Kohichi Yamashita

Micellar promoted photoreduction of ethylenediaminetetraacetato cobaltate(III) by 1-benzyl-1,4-dihydronicotinamide

The photoreduction of ethylenediaminetetraacetato cobaltate(III)([Co(edta)]–) by 1-benzyl-1,4-dihydronicotinamide (BNAH) has been studied in the presence of the surfactants cationic dodecyltrimethylammonium chloride (DTAC) and anionic sodium dodecylsulphate (SDS). Cationic DTAC accelerates the photoreduction of [Co(edta)]–, while anionic SDS suppresses the reaction The micellar effects on the reaction rates have been analysed by distribution of the reactants between an aqueous phase and a micellar phase and monitoring the difference in rate. In the DTAC micellar system, cationic micelles promote the reaction significantly by concentrating the photoexcited BNAH and [Co(edta)]– through hydrophobic and electrostatic interactions in spite of the smaller rate constant in the micellar phase compared with that in the aqueous phase. On the other hand, anionic SDS hinders approach of the anionic [Co(edta)]– to the photoexcited BNAH in the SDS micelles, so as to retard the reaction rate.

Micellar effects on the reduction of tris(acetylacetonato)cobalt (III) by 1-benzyl-1,4-dihydronicotinamide with ionic surfactants

The reduction of tris(acetylacetonato)cobalt(III)[Co(acac)3] by 1-benzyl-1,4-dihydronicotinamide (BNAH) has been studied in 4–100%(v/v) methanol-borate buffer with or without surfactants of anionic sodium dodecylsulphate (SDS) or cationic dodecyltrimethylammonium chloride (DTAC) micelles. The second-order reaction rate decreases on increased methanol content of the solvent system and in the presence of the surfactant micelles in the reaction system; it is retarded in the order of DTAC > SDS > none, and the rate constants of the electron transfer in the micellar pseudophase are 15–70 times smaller than that in the bulk aqueous phase. The binding constants of BNAH to SDS and DTAC micelles were determined to be 236 and 102 mol–1 dm3, respectively, at 30 °C by fluorescence measurements. It was also well reflected, by the estimated values of the activation parameters, that the micellar reaction of both the incorporated reactants [BNAH and Co(acac)3] proceeded at the water-micelle interface, in which low polar environments suppress the reaction through the unfavourable orientation of the reactants and the destabilization of the transition state of the reaction.

Enantioselective photoreduction of tris(acetylacetonato)cobalt(III) by 1,4-dihydropyridine derivatives in the presence of molecular aggregates

Enantioselective photoreduction of Δ- and Λ-tris(acetylacetonato)cobalt(III)([Co(acac)3]) by achiral or chiral 1,4-dihydronicotinamide derivatives has been performed in the presence of molecular aggregates. Although appreciable enantioselectivity is not observed for the photoreduction of [Co(acac)3] by chiral 1,4-dihydronicotinamides in micellar systems, achiral 1-benzyl-1,4-dihydronicotinamide (BNAH) reduces Δ-[Co(acac)3] in preference to the Λ-form in the presence of bovine serum albumin (BSA). The 1,4-dihydronicotinamide derivative covalently bound to BSA also reduces [Co(acac)3] enantioselectively. Activation parameters for the highly enantioselective reaction between photoexcited BNAH and [Co(acac)3] in the presence of BSA have been estimated and are discussed.

Micellar-accelerated photoreduction of tris(acetylacetonato)cobalt(III) by 1,4-dihydronicotinamide derivatives

The photo-accelerated reduction of tris(acetylacenato)cobalt(III) by 1,4-dihydronicotinamide derivatives (BNAH and DNAH) was considerably promoted by anionic micelles of sodium dodecylsulphate through the condensation (or incorporation) of the reductant and the substrate and through the stabilization of cationic species (such as BNAH·+ and BNA+), generated during the reaction.