Recent Advances on Computational Investigations ofN-Heterocyclic Carbene Catalyzed Cycloaddition/Annulation Reactions: Mechanism and Origin of Selectivities

Insights into N-Heterocyclic Carbene-Catalyzed Oxidative α-C(sp3)-H Activation of Aliphatic Aldehydes and Cascade [2 + 2] Cycloaddition with Ketimines

Predicting the chemoselectivity of [2 + 2] cyclizations is an important challenge in organic chemistry. Herein, we provided a valuable case for this issue. Density functional theory calculations were performed to systematically study the possible mechanisms and origin of selectivities for the N-heterocyclic carbene (NHC)-catalyzed oxidative α-C(sp³)-H activation of aliphatic aldehydes and the cascade [2 + 2] cycloaddition with ketimines. The [2 + 2] cycloaddition of azolium enolate intermediates to the C═N bond, rather than the C═O bond of ketimine, is revealed to be determined by chemo- and stereoselectivity. By comparing the energy gap between the frontier molecular orbitals (FMOs) of the two reacting parts involved in the [2 + 2] cycloaddition transition states, we propose a new strategy to determine the origin of the reaction chemoselectivity. Moreover, the local nucleophilic index can efficiently predict the active site of ketimines. Further analyses illustrate that NHC can increase the nucleophilicity of aldehydes and the acidity of the α-C(sp³)-H bond, and 3,3',5,5'-tetra- tert-butyl diphenoquinone (DQ) acts as an oxidant and promotes α-C(sp³)-H bond deprotonation. This work is useful not only for understanding the NHC-catalyzed oxidative [2 + 2] annulation but also for developing new applications of the FMO theory in organocatalytic cyclizations.

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.

Catalytic formal [4 + 1] isocyanide-based cycloaddition: an efficient strategy for the synthesis of 1H-cyclopenta[b]quinolin-1-one derivatives

An unprecedented catalytic isocyanide-based cycloaddition of alkyl and aryl isocyanides with (Z)-2-(2-hydroxy-2-alkylvinyl)quinoline-3-carbaldehydes was developed for the direct and efficient synthesis of 1H-cyclopenta[b]quinolin-1-one derivatives. On the basis of DFT calculations, a mechanism is proposed involving formal [4 + 1] cycloaddition of isocyanides, followed by imine-enamine tautomerization and air oxidation.

An unexpected cascade reaction of 3-hydroxyoxindoles with coumarin-3-carboxylates to construct 2,3-dihydrobenzofuran spirooxindoles

An unexpected Michael addition-inspired ring-opening/closure cascade reaction of 3-hydroxyoxindoles with coumarin-3-carboxylates was developed.

NHC-catalyzed green synthesis of functionalized chromones: DFT mechanistic insights and in vitro activities in cancer cells

A simple green protocol for the synthesis of 3-aminochromone derivatives using a NHC catalyzed intramolecular hydroacylation reaction was developed. Further functional 3-aminochromes were evaluated for their anticancer activity.

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.

An enantioselective assembly of naphthopyran via NHC-catalyzed [3 + 3] annulation of bromoenal with β-tetralone

An asymmetric assembly of naphthopyran was realized via the N-heterocyclic carbene (NHC)-catalyzed formal [3 + 3] annulation of bromoenal and β-tetralone. The key advantages of this protocol include ready availability of starting materials, mild reaction conditions, good yields.

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.

Prediction on the origin of chemoselectivity in Lewis base-mediated competition cyclizations between allenoates and chalcones: a computational study

The HOMO_TSs and p-orbital contributions of the center atoms were used to predict the origin of chemoselectivity in this work.

Understanding theZselectivity of the metal-free intermolecular aminoarylation of alkynes: a DFT study

The unusualZselectivity of the title reaction is revealed to be determined by the inherent requirements of dynamic preference of pathways that promise theZ-geometry product, rather than the isomerization of theE-productin situ.

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.

A theoretical study on NHC-catalysed enantioselective cycloaddition of ketenes and 3-aroylcoumarins: mechanism and enantioselectivity

NHC-catalysed enantioselective cycloaddition of ketenes to 3-aroylcoumarins to yield dihydrocoumarin-fused dihydropyranones has been investigated using DFT methods at the B3LYP/6-31G* and MPWB1K/6-311G** computational levels. Two plausible mechanisms have been studied: the "ketene-first" mechanism A and the "coumarin-first" mechanism B. An analysis of the activation Gibbs free energies involved in the two competitive pathways makes it possible to rule out the pathway associated with the "coumarin-first" mechanism B. The first step of the "ketene-first" mechanism A is the formation of zwitterionic intermediate IN1-Zvia a nucleophilic attack of NHC 1 on ketene 2. A [4 + 2] cycloaddition through the nucleophilic attack of enolate IN1-Z on the conjugated double bond of the benzoyl group of coumarin 3, viaTS3-SS-a2 or TS3-RR-a2, yields IN3. Finally, the extrusion of the catalyst through TS5 leads to the final products, either 4-SS or 4-RR. Enantioselectivity observed in the experimental results is determined in the transition states TS3-SS-a2/TS3-RR-a2. In this pathway, the intramolecular hydrogen-bonding between the hydroxyl group of the IN1-Z adduct and the carbonyl oxygen of the original ketene group directs the final stereochemistry throughout the entire process.

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.

E-Selective N-heterocyclic carbene-catalyzed reaction of aldehydes and butadienoates: effect of water and chloroform as the proton shuttle

An N-heterocyclic carbene-catalyzed reaction of aldehydes and butadienoates affording (E)-4-oxo-2-butenoates highly stereoselectively has been developed. Some key intermediates have been detected by an MS study.

Theoretical investigations of the Ir-catalyzed direct borylation of B(3,6)–H of o-carborane: the actual catalyst, mechanism, and origin of regioselectivity

The Ir^III species is proved to be the actual catalyst. The electron-delocalized structure and the inductive effects of the carbon centers is account for the regioselectivity.

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 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.

The conformational behavior of multivalent tris(imidazolium)cyclophanes in the hybrids with metal (pseudo)halides or polyoxometalates

Three 3D imidazole or benzimidazole-bearing cages as trivalent cationic templates react with metal (pseudo)halides or polyoxometalates to obtain a series of different organic–inorganic hybrids in which the cages exhibit breathing behavior of expansion and contraction.

A C1-symmetric N-heterocyclic carbene catalysed oxidative spiroannulation of isatin-derived enals: highly enantioselective synthesis of spirooxindole δ-lactones

A C₁-symmetric N-heterocyclic carbene (NHC)-catalysed activation of isatin-derived enals under oxidative conditions was achieved. The in situ generated α,β-unsaturated acyl azolium species was efficiently trapped by 1,3-dicarbonyl compounds via a Michael addition/spiroannualtion cascade, delivering a series of synthetically important spirooxindole δ-lactones with up to 96% enantioselectivity.

A NHC-catalysed spiroannulation of isatin-derived enals and 1,3-dicarbonyl compounds has been developed, enabling enantioselective synthesis of synthetically important spirooxindole δ-lactones.

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.