Theoretical Model for N-Heterocyclic Carbene-Catalyzed Desymmetrizing [4 + 1] and [4 + 2] Annulations of an Enal and Aryldialdehyde with 1,3-Cyclopentenedione

Mechanism and origin of enantioselectivity for organocatalyzed asymmetric heteroannulation of alkynes in the construction of axially chiral C2-arylquinoline

Axially chiral C2-arylquinoline has been successfully constructed via asymmetric heteroannulation of alkynes catalyzed by chiral phosphoric acid with high yield and high enantioselectivity. Inspired by this intriguing work, theoretical calculations have been carried out, and the detailed reaction mechanism has been elaborated, in which the whole reaction can be divided into steps including hydrogen transfer, C-N bonding, annulation reaction and the final dehydration processes. The initial hydrogen-transfer reaction has two possible pathways, while the subsequent C-N bonding process has eight possible pathways. Then, after the annulation reaction and the final dehydration processes, the major product and byproduct were formed. QTAIM and IGMH analyses were used to illustrate the role of weak intermolecular interactions in the catalytic process, and the distortion/interaction and EDA analyses provided a deeper understanding of the origin of enantioselectivity. The calculated results are consistent with the experimental results. This work would provide valuable insights into asymmetric reactions catalyzed by chiral phosphoric acid.

NHC-Catalyzed Transformation Reactions of Imines: Electrophilic versus Nucleophilic Attack

It is generally accepted that N-heterocyclic carbene (NHC)-catalyzed imine transformations are initiated by a nucleophilic attack (NA) by NHC. However, due to significant nucleophilicity of the iminyl nitrogen atom in imines, the electrophilic attack (EA) by electrophiles onto imine would also be a possible mechanism of these kinds of reactions. Therefore, we use the quantum mechanical approach to disclose that both the NA and EA modes could be switchable for a wide range of NHC-catalyzed transformations of imines.

Diastereoselective access to [4,4]-carbospirocycles: governance of thermodynamic enolates with an organocatalyst in vinylogous cascade annulation

A vinylogy concept driven annulation strategy is developed to access [4,4]-carbospirocycles from alkylidene malononitriles and cyclopentene-1,3-diones. The reaction is catalyzed by an inexpensive organocatalyst and products with three stereocenters were obtained as a single diastereomer in high yields. The spiro-selectivity originates from the reaction of the thermodynamic enolate intermediate which is fundamentally intriguing.

Prediction on chemoselectivity for selected organocatalytic reactions by the DFT version of the Hückel-defined free valence index

The DFT version of the Hückel-defined free valence (HFV) index has been suggested and successfully used for predicting the origin of chemoselectivity in the selected organocatalytic reactions.

Mechanism and origin of selectivities for NHC-catalyzed synthesis of axially chiral benzothiophene/benzofuran-fused biaryls

By performing density functional theory (DFT) calculations, we investigated and identified the fundamental pathway for N-heterocyclic carbene (NHC)-catalyzed synthesis of axially chiral benzothiophene-fused biaryl using enal and 2-benzyl-benzothiophene-3-carbaldehyde, which includes...

Carbene-enabled ether activation through the formation of oxonium: a theoretical view

Here, we report a theoretical investigation of the reactivity and chemoselectivity of carbene-enabled ether activation.

The chiral pyridoxal-catalyzed biomimetic Mannich reaction: the mechanism and origin of stereoselectivity

A biomimetic organocatalyst with a pyridoxal-like structure is one of the most successful examples of catalyzing organic reactions under mild conditions in an asymmetric synthesis field.

A DFT study on the mechanism and regioselectivity of NHC-catalyzed double acylation of aromatic 1,2-diketones with α,β-unsaturated ketones

The possible mechanisms and the origin of regioselectivity of the N-heterocyclic carbene (NHC)-catalyzed double acylation reaction of aromatic 1,2-diketones with α,β-unsaturated ketones have been theoretically studied using density functional theory.

Organocatalytic insertion into C–B bonds by in situ generated carbene: mechanism, role of the catalyst, and origin of stereoselectivity

The calculated results showed that C–H⋯X (X = I, Br) hydrogen bond interactions should be the determining factors of stereoselectivity, and FMO overlap mode and ELF analyses along IRC results were performed to explore the nature of carbene insertion.

The regioselectivity of the sulfonylation of tetrazoles: a theoretical view

DFT calculations were performed to reveal the regioselectivity for the sulfonylation of tetrazoles.

N-Heterocyclic Carbene-Catalyzed Asymmetric C-O Bond Construction Between Benzoic Acid and o-Phthalaldehyde: Mechanism and Origin of Stereoselectivity

A computational study was contributed to explore the origin of stereoselectivity of NHC-mediated cyclization reaction between benzoic acid and o-phthalaldehyde for asymmetric construction of phthalidyl ester. The most energetically favorable pathway mainly includes the following steps: (1) nucleophilic attack on carbonyl carbon of o-phthalaldehyde by catalyst NHC, (2) formation of Breslow intermediate, (3) oxidation by DQ, (4) asymmetric formation of dual C-O bonds, and (5) dissociation of catalyst with the product. The C-O bond formation was testified as the stereoselectivity-determining step, the R-configurational pathway is more energetically favorable than the S-configurational one. The non-covalent interaction (NCI) and atom-in-molecule (AIM) analyses were performed to reveal that the O-H ⋅⋅⋅ O and C-H ⋅⋅⋅ O hydrogen-bond interactions are the key factors for controlling the stereoselectivity. The detailed mechanism and origin of stereoselectivity give useful insights for understanding organocatalytic reactions for asymmetric construction of C-O bond.

Tale of the Breslow intermediate, a central player in N-heterocyclic carbene organocatalysis: then and now

N-Heterocyclic carbenes (NHCs) belong to the popular family of organocatalysts used in a wide range of reactions, including that for the synthesis of complex natural products and biologically active compounds. In their organocatalytic manifestation, NHCs are known to impart umpolung reactivity to aldehydes and ketones, which are then exploited in the generation of homoenolate, acyl anion, and enolate equivalents suitable for a plethora of reactions such as annulation, benzoin, Stetter, Claisen rearrangement, cycloaddition, and C-C and C-H bond functionalization reactions and so on. A common thread that runs through these NHC catalyzed reactions is the proposed involvement of an enaminol, also known as the Breslow intermediate, formed by the nucleophilic addition of an NHC to a carbonyl group of a suitable electrophile. In the emerging years of NHC catalysis, enaminol remained elusive and was largely considered a putative intermediate owing to the difficulties encountered in its isolation and characterization. However, in the last decade, synergistic efforts utilizing an array of computational and experimental techniques have helped in gaining important insights into the formation and characterization of Breslow intermediates. Computational studies have suggested that a direct 1,2-proton transfer within the initial zwitterionic intermediate, generated by the action of an NHC on the carbonyl carbon, is energetically prohibitive and hence the participation of other species capable of promoting an assisted proton transfer is more likely. The proton transfer assisted by additives (such as acids, bases, other species, or even a solvent) was found to ease the kinetics of formation of Breslow intermediates. These important details on the formation, in situ detection, isolation, and characterization of the Breslow intermediate are scattered over a series of reports spanning well over a decade, and we intend to consolidate them in this review and provide a critical assessment of these developments. Given the central role of the Breslow intermediate in organocatalytic reactions, this treatise is expected to serve as a valuable source of knowledge on the same.