Generating and Trapping Metalla‐N‐Heterocyclic Carbenes

By means of a combined experimental and theoretical approach, the electronic features and chemical behavior of metalla‐N‐heterocyclic carbenes (MNHCs, N‐heterocyclic carbenes containing a metal atom within the heterocyclic skeleton) have been established and compared with those of classical NHCs. MNHCs are strongly basic (proton affinity and pK_a values around 290 kcal mol^?1 and 36, respectively) with a narrow singlet–triplet gap (around 23 kcal mol^?1). MNHCs can be generated from the corresponding metalla‐imidazolium salts and trapped by addition of transition‐metal complexes affording the corresponding heterodimetallic dicarbene derivatives, which can serve as carbene transfer agents.     

9‐Boraanthracene Derivatives Stabilized by N‐Heterocyclic Carbenes

2009: A boraanthracene odyssey : General routes to boraanthracene derivatives—long proposed, but never observed—are disclosed, along with the structural and photophysical properties of these compounds and their remarkable reactivity towards oxygen.

     

Fusing N‐Heterocyclic Carbenes with Carborane Anions

Here we describe the fusion of two families of unusual carbon‐containing molecules that readily disregard the tendency of carbon to form four chemical bonds, namely N‐heterocyclic carbenes (NHCs) and carborane anions. Deprotonation of an anionic imidazolium salt with lithium diisopropylamide at room temperature leads to a mixture of lithium complexes of C‐2 and C‐5 dianionic NHC constitutional isomers as well as a trianionic (C‐2, C‐5) adduct. Judicious choice of the base and reaction conditions allows the selective formation of all three stable polyanionic carbenes. In solution, the so‐called abnormal C‐5 NHC lithium complex slowly isomerizes to the normal C‐2 NHC, and the process can be proton‐catalyzed by the addition of the anionic imidazolium salt. These results indicate that the combination of two unusual forms of carbon atoms can lead to unexpected chemical behavior, and that this strategy paves the way for the development of a broad new generation of NHC ligands for catalysis.     

Cationic,Two‐Coordinate Gold π Complexes

Cationic, two‐coordinate gold π complexes that contain a phosphine or N‐heterocyclic supporting ligand have attracted considerable attention recently owing to the potential relevance of these species as intermediates in the gold‐catalyzed functionalization of C? C multiple bonds. Although neutral two‐coordinate gold π complexes have been known for over 40 years, examples of the cationic two‐coordinate gold(I) π complexes germane to catalysis remained undocumented prior to 2006. This situation has changed dramatically in recent years and well‐defined examples of two‐coordinate, cationic gold π complexes containing alkene, alkyne, diene, allene, and enol ether ligands have been documented. This Minireview highlights this recent work with a focus on the structure, bonding, and ligand exchange behavior of these complexes.     

N‐Heterocyclic Carbenes: A New Concept in Organometallic Catalysis

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.     

Expanding the Chemistry of Cationic N‐Heterocyclic Carbenes: Alternative Synthesis,Reactivity, and Coordination Chemistry

A new synthetic route to complexes of the cationic N‐heterocyclic carbene ligand 2 has been developed by the attachment of a cationic pentamethylcyclopentadienylruthenium ([RuCp*]⁺) fragment to a metal‐coordinated benzimidazol‐2‐ylidene ligand. The coordination chemistry and the steric and electronic properties of the cationic carbene were investigated in detail by experimental and theoretical methods. X‐ray structures of three carbene–metal complexes were determined. The cationic ligand 2 is a poorer overall electron donor relative to the related neutral carbene, which is evident from cyclic voltammetry (CV) and IR measurements.     

Synthesis and Characterization of a Gold Vinylidene Complex Lacking π‐Conjugated Heteroatoms

Hydride abstraction from the gold (disilyl)ethylacetylide complex [( P )Au{η¹‐C?CSi(Me)₂CH₂CH₂SiMe₂H}] ( P =P(tBu)₂o‐biphenyl) with triphenylcarbenium tetrakis(pentafluorophenyl)borate at ?20 °C formed the cationic gold (β,β‐disilyl)vinylidene complex [( P )Au?C?CSi(Me)₂CH₂CH₂Si (Me)₂]⁺B(C₆F₅)₄^? with ≥90 % selectivity. ²⁹Si NMR analysis of this complex pointed to delocalization of positive charge onto both the β‐silyl groups and the ( P )Au fragment. The C1 and C2 carbon atoms of the vinylidene complex underwent facile interconversion (ΔG^≠=9.7 kcal mol^?1), presumably via the gold π‐disilacyclohexyne intermediate [( P )Au{η²‐C?CSi(Me)₂CH₂CH₂Si (Me)₂}]⁺B(C₆F₅)₄^?.     

Cyclic (Amino)(aryl)carbenes (CAArCs) as Strong σ‐Donating and π‐Accepting Ligands for Transition Metals

Cyclic (amino)(aryl)carbenes (CAArCs) result from the replacement of the alkyl substituent of cyclic (alkyl)(amino) carbenes (CAACs) by an aryl group. This structural modification leads to enhanced electrophilicity of the carbene center with retention of the high nucleophilicity of CAACs, and therefore CAArCs feature a small singlet–triplet gap. The isoindolium precursors are readily prepared in good yields, and deprotonation at low temperature, in the presence of [RhCl(cod)]₂ and [(Me₂S)AuCl] lead to air‐stable rhodium and gold CAArC‐supported complexes, respectively. The rhodium complexes promote the [3+2] cycloaddition of diphenylcyclopropenone with ethyl phenylpropiolate, and induce the addition of 2‐vinylpyridine to alkenes by CH activation. The gold complexes allow for the catalytic three‐component preparation of 1,2‐dihydroquinolines from aniline and phenyl acetylene. These preliminary results illustrate the potential of CAArC ligands in transition‐metal catalysis.     

N‐Heterocyclic Carbene–Phosphinidene Complexes of the Coinage Metals

Coinage metal complexes of the N‐heterocyclic carbene–phosphinidene adduct IPr ? PPh (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were prepared by its reaction with CuCl, AgCl, and [(Me₂S)AuCl], which afforded the monometallic complexes [(IPr ? PPh)MCl] (M=Cu, Ag, Au). The reaction with two equivalents of the metal halides gave bimetallic [(IPr ? PPh)(MCl)₂] (M=Cu, Au); the corresponding disilver complex could not be isolated. [(IPr ? PPh)(CuOTf)₂] was prepared by reaction with copper(I) trifluoromethanesulfonate. Treatment of [(IPr ? PPh)(MCl)₂] (M=Cu, Au) with Na(BAr^F) or AgSbF₆ afforded the tetranuclear complexes [(IPr ? PPh)₂M₄Cl₂]X₂ (X=BAr^F or SbF₆), which contain unusual eight‐membered M₄Cl₂P₂ rings with short cuprophilic or aurophilic contacts along the chlorine‐bridged M???M axes. Complete chloride abstraction from [(IPr ? PPh)(AuCl)₂] was achieved with two equivalents of AgSbF₆ in the presence of tetrahydrothiophene (THT) to form [(IPr ? PPh){Au(THT)}₂][SbF₆]₂. The cationic tetra‐ and dinuclear complexes were used as catalysts for enyne cyclization and carbene transfer reactions.     

Breslow Intermediates from Aromatic N‐Heterocyclic Carbenes (Benzimidazolin‐2‐ylidenes,Thiazolin‐2‐ylidenes)

We report the first generation and characterization of elusive Breslow intermediates derived from aromatic N‐heterocyclic carbenes (NHCs), namely benzimidazolin‐2‐ylidenes (NMR, X‐ray analysis) and thiazolin‐2‐ylidenes (NMR). In the former case, the diamino enols were generated by reaction of the free N,N‐bis(2,6‐diisopropylphenyl)‐ and N,N‐bis(mesityl)‐substituted benzimidazolin‐2‐ylidenes with aldehydes while the dimer of 3,4,5‐trimethylthiazolin‐2‐ylidene served as the starting material in the latter case. The unambiguous NMR identification of the first thiazolin‐2‐ylidene‐based Breslow intermediate rests on double ¹³C labeling of both the NHC and the aldehyde component. The acyl anion reactivity was confirmed by benzoin formation with excess aldehyde.     

N‐Heterocyclic Carbenes as Efficient Organocatalysts for CO2 Fixation Reactions

Getting a fix : N‐heterocyclic carbenes (NHCs) and NHC–CO₂ adducts serve as potent organocatalysts for carbonate synthesis by the addition of a CO₂ unit to propargylic alcohols or epoxides under mild and solvent‐free reaction conditions (see scheme). The enhanced Lewis basicity of imidazol‐2‐ylidenes bearing electron‐donating alkyl groups on the nitrogen atoms leads to utilizing CO₂ as a nucleophilic fragment in the chemical fixation processes.

     

Recent Advances in the Synthesis of Heterocycles and Related Substances Based on α‐Imino Rhodium Carbene Complexes Derived from N‐Sulfonyl‐1,2,3‐triazoles

In recent years, α‐imino rhodium carbene complexes derived by ring‐opening of N‐sulfonyl‐1,2,3‐triazoles have attracted much attention from organic chemists. Many transformations of these species have been reported that involve, in most cases, nucleophilic attack at the carbene center of the α‐imino rhodium carbene, facilitating the synthesis of a wide range of novel and useful compounds, particularly heterocycles. This Minireview mainly focuses on advances in the transformation of N‐sulfonyl‐1,2,3‐triazoles during the past two years.     

Oxidative Coupling of Anionic Abnormal N‐Heterocyclic Carbenes: Efficient Access to Janus‐Type 4,4′‐Bis(2H‐imidazol‐2‐ylidene)s

The oxidative coupling of anionic imidazol‐4‐ylidenes protected at the C2 position with [MnCp(CO)₂] or BH₃ led to the corresponding 4,4′‐bis(2H‐imidazol‐2‐ylidene) complexes or adducts, in which the two carbene moieties are connected through a single C?C bond. Subsequent acidic treatment of the later species led to the corresponding 4,4′‐bis(imidazolium) salts in good yields. The overall procedure offers practical access to a novel class of Janus‐type bis(NHC)s. Strikingly, the coplanarity of the two NHC rings within the mesityl derivative 4,4′‐bis(IMes), favored by steric hindrance along with stabilizing intramolecular C?H???π aryl interactions, allows the alignment of the π‐systems and, as a direct consequence, significant electron communication through the bis(carbene) scaffold.     

Main‐Chain Organometallic Microporous Polymers Bearing Triphenylene–Tris(N‐Heterocyclic Carbene)–Gold Species: Catalytic Properties

Two triphenylene‐based tris(N‐heterocyclic carbene)–gold–acetylide main‐chain organometallic microporous polymers (MOMPs) were obtained and fully characterized. Both materials show spherical shapes, and their size is highly dependent on the type of acetylene used in the synthetic protocol. The new solids were tested in the catalytic reduction of nitroarenes with NaBH₄ and in the three‐component Strecker reaction for the synthesis of α‐aminonitriles, and showed high activity in both processes. Whereas the activity of the solids in the reduction of nitroarenes may be attributed to the formation of Au nanoparticles due to the use of NaBH₄ as reducing agent, the activity in the Strecker reaction may originate from the Lewis acidic activation of the ketone or imine on coordination to Au.