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

Mechanism and Origins of Regio- and Stereoselectivities of NHC-Catalyzed Dearomative Annulation of Benzoazoles and Cinnamaldehydes from DFT

A theoretical study on the mechanism, regioselectivity, and enantioselectivity of NHC-catalyzed dearomatizing annulation of benzoxazoles with enals has been conducted using density functional theory calculations. Our calculated results indicate that the favored mechanism occurs through eight reaction steps: initial binding of the NHC to enals, followed by formation of the Breslow intermediate via proton transfer. Subsequent oxidation generates the α,β-unsaturated acylazolium intermediate, which can undergo Michael addition with benzoxazoles. Sequential protonation/deprotonation/cyclization produces the six-membered cyclic intermediate that undergoes catalyst elimination, leading to the final product. DABCO·H+ was found to play important roles in proton transfer and cyclization. Without DABCO·H+, the energy barrier up to 44.2 kcal/mol for step 2 is too high to be accessible. With DABCO·H+, the corresponding value is lowered to 18.6 kcal/mol. The energy barrier for cyclization can be lowered by 7.4 kcal/mol by using DABCO·H+. The Michael addition step determines both the enantioselectivity and the regioselectivity. According to NCI analysis, the enantioselectivity is controlled by the strong interactions (such as C-H···O, C-H···N, and π···π) between the α,β-unsaturated acylazolium intermediate and benzoxazoles. We also discuss the solvent and substituent effects on the enantioselectivity and the role of the NHC. The mechanistic insights obtained in the present study would help improving current reaction systems or designing new synthetic routes.

Mechanisms and Origins of Regio- and Stereoselectivities in NHC-Catalyzed [3 + 3] Annulation of α-Bromoenals and 5-Aminoisoxazoles: A DFT Study

Density functional theory (DFT) calculations were conducted to explore the mechanisms and origins of regio- and stereoselectivities underlying the [3 + 3] annulation reaction between α-bromoenals and 5-aminoisoxazoles with N-heterocyclic carbene (NHC) as the catalyst. The reaction occurs in nine steps: (1) nucleophilic addition of NHC to α-bromoenal, (2) Breslow intermediate formation through 1,2-proton transfer, (3) debromination, (4) α,β-unsaturated acyl azolium intermediate formation via 1,3-proton transfer, (5) addition of α,β-unsaturated acyl azolium intermediate to 5-aminoisoxazole, (6) deprotonation, (7) protonation, (8) ring closure, and (9) elimination of NHC. For the fifth step, 1,2-addition suggested in the experiment was not supported by our results. Instead, we found that Michael addition is energetically the most feasible pathway and the stereo-controlling step that preferentially provides the S-configuration product. DFT-computed results and experimental findings agree well. Analysis of distortion/interaction reveals that lower distortion energy leads to stability of the transition state corresponding to the S-configuration product. Global reactivity index analysis indicates that the behavior of the NHC catalyst differs significantly before and after the Breslow intermediate debromination. Before debromination, the nucleophilicity of α-bromoenal is enhanced by addition to NHC. However, after debromination, the α,β-unsaturated acyl azole generates and acts as an electrophilic reagent.

Mechanism and Origin of Stereoselectivity for the NHC-Catalyzed Desymmetrization Reaction for the Synthesis of Axially Chiral Biaryl Aldehydes

Organocatalytic desymmetrization reaction is a powerful tool for constructing axial chirality, but the theoretical study on the origin of stereoselectivity still lags behind even now. In this work, the N-heterocyclic carbene (NHC)-catalyzed desymmetrization reaction of biaryl frameworks for the synthesis of axially chiral aldehydes has been selected and theoretically investigated by using density functional theory (DFT). The fundamental pathway involves several steps, i.e., desymmetrization, formation of Breslow oxidation, esterification, and NHC regeneration. The desymmetrization and formation of Breslow processes have been identified as stereoselectivity-determining and rate-determining steps. Further weak interaction analyses proved that the C-H···O hydrogen bond and C-H···π interactions are responsible for the stability of the key stereoselective desymmetrization transition states. This research contributes to understanding the nature of NHC-catalyzed desymmetrization reactions for the synthesis of axially chiral compounds.

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...

Fragmentation and rearrangement of Breslow intermediates: branches to both radical and ionic pathways

Breslow intermediates are the key species in N-heterocyclic carbene-catalyzed reactions to promote the C-C bond formation. As the fragmentation and rearrangement of Breslow intermediates terminate the catalytic cycle of N-heterocyclic carbene, two mechanisms under debate have been proposed in terms of the radical channel and the ionic route. Theoretical calculations demonstrate herein that ionic and radical characteristics can coexist, depending on the protonation state of the hydroxyl group in Breslow intermediates: radicals are merely generated in the enol system, while both ionic and radical species exist in the enolate system with a lower barrier. Complete pathways for thiamin analogue and N-allyl benzothiazole Breslow intermediates are exclusively constructed considering experimental conditions. The growing population of the enolate under higher pH values rationalizes the increased rate of the fragmentation of thiamin. The fragmentation products of thiamin, namely pyrimidine and ketone, are the thermodynamic products, while the tertiary alcohol is both the kinetic and thermodynamic product for N-allyl benzothiazole Breslow intermediate via a Claisen-like rearrangement. Other NHCs used to synthesize tertiary alcohols could form the enolate due to the base, followed by the production of stable radicals and recombination to form tertiary alcohols. It is concluded that specific protonation states and chemical structures of NHCs account for the distinct mechanisms.

Insights into Organoamine-Catalyzed Asymmetric Synthesis of Axially Chiral Allenoates Using Morita-Baylis-Hillman Carbonates and Trisubstituted Allenoates: Mechanism and Origin of Stereoselectivity

A computational study was performed to explore the possible mechanisms of β-isocinchonine-catalyzed asymmetric C(sp²)-H allylation of trisubstituted allenoates using Morita-Baylis-Hillman (MBH) carbonates for synthesis of axially chiral tetrasubstituted allenoates. The calculated results indicate that the most energetically favorable pathway includes (1) nucleophilic attack on MBH carbonate by β-isocinchonine, (2) BocO^- dissociation, (3) stereoselective formation of the C-C bond, and (4) regeneration of the catalyst. By tracking the orbital overlap/interaction changes, the half shoulder-to-head orbital overlap mode can be smoothly switched to a head-to-head orbital overlap mode for the key C-C σ bond formation, which is also identified as the stereoselectivity-determining process. Further distortion/interaction, noncovalent interaction (NCI), and atom-in-molecule (AIM) analyses demonstrate that C-H···O and C-H···π interactions should be key for controlling the axial and central chirality. This work would be useful for rational design of organocatalytic allylic alkylation reactions for synthesis of axially chiral compounds in the future.

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.

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 model for N-heterocyclic carbene-catalyzed decarboxylation reactions

A general mechanism for N-heterocyclic carbene-catalyzed decarboxylation reactions has been studied using DFT calculations.

Access to dihydropyrano[3,2-b]pyrrol-5-ones skeletons by N-heterocyclic carbene-catalyzed [3+3] annulations

A novel and efficient NHC-catalyzed [3+3] annulation of enals with pyrrol-4-ones was developed, thus providing the dihydropyrano[3,2-b]pyrrol-5-one core structures with broad scope and good to excellent enantioselectivities. Notably, this strategy could also expand to the synthesis of axially chiral compounds and polysubstituted indoles.

Prediction of NHC-catalyzed chemoselective functionalizations of carbonyl compounds: a general mechanistic map

Generally, N-heterocyclic carbene (NHC) complexed with carbonyl compounds would transform into several important active intermediates, i.e., enolates, Breslow intermediates, or acylazolium intermediates, which act as either a nucleophile (Nu) or an electrophile (E) to react with the other E/Nu partner. Hence, the key to predicting the origin of chemoselectivity is to compute the activity (i.e., electrophilic index ω for E and nucleophilic index N for Nu) and stability of the intermediates and products, which are suggested in a general mechanistic map of these reactions. To support this point, we selected and studied different cases of the NHC-catalyzed reactions of carbonyl compounds in the presence of a base and/or an oxidant, in which multiple possible pathways involving acylazolium, enolate, Breslow, and α,β-unsaturated acylazolium intermediates were proposed and a novel index ω + N of the E and Nu partners was employed to exactly predict the energy barrier of the chemoselective step in theory. This work provides a guide for determining the general principle behind organocatalytic reactions with various chemoselectivities, and suggests a general application of the reaction index in predicting the chemoselectivity of the nucleophilic and electrophilic reactions.

A novel index ω + N can be used to predict the chemoselectivity according to the general NHC-catalyzed reaction mechanism.

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.

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.

Highly enantioselective 1,6-addition of dienolates to coumarins and chromones through N-heterocyclic carbene catalysis

An efficient protocol employing NHC-organocatalysis in asymmetric γ,δ-difunctionalization of cyanocoumarins and nitrochromones via a 1,6-addition of dienolates is established.

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.

Transition Metal-Free Intermolecular C(sp2)-H Direct Amination of Furanones via a Redox Pathway

A direct C(sp²)-H amination of 2-furanones under metal-free conditions was realized. This unprecedented intermolecular C-H to C-N conversion provides rapid access to 4-amino-furanone derivatives and novel aza-heterocycle fused furanone skeletons. A redox mechanism based on a double-Michael-addition intermediate INT2 is proposed and detected by spectrometry.

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.

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.

Can the Radical Channel Contribute to the Catalytic Cycle of N-Heterocyclic Carbene in Benzoin Condensation?

NHC can catalyze benzoin condensation via the key Breslow intermediate. EPR spectroscopy recently confirmed the existence of the radical species, but its catalytic role is still unclear. Herein, we use density functional approaches to study the radical-associated pathway in comparison with the nonradical mechanism reported previously. Theoretical investigations show that the nonradical path (Δ G^⧧ = 18.7 kcal/mol) is more kinetically favorable than the radical route (Δ G^⧧ = 27.6 kcal/mol), which is initialized by the hydrogen abstraction from the Breslow intermediate by benzaldehyde, leading to a radical pair. The product formation is thus dominated by the nonradical pathway. In addition, the Breslow intermediate is less stable than its keto form, which blocks the benzoin condensation, and the radical species could play an important role in assisting the tautomerization and promoting the catalytic reaction.