Role of Bronsted acids in promoting Pd(OAc)2-catalyzed chlorination of phenol carbamates using N-chlorosuccinimide

Numerous studies have demonstrated that Brønsted acids (HAs), such as HOTf and HOTs, can promote Pd(OAc)₂-catalyzed functionalization of C–H bonds. However, the rationale for using these acids as a promoter is not yet completely obvious. The purpose of this work is to provide a detailed explanation for this observation with the aid of density functional theory calculations. This is accomplished by investigating the chlorination mechanism of phenol carbamates (DG∼C–H) with N-chlorosuccinimide (NCS) using HOTf as a promoter and Pd(OAc)₂ as a catalyst. Typically, in order for Pd(OAc)₂ to activate the C–H bond, it is believed that the trinuclear precatalyst Pd₃(OAc)₆ reacts with the substrate DG∼C–H to generate the chelated complex [Pd(OAc)₂(DG∼C–H)], from which C–H activation occurs via a concerted metalation–deprotonation mechanism. Because the substrate DG∼C–H binds relatively weak to palladium, the corresponding chelated complex lies much higher in energy than the reference structure Pd₃(OAc)₆, resulting in a very high energy barrier for C–H activation. The Brønsted acid HA is capable of undergoing ligand-exchange reactions with both Pd₃(OAc)₆ and [Pd(OAc)₂(DG∼C–H)] to form Pd₃(OAc)_6–x(A)_x and [Pd(OAc)(A)(DG∼C–H)], respectively. Our calculations demonstrate that while the formation of [Pd(OAc)(A)(DG∼C–H)] from [Pd(OAc)₂(DG∼C–H)] is highly exergonic, that of Pd₃(OAc)_6–x(A)_x from Pd₃(OAc)₆ is either nearly thermoneutral or endergonic. This feature significantly reduces the energy difference between the reference structure and the chelated complex, resulting in a significant decreased energy barrier for C–H activation. We also found that the acidity of the employed HA influences the energy difference between the trinuclear reference structure and the chelated complex [Pd(OAc)(A)(DG∼C–H)]; the more acidic the HA, the smaller the energy difference, and the lower the activation energy of C–H activation. In addition, our calculations show that the presence of HA not only lowers the overall energy barrier for C–H activation but also accelerates the chlorination step by protonating one of the oxygen atoms in NCS rather than the N atom.