a with 78.2% ee (entry 1). This level of enantioselectivity
is comparable to that observed in the homogeneous catalysis using
[RuCl2{(R)-BINAP}{(R,R)-DPEN}] (∼80% ee of 5a) under similar
conditions.4b Inspired by this initial result, catalyst 3b, which
contains di(3,5-xylyl)phosphino moieties in the BINAP units, was
then employed for the hydrogenation of 4a. The enantioselectivity
of the reaction was remarkably enhanced to 97.4% with complete
conversion of 4a (entry 2). In contrast, the catalysis of hydrogena-
tion of 4a using the homogeneous counterparts resulted in the
formation of 5a in relatively lower enantioselectivity (95.5-96.4%)
(entries 3 and 4 versus 2). The enantioselectivity of the reaction
under the catalysis of the self-supported catalyst generated using
1b and the enantiomer of 2 dropped significantly, indicating a mis-
matched case of ligand combination (entry 5). All of these facts
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J. AM. CHEM. SOC. 2005, 127, 7694-7695
10.1021/ja050737u CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Enantioselective Hydrogenation of Aromatic Ketones
4a-h under the Catalysis of 3a,ba
Table 2. Recycling and Reuse of the Self-Supported Catalyst 3b
in Enantioselective Hydrogenation of 3ca
run
1
2
3
4
5
6
7
conv (%) >99
ee (%) 97.4
>99
97.6
>99
97.3
>99
96.5
>99
95.6
>99
97
96.1 95.4
a All of the reactions were carried out under the experimental conditions
of entry 2 in Table 1.
entry
catalyst
Ar in 4
ee (%) of 5b
1
2
3
4
5
6
7
8
9
10
11
12
13
3a
3b
c
d
e
3b
3b
3b
3b
3b
3b
3b
3bf
Ph (a)
Ph (a)
Ph (a)
Ph (a)
78.2 (R)
97.4 (R)
95.5 (R)
96.4 (R)
30.5 (R)
98.1 (R)
94.5 (R)
96.2 (R)
96.9 (R)
97.2 (R)
97.5 (R)
96.2 (R)
95.2 (R)
catalyst 3b were then examined in the hydrogenation of 4a. After
the completion of the hydrogenation, the separation of the catalyst
and product (Ru leaching in product <0.1 ppm) could be achieved
by simple filtration under Ar atmosphere. The separated solid
catalyst was recharged with solvent, base, and substrate again for
the next run of hydrogenation. As shown in Table 2, the self-
supported catalyst could be reused for seven cycles of hydrogenation
without obvious loss of enantioselectivities and catalytic activities.
In conclusion, we have demonstrated a new approach for
generation of self-supported chiral catalysts by programmed as-
sembly of two different multitopic ligands with metallic ions. The
self-supported heterogeneous Noyori-type catalysts are comparable
to those of their homogeneous counterpart in terms of both activity
and enantioselectivity, although several excellent organic polymer-
supported Ru(II) catalysts have been reported.5 This type of catalyst
can be readily recovered and reused with the retention of high
enantioselectivity and activity. Further studies on the simplification
of the synthetic procedures of multitopic ligands and the impact of
a spacer in the multitopic ligands on both the structure of assemblies
and their catalytic performance are underway in this laboratory.
Ph (a)
1-naphthyl (b)
2-naphthyl (c)
4′-F-Ph (d)
4′-Cl-Ph (e)
4′-Br-Ph (f)
4′-Me-Ph (g)
4′-MeO-Ph (h)
Ph (a)
a All reactions were carried out at 25 °C under 40 atm pressure of H2 at
a substrate concentration of 1.5 M with substrate/catalyst/KOtBu ) 1000:
1:20 for 20 h. The conversions of the substrates were determined by 1H
NMR to be >99%. b Determined by chiral GC. The absolute configuration
of the products was determined by the sign of optical rotation. c Catalyst:
[RuCl2{(S)-3,5-XylBINAP}{(S,S)-1,2-bis-(4-methoxyphenyl)ethylenedi-
amine}]. d Catalyst: [Ru2Cl4{1b}{(S,S)-1,2-bis-(4-methoxyphenyl)ethyl-
enediamine}2]. e Catalyst: [RuCl2{1b}{(R,R)-2}. f Substrate/catalyst/KOtBu
) 10000:1:20.
Acknowledgment. Financial support from the NSFC, CAS, the
Major Basic Research Development Program of China (Grant No.
G2000077506), and the Ministry of Science and Technology of
Shanghai Municipality is gratefully acknowledged.
Supporting Information Available: Experimental procedures for
ligand and catalyst preparation, PXD image of catalyst 3b, and chiral
GC analysis of the products. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 1. (a) Self-supported chiral Ru(II) catalyst 3b (pale brown solids
at the bottom of the reactor) in 2-propanol. (b) SEM image of the self-
supported Ru catalyst 3b. The scale bar indicates 2 µm.
References
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demonstrated the synergistic impact of the supramolecular archi-
tecture of the assemblies on their catalytic behaviors. Furthermore,
the catalyst 3b was used to catalyze the hydrogenation of a series
of aromatic ketones, 4b-h, affording the corresponding secondary
alcohols (5b-h) with excellent enantioselectivities (entries 6-12).
Moreover, acetophenone (4a) could also be hydrogenated at a
reduced catalyst loading (0.01 mol % of 3b) to give 1-phenylethanol
with complete conversion and 95.2% ee (entry 13). The TOF under
this circumstance is calculated to be ∼500 h-1, which illustrated
the high activity of the assembled solid catalyst.
The self-supported catalyst 3b has proven to be completely
insoluble in 2-propanol, as shown in Figure 1a. The inductively
coupled plasma (ICP) spectroscopic analysis of the supernatant of
the catalyst indicated that no detectable Ru leached into the organic
phase (<0.1 ppm). When the supernatant of catalyst 3b in
2-propanol was used as catalyst, the hydrogenation of acetophenone
(4a) did not occur at all. This experiment unambiguously demon-
strated the heterogeneous nature of the present catalytic system.
On the basis of this finding, the recovery and the reusability of
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