| Sumario: | The mechanisms of reductive functionalization of CO toformamide catalyzed by N-heterocyclic carbene (NHC) were comprehensively studied with DFT calculations. New activation mode with much lower energy barrier than those proposed before was discovered. In this reaction, NHC acts as neither a CO nor a silane activator, but as a precursor of the real catalyst, i.e., the in situ formed ionic liquid [NHCH][Carbamate]. In this loose contact ion pair, the negatively charged O atom of the carbamate anion becomes the new active site and is free to do nucleophilic attack. When amine is absent, CO will be converted into methanol. In this case, the NHC-CO adduct is the real catalytic species, the active site shifted from the carbene C atom to the negatively charged O atom. These new activation modes follow a pattern of "S2@Si- Acceptor", in which the Si-H bond is activated via concerted backside S2 nucleophilic attack by the negatively charged O atom, and the leaving hydride is directly accepted by a free CO molecule. The advantages of these new activation modes originate from the following points: (1) The ionic liquid [NHCH][Carbamate] and NHC-CO adduct are thermodynamically more stable than NHC. (2) The active site of the NHC catalyst is extended outside a lot. Consequently, the large steric effect between the NHC arms and the substrates in transition state can be avoided to some extent. (3) The O atom has good silicon affinity. In addition, a free CO molecule, whose carbon atom is more electrophilic than those of the CO moieties in NHC-COadduct and carbamate, acts as an efficient hydride acceptor.
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