Methods for the catalytic functionalization of C-H bonds are widely considered to possess the potential to revolutionize the synthesis of complex molecules, but the realization of this potential requires the selective functionalization of a singe C-H bond in compounds containing many other C-H bonds and functional groups that are typically more reactive than the C-H bond. The PI is developing an unusual strategy for the functionalization of C-H bonds involving the reactions of boranes and, more recently, of silanes with aryl, heteroaryl, and alkyl C-H bonds. The products from these reactions are valuable synthetic intermediates that can be converted to diverse final products containing new C-C, C-N, C-O, and C-S bonds. The fundamental innovation underlying the C-H bond functionalization in this proposal is the higher reactivity of complexes containing covalent transition metal-main group bonds toward C-H bond functionalization than those containing typical organometallic ligands. During the past several years, the PI's group has discovered a new class of catalyst for the silylation of aryl C-H bonds; silylmetal complexes that are catalytically competent and react with arenes to form arylsilanes; iridium-catalyzed borylations of aliphatic C-H bonds of cyclopropanes, amines and cyclic ethers; borylations of secondary alkyl and benzyl C-H bonds directed by alcohols and amines; a catalyst for broadly applicable borylations of heteroarenes; and one-pot combinations of borylation and subsequent functionalization to create sterically controlled functionalizations of arenes, alkylations of arenes, and mild access to unstable arylboronate intermediates. The proposed research will build upon these studies and additional preliminary data. The PI's group will prepare new ligand structures that enable metal-catalyzed borylations of primary C-H bonds with limiting substrate, increase the reactivity of the catalysts toward basic heteroarenes, and create practical methods for the silylation of functionalized arenes. In addition, they will use these catalysts to achieve directed borylations and silylations of alkyl C-H bonds and methods for the generation and characterization of intermediates in the iridium-catalyzed borylations and silylations. To achieve these goals, seven major aims are proposed: 1) to design new ligands for the borylation and silylation of aryl and alkyl C-H bonds by iridium and rhodium catalysts; 2) To discover new borylations of aliphatic C-H bonds enabled by the ligands of Aim 1; 3) To gain a mechanistic understanding of the relationships between iridium catalyst structure and activity for C-H borylation; 4) To broaden the scope of directed silylations of aliphatic C-H Bonds; 5) To create intermolecular silylations of aromatic C-H bonds that occur with remote steric effects and high functional group compatibility; 6) To reveal the mechanism of these intermolecular silylations of arenes, and 7) To develop new functionalizations of the main group products of the C-H bond functionalizations.
Many of the catalysts and reactions that are the subject of the proposed research will improve methods to prepare pharmaceutical candidates. New reactions, new applications of the catalytic reactions, and a mechanistic understanding of these systems will result from the proposed research. Thus, successful development of the proposed research will significantly increase the accessibility of compounds that improve human health.
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