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.     

Generalization of the Copper to Late‐Transition‐Metal Transmetallation to Carbenes beyond N‐Heterocyclic Carbenes

Carbene transition‐metal complexes have become a prevalent family of catalysts enabling numerous organic transformations. Their facile synthetic access is a matter of great importance. To this end, the Cu^I‐NHC transfer methodology has emerged as a powerful alternative presenting attractive advantages over other methods. Herein, we report the remarkable ability of copper to transfer not only NHCs but also other types of carbenes such as abnormal NHCs (aNHCs), cyclic (alkyl)(amino)carbenes (CAACs), and mesoionic carbenes (MICs) to various transition metal precursors.     

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.

     

Stable P‐Heterocyclic Carbenes: Scope and Limitations

The conjugate acids (PHCH⁺s) of P‐heterocyclic carbenes (PHCs) are prepared by formal [3+2] cycloaddition of a 1,3‐diphosphaallyl or 1,3‐phosphinophosphenium cation with various nitriles. The effect of the phosphorus substituent on the fate of the cyclization and on that of the counteranion and base in the subsequent deprotonation reaction are reported. Two PHCs that are indefinitely stable in the solid state are described. In solution, one of them, made from acetonitrile, undergoes a facile [3+2] cycloreversion, whereas the other, based on dimethyl cyanamide, is stable, presumably owing to its zwitterionic structure, which involves a tricoordinate pentavalent phosphorus atom. The reactivity of PHCs is strongly driven by the high electrophilicity of the phosphorus centers, as demonstrated by their reactivity with water and benzaldehyde. Although both PHCs reported in this paper are direct analogues of the least‐basic NHCs, their basicity is comparable to those of the more strongly basic NHCs (as determined by comparison of the carbonyl stretching frequencies of their corresponding cis‐[RhCl‐(CO)₂(L)] complexes).     

Comparative Theoretical Study of N‐Heterocyclic Carbenes and Other Ligands Bound to AuI

The bonding strength of N‐heterocyclic carbene (NHC) ligands to a neutral AuCl test moiety are compared to that of several phosphanes and other ligands. Of the ligands studied, the NHCs clearly form the strongest bonds to AuCl. A simplified triangular CN₂ model is also introduced for the NHCs.     

Selective Activation of Fluoroalkenes with N‐Heterocyclic Carbenes: Synthesis of N‐Heterocyclic Fluoroalkenes and Polyfluoroalkenyl Imidazolium Salts

Selective reactions between nucleophilic N,N′‐diaryl‐heterocyclic carbenes (NHCs) and electrophilic fluorinated alkenes afford NHC fluoroalkenes in high yields. These stable compounds undergo efficient and selective fluoride abstraction with Lewis acids to give polyfluoroalkenyl imidazolium salts. These salts react at Cβ with pyrrolidine to give ammonium fluoride‐substituted salts, which give rise to conjugated imidazolium‐enamine salts through loss of HF. Alternatively, reaction with 4‐(dimethylamino)‐pyridine provides a Cα‐pyridinium‐substituted NHC fluoroalkene. These compounds were studied using multinuclear NMR spectroscopy, mass spectrometry, and X‐ray crystallography. Insight into their electronic structure and reactivity was gained through the use of DFT calculations.     

Comprehensive Basicity Scales for N‐Heterocyclic Carbenes in DMSO: Implications on the Stabilities of N‐Heterocyclic Carbene and CO2 Adducts

A very broad acidity scale (≈40 pK units) for about 400 N‐heterocyclic carbene precursors (NHCPs) with various backbones and electronic features, including imidazolylidenes, 1,2,4‐triazolylidenes, cyclic diaminocarbenes (CDACs), diamidocarbenes (DACs), thiazolylidenes, cyclic (alkyl)(amino)carbenes (CAACs) and mesoionic carbenes (MICs), was established in DMSO by a well examined computational method. Varying the backbone structure or flanking N‐substituents can have different extent of acidifying effects, depending on both the nature and number of substituent(s). The Gibbs energies (ΔG_rs) for the reactions between the corresponding NHCs and CO₂ were also calculated. There is a good linear correlation between the pK_as of most NHCPs and ΔG_rs, suggesting that a greater basicity of NHC leads to a more stable NHC‐CO₂ adduct. Interestingly, the nearby asymmetric environment has virtually no differential effect on the acidities of the chiral NHCP enantiomers, but has a pronounced effect on the ΔG_r values.     

The Measure of All Rings—N‐Heterocyclic Carbenes

Quantification and variation of characteristic properties of different ligand classes is an exciting and rewarding research field. N‐Heterocyclic carbenes (NHCs) are of special interest since their electron richness and structure provide a unique class of ligands and organocatalysts. Consequently, they have found widespread application as ligands in transition‐metal catalysis and organometallic chemistry, and as organocatalysts in their own right. Herein we provide an overview on physicochemical data (electronics, sterics, bond strength) of NHCs that are essential for the design, application, and mechanistic understanding of NHCs in catalysis.     

Group 13 Element Trihalide Complexes of Anionic N‐Heterocyclic Carbenes

A series of group 13 complexes of the general type [{(WCA‐IDipp)EX₃}Li(solv)] (E=B, Al, Ga, In; X=Cl, Br) that bear an anionic N‐heterocyclic carbene ligand with a weakly coordinating borate moiety (WCA‐IDipp, WCA=B(C₆F₅)₃ and IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were prepared by the reaction of the respective group 13 trihalides (EX₃) with the lithium salt [(WCA‐IDipp)Li ? toluene]. The molecular structures of the BBr₃, AlCl₃, AlBr₃, GaCl₃ and InCl₃ adducts were established by X‐ray diffraction analyses, revealing the formation of coordination polymers linked by halide‐lithium interactions, except for the indium derivative, which consists of isolated [Li(THF)₄]⁺ and [(WCA‐IDipp)InCl₃]^? ions in the solid state.     

Mechanistic Insight into the Staudinger Reaction Catalyzed by N‐Heterocyclic Carbenes

Four zwitterions were prepared by treating 1,3‐dimesitylimidazolin‐2‐ylidene (SIMes) or 1,3‐dimesitylimidazol‐2‐ylidene (IMes) with either N‐tosyl benzaldimine or diphenylketene. They were isolated in high yields and characterized by IR and NMR spectroscopy. The molecular structures of three of them were determined by using X‐ray crystallography and their thermal stability was monitored by using thermogravimetric analysis. The imidazol(in)ium‐2‐amides were rather labile white solids that did not show any tendency to tautomerize into the corresponding 1,2,2‐triaminoethene derivatives. They displayed a mediocre catalytic activity in the Staudinger reaction of N‐tosyl benzaldimine with diphenylketene. In contrast, the imidazol(in)ium‐2‐enolates were orange‐red crystalline materials that remained stable over extended periods of time. Despite their greater stability, these zwitterions turned out to be efficient promoters for the model cycloaddition under scrutiny. As a matter of fact, their catalytic activity matched those recorded with the free carbenes. Altogether, these results provide strong experimental insight into the mechanism of the Staudinger reaction catalyzed by N‐heterocyclic carbenes. They also highlight the superior catalytic activity of the imidazole‐based carbene IMes compared with its saturated analogue SIMes in the reaction under consideration.     

Probing the Diastereoselectivity of Staudinger Reactions Catalyzed by N‐Heterocyclic Carbenes

The reaction of ethylphenylketene with 1,3‐dimesitylimidazol‐2‐ylidene (IMes) or 1,3‐dimesitylimidazolin‐2‐ylidene (SIMes) afforded the corresponding azolium enolates in high yields. The two zwitterions were fully characterized by various analytical techniques. Their thermal stabilities were monitored by thermogravimetric analysis and the molecular structure of SIMes ? EtPhC?C?O was determined by means of X‐ray crystallography. A mechanism was proposed to account for the trans‐diastereoselectivity observed in the [2+2] cycloaddition of ketenes and N‐protected imines catalyzed by N‐heterocyclic carbenes and an extensive catalytic screening was performed to test its validity. The steric bulk of the NHC catalyst markedly affected the cis/trans ratio of the model β‐lactam product. The nature of the solvent used to carry out the Staudinger reaction also significantly influenced its diastereoselectivity. Conversely, the nature of the substituent on the N‐sulfonated imine reagent and the reaction temperature were less critical parameters.     

CO2 and SnII Adducts of N‐Heterocyclic Carbenes as Delayed‐Action Catalysts for Polyurethane Synthesis

Catalytic rivals : Both CO₂‐protected tetrahydropyrimidin‐2‐ylidene‐based N‐heterocyclic carbenes (NHCs) and Sn^II‐1,3‐dimesitylimidazol‐2‐ylidene, as well as Sn^II‐1,3‐dimesitylimidazolin‐2‐ylidene complexes (example displayed), have been identified as truly latent catalysts for polyurethane (PUR) synthesis rivaling all existing systems both in activity and latency.