Shintaro Koumitsu

Bio: Shintaro Koumitsu is an academic researcher. The author has contributed to research in topics: Electron transfer & Photoinduced electron transfer. The author has an hindex of 2, co-authored 2 publications receiving 358 citations.

Energetic comparison between photoinduced electron-transfer reactions from NADH model compounds to organic and inorganic oxidants and hydride-transfer reactions from NADH model compounds to p-benzoquinone derivatives

Abstract: Kinetic studies on photoinduced electron-transfer reactions from dihydropyridine compounds (PyH/sub 2/) as being NADH model compounds to organic and inorganic oxidants and hydride-transfer reactions from PyH/sub 2/ to p-benzoquinone derivatives (Q) in the absence and presence of Mg/sup 2 +/ ion are reported by determining over 150 rate constants. These results, combined with the values of Gibbs energy change of the photoinduced electron-transfer reactions as well as those of each step of the hydride-transfer reactions as being the e/sup -/-H/sup +/-e/sup -/ sequence, which are determined independently, revealed that the rate constants of the photoinduced electron-transfer reactions obey the Rehm-Weller-Gibbs energy relationship and that the activation barrier of the hydride-transfer reactions from PyH/sub 2/ to Q is dependent solely on the Gibbs energy changes of the initial electron transfer from PyH/sub 2/ to Q and the following proton transfer from PyH/sub 2//sup .+/ to Q/sup .-/ and thus independent of the Gibbs energy change of the final electron transfer from PyH/sup ./ to QH/sup ./. The retarding effect of Mg/sup 2 +/ ion observed on the photoinduced electron transfer and hydride-transfer reactions of PyH/sub 2/ is ascribed to the positive shifts of the redox potentials of the ground and excitedmore » states of PyH/sub 2/ due to the complex formation with Mg/sup 2 +/ ion.« less

Energetic Comparison Between Photoinduced Electron-Transfer Reactions from NADH Model Compounds to Organic and Inorganic Oxidants and Hydride-Transfer Reactions from NADH Model Compounds to p-Benzoquinone Derivatives.

Abstract: Kinetic studies on photoinduced electron-transfer reactions from dihydropyridine compounds (PyH/sub 2/) as being NADH model compounds to organic and inorganic oxidants and hydride-transfer reactions from PyH/sub 2/ to p-benzoquinone derivatives (Q) in the absence and presence of Mg/sup 2 +/ ion are reported by determining over 150 rate constants. These results, combined with the values of Gibbs energy change of the photoinduced electron-transfer reactions as well as those of each step of the hydride-transfer reactions as being the e/sup -/-H/sup +/-e/sup -/ sequence, which are determined independently, revealed that the rate constants of the photoinduced electron-transfer reactions obey the Rehm-Weller-Gibbs energy relationship and that the activation barrier of the hydride-transfer reactions from PyH/sub 2/ to Q is dependent solely on the Gibbs energy changes of the initial electron transfer from PyH/sub 2/ to Q and the following proton transfer from PyH/sub 2//sup .+/ to Q/sup .-/ and thus independent of the Gibbs energy change of the final electron transfer from PyH/sup ./ to QH/sup ./. The retarding effect of Mg/sup 2 +/ ion observed on the photoinduced electron transfer and hydride-transfer reactions of PyH/sub 2/ is ascribed to the positive shifts of the redox potentials of the ground and excitedmore » states of PyH/sub 2/ due to the complex formation with Mg/sup 2 +/ ion.« less

Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

Abstract: A fundamental aim in the field of catalysis is the development of new modes of small molecule activation. One approach toward the catalytic activation of organic molecules that has received much attention recently is visible light photoredox catalysis. In a general sense, this approach relies on the ability of metal complexes and organic dyes to engage in single-electron-transfer (SET) processes with organic substrates upon photoexcitation with visible light. Many of the most commonly employed visible light photocatalysts are polypyridyl complexes of ruthenium and iridium, and are typified by the complex tris(2,2′-bipyridine) ruthenium(II), or Ru(bpy)32+ (Figure 1). These complexes absorb light in the visible region of the electromagnetic spectrum to give stable, long-lived photoexcited states.1,2 The lifetime of the excited species is sufficiently long (1100 ns for Ru(bpy)32+) that it may engage in bimolecular electron-transfer reactions in competition with deactivation pathways.3 Although these species are poor single-electron oxidants and reductants in the ground state, excitation of an electron affords excited states that are very potent single-electron-transfer reagents. Importantly, the conversion of these bench stable, benign catalysts to redox-active species upon irradiation with simple household lightbulbs represents a remarkably chemoselective trigger to induce unique and valuable catalytic processes. Open in a separate window Figure 1 Ruthenium polypyridyl complexes: versatile visible light photocatalysts.

Photoredox Catalysis in Organic Chemistry

Abstract: In recent years, photoredox catalysis has come to the forefront in organic chemistry as a powerful strategy for the activation of small molecules. In a general sense, these approaches rely on the ability of metal complexes and organic dyes to convert visible light into chemical energy by engaging in single-electron transfer with organic substrates, thereby generating reactive intermediates. In this Perspective, we highlight the unique ability of photoredox catalysis to expedite the development of completely new reaction mechanisms, with particular emphasis placed on multicatalytic strategies that enable the construction of challenging carbon–carbon and carbon–heteroatom bonds.

The merger of transition metal and photocatalysis

Abstract: The merger of transition metal catalysis and photocatalysis, termed metallaphotocatalysis, has recently emerged as a versatile platform for the development of new, highly enabling synthetic methodologies. Photoredox catalysis provides access to reactive radical species under mild conditions from abundant, native functional groups, and, when combined with transition metal catalysis, this feature allows direct coupling of non-traditional nucleophile partners. In addition, photocatalysis can aid fundamental organometallic steps through modulation of the oxidation state of transition metal complexes or through energy-transfer-mediated excitation of intermediate catalytic species. Metallaphotocatalysis provides access to distinct activation modes, which are complementary to those traditionally used in the field of transition metal catalysis, thereby enabling reaction development through entirely new mechanistic paradigms. This Review discusses key advances in the field of metallaphotocatalysis over the past decade and demonstrates how the unique mechanistic features permit challenging, or previously elusive, transformations to be accomplished. Transition metal catalysis is well established as an enabling tool in synthetic organic chemistry. Photoredox catalysis has recently emerged as a method to effect reactions that occur through single-electron-transfer pathways. Here we review the combination of the two to show how this provides access to highly reactive oxidation states of transition metals and distinct activation modes that further enable the synthetic chemist.

Photocatalysis in organic and polymer synthesis

Abstract: This review, with over 600 references, summarizes the recent applications of photoredox catalysis for organic transformation and polymer synthesis. Photoredox catalysts are metallo- or organo-compounds capable of absorbing visible light, resulting in an excited state species. This excited state species can donate or accept an electron from other substrates to mediate redox reactions at ambient temperature with high atom efficiency. These catalysts have been successfully implemented for the discovery of novel organic reactions and synthesis of added-value chemicals with an excellent control of selectivity and stereo-regularity. More recently, such catalysts have been implemented by polymer chemists to post-modify polymers in high yields, as well as to effectively catalyze reversible deactivation radical polymerizations and living polymerizations. These catalysts create new approaches for advanced organic transformation and polymer synthesis. The objective of this review is to give an overview of this emerging field to organic and polymer chemists as well as materials scientists.