Organic Reactions Promoted by Metal-Free Organic Dyes Under Visible Light Irradiation

Although dyes have received much attention as the visible light-activated photocatalysts, the use of metal-free organic dyes in synthetic organic chemistry is still limited. This chapter summarizes the recent progress in the visible light photocatalysis promoted by metal-free organic dyes. Eosin Y is the typical organic dyes to induce the photoredox catalysis. Recently, other organic dyes such as Rose Bengal, fluorescein, and methylene blue have been studied as photocatalysts to promote the single-electron transfer processes

Hydrogen-Bonding Activation in Chiral Organocatalysts

In a recent decade, various organocatalysts have been developed to be applicable to a wide range of asymmetric reactions. This review briefly summarizes the hydrogen-bonding activation by chiral noncovalent organocatalysts. First, the differences between hydrogen-bonding catalysts and Brønsted acid catalysts are addressed. Next, the effect of hydrogen-bonding interactions on the transition states is discussed. Finally, the hydrogen-bonding activations by the typical noncovalent organocatalysts, such as thiourea, diol, phosphoric acid, Brønsted acid-assisted chiral Brønsted acid, and N-triflyl phoshoramide, are shown

Recent Advances in Cooperative N-Heterocyclic Carbene Catalysis

The N-heterocyclic carbenes (NHCs) open the new field of organocatalysis, leading to the dramatic progress on the cooperative NHC catalysis with transition-metal catalysts or photocatalysts

Synthesis of Oxygen Heterocycles via Aromatic C-O Bond Formation Using Arynes

Most of the synthetic approaches to the benzo-fused heterocycles containing an oxygen atom have involved the use of phenol derivatives as a starting material. This review highlights the new synthetic approaches involving the aromatic C-O bond-forming process using arynes. The insertion of arynes into the C=O bond gives the unstable intermediates, [2 + 2] cycloaddition-type adducts, which can be easily converted into a variety of oxygen atom-containing heterocycles in a single operation. In this review, the syntheses of oxygen heterocycles, such as coumarin, chromene, xanthene, dihydrobenzofuran and benzofuran derivatives, via the insertion of arynes into the C=O bond of aldehydes or formamides are summarized

Transition-Metal-Free Activation of Amide Bond by Arynes

Highly reactive arynes activate the N–C and C=O bonds of amide groups under transition metal-free conditions. This review highlights the insertion of arynes into the N–C and C=O bonds of the amide group. The insertion of arynes into the N–C bond gives the unstable four-membered ring intermediates, which are easily converted into ortho-disubstituted arenes. On the other hand, the selective insertion of arynes into the C=O bond is observed when the sterically less-hindered formamides are employed to give a reactive transient intermediate. Therefore, the trapping reactions of transient intermediates with a variety of reactants lead to the formation of oxygen atom-containing heterocycles. As relative functional groups are activated, the reactions of arynes with sulfinamides, phosphoryl amides, cyanamides, sulfonamides, thioureas, and vinylogous amides are also summarized

Oxidative Radical Cyclization–Cyclization Reaction Leading to 1H-Benzo[f]isoindole Derivatives

The synthesis of 1H-benzo[f]isoindole derivatives was achieved by the cascade radical cyclization–cyclization reaction of the active methine substrate having an allyl group and phenyl group as different two radical acceptors. This oxidative transformation proceeded by using iron(III) chloride FeCl3 as a mild oxidant via the intramolecular radical addition to the allyl group followed by the second radical addition to the phenyl group

The Amino Thiourea-Catalyzed Asymmetric Nucleophilic Reactions

Bifunctional amino thiourea-catalyzed asymmetric additions of several nucleophiles into electron-deficient unsaturated compounds such as nitroolefins, ?,?-unsaturated imides, imines, and azodicarboxylates are described. We discovered that bifunctional thioureas bearing a tertiary amino group significantly accelerated several nucleophilic addition reactions of active methylene compounds to electron-deficient double bonds. In these reactions, a strong hydrogen-bonding ability of the thiourea moiety as well as an appropriate Brønsted basicity of the tertiary amine is crucial for high enantioselectivity. This dual activation of both nucleophiles and electrophiles by the bifunctional thiourea expanded the applicability of the thiourea-catalyzed enantioselective reaction. In addition, these organocatalyzed asymmetric reactions were successfully applied to the concise asymmetric synthesis of natural products and medicinal candidates such as epibatidine, baclofen, and CP-99,994

Three-Component Coupling Reactions of Arynes for the Synthesis of Benzofurans and Coumarins

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Reactivity of Trapped and Accumulated Electrons in Titanium Dioxide Photocatalysis

Electrons, photogenerated in conduction bands (CB) and trapped in electron trap defects (Tids) in titanium dioxide (TiO2), play crucial roles in characteristic reductive reactions. This review summarizes the recent progress in the research on electron transfer in photo-excited TiO2. Particularly, the reactivity of electrons accumulated in CB and trapped at Tids on TiO2 is highlighted in the reduction of molecular oxygen and molecular nitrogen, and the hydrogenation and dehalogenation of organic substrates. Finally, the prospects for developing highly active TiO2 photocatalysts are discussed