Sequential covalent bonding activation and general base catalysis: insight into N-heterocyclic carbene catalyzed formylation of N–H bonds using carbon dioxide and silane

Abstract

The detailed catalytic mechanisms of N-heterocyclic carbenes (NHCs) in the formylation of N–H bonds using carbon dioxide and silane were investigated using density functional theory (DFT) calculations. Among all the examined reaction pathways, we found that the most favorable pathway involves collaboration between the covalent bonding activation and general base catalysis. The overall reaction can be divided into four stages, including silane activation through a covalent bonding mechanism, CO₂ insertion into the Si–H bond of silane to yield a key intermediate formoxysilane (FOS), the NHC-catalyzed coupling of amine and FOS through a general base mechanism, and C–O bond breaking through general base catalysis to obtain the final amide product. The carbamic acid anion (Me₂NCOO⁻) is an inevitable intermediate from the side reactions, and its formation is almost barrier free. NHC can act as a base to abstract a proton from the nucleophiles (such as amines or alcohol), and facilitate C–N bond or C–O bond formation or cleavage, and such a general base mechanism is remarkably favorable over the covalent binding mechanism for C–N bond (or C–O) bond formation (or cleavage). The calculated thermodynamic properties are in good agreement with the available experimental findings.