Not just one but two carbenes of the same structure act cooperatively in oxidative acylations of alcohols with aldehydes by using a readily available cheap organic oxidant. Alcohols are selectively acylated in the presence of amines by cooperative carbene catalysis. Quantum chemical calculations support the suggested mechanism.

Transition‐metal‐free organocatalytic oxidations of various aldehydes proceed with the TEMPO radical as a mild organic oxidant; the resulting TEMPO esters are formed in moderate to excellent yields (see scheme). N‐Heterocyclic carbenes (NHCs) are efficient catalysts and activate aldehydes for electron‐transfer reactions. The TEMPO esters are readily hydrolyzed and the nitroxide can be regenerated by aerobic oxidation.

Synthetically useful functional groups, including ketones, amides, carbamates, carboxylic acids, aldehydes or ethers, have been identified as weakly coordinating directing groups in efficient ruthenium(II)-catalyzed C À H functionalizations. This strategy set the stage for versatile C À H bond olefinations, oxygenations, nitrogenations and oxidative alkyne annulations among others. Thereby, step-economical access to diversely decorated arenes and heteroarenes was provided in a sustainable fashion.

The past decade has witnessed the emergence of N-(acyloxy)phthalimides (NHPI esters) and its derivatives at the forefront of synthetic methods facilitating the construction of diverse molecular frameworks from the readily available carboxylic acid feedstock. The NHPI esters are predisposed to undergo reductive fragmentation via a single electron transfer (SET) process under thermal, photochemical, or electrochemical conditions to generate the corresponding carbon-or nitrogencentered radicals that participate in a multitude of synthetic transformations to forge carbon−carbon and carbon−heteroatom bonds. The chemistry involving NHPI esters has received broad applicability not only in well-designed cascade annulations but also in medicinal chemistry and natural product synthesis. This comprehensive Review, broadly categorized according to the nature of the bond formation, details the progress made in this field since the initial discovery by providing representative examples with mechanistic details, with an emphasis on challenges and future directions.

Acylated amino acid ligands enabled ruthenium(II)-catalyzed C-H functionalizations with excellent levels of meta-selectivity. The outstanding catalytic activity of the ruthenium(II) complexes derived from monoprotected amino acids (MPAA) set the stage for the first ruthenium-catalyzed meta-functionalizations with removable directing groups. Thereby, meta-alkylated anilines could be accessed, which are difficult to prepare by other means of direct aniline functionalizations. The robust nature of the versatile ruthenium(II)-MPAA was reflected by challenging remote C-H transformations with tertiary alkyl halides on aniline derivatives as well as on pyridyl-, pyrimidyl-, and pyrazolyl-substituted arenes. Detailed mechanistic studies provided strong support for an initial reversible C-H ruthenation, followed by a SET-type C-Hal activation through homolytic bond cleavage. Kinetic analyses confirmed this hypothesis through an unusual second-order dependence of the reaction rate on the ruthenium catalyst concentration. Overall, this report highlights the exceptional catalytic activity of ruthenium complexes derived from acylated amino acids, which should prove instrumental for C-H activation chemistry beyond remote functionalization.

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N-heterocyclic carbene catalyzed oxidative amidations of various aldehydes to the corresponding hexafluoroisopropylesters by using the readily available organic oxidant A are described. The hexafluoroisopropylesters prepared in situ are shown to be highly useful active esters for amide bond formation. In addition, oxidative azidation of aldehydes is presented. These mild organocatalytic processes do not use any transition metal.

This Concept article discusses the potential of oxidative carbene catalysis in synthesis and comprehensively covers pioneering studies as well as recent developments. Oxidative carbene catalysis can be conducted by using inorganic and organic oxidants. Applications in cascade processes, in enantioselective catalysis, and also in natural product synthesis are discussed.