Photoredox-Catalyzed Cascade sp2 C-H Bond Functionalization to Construct Substituted Acridine with Diarylamine and Hypervalent Iodine(III) Reagents

Visible Light-Induced Reactions of Diazo Compounds and Their Precursors

In recent years, visible light-induced reactions of diazo compounds have attracted increasing attention in organic synthesis, leading to improvement of existing reactions, as well as to the discovery of unprecedented transformations. Thus, photochemical or photocatalytic generation of both carbenes and radicals provide milder tools toward these key intermediates for many valuable transformations. However, the vast majority of the transformations represent new reactivity modes of diazo compounds, which are achieved by the photochemical decomposition of diazo compounds and photoredox catalysis. In particular, the use of a redox-active photocatalysts opens the avenue to a plethora of radical reactions. The application of these methods to diazo compounds led to discovery of transformations inaccessible by the classical reactivity associated with carbenes and metal carbenes. In most cases, diazo compounds act as radical sources but can also serve as radical acceptors. Importantly, the described processes operate under mild, practical conditions. This Review describes this subfield of diazo compound chemistry, particularly focusing on recent advancements.

Photoinduced Alkylsulfonylation and Cyanoalkylsulfonylation of Morita-Baylis-Hillman Adducts via Multicomponent Insertion of Sulfur Dioxide

General and convenient visible-light-promoted alkylsulfonylation and cyanoalkylsulfonylation of MBH adducts have been developed through the multicomponent insertion of sulfur dioxide, enabling the assembly of two C-S bonds to generate structurally diverse allylic alkylsulfones (43 examples in total). The reaction of MBH adducts with potassium alkyltrifluoroborates and 1,4-diazabicyclo[2.2.2]octane bis(sulfur dioxide) adduct afforded sulfones with generally good yields. Notably, the addition of N,N,N',N'-tetramethylethylenediamine as a base into the photocatalytic system led to yielding an alkyl sulfonyl unit and cyano group-anchored trisubstituted alkenes by utilizing cycloketone oxime esters as C-radical precursors. Both of these reactions have constructed two C-S bonds, and all desired products were obtained in moderate to excellent yields with complete stereospecificity.

Photoinduced radical tandem annulation of 1,7-diynes: an approach for divergent assembly of functionalized quinolin-2(1H)-ones

The first photocatalytic trichloromethyl radical-triggered annulative reactions of amide-linked 1,7-diynes with polyhalomethanes were established for the flexible assembly of functionalized quinolin-2(1H)-ones with generally acceptable yields. With the installation of the aryl group (R1) into the alkynyl moiety, C-center radical-initiated Kharasch-type addition/nucleophilic substitution/elimination cascade to produce quinolin-2(1H)-ones-incorporating gem-dihaloalkene, whereas three examples of polyhalogenated quinolin-2(1H)-ones were afforded when amide-linked 1,7-diynes bearing two terminal alkyne units were subjected to BrCX3 by exploiting dry acetonitrile as a solvent.

Photocatalytic Multicomponent Annulation of Amide-Anchored 1,7-Diynes Enabled by Deconstruction of Bromotrichloromethane

We present the first example of visible-light-mediated multicomponent annulation of 1,7-diynes by taking advantage of quadruple cleavage olf carbon-halogen bonds of BrCCl3 to generate a C1 synthon, which was adeptly applied to the preparation of skeletally diverse 3-benzoyl-quinolin-2(1H)-one acetates in moderate to good yields. Controlled experiments demonstrated that H2O acted as both oxygen and hydrogen sources, and gem-dichlorovinyl carbonyl compound exhibited as a critical intermediate in this process. The mechanistic pathway involves Kharasch-type addition/6-exo-dig cyclization/1,5-(SN")-substitution/elimination/binucleophilic 1,6-addition/proton transfer/tautomerization sequence.

Synthesis of Hypervalent Iodine Diazo Compounds and Their Application in Organic Synthesis

Diazomethyl-substituted iodine(III) compounds with electron-withdrawing groups (EWG) connected to diazo methyl center were a type of donor-acceptor diazo compounds with potential reaction abilities similar to ordinary diazo compounds. Although several diazomethyl-substituted iodine(III) compounds were synthesized and used in the nucleophilic substitution reactions as early as 1994, the synthesis and application of new iodine(III) diazo compounds have only been reported to a certain extent in recent years. In the presence of rhodium catalyst, photocatalyst, or nucleophiles, diazomethyl-substituted iodine(III) compounds can be converted into rhodium-carbenes, diazomethyl radicals, ester radicals or nucleophilic intermediates, which can be used as key intermediates for the formation of chemical bonds. The aim of this review is to give an overview of diazomethyl-substituted iodine(III) compounds in organic synthesis.

Electron Donor-Acceptor Complex Induced Fused Indoles with Hypervalent Iodine(III) Reagents

An operationally simple and efficient method for the cyclization of tertiary amines and hypervalent iodine reagents enabled by an EDA complex has been developed. A series of [1,2-α]indoles derivatives were obtained in good yields, including some key intermediates for the synthesis of biologically active molecules. In addition, this established strategy features a broad substrate scope and good functional group tolerance.

Remote Radical 1,3-, 1,4-, 1,5-, 1,6- and 1,7-Difunctionalization Reactions

Radical transformations are powerful in organic synthesis for the construction of molecular scaffolds and introduction of functional groups. In radical difunctionalization reactions, the radicals in the first functionalized intermediates can be relocated through resonance, hydrogen atom or group transfer, and ring opening. The resulting radical intermediates can undertake the following paths for the second functionalization: (1) couple with other radical groups, (2) oxidize to cations and then react with nucleophiles, (3) reduce to anions and then react with electrophiles, (4) couple with metal-complexes. The rearrangements of radicals provide the opportunity for the synthesis of 1,3-, 1,4-, 1,5-, 1,6-, and 1,7-difunctionalization products. Multiple ways to initiate the radical reaction coupling with intermediate radical rearrangements make the radical reactions good for difunctionalization at the remote positions. These reactions offer the advantages of synthetic efficiency, operation simplicity, and product diversity.