Achieving High Stereoselectivity in Ruthenium-Catalyzed Olefin Metathesis Reactions for Organic and Polymer Synthesis

Author: Ahmed, Tonia Sarah

Year: 2018

Degree: Dissertation (Ph.D.)

Advisor: Grubbs, Robert H.

Committee Members: Agapie, Theodor; Gray, Harry B.; Reisman, Sarah E.; Grubbs, Robert H.

Option: Chemistry

DOI: 10.7907/KCM2-6W92

Abstract

Transition metal-catalyzed olefin metathesis has emerged as a powerful tool for constructing C−C double bonds. This thesis delineates the development of Ru-based catalysts for the stereoselective formation of olefins and mechanistic studies used to examine how catalyst structure influences selectivity and activity.

Chapter 2 details the synthesis of Z-selective, cyclometalated catalysts bearing nitrite X-ligands. The activity and selectivity of these catalysts were examined in an array of ring-opening metathesis polymerization and cross metathesis reactions. Comparison of these catalysts with their nitrate-bound analogues is described.

Chapter 3 describes the examination of several Z-selective, cyclometalated catalysts in ring-opening metathesis polymerizations. The polymerizations of a variety of norbornene and norbornene derivatives were examined to determine the tacticity and microstructure of the resulting polymer. Computational studies were used to examine the mechanism of the polymerization reactions.

Chapter 4 examines the decomposition of Fischer carbene complexes derived from cyclometalated catalysts. In-depth NMR studies are used to determine the identity of the decomposition product, and the decomposition pathway is examined through computational studies.

Chapter 5 describes the first example of highly E-selective cross metathesis through kinetic control using stereoretentive, Ru-based catalysts bearing dithiolate catalysts. The preparation of additional stereoretentive catalysts is described for increasing catalyst activity while maintaining or increasing selectivity. A model for the observed stereoselectivity is proposed.

Chapter 6 delineates the preparation of a series of fast initiating, stereoretentive catalysts. These catalysts are assessed in an array of cross metathesis reactions, and significantly enhanced activity is observed in E-selective reactions. The examination of the relationships between the structure of a catalyst and its selectivity and activity is described.

Chapter 7 examines the use of stereoretentive catalysts in the synthesis of Z-macrocycles from diene starting materials bearing a Z-olefin and a terminal olefin. Initiation rate studies are conducted to examine the activity of these catalysts compared to previously reported cyclometallated catalysts used in this ring-closing metathesis reaction. The synthesis of twelve- to seventeen-membered rings with high Z-selectivity is described.

Chapter 8 explores the use of fast-initiating, stereoretentive catalysts for synthesizing E-macrocycles. The preparation of diene starting materials containing two E-olefins is described. Using these catalysts, twelve- to eighteen-membered rings are constructed with high E-selectivity.

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