Enamine/Carbene Cascade Catalysis in the Diastereo- and Enantioselective Synthesis of Functionalized Cyclopentanones

Catalytic stereoselective synthesis of cyclopentanones from donor-acceptor cyclopropanes and in situ-generated ketenes

A dual InBr3-EtAlCl2 Lewis acidic system was found to be optimal for promoting the diastereoselective (3 + 2)-cycloaddition of donor-acceptor cyclopropanes with in situ-generated ketenes to form cyclopentanones. The desired products were formed in good to excellent yields (70-93% for 16 examples) and with good to excellent diastereoselectivity and enantiospecificity.

A Modular Dual-Catalytic Aryl-Chlorination of Alkenes

Alkyl chlorides are a class of versatile building blocks widely used to generate C(sp3)-rich scaffolds through transformation such as nucleophilic substitution, radical addition reactions and metal-catalyzed cross-coupling processes. Despite their utility in the synthesis of high-value functional molecules, distinct methods for the preparation of alkyl chlorides are underrepresented. Here, we report a visible-light-mediated dual catalysis strategy for the modular synthesis of highly functionalized and structurally diverse arylated chloroalkanes via the coupling of diaryliodonium salts, alkenes and potassium chloride. A distinctive aspect of this transformation is a ligand-design-driven approach for the development of a copper(II)-based atom-transfer catalyst that enables the aryl-chlorination of electron-poor alkenes, complementing its iron(III)-based counterpart that accommodates non-activated aliphatic alkenes and styrene derivatives. The complementarity of the two dual catalytic systems allows the efficient aryl-chlorination of alkenes bearing different stereo-electronic properties and a broad range of functional groups, maximizing the structural diversity of the 1-aryl, 2-chloroalkane products.

The importance of four-membered NHCs in stabilizing Breslow intermediates on benzoin condensation pathway

N-heterocyclic carbenes (NHCs) have been established to be effective organocatalysts for facilitating the benzoin condensation and many other reactions. These reactions involve the formation of a Breslow intermediate (BI), which exhibits umpolung chemistry. To facilitate organocatalysis, several new cyclic carbenes are being introduced, four-membered NHCs are of special interest. Whether these NHCs can exhibit catalytic influence or not, can be evaluated by exploring the potential energy surface (PES) of the benzoin condensation reaction. Quantum chemical analysis has been carried out to compare the PES of these four-membered NHCs with that of standard five-membered NHCs to explore their catalytic ability. The barrier for the first step of the reaction for the formation of BI is comparable in all the cases. But the barrier for the second step of the reaction leading to the benzoin formation from BI is estimated to be very high for the four membered NHCs. These results indicate that the probability of identifying and isolating the BI is very high in comparison to the completion of benzoin condensation reaction in the case of the four-membered NHCs.

Asymmetric Organocatalysis: A Survival Guide to Medicinal Chemists

Majority of drugs act by interacting with chiral counterparts, e.g., proteins, and we are, unfortunately, well-aware of how chirality can negatively impact the outcome of a therapeutic regime. The number of chiral, non-racemic drugs on the market is increasing, and it is becoming ever more important to prepare these compounds in a safe, economic, and environmentally sustainable fashion. Asymmetric organocatalysis has a long history, but it began its renaissance era only during the first years of the millennium. Since then, this field has reached an extraordinary level, as confirmed by the awarding of the 2021 Chemistry Nobel Prize. In the present review, we wish to highlight the application of organocatalysis in the synthesis of enantio-enriched molecules that may be of interest to the pharmaceutical industry and the medicinal chemistry community. We aim to discuss the different activation modes observed for organocatalysts, examining, for each of them, the generally accepted mechanisms and the most important and developed reactions, that may be useful to medicinal chemists. For each of these types of organocatalytic activations, select examples from academic and industrial applications will be disclosed during the synthesis of drugs and natural products.

Remote N–H activation of indole aldehydes: an investigation of the mechanism, origin of selectivities, and role of the catalyst

Density functional theory investigation on the N-heterocyclic carbene-catalysed synthesis of oxazinoindole derivatives via N–H activation of indole aldehydes.

Recent developments on NHC-driven dual catalytic approaches

This article takes an in depth look at the early relay combination of NHC and other catalysts, and the latest progress in dual catalysis, analyzing the breakthroughs and limitations.

N-Heterocyclic Carbene-Catalyzed Asymmetric C-O Bond Construction Between Benzoic Acid and o-Phthalaldehyde: Mechanism and Origin of Stereoselectivity

A computational study was contributed to explore the origin of stereoselectivity of NHC-mediated cyclization reaction between benzoic acid and o-phthalaldehyde for asymmetric construction of phthalidyl ester. The most energetically favorable pathway mainly includes the following steps: (1) nucleophilic attack on carbonyl carbon of o-phthalaldehyde by catalyst NHC, (2) formation of Breslow intermediate, (3) oxidation by DQ, (4) asymmetric formation of dual C-O bonds, and (5) dissociation of catalyst with the product. The C-O bond formation was testified as the stereoselectivity-determining step, the R-configurational pathway is more energetically favorable than the S-configurational one. The non-covalent interaction (NCI) and atom-in-molecule (AIM) analyses were performed to reveal that the O-H ⋅⋅⋅ O and C-H ⋅⋅⋅ O hydrogen-bond interactions are the key factors for controlling the stereoselectivity. The detailed mechanism and origin of stereoselectivity give useful insights for understanding organocatalytic reactions for asymmetric construction of C-O bond.

Mechanism and regio- and stereoselectivity in an NHC-catalyzed Mannich/lactamization domino reaction

Density functional theory (DFT) calculations (M06-2X) have been employed to disclose the mechanisms and regio- and stereo-selectivities of the N-heterocyclic carbene (NHC)-catalyzed reaction of 2-benzothiazolimines and α-chloroaldehydes. The preferred mechanism is initiated by the nucleophilic attack of NHC on α-chloroaldehyde (first step), followed by 1,2-proton transfer which was assisted by the Brønsted acid DABCO·H+ to generate the Breslow intermediate (second step). The cleavage of the C-Cl bond (third step) and deprotonation (fourth step) form the enolate intermediate. This further reacts with 2-benzothiazolimine which leads to the formation of a new C-C bond (fifth step). Subsequent cyclization takes place via the formation of a new C-N bond (sixth step). Catalyst regeneration completes the whole catalytic cycle and affords the final product (seventh step). The DFT results indicate that the fifth step determines the stereochemistry of the reaction and leads to benzothiazolopyrimidinone with the SS configuration, which agrees well with experimental observations. Intramolecular cyclization is found to be the regioselectivity-determining step, for which the [4+2] annulation pathway is more preferred than that via [2+2] annulation, which again agrees well with experimental observations. Based on the mechanism proposed, the origins of regio- and stereoselectivities have also been investigated by performing distortion/interaction, natural bond orbital (NBO) and non-covalent interaction (NCI) analyses. The mechanistic insights gained in this work should be helpful in the rational design of potential catalysts for analogous reactions.

A radical [3 + 2]-cycloaddition reaction for the synthesis of difluorocyclopentanones

Described herein is a novel radical-mediated [3 + 2]-cycloaddition reaction of alkenes to afford difluorocyclopentanones.

Organocatalytic Name Reactions Enabled by NHCs

Giving reactions the names of their discoverers is an extraordinary tradition of organic chemistry. Nowadays, this phenomenon is much rarer, although already named historical reactions are still often developed. This is also true in the case of a broad branch of N-heterocyclic carbenes catalysis. NHCs allow many unique synthetic paths, including commonly known name reactions. This article aims to gather this extensive knowledge and compare historical reactions with current developed processes. Furthermore, this review is a great opportunity to highlight some of the unique applications of these procedures in the total synthesis of biologically active compounds. Hence, this concise article may also be a source of knowledge for scientists just starting their adventure with N-heterocyclic carbene chemistry.

Merging singlet-oxygen induced furan oxidations with organocatalysis: synthesis of enantiopure cyclopentanones and hydrindanes

A general, flexible and highly effective method for the regio-, diastereo- & enantioselective one-pot synthesis of important carbocycles, namely, enantiopure cyclopentanones and their hydrindane congeners is presented.

A dienamine-mediated deconjugative addition/cyclization cascade of γ,γ-disubstituted enals with carboxylic acid-activated enones: a rapid access to highly functionalized γ-lactones

An aminocatalytic deconjugative addition/cyclization cascade of γ,γ-disubstituted enals with carboxylic acid-activated enones was realized, giving rise to highly functionalized γ-lactones with excellent enantioselectivities.

Synergistic visible light photoredox catalysis

Within the last decade the combination of photoredox catalysis and other catalytic modes of activation has become a powerful tool for organic synthesis to enable transformations that are not possible using single catalyst systems and hence are complementary to traditional methodology. Especially reactions proceeding via synergistic catalysis where co-catalyst and photocatalyst simultaneously and separately activate different reaction partners greatly benefit from the special properties of molecules and transition metal complexes in their excited state being oxidizing and reducing in nature at the same time. Apart from allowing for the generation of radical (open-shell) reactive intermediates by SET under mild conditions from bench-stable, abundant precursors, the photocatalyst often acts to interweave the distinct catalytic cycles by interaction at multiple points of the reaction mechanism to provide overall redox-neutral processes by shuttling electrons within in this complex network of elementary reaction steps. Synergistic strategies moreover may allow to performing such reactions with enantioselectivity, while mostly the selectivity is achieved by the chiral co-catalyst. The merger of photocatalysis has been achieved with a broad range of alternative modes of catalysis including organocatalysis, Brønstedt and Lewis acid and base catalysis, enzyme catalysis as well as in the context of cross-coupling transition metal catalysis overcoming challenging steps in this methodology and therefore has contributed to considerably expand the repertoire of suitable coupling partners. While only selected examples will be discussed, this chapter will highlight various dual catalytic platforms focusing on the photocatalytically generated intermediates, but also illustrating the diverse roles of photocatalysts in the context of such synergistic multicatalysis reactions.

Asymmetric synthesis of cyclopentanones through dual Lewis acid-catalysed [3+2]-cycloaddition of donor–acceptor cyclopropanes with ketenes

A dual Lewis acid system promotes the formal [3+2]-cycloaddition of enantioenriched donor–acceptor cyclopropanes with ketenes to afford cyclopentanones.

Design, synthesis, molecular modelling, and in vitro evaluation of tricyclic coumarins against Trypanosoma cruzi

Chagas disease is caused by infection with the parasite protozoan Trypanosoma cruzi and affects about 8 million people in 21 countries in Latin America. The main form of treatment of this disease is still based on the use of two drugs, benznidazole and nifurtimox, which both present low cure rates in the chronic phase and often have serious side-effects. Herein, we describe the synthesis of tricyclic coumarins that were obtained via NHC organocatalysis and evaluation of their trypanocidal activity. Molecular docking studies against trypanosomal enzyme triosephosphate isomerase (TIM) were carried out, as well as a theoretical study of the physicochemical parameters. The tricyclic coumarins were tested in vitro against the intracellular forms of Trypanosoma cruzi. Among the 18 compounds tested, 10 were more active than the reference drug benznidazole. The trypanocidal activity of the lead compound was rationalized by molecular docking study which suggested the strong interaction with the enzyme TIM by T. cruzi and therefore indicating a possible mode of action. Furthermore, the selectivity index of eight tricyclic coumarins with high anti-T. cruzi activity was above 50 and thus showing that these lead compounds are viable candidates for further in vivo assays.

N-Heterocyclic Carbene and Chiral Brønsted Acid Cooperative Catalysis for a Highly Enantioselective [4+2] Annulation

A chiral NHC/Brønsted acid cooperative catalysis system has been developed for asymmetric annulation of functionalized benzaldehydes and activated ketones through dearomative generation of dienolate.

Recent advances in N-heterocyclic carbene (NHC)-catalysed benzoin reactions

N-Heterocyclic carbenes (NHCs) have emerged as a powerful class of organocatalysts that mediate a variety of organic transformations. The Benzoin reaction constitutes one of the earliest known carbon-carbon bond-forming reactions catalysed by NHCs. The rapid growth of NHC catalysis in general has resulted in the development of a variety of benzoin and benzoin-type reactions. An overview of such NHC-catalysed benzoin reactions is presented.

Catalytic cross-benzoin/Michael/acetalization cascade for asymmetric synthesis of trifluoromethylated γ-butyrolactones

A sequential NHC-amine catalytic cascade reaction has been developed. This approach incorporates a highly chemoselective intermolecular cross-benzoin reaction and a highly regioselective Michael-acetalization cascade.

Enantioselective Synthesis of Tertiary Propargylic Alcohols under N-Heterocyclic Carbene Catalysis

A straightforward procedure to carry out the enantioselective benzoin reaction between aldehydes and ynones by employing a chiral N-heterocyclic carbene (NHC) as catalyst was developed. Under the optimized reaction conditions, these ynones undergo a clean and selective 1,2-addition with the catalytically generated Breslow intermediate, not observing any byproduct arising from competitive Stetter-type reactivity. This procedure allows the preparation of tertiary alkynyl carbinols as highly enantioenriched materials, which have the remarkable potential to be used as chiral building blocks in organic synthesis.

Synthesis of functionalized γ-lactones via a three-component cascade reaction catalyzed by consecutive N-heterocyclic carbene systems

Two consecutive N-heterocyclic carbene (NHC) catalytic systems were combined in a one-pot cascade reaction for the assembly of aromatic aldehydes and 2-haloenals into a structurally complex γ-lactone backbone.

An organocatalytic Michael-cyclization cascade of 4-oxa-α,β-unsaturated carboxylic acids with aldehydes: facile synthesis of chiral γ-lactols and trisubstituted γ-lactones

An organocatalytic Michael-cyclization cascade of 4-oxa-α,β-unsaturated carboxylic acids with aldehydes has been developed, enabling highly enantioselective synthesis of γ-lactols, trisubstituted γ-lactones and γ-lactams.

Evolution of catalytic stereoselective olefin metathesis: from ancillary transformation to purveyor of stereochemical identity

There have been numerous significant advances in catalytic olefin metathesis (OM) during the past two decades. Such progress has transformed this important set of reactions to strategically pivotal processes that generate stereochemical identity while delivering molecules that cannot be easily prepared by alternative routes. In this Perspective, an analysis of the origin of the inception of bidentate benzylidene ligands for Ru-based OM catalysts is first presented. This is followed by an overview of the intellectual basis that culminated in the development of Mo-based diolates and stereogenic-at-Ru complexes for enantioselective OM. The principles accrued from the study of the latter Ru carbenes and Mo alkylidenes and utilized in the design of stereogenic-at-Mo, -W, and -Ru species applicable to enantioselective and Z-selective OM are then discussed. The influence of the recently introduced catalytic OM protocols on the design of synthesis routes leading to complex organic molecules is probed. The impact of a better understanding of the mechanistic nuances of OM toward the discovery of stereoselective catalysts is reviewed as well.

Access to oxoquinoline heterocycles by N-heterocyclic carbene catalyzed ester activation for selective reaction with an enone

Organocatalytic ester activation is developed for a highly selective cascade reaction between saturated esters and amino enones. The reaction involves activation of the β-carbon atom of the ester as a key step. This method allows a single-step access to multicyclic oxoquinoline-type heterocycles with high enantiomeric ratios.

Reactions of pyruvates: organocatalytic synthesis of functionalized dihydropyrans in one pot and further transformations to functionalized carbocycles and heterocycles

Concise cascade reactions of pyruvates with aldehydes that generate functionalized dihydropyran derivatives in one pot have been developed. The products, dihydropyrans, were further concisely transformed to various functionalized molecules.

Catalytic Kinetic Resolution of a Dynamic Racemate: Highly Stereoselective β-Lactone Formation by N-Heterocyclic Carbene Catalysis

This study describes the combined experimental and computational elucidation of the mechanism and origins of stereoselectivities in the NHC-catalyzed dynamic kinetic resolution (DKR) of α-substituted-β-ketoesters. Density functional theory computations reveal that the NHC-catalyzed DKR proceeds by two mechanisms, depending on the stereochemistry around the forming bond: 1) a concerted, asynchronous formal (2+2) aldol-lactonization process, or 2) a stepwise spiro-lactonization mechanism where the alkoxide is trapped by the NHC-catalyst. These mechanisms contrast significantly from mechanisms found and postulated in other related transformations. Conjugative stabilization of the electrophile and non-classical hydrogen bonds are key in controlling the stereoselectivity. This reaction constitutes an interesting class of DKRs in which the catalyst is responsible for the kinetic resolution to selectively and irreversibly capture an enantiomer of a substrate undergoing rapid racemization with the help of an exogenous base.