Immobilized and Fluorous Ru-Based Metathesis Catalysts

Northern Illinois University, Dekalb IL

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Abstract

One of the most significant recent advances in synthetic methodology is the discovery, independently by the groups of Schrock and Grubbs, of well-defined metal carbene complexes, and their applications as olefin metathesis catalysts in organic, materials and polymer chemistry. In particular, ring closing diene and enyne metatheses (RCM) have proven to be extremely powerful tools in the construction of highly functionalized carbo- and heterocyclic molecules of biological and pharmaceutical interest. Olefin metathesis mediated by these catalysts has been applied to the synthesis of a diverse array of complex targets, ranging from molecules with small, medium and large ring structures, to macro- and polycyclic systems and to molecules with carbohydrate and peptide backbones. Ring opening living polymerization has also found important applications in the preparation of new materials, including those with potential biological relevance. With the exception of structurally highly demanding substrates, which frequently call for the use of the more reactive but less tamable and air sensitive Mo-based catalyst, the commercially available Ru carbene catalysts of the Grubbs' type have arguably been the most widely used catalysts for olefin metathesis due to their broad functional group tolerance and remarkable stability towards air and moisture. This success notwithstanding, and with the scope of olefin metathesis still rapidly expanding, limitations and disadvantages of the currently available catalysts become apparent in several important settings. Substrates of biological relevance such as unprotected carbohydrates, amino acids and peptides have very limited solubilities in organic solvents and this in turn mandates an olefin metathesis in non-organic media. Reactions involving the use of Ru- containing catalysts invariably require vigorous purification by column chromatography to remove the various ligands and the residual metal complexes. Frequently, this purification stage can be impractical, as in the cases of library synthesis and of reactions on a large scale. The aim of this investigation is therefore to design and synthesize a diverse variety of ligands suitable for the immobilization of Ru catalysts. The new catalysts will have standard homogeneous behaviors in both organic and aqueous media, as well as under fluorous biphasic conditions. As the catalysts can be conveniently recycled and subsequently reused, they will offer many practical advantages over the existing catalysts. These universally applicable, highly reactive and recyclable catalysts will significantly expand the scope of application of olefin metathesis in organic synthesis, in drug discovery and in materials and polymer chemistry.