Iodine-catalyzed radical oxidative annulation for the construction of dihydrofurans and indolizines

Synergistic Copper-Aminocatalysis for Direct Tertiary α-Alkylation of Ketones with Electron-Deficient Alkanes

In this study, a novel approach for the tertiary α-alkylation of ketones using alkanes with electron-deficient C─H bonds is presented, employing a synergistic catalytic system combining inexpensive copper salts with aminocatalysis. This methodology addresses the limitations of traditional alkylation methods, such as the need for strong metallic bases, regioselectivity issues, and the risk of over alkylation, by providing a high reactivity and chemoselectivity without the necessity for pre-functionalized substrates. The dual catalytic strategy enables the direct functionalization of C(sp3)─H bonds, demonstrating remarkable selectivity in the presence of conventional C(sp3)─H bonds that are adjacent to heteroatoms or π systems, which are typically susceptible to single-electron transfer processes. The findings contribute to the advancement of alkylation techniques, offering a practical and efficient route for the construction of C(sp3)─C(sp3) bonds, and paving the way for further developments in the synthesis of complex organic molecules.

Palladium-Catalyzed [3 + 2] Cycloaddition of 4-Vinyl-4-butyrolactones with Ketenes Generated in Situ from Acyl Chlorides: A Method for the Synthesis of Dihydrofurans

In this paper, palladium-catalyzed [3 + 2] cycloaddition of 4-vinyl-4-butyrolactones with ketenes generated from easily available acyl chlorides was achieved. With Pd2(dba)3·CHCl3/XantPhos as the catalyst, the reaction proceeded smoothly under mild reaction conditions, affording a series of 2,3-dihydrofurans in moderate to high yields. The scale-up reaction and further transformations of the products are demonstrated, and a plausible mechanism is proposed as well.

Controllable Synthesis of N- and O-Containing Heterocycles via Formal [3 + 2] and [5 + 2] Cyclizations

The controllable synthesis of spirooxindole-dihydrofurans and spirooxindole-benzazepines was developed through formal [3 + 2] and [5 + 2] cyclization reactions from 2-(2-oxoindolin-3-yl)malononitriles and ortho-aminobenzaldehydes, respectively. A variety of spirooxindole-benzazepines were facilely constructed via a furan ring-open-involved hydride transfer/cyclization process. It is noteworthy that the application of the hydride-transfer-involved [5 + 2] cyclization strategy for construction of spirobenzazepines was unprecedented. In addition, the spiro N- and O-containing heterocycles were highly functionalized by amino, amide, and cyano groups, which were conducive to late-stage functionalization.

Rhodium(II)-Catalyzed Desaturative [3+2] Tandem Cyclization of Arylcycloalkanes with β-Dicarbonyls

Synthetically important scaffolds, fused tricyclic frameworks containing a 2,3-cyclo[b]dihydrofuran unit, play a crucial role in drug discovery. In this study, we demonstrate that rhodium(II)/N-fluorobenzenesulfonimide can catalyze the in situ generation of highly reactive alkene intermediates from commonly accessible alkanes, which undergo intermolecular [3+2] tandem cyclization with the simultaneously generated β-dicarbonyl radical to synthesize a series of fused tricyclic frameworks containing a 2,3-cyclo[b]dihydrofuran unit with a quaternary carbon center.

Organoelectrocatalysis Enables Direct Cyclopropanation of Methylene Compounds

Cyclopropane is a prevalent structural unit in natural products and bioactive compounds. While the transition metal-catalyzed alkene cyclopropanation of functionalized compounds such as α-diazocarbonyl derivatives has been well established and provides straightforward access to cyclopropanes, cyclopropanation directly from the more stable and simpler methylene compounds has remained an unsolved challenge despite the highly desirable benefits of minimal prefunctionalization and increased operational safety. Herein we report an electrocatalytic strategy for the cyclopropanation of active methylene compounds, employing an organic catalyst. The method shows a broad substrate scope and excellent scalability, requires no metal catalyst or external chemical oxidant, and provides convenient access to several types of cyclopropane-fused heterocyclic and carbocyclic compounds. Mechanistic investigations suggest that the reactions proceed through a radical-polar crossover process to form the two new carbon-carbon bonds in the nascent cyclopropane ring.

Copper(I)-Catalyzed Direct Oxidative Annulation of 1,3-Dicarbonyl Compounds with Maleimides: Access to Polysubstituted Dihydrofuran Derivatives

An efficient annulation method for the synthesis of polysubstituted dihydrofurans from 1,3-dicarbonyl compounds and maleimides is described. The reactions can afford furo[2,3-c]pyrrole derivatives with satisfactory yields. The developed strategy realizes the direct oxidative double C(sp³)-H functionalization in the presence of copper(I) salts and 2-(tert-butylperoxy)-2-methylpropane. Meanwhile, this protocol features a mild reaction condition and simple catalytic system. A reaction mechanism involving a single electron oxidation is also proposed.

Difluorodiazoethane as a masked acetylene equivalent in formal [3 + 2] cycloadditions with ketones to access 2,3-functionalized furans

A transition-metal-free [3+2] cycloaddition of CF2HCHN2 with β-ketones is reported, which enables the synthesis of 2,3-functionalized furans. Sequential defluorination, nucleophilic addition, and cyclization are key elemental steps of the reaction.

Cobalt-catalyzed modular assembling toward multi-functionalized furan derivatives

A cobalt-catalyzed modular [3+2] assembling of unsaturated hydrocarbons and β-dicarbonyls is reported. This protocol features mild reaction conditions and a broad substrate scope, providing facile entries toward diverse multi-functionalized dihydrofuran...

Electrochemically-initiated intramolecular 1,2-amino oxygenation of alkynes: facile access to formyl- and acyl-substituted indolizines

An environmentally benign electrooxidative approach to the intramolecular aminooxygenation of alkynes through an electrophilic cyclization reaction has been developed, providing an efficient approach toward diverse formyl- and acyl-substituted indolizines.

Gold-catalyzed formal (3 + 2) and (4 + 2) cycloaddition reactions using propiolates: assembly of 2,3-dihydrofurans and 3,4-dihydropyrans via a multistep cascade process

A gold-catalyzed formal dipolar cycloaddition reaction was developed using polarized alkynes as dipolarophiles and butenediol or pentenediol derivatives as formal dipoles. Silyl groups were used to solve the selectivity issue of unsymmetrical diols.

Straightforward Stereoselective Synthesis of Seven-Membered Oxa-Bridged Rings through In Situ Generated Cycloheptenol Derivatives

An iodine-mediated stereoselective synthesis of seven-membered oxa-bridged rings via in situ generated cycloheptenols was reported. This process was realized through the combination of C-C σ-bond cleavage and C-O bond-forming reactions in a one-pot fashion from simple and easily accessible raw materials. The formation of carbon radicals initiated by I₂ was the key to the reaction.

Blue Light-Emitting Diode-Mediated In Situ Generation of Pyridinium and Isoquinolinium Ylides from Aryl Diazoesters: Their Application in the Synthesis of Diverse Dihydroindolizine

Blue light-emitting diode-mediated environmentally sustainable three component reactions among pyridine/isoquinoline 1/2, aryl diazoesters 3, and acrylic ester/3-alkenyl oxindoles 5/6 provide various dihydroindolizines 7 to 9 in excellent yield. The principle of the strategy is photolytic generation of nitrogen ylides from N-heteroarenes and aryl diazoesters and their subsequent [3 + 2] cycloaddition reaction with dipolarophiles. Detailed mechanistic analysis of the transformation through control experiments establishes this strategy as the foundation for the photolytic multicomponent reaction.

Iron-Catalyzed Tertiary Alkylation of Terminal Alkynes with 1,3-Diesters via a Functionalized Alkyl Radical

Direct oxidative C(sp)-H/C(sp³ )-H cross-coupling offers an ideal and environmentally benign protocol for C(sp)-C(sp³ ) bond formations. As such, reactivity and site-selectivity with respect to C(sp³ )-H bond cleavage have remained a persistent challenge. Herein is reported a simple method for iron-catalyzed/silver-mediated tertiary alkylation of terminal alkynes with readily available and versatile 1,3-dicarbonyl compounds. The reaction is suitable for an array of substrates and proceeds in a highly selective manner even employing alkanes containing other tertiary, benzylic, and C(sp³ )-H bonds alpha to heteroatoms. Elaboration of the products enables the synthesis of a series of versatile building blocks. Control experiments implicate the in situ generation of a tertiary carbon-centered radical species.

The copper-catalyzed selective monoalkylation of active methylene compounds with alkylsilyl peroxides

A novel method for a mild copper-catalyzed selective monoalkylation of active methylene compounds with various alkylsilyl peroxides has been developed. The reaction has a broad substrate scope and our mechanistic studies suggest the participation of radical species in this alkylation reaction.

Palladium catalyzed synthesis of indolizines via the carbonylative coupling of bromopyridines, imines and alkynes

We report herein the development of a palladium-catalyzed, multicomponent synthesis of indolizines. The reaction proceeds via the carbonylative formation of a high energy, mesoionic pyridine-based 1,3-dipole, which can undergo spontaneous cycloaddition with alkynes. Overall, this provides a route to prepare indolizines in a modular fashion from combinations of commercially available or easily generated reagents: 2-bromopyridines, imines and alkynes.

Catalytic Cycloisomerization onto a Carbonyl Oxygen

We have found that terminal N-vinylindoles bearing cycloalkanone substituents are excellent hydrogen atom acceptors, generating α-aminyl radicals with a variety of catalysts (Co(II)/H2 or Co(III)Cl precatalysts with silane reductants). These radicals can be converted to internal vinylindoles but eventually add to the oxygen of the cycloalkanone substituents. These cyclizations eventually furnish a densely functionalized dihydrofuran (a net cycloisomerization). The internal vinylindoles are slowly converted to the dihydrofurans, but the final cycloisomerization/isomerization ratio is affected by the size of the cycloalkanone ring (seven- and eight-membered rings give the highest ratio). These results demonstrate how HAT can isomerize substrates in nonintuitive ways, here leading to the first HAT-promoted formation of a C-O bond.

Revealing the Iron-Catalyzed β-Methyl Scission of tert-Butoxyl Radicals via the Mechanistic Studies of Carboazidation of Alkenes

We describe here a mechanistic study of the iron-catalyzed carboazidation of alkenes involving an intriguing metal-assisted β-methyl scission process. Although t-BuO radical has frequently been observed in experiments, the β-methyl scission from a t-BuO radical into a methyl radical and acetone is still broadly believed to be thermodynamically spontaneous and difficult to control. An iron-catalyzed β-methyl scission of t-BuO is investigated in this work. Compared to a free t-BuO radical, the coordination at the iron atom reduces the activation energy for the scission from 9.3 to 3.9 ~ 5.2 kcal/mol. The low activation energy makes the iron-catalyzed β-methyl scission of t-BuO radicals almost an incomparably facile process and explains the selective formation of methyl radicals at low temperature in the presence of some iron catalysts. In addition, a radical relay process and an outer-sphere radical azidation process in the iron-catalyzed carboazidation of alkenes are suggested by density functional theory (DFT) calculations.

Photoredox Catalysis of Aromatic β-Ketoesters for in Situ Production of Transient and Persistent Radicals for Organic Transformation

Radical formation is the initial step for conventional radical chemistry. Reported herein is a unified strategy to generate radicals in situ from aromatic β-ketoesters by using a photocatalyst. Under visible-light irradiation, a small amount of photocatalyst fac-Ir(ppy)₃ generates a transient α-carbonyl radical and persistent ketyl radical in situ. In contrast to the well-established approaches, neither stoichiometric external oxidant nor reductant is required for this reaction. The synthetic utility is demonstrated by pinacol coupling of ketyl radicals and benzannulation of α-carbonyl radicals with alkynes to give a series of highly substituted 1-naphthols in good to excellent yields. The readily available photocatalyst, mild reaction conditions, broad substrate scope, and high functional-group tolerance make this reaction a useful synthetic tool.

Tritylium assisted iodine catalysis for the synthesis of unsymmetrical triarylmethanes

The combined Lewis acid catalytic system, generated from molecular iodine and tritylium tetrafluoroborate effectively catalyzed the Friedel-Crafts (FC) arylation of diarylmethyl sulfides providing an efficient access to various unsymmetrical triarylmethanes. The addition of tritylium and iodine created a more active catalytic system to promote the cleavage of sulfidic C-S bonds.

Copper-catalyzed formal [1 + 2 + 2]-annulation of alkyne-tethered diazoacetates and pyridines: access to polycyclic indolizines

A copper-catalyzed formal [1 + 2 + 2]-annulation of alkyne-tethered diazo compounds with pyridines, which affords polycyclic fused indolizine derivatives with broad substrate generality, has been reported.

DBU-Mediated Addition of α-Nitroketones to α-Cyano-enones and α,β-Unsaturated α-Ketoesters: Synthesis of Dihydrofurans and Conjugated Dienes

1,8-Diazabicyclo[5.4.0]undec-7-ene-mediated reactions of α-nitroketones with α-cyano-enones and α,β-unsaturated α-ketoesters have been developed. The products, namely, dihydrofurans and conjugated dienes, were isolated in moderate to good yields, and a range of substitutions were tolerated.

Mn-mediated sequential three-component domino Knoevenagel/cyclization/Michael addition/oxidative cyclization reaction towards annulated imidazo[1,2-a]pyridines

The sequential three-component reaction between o-hydroxybenzaldehydes, N-(cyanomethyl)pyridinium salts and a nucleophile towards substituted chromenoimidazopyridines under oxidative conditions has been developed. The employment of Mn(OAc)₃·2H₂O or KMnO₄ as stoichiometric oxidants allowed the use of a wide range of nucleophiles, such as nitromethane, (aza)indoles, pyrroles, phenols, pyrazole, indazole and diethyl malonate. The formation of the target compounds presumably proceeds through a domino Knoevenagel/cyclization/Michael addition/oxidative cyclization reaction sequence.

Catalyst-Free Annulation of 2-Pyridylacetates and Ynals with Molecular Oxygen: An Access to 3-Acylated Indolizines

A catalyst and additive-free annulation of 2-pyridylacetates and ynals under molecular oxygen was the first developed, affording 3-acylated indolizines in good to excellent yields. Molecular oxygen was used as the source of the carbonyl oxygen atom in indolizines. This approach was compatible with a wide range of functional groups, and especially it has been successfully extended to unsaturated double bonds and triple bonds, which were difficult to prepare by previous methods in a single step.

Iron-catalyzed carboazidation of alkenes and alkynes

Carboazidation of alkenes and alkynes holds the promise to construct valuable molecules directly from chemical feedstock therefore is significantly important. Although a few examples have been developed, there are still some unsolved problems and lack of universal methods for carboazidation of both alkenes and alkynes. Here we describe an iron-catalyzed rapid carboazidation of alkenes and alkynes, enabled by the oxidative radical relay precursor t-butyl perbenzoate. This strategy enjoys success with a broad scope of alkenes under mild conditions, and it can also work with aryl alkynes which are challenging substrates for carboazidation. A large number of diverse structures, including many kinds of amino acid precursors, fluoroalkylated vinyl azides, other specific organoazides, and 2H-azirines can be easily produced.

Carboazidation of alkenes and alkynes holds the promise to construct valuable, functionalized molecules. Here, the authors report a general and efficient iron-catalyzed carboazidation of both alkenes and alkynes enabled by t-butyl perbenzoate.

Synthesis of 2-acetyl trisubstituted furans via copper-mediated deacylation cleavage of unstrained C(sp3)–C(sp2) bonds

A copper-mediated tandem addition/cyclization/carbon–carbon cleavage reaction for the convenient synthesis of 2-acyl trisubstituted furans has been developed.