Chemistry Of N-heterocyclic Carbenes Research Articles

The N-heterocyclic carbene chemistry of transition-metal carbonyl clusters

In the last decade, chemists have dedicated many efforts to investigate the coordination chemistry of N-heterocyclic carbenes (NHCs). Although most of that research activity has been devoted to mononuclear complexes, transition-metal carbonyl clusters have not escaped from these investigations. This critical review, which is focussed on the reactivity of NHCs (or their precursors) with transition-metal carbonyl clusters (mostly are of ruthenium and osmium) and on the transformations underwent by the NHC-containing species initially formed in those reactions, shows that the polynuclear character of these metallic compounds or, more precisely, the close proximity of one or more metal atoms to that which is or can be attached to the NHC ligand, is responsible for reactivity patterns that have no parallel in the NHC chemistry of mononuclear complexes (74 references).

Fascinating frontiers of N/O-functionalized N-heterocyclic carbene chemistry: from chemical catalysis to biomedical applications

The in vogue N-heterocyclic carbenes (NHCs) have attracted attention largely for its new found popularity in chemical catalysis and also for displaying promising traits in biomedical applications. The current perspective provides an account of our efforts in exploring the utility of N/O-functionalized N-heterocyclic carbenes in these two areas. On the catalysis front, we have employed the N/O-functionalized N-heterocyclic carbene based precatalysts for the C-C and C-N bond forming reactions like the Suzuki-Miyaura, Sonogashira and Hiyama cross-couplings, the base-free Michael addition, the alkene and alkyne hydroamination reactions and the ring-opening polymerization (ROP) of L-lactides that produce biodegradable polylactide polymers while on the biomedical application front, the anticancer and antimicrobial properties of these N/O-functionalized N-heterocyclic carbene complexes were evaluated. Towards this objective, the N-heterocyclic carbene chemistry of a variety of transition metals like Ag, Au, Ni and Pd has been investigated.

A DFT study on pyridine-derived N-heterocyclic carbenes

To appreciate the chemistry of N-heterocyclic carbenes (NHCs), eight carbenic tautomers of pyridine (azacyclohexadienylidenes) are studied at B3LYP/AUG-cc-pVTZ//B3LYP/6-31+G ∗ and B3LYP/6-311++G ∗∗//B3LYP/6-31+G ∗ levels of theory. Various thermodynamic parameters are calculated for these minima, along with a kinetic focus on carbene–pyridine tautomerization. Appropriate isodesmic reactions show stabilization energies of 2-azacyclohexa-3,5-dienylidene ( 1) and 4-azacyclohexa-2,5-dienylidene ( 6) as 119.4 and 104.1 kcal/mol, rather close to that of the synthesized 1,3-dimethylimidazol-2-ylidene (129.2 kcal/mol). Three different mechanisms are suggested for the tautomerizations including: [1,2]-H shift, [1,4]-H shift, and three sequential [1,2]-H shifts. The calculated energy barrier for [1,2]-H shift of 1 is 44.6 kcal/mol, while the first [1,2]-H shift for the proposed sequential mechanism of 6 requires 65.1 kcal/mol. Three preliminary minimum templates are introduced, which may possess the potential of synthetic consideration: 2,6-di(X)-3,5-dichloro-4-azacyclohexa-2,5-dienylidene for X=Mes, t-Bu, and Ad.

Synthesis of 2-aryl-5,5-dimethyl-5,6-dihydro-1,2,4-triazolo-[3,4-a]isoquinolinium tetrafluoroborates

Recently the chemistry of N-heterocyclic carbenes made rapid progress, as seen from a large number of freshly published reviews [1–4] and a monograph [5]. In the modern organic synthesis the N-heterocyclic carbenes are used either directly [1, 2] or in complexes with transition and nontransition metals [3]. N-heterocyclic carbenes known now and their derivatives known now contain metal complexes based on salts of 1,2,4-triazole substituted with chiral functional groups [6] and of 1,2,4-triazole fused with pyrrolidine [7], oxazole [8], oxazine [9], morpholine [10], indeno[2,1-b]morpholine [11], and also with indeno[1,2-b]pyrroline, naphtho[1,2-b]pyrroline, and naphtho[2,1-b]morpholine [2]. Yet the heterocyclic system of triazolo[3,4-a]isoquinoline was notused as a precursor of N-heterocyclic carbenes. The target of the present study was the preparation of 2-aryl-5,5-dimethyl-5,6-dihydro-1,2,4-triazolo[3,4-a]isoquinolinium salts and their 8,9-dimethoxy analogs as precursors N-heterocyclic carbenes. We chose for counterion the tetrafluoroborate anion since unlike chlorides and bromides 2-aryl-5,5-dimethyl-5,6-dihydro-1,2,4-triazolo[3,4-a]isoquinolinium tetrafluoroborates were nonhydroscopic, heat-resistant, and formed well-developed crystals. The developed method of preparation of the target compound consisted in the cyclization of amidrazones IIa–IIe, obtained via methylsulfanyl derivatives Ia and Ib, carried out in excess ethyl orthoformate in the presence of ammonium tetrafluoroborate. Amidrazones IIa–IIe were obtained by heating 1methylsulfanyl-3,4-dihydroisoquinolines Ia and Ib [12] with arylhydrazines in an argon atmosphere at ~170– 180°C for 0.5–1 h. Compounds IIa and IIb have been isolated as yellow oily substances darkening in air and stable at room temperature for several days; amidrazone IIe can be stored at room temperature for 6 months without visible signs of decomposition . The attempts to isolate intermediate amidrazones IIc and IId in pure state failed due to their fast oxidation in air, therefore we further used them direct after the synthesis in the cyclization into the target compounds IIIc and IIId. Amidrazones IIa–IIe may exist in two tautomer forms but 1H NMR spectra detect the form shown in the scheme as indicated by the downfield shift of the signal belonging to proton HC8 [13]. Salts IIIa–IIIe are colorless or light-yellow (IIIb and IIIe) crystalline substances soluble in DMF, DMSO,

N-Heterocyclic carbene containing complexes in catalysis

In this report, recent advancements in metal-catalysed homogeneous transformations involving N-heterocyclic carbene (NHC) are described. In spite of the efficiency of NHCs in organocatalysis1 and their advantageous properties in coordination chemistry,2 and because of the ever growing interest in NHC chemistry and space limitations, the focus of this report is transition-metal mediated catalysis.

含窒素複素環カルベン触媒を用いた不斉ベンゾイン縮合

The discovery of stable carbenes in the last decade of the 20th century has brought about extensive research on the chemistry of N-heterocyclic carbenes. Recently the powerful catalytic activity of N-heterocyclic carbenes has been demonstrated by new organocatalytic transformations. This review focuses on the asymmetric benzoin reactions catalyzed by N-heterocyclic carbenes. Chiral nucleophilic carbenes catalyze the benzoin condensation in good yields and exellent enantioselectivities.

Anionic tethered N-heterocyclic carbene chemistry

Since the discovery of a stable "bottleable" N-heterocyclic carbene (NHC), there has been a spectacular explosion of interest in this ligand class. This interest stems from a desire to understand the fundamentals of the structure and bonding of these systems, but also because of their numerous and emerging applications in small molecule activation, homogeneous catalysis and Lewis acid-catalysed reactions. In this Tutorial Review, we introduce the reader to NHCs, cover general synthetic methods to prepare anionic tethered NHCs and their metal complexes, and discuss emerging applications in reactivity and catalytic studies.

N‐Heterocyclic Carbenes: Reagents, Not Just Ligands!

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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.

Synthesis, structural and theoretical studies of an iron–gallium(i) heterocycle complex: Analogies with N-heterocyclic carbene chemistry

The first iron complex of an anionic gallium N-heterocyclic carbene analogue, [Fe(CO)4{Ga[N(Ar)C(H)]2}]− Ar = C6H3Pri2-2,6, has been prepared and shown to have an unusual polymeric structure in the solid state; theoretical studies have pointed toward minimal Fe → Ga back-bonding in this complex.

A new entry to N-heterocyclic carbene chemistry: synthesis and characterisation of a triscarbene complex of thallium(I).

The synthesis and characterisation of a thallium(I) triscarbene complex of the chelating, tripodal carbene ligand 1,3,5-[tris(3-tert-butylimidazol-2-ylideno)methyl]-2,4,6-trimethylbenzene is reported, in which the thallium ion is coordinated by three N-heterocyclic carbene donors in a distorted trigonal planar environment.