Thesis (Ph. D.)--University of Hong Kong, 1970. / Also available in print.

Thesis (doctoral)--Ludwig-Maximilians-Universität München 1962. / Vita. Includes bibliographical references (p. 57-59).

No description available.

No description available.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Student Number : 9006831D - PhD thesis - School of Chemistry - Faculty of Science / The ligand substitution reactions of aquacobalamin are fast and hence the usual inertness of the d6 Co(III) ion has been modified. It is well established that the reactions proceed through a dissociative interchange mechanism; however, previous ligand studies were performed in a KCl medium, which led to the formation of the more substitution-inert chloro complex. The kinetics of aquacobalamin were reinvestigated with the ligands N3–, NO2–, SCN–, S2O32–, OCN– and SeCN– in a NaNO3 medium. The reactions proceeded too rapidly for saturation kinetics to be observed and hence only the second-order rate constants could be obtained. These were corrected for pH and determined as a function of temperature, from which the activation parameters were determined. The donor atom of the ambidentate ligands were investigated and correlations were found between the Mulliken population on the donor atom, the energy of the highest occupied molecular orbital (HOMO) with σ symmetry, and Δ, the enthalpy of activation, and Δ, the entropy of activation, respectively. Good correlations occurred when the donor atoms were taken to be N for SCNII‡kHII‡kS– and NO2–; S for S2O32–; O for OCN– and Se for SeCN–. The effect that changing the environment of aquacobalamin has on its kinetics was observed by determining the rate constants for the reaction of pyridine with aquacobalamin in water and 70% ethanol. The rates were faster in water and the activation parameters obtained for the reaction of aquacobalamin with pyridine in 70% ethanol are larger than they are for the reaction in water. The larger ΔH‡ arises due to less bond formation between pyridine and Co in the transition state and ΔS‡ is larger because it is dominated by the freeing of the coordinated water i.e. bond breaking is the dominant process in the transition state. The effects of a bulkier ligand than water on the kinetics of aquacobalamin were investigated. The temperature dependence of the kinetics of the substitution of I– in iodocobalamin by imidazole, N3– and S2O32– was studied. Despite the increase in size of the departing ligand there is still nucleophilic participation of the incoming ligand in the transition state and hence the reaction still proceeds via an Id mechanism. In order to probe the cis-effect of the corrin in vitamin B12 derivatives, comparative studies were undertaken of the reactions of aquacobalamin and aqua-10-Xcobalamin, X = Cl, NO, NH2, where the H at C10 was replaced with an electron-donating (Cl, NH2) or electron-withdrawing (NO) group. Formation constants were obtained for aquacobalamin and aqua-10-chlorocobalamin for the substitution of coordinated H2O with various anions (N3–, NO2–, SCN–, S2O32–, OCN–, SeCN–) and neutral N-donor ligands (CH3NH3, pyridine, imidazole). The anionic ligands bind more strongly to aqua-10-chlorocobalamin than to aquacobalamin with log K values larger by between 0.10 and 0.63 (average 0.26) larger. The converse is true for the neutral N-donor ligands, where log K is smaller by between 0.17 and 0.3 (average 0.25). Semi-empirical molecular orbital (SEMO) calculations using the ZINDO/1 model on the hydroxo complexes show that charge density is delocalised from the axial donor atom to the metal and Cl. Thus the anionic ligands bind more strongly to aqua-10-chlorocobalamin because of the ability of the metal and the Cl at C10 to accept charge density from the ligand. The cobalt ion in aqua-10-chlorocobalamin is more electron rich than it is in aquacobalamin and so it is less likely to accept further electron density from a neutral axial donor ligand. This results in the stability being lower than that of aquacobalamin. The reaction kinetics of the substitution of H2O in aqua-10-chlorocobalamin were determined for the ligands N3– and pyridine. The reaction proceeds via a dissociative interchange mechanism since saturation was seen for pyridine and not for N3–. The activation parameters, ΔH‡ and ΔS‡, are lower for aqua-10-chlorocobalamin than aquacobalamin and hence it can be deduced that bond breaking between the coordinated water and the cobalt atom is more dominant in aquacobalamin. The rates of reaction are faster for aquacobalamin than they are for aqua-10-chlorocobalamin. SEMO calculations show that as the Co–O bond is stretched, the charge density on Co in aquacobalamin is always lower than that on aqua-10-chlorocobalamin, suggesting that aquacobalamin is a better electrophile towards the incoming ligand, thereby explaining the faster kinetics. Aqua-10-nitrosocobalamin was synthesised and characterised by FAB(MS), NMR and UV-vis spectroscopy. The strongly electron-withdrawing NO group has deactivated the metal ion towards ligand substitution, with neither 1.2 M pyridine nor 0.7 M N3– showing any spectroscopic evidence for the displacement of the axial H2O ligand. This provides further evidence that the electronic structure of the corrin ring can directly influence the ligand-binding properties of the metal. Aqua-10-aminocobalamin was synthesised from aqua-10-nitrosocobalamin but is unstable in solution. Hence, only a preliminary UV-vis study could be undertaken with the compound. This study shows that the shifts in the bands occur towards longer wavelengths than that of aqua-10-chlorocobalamin, suggesting that the amino group at the C10 position donates more electron density to the cobalt centre than the chloro group.

Thesis (M.S.)--State University of New York at Binghamton, Department of Chemistry, 2006. / Includes bibliographical references.

Thesis--Ph. D., University of Hong Kong. / Includes The Sn mechanism in inorganic reactions. Pt. I. Hydroxide-ion substitution in halogenopentamminecobalt (III) complexes, by the author and others reprinted from Journal of the Chemical Society, May, 1965. Typewritten.

Thesis (M. Phil.)--University of Hong Kong, 1978.

Examination of the scope of the [Rh(CO)₂Cl]₂-catalyzed allylic substitution reaction as well as the development of a domino [Rh(CO)₂Cl]₂-catalyzed allylic alkylation/Pauson Khand reaction is described. A number of experiments were carried out in order to explore the novel regioselectivity in the [Rh(CO)₂Cl]₂-catalyzed allylic substitution reaction, and the [Rh(CO)₂Cl]₂-catalyzed allylic substitution reaction was found to give products resulting from attack of the nucleophile on the carbon bearing the leaving group in a highly regioselective fashion in most cases. Examination of allylic carbonate substrates containing similar substitution at each allylic site was carried out, and conditions that minimize equilibration of active intermediates were determined. Intramolecular [Rh(CO)₂Cl]₂-catalyzed allylic alkylation was accomplished to synthesize challenging eight-membered lactone ring systems. Nucleophile scope was explored with regards to the [Rh(CO)₂Cl]₂-catalyzed allylic substitution reaction, and malonates, substituted malonates, aliphatic amines, and ortho-substituted phenols were all determined to be effective in the reaction. A domino [Rh(CO)₂Cl]₂-catalyzed allylic alkylation/Pauson-Khand reaction was developed which allows the rapid synthesis of bicyclopentenone products from simple, readily available starting materials. The first application of the Pauson-Khand reaction to the synthesis of azabridged bicyclic structures is also described. Various cis-2,6-disubstituted piperidines were cyclized to the corresponding azabridged bicyclopentenones is high yields often in high diastereoselectivities. The effect of ring size, nitrogen substituent, and remote functionality on the Pauson-Khand substrates was studied. The methodology developed was applied to the concise, enantioselective total synthesis of the antimalarial and anticancer indole alkaloid (-)-alstonerine. Pauson-Khand reaction of a readily available enyne synthesized in four steps from L-tryptophan provided a cyclopentenone in high yield as one diastereomer. Elaboration of the Pauson-Khand product required the development of a one pot conversion of a five-membered cyclic silyl enol ether to a sixmembered lactone and the mild acylation of a glycal.