Conformational Isomerization as a Process to Determine Selectivity over Reaction Pathways: Effect of Alkene Rotation in Chain Walking via Cis Alkene Intermediates

In organic reactions, bond-forming and bond-cleaving
processes
are generally considered to be more important than other processes
such as conformational isomerization. We report herein an example
where a conformational isomerization process, propeller-like alkene
rotation, is considered to determine the selectivity over the reaction
pathways. The transition state with the highest energy barrier in
some alkylpalladium isomerization (chain walking) events was theoretically
indicated to correspond to alkene rotation, while transition states
for bond-cleaving β-hydride elimination and bond-forming migratory
insertion were not even observed. It was also suggested both theoretically
and experimentally that the palladium chain walking over internal
carbons in alkyl chains proceeds via cis alkene intermediates rather
than thermodynamically more stable trans alkene intermediates, due
to their relative difficulty to undergo alkene rotation.