Multi-electron transfer processes for molecular conversions

Multi-electron transfer plays an important role in many chemical reactions. A collection of examples, both of metal complexes that exhibit one-step, multi-electron transfer process, and of chemical reactions based on a multi-electron transfer system, is presented. Emphasis is placed on the role of multi-electron transfer process in a sense that some molecular conversion systems such as reduction of O₂ and oxidation of H₂O become feasible through multi-electron transfer. As an important example of molecular conversion, oxidative polymerization of sulfur-containing compounds through two-electron transfer, is also reviewed.     

Synthesis of poly(sulfonium cation) as an alkylating reagent

Methyl 4-phenylthiophenyl sulfoxide is polymerized to poly(methyl-4-phenylthiophenylsulfonium) in poly(phosphoric acid) with a yield higher than 90%. The demethylation of poly(sufonium cation) is examined to control the composition ratio of the thiophenylene unit in the polycation. The polycation is soluble in common solvents due to the alternative structure of phenylene sufide and phenylenesulfonium cation and is easily converted to poly(phenylene sulfide) through the demethylation with a nucleophile. The poly(sulfonium cation) can be applicable as an alkylating agent for phenol, aniline and benzoic acid to the corresponding to anisole, N-methyl aniline, N,N-dimethyl aniline and benzoic methyl ester, respectively, with high conversion through the demethylation. These products can be isolated without a complicated purification because poly(phenylene sulfide) is precipitated in the mixture as the side product due to the poor solvent solubility.     

Reaction of acetoxyazetidinones with trimethylsilylacetyl thiolesters: preparation of azetidinone-thiolester precursors to carbapenems

Reaction of acetoxyazetidinones, $\text{1}$ and $\text{2}$, with trimethylsilyl- acetyl thiolesters $\text{10}$ afforded azetidinone-thiolesters, $\text{11}$ and $\text{12}$, which are useful intermediates in the carbapenem synthesis.     

Association of calcium phosphate and fibroblast growth factor-2: a dynamic light scattering study

The size distributions of fibroblast growth factor-2 (FGF-2) in aqueous solutions with neutral pH were investigated with a dynamic light scattering technique. We found that the FGF-2 was distributed in dimer or trimer form at concentrations of 0.1-1.0 mg . mL(-1). An aggregate with a hydrodynamic radius of approximately 90 nm coexisted with this and its proportion increased with a decrease in concentration. At lower concentrations (less than 0.10 mg . mL(-1)) FGF-2 aggregates with an average radius of 80-100 nm were dominant and were stable for more than a day. These FGF-2 solutions were mixed with calcium phosphate solutions to produce a sub-micron sized compound of FGF-2 and hydroxyapatite, which could be used as a biological implant that possessed a pharmacological function for bone formation. By utilizing a transformation from amorphous calcium phosphate to hydroxyapatite, FGF-2 was effectively incorporated into polycrystals of hydroxyapatite.SEM photograph of a mixture of hydroxyapatite and FGF-2.     

Role of polymer ligands in the oxygenation of metal chlorophyllins

In order to obtain information about the characteristics of polymer-(metal chlorophyllins), their redox and oxygenation reactions were studied. Complexes composed of metal chlorophyllins (MChn) and partially quaternized poly(4-vinylpyridine) (QPVP) exhibited an increased stability due to electrostatic interaction between polymer cation and carboxylate anions of side groups in the MChn. The reduction of these polymer complexes by S₂O₄^2- occurred easily with an decrease in their apparent reduction potentials. The resulting low-valence complexes had an ability to absorb molecular oxygen. The oxygenation of these polymer complexes was not completely reversible; however the rate of the irreversible reoxidation was less by a factor of 10¹⁰ for the polymeric complex than that for the monomeric MChn-pyridine complex. The stability of these oxygen complexes is discussed in relation to the hydrophobicity of QPVPs.     

Surface-grafted multiporphyrin arrays as light-harvesting antennae to amplify photocurrent generation

Organized multiporphyrin arrays were developed on the conductive surface by a novel coordination-directed molecular architecture aiming at efficient photoelectric conversion. The basic strategy employs the mutual coordination of two imidazolylporphyrinatozinc(II) units to form a cofacial dimer. Thus, meso,meso-linked bis(imidazolylporphyrinatozinc) (Zn2(ImP)2) was organized onto imidazolylporphyrinatozinc on the gold substrate as a self-assembled monolayer. The organized Zn2(ImP)2 bearing allyl side chains was covalently linked by ring-closing olefin metathesis catalyzed with Grubbs catalyst. Alternating coordination/metathesis reactions allow the stepwise accumulation of multiporphyrin arrays on the gold electrode. A successive increase in absorption over a wide wavelength range occurred after each accumulation step of Zn2(ImP)2 on the gold electrode, and cathodic photocurrent generation was enhanced in the aqueous electrolyte system, containing viologen as an electron carrier. The significant increase of the photocurrent indicates that the multiporphyrin array works as a "light-harvesting antenna" on the gold electrode.     

Absorption spectra of amorphous organic films

Using a low-temperature evaporation technique, amorphous-state films of naphthacence, perylene, coronene and 1,12-benzoperylene were prepared, and their absorption and fluorescence spectra were measured. The obtained results are discussed and compared with the corresponding crystal-state spectra.