UBC Theses and Dissertations
Catalytic synthesis of N-heterocycles and alpha-alkylated amines by hydroamination and hydroaminoalkylation Lau, Ying Yin
The research presented in this thesis emphasizes the versatility and utility of N,O-chelated early transition metals for the catalytic synthesis of -alkylated amines. Two major transformations were studied extensively in this work, hydroamination and hydroaminoalkylation. For both reactions, the synthetic utility and substrate scope has been expanded by the work presented herein. In the field of hydroamination, N-heterocycles with more than one heteroatom can now be synthesized using early transition metal catalysts from prochiral substrates. Hydroamination with a bis(amidate)bis(amido) complex of titanium of ether-containing aminoalkyne substrates yield cyclic imines, which are subsequently reduced via asymmetric transfer hydrogenation using the Noyori-Ikariya catalyst, RuCl [(S,S)-Ts-DPEN] (η⁶-p-cymene). 3-Substituted morpholines are synthesized using a one-pot sequential catalysis protocol, in good yields and high enantiomeric excesses. Substrate scope investigations reveal that high enantioselectivities in the asymmetric transfer hydrogenation reaction arise from key hydrogen bonding interactions between the oxygen heteroatom of the ether-containing cyclic imine and the [(S,S)-Ts-DPEN] ligand of Noyori-Ikariya catalyst. This mechanistic insight informed the proposal that this synthetic strategy can be extended to other substrates containing functional groups with hydrogen bond acceptors. As such, 3-substituted piperazines are also prepared with high enantioselectivities using this one-pot protocol. Advances to the hydroaminoalkylation transformation have also been made with the first reported example of room temperature reactivity observed using a phosphoramidate-tantalum complex. The preparation and characterization of a series of N,O-chelated phosphoramidate-tantalum complexes is described. These complexes were easily synthesized from either Ta(NMe₂)₅ by protonolysis or a simple organometallic precursor, TaMe₃Cl₂, by salt metathesis. Reactivity towards catalytic hydroaminoalkylation was explored and the results highlight that the choice of tantalum starting material dramatically affects the reaction temperatures required for catalytic turnover. N,O-chelated phosphoramidate dimethylamido tantalum complexes showed reactivity occurred only at elevated temperatures (≥ 90 °C), whereas phosphoramidate-tantalum complexes derived from TaMe₃Cl₂ exhibited unprecedented catalytic activity at room temperature. Preliminary efforts indicate that there is potential for an asymmetric version of hydroaminoalkylation at room temperature. Chiral phosphoramidate-tantalum complexes were prepared and studied as the first examples of asymmetric hydroaminoalkylation reactions at room temperature.
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