Project Summary/Abstract The generation of chemical space and its translation into new functions is one of the main goals of synthetic chemistry. The BN/CC isosterism (i.e., replacement of a carbon-carbon bond with a boron-nitrogen bond) has emerged as a viable strategy to increase the chemical space of compounds relevant to biomedical research. This proposal describes a new untapped avenue of exploration for BN/CC isosterism, namely a program geared toward developing BN isosteres of arenes as useful reagents and catalysts for synthetic organic chemistry applications. Potential benefits of research into these BN heterocycles (also known as azaborines) include discovery of novel reactivity and selectivity that are unattainable by using conventional organic compounds and the development of new mechanistic principles, both as a consequence of azaborine's unique electronic structure. Specifically, we seek to: 1) establish azaborines as four-carbon (4C) + 1 nitrogen (1N) + 1 boron (1B) synthon for synthetic applications that exploit the preinstalled functional density of an azaborine, i.e., a nitrogen atom, a boron atom, and a 1,3- butadiene unit to efficiently produce new aminoborylated compounds for diversity-oriented synthesis. 2) establish azaborines as a new family of ligands that can engage in distinct coordination modes with transition metals to affect new reactivity and selectivity in metal-catalyzed transformations. Completion of the proposed aims will yield new readily accessible and utilizable reagents and catalyst systems that can accomplish synthetic transformations not readily achievable by existing methods.