Functionalized cyclopentenes through a tandem NHC-catalyzed dynamic kinetic resolution and ambient temperature decarboxylation: mechanistic insight and synthetic application

N-Heterocyclic Carbene (NHC)-Catalyzed Transformations Involving Azolium Enolates

The recent advances in the N-heterocyclic carbene (NHC)-organocatalyzed generation of azolium enolate intermediates and their subsequent interception with electrophiles are highlighted. The NHC-bound azolium intermediates are generated by the addition of NHCs to suitably substituted aldehydes, acid derivatives or ketenes. A broad range of coupling partners can intercept the azolium enolates to form [2+n] cycloadducts (n=2,3,4) and various α-functionalized compounds. The enantioselective synthesis of the target compounds are achieved with the use of chiral NHCs. Herein, we summarized the development that occurred in this subclass of NHC catalysis.

Stereodivergent Nucleophilic Additions to Racemic β-Oxo Acid Derivatives: Fast Addition Outcompetes Stereoconvergence in the Archetypal Configurationally Unstable Electrophile

Additions of carbon nucleophiles to racemic α-stereogenic β-oxo acid derivatives that deliver enantiomerically enriched tertiary alcohols are valuable, but uncommon. This article describes stereodivergent Cu-catalyzed borylative cyclizations of racemic β-oxo acid derivatives bearing tethered pro-nucleophilic olefins to deliver highly functionalized cyclopentanols containing four contiguous stereogenic centers. The reported protocol is applicable to a range of β-oxo acid derivatives, and the diastereomeric products are readily isolable by typical chromatographic techniques. α-Stereogenic-β-keto esters are typically thought to have extreme or spontaneous configurational fragility, but mechanistic studies for this system reveal an unusual scenario wherein productive catalysis occurs on the same time scale as background substrate racemization and completely outcompetes on-cycle epimerization, even under the basic conditions of the reaction.

N-Heterocyclic Carbene (NHC)-Organocatalyzed Kinetic Resolutions, Dynamic Kinetic Resolutions, and Desymmetrizations

The last couple of decades have witnessed tremendous development within N-heterocyclic carbene (NHC) organocatalysis. NHCs have been used as powerful organic catalysts in asymmetric synthesis. Although great achievements have been made in asymmetric NHC catalysis, their applications in kinetic resolution (KR), dynamic kinetic resolution (DKR), and desymmetrization processes have been relatively less developed. Moreover, limited activation modes have been involved in these processes. This review provides an overview of the NHC-organocatalyzed KR, DKR, and desymmetrization reactions in the preparation of chiral functional molecules. The aim is to highlight both the importance and elegance of these methods in the construction of challenging chiral compounds and to provide our own perspective on future development in this direction.

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.

Enantioselective Synthesis of Functionalized β-Lactones by NHC-Catalyzed Aldol Lactonization of Ketoacids

N-Heterocyclic carbene (NHC)-catalyzed intramolecular aldol lactonization of readily available ketoacids leading to the enantioselective synthesis of cyclopentane-fused β-lactones is presented. The reaction proceeds via the generation of NHC-bound enolate intermediates formed from the ketoacids in the presence of the peptide coupling reagent HATU and NHC generated from the chiral triazolium salt. The functionalized β-lactones are formed under mild conditions in high yields and enantioselectivities.

Influence of the N-Substituents on the Nucleophilicity and Lewis Basicity of N-Heterocyclic Carbenes

The ability to modulate the nucleophilicity and Lewis basicity of N-heterocyclic carbenes is pivotal to their application as organocatalysts. Herein we examine the impact of the N-substituent on the nucleophilicity and Lewis basicity. Four N-substituents popular in NHC organocatalysis, namely, the N-2,6-(CH3O)2C6H3, N-Mes, N-4-CH3OC6H4, and N-tert-butyl groups, have been examined and found to strongly affect the nucleophilicity. Thus, the N-2,6-(CH3O)2C6H3 group provides the most nucleophilic imidazolylidene NHC reported and the N-tert-butyl group one of the least. This difference in nucleophilicity is reflected in the catalyst efficiency, as observed with a recently reported trienyl ester rearrangement.

Recent advances in employing homoenolates generated by N-heterocyclic carbene (NHC) catalysis in carbon-carbon bond-forming reactions

The use of NHCs for generating homoenolate species has gained widespread popularity in recent years. A number of highly stereoselective processes of NHC-homoenolates have emerged. Homoenolate reactions have also been employed as key steps in the total synthesis of a number of natural products. The use of compatible co-catalysts, improved NHC-catalyst design and the use of novel precursors for homoenolate generation are among the major developments in this area that were disclosed recently. This tutorial review organises and presents the advancements in this rapidly growing area of catalysis and in the process updates a previous account published in 2011 in this journal.

N-Heterocyclic carbene-catalyzed diastereoselective synthesis of β-lactone-fused cyclopentanes using homoenolate annulation reaction

NHC-catalyzed annulation of enals with 2-enoylpyridines or 2-enoylpyridine N-oxides leading to the diastereoselective synthesis of β-lactone-fused cyclopentanes is reported.