N-Heterocyclic carbenes have become universal ligands in organometallic and inorganic coordination chemistry. They not only bind to any transition metal, be it in low or high oxidation states, but also to main group elements such as beryllium, sulfur, and iodine. Because of their specific coordination chemistry, N-heterocyclic carbenes both stabilize and activate metal centers in quite different key catalytic steps of organic syntheses, for example, C-H activation, C-C, C-H, C-O, and C-N bond formation. There is now ample evidence that in the new generation of organometallic catalysts the established ligand class of organophosphanes will be supplemented and, in part, replaced by N-heterocyclic carbenes. Over the past few years, this chemistry has been the field of vivid scientific competition, and yielded previously unexpected successes in key areas of homogeneous catalysis. From the work in numerous academic laboratories and in industry, a revolutionary turning point in oraganometallic catalysis is emerging.

http://aether.cmi.ua.ac.be/artikels/Artikels%20Gitte/Artikels/Reviews/NHC-%20liganden%20Herrmann.pdf

N-heterocyclic carbenes: a new concept in organometallic catalysis.

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

Polyketide biosynthesis: a millennium review.

Palladium-catalyzed C-C and C-N bond-forming reactions are among the most versatile and powerful synthetic methods. For the last 15 years, N-heterocyclic carbenes (NHCs) have enjoyed increasing popularity as ligands in Pd-mediated cross-coupling and related transformations because of their superior performance compared to the more traditional tertiary phosphanes. The strong sigma-electron-donating ability of NHCs renders oxidative insertion even in challenging substrates facile, while their steric bulk and particular topology is responsible for fast reductive elimination. The strong Pd-NHC bonds contribute to the high stability of the active species, even at low ligand/Pd ratios and high temperatures. With a number of commercially available, stable, user-friendly, and powerful NHC-Pd precatalysts, the goal of a universal cross-coupling catalyst is within reach. This Review discusses the basics of Pd-NHC chemistry to understand the peculiarities of these catalysts and then gives a critical discussion on their application in C-C and C-N cross-coupling as well as carbopalladation reactions.

Palladium Complexes of N-Heterocyclic Carbenes as Catalysts for Cross-Coupling Reactions—A Synthetic Chemist's Perspective

Organic chemists have always been fascinated by the cyclopropane subunit.1 The smallest cycloalkane is found as a basic structural element in a wide range of naturally occurring compounds.2 Moreover, many cyclopropane-containing unnatural products have been prepared to test the bonding features of this class of highly strained cycloalkanes3 and to study enzyme mechanism or inhibition.4 Cyclopropanes have also been used as versatile synthetic intermediates in the synthesis of more functionalized cycloalkanes5,6 and acyclic compounds.7 In recent years, most of the synthetic efforts have focused on the enantioselective synthesis of cyclopropanes.8 This has remained a challenge ever since it was found that the members of the pyrethroid class of compounds were effective insecticides.9 New and more efficient methods for the preparation of these entities in enantiomerically pure form are still evolving, and this review will focus mainly on the new methods that have appeared in the literature since 1989. It will elaborate on only three types of stereoselective cyclopropanation reactions from olefins: the halomethylmetal-mediated cyclopropanation reactions (eq 1), the transition metal-catalyzed decomposition of diazo compounds (eq 2), and the nucleophilic addition-ring closure sequence (eqs 3 and 4). These three processes will be examined in the context of diastereoand enantiocontrol. In the last section of the review, other methods commonly used to make chiral, nonracemic cyclopropanes will be briefly outlined.

Stereoselective Cyclopropanation Reactions

Trifluoromethyl substituted ring systems: synthesis, and reactivity towards metal carbonyls

Some diels—alder reactions of η1-bonded cyclopentadienylmetal compounds

A novel compound, 1,6,17,21-octahydrobotryococcene, apparently derived from the green alga Botryococcus braunii, is identified in a sediment from Sacred Lake, Mount Kenya.

Identification of 1,6,17,21-octahydrobotryococcene in a sediment

Treatment of [Os3Pt(µ-H)2(CO)10{P(C6H11)3}] with diazomethane affords, under kinetic control, a single isomer [Os3Pt(µ-H)2(µ-CH2)(CO)10{P(C6H11)3}], which in solution isomerises (days) to a second isomer; the structures in the solid state of both isomers have been determined by X-ray crystallography, and those in solution by 1H n.m.r. spectroscopy.

Synthesis, dynamic behaviour, and molecular structures of µ-methylene platinumtriosmium complexes; X-ray crystal structures of two isomers of [Os3Pt(µ-H)2(µ-CH2)(CO)10{P(C6H11)3}]

The nitrosonium salt [NO][PF6] reacts with [Fe(CO)L(NO)(η3-allyl)][L = P(OPh)3, P(OMe)3, or PPh3; allyl = C3H5, C3H4Me–1, or C3H4Me–2] to give [FeL(NO)2(η3-allyl)][PF6]. Spin saturation transfer 1H n.m.r. spectroscopy shows that the fluxionality of the products involves slow allyl rotation at room temperature.

Martin Murray

Papers

Trifluoromethyl substituted ring systems: synthesis, and reactivity towards metal carbonyls

Some diels—alder reactions of η1-bonded cyclopentadienylmetal compounds

Identification of 1,6,17,21-octahydrobotryococcene in a sediment

Synthesis, dynamic behaviour, and molecular structures of µ-methylene platinumtriosmium complexes; X-ray crystal structures of two isomers of [Os3Pt(µ-H)2(µ-CH2)(CO)10{P(C6H11)3}]

Cationic allyldinitrosyl complexes of iron