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

Strain-Release-Driven Electrochemical Skeletal Rearrangement of Non-Biased Alkyl Cyclopropanes/Butanes

Capitalizing the inherent strain energy within molecules, strain-release-driven reactions have been widely employed in organic synthesis. Small cycloalkanes like cyclopropanes and cyclobutanes, with their moderate ring strain, typically require dense functionalization to induce bias or distal activation of (hetero) aromatic rings via single-electron oxidation for relieving the tension. In this study, we present a pioneering direct activation of alkyl cyclopropanes/butanes through electrochemical oxidation. This approach not only showcases the potential for ring-opening of cyclopropane/butane under electrochemical conditions but also streamlines the synthesis of diverse oxazolines and oxazines. The applicability of our method is exemplified by its broad substrate scopes. Notably, the products derived from cyclobutanes undergo a formal ring contraction to cyclopropanes, introducing an intriguing aspect to our discoveries. These discoveries mark a significant advancement in strain-release-driven skeletal rearrangement reactions of moderately strained rings, offering sustainable and efficient synthetic pathways for future endeavours.

Electrochemical Deconstructive Methoxylation of Arylalcohols-A Synthetic and Mechanistic Investigation

Herein, we report a mechanistic investigation of a recently developed electrochemical method for the deconstructive methoxylation of arylalcohols. A combination of synthetic, electroanalytical, and computational experiments have been performed to gain a deeper understanding of the reaction mechanism and the structural requirements for fragmentation to occur. It was found that 2-arylalcohols undergo anodic oxidation to form the corresponding aromatic radical cations, which fragment to form oxocarbenium ions and benzylic radical intermediates via mesolytic cleavage, with further anodic oxidation and trapping of the benzylic carbocation with methanol to generate the observed methyl ether products. It was also found that the electrochemical fragmentation of 2-arylalkanols is promoted by structural features that stabilize the oxocarbenium ions and/or benzylic radical intermediates formed upon mesolytic cleavage of the aromatic radical cations. With an enhanced understanding of the reaction mechanism and the structural features that promote fragmentation, it is anticipated that alternative electrosynthetic transformations will be developed that utilize this powerful, yet underdeveloped, mode of substrate activation.

Electrochemical Synthesis of C(sp3)-Rich Heterocycles via Mesolytic Cleavage of Anodically Generated Aromatic Radical Cations

Herein we report an electrochemical deconstructive functionalization approach for the synthesis of C(sp3)-rich heterocycles. The reaction proceeds via the mesolytic cleavage of anodically generated aromatic radical cations and the trapping of formed carbocation intermediates with internal nucleophiles. The method has been demonstrated across various arylalcohol substrates to access a diverse range of C(sp3)-rich heterocycles including tetrahydrofuran, tetrahydropyran, and pyrrolidine scaffolds (26 examples). The electrochemical method was demonstrated on a 5 mmol scale via single pass continuous flow, which utilized lower supporting electrolyte concentration and exhibited increased productivity in relation to the batch process.

Electrochemical deconstructive functionalization of arylcycloalkanes via fragmentation of anodically generated aromatic radical cations

This highlight summarises electrochemical approaches for the deconstructive functionalization of arylcycloalkanes via the fragmentation of anodically generated aromatic radical cations. A diverse range of deconstructive functionalization processes is described, including discussion on the electrochemical reaction conditions employed, scope and limitations, and reaction mechanisms, in addition to highlighting future opportunities in this burgeoning area of sustainable synthesis.

Converting Strain Release into Aromaticity Loss for Activation of Donor-Acceptor Cyclopropanes: Generation of Quinone Methide Traps for C-Nucleophiles

Here, we present a new approach for the activation of donor-acceptor cyclopropanes in ring-opening reactions, which does not require the use of a Lewis or Brønsted acid as a catalyst. Donor-acceptor cyclopropanes containing a phenolic group as the donor undergo deprotonation and isomerization to form the corresponding quinone methides. This innovative strategy was applied to achieve (4 + 1)-annulation of cyclopropanes with sulfur ylides, affording functionalized dihydrobenzofurans. Additionally, the generated ortho- and para-(aza)quinone methides can be trapped by various CH-acids.

Electrooxidative 1,3-Oxo/Carboamination of Arylcyclopropanes

Herein, the work demonstrates an electrochemically paired electrolysis approach facilitating the efficient achievement of the electrooxidative 1,3-oxo/carboamination of arylcyclopropanes under mild conditions. The formation of 1,3-arylamination of arylcyclopropanes involves commercially available amine redox mediators through a radical-radical process. In addition, the successful execution of β-amino ketones also occurs under atmospheric conditions. The control experiments supported the existence of key benzylic radical intermediates in the reaction pathway.

Cooperative Photoredox and Cobalt-Catalyzed Acceptorless Dehydrogenative Functionalization of Cyclopropylamides towards Allylic N,O-Acyl-acetal Derivatives

We disclose herein a novel photoredox and cobalt co-catalyzed ring-opening/acceptorless dehydrogenative functionalization of mono-donor cyclopropanes. This sustainable and atom-economic approach allows the rapid assembly of a wide range of allylic N,O-acyl-acetal derivatives. The starting materials are readily available and the reaction features mild conditions, broad substrate scope, and excellent functional group compatibility. The optimized conditions accommodate assorted cycloalkylamides and primary, secondary, and tertiary alcohols, with applications in late-stage functionalization of pharmaceutically relevant compounds, stimulating further utility in medicinal chemistry. Moreover, selective nucleophilic substitutions with various carbon nucleophiles were achieved in a one-pot fashion, offering a reliable avenue to access some cyclic and acyclic derivatives.

An Intermolecular Hydroarylation of Unactivated Arylcyclopropane via Re2O7/HFIP-Mediated Ring Opening

In this paper, we describe a Re2O7-mediated ring-opening arylation of unactivated arylcyclopropane because of its functionalization with various arenes via Friedel-Crafts-type reactivity. This protocol allows facile access to functionalized 1,1-diaryl alkanes and is characterized by a broad substrate scope, mild reaction conditions, high efficiency, and high atom economy. Both density functional theory calculations and deuterium labeling experiments were carried out to justify the indispensable role of HFIP in this transformation and pointed to Re2O7-mediated ring opening being the rate-determining step.

Electrochemical Synthesis and Reactivity of Three-Membered Strained Carbo- and Heterocycles

Three-membered carbocyclic and heterocyclic ring structures are versatile synthetic building blocks in organic synthesis with biological importance. Moreover, the inherent strain of these three-membered rings leads to their ring-opening functionalization through C->C, C->N, and C-O bond cleavage. Traditional synthesis and ring-opening methods for these molecules require the use of acid catalysts or transition metals. Recently, electro-organic synthesis has emerged as a powerful tool for initiating new chemical transformations. In this review, the synthetic and mechanistic aspects of electro-mediated synthesis and ring-opening functionalization of three-membered carbo- and heterocycles are highlighted.

Electrochemical 1,3-Alkyloxylimidation of Arylcyclopropane Radical Cations: Four-Component Access to Imide Derivatives

Herein, a general electrochemical radical-cation-mediated four-component ring-opening 1,3-alkyloxylimidation of arylcyclopropanes, acetonitrile, carboxylic acids, and alcohols is described, providing a facile and sustainable approach to quickly construct structurally diverse imide derivatives from easily available raw materials in an operationally simple undivided cell. This metal-catalyst- and oxidant-free single-electron oxidation strategy offers a green alternative for the formation of highly reactive cyclopropane-derived radical cations, and this protocol features a broad functional group tolerance.

Electrochemical Oxidation Enables Regioselective 1,3-Hydroxyfunctionalization of Cyclopropanes

The direct construction of 1,3-hydroxyfunctionalized molecules is still a significant challenge, as they can currently be obtained through multiple synthetic steps. Herein, we report a general and efficient 1,3-hydroxyfunctionalization of arylcyclopropanes by electrochemical oxidation with a strategic choice of nucleophiles and H2O. 1,3-Amino alcohols, 1,3-alkynyl alcohols, 1,3-hydroxyesters, and 1,3-halo alcohols are achieved with high levels of chemo- and regio-selectivity, opening a new dimension for 1,3-difunctionalization reaction.

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.

Electroreductively Induced Radicals for Organic Synthesis

Organic electrochemistry has attracted tremendous interest within the novel sustainable methodologies that have not only reduced the undesired byproducts, but also utilized cleaner and renewable energy sources. Particularly, oxidative electrochemistry has gained major attention. On the contrary, reductive electrolysis remains an underexplored research direction. In this context, we discuss advances in transition-metal-free cathodically generated radicals for selective organic transformations since 2016. We highlight the electroreductive reaction of alkyl radicals, aryl radicals, acyl radicals, silyl radicals, fluorosulfonyl radicals and trifluoromethoxyl radicals.

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.

Photoredox-catalyzed C-C bond cleavage of cyclopropanes for the formation of C(sp3)-heteroatom bonds

Sterically congested C-O and C-N bonds are ubiquitous in natural products, pharmaceuticals, and bioactive compounds. However, the development of a general method for the efficient construction of those sterically demanding covalent bonds still remains a formidable challenge. Herein, a photoredox-driven ring-opening C(sp³)-heteroatom bond formation of arylcyclopropanes is presented, which enables the construction of structurally diversified while sterically congested dialkyl ether, alkyl ester, alcohol, amine, chloride/fluoride, azide and also thiocyanate derivatives. The selective single electron oxidation of aryl motif associated with the thermodynamic driving force from ring strain-release is the key for this transformation. By this synergistic activation mode, C-C bond cleavage of otherwise inert cyclopropane framework is successfully unlocked. Further mechanistic and computational studies disclose a complete stereoinversion upon nucleophilic attack, thus proving a concerted S_N2-type ring-opening functionalization manifold, while the regioselectivity is subjected to an orbital control scenario.

Reductive opening of a cyclopropane ring in the Ni(II) coordination environment: a route to functionalized dehydroalanine and cysteine derivatives

The involvement of an α,α-cyclopropanated amino acid in the chiral Ni(II) coordination environment in the form of a Schiff base is considered as a route to electrochemical broadening of the donor-acceptor cyclopropane concept in combination with chirality induction in the targeted products. A tendency to the reductive ring-opening and the follow-up reaction paths of thus formed radical anions influenced by substituents in the cyclopropane ring are discussed. Optimization of the reaction conditions opens a route to the non-proteinogenic amino acid derivatives containing an α-β or β-γ double C=C bond in the side chain; the regioselectivity can be tuned by the addition of Lewis acids. One-pot combination of the reductive ring opening and subsequent addition of thiols allows obtaining the cysteine derivatives in practical yields and with high stereoselectivity at the removed β-stereocenter.

Electrochemical Ring-Opening Dicarboxylation of Strained Carbon-Carbon Single Bonds with CO2: Facile Synthesis of Diacids and Derivatization into Polyesters

Diacids are important monomers in the polymer industry to construct valuable materials. Dicarboxylation of unsaturated bonds, such as alkenes and alkynes, with CO₂ has been demonstrated as a promising synthetic method. However, dicarboxylation of C─C single bonds with CO₂ has rarely been investigated. Herein we report a novel electrochemical ring-opening dicarboxylation of C─C single bonds in strained rings with CO₂. Structurally diverse glutaric acid and adipic acid derivatives were synthesized from substituted cyclopropanes and cyclobutanes in moderate to high yields. In contrast to oxidative ring openings, this is also the first realization of an electroreductive ring-opening reaction of strained rings, including commercialized ones. Control experiments suggested that radical anions and carbanions might be the key intermediates in this reaction. Moreover, this process features high step and atom economy, mild reaction conditions (1 atm, room temperature), good chemoselectivity and functional group tolerance, low electrolyte concentration, and easy derivatization of the products. Furthermore, we conducted polymerization of the corresponding diesters with diols to obtain a potential UV-shielding material with a self-healing function and a fluorine-containing polyester, whose performance tests showed promising applications.

Diastereoselective Synthesis of Tetrahydrobenzo[b]azocines by Lu(OTf)3 Catalyzed [4 + 4] Cycloaddition of Donor–Acceptor Cyclobutanes with Anthranils

The construction of N-heterocyclic eight-membered rings remains a challenging due to unfavorable transannular strain. Herein, we report a Lu(OTf)3 catalyzed formal [4 +4] cycloaddition reaction of cyclobutane 1,1-diesters with anthranils...

Lewis acid-catalyzed [4 + 2] cycloaddition of donor–acceptor cyclobutanes with iminooxindoles: access to spiro[piperidine-3,2′-oxindoles]

A Lewis acid-catalyzed [4 + 2] cycloaddition reaction from D–A cyclobutanes and iminooxindoles, providing the corresponding spiro[piperidine-3,2′-oxindoles] under mild conditions.

Lewis-Acid-Catalyzed (3+2)-Cycloadditions of Donor-Acceptor Cyclopropanes with Thioketenes

The reactivity of donor-acceptor (D-A) cyclopropanes towards thioketenes was investigated. In a (3+2)-cycloaddition using Sc(OTf)3 as a Lewis acidic catalyst, the corresponding exocyclic thioenol ethers (2-methylidene tetrahydrothiophenes) were formed in moderate to good yields. Unsymmetrical thioketenes provided E/Z mixtures at the double bond, with the Z isomer being preferred.