Palladium-catalyzed aerobic homocoupling of aliphatic olefins to dienes: evidence for rate-limiting concerted metalation-deprotonation.

Palladium(ii)-catalyzed dehydrogenative coupling of aliphatic olefins would enable an efficient route to (conjugated) dienes, but remains scarcely investigated. Here, 2-hydroxypyridine (2-OH-pyridine) was found to be an effective ligand for Pd(ii) in the activation of vinylic C(sp)-H bonds. While reoxidation of Pd(0) is challenging in many catalytic oxidations, one can avoid in this reaction that the reoxidation becomes rate-limiting, even under ambient O pressure, by working in coordinating solvents. kinetic studies the elementary steps governing this reaction were elucidated, resulting in enhanced performance (turnover frequency) of the Pd(ii)/2-OH-pyridine system. The diene product is formed a consecutive activation of two olefins on the same Pd atom, followed by a β-hydride elimination. The first olefin activation, the C-H activation, determines the overall reaction rate under these conditions. The catalytic complex was studied by ESI-MS and X-ray absorption spectroscopy, revealing that the coordination sphere of the working palladium complex contains two 2-OH-pyridine ligands.