Yukiko Hayashi

Efficient cyclic carbonate synthesis catalyzed by zinc cluster systems under mild conditions

An efficient catalytic system of a zinc cluster and tetrabutylammonium iodide (TBAI) was developed for cyclic carbonate synthesis from epoxides and carbon dioxide (CO2) without the use of any organic solvents under very mild conditions (25 °C, 1 atm), even in the presence of impurities such as water and air. Electrophilicity of the central Zn(II) ion, the number of trifluoromethyl groups, nucleophilicity and leaving ability of the anion of alkylammonium salts, and various reaction parameters have a great effect on the catalytic activity of the bifunctional catalyst. Therefore, this solvent-free process represents an environmentally friendly example for the catalytic conversion of CO2 into value-added chemicals and also has the potential to contribute towards decreasing atmospheric CO2 emission from the burning of fossil fuels.

Sodium methoxide: a simple but highly efficient catalyst for the direct amidation of esters

A simple NaOMe catalyst provides superior accessibility to a wide variety of functionalized amides including peptides through direct amination of esters in an atom-economical and environmentally benign way.

Transesterification of Various Methyl Esters Under Mild Conditions Catalyzed by Tetranuclear Zinc Cluster

A new catalytic transesterification promoted by a tetranuclear zinc cluster was developed. The mild reaction conditions enabled the reactions of various functionalized substrates to proceed in good to high yield. A large-scale reaction under solvent-free conditions proceeded with a low E-factor value (0.66), indicating the high environmental and economical advantage of the present catalysis.

Copper complexes for catalytic, aerobic oxidation of hydrocarbons

Catalytic oxidation of hydrocarbons can be performed efficiently upon treatment with tert-butylhydroperoxide or peracetic acid in the presence of a low-valent ruthenium catalyst. Furthermore, aerobic oxidation of hydrocarbons can be performed in the presence of acetaldehyde using ruthenium, iron, and copper catalysts. Copper derived from copper chloride/crown ether or copper chloride/crown ether/alkaline metal salts have proved to be efficient catalysts. Further study revealed that specific copper complexes formed from copper salts and acetonitrile are convenient and highly useful catalysts for the aerobic oxidation of unactivated hydrocarbons.

A tetranuclear-zinc-cluster-catalyzed practical and versatile deprotection of acetates and benzoates

A new catalytic deacylation of acetates and benzoates through transesterification with methanol was developed (see scheme). Reactions with various acid- and nucleophile-sensitive functional groups proceeded efficiently in the presence of a catalytic amount of the tetranuclear zinc cluster. The present catalysis is applicable to less-reactive tertiary acetates, the deacylation of which is difficult to achieve by transesterification with other catalysts.

Reductive coupling reaction induced by remote-site oxidation in titanocene bis(metallocenylacetylide), where metallocenyl = ferrocenyl or ruthenocenyl: a novel route to Cn (n = 4, 6, and 8) wire with the metallocenyl groups at both terminals

Abstract The titanocene bis(acetylide) complexes (ν5-C5H4)2Ti[C  C)m−Mc)[C  C) n−Mc′] (R  H, SiMe3: m, n = 1 or 2; Mc, Mc′ = ferroacenyl or ruthenocenyl) have been prepared and found to be easily oxidized with 2 equiv. amount of AgPF6 liberating a neutral product, Mc−(CC)m(CC)n−Me′. Electrochemical studies indicate that the reaction is induced by initial oxidation of Me and Me′ followed by unprecedented reductive coupling of the two alkynyl carbons bound to titanocene.

THE REACTION OF TITANOCENE BIS(FERROCENYLACETYLIDE) AND BIS(RUTHENOCENYLACETYLIDE) WITH SILVER CATION : FORMATION OF BIS(TI-TWEEZERS) SILVER COMPLEXES

Titanocene bis(acetylide) where the acetylide has ferrocenyl or ruthenocenyl terminal group (C 5 H 4 R) 2 Ti{(CC) n -Mc} 2 , reacted with 0.5 equivalent amount of AgPF 6 giving cationic heptanuclear complexes of the form [(C 5 H 4 R) 4 Ti 2 {(CC) n -Mc} 4 Ag](PF 6 ) ( 1a : R=SiMe 3 , n =1, Mc=ferrocenyl, 2a : R=H, n =2, Mc=ferrocenyl, 3a : R=SiMe 3 , n =2, Mc=ferrocenyl, 4a : R=SiMe 3 , n =1, Mc=ruthenocenyl) in good yields. Likewise, complex [(C 5 H 4 SiMe 3 ) 4 Ti 2 (CCPh) 4 Ag](PF 6 ) ( 5a ) was isolated. The Ti(IV)–Fe(II)–Ag(I) complex 3a was structurally characterized to confirm the solid-state geometry of the first bis(Ti-tweezers) type chelate complex with silver cation in which the silver was coordinated by four triple bonds of the two titanocene bis(acetylide) moieties. The similar Ti(IV)–Ru(II)–Ag(I) complex 4a was also characterized by single-crystal X-ray analysis. Complex 3a reacted with 1.5 equivalents (with respect to Ti) of AgPF 6 to liberate Fc–(CC) 4 -Fc (Fc=ferrocenyl) quantitatively.

Studies of the Electronic Effects of Zinc Cluster Catalysts and Their Application to the Transesterification of β-Keto Esters

The electronic effects of tetranuclear zinc cluster catalysts on transesterification were investigated by changing the carboxylate ligands in the clusters. High catalyst activity crucially depended on the balance between Lewis acidity and Brønsted basicity of the catalyst; this was consistent with the dual activation of both the electrophile and nucleophile by the cooperative zinc centers. In addition, tetranuclear zinc cluster catalysts achieved the transesterification of β-keto esters with unprecedented levels of broad substrate generality, in which a newly developed pentafluoropropionate-bridged zinc cluster and 4-dimethylaminopyridine additive greatly improved the reactivity of sterically congested α- and α,α-disubstituted β-keto esters.

Transition-metal clusters as catalysts for chemoselective transesterification of alcohols in the presence of amines

Abstract Acylation is one of the most abundant organic transformations of alcohols (esterification) and amines (amidation). Because of the greater nucleophilicity of the amino group compared to the hydroxyl group and the stability of amides compared to esters, N-acylation occurs predominantly in organic synthetic reactions. We reported that the μ-oxo-tetranuclear zinc cluster Zn4(OCOCF3)6O efficiently catalyzes highly chemoselective acylation of hydroxyl groups in the presence of primary and secondary alkyl amino groups to afford the corresponding esters in high yields. Not only zinc carboxylate complexes but also various carboxylate complexes of first-row late transition metals, such as Mn, Fe, Co, and Cu, become catalysts for such the hydroxy group-selective acylation in the presence of amines. Among these carboxylate compounds, we found that the combination of an octanuclear cobalt carboxylate cluster [Co4(OCOR)6O]2 (R = CF3, CH3, and tBu) with nitrogen-containing ligands such as 2,2′-bipyridine show sufficient catalytic activity toward O-selective transesterification. Notably, an alkoxide-bridged dinuclear complex, Co2(OCOtBu)2(bpy)2(μ2-OCH2-C6H4-4-CH3)2, was successfully isolated as a key intermediate that proceeds with Michaelis–Menten behavior through an ordered ternary complex mechanism similar to dinuclear metallo-enzymes, suggesting that the formation of alkoxides, followed by coordination of the ester, is responsible for the unique O-selective acylation.

Reductive elimination by remote electron transfer activation in C4-bridged titanocene-ferrocenyl complexes

Of the new titanocene derivatives [Ti(CCCCR)2(η5-C5H4R′)2][R = ferrocenyl (Fc); R′= H 1, SiMe3 2; R = R′= SiMe3 3], 3 is inert towards electrochemical or chemical oxidation while both 1 and 2 readily release a formal reductive elimination product Fc(CC)4Fc via initial oxidation of the Fc units, suggesting that the process is induced by electron transfer from the Ti–C fragment to Fc+ through the conjugated diacetylide bridges.