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

N atom, thus providing carbenes derived from pyrazolium, isothiazolium, and even quinolinium salts that contain a stabilizing heteroatom in a remote position (G-J in Figure 1). Recently, carbenes such as K, which are comprised of only one heteroatom and lack delocalization through the heterocycle, have been discovered as versatile ligands, thus constituting another important class of carbenes with low heteroatom stabilization. Both the synthesis of the organometallic complexes of these ligands as well as the (catalytic) properties of the coordinated metal centers generally show distinct differences, compared to the more classical NHC complexes, such as C2-metallated imidazolylidenes. This review intends to describe such differences and highlights the chemical peculiarities of these types of N-heterocyclic carbene complexes. It introduces, in a qualitative manner, the synthetic routes that have been established for the preparation of such complexes, covering the literature from the very beginning of activities in this area up to 2008. While specialized reviews on some aspects of the present topic have recently appeared,7 a comprehensive overview of the subject has not been available thus far. Rather than just being descriptive, the present account is mainly directed toward the impact of these still unusual metal-carbene bonding modes on the electronic properties and on the new catalytic applications that have been realized by employing such new carbene complexes. As a consequence of our focus on complexes with less-stabilized heterocyclic ligands, systems comprising acyclic carbenes have not been included, and the interested reader is, instead, referred to the pioneering and

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Beyond Conventional N-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

Heterocyclische Carbene – Synthese und Koordinationschemie

Since the isolation and crystallographic characterization of the first stable N-heterocyclic carbene in early 1991,1 nucleophilic diaminocarbenes (also known as “Arduengotype” carbenes) and their analogues have emerged as a powerful class of carbon-based ligands with broad applications in metal-based catalytic reactions.2-12 The main reason for their success in catalysis is their superior properties as ligands in comparison with their phosphine counterparts.11,13-18 In addition, carbene organocatalysis has emerged as an extremely fruitful area of research in synthetic organic chemistry. The benzoin condensation, the Stetter reaction, transformations involving homoenolates, 1,2-additions, transesterifications, and ring opening polymerizations are among the many reactions promoted by nucleophilic carbenes. Several excellent reviews, chapters, and books highlighting the recent progress of nucleophilic carbenes in metal-based catalysis and organocatalysis are available.7,9,11-16,19-24 Significant efforts have been made to better understand the unique properties associated with the free nucleophilic carbenes using a wide range of experimental techniques. X-ray diffraction,1,25 neutron diffraction,26,27 photoelectron spectroscopy,28 cyclic voltammetry,29-34 NMR spectroscopy, and IR spectroscopy10,35-40 are among the experimental techniques employed in these studies. The experimental methods have been complemented by theoretical investigations, which have become extremely important because they enable the study of a great number of related systems, a task that would be difficult or sometimes impossible to achieve experimentally. Density functional theory and molecular orbital theory have been employed for the study of these intriguing species.26-28,41-49

13C NMR Spectroscopy of “Arduengo-type” Carbenes and Their Derivatives

N‐Heterocyclic Carbene Ligands: Coordination Chemistry, Reactivity, and Applications Steṕhane Bellemin-Laponnaz*,†,‡ and Samuel Dagorne* †IPCMS (Institut de Physique et Chimie des Mateŕiaux de Strasbourg), CNRS-Universite ́ de Strasbourg, 23 rue du Loess BP 43, F-67034 Strasbourg, France ‡University of Strasbourg Institute for Advanced Study (USIAS), 5 alleé du Geńeŕal Rouvillois, F-67083 Strasbourg, France Institut de Chimie de Strasbourg, CNRS-Universite ́ de Strasbourg, 1 rue Blaise Pascal, F-67000 Strasbourg, France

Group 1 and 2 and Early Transition Metal Complexes Bearing N-Heterocyclic Carbene Ligands: Coordination Chemistry, Reactivity, and Applications

Group 6 carbene complexes derived from lithiated azoles and the crystal structure of a molybdenum thiazolinylidene complex

A series of palladium, platinum and manganese di(organo)carbene complexes have been prepared from 4-chloro-N-methylquinolinone by processes that involve alkylation before or after attachment to the metal unit; the nucleophilic heteroatoms are separated from the C-donor atom by three bonds.

Maggel Deetlefs

Papers

Group 6 carbene complexes derived from lithiated azoles and the crystal structure of a molybdenum thiazolinylidene complex

Preparation and characterisation of palladium, platinum and manganese di(organo)carbene complexes from quinolinone and quinolinium precursors