Cyclomerization of Alkynes

Generation of Metallacyclic Structures from the Reactions of Vinyl Ethers with a TpMe2IrIII Compound

“…To close this section, some comments on the iridacyclic linkages of 2 − 4 are worthy of note. Metallacyclopentene and metallacyclopentadiene derivatives of the transition elements have been intensively studied as important intermediates for the cyclooligomerization of alkynes. , Tp′Ir compounds in which the metal is part of an unsaturated five-membered ring are known. ,, An interesting observation by O’Connor and co-workers is the conversion of an iridacyclopentene complex into an iridacyclopentadiene−alkene complex (Scheme ) . While this 2,4-metallacyclopentadiene group is a relatively common, well-known structural motif, metallacyclopenta-1,3-dienes of type 4 are much rarer and, to our knowledge, are unknown for iridium.…”

Cp*RuCl-Catalyzed [2 + 2 + 2] Cycloadditions of α,ω-Diynes with Electron-Deficient Carbon−Heteroatom Multiple Bonds Leading to Heterocycles

“…In this respect, transition-metal catalysis is a powerful tool to construct heterocyclic frameworks under neutral and mild conditions . In particular, the catalyzed cyclocotrimerizations of alkynes with carbon−heteroatom multiple bonds have received growing attention, since they can form multiple carbon−carbon and carbon−heteroatom bonds simultaneously without any reagents other than a catalyst . In other words, the catalytic [2 + 2 + 2] cycloaddition strategy is a highly convergent and atom-economical approach to heterocyclic compounds.…”

“…The transition-metal-mediated cyclocotrimerization of alkynes with carbon−heteroatom multiple bonds was first pioneered by Wakatsuki and Yamazaki in their work on the stoichiometric reactions of cobaltacyclopentadienes with nitriles, carbon disulfide, or methyl isothiocyanate, leading respectively to pyridines, a dithiopyrone, and a thiopyridone . Since then, various stoichiometric and catalytic cyclocotrimerizations have been developed, but the control of chemo- and regioselectivity has been a crucial problem for the catalytic methods. In this context, the cobalt-catalyzed partially intramolecular cycloadditions of α,ω-diynes or ω-cyanoalkynes leading to bicyclic pyridines have been developed by Vollhardt and others, , providing access to the syntheses of natural and artificial molecules .…”

Theoretical Study on the Reaction Mechanism for the Formation of 2-Methylpyridine Cobalt(I) Complex from Cobaltacyclopentadiene and Acetonitrile

“…Pyridine derivatives were thermally prepared one century ago by heating ketoximes at 300 °C . Also, they are prepared by condensation of alcohols, aldehydes, and ammonia in the presence of Lewis acids as catalysts and by the reaction of aldehydes with amines in the presence of heterogeneous catalysts such as cobalt catalysts. , Since preparation from alkynes and nitriles catalyzed by CpCo(PPh 3 ) 2 had been reported, pyridine derivatives were prepared in the presence of different transition metal catalysts such as Ti, Zr, Co, Rh, Pd, Ru, and Ta catalysts. ,− Of these transition metal catalysts, cobalt catalysts are extensively used as highly regioselective and active catalysts . For example, CoCl 2 ·6H 2 O, Cp 2 Co, CpCo(COD), cobalt vapor, and CpCoL 2 (Cp = cyclopentadienyl, COD = cyclooctadiene, L = CO, olefin, PR 3 ) are used for the preparation of pyridine derivatives from substituted alkynes and nitriles. − Using CpCo(PPh 3 ) 2 , pyridine derivatives were synthesized from substituted alkynes and nitriles as well as alkynes and isocyanates. , Vollhardt et al prepared annulated pyridines by cocyclotrimerization of diacetylenes with nitrile molecules catalyzed by CpCo(CO) 2 .…”