Mechanistic Study of Suzuki-Miyaura Cross-Coupling Reactions of Amides Mediated by [Pd(NHC)(allyl)Cl] Precatalysts

We report a combined experimental and computational investigation of the Suzuki-Miyaura cross-coupling of amides enabled by [Pd(NHC)(allyl)Cl] precatalysts. Most crucially, mechanistic details pertaining to the Pd-0/NHC catalytic cycle were elucidated by computational methods. Mechanistic insights shed light on the role of each ligand about the metal. Sterics play a key role in the initial activation of the catalyst. As a key insight, we have shown that water participates in the activation of the Pd-NHC catalytic system. Easier activation has led to effect room temperature cross-coupling of a broad range of amides through selective N-C bond scission under the mildest conditions reported to date. The use of sterically hindered [Pd(IPr*)(cin)Cl] reported herein for the first time in the amide cross-coupling indicates that increasing flexible steric bulk of the isopropyl wingtip groups of the NHC ligand provides a modular scaffold for promoting amide cross-coupling in high yields. The precatalytic pathway involving both NHC ligands as well as the catalytic cycle beginning from the Pd-0 species are discussed. The mechanistic details provide insight into the amide bond twist (distortion) that leads to N-C cross-coupling reactions and is required for the efficient N-C bond activation.