Understanding and Practical Use of Ligand and Metal Exchange Reactions in Thiolate‐Protected Metal Clusters to Synthesize Controlled Metal Clusters

It is now possible to accurately synthesize thiolate (SR)‐protected gold clusters (Au_n(SR)_m) with various chemical compositions with atomic precision. The geometric structure, electronic structure, physical properties, and functions of these clusters are well known. In contrast, the ligand or metal atom exchange reactions between these clusters and other substances have not been studied extensively until recently, even though these phenomena were observed during early studies. Understanding the mechanisms of these reactions could allow desired functional metal clusters to be produced via exchange reactions. Therefore, we have studied the exchange reactions between Au_n(SR)_m and analogous clusters and other substances for the past four years. The results have enabled us to gain deep understanding of ligand exchange with respect to preferential exchange sites, acceleration means, effect on electronic structure, and intercluster exchange. We have also synthesized several new metal clusters using ligand and metal exchange reactions. In this account, we summarize our research on ligand and metal exchange reactions.     

Fabrication of Dendrimer‐Based Polyion Complex Submicrometer‐Scaled Structures with Enhanced Stability under Physiological Conditions

Submicrometer‐scaled (subμ‐) self‐assembled materials have been developed based on polyion complex (PIC) formation, in particular for biomedical‐applications. However, sufficient stability under physiological conditions is required for their practical use. In this study, PIC formation behavior is examined using a block aniomer, poly(ethylene glycol)‐b‐poly(aspartic acid), and homocatiomers, poly(l ‐lysine) (LPK) and dendritic poly(l ‐lysine) (DPK) with different generations, to elucidate the contribution of the dendritic architecture to stability enhancement. LPK‐based PIC shows a subμ‐vesicular structure only at 25 °C in the absence of NaCl; in contrast, DPK‐based PIC forms a subμ‐structure under physiological salt concentration and temperature conditions, even when the number of charges of a single molecule is much smaller than that of LPK. Moreover, the formation of subμ‐vesicular and ‐spherical micellar structures is dependent on DPK generation. Thus, the molecular backbone architecture of the PIC component plays an important role not only in expanding the preparation conditions and enhancing stability, but also in controlling the self‐assembled structures, mainly due to the spatially restricted structures of dendrimers.

     

Synthesis of ACAT inhibitors through substitution using allylic picolinate and copper reagent

Amide of an octanoic acid possessing an aryl group at C3 position is a highly potent ACAT inhibitor. In this paper, we describe a synthetic access to this class of compounds as optically active forms. The key reaction is substitution of the allylic picolinate of (S,Z)-8-(benzyloxy)oct-5-en-4-ol with a copper reagent derived from (benzo[d][1,3]dioxol-4-yl)MgBr and CuBr·Me₂S to produce anti S_N2′ product regio- and stereo-selectively. The product was hydrogenated to afford (S)-3-benzo[d][1,3]dioxol-4-yloctan-1-ol, which upon oxidation furnished the octanoic acid. Finally, the acid was converted with 2,6-(i-Pr)₂C₆H₃NH₂ to the target amide via acid chloride. In a similar way, the one-carbon long homolog was synthesized.     

Global status of trace elements in the ocean

Trace elements in seawater can be limiting factors of biological productivity, tracers of ocean circulation and biogeochemical processes, and proxies for paleoceanography. The global status of trace elements and their isotopes (TEIs) in the ocean is being explored this decade through an international study of the global marine biogeochemical cycles of TEIs (GEOTRACES). Such an international study has become possible due to recent methodological developments in sampling, preconcentration, and measurement of TEIs. Here, we present an overview of recent methodological developments and initial GEOTRACES intercalibration activities for obtaining data about TEIs that are accurate, precise, and intercomparable.     

Dielectric study of acetophenone and its derivatives

Complex permittivity measurements on acetophenone and its derivatives o-hydroxybenzaldehyde, o-methylacetophenone, and o-hydroxyacetophenone are performed at frequencies between 1 MHz and 20 GHz at temperatures from 273 to 323 K. The parameters obtained from the fitting of the complex permittivity are analyzed in order to study the effects of the hydroxyl group within a molecule on the dielectric relaxation phenomenon in these liquids. The analysis indicates that dynamical properties are affected not only by the intermolecular hydrogen bond but also by the slight change in molecular structure. This conclusion differs from those obtained from the results of other experiments.     

Kinetics of Si2H2 produced by the 193 nm photolysis of disilane

Si₂H₂ species were produced through the 193 nm excimer laser photolysis of Si₂H₆. Time‐resolved photoionization mass spectrometry was employed to study the reaction kinetics of Si₂H₂. The lower limit of self‐reaction of Si₂H₂ was estimated, k(Si₂H₂ + Si₂H₂) ≥ (1.7 ± 0.5) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, through analysis of the decay traces. This rate constant was independent of a total pressure on the entire pressure range (p_total = 1–7 Torr). No reaction of Si₂H₂ with H₂, CH₄, SiH₄, and Si₂H₆ was confirmed. The decay rates of Si₂H₂ reacting with O₂, NO, and HCl exhibit negative dependence on the total pressure. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 136–141, 2001