A mild and general route for preparing pyridines from nitriles and diynes is described. Ni/imidazolyidene complexes were used to mediate cyclization alkynes and both aryl and alkyl nitriles at ambient temperature. In addition, the efficacy of this protocol allows for the preparation of a fused seven-membered pyridone and for intermolecular cyclizations. When an asymmetrical diyne was employed, cyclization afforded a single pyridine regioisomer.

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The Ni/NHC-catalyzed rearrangement of cyclopropylen-ynes is described. Two different heterocycles, cyclopentane- or cycloheptene-based, were obtained. However, by employing ItBu as the NHC ligand, cyclopentane products were obtained selectively under mild conditions.

A combination of physical organic experiments and quantum chemical calculations were used to construct a detailed mechanistic model for the Ni(0)-N-heterocyclic carbene-catalyzed vinylcyclopropane-cyclopentene rearrangement that involves a muti-step oxidative addition/ haptotropic shift/reductive elimination pathway. No evidence for the intermediacy of radicals or zwitterions was found. The roles of substituents on the vinylcyclopropane substrate and variations in the ligands on Ni were evaluated. It is postulated that bulky carbene ligands facilitate formation of the active catalyst species.

A convenient method for preparing pyridines from air-stable, commercially available catalyst precursors is described. The addition of n-BuLi to Ni(acac)2 and an NHC salt (such as IPr.HCl or SIPr.HCl) rapidly generates an active Ni0/NHC catalyst for the cycloaddition of diynes and nitriles that affords pyridines without a decrease in observed yields. The in situ method also converts diynes and carbon dioxide to the corresponding pyrones.

Abstract2a,4‐Disubstituted 5‐benzoyl‐2‐chloro/2,2‐dichloro‐2a,3,4,5‐tetrahydro‐azeto [1,2‐a] [1,5]benzodiazepin‐1 (2H)‐ones (3a–h) were synthesized by cycloaddition reactions of 2,4‐disubstituted 1‐benzoyl‐2,3‐dihydr o‐1H‐1,5‐benzodiazepines (2a–h) and ketenes, generated from chloroacetyl chloride or dichloroacetyl chloride in the presence of triethylamine, in anhydrous benzene. In some cases, ring contraction of benzodiazepines has also been observed. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:636–640, 2001

Aufblasbare Ringe: Niedervalente Ni0‐Katalysatoren mit N‐heterocyclischen Carbenliganden isomerisieren aktivierte und nichtaktivierte Vinylcyclopropane. Unter milden Bedingungen werden die entsprechenden substituierten Cyclopentene in hohen Ausbeuten erhalten (siehe Schema, cod=Cycloocta‐1,5‐dien).

(S,S)-2- octanes were synthesized diastereospecifically from aldehydes and (S)-2-(hydroxydiphenylmethyl)pyrrolidine, which was obtained from L-proline, under acid-catalyzed conditions in anhydrous toluene. The stereospecificity of the condensation reaction is discussed.