Reaction of the N‐heterocyclic carbene, 1,3‐dimesityl‐ imidazol‐2‐ylidene,with a uranyl triflate complex,UO2(OTf)2(thf)3

The N‐heterocyclic carbene, 1,3‐dimesityl‐imidazol‐2‐ylidene (IMes) reacts with tetrahydrofuran (THF) in the presence of an oxidizing uranyl triflate complex, UO₂(OTf)₂(thf)₃ (^?OTf = ^?OSO₂CF₃), to give 1,4‐bis(1,3‐dimesityl‐2‐imidazolium)‐1,3‐butadiene bis(trifluoromethanesulfonate), formally understood as the coupling product of two equivalents of IMes with [CH?CH? CH?CH](OTf)₂. Copyright © 2005 John Wiley & Sons, Ltd.     

Aggregation and Degradation of White Phosphorus Mediated by N‐Heterocyclic Carbene Nickel(0) Complexes

The reaction of zerovalent nickel compounds with white phosphorus (P₄) is a barely explored route to binary nickel phosphide clusters. Here, we show that coordinatively and electronically unsaturated N‐heterocyclic carbene (NHC) nickel(0) complexes afford unusual cluster compounds with P₁, P₃, P₅ and P₈ units. Using [Ni(IMes)₂] [IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene], electron‐deficient Ni₃P₄ and Ni₃P₆ clusters have been isolated, which can be described as superhypercloso and hypercloso clusters according to the Wade–Mingos rules. Use of the bulkier NHC complexes [Ni(IPr)₂] or [(IPr)Ni(η⁶‐toluene)] [IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene] affords a closo‐Ni₃P₈ cluster. Inverse‐sandwich complexes [(NHC)₂Ni₂P₅] (NHC=IMes, IPr) with an aromatic cyclo‐P₅^? ligand were identified as additional products.     

A Masked Cuprous Hydride as a Catalyst for Carbonyl Hydrosilylation in Aqueous Solutions

Redox‐unstable cuprous hydridotriphenylborate was isolated as an N‐heterocyclic carbene adduct [(IPr)Cu(HBPh₃)] (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) with good thermal stability. Although this compound displays a contact ion‐pair structure, Cu^IH‐like catalytic activity was envisaged in carbonyl hydrosilylation. Sufficient moisture stability allowed the catalysis in aqueous/organic media. Mechanistic study further showed that a phenyl group on the borate anion is abstracted by [(IPr)Cu]⁺ to give the cationic organocopper complex [(IPr)₂Cu₂(μ‐Ph)][BPh₄].     

Investigation of the Catalytic Activity of a 2‐Phenylidenepyridine Palladium(II) Complex Bearing 4,5‐Dicyano‐1,3‐bis(mesityl)imidazol‐2‐ylidene in the Mizoroki‐Heck Reaction

The phenylidenepyridine (ppy) palladacycles [PdCl(ppy)(IMes)] ( 4 ) [IMes = 1,3‐bis(mesityl)imidazol‐2‐ylidene] and [PdCl(ppy){(CN)₂IMes}] ( 6 ) [(CN)₂IMes = 4,5‐dicyano‐1,3‐bis(mesityl)imidazol‐2‐ylidene] were prepared by facile two step syntheses, starting with the reaction of palladium(II) chloride with 2‐phenylpyridine followed by subsequent addition of the NHC ligand to the precatalyst precursor [PdCl(ppy)]₂. Suitable crystals for the X‐ray analysis of the complexes 4 and 6 were obtained. It was shown that 6 has a shorter NHC‐palladium bond than the IMes complex 4 . The difference of the palladium carbene bond lengths based on the higher π‐acceptor strength of (CN)₂IMes in comparison to IMes. Thus, (CN)₂IMes should stabilize the catalytically active central palladium atom better than IMes. As a measure for the π‐acceptor strength of (CN)₂IMes compared to IMes, the selone (CN)₂IMes · Se ( 7 ) was prepared and characterized by ⁷⁷Se‐NMR spectroscopy. The π‐acceptor strength of 7 was illuminated by the shift of its ⁷⁷Se‐NMR signal. The ⁷⁷Se‐NMR signal of 7 was shifted to much higher frequencies than the ⁷⁷Se‐NMR signal of IMes · Se. Catalytic experiments using the Mizoroki‐Heck reaction of aryl chlorides with n‐butyl acrylate showed that 6 is the superior performer in comparison to 4 . Using complex 6 , an extensive substrate screening of 26 different aryl bromides with n‐butyl acrylate was performed. Complex 6 is a suitable precatalyst for para‐substituted aryl bromides. The catalytically active species was identified by mercury poisoning experiments to be palladium nanoparticles.     

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.     

The Reactivities of Iminoboranes with Carbenes: BN Isosteres of Carbene–Alkyne Adducts

The first examples of adducts of cyclic alkyl(amino) carbenes (CAAC) and N‐heterocyclic carbenes (NHCs) with iminoboranes have been synthesized and isolated at low temperature (?45 °C). The adducts show short B?N bonds and planarity at boron, mimicking the structures of the isoelectronic imine functionality. When di‐tert‐butyliminoborane was reacted with 1,3‐bis(isopropyl)imidazol‐2‐ylidene (IPr), the initially formed Lewis acid–base adduct quickly rearranged to form a new carbene substituted with an aminoborane at the 4‐position. Warming the iminoborane–CAAC adduct to room temperature resulted in an intramolecular cyclization to give a bicyclic 1,2‐azaborilidine compound.     

Synthesis and Reactivity of Nickel‐Stabilised μ2:η2,η2‐P2, As2 and PAs Units

The reactivity of two paramagnetic nickel(I) compounds, CpNi(NHC) (where Cp=cyclopentadienyl; NHC=1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene (IMes) or 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr)), towards [Na(dioxane)_x][PnCO] (Pn=P, As) is described. These reactions afford symmetric bimetallic compounds (μ²:η²,η²‐Pn₂){Ni(NHC)(CO)}₂. Several novel intermediates en route to such species are identified and characterised, including a compound containing the PCO^? anion in an unprecedented μ²:η²,η²‐binding mode. Ultimately, on treatment of the (μ²:η²,η²‐Pn₂){Ni(IMes)(CO)}₂ compounds with carbon monoxide, the Pn₂ units can be released, affording P₄ in the case of the phosphorus‐containing species, and elemental arsenic in the case of (μ²:η²,η²‐As₂){Ni(IMes)(CO)}₂.     

Di‐tert‐butyldiphosphatetrahedrane: Catalytic Synthesis of the Elusive Phosphaalkyne Dimer

While tetrahedranes as a family are scarce, neutral heteroatomic species are all but unknown, with the only reported example being AsP₃. Herein, we describe the isolation of a neutral heteroatomic X₂Y₂ molecular tetrahedron (X, Y=p‐block elements), which also is the long‐sought‐after free phosphaalkyne dimer. Di‐tert‐butyldiphosphatetrahedrane, (tBuCP)₂, is formed from the monomer tBuCP in a nickel‐catalyzed dimerization reaction using [(NHC)Ni(CO)₃] (NHC=1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene (IMes) and 1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene (IPr)). Single‐crystal X‐ray structure determination of a silver(I) complex confirms the structure of (tBuCP)₂. The influence of the N‐heterocyclic carbene ligand on the catalytic reaction was investigated, and a mechanism was elucidated using a combination of synthetic and kinetic studies and quantum chemical calculations.     

Spectroscopic Evidence for a Monomeric Copper(I) Hydride and Crystallographic Characterization of a Monomeric Silver(I) Hydride

(1,3‐bis[2,6‐bis[di(4‐tert‐butylphenyl)methyl]‐4‐methylphenyl]imidazol‐2‐ylidene)CuOPh [(IPr**)CuOPh] reacts with poly(methylhydrosiloxane) as the hydride donor to afford the monomeric (IPr**)CuH complex, which was spectroscopically characterized. The latter is in equilibrium in solution with [(IPr**)CuH]₂, the dimer being exclusively present in the solid state. These results support the hypothesis that copper hydride aggregates dissociate in solution. In contrast, addition of pinacolborane to [(IPr**)AgOPh] at −40 °C allows the isolation of the monomeric (IPr**)AgH complex, which was crystallographically characterized.     

Large yet Flexible N‐Heterocyclic Carbene Ligands for Palladium Catalysis

A straightforward and scalable eight‐step synthesis of new N‐heterocyclic carbenes (NHCs) has been developed from inexpensive and readily available 2‐nitro‐m‐xylene. This process allows for the preparation of a novel class of NHCs coined ITent (“Tent” for “tentacular”) of which the well‐known IMes (N,N′‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene), IPr (N,N′‐bis(2,6‐di(2‐propyl)phenyl)imidazol‐2‐ylidene) and IPent (N,N′‐bis(2,6‐di(3‐pentyl)phenyl)imidazol‐2‐ylidene) NHCs are the simplest and already known congeners. The synthetic route was successfully used for the preparation of three members of the ITent family: IPent (N,N′‐bis(2,6‐di(3‐pentyl)phenyl)imidazol‐2‐ylidene), IHept (N,N′‐bis(2,6‐di(4‐heptyl)phenyl)imidazol‐2‐ylidene) and INon (N,N′‐bis(2,6‐di(5‐nonyl)phenyl)imidazol‐2‐ylidene). The electronic and steric properties of each NHC were studied through the preparation of both nickel and palladium complexes. Finally the effect of these new ITent ligands in Pd‐catalyzed Suzuki–Miyaura and Buchwald–Hartwig cross‐couplings was investigated.     

Palladium‐Catalyzed Direct C2‐Arylation of an N‐Heterocyclic Carbene: An Atom‐Economic Route to Mesoionic Carbene Ligands

Blocking the C2 position of an imidazole‐derived classical N‐heterocyclic carbene (NHC) with an aryl group is an essential strategy to establish a route to mesoionic carbenes (MICs), which coordinate to the metal via the C4 (or C5) carbon atom. An efficient catalytic route to MIC precursors by direct arylation of an NHC is reported. Treatment of 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr) with an aryl iodide (RC₆H₄I) in the presence of 0.5 mol % of [Pd₂(dba)₃] (dba=dibenzylideneacetone) precatalyst affords the C2‐arylated imidazolium salts {IPr(C₆H₄R)}I (R=H, 4‐Me, 2‐Me, 4‐OMe, 4‐COOMe) in excellent (up to 92 %) yields. Treatment of {IPr(C₆H₅)}I with CuI and KN(SiMe₃)₂ exclusively affords the MIC–copper complex [(IPrPh)CuI].     

Cationic Silica‐Supported N‐Heterocyclic Carbene Tungsten Oxo Alkylidene Sites: Highly Active and Stable Catalysts for Olefin Metathesis

Designing supported alkene metathesis catalysts with high activity and stability is still a challenge, despite significant advances in the last years. Described herein is the combination of strong σ‐donating N‐heterocyclic carbene ligands with weak σ‐donating surface silanolates and cationic tungsten sites leading to highly active and stable alkene metathesis catalysts. These well‐defined silica‐supported catalysts, [(≡SiO)W(=O)(=CHCMe₂Ph)(IMes)(OTf)] and [(≡SiO)W(=O)(=CHCMe₂Ph)(IMes)⁺][B(Ar^F)₄^?] [IMes=1,3‐bis(2,4,6‐trimethylphenyl)‐imidazol‐2‐ylidene, B(Ar^F)₄=B(3,5‐(CF₃)₂C₆H₃)₄] catalyze alkene metathesis, and the cationic species display unprecedented activity for a broad range of substrates, especially for terminal olefins with turnover numbers above 1.2 million for propene.     

Gold‐Catalyzed Oxa‐Povarov Reactions for the Synthesis of Highly Substituted Dihydrobenzopyrans from Diaryloxymethylarenes and Olefins

Oxa‐Povarov reactions involving readily available diaryloxymethylarenes and aryl‐substituted alkenes are reported. Their [4+2] cycloadditions were efficiently catalyzed by IPrAuSbF₆ (IPr=1,3‐bis(diisopropylphenyl)imidazol‐2‐ylidene) with high diastereoselectivity. Product analysis revealed that the reactions likely proceed by a stepwise ionic mechanism, because both E‐ and Z‐configured β‐methylstyrene gave the same cycloadducts in the same proportions.     

Development of versatile and silver-free protocols for gold(I) catalysis

The use of a versatile N‐heterocyclic carbene (NHC) gold(I) hydroxide precatalyst, [Au(OH)(IPr)], (IPr=N,N′‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) permits the in situ generation of the [Au(IPr)]⁺ ion by simple addition of a Brønsted acid. This cationic entity is believed to be the active species in numerous catalytic reactions. ¹H NMR studies in several solvent media of the in situ generation of this [Au(IPr)]⁺ ion also reveal the formation of a dinuclear gold hydroxide intermediate [{Au(IPr)}₂(μ‐OH)], which is fully characterized and was tested in gold(I) catalysis.     

The first example of a two‐coordinated AuI atom bonded to an FeII atom and an N‐heterocyclic carbene (NHC) ligand

The aurophilicity exhibited by Au^I complexes depends strongly on the nature of the supporting ligands present and the length of the Au–element (Au—E) bond may be used as a measure of the donor–acceptor properties of the coordinated ligands. A binuclear iron–gold complex, [1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene‐2κC²]dicarbonyl‐1κ²C‐(1η⁵‐cyclopentadienyl)gold(I)iron(II)(Au—Fe) benzene trisolvate, [AuFe(C₅H₅)(C₂₇H₃₆N₂)(CO)₂]·3C₆H₆, was prepared by reaction of K[CpFe(CO)₂] (Cp is cyclopentadienyl) with (NHC)AuCl [NHC = 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]. In addition to the binuclear complex, the asymmetric unit contains three benzene solvent molecules. This is the first example of a two‐coordinated Au atom bonded to an Fe and a C atom of an N‐heterocyclic carbene.     

Stable ‐ESiMe3 Complexes of CuI and AgI (E=S,Se) with NHCs: Synthons in Ternary Nanocluster Assembly

As a part of efforts to prepare new “metallachalcogenolate” precursors and develop their chemistry for the formation of ternary mixed‐metal chalcogenide nanoclusters, two sets of thermally stable, N‐heterocyclic carbene metal–chalcogenolate complexes of the general formula [(IPr)Ag?ESiMe₃] (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene; E=S, 1 ; Se, 2 ) and [(iPr₂‐bimy)Cu?ESiMe₃]₂ (iPr₂‐bimy=1,3‐diisopropylbenzimidazolin‐2‐ylidene; E=S, 4 ; Se, 5 ) are reported. These are prepared from the reaction between the corresponding carbene metal acetate, [(IPr)AgOAc] and [(iPr‐bimy)CuOAc] respectively, and E(SiMe₃)₂ at low temperature. The reaction of [(IPr)Ag?ESiMe₃] 1 with mercury(II) acetate affords the heterometallic complex [{(IPr)AgS}₂Hg] 3 containing two (IPr)Ag?S^? fragments bonded to a central Hg^II, representing a mixed mercury–silver sulfide complex. The reaction of [(iPr₂‐bimy)Cu‐SSiMe₃]₂, which contains a smaller N‐heterocyclic‐carbene, with mercuric(II) acetate affords the high nuclearity cluster, [(iPr₂‐bimy)₆Cu₁₀S₈Hg₃] 6 . The new N‐heterocyclic carbene metal–chalcogenolate complexes 1 , 2 , 4 , 5 and the ternary mixed‐metal chalcogenolate complex 3 and cluster 6 have been characterized by multinuclear NMR spectroscopy (¹H and ¹³C), elemental analysis and single‐crystal X‐ray diffraction.     

Alkali‐Metal‐Mediated Magnesiations of an N‐Heterocyclic Carbene: Normal,Abnormal, and “Paranormal” Reactivity in a Single Tritopic Molecule

Herein the sodium alkylmagnesium amide [Na₄Mg₂(TMP)₆(nBu)₂] (TMP=2,2,6,6‐tetramethylpiperidide), a template base as its deprotonating action is dictated primarily by its 12 atom ring structure, is studied with the common N‐heterocyclic carbene (NHC) IPr [1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]. Remarkably, magnesiation of IPr occurs at the para‐position of an aryl substituent, sodiation occurs at the abnormal C4 position, and a dative bond occurs between normal C2 and sodium, all within a 20 atom ring structure accommodating two IPr²⁻. Studies with different K/Mg and Na/Mg bimetallic bases led to two other magnesiated NHC structures containing two or three IPr⁻ monoanions bound to Mg through abnormal C4 sites. Synergistic in that magnesiation can only work through alkali‐metal mediation, these reactions add magnesium to the small cartel of metals capable of directly metalating a NHC.     

Alkali‐Metal‐Mediated Magnesiations of an N‐Heterocyclic Carbene: Normal,Abnormal, and “Paranormal” Reactivity in a Single Tritopic Molecule

Herein the sodium alkylmagnesium amide [Na₄Mg₂(TMP)₆(nBu)₂] (TMP=2,2,6,6‐tetramethylpiperidide), a template base as its deprotonating action is dictated primarily by its 12 atom ring structure, is studied with the common N‐heterocyclic carbene (NHC) IPr [1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]. Remarkably, magnesiation of IPr occurs at the para‐position of an aryl substituent, sodiation occurs at the abnormal C4 position, and a dative bond occurs between normal C2 and sodium, all within a 20 atom ring structure accommodating two IPr^2?. Studies with different K/Mg and Na/Mg bimetallic bases led to two other magnesiated NHC structures containing two or three IPr^? monoanions bound to Mg through abnormal C4 sites. Synergistic in that magnesiation can only work through alkali‐metal mediation, these reactions add magnesium to the small cartel of metals capable of directly metalating a NHC.     

Insight into the role of the counteranion of an imidazolium salt in organocatalysis: a combined experimental and computational study

N‐Heterocyclic carbenes (NHCs) can serve as very reactive nucleophilic catalysts and exhibit strong basicity. Herein, we initiate a combined experimental and computational investigation of the NHC‐catalyzed ring‐closing reactions of 4‐(2‐formylphenoxy)but‐2‐enoate derivatives 1 to uncover the relationship between the counteranion of an azolium salt, the nucleophilicity and basicity of the carbene species, and the catalytic performance of the carbene species by taking imidazolium salts IPr ? HX (X=counteranion, IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) as the representative precatalysts. The plausible mechanisms of IPr‐mediated ring‐closing reactions have been investigated by using DFT calculations. The hydrogen‐accepting ability, assigned as the basicity of the counteranion of IPr ? HX and evaluated by DFT calculations, is correlated with the rate of deprotonation of C2 in IPr ? HX, which could be monitored by the capture of the free carbene formed in situ with elemental sulfur. The deprotonation of C2 in IPr ? HX with a more basic anion gives rise to a higher concentration of the free carbene and vice versa. At a relatively low concentration, IPr prefers to show a nucleophilic character to induce the intramolecular Stetter reaction. At a relatively high concentration, IPr primarily acts as a base to afford benzofuran derivatives. These data comprehensively disclose, for the first time, that the counteranions of azolium salts significantly influence not only the catalytic activity, but also possibly the reaction mechanism.     

Rational Exploration of N‐Heterocyclic Carbene (NHC) Palladacycle Diversity: A Highly Active and Versatile Precatalyst for Suzuki–Miyaura Coupling Reactions of Deactivated Aryl and Alkyl Substrates

As less attention has been focussed on the design of highly efficient palladium precatalysts to ensure the smooth formation of the active catalyst for metal‐mediated cross coupling reactions, we herein demonstrate that combining the bulky N‐heterocyclic carbene (NHC) 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr) with cyclopalladated acetanilide as the optimal palladium precatalyst leads to superior catalytic activity compared with the state‐of‐the‐art NHC–Pd catalysts. The complex was discovered through the evaluation of a small, rationally designed library of NHC–palladacycles prepared by a novel, practical and atom‐economic method, the direct reaction of IPr?HCl with palladacycle acetate dimers.