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UBC Theses and Dissertations

Synthesis, structure, and reactivity of early transition metal precatalysts bearing (N,O)-chelating ligands Payne, Philippa Robyn


The synthesis, structure, and reactivity of early transition metal complexes containing (N,O)-chelating ancillary ligands are described. The ligands investigated include ureates, pyridonates, amidates, and sulfonamidates. These related ligands generate four-membered metallacycles when bound to the metal center in a κ²-(N,O) fashion. The zirconium and tantalum complexes have been examined in terms of their activity and selectivity as precatalyst systems for hydroamination or hydroaminoalkylation. A chiral cyclic ureate ligand has been synthesized from enantiopure L-valine for application in zirconium-catalyzed asymmetric hydroamination of aminoalkenes. Chiral zirconium complexes, prepared in situ from two equivalents of the urea proligand and tetrakis(dimethylamido) zirconium, promote the formation of pyrrolidines and piperidines in up to 12% ee. Isolation of an asymmetric bimetallic zirconium complex containing three bridging ureate ligands confirms that ligand redistribution occurs in solution and is most likely responsible for the low enantioselectivities. Mechanistic investigations focusing on the hydroaminoalkylation reactivity promoted by a bis(pyridonate) bis(dimethylamido) zirconium precatalyst expose a complex catalytic system in solution. Stoichiometric investigations reveal the formation of polymetallic complexes upon addition of primary amines. The kinetic and stoichiometric investigations are most consistent with a bimetallic catalytically active species. A series of mono(amidate) tantalum amido complexes with varying steric and electronic properties have been synthesized via protonolysis. Solid-state and solution-phase characterization indicate that the amidate substituents influence the observed binding mode of the ligand. Salt metathesis and protonolysis routes to the synthesis of mixed tantalum chloro amidate complexes are investigated. Sulfonamide proligands react with pentakis(dimethylamido) tantalum to generate well-defined monomeric complexes containing a κ²-(N,O) bound sulfonamidate. The hemilabile (N,O)-chelating amidate ligands, which generate four-membered metallacycles, are the most active of the precatalysts examined for the intermolecular hydroaminoalkylation of terminal olefins with secondary amines. The substrate scope of a mono(amidate) tetrakis(dimethylamido) tantalum complex has been examined for the α-alkylation of unprotected piperidine, piperazine, and azepane N-heterocyclic amines. The lack of reactivity with pyrrolidine substrates is examined by quantum chemical calculations and isotopic labeling studies. Two (N,O)-chelating ureate ligands are also successful ancillary ligands for this transformation and, with a C₁-symmetric chiral ureate complex, enantioselective α-alkylation of piperidine is observed.

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