Decennary Update on Oxidative-Rearrangement Involving 1,2-Aryl C-C Migration Around Alkenes: Synthetic and Mechanistic Insights

In recent years, numerous methodologies on oxidative rearrangements of alkenes have been investigated, that produce multipurpose synthons and heterocyclic scaffolds of potential applications. The present review focused on recently established methodologies for oxidative transformation via 1,2-aryl migration in alkenes (2013-2023). Special emphasis has been placed on mechanistic pathways to understand the reactivity pattern of different substrates, challenges to enhance selectivity, the key role of different reagents, and effect of different substituents, and how they affect the rearrangement process. Moreover, synthetic limitations and future direction also have been discussed. We believe, this review offers new synthetic and mechanistic insight to develop elegant precursors and approaches to explore the utilization of alkene-based compounds for natural product synthesis and functional materials.

Solvent-Controllable C-F Bond Activation for Masked Formylation of α-Trifluoromethyl Alkenes via Organo-Photoredox Catalysis

A solvent-controllable organo-photoredox-catalyzed C-F bond activation for masked formylation of α-trifluoromethyl alkenes with low-priced 1,3-dioxolane as masked formyl radical equivalent has been described. Consequently, a diversity of masked formylated gem-difluoroalkenes and monofluoroalkenes are constructed in moderate to high yields. This approach merits readily available starting materials, mild reaction conditions, and broad substrate scope. The feasibility of this approach has been highlighted by the one-pot masked formylation/hydrolysis sequence to form γ,γ-difluoroallylic aldehydes and late-stage modification of pharmaceutical and natural product derivatives.

gem-Bromonitroalkane Involved Radical 1,2-Aryl Migration of α,α-Diaryl Allyl Alcohol TMS Ether via Visible-Light Photoredox Catalysis

A mild and efficient coupling method concerning the reactions of gem-bromonitroalkanes with α,α-diaryl allyl alcohol trimethylsilyl ethers was reported. A cascade consisting of visible-light-induced generation of an α-nitroalkyl radical and a subsequent neophyl-type rearrangement was key to realize the coupling reactions. Structurally diverse α-aryl-γ-nitro ketones, especially those bearing a nitrocyclobutyl structure, were prepared in moderate to high yields, which could be converted into spirocyclic nitrones and imines.

Carboazidation of Terminal Alkenes with Trimethylsilyl Azide and Cyclic Ethers Catalyzed by Copper Powder under Oxidative Conditions

Copper-catalyzed carboazidation of alkenes with trimethylsilyl azide and cyclic ethers has been achieved. The employment of naturally abundant copper catalysts allowed cyclic ethers to be used as alkylating reagents under oxidative conditions. The use of styrene derivatives and 1,1-diaryl alkenes afforded carboazidation products. In addition, application of five- and six-membered cyclic ethers to the present reaction gave target organic molecules bearing azide and cyclic ether groups with perfect regioselectivity. Radical trapping and clock experiments revealed that the present reaction proceeded via the radical pathway. To further demonstrate the utility of this carboazidation reaction, transformations from the azide group to the related nitrogen-containing compounds were also performed.

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

Radical selenation of C(sp3)-H bonds to asymmetric selenides and mechanistic study

Selenides are important structural motifs with a broad range of biological activities and versatile transformational abilities. In this study, a novel and mild method was developed for the facile synthesis of asymmetric selenides under metal-free conditions. The key features of this reaction include good functional-group tolerance, the use of readily available reagents and cheap, low-toxicity solvent, and amenability to gram-scale synthesis. The results of preliminary radical-trapping experiments and a kinetic isotope effect study support a radical process.

Aryl Transfer Strategies Mediated by Photoinduced Electron Transfer

Radical aryl migrations are powerful techniques to forge new bonds in aromatic compounds. The growing popularity of photoredox catalysis has led to an influx of novel strategies to initiate and control aryl migration starting from widely available radical precursors. This review encapsulates progress in radical aryl migration enabled by photochemical methods─particularly photoredox catalysis─since 2015. Special attention is paid to descriptions of scope, mechanism, and synthetic applications of each method.

Halogen bond promoted aryl migration of allylic alcohols under visible light irradiation

A simple and catalyst-free radical addition/1,2-aryl migration cascade process of ally alcohol driven by halogen bond was developed under visible light irradiation, featuring mild conditions, practical procedures, and broad substrate scope.

Metal-free visible-light-promoted C(sp3)-H functionalization of aliphatic cyclic ethers using trace O2

Presented is a light-promoted C-C bond forming reaction yielding sulfone and phosphate derivatives at room temperature in the absence of metals or photoredox catalyst. This transformation proceeds in neat conditions through an auto-oxidation mechanism which is maintained through the leaching of trace amounts of O2 as sole green oxidant.

Methylation Alkynylation of Terminal Alkenes via 1,2-Alkynyl Migration Using Dicumyl Peroxide as the Methyl Source

The metal-free oxidative alkene methylation/alkynylation of 1,4-enyn-3-ols with an organic peroxide as the methyl source has been developed, which provides straightforward and practical access to the challenging quaternary-carbon-containing but-3-yn-1-ones. The method is reasoned to go through methylation of functional alkenes utilizing dicumyl peroxide as the methylating reagent and subsequent intermolecular cyclization/1,2-alkynyl migration. This reaction has an excellent functional group tolerance, broad substrate scope, and exquisite selectivity.

Radical-mediated rearrangements: past, present, and future

Rearrangement reactions, one of the most significant transformations in organic chemistry, play an irreplaceable role in improving synthetic efficiency and molecular complexity. Concomitant cleavage and reconstruction of chemical bonds can display the great artistry and the glamour of synthetic chemistry. Over the past century, ionic rearrangement reactions, in particular those involving cationic pathways, have represented most of the research. Alongside the renaissance of radical chemistry, radical-mediated rearrangements have recently seen a rapid increase of attention from the chemical community. Many new radical rearrangements that extensively reveal the migratory behaviour of functional groups have been unveiled in the last decade. This Review provides a comprehensive perspective on the area from the past to present achievements, and brings up the prospects that may inspire colleagues to develop more useful synthetic tools based on radical rearrangements.

Radical Aryl Migration from Boron to Carbon

Radical aryl migration reactions represent a unique type of organic transformations that involve the intramolecular migration of an aryl group from a carbon or heteroatom to a C- or heteroatom-centered radical through a spirocyclic intermediate. Various elements, including N, O, Si, P, S, Sn, Ge, and Se, have been reported to participate in radical aryl migrations. However, radical aryl migration from a boron center has not been reported to date. In this communication, radical 1,5-aryl migration from boron to carbon in aryl boronate complexes is presented. C-radicals readily generated through radical addition onto alkenyl aryl boronate complexes are shown to engage in 1,5-aryl migration reactions to provide 4-aryl-alkylboronic esters. As boronate complexes can be generated in situ by the reaction of alkenylboronic acid esters with aryl lithium reagents, the aryl moiety is readily varied, providing access to a series of arylated products starting from the same alkenylboronic acid ester via divergent chemistry. Reactions proceed with high diastereoselectivity under mild conditions, and also the analogous 1,4-aryl shifts are feasible. The suggested mechanism is supported by DFT calculations.

Metal-free [3 + 2 + 1] annulation of allylic alcohols, ketones, and ammonium acetate: radical-involving synthesis of 2,3-diarylpyridine derivatives

A three-component [3 + 2 + 1] annulation strategy for the synthesis of biologically and pharmaceutically active 2,3-diarylpyridine derivatives by using a series of allylic alcohols, ketones, and ammonium acetate as substrates has been developed. The method proceeds efficiently under metal-free conditions, and the desired heterocycles could be obtained in a site-specific selectivity manner with good functional group tolerance.

SOMOphilic alkynylation using acetylenic sulfones as functional reagents

Recent advancements in SOMOphilic alkynylation reactions by using acetylenic sulfones as functional reagents are summarized.

Photocatalytic Radical Aroylation of Unactivated Alkenes: Pathway to β-Functionalized 1,4-, 1,6-, and 1,7-Diketones

We report the development of a photocatalytic strategy for the synthesis of β-functionalized unsymmetrical 1,4-, 1-6 and 1,7-diketones from aroyl chlorides and unactivated alkenes at room temperature. The mild reaction conditions not only tolerate a wide range of functional groups and structural moieties, but also enable migration of a variety of distal groups including (hetero)arenes, nitrile, aldehyde, oxime-derivative, and alkene. The efficiency of chirality transfer, factors that control the distal-group migration, and synthesis of carbo- and heterocycles from the diketones are also described. Mechanistic studies suggest a reaction pathway involving a photocatalytic radical aroylation of unactivated alkenes followed by a distal-group migration, oxidation, and deprotonation to afford the desired diketones.

Base-promoted domino radical cyclization of 1,6-enynes

A good regioselective, high atom-economical and transition-metal-free method for the synthesis of α-functionalized ether derivatives via the domino radical cyclization of 1,6-enynes is described. A series of α-functionalized ether derivatives could be easily obtained in good yields with wide functional group tolerance by using less toxic and inexpensive Cs₂CO₃ as the base. The control experiment results show that the reaction involves a radical process. This strategy provides a regioselective way toward the formation of dual C-C bonds in one step.

The visible-light-triggered regioselective alkylation of quinoxalin-2(1H)-ones via decarboxylation coupling

An efficient protocol to synthesize 3-alkylated quinoxalin-2(1H)-ones through photocatalytic decarboxylation coupling reactions of quinoxalin-2(1H)-ones with N-hydroxyphthalimide ester was developed.

Cleavage of carbon–carbon bonds by radical reactions

This review provides insights into the in situ generated radicals triggered carbon–carbon bond cleavage reactions.

A copper-catalyzed radical coupling/fragmentation reaction: efficient access to β-oxophosphine oxides

The development of a novel copper-catalyzed three-component radical coupling/fragmentation cascade reaction to generate diverse β-oxophosphine oxides is reported.

Catalytic, metal-free alkylheteroarylation of alkenes via distal heteroaryl ipso-migration

A new direct alkylheteroarylation of alkenes with simple alkylnitriles via distal heteroaryl ipso-migration has been accomplished under metal-free conditions, in which inexpensive alkylnitriles served as radical precursors, which together with ease of operation makes this process a very practical protocol.

Electrochemical Synthesis of Bisindolylmethanes from Indoles and Ethers

An electrochemical bisindolylation of ethers was developed. Carried out under ambient conditions and in the absence of any chemical oxidants, this reaction exhibits a broad substrate scope and good functional group compatibility.

Cyanotrifluoromethylthiolation of unactivated dialkyl-substituted alkynes via cyano migration: synthesis of trifluoromethylthiolated acrylonitriles

Herein a novel, elusive cyanotrifluoromethylthiolation of unactivated dialkyl-substituted alkynes by means of intramolecular distal cyano migration is reported.

tert-Butyl peroxybenzoate mediated formation of 3-alkylated quinolines from N-propargylamines via a cascade radical addition/cyclization reaction

A novel method for constructing 3-alkylated quinolines was developed via tert-butyl peroxybenzoate mediated cycloaddition between N-propargyl aromatic amines and ethers.