Rhodium‐Mediated [2 + 2 + 2] Cycloaddition

Theoretical Examination of the Plausible Reaction Process for Stereoselective Synthesis of Hexapole Helicene via a Palladium-Catalyzed [2 + 2 + 2] Cyclotrimerization of [5]Helicenyl Aryne

“…However, the reason for the preferential formation of the C 2 -symmetric kinetic product 2 among the 10 possible stereoisomers remains unclear. In addition, the mechanism for the incorporation of the third alkyne or arene into the metallacyclopentadiene in such transition-metal-catalyzed [2 + 2 + 2] cyclotrimerizations remains a disputed topic. − …”

Co(I)-Mediated Removal of Addends on the C60 Cage and Formation of the Monovalent Cobalt Complex CpCo(CO)(η2-C60)

“…Cyclopentadienylcobalt dicarbonyl CpCo­(CO) 2 has been used for the [2 + 2 + 2] cycloaddition, intra/intermolecular cyclization, and ring-opening reactions, which have been applied to the total synthesis of natural products as key steps . The typical reaction mechanism is explained as follows: CO ligand(s) of CpCo­(CO) 2 are replaced by other ligand(s) or reaction substrate(s), followed by oxidative coupling for formation of the cobaltacycle of Co­(III) and finally reductive elimination to afford target compounds and Co­(I) species.…”

Cβ–Cβ Bond Fission of Metallacyclopentadiene over a Low-Valent Ditantalum Scaffold

“…Metallacyclopentadiene complexes of transition metals have been extensively investigated as key intermediates in various catalytic reactions, such as cyclotrimerization of three alkynes and co-cyclotrimerization of two alkynes with nitriles and heterocumulenes, for the production of aromatic and heteroaromatic compounds. − In addition, application as synthetically useful reagents for some stoichiometric reactions is demonstrated for the reaction with sulfur sources, main group element halides, and halogen sources for constructing five-membered heterocycles, 1,4-dihalo-1,3-butadienes, diene-based polymers, and so on. − Metallacyclopentadienes of late transition metals can be generated by concerted oxidative cyclization of two alkynes at low-valent metal centers, while the formation of metallacyclopentadiene of early transition metals proceeds in a stepwise mechanism through the initial generation of η 2 -alkyne complexes followed by insertion of the second alkyne . Notably, retro-oxidative cyclization via C β –C β bond fission is observed for some early transition metal complexes upon introducing bulky substituents at the α-position of the metallacyclopentadienes, producing the isomerized metallacyclopentadiene with bulky substituents at the β-position via the η 2 -alkyne complex …”