Metalloporphyrin-catalyzed olefin epoxidation: Asymmetric catalysis, kinetics, and active intermediates

The focus of this thesis is the olefin epoxidation catalyzed by synthetic metalloporphyrins and other related transition metal complexes. It represents our effort to mimic the functions of cytochrome P450 enzymes that are responsible for selective oxygenations in nature. The aim of this research work is to gain a deeper understanding of the mechanisms of P450-catalyzed oxygen transfer and to develop and optimize practical and potent metal loporphyrin catalysts for important oxidation reactions.; Chapter One contains a brief introduction of cytochrome P450 enzymes and epoxidation catalyzed by synthetic metalloporphyrins. Chapter Two presents a kinetic study of a well-behaved system for homogeneous metalloporphyrin-catalyzed olefin epoxidation. Michaelis-Menten type kinetic results have been observed and the rate-determining step is thought to be breakdown of the putative substrate-oxidant complex to afford the epoxidation product and the resting state of the catalyst. Chapter Three describes our effort to develop effective chiral metal loporphyrins for enantioslective olefin epoxidations One chiral iron porphyrin, prepared from the tetraaminophenylporphyrin template and a chiral binaphthyl moiety, provides moderate enantioselectivities for terminal olefins (ee's 43--65%) and high enantioselectivities for cis-1,2-disubstituted aromatic olefins (ee's up to 89%). Chapter Four describes the competitive olefin epoxidation catalyzed by metalloporphyrins and metallocorroles and the existence of multiple active oxygenating species. Chapter Five addresses the donor ligand effect in manganese salen complex-catalyzed olefin epoxidations. We propose that two active oxidants competitively effect the epoxidation when the stereoselectivities are markedly dependent on the oxygen atom source and the donor ligands can influence the partitioning of the two active oxidants.; In summary, my thesis has been devoted to the biomimetic studies of cytochrome P450 enzymes with an emphasis on the catalytic epoxidation. The chiral metalloporphyrin-catalyzed asymmetric epoxidation mimics the stereoselective olefin epoxidation mediated by cytochrome P450 enzymes. The kinetic study and the investigations on the active intermediates in metalloporphyrin and manganese salen-catalyzed epoxidation shed further light on the mechanisms of oxygen atom transfer catalyzed by synthetic transition metal complexes, and the principles that have been uncovered are expected to have parallels in the Cytochrome P450-catalyzed oxygenation mechanisms.