Cascade C(sp3)–S Bond Cleavage and Imidoyl C–S Formation: Radical Cyclization of 2-Isocyanoaryl Thioethers toward 2-Substituted Benzothiazoles

Efficacy of radical reactions of isocyanides with heteroatom radicals in organic synthesis

Isocyanide is a promising synthetic reagent not only as a one-carbon homologation reagent but also as a nitrogen source for nitrogen-containing molecules. Because of their isoelectronic structure with carbon monoxide, isocyanides also react with nucleophiles, electrophiles, carbon radicals, and transition metal reagents, and are widely used in organic synthesis. On the other hand, the use of isocyanides in reactions with heteroatom radicals is limited. However, the reaction of isocyanides with heteroatom radicals is a promising synthetic tool for the construction of nitrogen-containing organic molecules modified with a variety of heteroatoms. In this Perspective, we review the addition and cyclization reactions of heteroatom radicals with isocyanides and discuss the synthetic prospects of the reaction of isocyanides with heteroatom radicals.

Visible-Light-Promoted Radical Cascade Sulfone Alkylation/Cyclization of 2-Isocyanoaryl Thioethers Enabled by Electron Donor-Acceptor Complex Formation

A photo-induced cascade sulfone alkylation/cyclization of 2-isocyanoaryl thioethers is explored. This visible-light-triggered reaction not only occurs under extremely mild reaction conditions but also does not require the presence of a photosensitizer. The photocatalytic process is triggered by the photochemical activity of in situ-generated electron donor-acceptor complexes, arising from the association of 2-isocyanoaryl thioethers and α-iodosulfones. The radical pathway was confirmed by UV-vis spectroscopy, radical trapping, Job's plot, and on/off irradiation experiments.

Recent advances in the radical cascade reaction for constructing nitrogen heterocycles using azides as radical acceptors

Due to the high conversion properties, azide compounds are widely utilized in organic synthesis. For instance, azide compounds readily release nitrogen to form a new N-C bond when they function as radical acceptors for the active intermediates in the reaction. Over the past decade, strategies employing azides as radical acceptors to construct nitrogen heterocycles have been extensively developed. This approach has emerged as a crucial method for synthesizing nitrogen heterocycles. Therefore, this paper provides a review of the research advancements in tandem cyclization reactions using azides as radical acceptors, summarizing the process of reaction design, exploration, reasoning of the mechanism, and prospects for further research of these reactions.

Silver-catalyzed P-centered anion nucleophilic addition to isocyanide: access to 2-phosphinoyl indoles/indol-3-ols

A silver-catalyzed chemoselective cascade nucleophilic addition of a P-centered anion to isocyanides and cyclization reaction was developed for the efficient and practical synthesis of a wide range of 2-phosphinoyl indole and indol-3-ol derivatives. Unlike the well-documented synthesis of phosphorus-functionalized heterocycles via a P-centered radical, an anionic reactivity profile of phosphine oxides is most likely involved in this domino transformation.

Synthesis of 2,4-Dicyanoalkylated Benzoxazines through the Radical-Mediated Cascade Cyclization of Isocyanides with AIBN under Metal- and Additive-Free Conditions

A general and novel method for the radical cascade cyclization of aryl isocyanides with AIBN has been described. This strategy provides straightforward access to various 2,4-dicyanoalkylated benzoxazines in moderate to good yields under metal- and additive-free conditions. The reaction can apply to a gram scale and tolerate diverse functional groups. 2,4-Dicyanoalkylated benzoxazine derivatives feature a large Stokes shift and intramolecular charge transfer properties.

Photocatalytic cyclization of 3-(2-isocyanophenyl)quinazolin-4(3H)-ones for the construction of quinoxalino[2,1-b]quinazolinones

A new kind of building unit, 3-(2-isocyanophenyl)quinazolin-4(3H)-ones, was designed and synthesized for the construction of novel quinoxalino[2,1-b]quinazolinones. The radical cyclization of 3-(2-isocyanophenyl)quinazolin-4(3H)-ones with ethers afforded ether-substituted tetracyclic quinoxalino[2,1-b]quinazolinones under photocatalytic and metal-free conditions. In the process, the isocyano accepts a carbon radical to give an imidoyl radical, which adds to the electron-deficient CN bond in quinazolin-4(3H)-one.

Stefano Marcaccini: a pioneer in isocyanide chemistry

Stefano Marcaccini was one of the pioneers in the use of isocyanide-based multicomponent reactions in organic synthesis. Throughout his career at the University of Florence he explored many different faces of isocyanide chemistry, especially those geared towards the synthesis of biologically relevant heterocycles. His work inspired many researchers who contributed to other important developments in the field of multicomponent reactions and created a school of synthetic chemists that continues today. In this manuscript we intend to review the articles on isocyanide multicomponent reactions published by Dr. Marcaccini and analyse their influence on the following works by other researchers. With this, we hope to highlight the immense contribution of Stefano Marcaccini to the development of isocyanide chemistry and modern organic synthesis as well as the influence of his research on future generations. We believe that this review will not only be a well-deserved tribute to the figure of Stefano Marcaccini, but will also serve as a useful inspiration for chemists working in this field.

"Boomerang" Strategy in Carbohydrate Chemistry: Diastereoselective Synthesis of C-Glycosylated Benzothiazoles from ortho-Isocyanophenyl Thioglycosides

This paper reveals a novel "boomerang" strategy in the expedient and diastereoselective synthesis of C-nucleoside analogues. Bench-stable ortho-isocyanophenyl thioglycosides can be converted to glycosyl radicals through rapid and efficient C-S bond homolysis when they are irradiated by visible light. The glycosyl radicals are subsequently trapped by the corresponding leaving group or other radical acceptors to provide diverse C-nucleoside analogues under mild conditions.

Photoredox-Catalyzed Synthesis of C-Benzoselenazolyl/Benzothiazolyl Glycosides from 2-Isocyanoaryl Selenoethers/Thioethers and Glycosyl Bromides

Molecules containing heteroatoms, such as Se and S, play an indispensable role in the discovery and design of pharmaceuticals, whereas Se has been less studied. Here, we described a photoredox strategy to synthesize C-benzoselenazolyl (Bs) glycosides from 2-isocyanoaryl selenoethers and glycosyl bromides. This reaction was carried out under mild conditions with high efficiency. C-Benzothiazolyl (Bt) glycosides could also be synthesized from 2-isocyanoaryl thioethers using this strategy. This method can access novel seleno/thiosugars, which will benefit Se/S-containing drug discovery.

Copper(II)-catalyzed cascade Csp2-P/C-C bond formation to construct benzo[d]thiazol-2-ylphosphonates

A novel, copper(II)-catalyzed cascade Csp2-P/C-C bond formation in o-haloaryl isothiocyanates with organophosphorus esters has been developed under mild conditions. A series of benzo[d]thiazol-2-ylphosphonates were synthesized in moderate to good yields. Different from the traditional method of obtaining these scaffolds with radical reactions, the method proposed allows accessing them via ionic reactions and has the advantages of easy access to raw materials and simple operation. Finally, we carried out a gram-scale experiment to further demonstrate the scalability of this strategy in the efficient synthesis of benzo[d]thiazol-2-ylphosphonates.

Visible-Light-Induced Annulation of Iodonium Ylides and 2-Isocyanobiaryls to Access 6-Arylated Phenanthridines

A 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4-CzIPN)-photocatalyzed cascade arylation/cyclization reaction of 2-isocyanobiaryls and iodonium ylides was established for the synthesis of 6-arylated phenanthridines. This is the first example of employing iodonium ylides as aryl radical sources in a visible-light-induced radical cascade cyclization reaction.

Visible Light-Induced Decomposition of Acyl Peroxides Using Isocyanides: Synthesis of Heteroarenes by Radical Cascade Cyclization

Visible light-mediated facile synthesis of heteroarenes, namely, isoquinolines, benzothiazoles, and quinazolines, is demonstrated by employing isocyanides and inexpensive acyl peroxides. It is shown for the first time that singlet-excited isocyanides decompose acyl peroxides into aryl/alkyl radicals. The latter attack isocyanides, yielding imidoyl radicals that subsequently cyclize to afford heteroarene products. The protocol involving radical cascade reactions obviates the requirement of any external photocatalyst, oxidant, additive, and base.

Enantioselective Copper-Catalyzed Fukuyama Indole Synthesis from 2-Vinylphenyl Isocyanides

Enantioenriched chiral indoles are of high interest for the pharmaceutical and agrochemical industries. Herein, we present an asymmetric Fukuyama indole synthesis through a mild and efficient radical cascade reaction to access 2-fluoroalkylated 3-(α-cyanobenzylated) indoles by stereochemical control with a chiral copper-bisoxazoline complex using 2-vinylphenyl arylisocyanides as radical acceptors and fluoroalkyl iodides as C-radical precursors. Radical addition to the isonitrile moiety, 5-exo-trig cyclization, and Cu-catalyzed stereoselective cyanation provide the targeted indoles with excellent enantioselectivity and good yields. Due to the similar electronic and steric properties of the two aryl substituents to be differentiated, the enantioselective construction of the cyano diaryl methane stereocenter is highly challenging. Mechanistic studies reveal a negative nonlinear effect which allows proposing a model to explain the stereochemical outcome. Scalability and potential utility of the enantioenriched 3-(α-cyanobenzylated) indoles as hubs for chiral tryptamines, indole-3-acetic acid derivatives, and triarylmethanes are demonstrated, and a formal synthesis of a natural product analogue is disclosed.

Electro-Triggered Cascade Cyclization to Access Phosphinyl-Substituted N-Containing Heterocycles

An electro-triggered cascade cyclization strategy was disclosed with concomitant phosphinylation and N-heterocycle construction. It provides a novel and environmentally friendly approach to access phosphinyl-substituted N-heterocycles with no external metal catalyst, oxidant, or heating. Mechanistic studies have revealed that anodic oxidation of H-phosphorus compounds occurs first to generate the key P-centered radical directly and cathodic reduction leads to the concurrent release of molecular hydrogen or methane. This protocol features simple operation, broad substrate scope, clean and mild conditions, and atom and step economy to form heterocycle-containing organophosphorus scaffolds.

Mn(III)-Mediated Radical Addition/Cyclization of Isocyanides with Aryl Boronic Acids/Diarylphosphine Oxides: Access to 11-Functionalized Dibenzodiazepines

A Mn(III)-mediated radical addition/cyclization reaction of isocyanides with aryl boronic acids/diarylphosphine oxides has been developed. A series of 11-arylated/-phosphorylated dibenzodiazepines were efficiently constructed in moderate to excellent yields under mild reaction conditions via imidoyl radical process. The present protocol offers novel access to functionalized seven-membered N-heterocycles.

Silyl Radical Cascade Cyclization of 2-Isocyanothioanisole toward 2-Silylbenzothiazoles through Radical Initiator-Inhibitor Symbiosis

A demethylative silyl radical cascade cyclization of 2-isocyanothioanisoles toward 2-silylated benzothiazole building blocks has been developed. The development of a "radical initiator-inhibitor symbiosis" system solves the challenge of otherwise dominant methyl radical-triggered side reactions brought about by kinetically unfavored generation of reactive silyl radical species. The products accessed in this protocol are amendable to various downstream functionalization reactions, including the quick construction of a topoisomerase II inhibitor via a Hiyama cross-coupling reaction and of an antiviral agent via a fluoride-/hydroxide-free nucleophilic substitution to acyl chloride.

Metal-catalyzed reactions of organic nitriles and boronic acids to access diverse functionality

The nitrile or cyano (-CN) group is one of the most appreciated and effective functional groups in organic synthesis, having a polar unsaturated C-N triple bond. Despite sufficient stability and being intrinsically inert, the nitrile group can be easily transformed into many other functional groups, such as amines, carboxylic acids, ketones, etc. which makes it a vital group in organic synthesis. On the other hand, despite several boronic acids having a low level of genotoxicity, they have found wide applicability in the field of organic synthesis, especially in transition metal-catalyzed cross-coupling reactions. Recently, transition-metal-catalyzed cascade additions or addition/cyclization processes of boronic acids to the nitrile group open up exciting and useful strategies to prepare a variety of functional molecules through the formation of C-C, C-N and CO bonds. Boronic acids can be added to the cyano functionality through catalytic carbometallation or through a radical cascade process to provide newer pathways for the rapid construction of various important acyclic ketones or amides, carbamidines, carbocycles and N,O-heterocycles. The present review focuses on various transition-metal-catalyzed additions of boronic acids via carbometallation or radical cascade processes using the cyano group as an acceptor.

Advances in free-radical alkylation and arylation with organoboronic acids

Organoboronic acids act as carbon-centered radical precursors that are widely utilized to construct diverse C-C bonds. This review summarizes the advances in this field. The content is divided into four parts according to the different categories of coupling partners with organoboronic acids. The reaction conditions as well as the mechanisms are demonstrated in each part.

Synthetic Approaches to Biologically Active C-2-Substituted Benzothiazoles

Numerous benzothiazole derivatives are used in organic synthesis, in various industrial and consumer products, and in drugs, with a wide spectrum of biological activity. As the properties of the benzothiazole moiety are strongly affected by the nature and position of substitutions, in this review, covering the literature from 2016, we focus on C-2-substituted benzothiazoles, including the methods of their synthesis, structural modification, reaction mechanisms, and possible pharmacological activity. The synthetic approaches to these heterocycles include both traditional multistep reactions and one-pot atom economy processes using green chemistry principles and easily available reagents. Special attention is paid to the methods of the thiazole ring closure and chemical modification by the introduction of pharmacophore groups.

Radical modulated regioselective difunctionalization of vinyl enynes: tunable access to naphthalen-1(2H)-ones and allenic alcohols

A novel and efficient radical-modulated difunctionalization of vinyl enynes has been disclosed using TEMPO as a radical regulator. Facile access to structurally diverse 3-bromo-naphthalen-1(2H)-ones and 4-bromo-allenic alcohols was realized via 1,2-addition/1,2-migration or 1,4-addition, respectively. This protocol represents the first example of radical-modulated metal-free difunctionalization of 1,3-enynes with high regioselectivity.

Ionic Reactivity of 2-Isocyanoaryl Thioethers: Access to 2-Halo and 2-Aminobenzothia/Selenazoles

An ionic cascade insertion/cyclization reaction of thia-/selena-functionalized arylisocyanides has been successfully developed for the efficient and practical synthesis of 2-halobenzothiazole/benzoselenazole derivatives. This synthetic protocol, incorporating a halogen atom when forming the five-membered ring of benzothia/selenazoles, is different from the existing ones, where halogenation of the preformed benzothia/selenazole precursors happens. Additionally, a facile access to 2-aminobenzothiazoles is also achieved by the one-pot cascade reaction of 2-isocyanoaryl thioethers, iodine, and amines.

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...

Radical spirocyclization of biaryl ynones for the construction of NO2-containing spiro[5.5]trienones

An efficient 6-exo-trig radical cascade reaction of biaryl ynones with NaNO2 was developed to afford nitro-functionalized spiro[5.5]trienones with yields of up to 88%.

Mn(iii)-Catalyzed cascade cyclization reaction of o-acyl aromatic isocyanides with boronic acids

A Mn(iii)-catalyzed cascade cyclization of o-acyl aromatic isocyanides with boronic acids was examined to give a series of 3-hydroxyindolenines in single-step. This cascade process involved a transmetalation/nucleophilic addition/intramolecular cyclization sequence.

DMF-Assisted Radical Cyclization of o-Isocyanodiaryl Ethers via 1,5-Aryl Migration: Construction of 2-Arylbenzoxazoles

A novel DMF-assisted radical cyclization of o-isocyanodiaryl ethers via 1,5-aryl migration has been developed for the synthesis of a series of 2-arylbenzoxazoles by the FeCl₃/TBHP/Et₃N catalytic system in DMF. However, N,N-dimethylbenzo[d]thiazole-2-carboxamide and N,N-dimethylbenzo[d]selenazole-2-carboxamide were obtained from the corresponding substrate 2-isocyanophenyl p-methoxyphenyl thioether and 2-isocyanodiphenyl selenoether under the same conditions. A possible mechanism may involve aryl 1,5-migration and DMF-assisted radical cyclization of o-isocyanodiaryl ethers.

Divergent S- and C-Difluoromethylation of 2-Substituted Benzothiazoles

Two unprecedented and complementary synthetic strategies for S- and C-difluoromethylation of 2-substituted benzothiazoles have been developed by taking advantage of the remarkably different reactivity of CF₂H^- and 2-PySO₂CF₂^- nucleophiles. A variety of structurally diverse difluoromethyl 2-isocyanophenyl sulfides and 2-difluoromethylated benzothiazoles were synthesized with these two new synthetic protocols.

Electrochemical Trifluoromethylthiolation and Spirocyclization of Alkynes with AgSCF3: Access to SCF3-Containing Spiro[5,5]trienones

A novel and efficient strategy for trifluoromethylthiolation and dearomatization of activated alkynes with stable and readily available AgSCF₃ has been developed. Reported herein is the unprecedented electrochemical generation of the SCF₃ radical in the absence of persulfate for the synthesis of SCF₃-containing spiro[5,5]trienones in good yields via a 6-exo-trig radical cyclization.

Mn(III)-Mediated Radical Cyclization of o-Alkenyl Aromatic Isocyanides with Boronic Acids: Access to N-Unprotected 2-Aryl-3-cyanoindoles

The synthesis of N-unprotected 2-aryl-3-cyanoindoles was realized via the Mn(III)-mediated radical cascade cyclization of o-alkenyl aromatic isocyanides with boronic acids. A possible mechanism involving a sequential intermolecular radical addition, intramolecular cyclization, and cleavage of the C-C bond under mild reaction conditions is proposed. Mechanism studies show that H₂O or O₂ might provide the oxygen source for the elimination of benzaldehyde.

Selectfluor-Promoted Intramolecular N-S Bond Formation of α-Carbamoyl Ketene Dithioacetals in the Presence of Water: Synthesis of Multifunctionalized Isothiazolones

A practical and efficient protocol toward fully substituted isothiazolones through Selectfluor-mediated intramolecular oxidative annulation of α-carbamoyl ketene dithioacetals has been developed in the presence of H₂O and metal-free conditions. Notably, the experimental results reveal that H₂O was crucial to the formation of new N-S bonds and the elimination of alkyl group from the sulfur atom. This protocol provides readily prepared substrates and possesses good functional group tolerance, mild reaction conditions, and operational simplicity, which provides potential access to applications in the pharmaceutical chemistry.

Radical Borylative Cyclization of Isocyanoarenes with N-Heterocyclic Carbene Borane: Synthesis of Borylated Aza-arenes

Borylated aza-arenes are of great importance in the area of organic synthesis. A radical borylative cyclization of isocyanoarenes with N-heterocyclic carbene borane (NHC-BH₃) under metal-free conditions was developed. The reaction allows the efficient assembly of several types of borylated aza-arenes (phenanthridines, benzothiazoles, etc.), which are difficult to access using alternative methods. Mild reaction conditions, a good functional-group tolerance, and generally good efficiencies were observed. The utility of these products is demonstrated, and the mechanism is discussed.

The Synthesis and Mechanistic Considerations of a Series of Ammonium Monosubstituted H-Phosphonate Salts

A series of ammonium monosubstituted H-phosphonate salts were synthesized by combining H-phosphonate diesters with amines in the absence of solvent at 80 °C. Variation of the ester substituent and amine produced a range of ionic liquids with low melting points. The products and by-products were analyzed by spectroscopic and spectrometric techniques in order to get a better mechanistic picture of the dealkylation and formal dearylation observed. For dialkyl H-phosphonate diesters, (RO)₂ P(O)H (R=alkyl), the reaction proceeds via direct dealkylation with the reactivity increasing in the order R=iPr<Et<Me corresponding to DFT calculated activation enthalpies of 22.6, 20.8, and 17.9 kcal mol^-1 . For the diphenyl H-phosphonate diesters, (PhO)₂ P(O)H, the dearylation was found to proceed via phenol-assisted formation of a 5-coordinate intermediate, (PhO)₃ PH(OH), from which P(OPh)₃ and water were eliminated. The presence of an equivalent of water then facilitated the formation of P(OH)₂ OPh and the amine, R'NH₂ , subsequently abstracted a proton from it to yield [(PhO)PH(O)O]^- [R'NH₃ ]⁺ .

Mn(iii)-mediated cascade cyclization of 1-(azidomethyl)-2-isocyanoarenes with organoboronic acids: construction of quinazoline derivatives

A novel and efficient Mn(iii)-mediated oxidative radical cascade reaction of 1-(azidomethyl)-2-isocyanoarenes with organoboronic acids is reported.

Cascade Radical Annulation of 2-Alkynylthio(seleno)anisoles with Acetone or Acetonitrile: Synthesis of 3-Acetomethyl- or Cyanomethyl-Substituted Benzothio(seleno)phenes

An efficient method for the direct preparation of 3-aceto(cyano)methyl-substituted benzothio(seleno)phenes has been achieved through C(sp³)-H bond activation of easily available acetone or acetonitrile and cascade radical cyclization reaction. In this cascade radical cyclization reaction, C(sp²)-C(sp³) and C(sp²)-S bonds, as well as benzenethio(seleno)phene skeletons, can be built along with the cleavage of the C(sp³)-S bond, demonstrating the high step-economics and efficiency of this approach.