Asymmetric homoenolate additions to acyl phosphonates through rational design of a tailored N-heterocyclic carbene catalyst

CAMDOL-enabled diastereoselective synthesis of α-substituted phosphonates

Enantiopure α-substituted phosphonic acids are widely utilized as drugs, pesticides, and ligands. Despite numerous synthetic approaches having been investigated, precise construction of P-adjacent chiral tertiary carbon centres by the employment of recoverable chiral auxiliaries is traditional and still one of the most reliable and practical synthetic methodologies so far. Herein, we present a highly diastereoselective synthesis of α-substituted phosphonates via the unique CAMDOL-derived P-substrates by an efficient sequential deprotonation with LiHMDS and alkylation/arylation with RI. A wide range of 30 structurally diverse α-substituted phosphonate products, including the well-known P-analogues of naproxen and ibuprofen, were thus afforded conveniently in up to 92% yields and 99 : 1 diastereomeric ratios. The related chiral phosphonic acid could be easily obtained by simple acidic hydrolysis with fully recovered auxiliary. This CAMDOL-enabled asymmetric synthetic protocol exhibits comparative advantages over known chiral-induction methods with easy accessibility and compatibility of furnishing a variety of C-stereogenic centres in the proximity of the phosphorus atom, including some rare examples.

Carbene-Catalyzed Direct O-Functionalization of Ketone: Atroposelective Access to Non-C2-Symmetric Binaphthyls

Disclosed here is NHC-catalyzed direct intermolecular trapping of the ketone oxygen atom with the acyl azolium intermediate. The overall reaction is a dynamic kinetic resolution process that converts ketone to the corresponding enol ester with well-controlled axial chirality. Our reaction eventually affords non-C2-symmetric binaphthyl derivatives with important applications, such as in the area of asymmetric catalysis.

Recent Advances in Enantioselective Organocatalytic Reactions Enabled by NHCs Containing Triazolium Motifs

N-Heterocyclic carbenes (NHCs) containing triazolium motifs have emerged as a powerful tool in organocatalysis. Recently, various NHC pre-catalyst mediated organic transformations have been developed successfully. This article aims to compile the current state of knowledge on NHC-triazolium catalysed enantioselective name reactions and introduce newly developed catalytic methods. Furthermore, this review article framework provides an excellent opportunity to highlight some of the unique applications of these catalytic procedures in the natural product synthesis of biologically active compounds, notably the wide range of preparation of substituted chiral alcohols, and their derivatives. This article provides an overview of chiral NHC triazolium-catalyst libraries synthesis and their catalytic application in enantioselective reactions.

Enantioselective functionalization at the C4 position of pyridinium salts through NHC catalysis

A catalytic method for the enantioselective and C4-selective functionalization of pyridine derivatives is yet to be developed. Herein, we report an efficient method for the asymmetric β-pyridylations of enals that involve N-heterocyclic carbene (NHC) catalysis with excellent control over enantioselectivity and pyridyl C4-selectivity. The key strategy for precise stereocontrol involves enhancing interactions between the chiral NHC-bound homoenolate and pyridinium salt in the presence of hexafluorobenzene, which effectively differentiates the two faces of the homoenolate radical. Room temperature is sufficient for this transformation, and reaction efficiency is further accelerated by photo-mediation. This methodology exhibits broad functional group tolerance and enables facile access to a diverse range of enantioenriched β-pyridyl carbonyl compounds under mild and metal-free conditions.

A catalytic method for the enantioselective and C4-selective functionalization of pyridine derivatives is yet to be developed. Here the authors report an efficient method for the asymmetric β-pyridylations of enals that involve N-heterocyclic carbene (NHC) catalysis with excellent control over enantioselectivity and pyridyl C4-selectivity.

Electrogenerated NHCs in Organic Synthesis: Ionic Liquids vs Organic Solvents Effects

In the last twenty years, N-heterocyclic carbenes (NHCs) have been extensively studied for their application as organocatalysts in stereoselective synthesis as well as ligands for transition metals-promoted synthetic methodologies. Derived mainly from azolium salts, NHCs have demonstrated exceptional versatility in their generation usually performed by deprotonation or reduction (chemical or electrochemical). In particular, the generation of NHC under electrochemical conditions, starting from azolium-based ionic liquids, has proven to be a successful green approach and demonstrated wide applicability in organic synthesis. In this Personal Account, the application of electrogenerated NHCs in organic synthesis will be discussed, with a particular attention to the different reactivity in ionic liquids compared to classical organic solvents.

Unveiling the Chemo- and Stereoselectivities of NHC-Catalyzed Reactions of an Aliphatic Ester with Aminochalcone

The possible mechanisms and origin of selectivities in N-heterocyclic carbene (NHC)-catalyzed reactions of an aliphatic ester with aminochalcone were studied using density functional theory. Herein, a general mechanistic map involving various types of possible intermediates was discovered, and the corresponding chemoselective pathways were systematically investigated. Based on the computational results, the most energetically favorable reaction pathway mainly involved in the following processes: formation of a homoenolate intermediate via α/β-H elimination, formal Michael addition of a homoenolate intermediate to aminochalcone, intramolecular aldol-type reaction, and ring closure to form the lactam product. Furthermore, the formal Michael addition process was shown to be the stereoselectivity-determining step, generating the RR-configured product preferentially. The chemoselectivity was successfully predicted by comparing the ω + N index of reacting ability for a nucleophile (N) and electrophile (E). This work would provide a general guideline for predicting chemoselectivity in NHC-catalyzed reactions.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Access to enantioenriched 4-phosphorylated δ-lactones from β-phosphorylenones and enals via carbene organocatalysis

N-heterocyclic carbene (NHC) catalyzed direct access to enantioenriched 4-phosphorylated δ-lactones from β-phosphorylenones and enals has been achieved.

Synthesis of polysubstituted arenes through organocatalytic benzannulation

Polysubstituted arenes serve as ubiquitous structural cores of aromatic compounds with significant applications in chemistry, biological science, and materials science. Among all the synthetic approaches toward these highly functionalized arenes, organocatalytic benzannulation represents one of the most efficient and versatile transformations in the assembly of structurally diverse arene architectures under mild conditions with exceptional chemo-, regio- or stereoselectivities. Thus, the development of new benzannulation reactions through organocatalysis has attracted much attention in the past ten years. This review systemically presents recent advances in the organocatalytic benzannulation strategies, categorized as follows: (1) Brønsted acid-catalysis, (2) secondary amine catalysis, (3) primary amine catalysis, (4) tertiary amine catalysis, (5) tertiary phosphine catalysis, and (6) N-heterocyclic carbene catalysis. Each part is further classified into several types according to the number of carbon atoms contributed by different synthons participating in the cyclization reaction. The reaction mechanisms involved in different benzannulation strategies were highlighted.

Organocatalytic benzannulation represents one of the most efficient transformations for assembling polysubstituted arenes, this review presents recent advances in organocatalytic benzannulation strategies to construct functionalized benzenes.

Chemoselective Umpolung of Enals for Asymmetric Homoenolate Cross-Annulation of Enals and Aldehydes Catalyzed by N-Heterocyclic Carbene

An asymmetric homoenolate cross-annulation of enals and aldehydes with high enantioselectivity is realized by NHC-catalyzed chemoselective umpolung of enals. The reaction proceeds in a highly chemoselective manner, selectively generating the conjugated Breslow intermediates from enals rather than aldehydes, enabling the homoenolate addition of enals to aldehydes in preference to competing acyl anion-mediated reactions. Enantioenriched substituted γ-butyrolactones are formed in good yields with high enantioselectivities.

Highly enantioselective synthesis of functionalized azepino[1,2-a]indolesviaNHC-catalyzed [3+4] annulation

The enantioselective [3+4] annulation of 3-formylindol-2-methylmalonates with 2-bromoenals catalyzed by NHCs is described to efficiently synthesise functionalized azepino[1,2-a]indoles.

A mechanistic investigation into N-heterocyclic carbene (NHC) catalyzed umpolung of ketones and benzonitriles: is the cyano group better than the classical carbonyl group for the addition of NHC?

The cooperative participation of NHC-H₂O-activated cyano carbon is more preferred than the classical NHC-activated carbonyl carbon.

N-Heterocyclic carbene-catalyzed β-addition of enals to 3-alkylenyloxindoles: synthesis of oxindoles with all-carbon quaternary stereocenters

The N-heterocyclic carbene-catalyzed β-addition of enals to 3-alkylenyloxindoles was developed. All of the β-aryl, β-alkenyl and β-alkyl enals worked well for the reaction to give the corresponding 3,3-disubstituted oxindoles bearing contiguous all-carbon quaternary and tertiary stereocenters in good yields with moderate to high diastereoselectivities and excellent enantioselectivities.

Phosphonate-Directed Catalytic Asymmetric Hydroboration: Delivery of Boron to the More Substituted Carbon, Leading to Chiral Tertiary Benzylic Boronic Esters

Phosphonate-directed catalytic asymmetric hydroboration (CAHB) of β-aryl/heteroaryl methylidenes and trisubstituted alkenes by pinacolborane enables facile access to functionalized, chiral tertiary benzylic boronic esters. Hydroboration is catalyzed by a chiral rhodium catalyst prepared in situ from a Rh(I)-precursor in combination with a simple TADDOL-derived chiral cyclic monophosphite in a 1:1 ratio. The regio- and stereochemistry arises from the combined effects of the relative disposition of the directing group to the alkene, the alkene substitution pattern, and the necessity of an aryl substituent attached to the alkene. A range of aryl and heteroaryl substituents can be accommodated, and for several chiral substrates, the reactions are efficiently catalyst-controlled enabling the choice of diastereomeric products as desired. Stereospecific transformations of the chiral boronic ester afford chiral phosphonates bearing a quaternary carbon stereocenter. The synthetic utility of the products is further demonstrated by α-oxidation of the phosphonate leading to hydroxy- and oxo-phosphonates; the latter readily undergo elimination/substitution reactions to unmask the phosphonate functionality with the formation of aldehydes, alcohols, esters, amides, acids and ketones.

Carbene-Catalyzed [4 + 2] Cycloadditions of Vinyl Enolate and (in Situ Generated) Imines for Enantioselective Synthesis of Quaternary α-Amino Phosphonates

A carbene-catalyzed enantioselective addition of enals to five-membered cyclic imines is developed. The reaction gives chiral quaternary α-amino phosphonates bearing tetrasubstituted carbon centers with excellent enantioselectivities. The imine substrates can be generated in situ from the corresponding amines under an oxidative condition that is compatible with the carbene catalysis. Thus, a one-pot cross-dehydrogenative-coupling (CDC) reaction between enals and amines is also realized with high enantioselectivity remaining. The method provides quick enantioselective access to amino phosphonates with potential applications in medicines and pesticides.

Enantioselective Synthesis of α-Amidoboronates Catalyzed by Planar-Chiral NHC-Cu(I) Complexes

The first highly selective catalytic hydroboration of alkyl-substituted aldimines to provide medicinally relevant α-amidoboronates is disclosed. The Cu(I)-catalyzed borylation proceeds with excellent facial selectivity when a set of planar-chiral N-heterocyclic carbenes (NHCs) were employed as ligands. Density functional theory computations suggest that interactions between BPin and the planar-chiral catalyst are responsible for the observed stereoselectivity. Important pharmacophores, such as the boronate analogue of isoleucine, can be prepared using a chromatography-free protocol starting from commercially available reagents. The application of these NHC ligands in these Cu(I)-catalyzed processes offers a significant contribution to existing strategies for laboratory-scale preparation of enantioenriched α-amidoboronates.

Carbene-catalyzed enal γ-carbon addition to α-ketophosphonates for enantioselective access to bioactive 2-pyranylphosphonates

A carbene-catalyzed enantioselective [4+2] cycloaddition reaction between α,β-unsaturated aldehydes and α-ketophosphonates is developed. The reaction affords chiral 2-pyranylphosphonates with excellent enantioselectivities. The optically enriched phosphonate products bear multiple functional groups, including unsaturated lactone and phosphonate moieties that often lead to unique bio-activities. Preliminary studies show that the products from our reactions exhibit anti-bacterial (X. oryzae pv. oryzae) and anti-viral (Tobacco Mosaic Virus) activities for potential use in plant protection.

A continuing challenge: N-heterocyclic carbene-catalyzed syntheses of γ-butyrolactones

Catalytic, stereoselective N-heterocyclic carbene-catalyzed reactions facilitate efficient construction of many different heterocyclic compounds, such as the enantioenriched 5-membered (γ) lactones highlighted in this tutorial review. Herein, various strategies to enable formal [3+2] type annulations between electrophilic carbonyl equivalents and homoenolate nucleophiles for the synthesis of γ-lactones are summarized.

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.

Enantioselective access to multi-cyclic α-amino phosphonates via carbene-catalyzed cycloaddition reactions between enals and six-membered cyclic imines

A carbene-catalyzed enantioselective [4 + 2] cycloaddition reaction is developed and quarternary α-amino phosphonates are afforded with encouraging anti-bacterial activities.

A Cooperative Ternary Catalysis System for Asymmetric Lactonizations of α-Ketoesters

A general and enantioselective N-heterocyclic carbene (NHC)-catalyzed lactonization of simple enals and α-ketoesters has been discovered using a new ternary cooperative catalytic system. The highly selective annulation was achieved by using a combination of a chiral NHC, a hydrogen-bond donor, and a metal salt, facilitating self-assembly of the reactive partners. A proposed model for this new mode of NHC chiral relay catalysis is supported by experimental and computational mechanistic studies.

Organocatalytic Asymmetric Michael-Hemiacetalization Reaction Between 2-Hydroxyacetophenones and Enals: A Route to Chiral β,γ-Disubstituted γ-Butyrolactones

The first highly enantioselective organocatalytic reaction employing 2-hydroxyacetophenones is disclosed, namely Michael-hemiacetalization reaction of 2-hydroxyacetophenones with enals. The combination of a primary amine and a secondary amine catalyst was found to be the best choice for this methodology. The products of this reaction were obtained in high enantio- and diastereoselectivities and were converted to a variety of biologically important γ-butyrolactones.

Computational Insights into the Central Role of Nonbonding Interactions in Modern Covalent Organocatalysis

The flexibility, complexity, and size of contemporary organocatalytic transformations pose interesting and powerful opportunities to computational and experimental chemists alike. In this Account, we disclose our recent computational investigations of three branches of organocatalysis in which nonbonding interactions, such as C-H···O/N interactions, play a crucial role in the organization of transition states, catalysis, and selectivity. We begin with two examples of N-heterocyclic carbene (NHC) catalysis, both collaborations with the Scheidt laboratory at Northwestern. In the first example, we discuss the discovery of an unusual diverging mechanism in a catalytic kinetic resolution of a dynamic racemate that depends on the stereochemistry of the product being formed. Specifically, the major product is formed through a concerted asynchronous [2 + 2] aldol-lactonization, while the minor products come from a stepwise spiro-lactonization pathway. Stereoselectivity and catalysis are the results of electrophilic activation from C-H···O interactions between the catalyst and the substrate and conjugative stabilization of the electrophile. In the second example, we show how knowledge and understanding of the computed transition states led to the development of a more enantioselective NHC catalyst for the butyrolactonization of acyl phosphonates. The identification of mutually exclusive C-H···O interactions in the computed major and minor TSs directly resulted in structural hypotheses that would lead to targeted destabilization of the minor TS, leading to enhanced stereoinduction. Synthesis and evaluation of the newly designed NHC catalyst validated our hypotheses. Next, we discuss two works related to Lewis base catalysis involving 4-dimethylaminopyridine (DMAP) and its derivatives. In the first, we discuss our collaboration with the Smith laboratory at St Andrews, in which we discovered the origins of the regioselectivity in carboxyl transfer reactions. We disclose how different Lewis base catalysts (NHC or DMAP) can lead to different regiomeric products as a result of differing magnitudes of aromatic and C-H···O interactions present in the respective transition states. In the second example, we discuss the mechanism and origins of the stereoselectivity of a reaction catalyzed by a planar-chiral 4-(pyrrolidino)pyridine derivative, namely, the coupling of ketenes with cyanopyrrole. We discovered that the chiral base mechanism is operative, in contrast to the originally proposed Brønsted acid mechanism. The selectivity is determined by the ease with which the major and minor TSs can realize strong stabilizing C-H···N interactions between the pyrrole cyano group and the catalyst. These interactions induce increased catalyst distortion in the minor TS, thereby leading to enantioselectivity. Finally, we discuss our computations related to amine-based organocatalysis in collaboration with the Carter laboratory at Oregon State. We probed the mechanism and stereoselectivity of a bifunctional amine thiourea-catalyzed Michael reaction. Our computations led to the design of an improved catalyst. However, synthesis and tests revealed that this catalyst was prone to degradation to side products that also catalyze the reaction, ultimately reducing the observed enantioselectivity. Lastly, we discuss our study of the mechanism and stereoselectivity of a proline sulfonamide-catalyzed Robinson annulation, in which we discovered that the enantioselectivity is controlled by the first Michael step but the diastereoselectivity is controlled by the following Mannich step.

Formal [3 + 3] annulation of isatin-derived 2-bromoenals with 1,3-dicarbonyl compounds enabled by Lewis acid/N-heterocyclic carbene cooperative catalysis

Several novel isatin-derived 2-bromoenals were applied for formal [3 + 3] annulation with 1,3-dicarbonyl compounds enabled by NHC/Lewis acid cooperative catalysis.

Stereospecific cyclization reaction of alkenyl esters and amides of ethenetricarboxylate

The stereospecificity of the cyclization reaction of (E)- and (Z)-2-alkenyl esters and amides of ethenetricarboxylate has been examined. The reaction of (E)/(Z)-2-butenyl esters with AlCl3 or FeCl3 gave trans-substituted chlorinated γ-lactone diastereomers stereospecifically. (E)/(Z)-2-butenyl and pentenyl amides undergo an intramolecular ene reaction at room temperature gradually. At 80 °C, the (Z)-alkenyl amides were transformed to cis-substituted ene adducts and the (E)-amides were transformed to cis- and trans-substituted ene adduct mixtures. The reaction of (E)/(Z)-2-alkenyl amides with ZnI2 gave trans-substituted γ-lactam diastereomers stereospecifically along with the ene adducts.

Insights into the Unexpected Chemoselectivity for the N-Heterocyclic Carbene-Catalyzed Annulation Reaction of Allenals with Chalcones

Lewis base N-heterocyclic carbene (NHC)-catalyzed annulation is the subject of extensive interest in synthetic chemistry, but the reaction mechanisms, especially the unexpected chemoselectivity of some of these reactions, are poorly understood. In this work, a systematic theoretical calculation has been performed on NHC-catalyzed annulation between allenals and chalcone. Multiple possible reaction pathways (A-E) leading to three different products have been characterized. The calculated results reveal that NHC is more likely to initiate the reaction by nucleophilic attack on the center carbon atom of the allene group but not the carbonyl carbon atom in allenals leading to the Breslow intermediate, which is remarkably different from the other NHC-catalyzed annulations of unsaturated aldehydes with chalcones. The computed energy profiles demonstrate that the most energetically favorable pathway (A) results in polysubstituted pyranyl aldehydes, which reasonably explains the observed chemoselectivity in the experiment. The observed chemoselectivity is demonstrated to be thermodynamically but not kinetically controlled, and the stability of the Breslow intermediate is the key for the possibility of homoenolate pathway D and enolate pathway E. This work can improve our understanding of the multiple competing pathways for NHC-catalyzed annulation reactions of unsaturated aldehydes with chalcones and provide valuable insights for predicting the chemoselectivity for this kind of reaction.

Oxidatively Initiated NHC-Catalyzed Enantioselective Synthesis of 3,4-Disubstituted Cyclopentanones from Enals

An unprecedented N-heterocyclic carbene (NHC)-catalyzed annulation of enals to form 3,4-disubstituted cyclopentanones has been discovered. Aryl enals undergo dimerization in the presence of a single-electron oxidant to form C2 symmetric cyclopentanones. A cross-reaction has also been developed, allowing for the synthesis of differentially substituted cyclopentanones. Mechanistically, the reaction is thought to proceed through radical intermediates, further establishing the synthetic utility of this class of reactivity.

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.

Nucleophilic β-Carbon Activation of Propionic Acid as a 3-Carbon Synthon by Carbene Organocatalysis

Direct β-carbon activation of propionic acid (C2H5CO2H) by carbene organocatalysis has been developed. This activation affords the smallest azolium homoenolate intermediate (without any substituent) as a 3-carbon nucleophile for enantioselective reactions. Propionic acid is an excellent raw material because it is cheap, stable, and safe. This approach provides a much better solution to azolium homoenolate synthesis than the previously established use of acrolein (enal without any substituent), which is expensive, unstable, and toxic.

N-Heterocyclic Carbene-Catalyzed δ-Carbon LUMO Activation of Unsaturated Aldehydes

An N-heterocyclic carbene (NHC) catalyzed domino reaction triggered by a δ-LUMO activation of α,β-γ,δ-diunsaturated enal has been developed for the formal [4 + 2] construction of multisubstituted arenes and 3-ylidenephthalide. These two products, formed in a highly chemo- and regioselective manner, were obtained via different catalytic pathways due to a simple change of the substrate. The activation of the remote δ-carbon of unsaturated aldehydes expands the synthetic potentials of NHC organocatalysis.