Photo-oxidative Cross-Dehydrogenative Coupling-Type Reaction of Thiophenes with α-Position of Carbonyls Using a Catalytic Amount of Molecular Iodine

Recent Advances in Aerobic Oxidative of C-H Bond by Molecular Oxygen Focus on Heterocycles

Aerobic oxidative cross-coupling represents one of the most straightforward and atom-economic methods for construction of C-C and C-X (X=N, O, S, or P) bonds using air as a sustainable external oxidant. The oxidative coupling of C-H bonds in heterocyclic compounds can effectively increase their molecular complexity by introducing new functional groups through C-H bond activation, or by formation of new heterocyclic structures through cascade construction of two or more sequential chemical bonds. This is very useful as it can increase the potential applications of these structures in natural products, pharmaceuticals, agricultural chemicals, and functional materials. This is a representative overview of recent progress since 2010 on green oxidative coupling reactions of C-H bond using O2 or air as internal oxidant focus on Heterocycles. It aims to provide a platform for expanding the scope and utility of air as green oxidant, together with a brief discussion on research into the mechanisms behind it.

Oxindole synthesis via polar-radical crossover of ketene-derived amide enolates in a formal [3 + 2] cycloaddition

Herein we introduce a simple, efficient and transition-metal free method for the preparation of valuable and sterically hindered 3,3-disubstituted oxindoles via polar-radical crossover of ketene derived amide enolates. Various easily accessible N-alkyl and N-arylanilines are added to disubstituted ketenes and the resulting amide enolates undergo upon single electron transfer oxidation a homolytic aromatic substitution (HAS) to provide 3,3-disubstituted oxindoles in good to excellent yields. A variety of substituted anilines and a 3-amino pyridine engage in this oxidative formal [3 + 2] cycloaddition and cyclic ketenes provide spirooxindoles. Both substrates and reagents are readily available and tolerance to functional groups is broad.

Azobenzene Photoswitching with Near-Infrared Light Mediated by Molecular Oxygen

Efficient photoisomerization between the cis and the trans states of azobenzenes using low-energy light is desirable for a range of applications in, e.g., photobiology yet challenging to accomplish directly with modified azobenzenes. Herein, we utilize molecular iodine as a photocatalyst to induce indirect cis-to-trans isomerization of 4,4'-dimethoxyazobenzene with 770 nm near-infrared light, showing robustness during more than 1000 cycles in ambient conditions. Intriguingly, the catalysis is mediated by molecular oxygen, and we demonstrate that other singlet-oxygen-generating photosensitizers besides iodine, i.e., palladium phthalocyanine, catalyze the isomerization as well. Thus, we envision that the approach can be further improved by employing other catalysts with suitable photoelectrochemical properties. Further studies are needed to explore the applicability of the approach with other azobenzene derivatives.

Enantioselective Syntheses of C2-Symmetric Pyrazolones and Diones via One-Pot Organo-/Iodine Sequential Catalysis

The catalytic diastereo- and enantioselective syntheses of C₂-symmetric axially chiral 1,4-dicarbonyl derivatives with 2,3-quaternary stereocenters were achieved by utilizing an organo-/iodine binary catalytic strategy. The reactions proceeded well under mild conditions without metals or strong bases.

Cu(I)-Catalyzed Three-Component Cyclization for the Construction of Functionalized Thiazoles

A novel and straightforward strategy for the synthesis of functionalized thiazoles from thioamides, ynals, and alcohols via a copper(I)-catalyzed three-component reaction has been described. Through the formation of new C-S, C-N, and C-O bonds in one pot, it is easy to produce various valuable thiazoles fixed with aryl or heteroaryl groups. In addition, the reaction also exhibits other unique advantages, such as high step economics, good functional group tolerance, and good regioselectivity.

Visible light-mediated ipso-annulation of activated alkynes: access to 3-alkylated spiro[4,5]-trienones, thiaspiro[4,5]-trienones and azaspiro[4,5]-trienones

A novel method for chemoselective difunctionalization of activated alkynes for synthesizing 3-alkylated spiro[4,5]-trienones, thiaspiro[4,5]-trienones and spirolactams has been uncovered using photoredox catalysis under visible light conditions.

Metal-Free Photocatalytic Synthesis of exo-Iodomethylene 2-Oxazolidinones: An Alternative Strategy for CO2 Valorization with Solar Energy

A visible-light-promoted metal-free carboxylative cyclization of propargylic amines with CO₂ was shown to offer exo-iodomethylene 2-oxazolidinones. Incorporation of both CO₂ and iodo moieties into these compounds was realized efficiently. The mechanism study revealed that this carboxylative cyclization proceeds through a radical pathway. Notably, the iodine-functionalized 2-oxazolidinone as a platform molecule could be easily converted into a wide range of value-added chemicals through Buchwald-Hartwig, Suzuki, Sonogashira, photocatalytic ene, and photoreduction reactions. As a result, the plentiful downstream transformations remarkably enhance the range of chemicals derived from CO₂ and open a potential avenue for CO₂ functionalization to circumvent energy challenges in this field.

Low-Temperature, Transition-Metal-Free Cross-Dehydrogenative Coupling Protocol for the Synthesis of 3,3-Disubstituted Oxindoles

A new low-temperature procedure for the synthesis of 3,3-disubstituted 2-oxindoles via cross-dehydrogenative coupling (CDC) is reported. The use of a strong, nonreversible base in these reactions has been found to effect a dramatic drop in reaction temperature (to room temperature) relative to the current state-of-the-art (>100 °C) procedure. When employing iodine as an "oxidant", new evidence suggests that this transformation may occur via a transiently stable iodinated intermediate rather than by direct single-electron oxidation.

Direct intramolecular amination of tryptophan esters to prepare pyrrolo[2,3-b]indoles

A metal-free iodine-catalyzed intramolecular amination has been developed for the practical synthesis of pyrrolo[2,3-b]indoles from readily available tryptophan esters. The transformation has been applied to a wide array of substrates and can be performed on gram scale under very mild conditions.