Mechanistic Investigation of [Co₂(CO)₈] Catalyzed Photoaminocarbonylation

Recent advances in cobalt-photocatalyzed carbonylation methods offer an efficient synthetic route to a range of amides and esters. Although preliminary studies provide important mechanistic insights, the kinetics of these reactions have not been studied in detail. Herein, we report an in-depth mechanistic investigation into the [Co₂(CO)₈]-catalyzed aminocarbonylation of aryl halides. Rarely used ⁵⁹Co NMR spectroscopy provided evidence for [Co(CO)₄]^- as an on-cycle catalytic intermediate. Orders in reagents, Hammett analysis, and Eyring analysis gave unique insight into both photon-limited and photon-unlimited regimes. Data strongly suggested that the turnover-limiting step of catalysis is the reaction of [Co(CO)₄]^- with the aryl bromide. Plausible mechanisms for this step were considered and differentiated by experimental data, including quantum yield measurements, radical probes, and competition experiments. It is concluded that this step most likely occurs through a light-promoted nucleophilic attack of the cobalt anion on the aryl bromide. In turn, this knowledge was used to expand catalysis to more challenging reagents such as aryl chlorides, low nucleophilicity amines, and acyclic vinyl halides. Enhanced catalyst efficiency (TON and TOF improved >10-fold) was also achieved for established substrates, strengthening the industrial use case of this technology.