Development of group 4 and 5 complexes with N,O chelating supporting ligands as catalysts for the alpha-alkylation of amines

• Main Author: Lauzon, Jean Michel
• Language: English
• Published: University of British Columbia 2013
• Online Access: http://hdl.handle.net/2429/44509

The use of stoichiometric, catalytic and theoretical methods in the development of an early transition metal catalyst for the α-alkylation of amines is described herein. The investigation is primarily focused on a series of mono(amidate) complexes of tantalum with varying steric and electronic properties. The amidate binding mode and catalytic activity of these complexes is significantly influenced by sterics. Corresponding bis(amidate) complexes are less active as catalysts for the α-alkylation of amines but offers a platform to study the hemi-lability of amidate ligands as well as tantalaziridine formation in these systems. A model 5-membered metallacycle is synthesized and characterized. Isotopic labeling studies with the most active mono(amidate) precatalyst reveal off-cycle reactions and suggest that tantalaziridine formation is rapid and reversible. Preliminary kinetic investigations implicate alkene insertion as the turnover limiting step, consistent with stoichiometric investigations. In addition, the use of radical probes in ligand backbones and an alkene substrate contradicts a one electron mechanism. Quantum chemical calculations are used to develop a theoretical model of the proposed catalytic cycle. The hemi-lability of amidate ligands is highlighted with the optimization of both κ¹(O) and κ²(N,O) minima and transition states. Here, protonolysis is calculated to be the turnover limiting step with small changes in geometry having a significant effect on the potential energy surface. The unlikelihood of a radical mechanism is supported by the computations of triplet species. A survey of established steric parameters has been completed for asymmetric amidate ligands to be used as a predictive tool for catalyst design. The calculated values can be related to the catalytic activity of mono(amidate) and axially chiral tantalum precatalysts. Diamide and diurea proligands featuring a neutral chalcogen atom tether are installed on zirconium and tantalum. The zirconium species form well-defined κ⁴(N,N,O,O) complexes with fluxional behaviour observed for the tantalum species in solution. No evidence of bonding is observed between the chalcogen donor and any metal centre. Fundamental differences in the redox potentials for ligands and complexes are investigated using cyclic voltammetry. The tantalum complexes are found to catalyze the α-alkylation of amines with the zirconium species being competent precatalysts for hydroamination.