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

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)2S­(O)­Ph. The coordination takes place by an associative path over
a trigonal-bipyramidal transition state. The LL ligand undergoes a
coordination shift from κ2S,S to κ1S, 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 σ-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 σ-allyl converts easily in the π-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.