Showing papers by "Shiro Hikichi published in 2007"
TL;DR: The tetra-n-butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA](4)[gamma-SiW(10)O(34)(H(2)O)(2)] catalyzes the oxygen-transfer reactions of olefins, allylic alcohols, and sulfides with 30 % aqueous hydrogen peroxide as mentioned in this paper.
Abstract: The tetra-n-butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA](4)[gamma-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 rho(+) (-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)[gamma-SiW(10)O(32)(O(2))(2)] (II), with retention of a gamma-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.
122 citations
TL;DR: The catalytic properties of disilicoicosatungstates, [{γ-SiW10O32(H2O)2}2(μ-O)4− (2) and their parent divacant γ-Keggin type silicodecatungstate as discussed by the authors, toward C-C bond formation reactions have been investigated.
41 citations
TL;DR: In this article, the strong electrostatic interaction between the highly negatively charged polyoxometalates and the +2- or +3-charged [M(tacn)2]n+ as well as the hydrophobicity of [M[Tacn]2] n+ results in the formation of the water-insoluble binary composites.
Abstract: Complexation of Keggin-type polyoxometalates (POMs), [α-PW12O40]3- (PW), [α-SiW12O40]4- (SiW), and [γ-SiV2W10O38(OH)2]4- (SiVWH) with cationic transition-metal complexes having a triazacyclononane ligand, [M(tacn)2]n+ (M−tacn; M = CoIII and NiII; tacn = 1,4,7-triazacyclononane), yields fine particles of inorganic−organic composites. The strong electrostatic interaction between the highly negatively charged POMs and the +2- or +3-charged [M(tacn)2]n+ as well as the hydrophobicity of [M(tacn)2]n+ results in the formation of the water-insoluble binary composites. The crystal structure of the 1:1 (=the molar ratio of POM to M−tacn) composite [Co(tacn)2][α-PW12O40]·2H2O (1·H2O) shows the close packing of PW and Co−tacn in the crystal lattice. The guest sorption properties of the corresponding anhydrous compound 1 show that the amounts of hydrophobic molecules sorbed are comparable to that of water. The reaction of SiVWH with Co−tacn yields fine particles of [Co(tacn)2]2[γ-SiV2W10O40]·6H2O (2·H2O), and the mola...
28 citations
TL;DR: In this paper, the strong electrostatic interaction between the highly negatively charged polyoxometalates and the +2- or +3-charged [M(tacn)2]n+ as well as the hydrophobicity of [M[Tacn]2] n+ results in the formation of the water-insoluble binary composites.
Abstract: Complexation of Keggin-type polyoxometalates (POMs), [α-PW12O40]3- (PW), [α-SiW12O40]4- (SiW), and [γ-SiV2W10O38(OH)2]4- (SiVWH) with cationic transition-metal complexes having a triazacyclononane ligand, [M(tacn)2]n+ (M−tacn; M = CoIII and NiII; tacn = 1,4,7-triazacyclononane), yields fine particles of inorganic−organic composites. The strong electrostatic interaction between the highly negatively charged POMs and the +2- or +3-charged [M(tacn)2]n+ as well as the hydrophobicity of [M(tacn)2]n+ results in the formation of the water-insoluble binary composites. The crystal structure of the 1:1 (=the molar ratio of POM to M−tacn) composite [Co(tacn)2][α-PW12O40]·2H2O (1·H2O) shows the close packing of PW and Co−tacn in the crystal lattice. The guest sorption properties of the corresponding anhydrous compound 1 show that the amounts of hydrophobic molecules sorbed are comparable to that of water. The reaction of SiVWH with Co−tacn yields fine particles of [Co(tacn)2]2[γ-SiV2W10O40]·6H2O (2·H2O), and the mola...
2 citations
TL;DR: The tetra-n-butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA](4)[gamma-SiW(10)O(34)(H(2)O)(2)] catalyzes the oxygen-transfer reactions of olefins, allylic alcohols, and sulfides with 30 % aqueous hydrogen peroxide as mentioned in this paper.
Abstract: The tetra-n-butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA](4)[gamma-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 rho(+) (-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)[gamma-SiW(10)O(32)(O(2))(2)] (II), with retention of a gamma-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.
2 citations
TL;DR: In this article, the authors obtained disilicoeicosatung states, such as [γ-SiW 10 O 32 (H 2 O) 2 ) 2 (μ-O) 2 ] 4 - (2) and [H(γSiW10 O 32 ) 2(μ O)4] 7- (3), which are obtained by the dehydrative condensation of a partially protonated divacant lacunary Keggin-type polyoxometalate, and showed that only S-shaped cluster 2 catalyzes H 2
Abstract: Novel disilicoeicosatungstates, [(γ-SiW 10 O 32 (H 2 O) 2 ) 2 (μ-O) 2 ] 4 - (2) and [H(γ-SiW 10 O 32 ) 2 (μ-O)4] 7- (3), are obtained by the dehydrative condensation of a partially protonated divacant lacunary Keggin-type polyoxometalate, [γ-SiW 10 O 34 (H 2 O) 2 ] 4- (1). Interestingly, only S-shaped cluster 2 catalyzes H 2 O 2 -based Baeyer-Villiger oxidation of cycloalkanones, and catalytic performance of 2 (TON ∼1900) is much higher than those of previously reported catalysts. Also 2 efficiently promotes acid-catalyzed Mukaiyama aldol, carbonyl-ene, and Diels-Alder reactions. In contrast, monomeric cluster 1 and closed-shape dimer 3 promote the base-catalyzed Knoevenagel reaction. These results indicate that acid-base properties of 1 - 3 are clearly different, while they are composed of common (γ-SiW 10 O 32 ] fragments.
1 citations