Organic Pedagogical Electronic Network

Attachment of Molecular Catalysts on Solid Supports - Rh Complex on a Silica Support

Jones Group, Georgia Tech

Davies Group, Emory University

Introduction to Supported Molecular Catalysts

1) Chepiga, K. M.; Feng, Y.; Brunelli, N. A.; Jones, C. W.; Davies, H. M. L. Org. Lett. 2013, 15, 6136. 2) Tokunaga, M.; Larrow, J. F.; Kakiuchi, F.; Jacobsen, E. N. Science 1997, 277, 936. 3) Zheng, X.; Jones, C. W.; Weck, M. J. Am. Chem. Soc. 2007, 129, 1105.

• Increase catalyst turnover number (TON) by facilitating catalyst recovery and recycle1

• May allow enhanced utilization of expensive metals or ligands1

• Allows for design of experiments to probe key aspects of reaction mechanisms, e.g. prevention/limitation of key steps that are kinetically second order in catalyst concentration2,3

Porous oxides (e.g. silica, alumina)

Pros: tolerant to most organic solvents; high surface areas allow good solid-liquid contacting; broad temperature range;

Cons: hydroxyl groups on the surface mildly acidic and surface further covered with oxygen lone pairs (dative bonding to metal complex) - can interact with catalyst or reaction media;

Why Attach a Soluble, Molecular Catalyst to a Support?

Typical Support Characteristics

Polymers [e.g. poly(styrene) resins]

Pros: more tolerant to aqueous acidic or basic media compared to oxides; support backbone less likely to bond with metal complex than oxides;

Cons: resins swell differently in various solvents, affecting accessibility of catalyst sites; narrowing temperature range;

Catalyst Supporting Methodologies

1) Jones, C. W.; McKittrick, M. W.; Nguyen, J. V.; Yu, K. Top. Catal. 2005, 34, 67. 2) Tada, M.; Muratsugu, S.; Kinoshita, M.; Sasaki, T.; Iwasawa, Y. J. Am. Chem. Soc. 2010. 132. 713. 3) Nakazawa, J.; Smith, B. J.; Stack, T.D.P. J. Am. Chem. Soc. 2012. 134. 2750.

Immobilization Method

Covalent ligand binding

Physisorption Ion pair formation

Encapsulation

Applicability broad restricted restricted restricted

Drawbacks preparation competition with solventsor substrates

competition with polar or ionic substrates

substrate size, diffusion

General Immobilization Methods1

Selected Covalent Binding MethodsAlkene on Ligand and Surface2

Olefincoupling

Alkyne on Ligand and Azide on Surface3

Clickchemistry

support [Cat]AIBN

80 oC+ support [Cat]

support N3 +[Cat] [CuI]

ligandsupport N

N N

[Cat]

Covalent Attachment of Rhodium Catalyst

Chepiga, K. M.; Feng, Y.; Brunelli, N. A.; Jones, C. W.; Davies, H. M. L. Org. Lett. 2013, 15, 6136.

Supported catalyst (right) depicts product from two AIBN-initiated radicals reacting with each alkene, followed by coupling. Other products also possible.

Attaching Rh2(S-DOSP)4 analogue on silica

Silica supportwith functionalized

surface

N

S

C12H25

O O

O

O

Rh

Rh

N

S OO

O

O

3

Alkene Functionalized Rh2(S-DOSP)4Derivative

N

S

C12H25

O O

O

O

Rh

Rh

N

S OO

O

O

3

C12H25

Rh2(S-DOSP)4

Single-Ligand Exchange

Functional group for catalystgrafting by alkene coupling

SiOOO O

OMeSi Si

silica

1 eqiv. AIBN, toluene, 80°C

SiOOO O

OMe

silica

SiSi

CN

[Rh2]

CN

SiOOO O

OMe

silica

SiSi

or

NC

[Rh2]

NC

1 eqiv. [Rh2] per Si-styryl group

Supported Rh catalyst

Asymmetric Cyclopropanation: Catalyst Recycling

Chepiga, K. M.; Feng, Y.; Brunelli, N. A.; Jones, C. W.; Davies, H. M. L. Org. Lett. 2013, 15, 6136.

Ph

N2

PhPh Ph

CO2Me

CO2Me mesitylenehexanes,rt

+

1 mol%Rh(II)

cycle yield / % ee / %

1 72 80

2 78 81

3 75 79

4 76 77

5 78 79

Rh2(S-DOSP)4 88 88

Reaction, catalyst recovery via filtration, use in subsequent reaction; 5 cycles with consistent yield and enantiomeric excess (ee); Slightly reduced yield and ee compared to parent Rh2(S-DOSP)4 complex.

This reaction was catalyzed by the supported Rh catalyst, shown on the right of the previous slide.

Problems

1) What is the catalyst supporting methodology used in this work adding the Rh2(S-DOSP)4 analogue onto silica?

A. Covalent ligand binding;

B. Physisorption;

C. Ion pair formation;

D. Encapsulation.

2) What modification is needed for Rh2(S-DOSP)4 if click chemistry is used to bind the ligand onto solid supports? How might this be achieved?

N

S

C12H25

O O

O

O

Rh

Rh

N

S OO

O

O

3

C12H25

Rh2(S-DOSP)4