Computational Study on γ-C–H Functionalization of α,β-Unsaturated Ester Catalyzed by N-Heterocyclic Carbene: Mechanisms, Origin of Stereoselectivity, and Role of Catalyst

Mechanism and Origin of Stereoselectivity of N-Heterocyclic Carbene (NHC)-Catalyzed Transformation Reaction of Benzaldehyde with o-QDM as Key Intermediate: A DFT Study

N-heterocyclic carbene (NHC)-bound ortho-quinodimethane, served as a nucleophile, has occupied an important position for constructing various all-carbon or heterocyclic compounds and attracted increasing attention for the functionalization of benzylic carbon of aromatic aldehydes, whereas the mechanistic studies on the generation and transformations of dienolate intermediate are rare. In the present study, the mechanism of activation/transformation of aldehyde catalyzed by NHC was theoretically studied using the density functional theory (DFT) method. Based on the calculations, the nucleophilic addition process is the stereoselectivity-determining step with RS-configured product being generated preferentially. Furthermore, non-covalent index (NCI) and atoms-in-molecules (AIM) analyses have been performed to disclose the origin of stereoselectivity, by which the larger number and stronger weak interactions are the key for stabilizing the low-energy transition state and thus leading to the stereoselectivity inducing.

DFT investigation of the DDQ-catalytic mechanism for constructing C-O bonds

In this study, we investigated the photo-catalytic mechanisms for the construction of C-O bonds from arenes (benzene, 2',6'-dimethyl-[1,1'-biphenyl]-2-carboxylic acid, or 2,4-dichloro-1-fluorobenzene), catalyzed by 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). All the structures for the Gibbs free surfaces were calculated at the M06-2X-D3/ma-def2-SVP level in the SMD solvent model. Also, TDDFT calculations of DDQ were performed at the PBE1PBE-D3/ma-def2-SVP level in the SMD solvent model. The computational results indicated that DDQ, serving as a photo-catalyst, would be excited under visible light of 450 nm, aligning well with experimental observations as reflected in the UV-vis spectrum. Gibbs free energy surface analyses of the three reactions suggested that the path involving 3DDQ* activating the reactant (-COOH, H2O, or CH3OH) is favorable. Additionally, the role of O2 was investigated, revealing that it could facilitate the recycling of DDQ by lowering the energy barrier for the conversion of the DDQH˙ radical (not DDQH2) into DDQ. The use of ρhole and ρele can reveal the photo-catalytic reaction and charge transfer processes, while localized orbital locator isosurfaces and electron spin density isosurface graphs were employed to analyze structures and elucidate the single electron distribution. These computational results offer valuable insights into the studied interactions and related processes, shedding light on the mechanisms governing C-O bond formation from arenes catalyzed by DDQ.

Elucidating the mechanism and origin of stereoselectivity in the activation/transformation of an acetic ester catalyzed by an N-heterocyclic carbene

The activation of an ester by N-heterocyclic carbene (NHC) organocatalysis is an efficient and important approach for generating an NHC-bound enolate intermediate, an important active intermediate in the transformation of carbonyl compounds. Herein, we perform a theoretical study on the NHC-catalyzed activation and transformation reaction of an acetic ester in which the NHC-bound enolate intermediate is a key intermediate. Multiple activation and transformation pathways are proposed and analyzed to identify an energetically favorable pathway. The use of different substrates for the reaction is considered. When a chalcone substrate is used, [4+2] cycloaddition between the enolate intermediate and the chalcone is identified to be both the rate- and stereoselectivity-determining step for the reaction, with the R-configured product being generated as the major isomer. Noncovalent interaction (NCI) and atoms-in-molecules (AIM) analyses are performed to identify the origin of the stereoselectivity of the reaction, and a local reactivity analysis is conducted to explore substrate and catalyst effects on the reaction.

Distal meta-C-H functionalization of α-substituted cinnamates

Development of a novel strategy for the palladium-catalyzed selective meta-C-H activation of α-substituted cinnamates and their heterocyclic analogues with various alkenes using nitrile as a directing group (DG) has been described. Importantly, we introduced naphthoquinone, benzoquinones, maleimides and sulfolene as coupling partners in the meta-C-H activation reaction for the first time. Notably, allylation, acetoxylation and cyanation were also achieved through distal meta-C-H functionalization. This novel protocol also includes the coupling of various olefin-tethered bioactive molecules with high selectivity.

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.

Terpyridine-based Pd(ii)/Ni(ii) organometallic framework nano-sheets supported on graphene oxide-investigating the fabrication, tuning of catalytic properties and synergetic effects

Tailoring the structures of catalysts and the arrangement of organic bimetallic catalysts are essential in both fundamental research and applications. However, they still impose enormous challenges such as size and active species distribution, ordered uniformity, and controllable composition, which are critical in determining their specific activities and efficiency. Herein, a novel terpyridine-based hetero-bimetallic Ni/Pd nanosheet supported on graphene oxide (denoted as GO@Tpy-Ni/Pd) was fabricated, which exhibited higher catalytic activity, substrate applicability and recyclability for the Suzuki coupling reaction under mild conditions. The catalytic mechanism was heterogeneous catalysis at the interface and the synergetic effect between Pd and Ni resulted in a little Ni(0)/Pd(0) cluster including Pd(ii)/Ni(ii) as a whole being formed through electron transfer on the catalytic surface. This phenomenon could be interpreted as the nanoscale clusters of Ni/Pd being the real active centre stabilized by the ligand and GO and the synergetic effect. The absorption and desorption of different substrates and products on Ni/Pd clusters, as calculated by DFT, was proved to be another key factor.

The synergistic effect between Ni and Pd atom was the crucial factor for enhancing catalytic activity.

Computational study on NHC catalyzed [4+2] annulation between γ-chloroenals and pyrazolinones: mechanism and stereoselectivity

Mechanisms and origins of stereoselectivity of NHC-catalyzed reaction between γ-chloroenal and α-arylidene pyrazolinone.

Chiral phosphoric acid catalyzed asymmetric arylation of indolesvianucleophilic aromatic substitution: mechanisms and origin of enantioselectivity

Asymmetric arylation of indolesvianucleophilic aromatic substitution can be effectively achieved using chiral phosphoric acid as catalyst, where the mechanisms and origin of enantioselectivity were explored theoretically.

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.

Mechanism and stereoselectivity in NHC-catalyzed β-functionalization of saturated carboxylic ester

To understand the mechanism and origin of the stereoselectivity of the [3 + 2] annulation reaction between a carboxylic ester and an isatin generating spirooxindole lactone catalyzed by N-heterocyclic carbene (NHC), density functional theory (DFT) calculations have been carried out. DFT results indicate that the catalytic cycle begins with the coupling of the catalyst with benzotriazole ester, followed by α-deprotonation to produce the enolate intermediate. The subsequent 1,4-proton transfer affords the homoenolate intermediate. The next crucial step is the stereoselective C-C bond formation. Then proton transfer takes place leading to the formation of the lactone intermediate. Finally, the elimination of the catalyst furnishes the final product. The presence of 1-hydroxybenzotriazole (HOBt) dramatically accelerates the proton transfer step. More importantly, HOBt has a non-negligible impact on stereoselective C-C bond formation, and the SR-configured product is the major stereoisomer of the annulation product, which is in good agreement with the experimental observations. The differential π⋯π stacking, C-H⋯π, lone pair (LP)⋯π and repulsion interactions are found to be responsible for the stereoselectivity. The obtained mechanistic insights should provide valuable information for understanding the important roles of the NHC catalyst and HOBt additive and be helpful for designing better NHC catalysts for this kind of reaction.

Prediction on the origin of selectivities of NHC-catalyzed asymmetric dearomatization (CADA) reactions

The mechanism and origin of selectivities of NHC-catalyzed dearomatization reaction have been theoretically studied.

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.

Mechanistic studies on the N-heterocyclic carbene-catalyzed reaction of isatin-derived enals with hydrazones

The detailed mechanism and origin of stereoselectivity of the NHC-catalyzed annulation reaction were investigated.

Mechanistic insights and origin of chemoselectivity for S–O bond cleavage in dinitrobenzenesulfonic carbamates

A computational study on the NHC-catalyzed chemoselective S–O bond cleavage in dinitrobenzenesulfonic carbamates.

Why electrostatically enhanced thiourea is better than Schreiner's thiourea in both catalytic activity and regioselectivity?

Electrostatically enhanced thiourea is more active and efficient than Schreiner's thiourea in the ring-opening aminolysis of styrene oxide with aniline, and the underlying reasons were explored by DFT calculations.

Insights into NHC-catalyzed oxidative α-C(sp3)–H activation of aliphatic aldehydes and cascade [2 + 3] cycloaddition with azomethine imines

The NHC catalyst is identified to promote [2 + 3] cycloaddition by avoiding the poor FMO overlap mode in theory.

Theoretical Study of the Histidine-catalyzed Asymmetric Aldol Reaction of Acetone and Benzaldehyde

The mechanism of histidine-catalyzed asymmetrical aldol reaction of acetone with benzaldehyde was studied by using B3LYP method of density functional theory at the levels of 6-31G(d,p) and cc-pvdz basis sets. The calculation results showed that the reaction mechanism included four steps: (I) nucleophilic attack of histidine on acetone to form alcohol intermediate Inter-A through the transition state TS1 (considered a rate control step because the activation energy (49.95 kcal/mol) was relatively high); (II) dehydration of the alcohol intermediate to form the cis- or trans-enamine through the transition states TS3 and TS4 with the energy barriers of 36.12 and 38.15 kcal/mol; (III) electrophilic addition of cis-enamine or trans-enamine with benzaldehyde to form imine Inter-C or Inter-E through the transition states TS8, TS9, TS10, and TS11 (energy barriers 18.43, 22.34, 13.24, and 13.24 kcal/mol, respectively); (IV) after combination of the imine intermediate with water through the transition states TS12, TS13, TS14, and TS15 (energy barriers 22.79, 34.6, 28.2, 25.12 kcal/mol, respectively), removal of the histidine catalys to obtain the final S or R aldol product. Through analyzing the potential energy profile of reaction, we found that the histidine-catalyzed reaction of acetone with benzaldehyde was more energetically favorable to obtain the R-product (ee value >99%). Solvent effects computed with a polarizable continuum model (PCM) indicated that the DMSO and water can reduce the reaction energy barrier.

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.

Theoretical study on the mechanism and enantioselectivity of NHC-catalyzed intramolecular SN2′ nucleophilic substitution: what are the roles of NHC and DBU?

A possible catalytic mechanism was proposed and studied in very detail by using the DFT method for a recently reported enantioselective intramolecular S_N2′ substitution of aldehydes with trisubstituted allylic bromides.

Insights into Ag(i)-catalyzed addition reactions of amino alcohols to electron-deficient olefins: competing mechanisms, role of catalyst, and origin of chemoselectivity

The competing mechanisms of Ag(i)-catalyzed chemoselective addition reactions of amino alcohols and electron-deficient olefins leading to the O-adduct or N-adduct products were systematically studied with density functional theory methods. Calculations indicate that the AgHMDS/dppe versus AgOAc/dppe catalytic systems can play different roles and thereby generate two different products. The AgHMDS/dppe system works as a Brønsted base to deprotonate the amino alcohol OH to form the Ag–O bond, which leads to formation of the O-adduct. In contrast, the AgOAc/dppe system mainly acts as a Lewis acid to coordinate with O and N atoms of the amino alcohol, but it cannot act as the Brønsted base to further activate the OH group because of its weaker basicity. Therefore, the AgOAc/dppe catalyzed reaction has a mechanism that is similar to the non-catalyzed reaction, and generates the same N-adduct. The obtained insights will be important for rational design of the various kinds of cooperatively catalyzed chemoselective addition reactions, including the use of the less nucleophilic hydroxyl groups of unprotected amino alcohols.

The origin of the chemoselectivities of Ag(i)-catalyzed addition reactions of amino alcohols to olefin has been predicted for the first time.

Competing mechanisms and origins of chemo- and stereo-selectivities of NHC-catalyzed reactions of enals with 2-aminoacrylates

A DFT study on competing mechanisms of NHC-catalyzed reactions of enals with 2-aminoacrylates has been performed for the first time.

Insights into the isothiourea-catalyzed asymmetric [4 + 2] annulation of phenylacetic acid with alkylidene pyrazolone

A computational study on the isothiourea-catalyzed asymmetric [4 + 2] annulation of phenylacetic acid with alkylidene pyrazolone was performed using the DFT method.

Insights into N-heterocyclic carbene-catalyzed [3 + 4] annulation reactions of 2-bromoenals with N-Ts hydrazones

A DFT study toward N-heterocyclic carbene-catalyzed [3 + 4] annulation reactions of 2-bromoenals has been performed for the first time.

Insights into the NHC-catalyzed cascade Michael/aldol/lactamization reaction: mechanism and origin of stereoselectivity

A DFT study of the NHC-catalyzed cascade Michael/aldol/lactamization reaction has been performed for the first time.

Chiral bisoxazoline catalyzed decarboxylative aldol reactions between β-carbonyl acids and trifluoroacetaldehyde hemiacetals as well as trifluoroacetaldehyde: the mechanism, the origin of enantioselectivity and the role of a catalyst

The role of a catalyst in the decarboxylative aldol reactions between β-carbonyl acids and trifluoroacetaldehyde hemiacetals as well as trifluoroacetaldehyde catalyzed by chiral bisoxazoline were unveiled theoretically.

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.