Cobalt-Catalyzed Carbonylations
Résumé
In the past century, cobalt has been used as a catalyst in the carbonylation of methanol and hydrocarbonylation of alkenes under harsh conditions. The hydroformylation of alkenes is still currently utilized in the catalytic transformation of heavy alkenes, whereas lighter alkenes are more selectively transformed by rhodium catalysts. However, interesting and potentially powerful modifications of cobalt(I) or cobalt(II) coordination spheres are still under investigation. Cobalt catalysis has also found a significant role in fine chemistry. The Pauson – Khand [2+2+1] annulation reaction starting from an alkyne, an alkene, and carbon monoxide allows the formation of cyclopentenones via the formation of three C-C bonds. This reaction remains a central transformation in total synthesis of natural products and more so since the first reports of the enantioselective versions. Additionally, heterocycles can be carbonylated in the presence of a cobalt catalyst. In particular, the formation of chiral and enantioenriched lactones from epoxides is catalyzed by [Co(CO) 4 ] − and an enantiopure Lewis acid. Finally, beyond noble metals, cobalt is also efficient in the oxidative carbonylation of C-H bonds. Generally, the carbonylative cyclization is controlled by the presence of nitrogen-based directing groups. Cobalt catalysis is rooted in classical carbonylation transformations and now finds novel applications in fine organic synthesis.