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Bis(amidate) and bis(ureate) complexes of zirconium and tantalum : synthesis and catalytic application in C-N and C-C bond formation

Authors
Publisher
University of British Columbia
Publication Date
Disciplines
  • Design

Abstract

This is a study of early metal organometallic complexes for catalytic C-N and C-C bond formation using amines. The use of zirconium complexes as hydroamination catalysts is explored first. New axially-chiral bis(amide) and bis(urea) proligands are designed. Synthetic methods used to generate these compounds are described and X-ray crystallographic analysis of a bis(sulfonamide) establishes the absolute configuration of the chiral proligands. Installation of the new bis(amidate) and bis(ureate) ligands onto zirconium is undertaken and the structure of these complexes is examined in solution and, where possible, in the solid state. Where well-defined zirconium complexes can be obtained, those complexes are tested for their efficacy in enantioselective catalytic hydroamination. In the absence of well-defined zirconium complexes, an in situ catalyst generation protocol is employed. Catalysts featuring a bis(amidate) ligand derived from 2,2??-diamino-6,6??-dimethylbiphenyl achieve modest hydroamination activity with a primary aminoalkene at 110 ??C, with enantiomeric excesses (ee???s) of up to 25%. Catalysts featuring a bis(amidate) ligand derived from 3,3??,5,5??-tetrabromo-2,2??-diamino-6,6??-dimethylbiphenyl display impressive reactivity with a primary aminoalkene, including room temperature hydroamination, with ee???s ranging from 52-55%. Catalysts featuring a bis(ureate) ligand derived from 2,2??-diamino-6,6??-dimethylbiphenyl are shown to be capable of cyclizing secondary aminoalkenes with ee???s up to 63%. Capillary electrophoresis is developed as a method to determine the ee of tertiary amine products. A kinetic study of a catalyst featuring a bis(amidate) ligand derived from 2,2??-diamino-6,6??-dimethylbiphenyl supports established mechanistic proposals for neutral group 4 hydroamination catalysts. Solid state molecular structures, combined with existing knowledge of bonding and catalytic reaction pathways, are used to propose models for how enantioselectivity is achieved through the use of different ligand frameworks. The use of known ligands in the formation of tantalum complexes for hydroaminoalkylation catalysis is then explored. Installation of such axially-chiral bis(amidate) ligands onto tantalum centres is undertaken and the structure of these complexes is examined in solution and, where possible, in the solid state. Catalytic testing reveals general competence of these catalysts for the hydroaminoalkylation reaction.

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