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Supported catalysis is emerging as a cornerstone of transition metal complex catalysis as environmental awareness increases and transition metal resources get scarcer and more expensive. In collaboration with chemists, materials scientists, and chemical engineers at GeorgiaTech the Weck group is developing general principles for guiding catalyst design that will move the field of supported molecular catalysts out of the realm of empirical investigation into the field of predictive sciences.
The main emphasis in the Weck group is the development of new polymer-based solution-phase supports for different types of catalysts. We have designed, synthesized, and characterized polymer-supported metal salen catalysts for a variety of enantioselective transformations and identified structure property relationships of the supported systems. The fine-tuning of the catalyst-support interface in combination with a detailed understanding of catalytic reaction mechanisms has allowed us to develop not only reusable and recyclable polymer-supported catalysts but also facilitated the design and realization of supported catalysts that are significantly more active and selective than their non-supported counterparts. We found that through the optimization of four basic variables: (i) polymer backbone rigidity, (ii) nature of the linker (iii) catalyst site density and (iv) the nature of the catalyst attachment, polymer supports can be designed and tuned to enhance the catalytic activity or decrease/eliminate decomposition pathways of transition metal catalysts for reactions that follow either a monometallic or a bimetallic mechanism. The utilization of these design principles resulted in the creation of some of the most active and selective salen catalysts in the literature, in particular macrocyclic oligomers supported Co-salen complexes as catalysts for the hydrolytic kinetic resolution of terminal epoxides.
To mimic the high selectivity of enzymatic systems in nature, shell cross-linked micelle-supported catalytic systems based on well-defined block copolymers have been developed, which keep the catalysts in a confined core-shell structure stabilized by covalent linkages. Our system has unique substrate specificity for the hydrolytic kinetic resolution of epoxides. Currently, bifunctional core-shell micelles as well as trifunctional multicompartment micelles are being developed as multi catalyst supports for tandem reactions. These novel supports allow for the combination of otherwise incompatible catalytic systems and will serve as a test bed for the study of substrate diffusion between the different compartments.
To further the field of polymer-supported catalysis, the Weck group is also developing functionalized multicompartment micelles that can be used as a nanoreactor for three step catalytic tandem reactions. Unlike a “traditional” micelle which has a hydrophilic core and a single hydrophobic core, a multicompartment micelle has multiple microphase separated domains within its core, a result of the self-assembly of a triblock copolymer with three mutually incompatible polymer blocks. Multicompartment micelles prepared to date have used simple, non-functionalized monomers, thus limiting their applications. The Weck group has synthesized linear triblock copolymers with functionalized hydrophilic, lipophilic and fluorophilic blocks. These polymers self-assemble in an aqueous solution to form functionalized multicompartment micelles, a new kind of nanostructure with tremendous potential in tandem catalysis.