Ring-Opening 1,3-Dichlorination of Donor–Acceptor Cyclopropanes by Iodobenzene Dichloride

Electrocatalytic continuous flow chlorinations with iodine(I/III) mediators

Electrochemistry offers tunable, cost effective and environmentally friendly alternatives to carry out redox reactions with electrons as traceless reagents. The use of organoiodine compounds as electrocatalysts is largely underdeveloped, despite their widespread application as powerful and versatile reagents. Mechanistic data reveal that the hexafluoroisopropanol assisted iodoarene oxidation is followed by a stepwise chloride ligand exchange for the catalytic generation of the dichloroiodoarene mediator. Here, we report an environmentally benign iodine(I/III) electrocatalytic platform for the in situ generation of dichloroiodoarenes for different reactions such as mono- and dichlorinations as well as chlorocyclisations within a continuous flow setup.

Catalytic Intramolecular Ketone Haloacylation Enabled Stereoselective Heterolytic Cleavage of Cyclopropyl Ketones with Enhanced Reactivity and Regioselectivity beyond Electronics

By integration of oxocarbenium activation and Lewis acid coordination activation via conformational proximity-driven, Pd(II)- or Cu(I)-catalyzed intramolecular ketone haloacylation, regio- and stereoselective heterolytic ring-opening 1,5-haloacylation of cyclopropyl ketones, including those with weak single alkyl donors, has been developed for the synthesis of valuable α-quaternary halo-γ-butenolides. The vicinal carboxylic acid and ketone acceptors are no longer just spectator activators. Further, this reaction delivers a constant regioselectivity regardless of the electronic nature of substituents, even the malonate.

Sn(OTf)2-Catalyzed (3 + 2) Cycloaddition/Sulfur Rearrangement Reaction of Donor-Acceptor Cyclopropanes with Indoline-2-thiones

The (3 + 2) cycloaddition/sulfur rearrangement reaction of donor-acceptor cyclopropanes bearing a single keto acceptor with indoline-2-thiones has been realized. Under the catalysis of Sn(OTf)2, a series of functionalized 3-indolyl-4,5-dihydrothiophenes were synthesized with moderate to excellent yields.

Metal-free synthesis of 1,3-dichloro-1,5-diarylpentan-5-ones via cascade oxidative radical addition of styrenes with CHCl3

A novel and efficient metal-free cascade oxidative radical addition of styrenes is developed for the construction of 1,3-dichloro-1,5-diarylpentan-5-ones. This protocol presents a practical one-pot procedure that delivers highly functionalized 1,3-dichloro-1,5-diarylpentan-5-ones in moderate-to-good yields with a broad substrate scope under mild conditions.

AlCl3-mediated ring-opening reactions of indoline-2-thiones with acyl cyclopropanes, bi-cyclopropanes and spirocyclic cyclopropanes

AlCl3-mediated nucleophilic ring-opening reactions of indoline-2-thiones with various acyl cyclopropanes, bi-cyclopropanes and spirocyclic cyclopropanes were investigated. A series of ketones functionalized with indolylthio groups were synthesized in yields ranging from moderate to good. Moreover, chemical transformations of 4-indolylthio butan-1-ones to dihydro-2H-thiepino[2,3-b]indoles and sulfone were carried out to further expand both synthetic utility and structural complexity.

Benzotriazole-Triggered Three-Component Lewis Acid-Catalyzed Ring-Opening 1,3-Aminofunctionalization of Donor-Acceptor Cyclopropanes

The use of benzotriazoles as nucleophilic triggers in the three-component Yb(OTf)₃-catalyzed ring-opening 1,3-aminofunctionalization of donor-acceptor (D-A) cyclopropanes is presented. Using N-halo succinimide (NXS) as the third component, the reaction afforded the 1,3-aminohalogenation product in up to an 84% yield. Moreover, using alkyl halides or Michael acceptors as the third components, the 3,1-carboaminated products are formed in up to a 96% yield in a one-pot operation. Employing Selectfluor as the electrophile, the reaction furnished the 1,3-aminofluorinated product in a 61% yield.

Divergent Copper-salt-controlled Reactions of Donor-Acceptor Cyclopropanes and N-Fluorobenzene Sulfonimide: Access to the 1,3-Haloamines and Aminoindanes

We herein report a method for divergent copper salt controlled reactions of donor-acceptor cyclopropanes and N-fluorobenzene sulfonimide (NFSI). Specifically, in the presence of CuX₂ (X=Cl, Br), the cyclopropanes underwent formal umpolung 1,3-aminohalogenation bifunctionalization via a free radical mediated ring-opening process to afford 1,3-aminochlorination and 1,3-aminobromination products in moderate to good yields. In addition, by using CuI as a catalyst, we synthesized various aminoindane derivatives via 1,3-aminoarylation cyclization of D-A cyclopropanes, the reactions involved a free radical mediated ring-opening and subsequent ring expansion via C-H bond activation.

Synthesis of Indole-Fused Dihydrothiopyrano Scaffolds via (3 + 3)-Annulations of Donor-Acceptor Cyclopropanes with Indoline-2-Thiones

A new methodology for the synthesis of N-haloindole-fused dihydrothiopyrano derivatives via (3 + 3)-annulation of donor-acceptor cyclopropanes (DACs) with indoline-2-thiones in the presence of Sc(OTf)₃ as a Lewis acid catalyst has been developed. This protocol provides a variety of indole-fused dihydrothiopyrano molecules in good to excellent yields, which architecturally resemble other indole-fused tricyclic molecules having potential medicinal value. In addition, we have described a detailed reaction mechanism and transformation of the furnished product into N-fused thiazino indole molecule.

Donor-Acceptor Cyclopropanes: Activation Enabled by a Single, Vinylogous Acceptor

A novel class of highly activated donor-acceptor cyclopropanes bearing only a single, vinylogous acceptor is presented. These strained moieties readily undergo cycloadditions with aldehydes, ketones, thioketones, nitriles, naphth-2-ols and various other substrates to yield the corresponding carbo- and heterocycles. Diastereocontrol can be achieved through the choice of catalyst (Brønsted or Lewis acid). The formation of tetrahydrofurans was shown to be highly enantiospecific when chiral cyclopropanes are employed. A series of mechanistic and kinetic experiments was conducted to elucidate a plausible catalytic cycle and to rationalize the stereochemical outcome.

ICl-Mediated Functional Group Interconversion from Methyl Homopropargyl Ether to α-Iodo-γ-chloroketone

An ICl-mediated highly chemo- and regioselective functional group interconversion from methyl homopropargyl ether to α-iodo-γ-chloro-ketone is reported. Density functional theory (DFT)-calculated reaction coordinate and potential energy surface support the high chemo-selectivity observed for the formation of α-iodo-γ-chloroketone over furan. The five-membered oxonium ring formation-ring opening mechanism is a potential template for the preparation of polyfunctionalized carbonyl compounds.

The Cloke-Wilson rearrangement of aroyl-substituted donor-acceptor cylopropanes containing arylethyl donors

The chemistry of donor-acceptor (D-A) cyclopropanes containing alkyl donors has been scantily investigated. In the present work, we have synthesized new D-A cyclopropanes containing arylethyl donors and explored their reactivity in the presence of Lewis acids. Upon treatment with SnCl₄, these cyclopropanes underwent the Cloke-Wilson rearrangement to yield 3,4,5-trisubstituted γ-butyrolactones in good yields with high diastereoselectivity.

Ring-Opening 1,3-Carbothiolation of Donor-Acceptor Cyclopropanes Using Alkyl Halides and In Situ Generated Dithiocarbamates

Two-step, ring-opening 1,3-carbothiolation of donor-acceptor (D-A) cyclopropanes employing alkyl halides and in situ generated dithiocarbamates (from amines and CS2) has been demonstrated under mild conditions. The reaction is operationally simple and works with good functional group compatibility. Three new bonds including C-N, C-S, and C-C are formed in this 1,3-bifunctionalization strategy. Electron-poor olefins can also be used as electrophiles instead of alkyl halides. The use of enantiomerically pure D-A cyclopropane afforded enantiopure 1,3-carbothiolated product, thus demonstrating the stereospecificity of the reaction.

A novel type of donor-acceptor cyclopropane with fluorine as the donor: (3 + 2)-cycloadditions with carbonyls

gem-Difluorocyclopropane diester is disclosed as a new type of donor–acceptor cyclopropane, which smoothly participates in (3 + 2)-cycloadditions with various aldehydes and ketones. This work represents the first application of gem-difluorine substituents as an unconventional donor group for activating cyclopropane substrates in catalytic cycloaddition reactions. With this method, a wide variety of densely functionalized gem-difluorotetrahydrofuran skeletons, which are otherwise difficult to prepare, could be readily assembled in high yields under mild reaction conditions. Computational studies show that the cleavage of the C–C bond between the difluorine and diester moieties occurs upon a S_N2-type attack of the carbonyl oxygen.

A new type of donor–acceptor cyclopropane with gem-difluorine as an unconventional donor group undergoes (3 + 2)-cycloadditions with various aldehydes/ketones, affording densely functionalized gem-difluorotetrahydrofurans.

Reversal of regioselectivity in reactions of donor-acceptor cyclopropanes with electrophilic olefins

Reactivity of donor-acceptor cyclopropanes towards nucleophiles and electrophiles is determined by the specific philicity of the carbon atoms originating from the strong polarization of the central C-C bond. Herein, we report that vitamin B₁₂ catalysis enables the transformation of an initially electrophilic center into a nucleophilic radical that reacts with SOMOphiles. This radical-based strategy reverses the standard regioselectivity and thus complements the classical approaches.

Cycloadditions of Donor-Acceptor Cyclopropanes and -butanes using S=N-Containing Reagents: Access to Cyclic Sulfinamides, Sulfonamides, and Sulfinamidines

We present (3+2)‐ and (4+2)‐cycloadditions of donor–acceptor (D–A) cyclopropanes and cyclobutanes with N‐sulfinylamines and a sulfur diimide, along with a one‐pot, two‐step strategy for the formal insertion of HNSO₂ into D–A cyclopropanes. These are rare examples of cycloadditions with D–A cyclopropanes and cyclobutanes whereby the 2π component consists of two different heteroatoms, thus leading to five‐ and six‐membered rings containing adjacent heteroatoms.

Friedel-Crafts-Type Reactions with Electrochemically Generated Electrophiles from Donor-Acceptor Cyclopropanes and -Butanes

We describe a general electrochemical method to functionalize donor-acceptor (D-A) cyclopropanes and -butanes with arenes utilizing Friedel-Crafts-type reactivity. The catalyst-free strategy relies on the direct anodic oxidation of the strained carbocycles, which leads after C(sp³)-C(sp³) cleavage to radical cations that act as electrophiles for the arylation reaction. Broad reaction scopes in regard to cyclopropanes, cyclobutanes, and aromatic reaction partners are presented. Additionally, a plausible electrolysis mechanism is proposed.

Electrocatalytic Activation of Donor-Acceptor Cyclopropanes and Cyclobutanes: An Alternative C(sp3 )-C(sp3 ) Cleavage Mode

We describe the first electrochemical activation of D-A cyclopropanes and D-A cyclobutanes leading after C(sp3 )-C(sp3 ) cleavage to the formation of highly reactive radical cations. This concept is utilized to formally insert molecular oxygen after direct or DDQ-assisted anodic oxidation of the strained carbocycles, delivering β- and γ-hydroxy ketones and 1,2-dioxanes electrocatalytically. Furthermore, insights into the mechanism of the oxidative process, obtained experimentally and by additional quantum-chemical calculations are presented. The synthetic potential of the reaction products is demonstrated by diverse derivatizations.

Copper-Catalyzed Ring-Opening 1,3-Aminotrifluoromethylation of Arylcyclopropanes

The copper-catalyzed reaction of arylcyclopropanes, N-fluorobis(arenesulfonyl)imides, and (bpy)Zn(CF₃)₂ (bpy = 2,2'-bipyridine) at room temperature affords the corresponding ring-opening 1,3-aminotrifluoromethylation products in satisfactory yields. The protocol is highly regioselective, providing a convenient entry to γ-trifluoromethylated amines. A mechanism involving the trifluoromethylation of benzyl radicals is proposed.

Umpolung of donor–acceptor cyclopropanes via N-heterocyclic carbene organic catalysis

A carbene-catalyzed formal umpolung of donor–acceptor (D–A) cyclopropanes is disclosed with chiral spirocyclic lactones bearing multiple functional groups afforded with excellent enantio- and diastereoselectivities.

Synthesis of (Het)aryl 2-(2-hydroxyaryl)cyclopropyl Ketones

A simple general method for the synthesis of 1-acyl-2-(ortho-hydroxyaryl)cyclopropanes, which belong to the donor–acceptor cyclopropane family, has been developed. This method, based on the Corey–Chaykovsky cyclopropanation of 2-hydroxychalcones, allows for the preparation of a large diversity of hydroxy-substituted cyclopropanes, which can serve as promising building blocks for the synthesis of various bioactive compounds.

Stereoselective Synthesis of Tropanes via a 6π-Electrocyclic Ring-Opening/ Huisgen [3+2]-Cycloaddition Cascade of Monocyclopropanated Heterocycles

The synthesis of tropanes via a microwave-assisted, stereoselective 6π-electrocyclic ring-opening/ Huisgen [3+2]-cycloaddition cascade of cyclopropanated pyrrole and furan derivatives with electron-deficient dipolarophiles is demonstrated. Starting from furans or pyrroles, 8-aza- and 8-oxabicyclo[3.2.1]octanes are accessible in two steps in dia- and enantioselective pure form, being versatile building blocks for the synthesis of pharmaceutically relevant targets, especially for new cocaine analogues bearing various substituents at the C-6/C-7 positions of the tropane ring system. Moreover, the 2-azabicyclo[2.2.2]octane core (isoquinuclidines), being prominently represented in many natural and pharmaceutical products, is accessible via this approach.

Multicomponent, Tandem 1,3- and 1,4-Bisarylation of Donor-Acceptor Cyclopropanes and Cyclobutanes with Electron-Rich Arenes and Hypervalent Arylbismuth Reagents

A tandem catalytic process for 1,3- and 1,4-bisarylation of donor-acceptor (D-A) cyclopropanes and cyclobutanes is disclosed. This strategy capitalizes on the use of two distinct sources of nucleophilic and electrophilic arylating agents, affording the formation of two new C-C bonds in an orchestrated multicomponent fashion with the aid of a catalytic Lewis acid. Mechanistic investigations have revealed it to be a stereoselective process, and products could be easily elaborated into other useful compounds.

Uncovering the Neglected Similarities of Arynes and Donor-Acceptor Cyclopropanes

Arynes and donor-acceptor (D-A) cyclopropanes are two classes of strained systems having the potential for numerous applications in organic synthesis. The last two decades have witnessed a renaissance of interest in the chemistry of these species primarily because of the mild and robust methods for their generation or activation. Commonly, arynes as easily polarizable systems result in 1,2-disubstitution, whereas D-A cyclopropanes as polarized systems lead to 1,3-bisfunctionalization thereby showing striking similarities. Transformations with 1,2- and 1,3-dipoles afford cyclic structures. With arynes, emerging four-membered rings as intermediates might react further, whereas the analogous five-membered rings obtained from D-A cyclopropanes are most often the final products. However, there are a few cases where these intermediates behave surprisingly differently. This Minireview highlights the parallels in reactivity between arynes and D-A cyclopropanes thereby shedding light on the neglected similarities of these two reactive species.

Photo/N-Heterocyclic Carbene Co-catalyzed Ring Opening and γ-Alkylation of Cyclopropane Enal

An unprecedented photo/NHC-co-catalyzed ring-opening C-C bond cleavage of cyclopropane enal and the following γ-alkylation with a halogenated compound via radicals were established, affording the corresponding γ-alkylated α,β-unsaturated esters in moderate to good yields.

DBU-promoted ring-opening reactions of multi-substituted donor–acceptor cyclopropanes: access to functionalized chalcones with a quaternary carbon group

A strategy to synthesize highly stereoselective chalcones with alkylcyanoacetate subunits via DBU-promoted ring-opening reactions of multi-substituted D–A cyclopropanes has been developed without the requirement of a transition metal catalyst and extra solvent.

Ring-Opening 1,3-Aminochalcogenation of Donor-Acceptor Cyclopropanes: A Three-Component Approach

A 1,3-aminothiolation was realized by reacting 2-substituted cyclopropane 1,1-dicarboxylates with sulfonamides and N-(arylthio)succinimides. Under Sn(OTf)2 catalysis the transformation proceeded smoothly to the corresponding ring-opened products bearing the sulfonamide in the 1-position next to the donor and the arylthio residue in the 3-position next to the acceptor. The procedure was extended to the corresponding selenium analogues by employing N-(phenylseleno)succinimides as an electrophilic selenium source.

Ring-Opening Reactions of Donor-Acceptor Cyclobutanes with Electron-Rich Arenes, Thiols, and Selenols

Donor-acceptor (D-A) cyclobutanes with two geminal ester groups as acceptors are reacted with electron-rich arenes as nucleophiles to afford ring-opened products. AlCl₃ mediates this Friedel-Crafts-type reaction. A variety of donors and electron-rich arenes are used. Nucleophilic thiols and selenols also trigger this ring-opening reaction. Furthermore, a comparison of various physical parameters has been carried out for several D-A cyclobutanes.

New Dyes Based on Extended Fulvene Motifs: Synthesis through Redox Reactions of Naphthoquinones with Donor-Acceptor Cyclopropanes and Their Spectroelectrochemical Behavior

Novel dyes based on extended fulvene motifs are reported. The carbon skeleton was generated by a catalyzed addition of donor-acceptor cyclopropanes to naphthoquinone. The hydroxy group at the central ring of the tricyclic fulvene motif was converted into the triflate, which reacted efficiently with a wide range of nucleophiles, resulting in substitution and thereby providing new derivatives. The synthetic versatility allowed us to investigate the absorption, electrochemical, and UV/Vis-NIR spectroelectrochemical properties of these dyes as a function of the substituents. The dyes were shown to participate in reductive electrochemistry, the reversibility of which can be improved by appropriate selection of the substituents. Additionally, first signs of NIR electrochromism are presented, opening new avenues for the future investigations of such dyes.

Regio- and Stereoselective Synthesis of Functionalized Dihydropyridines, Pyridines, and 2H-Pyrans: Heck Coupling of Monocyclopropanated Heterocycles

A palladium-catalyzed coupling between aryl halides and monocyclopropanated pyrroles or furans has been developed, leading to valuable six-membered N- and O-heterocycles. As the key step, a selective cleavage of the non-activated endocyclic C-C bond of the 2-heterobicyclo-[3.1.0]hexane framework is achieved. The developed method offers access to highly functionalized piperidines, pyridines, and pyrans that are challenging to access by traditional methods.