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Bis(amidate) titanium, zirconium, and tantalum complexes : applications in catalytic synthesis of N-containing compounds

University of British Columbia

A two-part study involving the synthesis and catalytic investigations of amidate complexes of early transition metals is described. In the first part, the substrate scope of an achiral bis(amidate) titanium hydroamination precatalyst has been broaden to include challenging substrates. This complex efficiently catalyzes the hydroamination of heteroatom-containing allenes with arylamines. Control experiments rule out allene-alkyne isomerization as a reaction pathway during this catalysis. The hydrohydrazination of a variety of alkynes with 1,1-disubstituted hydrazines also proceed efficiently in the presence of this precatalyst giving the anti-Markovnikov hydrazone products predominantly. These hydrazones have been transformed into substituted indoles by a one-pot tandem sequential hydroamination/ZnCl₂-mediated cyclization. Importantly, the bis(amidate) titanium precatalyst can be generated in situ for these reactions with no impact on the reactivity or selectivity of the complex. The second part of this thesis focuses on the synthesis, characterization, stability and catalytic investigations of chiral zirconium and tantalum complexes ligated with amidate ancillary ligands. Seven new axially chiral proligands have been synthesized and used for in situ generation of zirconium hydroamination precatalysts. These chiral complexes efficiently produce N-heterocycles in up to 94% isolated yield with ee reaching 74%. Preparative scale synthesis and characterization of the zirconium complexes revealed coordination geometry that is greatly influenced by the steric properties of the ligand. Bulky proligands produce monomeric complexes, wherein the biphenyl ligand displays a κ²-O,O-bonding motif which accounts for the modest enantioselectivities realized with this system. The less sterically-congested proligands initially form similar monomeric complexes; however, these complexes dimerize diastereoselectively to κ⁴-N,O,O-N-bonding amidate complexes within a few hours in solution. The binding motif of the amidate ligand of the chiral biphenyl tantalum complexes is also dictated by the size of the N-substituent of the ligand. While a bulky proligand results in a discrete tantalum κ²-O,O-bonding amidate complex, less sterically-encumbered proligands produce a mixture of κ²-O,O-bonding and κ³-N,O,O-bonding amidate complexes. Using these tantalum complexes as precatalysts, alkenes undergo hydroaminoalkylation reactions with secondary amines to give branched alkylated secondary amines in isolated yields of up to 92% and enantiomeric excesses reaching 66%, for the first examples of an enantioselective hydroaminoalkylation reaction.

Text

2013-06-30

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/39851

Chemistry

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/