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UBC Theses and Dissertations

Titanium-catalyzed hydroamination : mechanism, reactivity and applications Hao, Han


This thesis focuses on investigating a bisamidate bisamido titanium complex as a pre-catalyst for the hydroamination of alkynes with primary amines. It begins with revealing the reaction mechanism using both theoretical and experimental tools. Next are efforts to extend the use of this catalyst for the synthesis of interesting functional materials and exploring its reactivity with α-heteroatom-substituted alkynes. Computational studies were performed to understand the high reactivity and regioselectivity of the titanium pre-catalyst, which could not be fully answered previously. The hemilabile amidate ligand was found to play a key role by both creating the right geometric environment to lower the transition state energies and to promote hydrogen bonding to allow metal-ligand cooperativity. Experiments were designed and conducted to test the computationally proposed mechanism. Conjugated enamine small molecules and polymers were successfully prepared using titanium-catalyzed hydroamination, and more importantly isolated as characterizable functional materials. They exhibit interesting photophysical properties including emission wavelengths and fluorescent quantum yields that are tunable with substituent modification. Moreover, computational analysis was found to be precise and useful in predicting and designing conjugated enamines as emissive materials. Early stage investigation of the synthesis of multi-broron-nitrogen-substituted polycyclic systems from conjugated enamines was presented together with their interesting photophysical properties. The first example of catalytic hydroamination of α-phosphino alkynes was achieved with the same titanium pre-catalyst. Phosphino enamines/imines and β-aminophosphines are important phosphorous-nitrogen-containing ligands. These compounds could be prepared easily with good functional group tolerance, high reactivity and high regioselectivity. The synthesis of nitrogen-phosphorous-nitrogen (NPN) type compounds and complexation using in situ generated ligands were found to be a successful route to prepare metal-NPN complexes. Additionally, an iron bisenamido complex was prepared via a sequential hydroamination-salt metathesis, and showed catalytic reactivity in ammonia borane dehydrogenation. Hydroamination of copper acetylide was successfully achieved with the titanium pre-catalyst as one of the few examples of metal acetylide hydroamination. Copper enamide product was isolated at almost quantitative yield and its structure was characterized. Mechanistic studies showed the reaction is a direct functionalization of the acetylide moiety by the [2+2] cycloaddition mechanism, which was supported by further kinetic and isotope competition studies.

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