An aqua-Ru-quinone complex, [Ru(trpy)(Q)(OH2)]2+ (trpy=2,2’: 6’,2”-terpyridine, Q=3,5-di-tert-butyl-1,2-benzoquinone, or 3,6-di-tert-butyl-1,2-benzoquinone) dissociates two protons in a stepwise fashion. The pKavalues of [Ru(trpy)(Q)(OH2)]+ and [Ru(trpy)(Q)(OH)]+ are 5.5 and 10.5, respectively. Proton dissociation of the latter is coupled with electron transfer from the resultant O2− to Q to produce an unusual O-radical complex, [Ru(trpy)(Sq)(O−·)]0 (Sq=3,5-di-tert-butyl-1,2-benzosemiquinone). Dissociation of an amino proton of analogous [Ru(trpy)(Q)(NH3)]2+ (pKa6.2) results in similar intramolecular electron transfer from NH2− to Q with generating an N-radical complex, [Ru(trpy)(Sq)(NH2·)]+. One-electron oxidation of [Ru(trpy)(Sq)(NH2·)]2+ produces [Ru(trpy)(Q)(NH2·)]2+, which reacts with alcohols to give the correspondent aldehyde or ketone with regeneration of [Ru(trpy)(Sq)(NH3)]+. On the other hand, two electron oxidation of [Ru(trpy)(Sq)(O−·)]0 generates a catalytic ability to oxidize alcohols. The catalytic activity of the O-radical complex is less than that of N-radical one. Dinuclear Ru complexes bridged with an anthracene framework, [Ru2(btpyan)(Q)2(OH)2]2+ and [Ru2(btpyan)(bpy)2(OH)2]2+ (btpyan=1,8-bis(2,2’: 6’,2”-terpyrid-4’-yl)anthracene) catalyze four-electron oxidation of water. The catalytic activity for O2 production by [Ru2(btpyan)(Q)2(OH)2]2+ is much higher than [Ru2(btpyan)(bpy)2(OH)2]2+. Proton dissociation of [Ru2(btpyan)(Q)2(OH)2]2+ spontaneously produces [Ru2(btpyan)(Sq)2(O−·)2]0, and the subsequent oxidation of the two Ru(Sq)(O−·) groups induces O-O bond formation through the coupling reaction of two O-radicals. On the other hand, deprotonation of [Ru2(btpyan)(bpy)2(OH)2]2+ is caused by oxidation of [Ru2(btpyan)(bpy)2(OH)2]2+, and four electron oxidation affords [RuIV2(btpyan)(bpy)2(=O)2]4+ without passing through an O-radical complex. High catalytic activity of [Ru2(btpyan)(Q)2(OH)2]2+ compared with [Ru2(btpyan)(bpy)2(OH)2]2+ as to the O2 evolution is associated with the easiness of the O-O bond formation by the coupling reaction of two Ru-O· radicals compared with that of two RuIV=O groups. A new pbn (pbn=2-pyridylbenzo[b]-1,5-naphthyridine) ligand was designed to simulate the NAD+/NADH redox reaction as a possible renewable hydride donor. A series of [Ru(bpy)3−n(pbn)n]2+ (n=1, 2, 3) complexes undergoes photo-induced two-(n=1), four-(n=2), and six-electron reductions (n=3) under irradiation of visible light in the presence of N(CH2CH2OH)3.
[Ru(terpy)(bpy)(OH2)]2+ and its analogues were found to be highly active as catalysts toward water oxidation in the presence of Ce4+ as an oxidizing reagent in acidic aqueous media. These findings were considered as a significant breakthrough in this field because there had been a long belief that the four-electron process (i.e., 2H2O→O2+4H++4e−) is much more effectively accelerated by dinuclear or tetranuclear metal complexes. The kinetics of O2 evolution is investigated as a function of either the catalyst concentration or the oxidant concentration, revealing that these catalysts can be classified into two groups exhibiting different rate laws for O2 evolution. Moreover, the singlet biradical character of the hydroxocerium(IV) ion is realized, indicating that the radical coupling of the oxygen atoms of a RuV=O species and a hydroxocerium(IV) ion is the key step for the catalysis. Several important insights into the mechanism of oxygen evolution from water catalyzed by the mononuclear aquaruthenium complexes will also be discussed.
A plenty of human efforts is necessary to perform simply repeating processes such as preparations of synthetic intermediates and building blocks, optimizations of reaction conditions and couplings of building blocks in syntheses of natural products and their analogue libraries. These efforts can be reduced by using an automated synthesis apparatus. It is expected that laboratory automation can realize highly efficient, reproducible and safe synthesis of organic compounds. We report here the application of single reactor automated synthesizers to supply of a synthetic key intermediate of taxol and to preparation of monosaccharide building blocks toward combinatorial oligosaccharide library synthesis. In addition, we also applied multi reactor automated synthesizers to optimization of a primary amide N-glycosylation and to one-pot parallel syntheses of phytoalexin elicitor active oligosaccharides library and LewisX/dimeric LewisX library.
Nuclear export signal (NES)-mediated export of MEK and Rev from the nucleus to the cytoplasm is revealed to be essential for proliferation of tumor cells and viral multiplication of HIV. In this context, we have been engaged in explore medicinal leads by using medicinal plant originated natural products with inhibitory potency for nuclear export of NES-containing proteins as seed principles. By use of the bioassay monitoring MEK directly, the unprecedented NES non-antagonistic MEK-export inhibitor, peumusolide A (5), was disclosed. Furthermore, this principle was shown to act in the NES non-antagonistic mode by the synthesized probe and display selective cytotoxicity for MEK-activated tumor cells. Additionally, we developed the stereo-controlled synthesis of the core structure of 5, presenting all four stereoisomers from the common starting material. By application of this procedure, the first total synthesis of 5 as well as exploration of the antitumor leads with the novel mechanism of action has been accomplished. With respect to Rev-export inhibitor valtrate (3), the synthesis of 5,6-dihydroanalog (24), rationally designed with the aid of MO calculation, presented the alternative bioisosteric seed principle. By utilizing this bioisotere as the scaffold, the new anti-HIV lead with enhanced Rev-export inhibitory potency in comparison with 3 and 24 has been disclosed.
We have recently developed novel resist materials for micro-lithographic patterning. One is a hydroxystyrene derivative, which is a key compound for KrF excimer laser resists. We found that microwave-assisted decarboxylation of hydroxycinnamic acids smoothly proceeded under the presence of a catalytic amount of amine base, to afford hydroxystyrene derivatives in good yield. The other is positive and negative-tone molecular resists utilizing the unique character of a furan ring. The synthesized compounds showed relatively high glass transition temperature and readily formed uniform amorphous films on a silicon wafer. The sensitivity as an EB resist of both positive and negative-tone resists was below 10 µC/cm2 and line and space patterns of 200 nm could be fabricated. The promising feature of the positive-tone resist is that no outgassed products from base matrixes are theoretically produced under the exposure and post-exposure bake procedure.
Since the first synthesis of silicon compound in mid-19th century, silicon compounds have been widely utilized in organic synthesis. Among them, silanols are recognized as reactive intermediates to form siloxanes. Before our research, studies on silanols were limited to the isolation and structure determination by introducing bulky substituents for stabilization. Therefore the reactions starting from silanols were mostly undiscovered. We have found that introducing medium-size substituents is effective for isolation as well as utilization of silanols as starting materials. In this article, our 15 years of research on silanol chemistry is summarized. That includes composition of supramolecular aggregates and usages as starting compounds of the siloxanes or silsesquioxanes with highly regulated structures. Those compounds show various outstanding features like thermal stability, transparency, and physiological inertness. Among these compounds, ladder-type silsesquioxanes are highlighted in this article.
Asymmetric hydrogenation is a powerful tool for stereoselective synthesis. Despite the wide use of rhodium and ruthenium catalysts, the substrates are still limited. Recently, iridium catalysts have emerged as a new entry to the field, giving high reactivity to give high yield and high stereoselectivity for low reactive substrates.