New Organic Radical Reactions

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Proposed work and Significance: N-heterocyclic carbene boranes (NHC boranes) have been shown to have weak B-H bonds and accordingly behave as radical hydrogen atom donors in xanthate reductions. A new class of boron radical, the NHC-boryl radical, has been demonstrated to be an intermediate in these reactions. This early work raises many questions: What are the best NHC-borane reagents for radical reactions? Can NHC boranes replace tin hydrides as the long-elusive "ideal" mediators of radical reductions (inexpensive, low molecular weight, green, easy to separate, etc.)? What are the structures and reactivities of the new boranes and especially the new boryl radicals? These and other fundamental questions will be addressed by synthesizing diverse new NHC-boranes with assorted NHC and boron substituents and characterizing their structures and reactions. The most promising compounds will be used as reagents for radical reductions of xanthates, halides and pseudohalides, for new radical hydroborations, and as coinitiators in radical polymerizations. The structures of the most important radicals will be probed by electron paramagnetic resonance (EPR) spectroscopy, which will also be used along with other tools for kinetic measurements. Broader Impact: Societal benefits from this project accrue over the long term in two areas. First, organic synthesis is a central discipline that contributes to the development of most biological and physical sciences by providing new molecules and materials for study and sale. The concepts and methods of this work deepen and broaden the discipline of synthesis and help it in its quest to provide the needed reactions and technologies for other fields that rely on it to progress. Second, there is a strong coupling on this project of advancing the frontiers of science while at the same time promoting teaching, training and learning of the diverse group of undergraduate students and graduate students who do the day-today research work. They move into the workplace to become the human capital that drives progress in many areas that are vital to the health of our citizenry and the economy of our nation. The project also regularly uses cutting edge technology like videoconferences to manage the intercontinental collaborations. These collaborations expand the teaching/training impact of the proposal from Pittsburgh to Paris and Mulhouse in France, St. Andrews in Scotland, and Tokyo in Japan.