Oxidative C-C Bond Functionalization of Methylenecyclopropanes with Aldehydes for the Formation of 2-Acyl-3,4-dihydronaphthalenes

Regioselective synthesis of N-containing polycyclic compounds via radical annulation cyclization of 1,7-dienes with aldehydes

A convenient method for oxidant-promoted radical cascade acylation or decarbonylative alkylation of 1,7-dienes with aldehydes has been established. This method allows for the rapid construction of N-containing polycyclic skeletons in a highly regio- and stereoselective manner. This transformation provides a simple and efficient method for the preparation of a range of tetrahydro-6H-indeno[2,1-c]quinolinone derivatives by sequential formation of three new carbon-carbon bonds. Additionally, this radical cascade cyclization can selectively convert aldehydes into aroyl/primary aliphatic acyl radicals and secondary or tertiary alkyl radicals.

Recent advances in ring-opening of cyclobutanone oximes for capturing SO2, CO or O2via a radical process

Cyclobutanone oximes and their derivatives are pivotal core structural motifs in organic chemistry. Iminyl-radical-triggered C-C bond cleavage of cyclobutanone oximes delivers an efficient strategy to produce stable distal cyano-substituted alkyl radicals, which can capture SO2, CO or O2 to form cyanoalkylsulfonyl radicals, cyanoalkylcarbonyl radicals or cyanoalkoxyl radicals under mild conditions. In the past several years, cyanoalkylsulfonylation/cyanoalkylcarbonyaltion/cyanoalkoxylation has attracted a lot of interest. In this updated report, the strategies for trapping SO2, CO or O2via iminyl-radical-triggered ring-opening of cyclobutanone oximes are summarized.

Visible-Light-Promoted Decarboxylative Alkylation/Cyclization of Vinylcycloalkanes

An efficient strategy for visible-light-promoted decarboxylative alkylation of vinylcyclopropanes with alkyl N-(acyloxy)phthalimide esters through the dual C-C bond and single N-O bond cleavage, employing triphenylphosphine and lithium iodide as the photoredox system to synthesize 2-alkylated 3,4-dihydronaphthalenes, has been established. This alkylation/cyclization involves a radical process and undergoes a sequence of N-(acyloxy)phthalimide ester single-electron reduction, N-O bond cleavage, decarboxylative, alkyl radical addition, C-C bond cleavage, and intramolecular cyclization. Moreover, using the photocatalyst Na₂-Eosin Y instead of triphenylphosphine and lithium iodide, the vinyl transfer products are acquired when vinylcyclobutanes or vinylcyclopentanes are utilized as alkyl radical receptors.

Visible-light-induced reactions of methylenecyclopropanes (MCPs)

Diverse, visible-light-induced transformations of methylenecyclopropanes (MCPs) have been reported in recent years, attracting significant attention from synthetic chemists. As readily accessible strained molecules, MCPs have sufficient reactivity to selectively generate different target products, through reactions with various radical species upon visible-light irradiation under regulated reaction conditions. These transformations can be classified into three subcategories of reaction pathway, forming ring-opened products, cyclopropane derivatives, and alkynes. These products include pharmaceutical intermediates and polycyclic/heterocyclic compounds that are challenging to obtain using traditional methods. This review summarizes the recent advancements in this field.

Oxidative functionalization of alkylidenecyclopropanes and alkylidenecyclobutanes: a versatile platform to access nitrated cyclopropanes and cyclobutanes

A divergent radical nitration of alkylidenecyclopropanes (ACPs) and alkylidenecyclobutanes (ACBs) with Fe(NO₃)₃·9H₂O or AgNO₂ has been achieved, affording three categories of products including β-nitro alcohol, α-nitro ketone and nitro nitratosation products with yields up to 90%. Particularly, the cyclopropyl and cyclobutyl rings were conserved in the products. The applicability of this method was demonstrated by the scale-up experiment and reduction of the nitro into an amino group. Preliminary mechanistic studies suggested that the nitro radical was involved in the reaction process.

Metal-/solvent-free oxidative [4+2]/[3+2] annulation of 2-ethynylbenzaldehydes with arylalkenes: Facile synthesis of benzo[a]fluoren-5-ones

We report here a DTBP-mediated oxidative [4+2]/[3+2] annulation between 2-alkynylbenzaldehydes and terminal arylalkenes enabled by aldehyde C(sp2)-H functionalization for assembling a range of benzo[a]fluoren-5-ones with high step-economy, simple operations under...

Visible-Light-Induced Photoannulation of α-Naphthyl Cyclopropane Carboxylic Esters to Functionalized Dihydrophenalenes

A general synthetic entry to functionalized dihydrophenalenes has been found using naphthyl-cyclopropane esters as starting materials. The desired annulation was possible with visible light, Ir(Fppy)₃ as photocatalyst, BnNMe₂ or DABCO as electron donor, HAT-catalyst, and proton source. A broad scope of substituted naphthyl and azanaphthyl derivatives provided the photoannulation products in high yield. Deuteration studies support a photoredox mechanism involving the photoreductive cyclopropane opening to an enolate radical followed by an aryl radical trapping.

Photocatalytic Decarboxylative [3 + 2] and [4 + 2] Annulation of Enynals and γ,σ-Unsaturated N-(Acyloxy)phthalimides by NaI/PPh3 Catalysis

A practical and eco-friendly strategy for the radical-mediated decarboxylative [3 + 2] and [4 + 2] annulation of enynals and γ,σ-unsaturated N-(acyloxy)phthalimides through the photoactivation of an electron donor-acceptor (EDA) complex has been developed. A wide range of primary, secondary, and tertiary alkyl N-hydroxyphthalimide (NHP) esters can be used as suitable substrates for the synthesis of fused ketones without any transition-metal catalysts or oxidants. This protocol features a broad substrate scope, excellent selectivity, and clean reaction conditions.

Visible-Light-Induced Transition-Metal-Free Nitrogen-Centered Radical Strategy for the Synthesis of 2-Acylated 9H-Pyrrolo[1,2-a]indoles

A convenient and efficient visible-light-induced tandem acylation/cyclization of N-propargylindoles with aryl- or alkyl-substituted acyl oxime esters for the synthesis of 2-acyl-substituted 9H-pyrrolo[1,2-a]indoles under transition-metal-free conditions, which proceeds via nitrogen-centered radical-mediated cleavage of the C-C σ-bond in acyl oxime esters, is established. The aryl or alkyl acyl radicals, which come from acyl oxime esters, attack the C-C triple bonds in N-propargylindoles and then go through intramolecular cyclization/isomerization.

SbCl3 initiated conjunctive C-H bond functionalization and carbochlorination between glycine esters and methylenecyclopropanes (MCPs)

In the presence of dioxygen, an antimony trichloride enabled conjunctive sp³ C-H bond functionalization and carbochlorination of glycines was realized, providing a series of chlorinated quinolines in high yields. The mechanistic study shows that the antimony(V) species might be involved in the oxidation of the sp³ C-H bond and is followed by carbochlorination through a radical intermediate.

Cascade Reactions Assisted by Microwave Irradiation: Ultrafast Construction of 2-Quinolinone-Fused γ-Lactones from N-(o-Ethynylaryl)acrylamides and Formamide

An ultrafast (10 s) methodology to construct novel highly functionalized 2-quinolinones from N-(o-ethynylaryl)acrylamides (1,7-enynes) is described for the first time. Microwave irradiation enabled the ultrafast synthesis of 2-quinolinone-fused γ-lactones from Fenton's reagents in formamide. After six key consecutive reactions, including a diastereoselective step, 2-quinolinone-fused γ-lactones were obtained in good overall yield (up to 46%; 10 s).

Recent advances in acyl radical enabled reactions between aldehydes and alkenes

Radical-mediated functionalization of alkenes has been emerging as an elegant and straightforward protocol to increase molecule complexity. Moreover, the abstraction of a hydrogen atom from aldehydes to afford acyl radicals has evolved as a rising star due to its high atom-economy and the ready availability of aldehydes. Considering the great influence and synthetic potential of acyl radical enabled reactions between aldehydes and alkenes, we provide a summary of the state of the art in this field with a specific emphasis on the working models and corresponding mechanisms. The discussion is divided according to the kind of alkenes and reaction type.

Intramolecular difunctionalization of methylenecyclopropanes tethered with carboxylic acid by visible-light photoredox catalysis

We have developed a visible-light photoredox catalyzed intramolecular difunctionalization of MCPs to access spiro[cyclopropane-1,2-indan]one from easily prepared methylenecyclopropanes tethered with carboxylic acid.

Visible-light-promoted divergent functionalizations of methylenecyclopropanes

Visible-light-induced monofunctionalization and difunctionalization of MCPs via ring-opening and cyclization processes have been developed for the syntheses of difluoromethyl compounds, alkyl compounds, halides, and sulfonyl compounds.

Recent Advances on Synthetic Methodology Merging C-H Functionalization and C-C Cleavage

The functionalization of C-H bonds has become a major thread of research in organic synthesis that can be assessed from different angles, for instance depending on the type of catalyst employed or the overall transformation that is carried out. This review compiles recent progress in synthetic methodology that merges the functionalization of C-H bonds along with the cleavage of C-C bonds, either in intra- or intermolecular fashion. The manuscript is organized in two main sections according to the type of substrate in which the cleavage of the C-C bond takes place, basically attending to the scission of strained or unstrained C-C bonds. Furthermore, the related research works have been grouped on the basis of the mechanistic aspects of the different transformations that are carried out, i.e.,: (a) classic transition metal catalysis where organometallic intermediates are involved; (b) processes occurring via radical intermediates generated through the use of radical initiators or photochemically; and (c) reactions that are catalyzed or mediated by suitable Lewis or Brønsted acid or bases, where molecular rearrangements take place. Thus, throughout the review a wide range of synthetic approaches show that the combination of C-H and C-C cleavage in single synthetic operations can serve as a platform to achieve complex molecular skeletons in a straightforward manner, among them interesting carbo- and heterocyclic scaffolds.

Copper-promoted cyanoalkylation/ring-expansion of vinylcyclopropanes with α-C-H bonds in alkylnitriles toward 3,4-dihydronaphthalenes

A copper-promoted oxidative cyanomethylation/ring-expansion of vinylcyclopropanes with α-C(sp3)-H bonds in alkyl nitriles is established for the generation of 1-cyanoethylated 3,4-dihydronaphthalenes. This cyanomethylation/ring-expansion involves a radical pathway and proceeds via cyanomethyl radical formation, radical addition and ring-expansion. This ring-expansion strategy offers a highly atom-economical route for the construction of nitrile-containing 3,4-dihydronaphthalenes, which can be transformed into other useful products under simple conditions.

Silver-promoted oxidative sulfonylation and ring-expansion of vinylcyclopropanes with sodium sulfinates leading to dihydronaphthalene derivatives

Silver-promoted sulfonylation and ring-expansion of vinylcyclopropanes with sodium sulfinates is established for the construction of 1-sulfonylmethylated 3,4-dihydronaphthalenes. This sulfonylation process involves a radical pathway, including sulfonyl radical formation, radical addition, ring-opening and cyclization. The 1-sulfonylmethylated 3,4-dihydronaphthalenes can be converted into other useful products.

Rapid construction of cyclopenta[b]naphthalene frameworks from propargylic alcohol tethered methylenecyclopropanes

We have developed a new synthetic methodology for the rapid construction of cyclopenta[b]naphthalene frameworks from the reaction of propargylic alcohol tethered methylenecyclopropanes with mesyl chloride in the presence of triethylamine through cascade cyclization. The reaction can be performed under mild conditions without the use of transition metals, affording the target products in moderate to good yields, and this cyclization reaction process can be scaled up to a gram-scale synthesis.

A highly efficient method for the construction of cyclopropane-containing dihydroindole derivatives from indolemethylenecyclopropanes with DIAD and DEAD

A highly efficient method for the construction of cyclopropane-containing dihydroindole derivatives from indolemethylenecyclopropanes with DIAD and DEAD has been disclosed. The transformation could occur under catalyst-free conditions at ambient temperature to afford dihyroindole derivatives in good yields. It has been proved that the strained moiety of methylenecyclopropane in the substrate of indolemethylenecyclopropane is critical and DFT calculations reveal that the reaction proceeds through a two-step pathway.

Visible-light photoredox-catalyzed dual C–C bond cleavage: synthesis of 2-cyanoalkylsulfonylated 3,4-dihydronaphthalenes through the insertion of sulfur dioxide

A novel visible-light photoredox-catalyzed dual C–C bond cleavage of methylenecyclopropanes and cycloketone oximes for accessing 2-cyanoalkylsulfonated 3,4-dihydronaphthalenes is established.

Lewis or Brønsted acid-catalysed reaction of propargylic alcohol-tethered alkylidenecyclopropanes with indoles and pyrroles for the preparation of polycyclic compounds tethered with indole or pyrrole motif

We developed a facile synthetic method to access the cyclopenta[b]naphthalene skeleton from Lewis or Brønsted acid-catalysed propargylic alcohol-tethered alkylidenecyclopropanes with indole and pyrrole derivatives.

Oxidative radical ring-opening/cyclization of cyclopropane derivatives

The ring-opening/cyclization of cyclopropane derivatives has drawn great attention in the past several decades. In this review, recent efforts in the development of oxidative radical ring-opening/cyclization of cyclopropane derivatives, including methylenecyclopropanes, cyclopropyl olefins and cyclopropanols, are described. We hope this review will be of sufficient interest for the scientific community to further advance the application of oxidative radical strategies in the ring-opening/cyclization of cyclopropane derivatives.

Silver-mediated oxidative C–C bond sulfonylation/arylation of methylenecyclopropanes with sodium sulfinates: facile access to 3-sulfonyl-1,2-dihydronaphthalenes

The novel AgNO₃-mediated sulfonylation/arylation of a C–C σ-bond in MCPs with sodium sulfinates to synthesize 3-sulfonylated 1,2-dihydronaphthalenes is reported.