Development of Stereospecific Nickel-Catalyzed Cross-Electrophile Coupling Reactions for Cyclopropane Synthesis
Transition metal-catalyzed cross-coupling (XC) reactions are a critical component of the organic chemist’s toolbox. The eminence of these methods is illustrated by the 2010 Nobel Prize in Chemistry, which was awarded to Heck, Negishi, and Suzuki for their work on palladium-catalyzed XC reactions. Although palladium is esteemed for its robust and well-understood reactivity, organonickel chemistry has gained significant attention due to the many unique advantages provided by nickel. Nickel can easily access oxidation states of 0, +1, +2, and +3, enabling both radical and polar reaction pathways, whereas palladium is only stable in oxidation states of 0, +2, and +4. Because nickel is less electronegative than palladium, it is capable of undergoing more facile oxidative addition, and therefore has the ability to cleave stronger bonds. Additionally, nickel is a sustainable and inexpensive catalyst due to its natural abundance. The work presented in this dissertation harnesses the unique capabilities of nickel in order to discover new fundamental reactivity.
Cross-electrophile coupling (XEC) provides several advantages over traditional XC, including improved functional group compatibility, greater commercial availability of starting materials, and fewer safety precautions. The advancement of XEC has lagged behind traditional XC due to the challenge of obtaining cross-selectivity between two substrates with similar reactivity. Recently, the design of catalyst systems capable of distinguishing between two unlike electrophiles has allowed for development of the first stereoselective XECs. The Weix and Reisman laboratories have led the design of stereoconvergent XECs, and our group has developed stereospecific variants. This dissertation details the development of stereospecific nickel-catalyzed XECs of benzylic ethers, benzylic sulfonamides, and alkyl fluorides for cyclopropane synthesis. Moreover, mechanistic investigations are chronicled which uncover the foundation for the observed reactivity and advance our understanding of low-valent nickel catalysis.