The Real Role of N-Heterocyclic Carbene in Reductive Functionalization of CO₂: An Alternative Understanding from Density Functional Theory Study

Abstract

The mechanisms of reductive functionalization of CO 2 to formamide 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 2 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 2 will be converted into methanol. In this case, the NHC-CO 2 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 "S N 2@Si-Acceptor", in which the Si-H bond is activated via concerted backside S N 2 nucleophilic attack by the negatively charged O atom, and the leaving hydride is directly accepted by a free CO 2 molecule. The advantages of these new activation modes originate from the following points: (1) the ionic liquid [NHCH] + [Carbamate] -and NHC-CO 2 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) O atom has good silicon-affinity. In addition, a free CO 2 molecule, whose carbon atom is more electrophilic than those of the CO 2 moieties in NHC-CO 2 adduct and carbamate, acts as an efficient hydride acceptor.

Carbon Dioxide Based N‐Formylation of Amines Catalyzed by Fluoride and Hydroxide Anions

“…With polymethylhydrosiloxane (PMHS), an N‐formylation product was not observed, and instead, cross‐linking of the polymer occurred. The decrease in the reaction rate observed with Ph 2 SiH 2 is consistent with the proposed S N 2‐type mechanism for the reaction of hydrosilanes with hydroxide anions and the calculated S N 2‐type mechanism for CO 2 reduction with silicon hydride, as well as the formation of an axial silicon hydride bond. The steric congestion induced by an additional phenyl ring hinders nucleophilic attack by the anion on the silicon center and prevents its activation.…”

“…A number of different CO 2 reduction mechanisms are possible by the silicon hydride. However, whereas direct CO 2 insertion into the polarized Si δ + −H δ − bond was previously suggested and is partly supported by the detection of the hypervalent silicon intermediate, a recent DFT study of related systems indicated that in the presence of CO 2 a concerted hydride transfer proceeding though an S N 2‐type mechanism with a structurally equivalent five‐coordinate silicon transition state followed by reinsertion of a formate anion was energetically favored …”

Coupling Reactions of Alkynyl Indoles and CO2 by Bicyclic Guanidine: Origin of Catalytic Activity?

“…Theoretical studies on organocatalyzed CO 2 functionalization reactions can be used to reveal mechanistic details at the molecular level . Furthermore, in certain cases, calculations can be used to reveal novel activation modes . With this in mind, we envisaged that density functional theory (DFT) calculations could be used to elucidate the mechanism of this useful guanidine‐catalyzed CO 2 transformation.…”

Design principles in frustrated Lewis pair catalysis for the functionalization of carbon dioxide and heterocycles

“…The first example of metal-free catalytic reduction of carbon dioxide was reported by Ying and coworkers in 2009 when they demonstrated that N-heterocyclic carbenes can catalyze the hydrosilylation of CO2 to methoxysilanes [52]. While the mechanism is debated [53][54], recently published computations suggest that the NHC-CO2 adduct serves as a Lewis base in order to activate silanes by hyper-coordination, thus favoring hydride transfer [55]. In that regard, we also reported that phosphazenes could act as efficient catalysts for CO2…”