N-Heterocyclic carbenes: VI. Abietane-based chiral benzimidazolium salts as N-heterocyclic carbene ligands in the heck reaction

New chiral benzimidazolium salts were synthesized on the basis of abietane and were used as N-heterocyclic carbene ligands in the Heck reaction.     

N-heterocyclic carbenes: VIII. Benzimidazolium diterpene salts with an oxo group in substituent

Benzimidazolium salts with an oxo group in the substituent are synthesized from a diterpene ketone.     

Dual reactivity of N-heterocyclic carbenes towards copper(II) salts

Complexes of N-heterocyclic carbenes (NHCs) with copper(II) halogenides are unstable. Upon formation, these complexes decompose to give haloamidinium salts. Contrastingly, O-substituted copper(II) NHC complexes are fairly stable. A series of new five-, six- and seven-membered ring NHC complexes of Cu(OAc)(2) have been synthesised and characterised in the solid state.     

N-heterocyclic carbenes as organocatalysts

Organocatalyzed reactions represent an attractive alternative to metal-catalyzed processes notably because of their lower cost and benign environmental impact in comparison to organometallic catalysis. In this context, N-heterocyclic carbenes (NHCs) have been studied for their ability to promote primarily the benzoin condensation. Lately, dramatic progress in understanding their intrinsic properties and in their synthesis have made them available to organic chemists. This has resulted in a tremendous increase of their scope and in a true explosion of the number of papers reporting NHC-catalyzed reactions. Here, we highlight the ever-increasing number of reactions that can be promoted by N-heterocyclic carbenes.     

N-heterocyclic carbenes: III. N-heterocyclic carbene ligands based on abietane in Suzuki-Miyaura reaction

By reactions of N-alkyl- and N-arylimidazoles with methyl 12-bromoacetyldehydroabietate a series of unsymmetrically substituted chiral imidazolium bromides with the abietane fragment was synthesized. The salts obtained were suggested as new N-heterocyclic carbene ligands in the Suzuki-Miyaura reaction.     

Reversible carboxylation of N-heterocyclic carbenes

Spectroscopic analysis, thermogravimetric analysis, and crossover experiments performed on a series of imidazolium carboxylates revealed carboxylation was reversible with N-aryl substituted adducts.     

Organocatalytic umpolung: N-heterocyclic carbenes and beyond

The umpolung strategy encompasses all the methods that make organic molecules react in an inverse manner compared to their innate polarity-driven reactivity. This concept entered the field of organocatalysis when it was recognized that N-heterocyclic carbenes (NHCs) can provide catalytic access to acyl anion equivalents. Since then, tremendous efforts have followed to develop a broad variety of NHC-catalyzed reactions. In addition to this, more recent research developments have shown that other families of organocatalysts are also able to mediate transformations in which inversion of polarity is involved. This tutorial review aims at offering a didactic overview of organocatalytic umpolung and should serve as an inspiration for further progress in this field.     

N-heterocyclic carbenes: Reaction to give anilines

The attempted synthesis of bis(NHC)palladium complexes via the direct reaction of an imidazolium salt with palladium acetate results in the formation of a mixed NHC/aniline complex. The route by which the aniline is formed has not been completely elucidated, but it does originally derive from an imidazolium salt.     

N-heterocyclic carbenes in gold catalysis

The appealing properties of N-heterocyclic carbenes (NHC) as ancillary ligands and the high potential of gold as an organometallic catalyst have made their encounter inevitable. Still in its infancy, NHC-gold catalysis is nevertheless growing rapidly. In this tutorial review, catalytic transformations involving NHC-containing gold(i) and gold(iii) complexes are presented. Particular attention is drawn to the versatility and selectivity of these catalysts.     

An efficient synthesis of achiral and chiral 1,2,4-triazolium salts: bench stable precursors for N-heterocyclic carbenes

[reaction: see text] The promising utility of triazolyl N-heterocyclic carbene catalysts in umpolung aldehyde chemistry requires a straightforward reliable synthesis from readily available materials. Herein, we describe the synthesis of a variety of triazolyl N-heterocyclic carbene precursors. The reactions commence from commercially available amino acids and proceed in 44-68% overall yields. The N-heterocyclic salts are air-stable crystalline solids that can be stored with no special precaution and can generate the active catalyst when treated with an appropriate base.     

N-heterocyclic carbenes mediated cyano-phosphorylation of ketones

Cyanation-O-protection reaction of ketones with different cyanide sources catalyzed by N-heterocyclic carbenes is reported. Under the catalysis of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, various ketones coupled with diethyl cyanophosphonate to produce cyanohydrin-O-phosphates with a quaternary carbon center in moderate to excellent yield. Furthermore, the reaction can be scaled-up easily and high yield maintained.     

pi-Face donor properties of N-heterocyclic carbenes

The donor properties of aryl substituted N-heterocyclic carbenes are characterized by lone pair donation from the carbene carbon and, as is shown here, by donation of electron density of the aromatic pi-face of the NHC aryl groups towards the metal.     

The chemistry of functionalised N-heterocyclic carbenes

This tutorial review presents the synthesis, chemistry and applications of functionalised N-heterocyclic carbenes NHC and their transition metal complexes. Functionalised NHC comprise those carrying a phosphino-, amino-, imino- or oxygen-containing functionality on the imidazole sidechain. Main applications have been the modification of catalysts and their immobilisation by fixation on a polymeric support using the functional group. Whereas the functionalisation of the NHC has not improved their performance in catalysis, new developments have occurred in the use of imidazole-containing biomolecules such as L-histidine or caffeine as precursors for NHC.     

N-heterocyclic carbenes: reagents, not just ligands!

The unique properties of N-heterocyclic carbenes (NHCs) have attracted much attention, mainly from theorists and organometallic chemists, the latter using them impressively as ligands for metals. Less well known, however, has been their suitability as excellent catalysts and nucleophilic reagents. Transesterification, nucleophilic aromatic substitution, and cycloaddition reactions are examples in which NHCs can play an important role. Asymmetric reactions using catalytic amounts of chiral NHCs are an efficient approach to optically active compounds. This minireview focuses on this aspect of the chemistry of NHCs.     

Proton affinities of phosphines versus N-heterocyclic carbenes

The gas-phase proton affinities of unusually basic phosphines and N-heterocyclic carbenes are compared and contrasted both computationally and experimentally.     

Complexes featuring N-heterocyclic carbenes with bowl-shaped wingtips

Three imidazolium salts having their two N-substituents equipped with remote calix[4]arenyl termini have been synthesised and converted into N-heterocyclic carbene (NHC) complexes of the type [PdCl₂(NHC)(pyridine)]. An X-ray diffraction study carried out for one of the complexes, namely, trans-[1,3-bis(4-{25,26,27,28-tetrapropyloxycalix[4]aren-5-yl}phenyl)imidazol-2-ylidene](pyridine)palladium(II) dichloride, revealed that in the solid state the complex crystallises as a self-assembled dimer, its components being held together by dispersion forces involving the polyphenoxy units, the pyridine ligands and phenylene rings. This structure provides a new example of the diversity of interactions that may occur in NHC complexes of catalytic relevance, which are thus not limited to intramolecular ones. There was no spectroscopic indication that such interactions also occur in solution.     

Regioselective C-H activation of lanthanide-bound N-heterocyclic carbenes

The reaction of Ln(L)N'2 (Ln = Nd, Ce; L = t-BuNCH2CH2[C{NCHCHNt-Bu}], N' = N(SiMe3)2) with trimethylsilyl iodide regiospecifically functionalises the carbene backbone at the C4-carbene ring position to afford the silylated complex Ln(L')N'I; Ln(L')N'2 is isolated after attempted reduction (L' = t-BuNCH2CH2[C{NC(SiMe3)CHNt-Bu}]) which allows a comparison of the structurally characterised complexes Nd(L)N'2, [Nd(L')N'I]2, and Nd(L')N'2.     

Tuning the electronic properties of N-heterocyclic carbenes

The electron-donating properties of N-heterocyclic carbenes ([N,N'-bis(2,6-dimethylphenyl)imidazol]-2-ylidene and the respective dihydro ligands) with 4,4'-R-substituted aryl rings (4,4'-R=NEt2, OC(12)H(25), Me, H, Br, S(4-tolyl), SO(4-tolyl), SO2(4-tolyl)) were studied. Twelve new N-heterocyclic carbene (NHC) ligands were synthesized as well as the respective iridium complexes [IrCl(cod)(NHC)] and [IrCl(CO)2(NHC)]. Cyclic voltammetry (DeltaE1/2) and IR (nu (CO)) can be used to measure the electron-donating properties of the carbene ligands. Modifying the 4-positions with electron-withdrawing substituents (4-R=-SO(2)Ar, DeltaE1/2=+0.92 V) results in NHC ligands with virtually the same electron-donating capacity as a trialkylphosphine in [IrCl(cod)(PCy3)] (DeltaE1/2 =+0.95 V), while [IrCl(cod)(NHC)] complexes with 4-R=NEt2 (DeltaE1/2= +0.59 V) show drastically more cathodic redox potentials and significantly enhanced donating properties.     

Chiral N-heterocyclic carbenes as asymmetric acylation catalysts

Chiral N-heterocyclic carbenes are generated from C₂-symmetric 1,3-bis(1-arylethyl)imidazolium salts and potassium tert-butoxide. These C₂-symmetric imidazolidenyl carbenes catalyze enantioselective acylation of racemic secondary alcohols. The asymmetric acylation of 1-(1-naphthyl)ethanol was achieved in up to 68% ee of the acylated product, using (R,R)-1,3-bis[(1-naphthyl)ethyl]imidazolium tetrafluoroborate as a precursor of the chiral N-heterocyclic carbene and vinyl propionate as the acyl donor.