Alternate Geometries in the Cobalt-Catalyzed Hydroacylation of Dienes Facilitate a High Spin Mechanism: A Density Functional Theory Study

In contrast to the more common rhodium-catalyzed hydroacylation reaction, which is widely accepted to proceed via a low-spin singlet mechanism that passes through the familiar steps of oxidative addition → alkene insertion → reductive elimination, the cobalt-catalyzed hydroacylation reaction of dienes reported by Dong et al. ( J. Am. Chem. Soc., 2014) has been calculated to proceed via a high-spin triplet mechanism. The initial minimum energy pathway evaluated was the singlet, as two prior studies had also examined that pathway. The use of nudged elastic band methods enabled location of additional intermediates relative to the previous studies but also revealed that the isomeric product distribution was not accurately reproduced and that at least one intermediate appeared to prefer a different geometry. Subsequent examination of the triplet minimum energy pathway showed the intermediates are accompanied by geometries not typically associated with the singlet mechanism, which facilitates a very different pathway that involves oxidative cyclization and a direct reductive hydrogen atom transfer, thus avoiding the metal-hydride intermediates and reductive elimination steps that characterize the singlet pathway entirely.