PROJECT SUMMARY/ABSTRACT Advances in catalytic science and technology enable the preparation of pharmaceutical agents used to treat human disease. This project has the long-term goal of developing a broad class of inexpensive nonmetal catalysts that promote catalytic transformations via formal oxidation state cycling in qualitative analogy to transition metal catalysis. Within this overarching goal, the primary focus of this proposal is the design and application of phosphorus-based catalysts that function in the P(III)⇌P(V) redox couple. While phosphines are well-established in catalysis as spectator ligands for transition metal catalysis and as nucleophilic catalysts, this research describes innovative phosphorus-based catalysts of novel com- position and structure that explore the structural and electronic conditions required to enable new catalyt- ically-relevant reactivity via reversible P(III)⇌P(V) oxidation state cycling. The first major effort is the de- velopment of phosphine-catalyzed O-atom transfer methods that result in reductive functionalization of nitroarene compounds through the formation of new carbon-nitrogen bonds. The second major effort is the development of net redox-neutral (cyclo)dehydration reactions that are accomplished by phosphine catalysis in the P(III)⇌P(V) redox couple. The proposed research is expected to yield new practical cata- lytic methods for the construction of pharmacologically-relevant small molecules that meet the challenges of sustainable synthesis, and an improved fundamental understanding the interplay between structure and reactivity in the p-block that will underpin future development of nonmetals for atom transfer, bond activation, and catalysis. Taken together, these outcomes will advance nonmetal-based redox catalysis as a new and powerful modality in pharmaceutical synthesis.