Pd(0)-Catalyzed radical aryldifluoromethylation of activated alkenes

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.

Synthesis of 3-substituted 2-oxindoles from secondary α-bromo-propionanilides via palladium-catalyzed intramolecular cyclization

In contrast to aromatic halides, coupling reactions involving oxidative addition of alkyl halides, especially secondary or tertiary halides, to transition metals tend to be more challenging. Herein a palladium-catalyzed intramolecular cyclization of α-bromo-propionanilides has been developed, delivering a series of 3-substituted 2-oxindoles in high yields. The method features easy to prepare starting materials, broad substrate scope and excellent functional group tolerance. A detailed mechanistic investigation has been performed.

Copper-Catalyzed Difluoroalkylation of Alkene/Nitrile Insertion/Cyclization Tandem Sequences: Construction of Difluorinated Bicyclic Amidines

A copper-catalyzed difluoroalkylation of an alkene/nitrile insertion/cyclization tandem sequence of N-cyanamide alkene was described, which provided a convenient synthetic approach for accessing difluorinated bicyclic amidines bearing imine moieties in a sustainable fashion. This protocol is characterized by high yields, a broad substrate scope, and good functional group compatibility. In addition, the desired product can be readily converted into other valuable functionalized fluorinated aza-heterocycles.

Electrooxidative dearomatization of biaryls: synthesis of tri- and difluoromethylated spiro[5.5]trienones

Radical spirocyclization via dearomatization has emerged as an attractive strategy for the rapid synthesis of structurally diverse spiro molecules. We report the use of electrochemistry to perform an oxidative dearomatization of biaryls leading to tri- and difluoromethylated spiro[5.5]trienones in a user friendly undivided cell set-up and a constant current mode. The catalyst- and chemical oxidant-free dearomatization procedure features ample scope, and employs electricity as the green and sole oxidant.

Palladium-catalyzed cascade 5-exo-trig radical cyclization/aromatic C-H alkylation with unactivated alkyl iodides

A novel and convenient palladium-catalyzed cascade 5-exo-trig radical cyclization/aromatic C-H alkylation with unactivated alkyl iodides has been described. This strategy provides an efficient access to a variety of 3a-methyl-1,2,3,3a,4,8b-hexahydroindeno[1,2-b]pyrrole derivatives, which facilitate access to a series of medically important heterocyclic bioactive molecules. This protocol involves mild catalytic reaction conditions and shows high functional group tolerance with high stereoselectivity. Mechanistic investigations reveal that an alkyl radical pathway is involved in this reaction.

Fluorination and fluoroalkylation of alkenes/alkynes to construct fluoro-containing heterocycles

This review summarize the established strategies through fluorination and fluoroalkylation of alkenes/alkynes for constructing fluoro-containing heterocycles. Reaction scopes, mechanisms and some shortcomings are also discussed.

Silver-catalyzed decarboxylative radical relay difluoroalkylation–carbocyclization: convenient access to CF2-containing quinolinones

A practical Ag-catalyzed formal decarboxylation and radical difluoroalkylation–carbocyclization–hydrolysis route is established to construct a series of structurally diverse CF2-containing N-heterocycles.

Photoredox initiated azole-nucleophilic addition: oxo-azolation of gem-difluoroalkenes

A protocol for photoredox initiated oxo-azolation of gem-difluoroalkenes has been demonstrated. The strategy offers a facile access to a series of synthetically useful azolated difluoroacetonarenes and tolerates a wide range of functional groups.

Copper-Catalyzed Bromodifluoroacetylative Cyclization of Enynes

A copper-catalyzed bromodifluoroacetylative cyclization reaction is described. The treatment of bromodifluoroacete derivatives by CuI and B₂Pin₂ enables difluoroalkyl radical generation and triggers the radical addition/cyclization/bromination sequences. Bromodifluoroacetyl-derived ester, amide, and ketone were compatible and gave various vinyl C-Br bonds containing functionalized heterocycles in good yields.

Metal-Catalyzed Spiroannulation-Fluoromethylfunctionaliztions of 1,5-Enynes for the Synthesis of Stereodefined (Z)-Spiroindenes

Two classes of new catalytic spiroannulation-fluormethylfunctionaliztions of para-quinone methide (p-QM)-containing 1,5-enynes have been established under redox-neutral conditions. Palladium-catalyzed spiroannulation-iododifluoromethylation with ethyl difluoroiodoacetate oriented completely stereoselective access to (Z)-spiroindenes and the latter included copper-catalyzed three-component spiroannulation-cyanotrifluoromethylation starting from Togni's reagent and trimethylsilanecarbonitrile (TMSCN). Both reaction pathways involve fluoroalkyl radical-triggered 1,6-addition/5-exo-dig annulation/metal radical cross-coupling/reductive elimination sequence, providing practical and stereoselective protocols for rapidly constructing cyclohexadienone-containing spiroindenes with generally good yields.

Pd-Catalyzed Domino Reactions Involving Alkenes To Access Substituted Indole Derivatives

Palladium-catalyzed domino reactions are advanced tools in achieving various nitrogen-containing heterocycles in an efficient and economical manner due to the reduced number of steps in the process. This review highlights recent advances in domino processes aimed at the synthesis of indole derivatives and polycyclic systems containing the indole nucleus in intra/intra- or intra/intermolecular reactions. In particular, we consider domino processes that involve a double bond in a step of the sequence, which allow the issue of regioselectivity in the cyclization to be faced and overcome. The different sections in this review focus on the synthesis of the indole nucleus and functionalization of the scaffold starting from different substrates that have been identified as activated starting materials, which involve a halogenated moiety or unactivated unsaturated systems. In the former case, the reaction is under Pd(0) catalysis, and in the second case a Pd(II) catalytic species is required and then an oxidant is necessary to reconvert the Pd(0) into the active Pd(II) species. On the other hand, the second method has the advantage that it uses easy available and inexpensive substrates.

1 Introduction

2 Indole Scaffold Synthesis

2.1 Activated Substrates

2.2 Unactivated Substrates

3 Functionalization of Indole Scaffold

3.1 Activated Substrates

3.2 Unactivated Substrates

4 Conclusions

Copper-catalyzed C–H [3 + 2] annulation of N-substituted anilines with α-carbonyl alkyl bromides via C(sp3)–Br/C(sp2)–H functionalization

A copper-catalyzed C–H [3 + 2] annulation of N-substituted anilines with α-carbonyl alkyl bromides for the synthesis of 3,3′-disubstituted oxindoles is developed.

Visible light photocatalytic acyldifluoroalkylation of unactivated alkenes for the direct synthesis of gem-difluorinated ketones

A photoredox-catalyzed alkene acyldifluoroalkylation with difluoroacetic acids for the direct synthesis of gem-difluorinated cyclic ketones is developed.

Synthesis of difluoromethylated 2-oxindoles and quinoline-2,4-diones via visible light-induced tandem radical cyclization of N-arylacrylamides

Visible light-induced difluoromethylation of N-arylacrylamides to afford difluoromethylated 2-oxindoles and quinoline-2,4-diones with difluoromethyl 2-pyridyl sulfones as radical precursors has been disclosed.

Pd-Catalyzed para-selective C–H difluoromethylation of aromatic carbonyls

A novel Pd-catalyzed para-selective C–H bond difluoromethylation of diverse aromatic carbonyls was developed.

Organic amine-mediated free-radical carbocyclization reactions of 2,2,2-trihalogeno-substituted N-(2-alkynylphenyl)acetamides

An efficient method for the synthesis of 3-halogeno-substituted 4-benzoylquinolin-2-(1H)-ones from N-(2-alkynylphenyl)-substituted trihaloacetamides has been developed, in which organic amines (TNPA and DIEA) act as the electron donors. In this carbocyclization reaction, a new C-C bond formation occurred regioselectively via a 6-exo-dig radical cyclization. A variety of useful functional groups are compatible with the reaction conditions. In this process, readily removable organic amines were employed and no heavy metal catalysts were required.

Ppm Cu Catalyst Enables tert-Alkylation Followed by C-H Cyclization To Synthesize Substituted Oxindoles

In this paper, we established highly efficient Cu-catalyzed tandem tert-alkylation C-H cyclization of α-bromocarbonyls and methacrylamides to produce substituted oxindoles. The maximum turnover number was up to 48 000 with reasonable yield. Although the catalyst loadings were very low, the reaction was not involving radical chain reaction. The resulting oxindoles were able to transform into aza-multicyclic compound via a reduction.

Transition-Metal-Free Synthesis of N-Hydroxy Oxindoles by an Aza-Nazarov-Type Reaction Involving Azaoxyallyl Cations

A novel transition-metal-free method to construct N-hydroxy oxindoles by an aza-Nazarov-type reaction involving azaoxyallyl cation intermediates is described. A variety of functional groups were tolerated under the weak basic reaction conditions and at room temperature. A one-pot process was also developed to make the reaction even more practical. This method provides alternative access to oxindoles and their biologically active derivatives.

Transition metal free decarboxylative fluoroalkylation of N-acrylamides with 3,3,3-trifluoro-2,2-dimethylpropanoic acid (TFDMPA)

1,1-Dimethyl-2,2,2-trifluoroethyl substituted oxindoles were prepared by a novel transition metal-free decarboxylative fluoroalkylation of activated alkenes and C–H functionalization cascade process.

Visible-light induced three-component alkynyl-difluoroalkylation of unactivated alkenes

A visible-light induced three-component alkynyl-fluoroalkylation of unactivated alkenes is described, giving a facile access to β-fluoroalkylated alkynes under room temperature conditions.

Computational Study of Ni-Catalyzed C-H Functionalization: Factors That Control the Competition of Oxidative Addition and Radical Pathways

The mechanisms of Ni-catalyzed C-H arylation, alkylation, and sulfenylation with N,N-bidentate directing groups are investigated using density functional theory (DFT) calculations. While the C-H cleavage occurs via the concerted metalation-deprotonation (CMD) mechanism in all types of reactions, the subsequent C-C and C-X bond formation steps may occur via either oxidative addition to form a Ni(IV) intermediate or radical pathways involving Ni(III) complexes generated from homolytic dissociation of disulfides/peroxides or halide-atom transfer from alkyl halides. DFT calculations revealed that radical mechanisms are preferred in reactions with sterically hindered coupling partners with relatively low bond dissociation energies (BDE) such as dicumyl peroxide, heptafluoroisopropyl iodide and diphenyl disulfide. In contrast, these radical processes are highly disfavored when generating unstable phenyl and primary alkyl radicals. In such cases, the reaction proceeds via an oxidative addition/reductive elimination mechanism involving a Ni(IV) intermediate. These theoretical insights into the substrate-controlled mechanisms in the C-H functionalizations were employed to investigate a number of experimental phenomena including substituent effects on reactivity, chemo- and regioselectivity and the effects of oxidant in the intermolecular oxidative C-H/C-H coupling reactions.

Visible-light-induced and iron-catalyzed methylation of N-arylacrylamides with dimethyl sulphoxide: a convenient access to 3-ethyl-3-methyl oxindoles

An efficient synthesis of 3-ethyl-3-methyl oxindoles by visible-light promoted and iron-catalyzed difunctionalization of N-arylacrylamides with dimethyl sulphoxide was developed.

Visible-light-mediated direct difluoromethylation of alkynoates: synthesis of 3-difluoromethylated coumarins

A photoredox-catalyzed direct difluoromethylation of alkynoates has been developed.