Topology of electrostatic potential and electron density reveals a covalent to non-covalent carbon-carbon bond continuum

The covalent and non-covalent nature of carbon-carbon (CC) interactions in a wide range of molecular systems can be characterized using various methods, including the analysis of molecular electrostatic potential (MESP), represented as V(r), and the molecular electron density (MED), represented as ρ(r). These techniques provide valuable insights into the bonding between carbon atoms in different molecular environments. By uncovering a fundamental exponential relationship between the distance of the CC bond and the highest eigenvalue (λv1) of V(r) at the bond critical point (BCP), this study establishes the continuum model for all types of CC interactions, including transition states. The continuum model is further delineated into three distinct regions, namely covalent, borderline cases, and non-covalent, based on the gradient, , with the bond distance of the CC interaction. For covalent interactions, this parameter exhibits a more negative value than -5.0 a.u. Å-1, while for non-covalent interactions, it is less negative than -1.0 a.u. Å-1. Borderline cases, which encompass transition state structures, fall within the range of -1.0 to -5.0 a.u. Å-1. Furthermore, this study expands upon Popelier's analysis of the Laplacian of the MED, denoted as ∇2ρ, to encompass the entire spectrum of covalent, non-covalent, and borderline cases of CC interactions. Therefore, the present study presents compelling evidence supporting the concept of a continuum model for CC bonds in chemistry. Additionally, this continuum model is further explored within the context of C-N, C-O, C-S, N-N, O-O, and S-S interactions, albeit with a limited dataset.

Theoretical Insights into Enantioselective [3 + 2] Cycloaddition between Cinnamaldehyde and Cyclic N-Sulfonyl Trifluoromethylated Ketimine Catalyzed by N-Heterocyclic Carbene

The density functional theory (DFT) method was used to investigate the mechanism and the origin of stereoselectivity of N-heterocyclic carbene (NHC)-catalyzed [3 + 2] cycloaddition between enals and cyclic imine N-sulfonyl trifluoromethyl ketimines at the M06-2X/SMD/6-311+G(d,p)//M06-2X/SMD/6-31G (d,p) level. The results show that the favorable reaction path consists of five steps: nucleophilic attack, proton transfer, the formation of the C-C bond, the tautomerism of the enol intermediate, the formation of the five-membered ring, and the regeneration of the catalyst. For the process of proton transfer, the base-assisted reaction can reduce the activation free energy and make the reaction easier to occur compared with the direct proton transfer process. The formation of the C-C bond is the crucial step of stereoselectivity, in which two chiral centers and four configurations of intermediates (RR/RS/SR/SS) were generated. The free energy barriers obtained and the noncovalent interaction analysis confirm that the dominant configuration is SS, becoming the final trans-type product observed in experiment. Furthermore, through the analyses of the conceptual DFT and natural atomic charges, it is revealed that NHC acts as a double catalyst, which can not only increase the nucleophilicity of reactants by Lewis base but also activate the C-H bond and promote the proton transfer process. The understanding of the mechanism obtained in this study should be helpful to the other organic catalytic reactions with high stereoselectivity.

Insights into highly selective ring expansion of oxaziridines under Lewis base catalysis: a DFT study

The possible mechanism and stereoselectivity of the NHC-catalyzed ring expansion reaction of oxaziridines have been theoretically studied for the first time.

Insights into the N-Heterocyclic Carbene (NHC)-Catalyzed Oxidative γ-C(sp3)-H Deprotonation of Alkylenals and Cascade [4 + 2] Cycloaddition with Alkenylisoxazoles

The N-heterocyclic carbene (NHC)-catalyzed oxidative C-H deprotonations have attracted increasing attention; however, the general mechanism regarding this kind of oxidative organocatalysis remains unclear. In this paper, the competing mechanisms and origin of the stereoselectivity of the NHC-catalyzed oxidative γ-C(sp³)-H deprotonation of alkylenals and cascade [4 + 2] cycloaddition with alkenylisoxazoles were systematically investigated for the first time using density functional theory (DFT). The computed results indicate that the oxidation of the Breslow intermediate by 3,3',5,5'-tetra- tert-butyl diphenoquinone (DQ) via a hydride transfer to oxygen (HTO) pathway is the most favorable among the four competing pathways. In addition, the analyses demonstrate that oxidant DQ plays a double role, i.e., strengthening the acidity of the hydrogen of the γ-carbon of alkylenal and forming π···π interactions with conjugated C═C bonds to promote the γ-C(sp³)-H deprotonation. The NHC catalyst acts as a Lewis base, and the hydrogen-bond network between the NHC and the substrate formed in the key Michael addition step is responsible for the origin of the stereoselectivity. Further DFT calculations reveal that the nonpolar solvent can stabilize the nonpolar R isomer but destabilize the polar S isomer for the stereoselectivity-determining transition states, thus improving the stereoselectivity.

Insights into the N-Heterocyclic Carbene (NHC)-Catalyzed Intramolecular Cyclization of Aldimines: General Mechanism and Role of Catalyst

One of the most challenging questions in the Lewis base organocatalyst field is how to predict the most electrophilic carbon for the complexation of N-heterocyclic carbene (NHC) and reactant. This study provides a valuable case for this issue. Multiple mechanisms (A, B, C, D, and E) for the intramolecular cyclization of aldimine catalyzed by NHC were investigated by using density functional theory (DFT). The computed results reveal that the NHC energetically prefers attacking the iminyl carbon (AIC mode, which is associated with mechanisms A and C) rather than attacking the olefin carbon (AOC mode, which is associated with mechanisms B and D) or attacking the carbonyl carbon (ACC mode, which is associated with mechanism E) of aldimine. The calculated results based on the different reaction models indicate that mechanism A (AIC mode), which is associated with the formation of the aza-Breslow intermediate, is the most favorable pathway. For mechanism A, there are five steps: (1) nucleophilic addition of NHC to the iminyl carbon of aldimine; (2) [1,2]-proton transfer to form an aza-Breslow intermediate; (3) intramolecular cyclization; (4) the other [1,2]-proton transfer; and (5) regeneration of NHC. The analyses of reactivity indexes have been applied to explain the chemoselectivity, and the general principles regarding the possible mechanisms would be useful for the rational design of NHC-catalyzed chemoselective reactions.

Computational study on the NHC-catalyzed synthesis of 2,3-disubstituted indoles: mechanism, key intermediate and the role of the catalyst

Mechanism, key intermediate and role of NHC for NHC-catalyzed umpolung of imines are clarified through our calculations.

Access to Enantiopure Triarylmethanes and 1,1-Diarylalkanes by NHC-Catalyzed Acylative Desymmetrization

We present herein an unprecedented, efficient and enantioselective synthesis of triarylmethanes and 1,1-diarylalkanes through N-heterocyclic carbene-catalyzed acylative desymmetrization of bisphenols. This method utilizes readily available substrates, reagents and a simple procedure to deliver the valuable products in excellent enantiopurity. DFT calculations reveal that the selectivity is governed by the C-C bond cleavage step of the tetrahedral intermediate leading to the ester product. A transition state model featuring a combination of intramolecular hydrogen bond and steric effect is developed to explain the enantioselectivity.

Understanding the regioselectivity of Michael addition reactions to asymmetric divinylic compounds

In the present paper, we describe the synthesis of novel monomers prepared by regioselective Michael addition to asymmetric divinylic compounds.

Insights into the Competing Mechanisms and Origin of Enantioselectivity for N-Heterocyclic Carbene-Catalyzed Reaction of Aldehyde with Enamide

Hydroacylation reactions and aza-benzoin reactions have attracted considerable attention from experimental chemists. Recently, Wang et al. reported an interesting reaction of N-heterocyclic carbene (NHC)-catalyzed addition of aldehyde to enamide, in which both hydroacylation and aza-benzoin reactions may be involved. Thus, understanding the competing relationship between them is of great interest. Now, density functional theory (DFT) investigation was performed to elucidate this issue. Our results reveal that enamide can tautomerize to its imine isomer with the assistance of HCO3-. The addition of NHC to aldehydes formed Breslow intermediate, which can go through cross-coupling with enamide via hydroacylation reaction or its imine isomer via aza-benzoin reaction. The aza-benzoin reaction requires relatively lower free energy barrier than the hydroacylation reaction. The more polar characteristic of C=N group in the imine isomers, and the more advantageous stereoelectronic effect in the carbon-carbon bond forming transition states in aza-benzoin pathway were identified to determine that the imine isomer can react with the Breslow intermediate more easily. Furthermore, the origin of enantioselectivities for the reaction was explored and reasonably explained by structural analyses on key transition states. The work should provide valuable insights for rational design of switchable NHC-catalyzed hydroacylation and aza-benzoin reactions with high stereoselectivity.

A DFT study on PBu3-catalyzed intramolecular cyclizations of N-allylic substituted α-amino nitriles for the formation of functionalized pyrrolidines: mechanisms, selectivities, and the role of catalysts

The detailed mechanisms and stereoselectivities of PBu₃-catalyzed intramolecular cyclizations for the formation of functionalized pyrrolidines have been investigated using a DFT method.

A DFT study of the mechanism of NHC catalysed annulation reactions involving α,β-unsaturated acyl azoliums and β-naphthol

The mechanism of NHC catalysed annulation reactions involving an α,β-unsaturated acyl azolium and β-naphthol has been studied using DFT methods at the MPWB1K/6-311G(d,p) level in toluene.

Highly Stereoselective Synthesis of 1,6-Ketoesters Mediated by Ionic Liquids: A Three-component Reaction Enabling Rapid Access to a New Class of Low Molecular Weight Gelators

In organic chemistry ionic liquids (ILs) have emerged as safe and recyclable reaction solvents. In the presence of a base ILs can be deprotonated to form catalytically active N-Heterocyclic Carbenes (NHCs). Here we have used ILs as precatalysts in the addition of α,β-unsaturated aldehydes to chalcones to form 1,6-ketoesters, incorporating an anti-diphenyl moiety in a highly stereoselective fashion. The reaction has a broad substrate scope and several functional groups and heteroaromatics can be integrated into the ketoester backbone in generally good yields with maintained stereoselectivity. The reaction protocol is robust and scalable. The starting materials are inexpensive and the products can be obtained after simple filtration, avoiding solvent-demanding chromatography. Furthermore, the IL can be recycled up to 5 times without any loss of reactivity. Moreover, the 1,6-ketoester end product is a potent gelator in several hydrocarbon based solvents. The method enables rapid access to and evaluation of a new class of low molecular weight gelators (LMWGs) from recyclable and inexpensive starting materials.

Insights into the Unexpected Chemoselectivity for the N-Heterocyclic Carbene-Catalyzed Annulation Reaction of Allenals with Chalcones

Lewis base N-heterocyclic carbene (NHC)-catalyzed annulation is the subject of extensive interest in synthetic chemistry, but the reaction mechanisms, especially the unexpected chemoselectivity of some of these reactions, are poorly understood. In this work, a systematic theoretical calculation has been performed on NHC-catalyzed annulation between allenals and chalcone. Multiple possible reaction pathways (A-E) leading to three different products have been characterized. The calculated results reveal that NHC is more likely to initiate the reaction by nucleophilic attack on the center carbon atom of the allene group but not the carbonyl carbon atom in allenals leading to the Breslow intermediate, which is remarkably different from the other NHC-catalyzed annulations of unsaturated aldehydes with chalcones. The computed energy profiles demonstrate that the most energetically favorable pathway (A) results in polysubstituted pyranyl aldehydes, which reasonably explains the observed chemoselectivity in the experiment. The observed chemoselectivity is demonstrated to be thermodynamically but not kinetically controlled, and the stability of the Breslow intermediate is the key for the possibility of homoenolate pathway D and enolate pathway E. This work can improve our understanding of the multiple competing pathways for NHC-catalyzed annulation reactions of unsaturated aldehydes with chalcones and provide valuable insights for predicting the chemoselectivity for this kind of reaction.

Theoretical study of stereoselectivity of the [1 + 2] cycloaddition reaction between (1S,3R,8S)-2,2-dichloro-3,7,7,10-tetramethyltricyclo[6,4,0,0(1.3)]dodec-9-ene and dibromocarbene using density functional theory (DFT) B3LYP/6-31G*(d)

In this work we used density functional theory (DFT) B3LYP/6-31G*(d) to study the stoichiometric reaction between the product (1S,3R,8S)-2,2-dichloro-3,7,7,10-tetramethyltricyclo[6,4,0,0(1.3)]dodec-9-ene (referred to here as P1) and dibromocarbene. We have shown that P1 behaves as a nucleophile, while dibromocarbene behaves as an electrophile; that the chemical potential of dibromocarbene is superior to that of P1 in absolute terms; and that P1 reacts with an equivalent quantity of dibromocarbene to produce two products: (1S,3R,8R,9S,11R)-10,10-dibromo-2,2-dichloro-3,7,7,11-tetramethyltetracyclo[6,5,0,0(1.3),0(9.11)] tridecane (referred to here as P2) and (1S,3R,8R,9R,11S)-10,10-dibromo-2,2-dichloro-3,7,7,11-tetramethyltetracyclo[6,5,0,0(1.3),0(9.11)] tridecane (referred to here as P3). P2 and P3 are formed at the α and β sides, respectively, of the C2 = C3 double bond of P1. This reaction is exothermic, stereoselective and chemospecific, and is controlled by charge transfer. Regioselectivity of the reaction was interpreted using the Lee-Yang-Parr functional.

Mechanism of N-heterocylic carbene-catalyzed annulation of allenals with chalcones to 3-pyrancarbaldehydes or cyclopentene

The reaction mechanisms of N-heterocylic carbene (NHC)-catalyzed annulation to 3-pyrancarbaldehydes or cyclopentene were theoretically analyzed.

Silver(i)–N-heterocyclic carbene catalyzed multicomponent reactions: a facile synthesis of multisubstituted pyridines

A room temperature four component reaction between aromatic aldehydes, CH₂(CN)₂and NH₄OAc with ketones mediated by Ag(i)–NHC pre-catalysts to produce multisubstituted pyridines in a short reaction time in eco-friendly ethanol was described.

Ionic liquids as precatalysts in the highly stereoselective conjugate addition of α,β-unsaturated aldehydes to chalcones

Imidazolium-based ionic liquids (ILs) serve both as recyclable reaction media and as precatalysts for the N-heterocyclic carbene-catalyzed conjugate addition of α,β-unsaturated aldehydes to chalcones. The reaction produces a broad scope of 1,6-ketoesters incorporating an anti-diphenyl moiety in high yields and with high stereoselectivity. In recycling experiments, the IL can be reused up to five times with retained reactivity and selectivity. Moreover, the 1,6-ketoesters form self-assembled organogels in aliphatic hydrocarbons. The reaction protocol is robust, easily operated, scalable and highly functionalized compounds can be obtained from inexpensive and readily accessible starting materials.

On the mechanism of N-heterocyclic carbene-catalyzed reactions involving acyl azoliums

Catalytic reactions promoted by N-heterocyclic carbenes (NHCs) have exploded in popularity since 2004 when several reports described new fundamental reactions that extended beyond the long-studied generation of acyl anion equivalents. These new NHC-catalyzed reactions allow chemists to generate unique reactive species from otherwise inert starting materials, all under simple, mild reaction conditions and with exceptional selectivities. In analogy to transition metal catalysis, the use of NHCs has introduced a new set of elementary steps that operate via discrete reactive species, including acyl anion, homoenolate, and enolate equivalents, usually generated by oxidation state reorganization ("redox neutral" reactions). Nearly all NHC-catalyzed reactions offer operationally simple reactions, proceed at room temperature without the need for stringent exclusion of air, and do not generate reaction byproducts. Variation of the catalyst or reaction conditions can profoundly influence reaction outcomes, and researchers can tune the desired selectivities through careful choice of NHC precursor and base. The catalytically generated homoenolate and enolate equivalents are nucleophilic species. In contrast, the catalytically generated acyl azolium and α,β-unsaturated acyl azoliums are electrophilic cationic species with unique and unprecedented chemistry. For example, when generated catalytically, these species transformed an α-functionalized aldehyde to an ester under redox neutral conditions without coupling reagents or waste. In addition to providing new approaches to catalytic esterifications, acyl azoliums offer unique reactivities that chemists can exploit for selective reactions. This Account focuses on the discovery and mechanistic investigation of the catalytic generation of acyl azoliums and α,β-unsaturated acyl azoliums. These chemical species are fascinating, and their catalytic generation is an important development. Studies of their unusual chemistry, however, date back to the intense investigation of thiamine-dependent enzymatic processes in the 1960s. Acyl azoliums are remarkably reactive in acylation chemistry and are unusually chemoselective. These two properties have led to a new wave of reactions such as redox esterification reaction (1) and the catalytic kinetic resolution of challenging substrates (i.e., 3). Our group and others have also developed methods to generate and exploit α,β-unsaturated acyl azoliums, which have facilitated new C-C bond-forming annulations, including a catalytic, enantioselective variant of the Claisen rearrangement (2). From essentially one class of catalysts, the N-mesityl derived triazolium salts, researchers can easily prepare highly enantioenriched dihydropyranones and dihydropyridinones. Although this field is now one of the most explored areas of enantioselective C-C bond forming reactions, many mechanistic details remained unsolved and in dispute. In this Account, we address the mechanistic inquiries about the characterization of the unsaturated acyl triazolium species and its kinetic profile under catalytically relevant conditions. We also provide explanations for the requirement and effect of the N-mesityl group in NHC catalysis based on detailed experimental data within given specific reactions or conditions. We hope that our studies provide a roadmap for catalyst design/selection and new reaction discovery based on a fundamental understanding of the mechanistic course of NHC reactions.

A new C–C bond formation model based on the quantum chemical topology of electron density

Pseudodiradical structures and GEDT involved in the C–C single bond formation in non-polar, polar and ionic organic reactions.