Understanding and Improving Efficiency in Ruthenium Olefin Metathesis

Citation

Kuhn, Kevin Michael (2010) Understanding and Improving Efficiency in Ruthenium Olefin Metathesis. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/XT5J-TA64. https://resolver.caltech.edu/CaltechETD:etd-07142009-090835

Abstract

Olefin metathesis has become an increasingly important and powerful reaction. The development of the well-defined ruthenium alkylidene complexes, in particular, has broadened the scope and utility of the olefin metathesis reaction in both organic synthesis and polymer science. Despite these advances, complete control of the parameters (activity, stability, and selectivity) that affect efficiency in olefin metathesis remains a major challenge, and the development of more efficient catalysts for a variety of applications remains a very important goal. With that in mind, this thesis primarily focuses on understanding the requirements for and improving the efficiency of ruthenium-based olefin metathesis.

In chapter two, a series of ruthenium olefin metathesis catalysts bearing N-heterocyclic carbene (NHC) ligands with varying degrees of backbone and N-aryl substitution were prepared. These complexes show greater resistance to decomposition through C-H activation of the N-aryl group, resulting in increased catalyst lifetimes. This work utilized robotic technology to examine the activity and stability of each catalyst in metathesis, providing insights into the relationship between ligand architecture and catalyst efficiency.

In chapter three, the high-throughput assay developed in the previous chapter was utilized to screen a series of ruthenium catalysts for the ring-closing metathesis (RCM) of acyclic carbamates to form the corresponding di-, tri-, and tetrasubstituted five-, six-, and seven-membered cyclic carbamates. While disubstituted cyclic olefins were easily formed by a variety of catalysts, NHC-bearing catalysts were required to produce trisubstituted cylic olefin products at low catalyst loadings. Furthermore, only catalysts bearing small N-aryl bulk on the NHC ligands were found to effectively accomplish the RCM reaction for sterically challenging substrates, providing a reminder that more-efficient catalysts still need to be developed.

A process for the preparation of symmetric and unsymmetric imidazolinium chlorides that involves reaction of a formamidine with dichloroethane and a base is described in chapter four. This method makes it possible to obtain numerous imidazolinium chlorides under solvent-free reaction conditions and in excellent yields with purification by simple filtration.

In chapter five, both chiral triazolylidenes and cyclic alkyl amino carbenes (CAACs) were chosen as ligands for the preparation of chiral ruthenium olefin metathesis catalysts. These C1 symmetric ligands were chosen to create non-conformationally flexible environments in proximity to the ruthenium center, potentially bringing chirality extremely close to the site of catalysis. These new motifs for ligand architecture show great promise. The moderate enantioselectivies obtained for AROCM and ARCM indicate potential utility toward both synthetic methodology and mechanistic insight.

Finally, appendix A describes the preparation of a series of ruthenium olefin metathesis catalysts bearing acenapthylene-annulated NHC ligands with varying degrees of N-aryl substitution. Initial evaluation of their performance in olefin metathesis demonstrated that these complexes show greater resistance to decomposition, resulting in increased catalyst lifetimes. While this work has significant potential, the results are preliminary.

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