Diastereoselective Formation of 5-Vinylcyclopentenes from 1,6-Enynes: Cobalt-Mediated C−H Allylic Activation and Formal 5-Endo-Dig Cyclization

A novel intramolecular cyclization reaction mediated by dicobalt octacarbonyl (Co(2)(CO)(8)) is reported. Thermolysis in an argon atmosphere transforms the cobalt complex of 1-trimethylsilyl-6-hepten-1-ynes into 1-trimethylsilyl-5-vinylcyclopentenes in good yield and in a highly diastereoselective manner. This formal 5-endo-dig cyclization is proposed to proceed via an allylic C-H oxidative addition.