Dinuclear Zinc Hydride Supported by an Anionic Bis(N‐Heterocyclic Carbene) Ligand

Methylene‐linked bis(N,N′‐di‐tert‐butylimidazol‐2‐ylidene) 1 reacted with diethylzinc to give dinuclear zinc ethyl compound 2 , which contains a formally anionic bis(carbene) ligand as a result of deprotonation of the methylene bridge. The reaction of 2 with PhSiH₃ gave the phenylsilyl compound 3 . The zinc hydride 4 was obtained by the reaction of 2 with LiAlH₄ or Ph₃SiOH followed by treatment with PhSiH₃. X‐ray diffraction studies show that compounds 2 , 3 , and 4 all have a similar dimeric structure with D_2h symmetry. The reaction of hydride 4 with carbon dioxide and N,N′‐diisopropylcarbodiimide gave formato ( 5 ) and formamidinato ( 7 ) derivatives as a result of the insertion of the heterocumulene into both Zn? H bonds. Reaction with Ph₂CO gave the diphenylmethoxy compound 6 . Hydride 4 shows catalytic activity in the hydrosilylation of 1,1‐diphenylethylene and methanolysis of silanes.     

Conversion of Carbon Dioxide into Methanol with Silanes over N‐Heterocyclic Carbene Catalysts

Activate and reduce : Carbon dioxide was reduced with silane using a stable N‐heterocyclic carbene organocatalyst to provide methanol under very mild conditions. Dry air can serve as the feedstock, and the organocatalyst is much more efficient than transition‐metal catalysts for this reaction. This approach offers a very promising protocol for chemical CO₂ activation and fixation.

     

Synthesis of Mixed Silylene–Carbene Chelate Ligands from N‐Heterocyclic Silylcarbenes Mediated by Nickel

The Ni^II‐mediated tautomerization of the N‐heterocyclic hydrosilylcarbene L²Si(H)(CH₂)NHC 1 , where L²=CH(C?CH₂)(CMe)(NAr)₂, Ar=2,6‐iPr₂C₆H₃; NHC=3,4,5‐trimethylimidazol‐2‐yliden‐6‐yl, leads to the first N‐heterocyclic silylene (NHSi)–carbene (NHC) chelate ligand in the dibromo nickel(II) complex [L¹Si:(CH₂)(NHC)NiBr₂] 2 (L¹=CH(MeC?NAr)₂). Reduction of 2 with KC₈ in the presence of PMe₃ as an auxiliary ligand afforded, depending on the reaction time, the N‐heterocyclic silyl–NHC bromo Ni^II complex [L²Si(CH₂)NHCNiBr(PMe₃)] 3 and the unique Ni⁰ complex [η²(Si‐H){L²Si(H)(CH₂)NHC}Ni(PMe₃)₂] 4 featuring an agostic Si? H→Ni bonding interaction. When 1,2‐bis(dimethylphosphino)ethane (DMPE) was employed as an exogenous ligand, the first NHSi–NHC chelate‐ligand‐stabilized Ni⁰ complex [L¹Si:(CH₂)NHCNi(dmpe)] 5 could be isolated. Moreover, the dicarbonyl Ni⁰ complex 6 , [L¹Si:(CH₂)NHCNi(CO)₂], is easily accessible by the reduction of 2 with K(BHEt₃) under a CO atmosphere. The complexes were spectroscopically and structurally characterized. Furthermore, complex 2 can serve as an efficient precatalyst for Kumada–Corriu‐type cross‐coupling reactions.     

A Continuum of Progress: Applications of N‐Hetereocyclic Carbene Catalysis in Total Synthesis

N‐Heterocyclic carbene (NHC) catalyzed transformations have emerged as powerful tactics for the construction of complex molecules. Since Stetter’s report in 1975 of the total synthesis of cis‐jasmon and dihydrojasmon by using carbene catalysis, the use of NHCs in total synthesis has grown rapidly, particularly over the last decade. This renaissance is undoubtedly due to the recent developments in NHC‐catalyzed reactions, including new benzoin, Stetter, homoenolate, and aroylation processes. These transformations employ typical as well as Umpolung types of bond disconnections and have served as the key step in several new total syntheses. This Minireview highlights these reports and captures the excitement and emerging synthetic utility of carbene catalysis in total synthesis.     

Neutral and Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Complexes: Reactivity in Selected Olefin Metathesis Reactions and Immobilization on Silica

The synthesis and single‐crystal X‐ray structures of the novel molybdenum imido alkylidene N‐heterocyclic carbene complexes [Mo(N‐2,6‐Me₂C₆H₃)(IMesH₂)(CHCMe₂Ph)(OTf)₂] ( 3 ), [Mo(N‐2,6‐Me₂C₆H₃)(IMes)(CHCMe₂Ph)(OTf)₂] ( 4 ), [Mo(N‐2,6‐Me₂C₆H₃)(IMesH₂)(CHCMe₂Ph)(OTf){OCH(CF₃)₂}] ( 5 ), [Mo(N‐2,6‐Me₂C₆H₃)(CH₃CN)(IMesH₂)(CHCMe₂Ph)(OTf)]⁺ BAr_F^? ( 6 ), [Mo(N‐2,6‐Cl₂C₆H₃)(IMesH₂)(CHCMe₃)(OTf)₂] ( 7 ) and [Mo(N‐2,6‐Cl₂C₆H₃)(IMes)(CHCMe₃)(OTf)₂] ( 8 ) are reported (IMesH₂=1,3‐dimesitylimidazolidin‐2‐ylidene, IMes=1,3‐dimesitylimidazolin‐2‐ylidene, BAr_F^?=tetrakis‐[3,5‐bis(trifluoromethyl)phenyl] borate, OTf=CF₃SO₃^?). Also, silica‐immobilized versions I1 and I2 were prepared. Catalysts 3 – 8 , I1 and I2 were used in homo‐, cross‐, and ring‐closing metathesis (RCM) reactions and in the cyclopolymerization of α,ω‐diynes. In the RCM of α,ω‐dienes, in the homometathesis of 1‐alkenes, and in the ethenolysis of cyclooctene, turnover numbers (TONs) up to 100 000, 210 000 and 30 000, respectively, were achieved. With I1 and I2 , virtually Mo‐free products were obtained (<3 ppm Mo). With 1,6‐hepta‐ and 1,7‐octadiynes, catalysts 3 , 4 , and 5 allowed for the regioselective cyclopolymerization of 4,4‐bis(ethoxycarbonyl)‐1,6‐heptadiyne, 4,4‐bis(hydroxymethyl)‐1,6‐heptadiyne, 4,4‐bis[(3,5‐diethoxybenzoyloxy)methyl]‐1,6‐heptadiyne, 4,4,5,5‐tetrakis(ethoxycarbonyl)‐1,7‐octadiyne, and 1,6‐heptadiyne‐4‐carboxylic acid, underlining the high functional‐group tolerance of these novel Group 6 metal alkylidenes.     

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.     

Oxidative Enantioselective α‐Fluorination of Aliphatic Aldehydes Enabled by N‐Heterocyclic Carbene Catalysis

Described is the first study on oxidative enantioselective α‐fluorination of simple aliphatic aldehydes enabled by N‐heterocyclic carbene catalysis. N‐fluorobis(phenyl)sulfonimide serves as a an oxidant and as an “F” source. The C? F bond formation occurs directly at the α position of simple aliphatic aldehydes, thus overcoming nontrivial challenges, such as competitive difluorination and nonfluorination, and proceeds with high to excellent enantioselectivities.     

N‐Heterocyclic Carbene,High Oxidation State Molybdenum Alkylidene Complexes: Functional‐Group‐Tolerant Cationic Metathesis Catalysts

We synthesized the first N‐heterocyclic carbene (NHC) complexes of Schrock’s molybdenum imido alkylidene bis(triflate) complexes. Unlike existing bis(triflate) complexes, the novel 16‐electron complexes represent metathesis active, functional‐group‐tolerant catalysts. Single‐crystal X‐ray structures of two representatives of this novel class of Schrock catalysts are presented and reactivity is discussed in view of their structural peculiarities. In the presence of monomer (substrate), these catalysts form cationic species and can be employed in ring‐closing metathesis (RCM), ring‐opening metathesis polymerization (ROMP), as well as in the cyclopolymerization of α,ω‐diynes. Monomers containing functional groups, which are not tolerated by the existing variations of Schrock’s catalyst, e.g., sec‐amine, hydroxy, and carboxylic acid moieties, can be used. These catalysts therefore hold great promise in both organic and polymer chemistry, where they allow for the use of protic monomers.     

Catalytic Asymmetric Hydroalkenylation of Vinylarenes: Electronic Effects of Substrates and Chiral N‐Heterocyclic Carbene Ligands

An asymmetric tail‐to‐tail cross‐hydroalkenylation of vinylarenes with terminal olefins was achieved by catalysis with NiH complexes bearing chiral N‐heterocyclic carbenes (NHCs). The reaction provides branched gem‐disubstituted olefins with high enantioselectivity (up to 94 % ee) and chemoselectivity (cross/homo product ratio: up to 99:1). Electronic effects of the substituents on the vinylarenes and on the N‐aryl groups of the NHC ligands, but not a π,π‐stacking mechanism, assist the steric effect and influence the outcome of the cross‐hydroalkenylation.     

Proton‐Transfer Polymerization (HTP): Converting Methacrylates to Polyesters by an N‐Heterocyclic Carbene

A new polymerization termed proton (H)‐transfer polymerization (HTP) has been developed to convert dimethacrylates to unsaturated polyesters. HTP is catalyzed by a selective N‐heterocyclic carbene capable of promoting intermolecular Umpolung condensation through proton transfer and proceeds through the step‐growth propagation cycles via enamine intermediates. The role of the added suitable phenol, which is critical for achieving an effective HTP, is twofold: shutting down the radically induced chain‐growth addition polymerization under HTP conditions (typically at 80–120 °C) and facilitating proton transfer after each monomer enchainment. The resulting unsaturated polyesters have a high thermal stability and can be readily cross‐linked to robust polyester materials.     

Chiral N‐Heterocyclic Carbene Ligands Bearing a Pyridine Moiety for the Copper‐Catalyzed Alkylation of N‐Sulfonylimines with Dialkylzinc Reagents

Amino acid‐derived chiral imidazolium salts, each bearing a pyridine ring, were developed as N‐heterocyclic carbene ligands. The copper‐catalyzed asymmetric alkylation of various N‐sulfonylimines with dialkylzinc reagents in the presence of these chiral imidazolium salts afforded the corresponding alkylated products with high enantioselectivity (up to 99 % ee). The addition of HMPA to the reaction mixture as a co‐solvent is critical in terms of chemical yield and enantioselectivity. A wide range of N‐sulfonylimines and dialkylzinc reagents were found to be applicable to this reaction.     

A Cooperative N‐Heterocyclic Carbene/Chiral Phosphate Catalysis System for Allenolate Annulations

The highly enantioselective NHC‐catalyzed [3+2] annulation reaction with α,β‐alkynals and α‐ketoesters has been developed. A new mode of cooperative catalysis involving the combination of a chiral Brønsted acid and a C₁‐symmetric biaryl saturated‐imidazolium precatalyst was required to generate the desired γ‐crotonolactones in high yields and levels of enantioselectivity.     

Hydroboration of Arynes with N‐Heterocyclic Carbene Boranes

Arynes were generated in situ from ortho‐silyl aryl triflates and fluoride ions in the presence of stable N‐heterocyclic carbene boranes (NHC? BH₃). Spontaneous hydroboration ensued to provide stable B‐aryl‐substituted NHC‐boranes (NHC? BH₂Ar). The reaction shows good scope in terms of both the NHC‐borane and aryne components and provides direct access to mono‐ and disubstituted NHC‐boranes. The formation of unusual ortho regioisomers in the hydroboration of arynes with an electron‐withdrawing group supports a hydroboration process with hydride‐transfer character.     

The Role of Free N‐Heterocyclic Carbene (NHC) in the Catalytic Dehydrogenation of Ammonia–Borane in the Nickel NHC System

Enders' N‐heterocyclic carbene (NHC) dehydrogenates ammonia–borane with a relatively low barrier, producing NH₂BH₂ and NHC–(H)₂. The nickel NHC catalyst present in the reaction media can activate the NHC–(H)₂ produced to regenerate the free NHC and release H₂. The release of free NHC enables further dehydrogenation of ammonia–borane.

     

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.