HETEROGENIZATION OF TRANSITION METAL CATALYSTS

 

While homogeneous transition metal catalysts have proven to be of enormous utility in organic synthesis, the often difficult separation of the catalyst from the desired reaction products is an inherent drawback to their use in applications requiring very pure product streams (for instance the pharmaceutical industry). Additionally, the wide-scale reuse of these expensive and sometimes quite toxic metal/ligand systems would result in significant monetary and environmental benefits. Therefore, replacement of current homogeneous processes with new, efficient heterogeneous equivalents is a field of study attracting much interest generally and in our group.

 

 

Heterogenization approach

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Whereas heterogeneous catalysts generally consist of inorganic materials containing active sites as part of the material’s matrix, heterogenized (or immobilized) catalysts comprise well-defined molecular species which have been tailored to remain in some phase other than that which contains the substrates and reaction products in a catalytic reaction. That “other” phase is commonly a solid support, such as silica or an insoluble polymer (left side of the figure); however, liquid support phases are also utilized (right side of the figure). An example of a liquid-immobilized catalyst is a water-soluble organometallic complex which, when added to a biphasic reaction mixture containing water and an organic solvent/substrate solution, catalyzes the desired reaction while remaining in the aqueous phase for easy separation and/or reuse.

 

There are potentially multiple advantages of employing an heterogenized catalytic system in place of an inorganic heterogeneous system. The principal advantage is the increased selectivity control which is possible when using well-defined complexes of tailor-made organic ligands: the work of decades invested in the development of effective ligands such as chiral phosphines is directly transferable to heterogenized systems with (and sometimes even without!) minor modifications to the ligand architecture. Another important aspect of the heterogenized catalytic method is the improved kinetic profile available when using a solid-supported catalyst with a long tether length (or especially a liquid-immobilized catalyst) due to the increased mobility of the catalyst and catalyst-substrate complexes.

Our group is currently investigating multiple approaches to this goal, using solid support, aqueous and fluorous-phase immobilization strategies together with the design and synthesis of novel ligand architectures.

 

 

CARBONYLATION PROCESSES


Rh/ Asymmetric hydroformylation of alkenes (diphosphite ligands)

Drawing on our expertise acquired in carbonylation reactions under homogeneous conditions, our group is currently working on the development of new series of chiral phosphorus-based ligands for their subsequent application in asymmetric hydroformylation of alkenes. Our aim is the preparation of new immobilized Rh and Ir catalysts that will provide relevant activity together with high regio- and enantioselectivity. These catalysts should be recoverable several times without affecting their performance.

The method proposed to achieve these objectives focuses on the development of a greater understanding of these heterogenized systems that will lead to rational modifications, first with respect to the ligands and finally to the catalytic system as a whole.

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Polymer supported chiral P-N ligands.

 

C-C BOND FORMATION REACTIONS


Pd-polymer/asymmetric allylic alkylation (P-N ligands)

In collaboration with the group of M. Pericas (ICIQ, Spain): Adv. Synth. Catal. 2011, 353, 3255 – 3261

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Conversions and enantiomeric excesses obtained in Pd catalyzed allylic alkylation using homogenous systems.


 

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Results of recycling experiments.


 

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Supported Fe catalysts synthesized in our lab.

 

 

OXIDATION PROCESSES


Fe/epoxidation of alkenes (N-donor ligands)

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