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Investigation of the Reaction Mechanism for the Epoxidation of Alkenes with Hydrogen Peroxide Catalyzed by a Protonated Tetranuclear Peroxotungstate with NMR Spectroscopy, Kinetics, and DFT Calculations
Author(s) -
Ishimoto Ryo,
Kamata Keigo,
Mizuno Noritaka
Publication year - 2013
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201201058
Subject(s) - chemistry , cyclooctene , alkene , catalysis , cyclohexene , protonation , medicinal chemistry , photochemistry , hydrogen peroxide , steric effects , nucleophile , reaction mechanism , cycloalkene , stereochemistry , organic chemistry , ion
Abstract For the epoxidation of cyclooctene with hydrogen peroxide (H 2 O 2 ), the catalytic activity of a dinuclear peroxotungstate, [{WO(O 2 ) 2 } 2 (μ‐O)] 2– , is strongly dependent on additives, and HClO 4 is the most effective. The reaction of [{WO(O 2 ) 2 } 2 (μ‐O)] 2– with HClO 4 (0.5 equiv.) gives a protonated tetranuclear peroxotungstate, [H{W 2 O 2 (O 2 ) 4 (μ‐O)} 2 ] 3– ( I ). The diastereoselectivity for epoxidation of 3‐methyl‐1‐cyclohexene shows that steric constraints of the active site of I are comparable to those of di‐ and tetranuclear peroxotungstates with XO 4 n – ligands (X = Se VI , As V , P V , S VI , and Si IV ). The lowest X SO [X SO = (nucleophilic oxidation)/(total oxidation)] value of 0.13 for the I ‐catalyzed oxidation of thianthrene 5‐oxide among peroxotungstates reveals that I has the most electrophilic active oxygen species. Kinetic and spectroscopic results show that an inactive species ( I′ ) is reversibly formed by the reaction of I with water. Reaction rates for the catalytic epoxidation show first‐order dependences on concentrations of alkene and I and a zero‐order dependence on concentrations of H 2 O 2 . All these results indicate that oxygen transfer from I to a C=C double bond in an alkene is the rate‐determining step. Computational studies support the proposed reaction mechanism.