Theoretical studies on structural and mechanistic aspects of transition-metal catalyzed reactions
by Lam King Chung
xx, 143 leaves : ill. ; 30 cm
Theoretical studies on structural and bonding features of transition-metal complexes and reaction mechanisms of transition-metal-catalyzed reactions are reported in this thesis....[ Read more ]
Theoretical studies on structural and bonding features of transition-metal complexes and reaction mechanisms of transition-metal-catalyzed reactions are reported in this thesis.
1-Zirconacyclopen-3-yne (1) was reported as a small and stable five-membered cyclic alkyne. We investigated the structural stability of 1 with the aid of DFT calculations. The calculations results suggest that the resonance hybrid between a cumulene complex form and a metallacyclopentyne Lewis structure is required to account for the structure and stability of 1.
The site preference of boryl ligands in five-coordinate transition metal boryl complexes has been investigated by DFT calculations. The preferred site for a boryl ligand depends on the electron count of the complex under consideration. Our studies show that the very strong σ-donating boryl ligands choose to occupy coordination sites such that those orbitals accommodating metal d electrons have minimal metal-boryl σ*-antibonding character.
The heterobimetallic complex (η5-C5H5)Ru(CO)(μ-dppm)Mn(CO)4 was found as the active catalyst for the coupling reactions of epoxides with carbon dioxide to yield cyclic carbonates. DFT calculations were carried out to understand the reaction mechanisms. We have shown how the two metal centers cooperate with each other to promote the catalytic reactions.
In 1997, Masuda et al. reported a palladium-catalyzed cross-coupling reaction of pinacolborane with aryl halides. DFT calculations have been preformed to study the reaction mechanism. Our calculations show that the metathesis process of [LnPd-Ar(η2-H-B(OR)2)]+ → [LnPd-H(η2-Ar-B(OR)2)]+ via a four-center transition state is a feasible way to transfer the boryl group from pinacolborane to the aryl group. Applying the newly proposed mechanism, we have also investigated the palladium-catalyzed coupling reactions of (dialkylamino)boranes with aryl halides to yield monomeric aryl(dialkylamino)boranes.
DFT calculations on the model systern [(PH3)(CO)(C1)Ru-CH2CH2B(OCH2CH2O)] provide insight into the process by which a boryl group, at the β-position of a metal bound alkyl moiety, can be transferred to the metal center. Implications of this pathway with regard to catalytic processes such as hydroboration, dehydrogenative borylation, diboration and addition of other B-X bonds to alkenes are noted.