Mechanistic studies and synthetic applications of oxygen atom transfer reactions catalyzed by rhenium(V) dithiolato complexes and methyltrioxorhenium (MTO)

55 Results and Discussion 55 References 61 CHAPTER III. OXIDATION OF SULFIDES BY TERT-BUTYL HYDROPEROXIDE CATALYZED BY AN OXORHENIUM(V) DITHIOLATO COMPLEX A GENERAL AND EFFICIENT SYNTHESIS OF SULFOXIDES AND SULFONES. 62 Abstract 62 iv Introduction 63 Results and Discussion 65 Conclusion 87 Experimental Section 87 References 90 CHAPTER IV. OXIDATION OF SYMMETRIC DISULFIDES WITH HYDROGEN PEROXIDE CATALYZED BY METHYLTRIOXORHENIUM 96 Abstract 96 Introduction 97 Experimental Section 98 Results 101 Discussion 104 References 106 GENERAL CONCLUSIONS 113 ACKNOWLEDGMENTS 115 1 Mechanistic studies and synthetic application of oxygen transfer reactions catalyzed by methyltrioxorhenium (MTO) and rhenium(V) dithiolato complexes Ying Wang Major Professor: James H. Espenson Iowa Sate University A series of Re(V) dithiolato complexes catalyze the rapid and efficient transfer of an oxygen atom from a wide range of ring-substituted pyridine N-oxides to triphenylphosphine, yielding the pyridines in high yield. Some of these complexes can also catalyze the oxidation of a variety of sulfides to sulfoxides and sulfones with /erf-butyl hydroperoxide in excellent yield with high selectivity and generality. Heavily substituted thiophenes can also be converted to the corresponding S-oxides with moderate to excellent yields using this catalytic system. The kinetics and mechanism of these two oxygen transfer reactions were also studied. The mechanisms feature a common aspect in that phosphines and sulfides, being the substrates, inhibit the reaction. Rate constants were obtained for the oxygen atom transfer reactions between pyridine N-oxides and phosphines catalyzed by Re(V) dithiolato complexes. In the case of oxygen atom transfer reaction between ferf-butyl hydroperoxide and sulfide catalyzed by Re(V) dithiolato compounds, an induction period is observed due to the slow ligand exchange step. Reaction schemes are proposed to interpret the kinetic data. In both cases, the active intermediates are Re(VII) dioxo species, which were detected at the low temperature. Organic disulfides with both alkyl and aryl substituents are oxidized by hydrogen peroxide when CHjReOs (MTO) is used as a catalyst. Thiosulfinate is formed in the first 2 step in about an hour with nearly quantitative yield. Kinetics studies of the first oxidation reaction established that two peroxorheniam compounds are the active forms of the catalyst. Rate constants were obtained and a mechanism was proposed in which the electron-rich sulfur attacks the peroxo oxygen of intermediates. 1 GENERAL INTRODUCTION Introduction Oxygen atom transfer reactions catalyzed by transition-metal compounds continue to category of oxygen transfer catalysts is high-valent transition metal-oxo complexes, with metals such as molybdenum, tungsten and rhenium. In comparison with Mo(VI)/Mo(IV) and W(VI)/W(IV) catalytic systems, one can envisage a parallel rhenium chemistry with a pair of Re(VII)/Re(V) species that can transfer an oxygen atom to the substrate by going through a Re(VII)/Re(V) loop. Indeed, some Re(V) compounds reported are good oxygen transfer catalysts.2"5 Recently, a series of Re(V) compounds was synthesized in our group and among them is the Re(V) dithiolato complex MeReO(mtp)(PPh3) 1 where mtpHi is 2(mercaptomethyl)thiophenol (0-HSC6H4CH2SH). This compound has been found to catalyze oxygen atom transfer reactions. The deprotection of pyridine N-oxides by phosphines and the syntheses of sulfoxide and sulfones from sulfides by /erf-butyl hydroperoxide have been investigated. attract great interest due to their fundamental role in chemistry and biology.1 One major