N-heterocydes

Langer and coworkers constructed diverse O- and N-heterocydic scaffolds, such as y-alkylidene-a-hydroxybutenolides and pyrrolo[3,2-b]pyrrol-2,5-diones, exploiting the well-established cyclization strategy of bisnucleophiles with oxalic acid derivatives [163], while Stockman s research group reported in this context on a novel oxime formation/Michael addition providing the structural core of the alkaloid perhydrohistrionicotoxin [164]. 

A pyridyl bis-N-heterocydic carbene (NHC) ligand has been prepared by Steel and Teasdale based on nudeophilic aromatic substitution of dichloroisonicotinic amides with N-methylimidazole (Scheme 6.122) [250]. Microwave heating of the neat reagents at 140 °C for 10 min provided a 91% yield of the corresponding bis-... 

Sovadinova, I., Blaha, L., Janosek, J., Hilscherova, K., Giesy, J.P., Jones, P.D. and Holoubek, I. (2006) Cytotoxicity and aryl hydrocarbon receptor-mediated activity of N-heterocydic polycyclic aromatic hydrocarbons structure-activity relationships. Environmental Toxicology and Chemistry, 25, 1291-1297. 

Further research is needed to identify additional N-heterocyclics in soils and to determine whether the N-heterocydics are present in the soil in the forms in which they were identified or whether they originate from more complex structures. If the latter is the case, we need to isolate these complex N-containing molecules and attempt to identify them. 

Peris E (2006) Routes to N-Heterocydic Carbene Complexes. 21 83-116 Fitter S, Dinjus E, lonescu C, Maniut C, Makarczyk P, Patcas F (2008) Evaluation of Supercritical Carbon Dioxide as a Tuneable Reaction Medium for Homogeneous Catalysis. 23 109-147... 

Both, the Kolbe-Schmitt reaction and the dialkyl carbonate route may be beneficial, notably when taking into account the former syntheses used to produce these carboxylato derivatives. Prior to the study of Tommasi et al., the formation of imi-dazolium carboxylates via highly reactive N-heterocydic carbenes (NHC) had been reported, although this rather challenging synthesis could not be extended to an industrial scale. The synthesis of l,3-diisopropyl-4,5-dimethyl-imidazohum-... 

Using enol ethers of acetophenone 207 or similar compounds in a reaction with 206 and 2 one primarily obtains 2-phenyl-substituted N-heterocydes 209, which can undergo hydrogenolysis under the reaction conditions to afford the corresponding open-chain compounds 210 as mixtures of several diastereomers (Scheme 5.41). 

The additive-modified catalyst mixture developed for intramolecular olefin coupling is sufficiently active to catalyze the analogous intermolecular reaction. A variety of five-membered N-heterocydes can be catalytically alkylated at the C-2 position (Table 2) [8], Additional functional groups on the heterocyde[15] (Table 2, Entry 1) and on the olefin (Table 3) are well tolerated. Products corresponding to linear addition are usually obtained exclusively, even when the olefin is rapidly isomerized under the reaction conditions. 

Catalytic Functionalization of N-Heterocydes via their Rhodium-Carbene Complexes 187... 

Keywords Catalysis Cross-coupling reaction Electronic properties Metathesis N-heterocydic carbene Topology... 

Application of N-Heterocydic Carbenes in Transition Metal Catalysis... 

Chiral N-Heterocydic Carbenes as Stereodirecting Ligands in Asymmetric Catalysis... 

The introduction of chirafity into NHCs will therefore follow different strategies than those that have proved to be successful in phosphine-based asymmetric catalysis. For example, N-heterocydic carbene units will not create an edge-to-face arrangement of their aryl substituents, a structural feature common to many chiral diphosphines, such as the derivatives of Diop, Binap, Josiphos, Chiraphos and others. Results obtained in asymmetric catalysis, using chiral phosphine ligands, are therefore not directly transferable to the respective NHC-analogues. 

NHC Ligands Containing Chiral Elements within the N-Heterocyde... 

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