Origin of Regio‐ and Stereoselectivity in the NHC‐catalyzed Reaction of Alkyl Pyridinium with Aliphatic Enal

Construction of 1,4-Dihydropyridines: The Evolution of C4 Source

The field of cascade cyclization for the construction of 1,4-dihydropyridines (1,4-DHPs) has been continuously expanding during the last decades because of their broad-spectrum biological and synthetic importance. To date, many methods have been developed, mainly including the Hantzsch reaction, Hantzsch-like reaction and newly developed cascade cyclization, in which various synthons have been successively developed as C4 sources of 1,4-DHPs. This review presents the cascade cyclization synthesis strategy for the construction of 1,4-DHPs according to various C4 sources from carbonyl compounds, alkenyl fragments, alcohols, aliphatic amines, glycines and other C4 sources.

Insights into Synergistic Effects of Counterion and Ligand on Diastereoselectivity Switch in Gold-Catalyzed Post-Ugi Ipso-Cyclization

The concept of diastereoselectivity switch in gold catalysis is investigated, which primarily depends on the effects of ligand and counterion. The origins of gold-catalyzed post-Ugi ipso-cyclization for the diastereoselective synthesis of spirocyclic pyrrol-2-one-dienone have been explored with density functional theory calculations. The reported mechanism emphasized the importance of the cooperation of ligand and counterion in diastereoselectivity switch, leading to the stereocontrolling transition states. Furthermore, the nonbonding interactions primarily between the catalyst and the substrate play a significant role in the cooperation of ligand and counterion. This work would be useful to further understand the reaction mechanism of gold-catalyzed cyclization and the effects of ligand and counterion.

The Role of the Fused Ring in Bicyclic Triazolium Organocatalysts: Kinetic, X-ray, and DFT Insights

Bicyclic triazolium scaffolds are widely employed in N-heterocyclic carbene (NHC) organocatalysis. While the incorporation of a fused ring was initially for synthetic utility in accessing chiral, modular triazolyl scaffolds, recent results highlight the potential for impact upon reaction outcome with the underpinning origins unclear. The common first step to all triazolium-catalyzed transformations is C(3)-H deprotonation to form the triazolylidene NHC. Herein, we report an analysis of the impact of size of the fused (5-, 6-, and 7-membered, n = 1, 2, and 3, respectively) ring on the C(3) proton transfer reactions of a series of bicyclic triazolium salts. Rate constants for the deuteroxide-catalyzed C(3)-H/D-exchange of triazolium salts, k_DO, were significantly influenced by the size of the adjacent fused ring, with the kinetic acidity trend, or protofugalities, following the order k_DO (n = 1) > k_DO (n = 2) ≈ k_DO (n = 3). Detailed analyses of X-ray diffraction (XRD) data for 20 triazolium salts (including 16 new structures) and of computational data for the corresponding triazolylidene NHCs provide insight on structural effects of alteration of fused ring size. In particular, changes in internal triazolyl NCN angle and positioning of the most proximal CH₂ with variation in fused ring size are proposed to influence the experimental protofugality order.

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...

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.

The regioselectivity of the sulfonylation of tetrazoles: a theoretical view

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

Ir-catalyzed enantioselective B-H alkenylation for asymmetric synthesis of chiral-at-cage o‑carboranes

The asymmetric synthesis of chiral-at-cage o-carboranes, whose chirality is associated with the substitution patterns on the polyhedron, is of great interest as the icosahedral carboranes have wide applications in medicinal and materials chemistry. Herein we report an intermolecular Ir-catalyzed enantioselective B-H alkenylation for efficient and facile synthesis of chiral-at-cage o-carboranes with new skeletons under mild reaction conditions. Generally very good to excellent yields with up to 99% ee can be achieved in this Ir-catalyzed B-H alkenylation. The enantiocontrol model is proposed based on Density Functional Theory calculations in which the use of chiral phosphoramidite ligand is essential for such asymmetric o-carborane B-H alkenylation.

Organocatalysis: A Tool of Choice for the Enantioselective Nucleophilic Dearomatization of Electron-Deficient Six-Membered Ring Azaarenium Salts

Nucleophilic dearomatization of azaarenium salts is a powerful strategy to access 3D scaffolds of interest from easily accessible planar aromatic azaarene compounds. Moreover, this approach yields complex dihydroazaarenes by allowing the functionalization of the scaffold simultaneously to the dearomatization step. On the other side, organocatalysis is nowadays recognized as one of the pillars of the asymmetric catalysis field of research and is well-known to afford a high level of enantioselectivity for a myriad of transformations thanks to well-organized transition states resulting from low-energy interactions (electrostatic and/or H-bonding interactions…). Consequently, in the last fifteen years, organocatalysis has met great success in nucleophilic dearomatization of azaarenium salts. This review summarizes the work achieved up to date in the field of organocatalyzed nucleophilic dearomatization of azaarenium salts (mainly pyridinium, quinolinium, quinolinium and acridinium salts). A classification by organocatalytic mode of activation will be disclosed by shedding light on their related advantages and drawbacks. The versatility of the dearomatization approach will also be demonstrated by discussing several chemical transformations of the resulting dihydroazaarenes towards the synthesis of structurally complex compounds.

Recent Advances in Theoretical Studies on Transition-Metal-Catalyzed Carbene Transformations

Metal carbene plays a vital role in modern organic synthesis. The neutral divalent carbon of metal carbene renders it an active intermediate throughout a range of reactions. In experiments, diverse metal carbene-related transformation reactions have been established, including transition-metal-catalyzed cross-coupling reactions using N-heterocyclic carbenes as ligands, metal carbene insertion into σ bonds, cyclopropanations, ylide formation, and so forth. The remarkable progress achieved in synthetic chemistry, in turn, has increased the demand for mechanistic studies of carbene chemistry. A thorough understanding of reaction mechanisms can extend the application scope of metal carbene compounds and inspire the rational design of new carbene transformation reactions.Density functional theory (DFT) calculations have been performed in our group to gain more mechanistic insights into metal carbene-related reactions. This account focuses on computational studies of transition-metal-catalyzed carbene transformation reactions with nucleophiles. The generation of metal carbene or metal-ligated free carbene and subsequent carbene transformation pathways is discussed. According to our mechanistic studies of carbene transformation with nucleophiles, three generalized reaction models are summarized, including the intramolecular migratory insertion of metal carbene, intermolecular nucleophilic addition toward metal carbene, and outer-sphere nucleophilic addition to the metal-ligated free carbene.In general, the intermolecular nucleophilic addition mechanism is commonly proposed since metal carbene has an electrophilic carbene carbon. From a mechanistic point of view, the intramolecular migratory insertion mechanism is also widely used because metal carbene insertion into σ bonds formally occurs through this mechanism. An outer-sphere nucleophilic addition mechanism is proposed for reactions that form a metal-ligated free carbene complex instead of the commonly proposed metal carbene. The metal-ligated free carbene complex contains a naked carbene carbon that is not coordinated with the metal center. In this case, a transition-metal catalyst is used only as a Lewis acid, and nucleophilic addition occurs directly at the free carbene carbon. Our computational results suggested that outer-sphere nucleophilic addition is a facile step because metal ligation could stabilize the transition state as well as the generated intermediate. The intramolecular migratory insertion mechanism also has a low energy barrier due to the lack of an entropy penalty. Carbene formation from carbene precursors is usually the rate-determining step, except in intermolecular nucleophilic addition, and the reactivity of nucleophiles has a significant influence on the overall reaction rate. We can also envision that the weak nucleophilicity of nucleophiles would suppress outer-sphere nucleophilic addition. These computational studies showcase the characteristics of three carbene transformation models, and we hope that it will spur the development of mechanistic studies of carbene chemistry.

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.

Mechanism and regio- and stereoselectivity in an NHC-catalyzed Mannich/lactamization domino reaction

Density functional theory (DFT) calculations (M06-2X) have been employed to disclose the mechanisms and regio- and stereo-selectivities of the N-heterocyclic carbene (NHC)-catalyzed reaction of 2-benzothiazolimines and α-chloroaldehydes. The preferred mechanism is initiated by the nucleophilic attack of NHC on α-chloroaldehyde (first step), followed by 1,2-proton transfer which was assisted by the Brønsted acid DABCO·H+ to generate the Breslow intermediate (second step). The cleavage of the C-Cl bond (third step) and deprotonation (fourth step) form the enolate intermediate. This further reacts with 2-benzothiazolimine which leads to the formation of a new C-C bond (fifth step). Subsequent cyclization takes place via the formation of a new C-N bond (sixth step). Catalyst regeneration completes the whole catalytic cycle and affords the final product (seventh step). The DFT results indicate that the fifth step determines the stereochemistry of the reaction and leads to benzothiazolopyrimidinone with the SS configuration, which agrees well with experimental observations. Intramolecular cyclization is found to be the regioselectivity-determining step, for which the [4+2] annulation pathway is more preferred than that via [2+2] annulation, which again agrees well with experimental observations. Based on the mechanism proposed, the origins of regio- and stereoselectivities have also been investigated by performing distortion/interaction, natural bond orbital (NBO) and non-covalent interaction (NCI) analyses. The mechanistic insights gained in this work should be helpful in the rational design of potential catalysts for analogous reactions.

Insights into the chiral sulfide/selenide-catalyzed electrophilic carbothiolation of alkynes: mechanism and origin of axial chirality

The FMO overlap mode, ELF, and AIM analyses along the formation process of thiiranium ion intermediate have been performed for the first time to explore the nature of the electronic structural changes and origin of stereoselectivity.

Predicting the origin of selectivity in NHC-catalyzed ring opening of formylcyclopropane: a theoretical investigation

Using density functional theory, we investigated the origin of selectivity in the N-heterocyclic carbene (NHC)-catalyzed transformation of formylcyclopropane with an alkylidene oxindole.

Theoretical study of the NHC-catalyzed C–S bond cleavage and reconstruction reaction: mechanism, stereoselectivity, and role of catalysts

The mechanism of the NHC-catalyzed C–S bond activation reaction has been theoretically studied and ELF analysis along with IRC results shows that the NHC promotes the cleavage of the C–S bond via an SN2 process.

Unveiling the Chemo- and Stereoselectivities of NHC-Catalyzed Reactions of an Aliphatic Ester with Aminochalcone

The possible mechanisms and origin of selectivities in N-heterocyclic carbene (NHC)-catalyzed reactions of an aliphatic ester with aminochalcone were studied using density functional theory. Herein, a general mechanistic map involving various types of possible intermediates was discovered, and the corresponding chemoselective pathways were systematically investigated. Based on the computational results, the most energetically favorable reaction pathway mainly involved in the following processes: formation of a homoenolate intermediate via α/β-H elimination, formal Michael addition of a homoenolate intermediate to aminochalcone, intramolecular aldol-type reaction, and ring closure to form the lactam product. Furthermore, the formal Michael addition process was shown to be the stereoselectivity-determining step, generating the RR-configured product preferentially. The chemoselectivity was successfully predicted by comparing the ω + N index of reacting ability for a nucleophile (N) and electrophile (E). This work would provide a general guideline for predicting chemoselectivity in NHC-catalyzed reactions.

A semipinacol rearrangement of vinylogous α-ketol cocatalyzed by a cinchona-based primary amine and N-Boc-phenylglycines: mechanisms, roles of catalysts and the origin of enantioselectivity

A semipinacol rearrangement of vinylogous α-ketol cocatalyzed by a cinchona-based primary amine and Brønsted acids can be achieved with good enantiocontrol, and theoretical investigations have been performed to uncover and understand it in detail.

Origin of stereoselectivity in an isothiourea catalyzed Michael addition reaction of aryl ester with vinyl disulfone

The origin of stereoselectivity in an isothiourea-catalyzed addition reaction of aryl ester with vinyl disulfone was explored for the first time.

A theoretical review for novel Lewis base amine/imine-catalyzed reactions

Recent advances in computational investigations of Lewis base amine/imine-catalyzed reactions have been systematically summarized and reviewed for the first time.