Takuya Kurahashi

Nickel-Catalyzed Decarbonylative Addition of Phthalimides to Alkynes

An intermolecular nickel-catalyzed addition reaction has been developed where N-arylphthalimides react with alkynes to afford substituted isoquinolones. A mechanistic rationale is proposed, implying nucleophilic attack of Ni(0) to an amide as the primary step of the catalytic cycle.

One-Electron Oxidation of Electronically Diverse Manganese(III) and Nickel(II) Salen Complexes: Transition from Localized to Delocalized Mixed-Valence Ligand Radicals

Ligand radicals from salen complexes are unique mixed-valence compounds in which a phenoxyl radical is electronically linked to a remote phenolate via a neighboring redox-active metal ion, providing an opportunity to study electron transfer from a phenolate to a phenoxyl radical mediated by a redox-active metal ion as a bridge. We herein synthesize one-electron-oxidized products from electronically diverse manganese(III) salen complexes in which the locus of oxidation is shown to be ligand-centered, not metal-centered, affording manganese(III)-phenoxyl radical species. The key point in the present study is an unambiguous assignment of intervalence charge transfer bands by using nonsymmetrical salen complexes, which enables us to obtain otherwise inaccessible insight into the mixed-valence property. A d(4) high-spin manganese(III) ion forms a Robin-Day class II mixed-valence system, in which electron transfer is occurring between the localized phenoxyl radical and the phenolate. This is in clear contrast to a d(8) low-spin nickel(II) ion with the same salen ligand, which induces a delocalized radical (Robin-Day class III) over the two phenolate rings, as previously reported by others. The present findings point to a fascinating possibility that electron transfer could be drastically modulated by exchanging the metal ion that bridges the two redox centers.

Geminal dimetalation of alkylidene-type carbenoids with silylboranes and diborons

Abstract A novel and efficient method for gem-dimetalation of carbenoids has been demonstrated. Treatment of alkylidene-type lithium carbenoids with such an interelement compound as silylborane or diboron to generate the corresponding borate complex, followed by warming to room temperature, induced migration of the silyl or boryl group from a negatively charged boron atom to the carbenoid carbon to afford 1-boryl-1-silyl-1-alkenes or 1,1-diboryl-1-alkenes in good yields. Carbon–carbon bond forming transformations of the gem-dimetalated compounds mediated by boron or silicon is also described.

Nickel-Catalyzed Decarbonylative Addition of Anhydrides to Alkynes

An intermolecular nickel-catalyzed addition reaction has been developed where phthalic anhydrides react with alkynes to afford substituted isocoumarins. A mechanistic rationale is proposed, implying reductive elimination of Ni(0) promoted by ZnCl(2) cocatalyst as the key step of the catalytic cycle.

Nickel-catalyzed decarboxylative carboamination of alkynes with isatoic anhydrides.

An intermolecular nickel-catalyzed addition reaction in which isatoic anhydrides react with alkynes to afford substituted quinolones has been developed. A mechanistic rationale is proposed, implying oxidative addition of Ni(0) to a carbamate, which allows intermolecular addition to alkynes via decarboxylation.

Nickel-Catalyzed Cycloaddition of o-Arylcarboxybenzonitriles and Alkynes via Cleavage of Two Carbon–Carbon σ Bonds

An intermolecular cycloaddition reaction has been developed, where o-arylcarboxybenzonitriles react with alkynes to afford coumarins in the presence of Ni(0)/P(CH(2)Ph)(3)/MAD as a catalyst. The reaction process displays an unusual mechanistic feature-the cleavage of two independent C-CN and C-CO bonds.

Transient Intermediates from Mn(salen) with Sterically Hindered Mesityl Groups: Interconversion between MnIV-Phenolate and MnIII-Phenoxyl Radicals as an Origin for Unique Reactivity

In order to reveal structure-reactivity relationships for the high catalytic activity of the epoxidation catalyst Mn(salen), transient intermediates are investigated. Steric hindrance incorporated to the salen ligand enables highly selective generation of three related intermediates, OMnIV(salen), HO-Mn IV(salen), and H2O-MnIII(salen (+*)), each of which is thoroughly characterized using various spectroscopic techniques including UV-vis, electron paramagnetic resonance, resonance Raman, electrospray ionization mass spectrometry, 2H NMR, and X-ray absorption spectroscopy. These intermediates are all one-electron oxidized from the starting MnIII(salen) precursor but differ only in the degree of protonation. However, structural and electronic features are strikingly different: The Mn-O bond length of HO-MnIV(salen) (1.83 A) is considerably longer than that of OMnIV(salen) (1.58 A); the electronic configuration of H2O-MnIII(salen (+*)) is MnIII-phenoxyl radical, while those of OMnIV(salen) and HO-MnIV(salen) are MnIV-phenolate. Among OMnIV(salen), HO-MnIV(salen), and H2O-MnIII(salen (+*)), only the OMnIV(salen) can transfer oxygen to phosphine and sulfide substrates, as well as abstract hydrogen from weak C-H bonds, although the oxidizing power is not enough to epoxidize olefins. The high activity of Mn(salen) is a direct consequence of the favored formation of the reactive OMnIV(salen) state.

Effect of Imidazole and Phenolate Axial Ligands on the Electronic Structure and Reactivity of Oxoiron(IV) Porphyrin π-Cation Radical Complexes: Drastic Increase in Oxo-Transfer and Hydrogen Abstraction Reactivities

To study the effect of axial ligands on the electronic structure and reactivity of compound I of peroxidases and catalases, oxoiron(IV) porphyrin pi-cation radical complexes with imidazole, 2-methylimidazole, 4(5)-methylimidazole, and 3-fluoro-4-nitrophenolate as the axial ligands were prepared by ozone oxidation of iron(III) complexes of 5,10,15,20-tetramesitylporphyrin (TMP) and 2,7,12,17-tetramethyl-3,8,13,18-tetramesitylporphyrin (TMTMP). These complexes were fully characterized by absorption, (1)H, (2)H, and (19)F NMR, electron paramagnetic resonance (EPR), and electrospray ionization mass spectrometry (ESI-MS) spectroscopy. The characteristic absorption peak of compound I at approximately 650 nm was found to be a good marker for estimation of the electron donor effect from the axial ligand. The axial ligand effect did not change the porphyrin pi-cation radical state, the a(2u) state of the TMP complexes, or the a(1u) radical state of both the TMTMP complexes and compound I. The ferryl iron and porphyrin pi-cation radical spins were effectively transferred into the axial ligands for the a(2u) complexes but not for the a(1u) complexes. Most importantly, the reactivity of the oxoiron(IV) porphyrin pi-cation radical complex was drastically increased by the imidazole and phenolate axial ligands. The reaction rate for cyclooctene epoxidation was increased 100- to 400-fold with axial coordination of imidazoles and phenolate. A similar increase was also observed for the oxidation of 1,4-cyclohexadiene,N,N-dimethyl-p-nitroaniline and hydrogen peroxide. These results suggest extreme enhancement of the reactivity of compound I by the axial ligand in heme enzymes. The functional role of axial ligands on the compound I in heme enzymes is discussed.

Functionalized DMAP catalysts for regioselective acetylation of carbohydrates

Abstract New functionalized DMAPs having carboxylic acid functionality are developed for regioselective acylation of carbohydrates. In these catalysts, DMAP (4-(N,N-dimethylamino)pyridine) is linked with –COOH (–COOMe or –OSO3H in reference catalysts) via methylene spacers of different length at the dimethylamino moiety. Utilizing one of these catalysts, 3-[N-decyl-N-(4-pyridyl)amino]propionic acid ( 1 ), regioselectivity for the primary 6-OH group in acetylation of 1-O-octyl β- d -glucopyranoside is increased from 16% to 89% with rather improved catalytic activity compared with the parent DMAP. Catalyst 1 regioselectively acetylates both anomers of 1-O-octyl glucopyranosides (89% and 88% regioselectivity for β- and α-anomer, respectively) and 1-O-octyl galactopyranosides (100% regioselectivity for both anomers) at position 6 in CHCl3, but gives nearly 1:1 mixtures of 4- and 6-monoacetates in the case of 1-O-octyl mannopyranosides. Control experiments are done to investigate the mechanistic aspects of regiocontrol.

Nickel-Catalyzed Cycloadditions of Thiophthalic Anhydrides with Alkynes

Nickel-catalyzed cycloadditions have been developed where thiophthalic anhydrides react with alkynes to afford substituted sulfur-containing heterocyclic compounds. Selective formations of thioisocoumarins, benzothiophenes, and thiochromones were accomplished with three different reaction conditions.