Enantioselective N-Heterocyclic Carbene Catalysis via Acyl Azolium without Exogenous Oxidants

Ionic Hydrogen Bond-Assisted Catalytic Construction of Nitrogen Stereogenic Center via Formal Desymmetrization of Remote Diols

The control of noncarbon stereogenic centers is of profound importance owing to their enormous interest in bioactive compounds and chiral catalyst or ligand design for enantioselective synthesis. Despite various elegant approaches have been achieved for construction of S-, P-, Si- and B-stereocenters over the past decades, the catalyst-controlled strategies to govern the formation of N-stereogenic compounds have garnered less attention. Here, we disclose the first organocatalytic approach for efficient access to a wide range of nitrogen-stereogenic compounds through a desymmetrization approach. Intriguingly, the pro-chiral remote diols, which are previously not well addressed with enantiocontrol, are well differentiated by potent chiral carbene-bound acyl azolium intermediates. Preliminary studies shed insights on the critical importance of the ionic hydrogen bond (IHB) formed between the dimer aggregate of diols to afford the chiral N-oxide products that feature a tetrahedral nitrogen as the sole stereogenic element with good yields and excellent enantioselectivities. Notably, the chiral N-oxide products could offer an attractive strategy for chiral ligand design and discovery of potential antibacterial agrochemicals.

NHC-Catalyzed Reaction of Carboxylic Acids Using Allene Ketones as Substrates and Activating Reagents

We present a new reaction between carboxylic acids and allene ketones mediated by N-heterocyclic carbene (NHC) catalysts, which exhibit, in principle, nearly perfect atom economy. In this new approach, allene ketones act as both an activating reagent and a reactant. All atoms in the substrates end up in the product without the need for coupling reagents. The present study aims to encourage further explorations of NHC catalytic reactions with alternative activation strategies and better atom economy.

Recent Advances in the Synthesis of 3,4-Dihydropyran-2-Ones Organocatalyzed by N-Heterocyclic Carbenes

In recent years, N-heterocyclic carbenes (NHC) have gained recognition as versatile molecules capable of acting as organocatalysts in various reactions, particularly through the activation of aldehydes via Breslow-type adducts. This organocatalytic activation has enabled the production of numerous 3,4-dihydropyran-2-ones and related derivatives. In this review, we provide an overview of the production of 3,4-dihydropyran-2-ones and derivatives via organocatalytic processes involving NHCs over the past eight years. These processes involve the use of a diverse range of substrates, catalysts, and reaction conditions, which can be classified into [4+2]-and [3+3]-type cycloadditions, primarily aimed at synthesizing this skeleton due to its biological activity and multiple stereocenters. These processes are scaled up to the gram scale, and the resulting products are often directed towards epimerization and functionalization to produce more complex molecules with potential applications in the biological field. Finally, we provide a perspective and the future directions of this topic in organic synthesis.

Recent Advances in C–F Bond Activation of Acyl Fluorides Directed toward Catalytic Transformation by Transition Metals, N-Heterocyclic Carbenes, or Phosphines

Numerous studies on the activation of carbon–fluorine bonds have been reported in recent years. For example, acyl fluorides have been utilized as versatile reagents for acylation, arylation, and even fluorination. In this review, we focus on acyl fluorides as compounds with carbon–fluorine bonds, and highlight recent advances in strategies for the activation of their C–F bonds via transition-metal catalysis, N-heterocyclic carbene (NHCs) catalysis, organophosphine catalysis, and classical nucleophilic substitution reactions.

1 Introduction

2 Transition-Metal-Mediated C–F Bond Activation

2.1 Acylation (Carbonyl-Retentive) Coupling Reactions

2.2 Decarbonylative Reactions

2.3 C–F Bond Activation by Other Transition Metals

3 C–F Bond Activation by N-Heterocyclic Carbenes (NHCs)

3.1 NHC-Catalyzed Cycloaddition of Acyl Fluorides

3.2 NHC-Catalyzed Radical Functionalization of Acyl Fluorides

3.3 NHC-Catalyzed Nucleophilic Fluorination of (Hetero)aromatics

4 C–F Bond Activation by Phosphines

4.1 Phosphine-Catalyzed Direct Activation of the C–F Bond of Acyl Fluorides

4.2 Phosphine-Catalyzed Indirect Activation of the C–F Bond of Acyl Fluorides

5 C–F Bond Activation by Classical Nucleophilic Substitution

6 Miscellaneous Examples

7 Summary and Perspective

A Benzannulation Strategy for Rapid Access to Quinazoline-2,4-diones via Oxidative N-Heterocyclic Carbene Catalysis

N-Heterocyclic carbene-catalyzed formal [4+2] benzannulation of enals with suitably substituted pyrimidine-2,4-diones allowing the mild and facile synthesis of functionalized quinazoline-2,4-diones is presented. This oxidative transformation proceeds via the simultaneous generation of dienolates and α,β-unsaturated acylazoliums and follows a vinylogous Michael/aldol/β-lactonization/decarboxylation/oxidation sequence to afford quinazoline-2,4-diones, including axially chiral ones with suitable substitutions, in an operationally simple procedure. In addition, substituted coumarins as dienolate precursors afforded benzochromen-6-one derivatives.

Single-electron carbene catalysis in redox processes

Inspired by the role of N-heterocyclic carbenes (NHCs) in natural enzymatic processes, chemists have harnessed the umpolung (polarity reversal) reactivity of these reactive, Lewis basic species over the past few decades to construct key chemical bonds. While NHCs continue to play a role in two-electron transformations, their unique redox properties enable a variety of useful, stabilized radical species to be accessed via single-electron oxidation or reduction. As a result, their utility in synthesis has grown rapidly concurrent with the revival of radical chemistry, highlighted by their extensive use as reactive single-electron species in recent years.

Prediction on chemoselectivity for selected organocatalytic reactions by the DFT version of the Hückel-defined free valence index

The DFT version of the Hückel-defined free valence (HFV) index has been suggested and successfully used for predicting the origin of chemoselectivity in the selected organocatalytic reactions.

Enantioselective synthesis of tetra-substituted tetralines and tetrahydro-indolizines by NHC-catalyzed azolium-enolate cascade

NHC-catalyzed cascade reaction of enals with suitably substituted β-(hetero)aryl enones allowing the enantioselective synthesis of tetra-substituted tetralines and tetrahydro indolizines is presented. The catalytically generated chiral α,β-unsaturated acylazoliums from enals under oxidative conditions reacted in a Michael-Michael-lactonization sequence to form the tricyclic δ-lactone products bearing four contiguous stereocentres.

Enantioselective Synthesis of 5,6-Dihydroindolizines by N-Heterocyclic Carbene (NHC)-Catalyzed Core-Structure-Inspired Strategy of Azolium-Enolate Cascade

The core-structure motivated design has allowed the enantioselective synthesis of 5,6-dihydroindolizines via N-heterocyclic carbene (NHC) catalysis. The NHC-catalyzed reaction of α,β-unsaturated aldehydes with the suitably substituted pyrrole derivatives proceed via the initial generation of α,β-unsaturated acylazoliums from enals, and enolates from pyrroles and the reaction culminated in an efficient cascade process involving the Michael-aldol-lactonization-decarboxylation sequence to afford the products in reasonable yields and high selectivities. The method is further extended to the construction of spirocyclic 5,6-dihydroindolizines.