Generation of self-supported Noyori-type catalysts using achiral bridged-BIPHEP for heterogeneous asymmetric hydrogenation of ketones

Article abstract of DOI:10.1002/adsc.200606143

The programmed assembly strategy has been applied to the generation of self-supported Noyori-type catalysts for asymmetric hydrogenation of ketones by spontaneous hetero-coordination of an achiral bridged diphosphine and chiral bridged diamine ligands wit

Full text of DOI:10.1002/adsc.200606143

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DOI: 10.1002/adsc.200606143
Generation of Self-Supported Noyori-Type Catalysts Using
Achiral Bridged-BIPHEP for Heterogeneous Asymmetric
Hydrogenation of Ketones
Yuxue Liang,a Zheng Wang,a and Kuiling Dinga,*
a
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 354 Fenglin Road, Shanghai 200032, Peopleꢀs Republic of China
Fax : (+86)-21-6416-6128; e-mail: kding@mail.sioc.ac.cn
Received: March 24, 2006; Accepted: June 22, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The programmed assembly strategy has
been applied to the generation of self-supported
Noyori-type catalysts for asymmetric hydrogenation
of ketones by spontaneous hetero-coordination of
an achiral bridged diphosphine and chiral bridged
diamine ligands with RuII metal ions. The immobi-
lized catalyst demonstrates good enantioselectivity
and activity in the heterogeneous catalysis of the
hydrogenation of aromatic ketones and can be re-
covered and recycled for 4 times without obvious
loss of selectivity and activity.
and activity.[3e] In asymmetric catalysis, the common
practice for construction of chiral catalysts is the use
of enantiopure ligands to coordinate with catalytically
active metal ions.[4] On the other hand, the pioneering
works reported by Brown, Yamamoto, Faller and
Mikami demonstrated that asymmetric catalysis could
also be achieved by using racemic or achiral ligands
on the basis of the chiral poisoning[5] or asymmetric
activation concept,[6] obviating the use of expensive
enantiopure chiral ligands. Research by Mikami and
Noyori showed the feasibility of a combinatorial use
of pro-atropisomeric BIPHEP [BIPHEP: 2,2’-bis(di-
phenylphosphino)-1,1’-biphenyl] ligands and an enan-
tiopure diamine DPEN (DPEN: 1,2-diphenylethyle-
nediamine) (Scheme 1) with Ru(II) to generate
Noyori-type catalysts. The resulting complexes dem-
onstrated good to excellent enantioselectivities in the
catalysis of hydrogenation of ketones.[6c] Inspired by
Keywords: achiral ligands; asymmetric catalysis; hy-
drogenation; immobilization; ketones
Heterogenization of homogeneous chiral catalysts has Mikami and Noyoriꢀs discovery, herein we report on
been an important topic in the area of asymmetric the generation of self-supported catalysts by assembly
catalysis in the past decades.[1] Recently, we have of achiral bridged diphosphine and chiral bridged di-
demonstrated a conceptually new strategy, i.e., a amine with Ru(II) metal ion on the basis of our self-
“self-supporting” approach, for the immobilization of supporting strategy[2,3e] for heterogeneous asymmetric
homogeneous catalysts through self-assembly of chiral catalysis of the hydrogenation of aromatic ketones.
multitopic ligands and metal ions without the use of
The bridged bis-BIPHEP-type ligands 8a and 8b
any support.[2,3] On the basis of this strategy, the were designed to possess a conformationally flexible
chiral multitopic ligand can spontaneously form a 1,4-phenylene-bis-methoxy linker assembled at the 4’-
chiral environment inside the cavities of or on the sur- position of BIPHEP backbones. As shown in
face of the solids for enantioselective control of the Scheme 2, the multitopic ligand 8a was synthesized
reaction, and the metal ions act as the catalytically
active centers. For example, self-supported Noyori-
type catalysts generated by spontaneous hetero-coor-
dination of bridged BINAP [BINAP: 2,2’-bis(diphe-
nylphosphino)-1,1’-binaphthyl] and bridged 1,2-diaryl-
ethylenediamine ligands with Ru(II) metal ions exhib-
ited excellent enantioselectivity and activity in the
heterogeneous catalysis of the hydrogenation of aro-
matic ketones and could be recovered and recycled
for at least 7 times without obvious loss of selectivity Scheme 1. BINAP, BIPHEP and DPEN.
Adv. Synth. Catal. 2006, 348, 1533 – 1538
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Scheme 2. Synthesis of Bridged BIPHEP 8a–b.
from the cheap starting materials 4-methoxylphenol (Scheme 3). After removal of the solvent under
(1) and 4-methylphenol (2) in six steps. Oxidative vacuum, the resulting brown solids were washed with
cross-coupling of 1 with 2 by DDQ (DDQ: 2,3-di- 2-propanol three times to afford catalysts 10a and b
chloro-5,6-dicyano-1,4-benzoquinone) in the presence in nearly quantitative yields. These assembled cata-
of AlCl3 afforded 2,2’-biphenol 3[7] which, upon treat- lysts were found to be completely insoluble in 2-prop-
ment with trifluoromethanesulfonic anhydride (Tf2O) anol (as shown for 10b in Figure 1a). Elemental C/H/
in the presence of Et3N, yielded its ditriflate deriva- Cl/N/P analysis of the resulting solids were consistent
tive 4 in good yield. Pd-catalyzed phosphinoylation of with the composition of the expected structure 10.
4 with Ph2P(O)H proceeded smoothly in DMSO at Moreover, the solid-state 31P NMR (31P CP/MAS)
1008C to afford the corresponding bisphosphine oxide spectral data (see Figure S2 in Supporting Informa-
5a in 48% yield. Demethylation of 5a with BBr3 at tion) of the free ligand 2,2’-bis[di(3,5-xylphosphino)-
À788C provided phenol derivative 6a in 95% yield. 5-methyl-5’-methoxy]-1,1’-biphenyl (À21.5 ppm), an
The coupling of 6a with 1,4-xylylene dibromide in the authentic sample of monomeric complex (RuCl
presence of anhydrous K2CO3 resulted in the forma- bis[di(3,5-xylphosphino)]-5-methyl-5’-methoxy-1,1’-
tion of the bridged bisphosphine oxide 7a in 87% biphenyl}[(S,S)-1,2-bis-(4-methoxyphenyl)ethylenedi-
2{2,2’-
AHCTREUNG
yield. Subsequent reduction of 7a with trichlorosilane amine]) (43.6 ppm) and the assembled polymeric cat-
afforded the target ligand 8a in 77% yield alyst 10b (43.2 ppm) clearly indicate that the selective
(Scheme 2). Ligand 8b, a structural analogue of 8a formation of hetero-ligands complex has occurred ex-
with bulkier di(3,5-xylyl)phosphino moieties in the clusively in the assembly of two different multi-topic
BIPHEP units, was synthesized by following a similar ligands 8b and 9 with Ru(II) ions. The FT-IR spectra
procedure.
(see Figure S3 in Supporting Information) of the free
À
The self-supported catalysts 10a and b were pre- ligand 9 [n
pared by reacting the bridged bis-BIPHEP ligands 8a complex
and b with [(C6H6)RuCl2]2 in DMF at 808C, followed methyl-5’-methoxy-1,1’-biphenyl}[(S,S)-1,2-bis-(4-me-
A
(RuCl
A
R
À
by treatment of the resulting reddish brown solution thoxyphenyl)ethylenediamine])
[n(N H)=3328,
C
with
1
equivalent of bridged bis-DPEN 9[3e] 3259 cmÀ1], and the assembled polymeric catalyst 10b
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Self-Supported Noyori-Type Catalysts Using Achiral Bridged-BIPHEP
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Scheme 3. Preparation of self-supported catalysts 10a-b
[n
(N H)=3331, 3259 cmÀ1]also support the conclu- acetophenones gave rise to the hydrogenation prod-
À
sion mentioned above.[8b] SEM images showed that ucts in different ee values depending on the position
the solids were composed of micrometer particles of methyl substituent on the phenyl ring (entries 6, 7
(Figure 1b) and the powder X-ray diffraction (PXD) and 8). With electron-deficient 4’-chloroacetophenone
pattern (see Figure S1 in Supporting Information) in- as substrate, only 71.9% ee of the product was ob-
dicated their non-crystalline nature.
tained (entry 9). These observations suggest that the
The self-supported catalysts 10a and b were then catalytic behavior of 10b is sensitive to the structures
examined for the heterogeneous catalysis of the enan- of both the supramolecular assembly and the sub-
tioselective hydrogenation of aromatic ketones. As strates.
shown in Table 1, all the hydrogenation reactions
It is obvious that the enantioselectivities achieved
were carried out in 2-propanol at 258C for 24 h in the in the hydrogenation of ketones with the present het-
presence of t-BuOK under 40 atm of H2, with com- erogeneous catalysts 10a and b composed of chirally
plete conversions observed for all substrates. While flexible biphenylphosphane liagnds 8a and b are
catalyst 10a promoted the hydrogenation of acetophe- lower than those obtained with chiral BINAP-con-
none (11a) to afford 12a with only a moderate ee taining heterogeneous catalysts.[3e] This might be ex-
(46.2%, entry 1), the enantioselectivity of the same plained using Mikamiꢀs mechanistic Scheme outlined
reaction with catalyst 10b was dramatically enhanced on the basis of 1H NMR study of the monomeric com-
to 83.2% (entry 2). As a comparison, the control ex- plex of DM-BIPHEP/RuCl /
periment using the homogeneous counterpart of 10b, found that in Mikamiꢀs system a maximum of 3:1
(RuCl2{2,2’-bis-[di(3,5-xylylphosphino)]-5-methyl-5’- ratio of matched (S)-DM-BIPHEP/RuCl2/(S,S)-
methoxy-1,1’-biphenyl}[(S,S)-1,2-bis-(4-methoxyphe- DPEN to mismatched (R)-DM-BIPHEP/RuCl2/(S,S)-
2 (S,S)-DPEN. It was
ACHTREUNG
A
ACHTREUNG
nyl)ethylenediamine]), resulted in a slightly lower ee DPEN catalyst diastereomers could be observed in
in 12a (entry 3). Subsequently, the catalyst 10b was solution under the optimized conditions.[6f] While the
tested for the catalytic hydrogenation of a variety of higher enantioselectivity can be expected with the
aromatic ketones 11b–f, affording the corresponding major diastereomeric complex, the minor mismatched
secondary alcohols (12b–f) with enantioselectivities species would deteriorate the enantioselection of the
ranging from 71.9% to 86.9% ee. For the reaction of catalysis.[6e] Such a kind of incomplete asymmetric in-
1-acetonaphthone, the heterogenized catalyst 10b af- duction may also occur in the formation of the pres-
fords the hydrogenation product 12b with a lower ee ent heterogeneous catalysts containing conformation-
than that of its homogeneous counterpart (entry 4 vs. ally flexible bisphosphine ligands, which accordingly
entry 5). The reactions of three methyl-substituted lead to relatively lower enantioselectivities in the cat-
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Table 1. Enantioselective hydrogenation of aromatic ketones
11a–f under the catalysis of 10a and b.[a]
Entry
Catalyst
Ar in 11
ee [%] of 12[b]
1
2
3
4
5
6
7
8
9
10a
Ph (11a)
Ph (11a)
Ph (11a)
1-naphthyl (11b)
1-naphthyl (11b)
2’-Me-C6H4 (11c)
3’-Me-C6H4 (11d)
4’-Me-C6H4 (11e)
4’-Cl-C6H4 (11f)
46.2 (R)
83.2 (R)
82.1 (R)
77.7 (R)
85.1 (R)
81.4 (R)
86.9 (R)
74.9 (R)
71.9 (R)
10b
[c]
10b
[c]
10b
10b
10b
10b
[a]
All reactions were carried out at 258C under 40 atm pres-
sure of H2 at a substrate concentration of 1.5M with sub-
strate/catalyst/t-BuOK=1000/1/20 for 24 h. The conver-
1
sions of the substrates were determined by H 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
ACHTREUNG
5’-methoxy-1,1’-biphenyl}
AHCTREUNG
phenyl)ethylenediamine]).
not occur at all, confirming the heterogeneous nature
of the present catalytic system. On the basis of this
finding, the recovery and the reusability of catalyst
10b were then examined in the hydrogenation of 11a.
After the completion of the hydrogenation, the sepa-
ration of the catalyst and product could be achieved
by simple filtration under an argon atmosphere. The
recovered solid catalyst was recharged with solvent,
base and substrate for the next run of hydrogenation.
As shown in Table 2, the self-supported catalyst could
be reused four times in the hydrogenation without ob-
vious loss of enantioselectivities and catalytic activi-
ties.
Figure 1. (a) Self-supported chiral Ru(II) catalyst 10b
(brown solids at the bottom of the reactor) in 2-propanol.
(b) SEM image of the self-supported Ru catalyst 10b. The
scale bar indicates 1 mm.
Table 2. Recycling and reuse of the self-supported catalysts
10b in enantioselective hydrogenation of 11a.[a]
Run
1
2
3
4
5
Conv. [%]
>99
>99
>99
97
82
ee [%]
83.6
82.7
83.1
79.1
73.2
alysis. This limitation may possibly be overcome by
judicious choice of the achiral bisphosphine ligands
with complete asymmetric induction in the catalyst
formation.[9b,c]
[a]
All of the reactions were carried out under the experi-
mental conditions of entry 2 in Table 1.
The inductively coupled plasma atomic emission
spectrometric analysis (ICP-AES) of supernatant of
In summary, hetero-combination of two different
the catalyst indicated that no detectable Ru was multi-topic ligands (i.e., the conformationally flexible
leached into the organic phase (<0.1 ppm). When the achiral bridged bis-BIPHEP-type ligands and the
supernatant of catalyst 10b in 2-propanol was used as chiral bridged bis-DPEN) with Ru metal ions was
catalyst, the hydrogenation of acetophenone (11a) did used to generate self-supported chiral Ru catalysts,
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Self-Supported Noyori-Type Catalysts Using Achiral Bridged-BIPHEP
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extension of this strategy to the development of prac-
tical catalysts for asymmetric hydrogenation using
achiral bis- or monophosphine ligands[9] is underway
in this laboratory.
Experimental Section
Typical Procedure for Heterogeneous Asymmetric
Hydrogenation of Ketones using Self-Supported
Catalyst 10b and Catalyst Recycle
To a mixture of the Ru complex 10b (4.2 mg, 3.0 mmol based
on Ru) and potassium tert-butoxide (6.8 mg, 60 mmol) in a
test tube were added acetophenone (0.36 g, 3.0 mmol) and
anhydrous 2-propanol (2 mL) under argon. The test tube
was placed in a stainless steel autoclave, and the autoclave
was sealed under N2 before purging with hydrogen for 6
times. The autoclave was then charged with hydrogen (40
atm), and the reaction mixture was stirred at room tempera-
ture for 24 h. The remaining hydrogen was released slowly
and the catalyst was recovered by filtration through a can-
nula under N2. The solid catalyst was washed with anhy-
drous degassed 2-propanol and then was recharged with
substrate, potassium tert-butoxide and 2-propanol for next
run of the hydrogenation. After removal of solvent from the
filtrate under the reduced pressure, the residue was submit-
ted to 1H NMR analysis to assess the conversion of the
starting materials. The enantiomeric excesses of the products
were determined by chiral GC.
Acknowledgements
Financial supports from the National Natural Science Foun-
dation of China, Chinese Academy of Sciences, the Major
Basic Research Development Program of China (Grant no.
G2000077506), and the Science and Technology Committee
of Shanghai Municipality are gratefully acknowledged.
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