pi-Face donor properties of N-heterocyclic carbenes in Grubbs II complexes

The electron-donating properties of eighteen 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, OMe, Me, H, SMe, F, Cl, Br, I) in the respective Grubbs II complexes were studied using electrochemical techniques. The nature of the 4-R substituent has a strong influence on the RuII/III redox potentials ranging between DeltaE1/2= +0.196 and +0.532 V. Three unsymmetrical Grubbs II complexes with 4-R not equal to 4-R' were also synthesized. Dynamic NMR spectroscopy revealed the restricted rotation around the (NHC)C-Ru bond (DeltaG = 89 kJ mol(-1) at 333 K) resulting in two atropisomers, respectively, with an isomer ratio close to unity. Each of the isomers, that is the two orientations of the 4-R/4-R' substituted mesityl ring with respect to the R=CHPh unit, gives rise to different redox potentials (4-R = NEt2, 4-R' = Br: DeltaE1/2= +0.232 and +0.451 V). In the oxidized Grubbs II complex (4-R = NEt2, H) and in the cathodic isomer the electron rich aryl ring is located above the Ru=CHPh unit. This orientational effect provides clear evidence for strong pi-pi through-space interactions in the RuIII complexes, assuming that the alternative through-bond transfer of electron density is equally efficient in both isomers.     

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.     

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.     

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.     

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.     

Stable planar six-pi-electron six-membered N-heterocyclic carbenes with tunable electronic properties

Carbene analogues of borazines are highly thermally stable. Keeping quasi-identical steric demands, the electronic properties of the carbene can be precisely tuned by varying the nature of the substituents at the boron centers.     

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 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.     

N-heterocyclic carbenes with inorganic backbones: electronic structures and ligand properties

The electronic structures of known N-heterocyclic carbenes (NHCs) with boron, nitrogen, and phosphorus backbones are examined using quantum chemical methods and compared to the experimental results and to the computational data obtained for a classical carbon analogue, imidazol-2-ylidene. The sigma-donor and pi-acceptor abilities of the studied NHCs in selected transition-metal complexes are evaluated using a variety of approaches such as energy and charge decomposition analysis, as well as calculated acidity constants and carbonyl stretching frequencies. The study shows that the introduction of selected heteroatoms into the NHC backbone generally leads to stronger metal-carbene bonds and therefore improves the ligand properties of these systems. The backdonation of pi-electron density from the metal to the ligand is found to be strong in complexes of the studied NHCs with group 11 metals, where it constitutes up to nearly 35% of the total orbital interaction energy. The ligand properties of the aluminum analogues of some of the reported NHCs with boron backbones are also assessed.     

Complexes of borane and N-heterocyclic carbenes: a new class of radical hydrogen atom donor

Calculations suggest that complexes of borane with N-heterocyclic carbenes (NHC) have B-H bond dissocation energies more then 20 kcal/mol less than free borane, diborane, borane-THF, and related complexes. Values are in the range of popular radical hydrogen atom donors like tin hydrides (70-80 kcal/mol). The resulting prediction that NHC borane complexes could be used as radical hydrogen atom donors was verified by radical deoxygenations of xanthates by using either AIBN or triethylborane as initiator.     

Ligand properties of N-heterocyclic and Bertrand carbenes: A density functional study

In order to probe the ligand properties we have examined a series of Cr(CO)₅L and Ni(CO)₃L complexes using density functional theory (DFT). The ligands included in our study are N-heterocyclic carbenes (NHCs) and Bertrand-type carbenes. Our study shows that the carbene–metal bonds of imidazol-2-ylidenes (1), imidazolin-2-ylidenes (2), thiazo-2-ylidenes (3), and triazo-5-ylidenes (4) are significantly stronger than those of Bertrand-type carbenes (5–7). The force constants of C–O in complexes are related to the property of isolated carbenes such as proton affinity (PA), electronegativity (χ), and charge transfer (ΔN). NHCs and Bertrand-type carbenes are identified as nucleophilic, soft ligands. Carbene stabilization energy (CSE) computations indicate that carbenes 1 and 4 are the most stable species, while 2 and 3 are less stable. In contrast to NHCs, CSE of carbenes 5–7 are much smaller, and their relative stabilities are in the order (amino)(aryl) carbenes 7e–7g > (amino)(alkyl) carbenes 7a–7d > (phosphino)(aryl) 6d–6e, and (phosphino)(silyl) carbenes 5a–5c > (phosphino)(alkyl) carbenes 6a–6c.     

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.     

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.     

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.     

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.     

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.     

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.     

Cycloadditions of anionic N-heterocyclic carbenes of sydnone imines

Sydnone imines were deprotonated with lithium bis(trimethylsilyl)amide at the C4 position to give the corresponding sydnone imine anions as lithium adducts. These can be represented as lithium stabilized anionic N-heterocyclic carbenes. Treatment with diisopropyl azodicarboxylate (DIAD) gave the corresponding C4 adducts, i.e. 4-hydrazinyl-sydnone imines, which form tautomers in solution. Reductive 1,3-dipolar cycloadditions of the sydnone imine anions with tetracyanoethylene (TCNE) resulted in the formation of pyrazoles, the mechanism of formation of which differs from known reactions. Reaction of the anion derived from the 2-methoxyphenyl sydnone imine with N,N-diisopropylcarbodiimide gave a ring-cleaved bisiminonitrile. Structure elucidations were accomplished by NMR spectroscopy and by four single crystal X-ray analyses.     

Nickel and palladium complexes of enolatefunctionalised N-heterocyclic carbenes

The reaction of chloroethyltrimethylsilylether with 1-methylimidazole furnishes an ionic liquid that undergoes methanolysis to crystalline 2-hydroxyethylimidazolium chloride (crystal structure presented). Conversion to defined hydroxyethylimidazol-2-ylidene nickel complexes failed, but was accomplished with 1-methyl-3-acetophenyl-imidazolium bromide. The bis(NHC^⋂O⁻) nickel(II) chelate is formed, rather than a methallylnickel monochelate, but with nickelocene a monochelate NiCp complex was detected. The bulky 1-(2,6-diisopropylphenyl)-3-(2’-phenyl-enolato)-imidazol-2-ylidene allylpalladium chloride was obtained in pure form. Attempts to generate catalysts for ethylene oligomerization by in situ techniques have failed so far whereas P^⋂O⁻ ligands, comparable by the P-C diagonal relationship, provide active catalysts.