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.