Mechanistic Study of the Direct Intramolecular Allylic Amination Reaction Catalyzed by Palladium(II)

摘要

DFT calculations (PBE1PBE/6-31G(d,p), Def2-TZVPPD) were performed to study the intramolecular C-H amination of an unsaturated carbamate catalyzed by [Pd(LL)(OAc)(2)] (2), where LL is the bis(sulfoxide) ligand PhS(O)(CH2)(2)S(O)Ph. The coordination takes place by an associative path over a trigonal-bipyramidal transition state. The LL ligand undergoes a coordination shift from kappa S-2,S to K'S, leaving an open position for binding of the substrate (C=C). In the next step, the C-H activation, the transition state for the hydrogen abstraction from the substrate to form the sigma-allyl complex has an energy of 124.0 kJ mol(-1), which is the highest energy in the whole mechanism (TOF-determining transition state). The sigma-allyl converts easily in the pi-allyl, the acetic acid molecule leaving the coordination sphere. The remaining acetate receives the second hydrogen from the NH group, while the newly formed acetic acid molecule is replaced by the pendant arm of the LL ligand, and the cyclization takes place (nucleophilic attack). During these changes, the metal is reduced to Pd(0) in the form of the Pd(0) complex of the oxazolidinone product, the most stable species in the cycle (TOF-determining intermediate). Either the C-H activation or the Pd(0) oxidation may be the step determining the energy span of the reaction, depending on reaction conditions.