Cobalt-Catalyzed Hydrogen-Atom Transfer Induces Bicyclizations that Tolerate Electron-Rich and Electron-Deficient Intermediate Alkenes

A novel Co-II-catalyzed polyene cyclization was developed that is uniquely effective when performed in hexafluoroisopropanol as the solvent. The process is presumably initiated by metal-catalyzed hydrogen-atom transfer (MHAT) to 1,1-disubstituted or monosubstituted alkenes, and the reaction is remarkable for its tolerance of internal alkenes bearing either electron-rich methyl or electron-deficient nitrile substituents. Electron-rich aromatic terminators are required in both cases. Terpenoid scaffolds with different substitution patterns are obtained with excellent diastereoselectivities, and the bioactive C20-oxidized abietane diterpenoid carnosaldehyde was made to showcase the utility of the nitrile-bearing products. Also provided are the results of several mechanistic experiments that suggest the process features an MHAT-induced radical bicyclization with late-stage oxidation to regenerate the aromatic terminator.