Late Transition Metal Complexes for E-H Bond Activation and Additions to Multiple Bonds
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The study of organometallic chemistry in the context of catalysis can be approached from a stoichiometric perspective, in which PGM complexes are examined in the context of understanding fundamental reactions to provide insight into established catalytic transformations. Alternatively, a catalytic perspective can be adopted, in which an unknown or underdeveloped transformation is identified and PGM catalysis is employed to assist in the further development of this area. In this regard, two general goals of this thesis are: 1) to explore alternative ligation strategies based on P,S-functionalized indene ligands, with a particular focus of studying divergent stoichiometric reactivity between related cationic and zwitterionic PGM complexes; and 2) to identify general PGM catalysts for the cyclohydroamination of alkylaminoalkenes and the hydroamination of internal alkynes with secondary alkylamines. The preparation and divergent reactivity of a previously unreported class of coordinatively unsaturated cationic and zwitterionic Cp*Ir(P,S) complexes that feature structurally analogous P,S-indene and mono-deprotonated P,S-indenide ancillary ligands, respectively, are discussed. The cationic complex was observed to activate organosilanes via the first well-documented H-Si addition across an M-SR linkage. In contrast, the unusual stoichiometric reactivity of the putative zwitterion with CH3CN or Ph2SiH2 can be viewed as resulting from the dual action of the Lewis acidic Cp*Ir fragment and the Lewis basic 10?-electron indenide unit within this formally charge-separated zwitterion. Building on these initial studies, the synthesis of structurally related (benzyl)Pt(P,S) borato- and carbanion-based zwitterions and cationic complexes featuring the P,S-indene and indenide ligand framework are also presented. In the context of hydroamination studies, [Ir(COD)Cl]2 was identified as an effective pre-catalyst for the efficient synthesis of pyrrolidine and piperidine heterocycles via the cyclohydroamination of tethered aminoalkenes. Following optimization studies of this catalyst system, a broad substrate scope that included the cyclization of primary and secondary alkyl- or arylamines was established. A kinetic and mechanistic evaluation of this reaction suggested the operative pathway as involving olefin activation in a manner that had not previously been documented for Ir-catalyzed alkene hydroamination. In the pursuit of a general catalyst for the alkyne hydroamination reaction, an effective gold pre-catalyst featuring a P,N-ligand was identified and was used in the addition of a variety of functionalized dialkylamines to internal alkynes. In particular, the first examples of the regioselective addition of dialkylamines to unsymmetrically substituted alkynes are discussed. A preliminary mechanistic survey, consisting of kinetic and stoichiometric experiments, has provided empirical evidence to support a mechanism comprised of turnover-limiting alkyne insertion into a Au?N bond followed by proto-deauration.