Transition-State Complexation in Palladium-Promoted [3,3] Sigmatropic Shifts

Herein we use quantum chemical methods (B3LYP/LANL2DZ) to chronicle the path of the Pd(II)-promoted Cope rearrangement. It was discovered that although the mechanism can proceed through a stepwise associative pathway (forming a cyclohexyl cation-like intermediate if appropriately substituted with an electron-donating group), a simple change in substituent position or nature (e.g., an aptly placed electron-withdrawing group) makes the concerted pathway energetically favorable. The origins of rate acceleration in these systems upon complexation are discussed in terms of “transition-state complexation”. Since the Pd(II)-promoted Cope rearrangement, much like its metal-free counterpart, changes mechanism on the basis of the nature and position of appended substituents, both should share the distinctive classification “chameleonic.”