Synthesis of Ligand-Centered Boron Cations to Access Tandem Reactions with Transition Metals

With the support of the Chemical Synthesis program in the Division of Chemistry and the Established Program to Stimulate Competitive Research (EPSCoR), Scott Daly of the University of Iowa and Jason Keith of Colgate University will study metal complexes that enable tandem, multistep molecular assembly of targeted organic compounds in one-pot reactions. The innovative concept to be explored is the development of reactive boron molecules that bind metals and allow two different catalytic reactions to be performed using a single metal catalyst. This strategy holds promise for establishing more efficient and cost-effective methods to transform simple chemical building blocks into complex molecular scaffolds used in pharmaceuticals, agrochemicals, and other fine chemicals. This research will also provide important workforce development and scientific training for graduate and undergraduate students from diverse backgrounds. The recruitment and support of nontraditional and at-risk STEM (science, technology, engineering and mathematics) majors will be facilitated by continued development of the Chemistry Platoon, an acclaimed outreach project at the University of Iowa aimed at assisting veterans and Military affiliates in introductory chemistry courses, and by intensive undergraduate research efforts at Colgate University. Tandem reactions are coupled catalytic reactions that sequentially transform a substrate via two or more mechanistically distinct processes. These reactions are often limited by co-catalyst incompatibility when two or more catalysts are required, as well as difficulties generalizing the reactivity for broad classes of important chemical transformations. It is proposed that these problems can be addressed using single multifunctional metal complexes containing reactive boron ligands capable of performing catalytic reactions in tandem with the metal. Using cycloaddition catalysis as a tool, the Daly team will assess how chemical changes at the boron ligand affect stereoselectivity and reaction yields, catalytic stability under different reaction conditions, and tandem cross-coupling scope at the metal. Synthetic efforts will be complemented by density functional theory calculations by the Keith research team and by cutting-edge synchrotron spectroscopy measurements-B and P K-edge XANES (X-ray absorption near edge spectroscopy)-to map out reaction mechanisms and underlying electronic structure variations that govern desirable reactivity. These collaborative efforts are expected to illuminate chemical factors that yield desirable tandem metal-ligand catalysis, and provides a possible alternative to established methods. These studies are anticipated to have cross-cutting relevance to main-group chemistry and catalysis while also affording new structure-reactivity insights germane to other types of catalytic reactions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.