Intramolecular 1,n Palladium Migrations in Polycyclic Aromatic Hydrocarbons. Palladium(II) versus Palladium(IV) Mechanisms: A Theoretical Study

Abstract

DFT/B3LYP calculations have been carried out to study intramolecular 1,n aryl-to-aryl palladium shifts (n = 3−6) in polyaryl systems. Such rearrangements, which are associated with a concomitant shift of a hydrogen atom, have been found experimentally to be a pivotal step of several organic transformations mediated by palladium complexes. From the calculations of the various transition states and intermediates involved in the process, the intimate mechanism for the 1,3 shift is shown to correspond to a Pd(IV) pathway, whereas a Pd(II) pathway is favored in the case of 1,5 and 1,6 migrations. In the case of 1,4 migrations, both mechanisms become competitive. The Pd(IV) pathway can involve either a true Pd(IV) intermediate (oxidative addition mechanism) or a Pd(IV) transition state (oxidative hydrogen migration mechanism). The energy barrier is very high for the 1,3 palladium shift, making this process very unlikely, in contrast to the other ones, which have enthalpy barriers ranging between 17.8 and 27.5 k...