The successful isolation and characterization of an N-heterocyclic carbene in 1991 opened up a new class of organic compounds for investigation. From these beginnings as academic curiosities, N-heterocyclic carbenes today rank among the most powerful tools in organic chemistry, with numerous applications in commercially important processes. Here we provide a concise overview of N-heterocyclic carbenes in modern chemistry, summarizing their general properties and uses and highlighting how these features are being exploited in a selection of pioneering recent studies.

An overview of N-heterocyclic carbenes

The use of coordinating moieties as directing groups for the functionalization of aromatic CH bonds has become an established tool to enhance reactivity and induce regioselectivity. Nevertheless, with regard to the synthetic applicability of CH activation, there is a growing interest in transformations in which the directing group can be fully abandoned, thus allowing the direct functionalization of simple benzene derivatives. However, this approach requires the disclosure of new strategies to achieve reactivity and to control selectivity. In this review, recent advances in the emerging field of non-chelate-assisted CH activation are discussed, highlighting some of the most intriguing and inspiring examples of induction of reactivity and selectivity.

Beyond directing groups: transition-metal-catalyzed C-H activation of simple arenes.

Organocatalytic Reactions Enabled by N-Heterocyclic Carbenes.

The umpolung strategy encompasses all the methods that make organic molecules react in an inverse manner compared to their innate polarity-driven reactivity. This concept entered the field of organocatalysis when it was recognized that N-heterocyclic carbenes (NHCs) can provide catalytic access to acyl anion equivalents. Since then, tremendous efforts have followed to develop a broad variety of NHC-catalyzed reactions. In addition to this, more recent research developments have shown that other families of organocatalysts are also able to mediate transformations in which inversion of polarity is involved. This tutorial review aims at offering a didactic overview of organocatalytic umpolung and should serve as an inspiration for further progress in this field.

Organocatalytic umpolung: N-heterocyclic carbenes and beyond

Surface functionalization of nano-magnetic nanoparticles is a well-designed way to bridge the gap between heterogeneous and homogeneous catalysis. The introduction of magnetic nanoparticles (MNPs) in a variety of solid matrices allows the combination of well-known procedures for catalyst heterogenization with techniques for magnetic separation. Magnetite is a well-known material, also known as ferrite (Fe3O4), and can be used as a versatile support for functionalization of metals, organocatalysts, N-heterocyclic carbenes, and chiral catalysts. It is used as a support for important homogeneous catalytically active metals such as Pd, Pt, Cu, Ni, Co, Ir, etc. to obtain stable and magnetically recyclable heterogeneous catalysts. Homogeneous organocatalysts can be successfully decorated with linkers/ligands on the surface of magnetite or alternatively the organocatalysts can be directly immobilized on the surface of magnetite. The functionalized magnetically retrievable catalysts or nanocatalysts that are increasingly being used in catalysis, green chemistry and pharmaceutically significant reactions are summarized in this review.

Nano-magnetite (Fe3O4) as a support for recyclable catalysts in the development of sustainable methodologies

For the first time, enantiomerically enriched atropoisomeric furans have been accessed using a central-to-axial chirality conversion strategy. Hence, oxidation of the enantioenriched dihydrofuran precursors gave rise to axially chiral furans with high enantiopurities accounting from excellent conversion percentages (cp) in most cases.

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Enantioselective Syntheses of Furan Atropisomers by an Oxidative Central-to-Axial Chirality Conversion Strategy

Palladium‐Catalyzed Selective Dehydrogenative Cross‐Couplings of Heteroarenes

Suitably substituted enantioenriched 4-aryl-1,4-dihydro-pyridines prepared by an organocatalytic enantioselective Michael addition were oxidized with MnO2 into axially chiral 4-arylpyridines with central-to-axial chirality conversion. Moderate to complete percentages (cp) were observed, and a model for the conversion of chirality is discussed.

Combining Organocatalysis with Central-to-Axial Chirality Conversion: Atroposelective Hantzsch-Type Synthesis of 4-Arylpyridines.

We report the first intermolecular NHC-catalyzed hydroacylation of electron-neutral olefins. Treatment of aromatic aldehydes with cyclopropenes under mild conditions affords valuable acylcyclopropanes in moderate to high yields with an excellent level of diastereocontrol. Preliminary mechanistic studies suggest that product formation occurs via a concerted syn hydroacylation pathway.

Xavier Bugaut

Papers

Organocatalytic umpolung: N-heterocyclic carbenes and beyond

Enantioselective Syntheses of Furan Atropisomers by an Oxidative Central-to-Axial Chirality Conversion Strategy

Palladium‐Catalyzed Selective Dehydrogenative Cross‐Couplings of Heteroarenes

Combining Organocatalysis with Central-to-Axial Chirality Conversion: Atroposelective Hantzsch-Type Synthesis of 4-Arylpyridines.

N-Heterocyclic Carbene (NHC)-Catalyzed Intermolecular Hydroacylation of Cyclopropenes