The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

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Stable Cyclic Carbenes and Related Species beyond Diaminocarbenes

The availability of catalysts to perform specific transformations is critical for both industry and academia. Over the years, the success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of a variety of metal-containing systems. Advances in ligand design have allowed not only for improvements of known processes in terms of scope, mildness, and catalyst loadings, but also for the discovery of new selective reactions. A good illustration is given by palladium-catalyzed coupling reactions, which are applied to a wide area of endeavors ranging from synthetic organic chemistry to materials science.[1] For these catalytic processes, which represent some of the most powerful and versatile tools available for synthetic chemists, major advances have recently been reported thanks to the use of bulky, electron-rich, phosphines A and cyclic diaminocarbenes (N-heterocyclic carbenes (NHCs)) B (Figure 1).[2] These ligands stabilize the active catalytic species, and accelerate the important catalytic steps, namely oxidative addition, transmetallation, and reductive elimination. On the other hand, excessive steric hindrance can present some drawbacks for the coupling of bulky reactants.[3] To overcome this problem Glorius and co-workers have successfully developed ligands with “flexible steric bulk” using the conformational flexibility of cyclohexane.[4]

Stable Cyclic (Alkyl)(Amino)Carbenes as Rigid or Flexible, Bulky, Electron-Rich Ligands for Transition-Metal Catalysts: A Quaternary Carbon Atom Makes the Difference

Heterocyclische Carbene – Synthese und Koordinationschemie

Renaissance of annulene chemistry.

Reactions of dimethoxycarbene and fluoromethoxycarbene with hydroxyl compounds. Absolute rate constants and the heat of formation of dimethoxycarbene

Direct observational studies of a singlet alkylcarbene: methylmethoxycarbene, a remarkably selective nucleophile

Methoxychlorocarbene. Matrix spectroscopy and photochemistry

La double liaison centrale du syn-sesquinorbornadiene, produit par la reaction de Diels Alder du (Z)-bis (benzenesulfonyl)-1,2 ethylene avec le trichloro [5.2.1.0 2,6 ] decatriene-2,5,8, est reduite regiospecifiquement avec le diimide. D'autres reactions permettent la fusion avec des sous-unites: oxiranne, aziridine, cyclopropane et methylocyclopropane, dans la region centrale de cette molecule. Tout ces produits subissent la transposition dyotropique etudiee

Quantitation of proximity effects on rate. A case study involving dyotropic hydrogen migration within syn-sesquinorbornene disulfones carrying central substituents having different spatial demands

The doubly-decked [8]annulene 11 has been prepared in 13 steps from octahydro 3a,9a-propenocyclopentacycloocten-2,11-dione. The scheme takes advantage of the annulating capacity of the Cook-Weiss reaction and the ability of semibullvalenes to undergo thermal isomerization to cyclooctatetraenes. Bis(semibullvalene) is both conformationally dynamic and subject to very rapid Cope rearrangement. The energetics of inversion within its interconnective cyclooctane ring have been elucidated by variable-temperature 1 H NMR spectroscopy

Multifaceted consequences of holding two [8]annulene rings face-to-face. Synthesis, structural characteristics, and reduction behavior of [22](1,5)cyclooctatetraenophane

Mark A. Kesselmayer

Papers

Reactions of dimethoxycarbene and fluoromethoxycarbene with hydroxyl compounds. Absolute rate constants and the heat of formation of dimethoxycarbene

Direct observational studies of a singlet alkylcarbene: methylmethoxycarbene, a remarkably selective nucleophile

Methoxychlorocarbene. Matrix spectroscopy and photochemistry

Quantitation of proximity effects on rate. A case study involving dyotropic hydrogen migration within syn-sesquinorbornene disulfones carrying central substituents having different spatial demands

Multifaceted consequences of holding two [8]annulene rings face-to-face. Synthesis, structural characteristics, and reduction behavior of [22](1,5)cyclooctatetraenophane