Gang LU

literature seminar on 080426

Asymmetric Heterogeneous Catalysis
Figure 1. Catalysis in chemical industry: heterogeneous vs. homogeneous
noncatalytic or
onlyheterogeneous catalysis

gas (H2, CO)gasline and other fuels

crude oil
Carbon-containing feed stockcoal
natural gas
other oils

ethylene and propylene

Ni/Al2O3 or MgOPt, Rh/Al2O3
zeolites......

aromatics and higher olefins

important chemicals
homogeneous catalysis
vs.
heterogeneous catalysis

basic building blocks
for most chemicals"In terms of total tonnage and dollar value, the contribution of homogeneous catalytic processes in the chemicalindustry is significantly smaller than that of heterogeneous catalytic reactions. All the basic raw materials orbuilding blocks for chemicals are manufactured by a small but very important set of heterogeneous catalytic
reactions. Roughly 85% of all catalytic processes are based on heterogeneous catalysis."(Bhaduri S. and Mukesh D., Homogeneous Catalysis: Mechanisms and Industrial Applications 2000, pp1−8.)
Advantages of heterogeneous catalysis:
easy seperation, efficient recycling, mimization of metal leaching, improved handling and process control, low costeven higher selectivity than homogeneous ones in some cases

Achiral heterogeneous catalyst

B*

Pioneer reviews on asymmetric heterogeneous catalysis:a) Blaser, H. U. Tetrahedron Asymmetry 1991, 9, 843;b) Chem Review 2002 102, 3215−3892;c) Hutchings, G. Chem. Soc. Rev. 2004, 33, 108;d) Glorius, F. et al. Angew. Chem. 2006, 45, 4732.

Today's topics:
1. Classical immobilization of chiral homogeneous catalysts
Immobilized (Supported) Catalysis
2. Self-assembly chiral metal-organic catalysts
3.1 Chiral modifiers
3. Catalysis at Chiral Surfaces
3.2 Macromolecular catalysis
Chiral substrate-induced catalysis, others

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1. Classical immobilization of chiral homogeneous catalysts
Figure 2. Strategies for immobilization of chiral homogeneous catalysts
solid support

Covalent
linkageligandMn

Noncovalent linkage

on polymeric resins

by copolymerization

on inorganic supports

The choice of a suitable support plays an important, although not fully understanding, role.
Possible problems can diminish the performance of catalyst:♦ undesired interactions between of the support and the homogeneous complex → low enantioinduction;♦ unstability of the linkage between the catalyst and support or the catalyst itself → metal leaching;♦ limited accessibility of the active site from substrate → low reactivity.
To achieve minimal levels of interaction, which has been the predominant strategy, the anchoring point in
the ligand structure should be as far away from the active site of the catalyst as possible. Furthermore, along and flexible linker between the catalyst and support or a highly swellable polymer should be chosen.
However, the catalyst can also be attached to the support in proximity to the active site, which has led to
improved catalytic performances in a few cases

1.1 Covalently immobilized catalysts on polymeric resins♦ Merrifield resins: poly(styrenedivinylbenzene) polymers♦ JandaJEL: polystyrene polymers containing a tetrahydrofuran-derived crosslinker♦ TentaGel: polystyrene-polyethyleneglycol-OC2H4-NHCOC2H5♦ PS-PEG: polystyrene polyethyleneglycol resins
Scheme 1.
linker1
Trost ligand

Han, A. H. ACIE 2002, 41, 3852.
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Scheme 2. Polystrene(PS)-bound binap ligandusing 3 (80 ppm loading)34

Noyori et al. Adv. Synth. Catal. 2001, 343, 369.
4

key intermediate for fragrance
and flavour industry

5Lemaire et al. Helv. Chim. Acta. 2001, 84, 230.

6Scheme 3. Immobilized Co-Salen complex
0.25 mol %8over 5 cycles
79
yield: 51−52%
ee: 92−95%

10ee: > 99%

Jacobsen et al.JACS 1999, 121, 4147.
toluene-HOAc washingresiduereaction mixture
filtrate

under airwater layer

active catalyst 7
diol 9product 10organic layer

7131112

Jacobsen et al.OL 1999, 1, 1245.

Scheme 4. Immobilized Pd-phosphine complex

OCOOMeX
1514Uozumi et al.PAC 2007, 79, 1481.

Nu

NuH (16), base, H2O
314 [PdCl(η -C3H5)]2X(2−10 mol % complex)

17

X = CH2, C2H4, NBoc

yield: up to 99%
ee: up to 99%

NuH: malonate, Bn2NH, phenol

1.2 Covalently immobilized by copolymerization♦ Radical polymerization of vinyl-modified ligands with styrene and divinylbenzene♦ Polymerization of amines with isocyanates

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Scheme 5. Copolymerization of pybox ligands

styrene18
divinylbenzene(cross linker)
x% = degree of cross-linkage

accessibility of aactive site

activity and selectivity

immobilized ligand[RuCl2(p-cymene)]219
x = 2: trans/cis: up to 90/10, ee (trans): up to 91

recycling use 6 times

Mayoral, J. A. JOC 2005, 70, 5536.

3.6 mol% 20
2 mol% Cu(OTf)2
yield: 60%, trans/cis: 67/33, ee: 93% (trans), 90%(cis)20 was found swelling in a solution of Cu(OTf)2 in THF. The
rapid uptake of 0.18-0.20 mmol/g of the metal salt took place,demonstrating that about 60% of the supported box units couldbe readily engaged in the metal complexation.
Salvadori et al. CC 2003, 2466.

202121(glyoxylate-ene reaction)

Salvadori et al. TA 2004, 3233.

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Scheme 6. Immobilization of binap with diisocyanate

232224

Lemaire et al. BMCL 2002, 12, 1841.
Lemaire et al. TA 2002, 13, 1141.

25
rigidity increasing
232425
Scheme 7. Salen copolymers

26282729

Kim et al. Catalyst Today 2003, 87, 145.
used for the Co-catalyzed hydrolytic kinetic resolution of epoxide, > 96% ee.
These copolymers could also be recycled and reused up to 7 times without any loss of activity andselectivity, and do not need to be regenerated.
1.3 Covalent immobilization on inorganic support♦ zeolites, amorphous silica gel♦ mesoporous silicas (2~50 nm pore size):
MCM-41 ("mobile crystalline material", ordered hexagonal, 3~4 nm pore diameter)
SBA-15 (ordered hexagonal, 1.2~3 nm pore diameter)♦ crystalline nanoparticles
advantage: rigid structure avoids the aggreation of active catalyst; insoluble in organic solvents
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Scheme 8.

silanol moiety
linker303132
33
31
Tu,Y. Q. OL 2004, 6, 169.
Ru:ligand:ketone = 1 : 1.7 : 100
Figure 4.

Figure 3.

34
super-paramagnetic
magnetite crystalline nanopart

dpen on carbosil
dpen in MCM-41
restricted approach of the reactant
to the active catalyst in concave porehomohetero

recycled by magnetic decantation

Thomas, B. F. G. ACIE 2003, 42, 4326.

Lin et al. JACS 2005, 127, 12486.

0.1 mol % catalyst loading

Figure 5.
5 mol %
Silica-supported BINOL(1.5 eq)

up to 94% ee
Abdi,SHR J. Mol. Cat. A 2008, 280, 106.
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Silica-supported BINOLScheme 9. Ru complex immobilized in a pore of SBA-15

Lin et al. CC 2004, 2284.

Scheme 10.

3535 35
Corma et al. CC 2002, 1058.

For other similar examples of recent, see:a) J. Cat. 2008, 254(1), 91.
b) J. Mol. Cat. A. 2008, 280(1-2), 106.
c) JOC 2007, 72, 9353.
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1.4 Noncovalent immobilization by adsorption♦ physisorption via van der Waals interaction (synthetic modification of ligand is no necessary)♦ immobilized by H-bonding on a polar support (eg. SiO2, stability is improved)♦ SAPC (supported aqeous-phase catalyst, immobilized on a hydrophilic support such as SiO2)
Figure 6.
H-bonding of support with ligand

H-bonding of support with counterion

Figure 7. SAPC model

Scheme 11.
CPG-240: controlled pore glass-240, pore diameter 242 Å363736

naproxen

without CPG-240 support: < 2% conversion

Davis et al. Nature 1994, 370, 449.
dissolved in ethylene glycol and adsorbed on CPG-240
ethylene glycol was the hydrophilic liquid phase containing the chiral catalyst,
which effectively prevents the leaching of the complex into the organic phase

1.5 Noncovalent immobilization by ion exchange
Ion exchange between a chiral, cationic metal¨Cligand complex and an acidic resin represents anelegant method for immobilization through electrostatic interactions. Moreover, ion exchange is theonly method that allows the direct immobilization of the metal itself. Therefore, it is a method ofchoice for the recycling of expensive or very toxic metal derivatives such as osmium tetroxide.
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Figure 8. Immobilization of OsO42−− by ion exchange(O = OH)z 2O layers,
LDH: layered double hydroxides, it contains cationic M(II)1-xM(III)x(OH)2x+ and anionic An−• H
the exchange of Cl− for OsO42− in Mg1-xAlx(OH)2(Cl)x• zH2O (x = 0.25) gives LDH-OsO4

Choudary et al. JACS 2001, 123, 9220.

(NMO: N-methylmorpholine-N-oxide)

1.6 Noncovalent immobilization by encapsulation or others
The encapsulation of chiral catalysts in a support, often referred to as ¡°ship in a bottle¡± (Figure 2),
is the only type of immobilization that does not require any favorable interaction between themetal¨Cligand complex and the support. In general, chiral complexes can either be successively
assembled in the pores of a mesoporous material or the presynthesized complex is entrapped bypolymerization or in a polydimethylsiloxane (PDMS) film. In both cases, reaction conditions arerequired that are well tolerated by the support and the chiral complex.
Scheme 12.

38
Geresh et al. CC 2002, 388.
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Scheme 13.

39 (0.5 mol %)Davis et al. JACS 2004, 126, 4271.
39♣ Summary
By choosing a suitable support, especially mesoporous materials, a heterogeneous catalyst can beprepared that gives similar or even enhanced selectivities and activities. In other words, the role of thesupport has changed from an inevitable appendage to a well-defined material that can be used tobeneficially influence the outcome of a catalyzed reaction. Is a covalent or a noncovalent immobilizationof the catalyst preferential? For a long time, covalent immobilization of chiral complexes was unrivaled
because of the stability and recyclability of the resulting catalysts. In contrast, catalysts prepared by theoften synthetically more facile noncovalent immobilization strategy most often suffered from severe
stability problems. However, recent results with cationic complexes immobilized by surface-supported
counteranions or ion exchange have demonstrated that these noncovalently immobilized catalysts canshow good stabilities, can be recycled several times, and in addition result in good selectivities andactivities.

2. Self-assembly chiral metal-organic catalysts
This new immobilization technique is based on the skillful application of multitopic ligands and metals
and allows for a simple and efficient assembly of solid metal¨Corganic structures by complexation
without the need additional support (Figure 9). This self-assembly can give highly porous, in some cases
very regular, coordination polymers that mainly consist of the metal and the chiral ligand.
Figure 9. Three types of chiral metal-organic catalysts

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2.1 One metal-one ligand self-assembly catalyst (type a)
Scheme 14. First example by Kim et al. on Nature 2000, 404, 982

4041
stable to air
41 (10%)(S)-44
18% yield, 8% ee

43 (racemic)

42
Scheme 15.

46

45
45 (20 mol %)or
46 (1 mol%)47a) Sasai et al. ACEI 2003, 42, 5711. (for cat. 45)b) Ding et al. TL 2004, 45, 2009. (for cat. 46)

4849a
50
49

Ding et al. CEJ 2005, 11, 4078.

Scheme 16.
51 (1 mol %)51
Ding et al. JACS 2004, 126, 10524.

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Scheme 17.
52a(83% ee)

52b(96% ee)

52e(3D tetratendate) (95% ee)

52c(84% ee)

52d(planar tridentate) (92% ee)(ee in each bracket is for substrate of R1 and R2 = Ph)Ding et al. ACIE 2005, 44, 6362.

Scheme 18.
53 (4 mol%)up to 82% ee
X = Cl or BF4Schulz, E. et al. J. Mol. Cat. A 2007, 272, 20.

53

2.2 Two metals-one ligand self-assembly catalyst (type b)
Scheme 19.

reflux

Ru-L + Zr(OtBu)454 (1 mol%)quant. yield and up to 95% ee
54amorphous, highly porous, large pore distribution

Lin et al. ACIE 2003, 42, 6000.
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2.3 One metal-two ligands self-assembly catalyst (type c)
Scheme 20.

Ding et al. JACS 2005, 127, 7694.

dpenbinap

dpen55binap55b♣ Summary
The attractive properties of these metal¨Corganic catalysts are their ready separation from theproduct and their reusability. Moreover, in contrast to homogeneous catalysts immobilized on an
external support, they have the advantage of possessing an especially high density of catalyticallyactive units. In some cases, the structure of these systems is highly ordered on the microscopic
level. Therefore, in contrast to most other heterogeneous catalysts, their structure can be solved andthe information used for a better mechanistic understanding.

3. Catalysis at Chiral Surfaces
3.1 Chiral modifiers
An achiral heterogeneous catalyst and small enantiomerically pure, organic molecules (chiralmodifiers) work together as catalysts. This kind of ¡°tandem catalysis¡± is at the boundary between
homogeneous and heterogeneous catalysis.
often used chiral modifiers: cinchona alkaloids, tartaric acid, glucose

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Scheme 21.

modifier: 0.3%~0.1%; Pt /modifier: 5~12

natural cinchina alkaloids and deriv.

review: Adv. Syn. Cat. 2003, 345, 45.

An expanded aromatic ring, a basic N atom, a properly located asymmetric center are benificial
3.2 Macromolecular catalysis (polypeptide catalysis)
Scheme 22. Julia-Colonna epoxidation

56a (n = 30)

56bthree phase(water,organic solvent,insoluble catalyst)

two phase56ab

Julia et al. T 1984, 40, 5207.
Roberts et al. CC 1998, 1159.

For application in the syntheses of diltiazem, Taxol side chain and SK&F 104353, see:a) JOC 1993, 58, 6247; b) JCSPT1 1997 3501.
4. Other asymmetric heterogeneous catalysis
eg. chiral substrate induced asymmetric heterogeneous catalysis:

pinene14/14