Mechanistic Studies And Applications Of The Copper-Catalyzed Rearrangement Of Vinyl Heterocycles

Abstract

The development of new synthetic methods advances the study of chemistry on many fronts, including new drugs for the treatment of human disease and new materials. The copper-catalyzed rearrangement of vinyl heterocycles is one such method, providing access to a wide array of five-membered oxygen, nitrogen, and sulfur-containing heterocycles. To harness the true potential of a new methodology, an understanding of the mechanistic underpinnings is required. Utilizing an array of mechanistic tools, the rearrangement of vinyl aziridines is studied in detail. Using mechanistically inspired metal additives, a dramatic acceleration of the rearrangement of vinyl aziridines was realized. Demonstrating the use of in situ reducing agents significantly accelerate the rearrangement, suggesting a copper(I) species is the active catalytic species. This was corroborated through the use of (COD)Cu(hfacac) as a copper(I) starting point. Detailed NMR kinetic studies revealed the relative importance of olefin and sulfonamide electronics, and that the rearrangement is first order in substrate and first order in catalyst. As a result, a new catalytic system was discovered, which provides enhanced chemoselectivity and milder reaction conditions towards five-membered heterocycles. In addition, studying the mechanism provided a new direction for future rational catalyst designs towards more active catalysts, and catalysts for chiral applications, such as asymmetric desymmetrization. To further demonstrate the utility of the new copper(I) catalytic system, an expedient and scalable approach was designed utilizing (COD)Cu(hfacac) in the first total synthesis of members of a family of heterocyclic labdane natural products. Through synthesis, conformation and clarification of the structural assignment of isolated smallmolecules was realized. The route provided access to quantities of material for biological screening purposes, and the versatility of the route could be used to provide synthetic analogues of the natural structures.