An unusual stereoretentive 1,3-quaternary carbon shift resulting in an enantioselective RhII-catalyzed formal [4+1]-cycloaddition between diazo compounds and vinyl ketenes

Asymmetric Carbene Transformations for the Construction of All-Carbon Quaternary Centers

Asymmetric catalytic carbene reactions have been well documented in the last few decades for the expeditious assembly of chiral molecules with structural diversity. However, the enantioselective construction of all-carbon quaternary centers remains a challenge in this area. In this review article, two types of asymmetric carbene reactions that beyond cyclopropanation, cyclopropenation, and Büchner reaction, have been summarized for the construction of all-carbon quaternary centers: 1) using carbene species as a 1C synthon that reacts with a trisubstituted prochiral center; 2) sequential installation of two different C-C bonds on the carbene position, which features a gem-difunctionalization reaction. Especially, the asymmetric metal carbene gem-dialkylation process, which has emerged as a practical and versatile method for the expeditious assembly of complex architectures from readily available chemical resources, is a complementary approach for the expeditious assembly of all-carbon quaternary centers.

Enhancing Flavins Photochemical Activity in Hydrogen Atom Abstraction and Triplet Sensitization through Ring-Contraction

The isoalloxazine heterocycle of flavin cofactors reacts with various nucleophiles to form covalent adducts with important functions in enzymes. Molecular flavin models allow for the characterization of such adducts and the study of their properties. A fascinating set of reactions occurs when flavins react with hydroxide base, which leads to imidazolonequinoxalines, ring-contracted flavins, with so far unexplored activity. We report a systematic study of the photophysical properties of this new chromophore by absorption and emission spectroscopy as well as cyclic voltammetry. Excited, ring-contracted flavins are significantly stronger hydrogen atom abstractors when compared to the parent flavins, which allowed the direct trifluoromethylthiolation of aliphatic methine positions (bond dissociation energy (BDE) of 400.8 kJ mol-1). In an orthogonal activity, their increased triplet energy (E(S0←T1)=244 kJ mol-1) made sensitized reactions possible which exceeded the power of standard flavins. Combining both properties, ring-contracted flavin catalysts enabled the one-pot, five-step transformation of α-tropolone into trans-3,4-disubstituted cyclopentanones. We envision this new class of flavin-derived chromophores to open up new modes of reactivity that are currently impossible with unmodified flavins.

A Proline-Squaraine Ligand Framework (Pro-SqEB) for Stereoselective Rhodium(II)-Catalyzed Cyclopropanations

A proline-squaraine ligand (Pro-SqEB) that demonstrates high levels of stereoselectivity in olefin cyclopropanations when anchored to a Rh2II scaffold is introduced. High yields and enantioselectivities were achieved in the cyclopropanation of alkenes with diazo compounds in the presence of Rh2(Pro-SqEB)4. Notably, the unique electronic and steric design of this catalyst enabled the use of polar solvents that are otherwise incompatible with most RhII complexes.

Recent Advances in Palladium-Catalyzed [4 + n] Cycloaddition of Lactones, Benzoxazinanones, Allylic Carbonates, and Vinyloxetanes

Palladium-catalyzed allylation cyclization reaction has recently emerged as an efficient and powerful synthetic platform for the construction of diverse and valuable carbo- and heterocycles. Thus the development of new allylic motifs for achieving this type of transformations in high reactivity and selectivity is of great importance. Generally, these substrates have been utilized as 1,3-, 1,4-, 1,5-, 1,6-dipoles in many reactions, which are applied to prepare highly functionalized products with complete control of chemo-, regio-, diastereo-, and enantioselectivity. In this review, we focus our attention on the development of palladium-catalyzed [4 + n] cycloaddition of allylic motifs and describe a comprehensive and impressive advances in this area. Meanwhile, the related mechanism and the application of these annulation strategies in natural product total synthesis will be highlighted in detail.

Iodine-promoted insertion of the oxygen atom from water in η4-vinylketene[Fe(CO)3] complexes

Iodine promotes the in situ formation of iron(II) species from η⁴-vinylketene[Fe(CO)₃] (3a-h) as a key intermediate for the synthesis of 2(5H)-furanones (4a-h) by a sequential water-insertion/carbon-oxygen coupling under mild reaction conditions. Compounds 4a-h were obtained in good to excellent yields. A possible reaction pathway was also proposed by DFT calculations. This methodology can be extended to the synthesis of (5H)-pyrrol-2-ones using anilines, with moderate yields and a few limitations.

Generation of Functionalized Azepinone Derivatives via a (4 + 3)-Cycloaddition of Vinyl Ketenes and α-Imino Carbenes Derived from N-Sulfonyl-triazoles

An intermolecular Rh^II-catalyzed, formal (4 + 3)-cycloaddition between vinyl ketenes and N-sulfonyl-1,2,3-triazoles for the construction of azepinone products is described. Employing vinyl ketenes as a 1,4-dipolar surrogate, instead of the more commonly used dienyl moieties, allows for the intermolecular and selective formation of azepinone products over a potential (3 + 2)-cycloadduct under mild reaction conditions allows for the generation of azepinone products in up to 98% yield.

[4+3]-Cycloaddition Reaction of Sulfilimines with Cyclobutenones: Access to Benzazepinones

A catalyst-free [4+3]-cycloaddition reaction of N-aryl sulfilimines with cyclobutenones is described, which provides a straightforward protocol for synthesizing 1,5-dihydro-2H-benzo[b]azepin-2-ones under mild reaction conditions. This reaction features a broad substrate scope and good functional group tolerance and does not require catalysts or additives. Moreover, using N-pyridinyl sulfilimine as the substrate, a series of pyridoazepinones have also been prepared.

Reactivity of an N-heterocyclic silylene with a 1,1'-ferrocenediyl backbone towards carbonyl compounds, including carbon suboxide

Reactions of a silylene with a ketene and with carbon suboxide are reported, respectively leading to the first silaallene oxide and to a silylketene, whose reaction with water affords the first structurally characterised stable methyleneketene and constitutes a unique type of single-crystal-to-single-crystal transformation of a molecular solid by a stoichiometric gas-solid chemical reaction.

Gold(I)-catalyzed intramolecular cyclization/intermolecular cycloaddition cascade as a fast track to polycarbocycles and mechanistic insights

Metal carbene is an active synthetic intermediate, which has shown versatile applications in synthetic chemistry. Although a variety of catalytic methods have been disclosed for the generation of carbene species from different precursors, there is an increasing demand for the development of efficient and practical approaches for the in-situ formation of metal carbene intermediates with structural diversity and unrevealed reactivity. Herein we report a gold-catalyzed cascade protocol for the assembly of polycarbocyclic frameworks in high yields under mild reaction conditions. Mechanistic studies indicate that the unique β-aryl gold-carbene species, generated via gold-promoted 6-endo-dig diazo-yne cyclization, is the key intermediate in this reaction, followed by a [4 + 2]-cycloaddition with external alkenes. In comparison to the well-documented metal carbene cycloadditions, this carbene intermediate serves as a 4-C synthon in a cycloaddition reaction. A variety of elusive π-conjugated polycyclic hydrocarbons (CPHs) with multiple substituents are readily accessible from the initially generated products by a mild oxidation procedure.

(4+1)-Cycloadditions Exploiting the Biphilicity of Oxyphosphonium Enolates and RhII/PdII-Stabilized Metallocarbenes for the Construction of Five-Membered Frameworks

(4+1)-Cyclizations are an underutilized disconnect for the formation of five-membered heterocyclic and carbocyclic frameworks. Herein we analyze methods employing oxyphosphonium enolates and RhII/PdII-metallocarbenes as C1 synthons in the presence of several four-atom components for the synthesis of 2,3-dihydrobenzofurans, 2,3-dihydroindoles, oxazolones, cyclopentenones, and pyrrolones.

1 Introduction

2 (4+1)-Cyclizations Employing Kukhtin–Ramirez-Like Reactivity

3 (4+1)-Cyclizations Employing a Cyclopropanation/Ring-Expansion Sequence

4 Pd-Catalyzed (4+1)-Cyclizations through Carbene Migratory Insertion/Reductive Elimination Processes

5 Summary

Supramolecular Halogen Bonds in Asymmetric Catalysis

Halogen bonding has received a significant increase in attention in the past 20 years. An important part of this interest has centered on catalytic applications of halogen bonding. Halogen bond (XB) catalysis is still a developing field in organocatalysis, although XB catalysis has outgrown its proof of concept phase. The start of this year witnessed the publication of the first example of a purely XB-based enantioselective catalytic reaction. While the selectivity can be improved upon, there are already plenty of examples in which halogen bonds, among other interactions, play a crucial role in the outcome of highly enantioselective reactions. This paper will give an overview of the current state of the use of XBs in catalytic stereoselective processes.

Diverting β-Hydride Elimination of a π-Allyl PdII Carbene Complex for the Assembly of Disubstituted Indolines via a Highly Diastereoselective (4 + 1)-Cycloaddition

A Pd⁰-catalyzed formal (4 + 1)-cycloaddition approach to 2,3-disubstituted dihydroindoles is described. The diastereoselective formation of dihydroindoles that is highlighted by a carbene migratory insertion/reductive elimination sequence proceeding via a π-allyl Pd^II-species compliments existing methods of indoline assembly.

Phosphine-Catalyzed [4+1] Cycloadditions of Allenes with Methyl Ketimines, Enamines, and a Primary Amine

Unprecedented phosphine-catalyzed [4+1] cycloadditions of allenyl imides have been discovered using various N-based substrates including methyl ketimines, enamines, and a primary amine. These transformations provide a one-pot access to cyclopentenoyl enamines and imines, or (chiral) γ-lactams through two geminal C-C bond or two C-N bond formations, respectively. Several P-based key intermediates including a 1,4-(bis)electrophilic α,β-unsaturated ketenyl phosphonium species have been detected by ³¹ P NMR and HRMS analyses, which shed light on the postulated catalytic cycle. The synthetic utility of this new chemistry has been demonstrated through a gram-scaling up of the catalytic reaction as well as regioselective hydrogenation and double condensation to form cyclopentanoyl enamines and fused pyrazole building blocks, respectively.

Copper-catalyzed cascade cyclization reaction of 3-aminocyclobutenones with electron-deficient internal alkynes: synthesis of fully substituted indoles

A novel copper-catalyzed cascade cyclization reaction of 3-aminocyclobutenones with electron-deficient internal alkynes has been developed. This reaction provides a new method for the synthesis of fully substituted indoles by formation of four new bonds and two rings in a single step.

Domino 1,4- and 1,6-Addition Reactions of Ketene Silyl Acetals to Dialkynyl Imines Promoted by Aluminum Chloride: Synthesis of Multifunctionalized β-Lactams

Domino 1,4- and 1,6-addition reactions of ketene silyl acetals to dialkynyl imines are disclosed. Aluminum chloride promoted domino 1,4- and 1,6-addition reactions of ketene silyl acetals to dialkynyl imines to give a variety of alkenyl iminocyclobutenones in moderate to good yields. The chemoselective reduction of alkenyl iminocyclobutenones and the subsequent thermal rearrangement of resulting alkenyl aminocyclobutenones in the presence of appropriate amines provided cis or trans multifunctionalized β-lactams in moderate to high yields with good to high diastereoselectivities.

Copper-catalyzed [4+1]-annulation of 2-alkenylindoles with diazoacetates: a facile access to dihydrocyclopenta[b]indoles

A copper-catalyzed [4+1]-annulation of 2-vinylindoles with diazoacetates: a facile access to the dihydrocyclopenta[b]indoles bearing two contiguous all-carbon quaternary centers.

Enantioselective Formal [4+1] Cycloaddition of Diazoarylacetates and the Danishefsky's Diene: Stereoselective Synthesis of (-)-1,13-Herbertenediol

Rodium chiral diene complex-catalyzed enantioselective cycloaddition of aryl α-diazoarylacetates and electron-enriched Danishefsky-type dienes afforded highly functionalized and optically enriched cyclopentenones in excellent yields (up to 97% yield) and with good to excellent enantioselectivities (60-92% ee). (-)-1,13-Herbertenediol was successfully synthesized in an overall 25% yield employing the optically enriched cyclopentenone with an all-carbon quaternary center as the key intermediate.