Through-space intramolecular palladium rearrangement in substituted aryl complexes: theoretical study of the aryl to alkylpalladium migration process.

DFT/B3LYP calculations have been carried out to study intramolecular 1,n palladium shifts (n = 3-5) between sp2 and sp3 carbon atoms in alkylarylpalladium systems. Such shifts, which also involve a concomitant exchange with a hydrogen atom of the alkylaryl ligand, are quite often a pivotal step of several organic transformations mediated by palladium complexes. We show that the intimate mechanism for the 1,3 shift corresponds to a Pd(IV) pathway, whereas a Pd(II) pathway is favored in the case of 1,5 migrations. In the case of 1,4 migrations, both mechanisms are competitive. The Pd(IV) pathway can involve either a true Pd(IV) intermediate (oxidative addition/reductive elimination 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 22.8 kcal mol-1 (for the 1,5 shift) and 31.9 kcal mol-1 (for the least favorable 1,4 shift studied here). All of these results are in line with our previous results for palladium shifts between two sp2 carbon atoms. In addition, the sp2 to sp3 shifts have been found to be rather exothermic owing to the possibility for the alkylaryl ligand in the product to achieve a eta3 coordination mode. This eta3 coordination mode results either from the shift itself (1,3 case) or from a subsequent rearrangement that comprises a chain-running mechanism within the alkyl chain bound to the metal (for n > 3).