Recent Advances in Polychloromethylation Reactions

Dinuclear gold-catalyzed divergent dechlorinative radical borylation of gem-dichloroalkanes

The enormous and widespread use of organoboronic acids has prompted the development of innovative synthetic methodologies to meet the demands on structural diversity and functional group tolerance. The existing photoinduced defunctionalization radical borylation, typically focused on the conversion of one C-X bond (X= Br, I, or other leaving group) into only one C-B bond. Herein, we disclose a divergent radical dechloroborylation reaction enabled by dinuclear gold catalysis with visible light irradiation. A wide range of structurally diverse alkyl boronic, α-chloroboronic, and gem-diboronic esters can be synthesized in moderate to good yields (up to 92%). Its synthetic robustness is further demonstrated on a preparative scale and applied to late-stage diversification of complex molecules. The process hinges on a C-Cl bond relay activation in readily available gem-dichloroalkanes through inner-sphere electron transfer, overcoming the redox potential limits of unreactive alkyl chlorides.

Metal-free radical bicyclization/chloroalkylarylation of 1,6-enynes with chloroalkanes

Free radical initiated bicyclization of 1,6-enynes with chloralkanes, is achieved via selective activation of the C(sp3)-H bond of the chloralkane, resulting in diverse polychlorinated/chlorinated polyheterocycles. Two kinds of transformations and a scaled-up experiment were performed to test the synthetic importance of the organic chlorides. Finally, a range of radical inhibition operations and radical clock tests were explored to support the reaction process.

Synthesis of 4-(trichloromethyl)pyrido[2',1':3,4]pyrazino[2,1-b]quinazolinones through a cyclized dearomatization and trichloromethylation cascade strategy

A variety of 4-(trichloromethyl)pyrido[2',1':3,4]pyrazino[2,1-b]quinazolinones were prepared in moderate to good yields with high regioselectivity through intramolecular 6-endo-dig cyclization and trichloromethylation of N3-alkynyl-2-pyridinyl-tethered quinazolinones in chloroform. Mechanistic studies revealed that chloroform might serve as a trichloromethyl anion precursor. Furthermore, the reaction could be easily performed on gram scales and an estrone-derived 4-(trichloromethyl)pyrido[2',1':3,4]pyrazino[2,1-b]quinazolinone was prepared over five steps. The present method features broad substrate scope, good functional group tolerance, new dearomatization of pyridine rings, and chloroform as the trichloromethylation reagent.

Metal-free synthesis of 1,3-dichloro-1,5-diarylpentan-5-ones via cascade oxidative radical addition of styrenes with CHCl3

A novel and efficient metal-free cascade oxidative radical addition of styrenes is developed for the construction of 1,3-dichloro-1,5-diarylpentan-5-ones. This protocol presents a practical one-pot procedure that delivers highly functionalized 1,3-dichloro-1,5-diarylpentan-5-ones in moderate-to-good yields with a broad substrate scope under mild conditions.

Metal-free polychloromethylation/cyclization of unactivated alkenes towards ring-fused tricyclic indolones and benzoimidazoles

Polychloromethylative cyclization of N-alkenyl indoles was developed under metal-free conditions to afford tricyclic pyridoindolones and pyrroloindolones in moderate to good yields. In the reaction, commercially available CHCl3 and CH2Cl2 were employed as tri- and dichloromethyl radical sources. Moreover, tri- and dichloromethylated polycyclic benzoimidazoles can also be obtained under standard conditions.

1,2-Alkynyl Functionalization of Unactivated Alkenes via Diverse Radical-Triggered Functional Group Migration

We have developed a transition-metal-free radical approach for 1,2-alkynyl functionalization of unactivated alkenes through the combination of 3-exo-dig cyclization with alkynyl migration triggered by in situ-generated diverse radical precursors. This strategy provides a robust toolkit to access a variety of synthetically important α-functionalized alkynyl ketones, simultaneously installing densely functionalized carbonyl, alkynyl, and other various functional groups into the alkenes. The broad substrate scope, which includes distinctly electron-donating or electron-withdrawing alkynyl migrating groups, excellent functional group compatibility, and remarkable selectivity make this protocol practical and attractive.

α-Amino Radical-Mediated Difunctionalization of Alkenes with Polyhaloalkanes and N-Heteroarenes

Herein, we report a mild and practical protocol for the α-perchloroalkyl β-heteroarylation of alkenes using available chloroform as the dichloromethyl source via α-amino radical-mediated halogen-atom transfer. Various substrates are compatible under mild reaction conditions, providing the corresponding products in moderate to good yields. This strategy gives an efficient and convenient method for the introduction of chloroalkyl motifs into N-heteroarenes. The control experiment demonstrates that the α-amino radical generated in situ is a key intermediate in the transformation.

Recent advances in C-H functionalization of 2H-indazoles

2H-Indazoles are one class of the most important nitrogen-containing heterocyclic compounds. The 2H-indazole motif is widely present in bioactive natural products and drug molecules that exhibit distinctive bioactivities. Therefore, much attention has been paid to access diverse 2H-indazole derivatives. Among them, the late-stage functionalization of 2H-indazoles via C-H activation is recognized as an efficient approach for increasing the complexity and diversity of 2H-indazole derivatives. In this review, we summarized recent achievements in the late-stage functionalization of 2H-indazoles, including the C3-functionalization of 2H-indazoles through transition metal-catalyzed C-H activation or a radical pathway, transition metal-catalyzed ortho C2'-H functionalization of 2H-indazoles and remote C-H functionalization at the benzene ring in 2H-indazoles.

The polychloromethylation/acyloxylation of 1,6-enynes with chloroalkanes and diacyl peroxides through dual-role designs

The novel polychloromethylation/acyloxylation of 1,6-enynes with chloroalkanes and diacyl peroxides through dual-role designs has been developed to prepare 2-pyrrolidinone derivatives with polychloromethyl units with the use of an inexpensive copper salt under mild conditions. This strategy includes two dual-role designs, not only improving atomic utilization but also allowing a cleaner process. The wide substrate scope and simple reaction conditions demonstrate the practicability of this protocol.

Metal-free polychloromethyl radical-initiated cyclization of unactivated N-allylindoles towards pyrrolo[1,2-a]indoles

A metal-free polychloromethyl radical-initiated cyclization of unactivated alkenes was developed using CH₂Cl₂ and CHCl₃ as the di- and trichloromethyl radical sources. Variously substituted N-allyl-indoles were successfully transformed into the corresponding C2-(di- and trichloromethyl) pyrrolo[1,2-a]indoles in moderate to good yields. This reaction has a broad substrate scope and good functional group tolerance. Dibromomethylated products can also be obtained using CH₂Br₂ under standard conditions.

Visible light promoted polyhalomethylation of alkenes: alkylation and cyclization

The report describes a visible light promoted carbochloromethylation of 2-bromomethyl acrylate and N-arylmethacrylamide, and a series of trihalomethyl substituted allylic acid esters and indolone derivatives were synthesized.

Selective polychloromethylation and halogenation of alkynes with polyhaloalkanes

We disclosed a selective polychloromethylation and halogenation reaction of alkynes via a radical addition/spirocyclization cascade sequence, in which applying polyhaloalkanes as the precursor of polyhalomethyl and halogen radical. Across this...

Photophysics, photochemistry and bioimaging application of 8-azapurine derivatives

New 2-aryl-1,2,3-triazolopyrimidines were designed, synthesized, and characterized. Their optical properties were thoroughly studied in the solid phase, in solution and in a biological environment. Density Functional Theory (DFT) based calculations were performed, including the molecular geometry optimization for both the ground state and the first singlet excited state, the prediction of the UV-Vis absorption and fluorescence spectra, the determination of the molecular electrostatic properties and the solvent effect on the optical properties. The emission intensity was revealed to increase in time upon irradiation. Mass spectrometric research, quantum mechanical calculations, and analysis of literature data suggested a possible photo-transformation pathway through the homolytic cleavage of one of the C-Cl bonds upon irradiation with UV light. The structure of the active intermediate was identified by the series of mass spectrometry experiments and via synthesis of putative transformation products. The kinetic parameters measured in different solvents allowed estimating the rate of these photo-transformations. Biological experiments demonstrated that 2-aryl-1,2,3-triazolopyrimidines penetrate cells and selectively accumulate in the cell membrane and the Golgi complex and endoplasmic reticulum. Their unique properties pave the way for new possible applications of fluorescent 8-azapurines in biology and medicine.

Free Radical-Promoted Monochloroalkylarylation of Alkenes with Chloralkanes

Free radical-initiated cascade cyclization of unactivated alkenes with chloralkanes, which undergoes selective activation of the α-C(sp³)-H bond of chloralkanes, provides a protocol for the synthesis of chlorinated heterocycles or polycyclic compounds. A series of radical inhibition experiments, radical capture operations, and radical clock tests were studied in this system.

Radical-mediated alkoxypolyhaloalkylation of styrenes with polyhaloalkanes and alcohols via C(sp3)–H bond cleavage

A radical-mediated alkoxypolyhaloalkylation of styrenes with polychloroalkanes and alcohols for the facile synthesis of complex polyhaloalkanes with excellent functional-group compatibility and a broad substrate scope.

Three-component photoredox 1,2-alkylamination of styrenes with alkanes and nitrogen nucleophiles via C(sp3)–H bond cleavage

A three-component photoredox 1,2-alkylamination of styrenes involving functionalization of C(sp3)–H bonds in alkyl halides instead of functionalization of C-halogen bonds is disclosed.

Recent advances in the application of tetrabromomethane in organic synthesis

This review article covers the use of tetrabromomethane as mediator, catalyst and reagents for organic synthesis for the period from 2007 to 2020.