Transition metal catalyzed reactions are ubiquitous in the realm of synthetic chemistry, allowing for the strategic construction of complex molecular frameworks of pharmaceuticals, natural products, and synthetic materials. Palladium-catalyzed cross-coupling reactions are part of the foundation of these transformations, insofar as they were recognized with the 2010 Nobel Prize in chemistry. Traditionally, these reactions have relied on aryl and vinyl electrophiles, whereas the alkyl counterparts have only recently begun to emerge in the literature. Nickel has been on the forefront of enantioconvergent alkyl cross-coupling reactions due to its propensity to undergo single electron chemistry. However, under special conditions, it has a unique ability to break strong carbon–oxygen bonds in a stereospecific manner, making research into its reactivity a valuable endeavor to the field of organometallic chemistry.
Chapter 1 describes the development of a stereospecific intramolecular alkyl-Heck cyclization of benzylic ethers. The reaction proceeds with inversion at the electrophilic carbon, for the synthesis of methylenecyclopentanes of both extended π-electron and simple aromatic systems. The enantioenriched products can be effectively derivatized to cyclic α-aryl ketones in good yields with good transfer of chirality. Avenues to expand the utility of this reaction have been identified and further studies are ongoing.
Chapter 2 discusses the development of nickel-catalyzed cross-electrophile coupling reactions of benzylic esters and aryl halides. An intermolecular reaction proceeds in high yields for primary benzylic esters for the synthesis of pharmacologically relevant diarylmethanes. The corresponding intramolecular cyclization proceeds under mild conditions, demonstrating the first example of a stereospecific cross-electrophile coupling of secondary benzylic esters. A variety of heterocyclic and functionalized substrates are tolerated under the reaction conditions.
Chapter 3 examines the development a regio- and stereoselective nickel-catalyzed hydroarylation of alkynes with arylboronic acids. The reaction is facilitated by propargyl carbamates as directing groups. The reaction is tolerant of a range of functional groups and heterocycles. Mechanistic studies reveal that the acidic protons of the arylboronic acid coupling partner serve as the origin of hydrogen. Furthermore, the synthesis of tamoxifen can be completed in two steps from a simple hydroarylation product.
