Premium
Olefin Epoxidation with Hydrogen Peroxide Catalyzed by Lacunary Polyoxometalate [γ‐SiW 10 O 34 (H 2 O) 2 ] 4−
Author(s) -
Kamata Keigo,
Kotani Miyuki,
Yamaguchi Kazuya,
Hikichi Shiro,
Mizuno Noritaka
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200600384
Subject(s) - chemistry , hydrogen peroxide , allylic rearrangement , cyclooctene , catalysis , cyclohexene , epoxide , medicinal chemistry , steric effects , polyoxometalate , peroxide , thianthrene , nucleophile , photochemistry , inorganic chemistry , organic chemistry
The tetra‐ n ‐butylammonium (TBA) salt of the divacant Keggin‐type polyoxometalate [TBA] 4 [γ‐SiW 10 O 34 (H 2 O) 2 ] ( I ) catalyzes the oxygen‐transfer reactions of olefins, allylic alcohols, and sulfides with 30 % aqueous hydrogen peroxide. The negative Hammett ρ + (−0.99) for the competitive oxidation of p ‐substituted styrenes and the low value of (nucleophilic oxidation)/(total oxidation), X SO =0.04, for I ‐catalyzed oxidation of thianthrene 5‐oxide (SSO) reveals that a strongly electrophilic oxidant species is formed on I . The preferential formation of trans ‐epoxide during epoxidation of 3‐methyl‐1‐cyclohexene demonstrates the steric constraints of the active site of I . The I ‐catalyzed epoxidation proceeds with an induction period that disappears upon treatment of I with hydrogen peroxide. 29 Si and 183 W NMR spectroscopy and CSI mass spectrometry show that reaction of I with excess hydrogen peroxide leads to fast formation of a diperoxo species, [TBA] 4 [γ‐SiW 10 O 32 (O 2 ) 2 ] ( II ), with retention of a γ‐Keggin type structure. Whereas the isolated compound II is inactive for stoichiometric epoxidation of cyclooctene, epoxidation with II does proceed in the presence of hydrogen peroxide. The reaction of II with hydrogen peroxide would form a reactive species ( III ), and this step corresponds to the induction period observed in the catalytic epoxidation. The steric and electronic characters of III are the same as those for the catalytic epoxidation by I . Kinetic, spectroscopic, and mechanistic investigations show that the present epoxidation proceeds via III .