New applications of molybdenum (VI) catalyzed oxygen atom transfer reactions

Abstract

The chemistry of dioxomolybdenum coordination complexes has been deeply investigated as a model for molybdoenzyme structure and activity, as well as for non-biological chemical applications. In this work, we present investigation into new applications of dioxomolybdenum reactivity and the mechanisms thereof. Chapter 1 gives a brief summary of metal oxo bonding structure, the reactivity of molybdoenzymes and synthetic molybdenum complexes, oxidative kinetic resolutions, metal-dioxygen complexes, and catalytic oxidation of thiols. Chapter 2 presents the use of a series of chiral dioxomolybdenum (VI) Schiff-base salen complexes as catalysts for the oxidative kinetic resolution of the P-chiral monophosphine, methylphenyl-tert-butyl phosphine. The studied complexes are shown to yield the chiral phosphine oxide in low to moderate enantiomeric excess (0-35% e.e.) employing pyridine N-oxide as the stoichiometric oxygen atom source. Use of a para-nitro substituent on the ligand salicylimine ring is found to increase catalyst activity, and enantioselectivity of the reaction is found to be controlled by steric bulk at the salen ortho-position. Density Functional Theory (DFT) study of the reaction finds the stereochemically-defining step is the O-transfer transition state involving nucleophilic attack of the phosphine on an oxo-group of the chiral LMoO2 complex. Chapter 3 reports the catalytic oxidation of phosphines by Schiff-base complexes of dioxomolybdenum under aerobic conditions. The activity of the complexes toward aerobic oxidation of phosphines is found to be the same as with pyridine N-oxide, with the presence of a para-nitro substituent greatly increasing the rate of oxidation. DFT studies are carried out to determine the mechanism of coordination and activation of dioxygen towards oxygen atom transfer, and to determine which of the available oxygen atom transfer pathways is most favorable for the oxidation of phosphines. Transfer of the oxo moiety is found to be most favorable, and the dioxo complex is regenerated through an unusual cleavage of the peroxo group. A computational investigation of oxygen atom transfer to sulfoxides and their lack of reactivity towards oxidation by dioxomolybdenum complexes is also reported. Chapter 4 extends the reactivity of Schiff-base dioxomolybdenum complexes to include the oxidation of thiols to disulfides under base-free conditions. This reactivity is found to encompass alkyl, benzyl, aryl, and amino acid-derived thiols. Alkyl and benzyl thiols are found to be oxidized to two primary products, of which the disulfide is the major (60-80% yield). The secondary products are found to differ due to substrate effects, with benzyl mercaptan oxidized to benzyl trisulfide and dodecane thiol oxidized to the previously unreported dodecane sulfenic anhydride. The selectivity of this oxidation toward formation of disulfide products is found to increase with addition of base, and variation of catalyst electronics (salen para-substituent = NO2, H, OMe) shows significant rate and product distribution effects. DFT study of possible reaction intermediates suggest this oxidation proceeds initially via a hydrogen atom transfer process forming a thiyl radical and a molybdenum (V) oxo-hydroxyl species. Disproportionation of two molecules of the molybdenum (V) species forms one dioxomolybdenum (VI) complex, one oxomolybdenum (IV) complex, and a molecule of water. Mechanistic pathways are suggested for formation of disulfide, trisulfide, and sulfenic acid products.