Catalyst and transition-metal free 1,6-conjugate addition of azobisisobutyronitrile: access to isobutyronitrile containing diarylmethanes

A catalyst and transition-metal free 1,6-conjugate addition of azobisisobutyronitrile to para-quinone methides for the synthesis of isobutyronitrile containing diarylmethanes has been achieved. This protocol enables the synthesis of isobutyronitrile containing diarylmethanes in good yields and with a broad substrate scope. This is the first example wherein azobisisobutyronitrile has been used as a cyanide source for 1,6-conjugate addition under catalyst and metal-free conditions.

Electrophilic C(sp2)-H Cyanation with Inorganic Cyanate (OCN-) by PIII/PV=O-Catalyzed Phase Transfer Activation

An organophosphorus -catalyzed method for the direct electrophilic cyanation of C(sp2)-H nucleophiles with sodium cyanate (NaOCN) is reported. The catalytic deoxyfunctionalization of the OCN- anion is enabled by the use of a small-ring phosphacyclic (phosphetane) catalyst in combination with a terminal hydrosilane O-atom acceptor and a malonate-derived bromenium donor. In situ spectroscopy under single-turnover conditions demonstrate that insoluble inorganic cyanate anion is activated by bromide displacement on a bromophosphonium catalytic intermediate to give a reactive N-bound isocyanatophosphonium ion, which delivers electrophilic "CN+" equivalents to nucleophilic (hetero)arenes and alkenes with loss of a phosphine oxide. These results demonstrate the feasibility of deoxyfunctionalization of insoluble inorganic salts by PIII/PV=O catalyzed phase transfer activation.

NBS-Promoted Synthesis of Thiocyanated Aminomaleimides and Site-Selective Intramolecular Cyclization Access to 1,4-Benzothiazepines via S-CN Bond Cleavage

A transition metal-free concise and efficient protocol for the synthesis of thiocyanated aminomaleimides and benzo[e][1,4]thiazepine derivatives has been developed. The method involves an initial α-C-H thiocyanation of aminomaleimides with KSCN and TEMPO-mediated tandem S-CN bond cleavage/intramolecular cyclization substitution processes, which enables the formation of seven-membered S/N-heterocycles. This synthetic strategy provides a reliable method for the synthesis of biologically interesting benzo[e][1,4]thiazepine derivatives by using KSCN as sulfur sources as well as expands the application of enaminones thiocyanation reactions in heterocycles synthesis.

Recent Advancement on the Indirect or Combined Alternative Thiocyanate Sources for the Construction of S-CN Bonds

The process of thiocyanation is a notable chemical conversion owing to the extensive range of applications associated with thiocyanate compounds in the field of organic chemistry. In past centuries, the thiocyanation reaction incorporated metal thiocyanates or thiocyanate salts as sources of thiocyanate, which are environmentally detrimental and undesirable. In recent literature, there have been numerous instances where combined or indirect alternative sources of thiocyanate have been employed as agents for thiocyanation, showcasing their noteworthy applications. The present literature review focuses on elucidating the ramifications associated with the utilization of indirect or combined alternative sources of thiocyanate in various thiocyanation reactions.

tert-Butylhydroperoxide mediated radical cyanoalkylation/cyanoalkenylation of 2-anilino-1,4-naphthoquinones with vinylarenes/arylalkynes and azobis(alkylcarbonitrile)s

A novel sustainable methodology based on one-pot cyanoalkylation/cyanoalkenylation of 2-anilino-1,4-naphthoquinones with vinylarenes/arylalkynes and azobis(alkylcarbonitrile)s involving a three-component radical cascade pathway has been achieved. Here, tert-butylhydroperoxide (TBHP) acts as an efficient oxidant, and it smoothly drives the reaction, producing the three-component products in very good to excellent yields. This cascade reaction eliminates the use of any base, additive, metal and hazardous cyanating agent. Additionally, this protocol exclusively delivers a stereospecific product in the case of arylalkynes. The involvement of radicals is evidenced through various radical trapping experiments.

Structural verification and new reactivity for Stang's reagent, [PhI(CN)][OTf]

The structure of Stang's reagent [PhI(CN)][OTf] is confirmed by X-ray crystallography and is determined to be best described as an ion-pair in organic solution. It is found to be a strong Lewis acid, but reaction with pyridine ligands gives [Pyr-CN][OTf] salts via oxidation of pyridine giving a new derivative of the CDAP reagent widely used as an activation agent for polysaccharides.

Synthesis of thiocyanato-containing phenanthrenes and dihydronaphthalenes via Lewis acid-activated tandem electrophilic thiocyanation/carbocyclization of alkynes

A tandem electrophilic thiocyanation and cyclization of arene-alkynes has been developed under mild conditions, affording thiocyanato-substituted phenanthrenes, dihydronaphthalenes, 2H-chromenes and dihydroquinolines in moderate to excellent yields. This reaction provides an efficient protocol for the construction of C-SCN and C-C bonds in one step. In this transformation, N-thiocyanato reagent serves as a convenient precursor to transfer SCN⁺ in the presence of trimethylchlorosilane, and the cyclization exhibited exclusive 6-endo-dig selectivity. Finally, a gram scale reaction and further derivatizations highlight the utility of this synthetic strategy.

3,5-Di(trifluoromethyl)phenyl(cyano)iodonium triflate as a novel and potential activator for p-tolyl thioglycoside donors

3,5-Di(trifluoromethyl)phenyl(cyano)iodonium triflate is described as an accessible, stable, and powerful thiophile that can activate batches of p-tolyl thioglycoside donors at room temperature. Various alcoholic acceptors were efficiently glycosylated, providing the desired glycosides. The novel activation protocol features mild conditions as well as high compatibility with some classic strategies for the stereoselective construction of some biologically relevant glycosidic linkages, as exemplified by α-idosides, α-galactoamines, β-mannosides, and β-rhamnosides.

Direct Cyanation of Thiophenols or Thiols to Access Thiocyanates under Electrochemical Conditions

A novel electrochemical cross-coupling method for the synthesis of thiocyanates via the direct cyanation of readily available thiophenols or thiols with trimethylsilyl cyanide (TMSCN) was developed. This approach was also suitable for selenols. External oxidant-free, transition-metal-free and mild operating conditions were the main advantages of this protocol. A series of thiocyanates and selenocyanates could be obtained in moderate to high yields.

Metal-free three-component cyanoalkylation of quinoxalin-2(1H)-ones with vinylarenes and azobis(alkylcarbonitrile)s

A K2S2O8-mediated C3 cyanoalkylation of quinoxalin-2(1H)-ones via a three-component radical cascade reaction with vinylarenes and azobis(alkylcarbonitrile)s has been achieved.

Asymmetric 1,4-Michael Addition Reaction of Azadienes with α-Thiocyanoindanones Catalyzed by Bifunctional Chiral Squaramide

In this paper, the organocatalytic asymmetric 1,4-Michael addition reaction of azadienes and α-thiocyanoindanones was investigated. A series of chiral benzofuran compounds containing thiocyano group and quaternary carbon center were synthesized in moderate yields with good enantioselectivities (up to 90:10 er) and high diastereoselectivities (up to >95:5 dr). This is the first case of 1,4-Michael addition reaction using α-thiocyanoindanones to obtain a series of chiral thiocyano compounds and further broaden the scope of application of azadiene substrates. In addition, a possible reaction mechanism is also described in the article.

Programmed Formation of HCN Oligomers through Organosulfur Catalysis

An efficient, inexpensive, and reliable synthesis of diaminomaleonitrile (DAMN, 1) is described starting from readily available acetone cyanohydrin as the source of hydrogen cyanide (HCN). Diaminomaleonitrile (DAMN) is known to be an important intermediate in heterocyclic and medicinal chemistry as well as being a possible precursor for the origin of life's hypothesis within prebiotic chemistry. The mechanism of its formation through organosulfur catalysis has been investigated by electrospray ionization mass spectrometry (ESI-MS) using two newly synthesized cationic "marker" molecules as a tool that allows for sensitive detection. As a result, the proposed mechanism of a thiocyanate-mediated synthesis of the HCN tetramer DAMN starting from organic disulfides was confirmed.

Transition-Metal-Free HFIP-Mediated Organo Chalcogenylation of Arenes/Indoles with Thio-/Selenocyanates

We have developed a protocol for the synthesis of diaryl thio-/selenoethers by the reaction of aryl chalcogenocyanates with electron rich arenes/hetero arenes via HFIP promoted C-H activation. The reaction produces chalcogenides in good to excellent yields under mild conditions without the need of a transition metal as a catalyst. The HFIP-mediated reactions tolerated a wide range of functional groups and set the stage for the synthesis of diversely decorated chalcogenides. A mechanism involving activation of the C-H bond through hydrogen bonding is proposed.

Design and Use of Electrophilic Thiocyanating and Selenocyanating Reagents: An Interesting Trend for the Construction of SCN- and SeCN-Containing Compounds

Organothiocyanate and organoselenocyanate compounds are of paramount importance in organic chemistry as they are key intermediates to access sulfur- and selenium-containing compounds. Therefore, among the different synthetic pathways to get SCN- and SeCN-containing molecules, original methodologies using electrophilic reagents have recently been explored. This Minireview will showcase the recent advances that have been made. In particular, the design of several electrophilic sources and their applications for the thiocyanation and the selenocyanation of various classes of compounds will be highlighted and discussed.

Straightforward Access to Thiocyanates via Dealkylative Cyanation of Sulfoxides

Thiocyanates, versatile building blocks in organic synthesis, are shown to be easily accessible via an interrupted Pummerer reaction of sulfoxides. This facile dealkylative functionalization proceeds under mild conditions through electrophilic activation of the sulfoxide partner. The resulting thiocyanate itself can serve as a handle for diversification in a straightforward one-pot procedure.

Mechanochemical Thiocyanation of Aryl Compounds via C-H Functionalization

Aryl thiocyanate compounds are important building blocks for the synthesis of bioactive compounds and intermediates for several functional groups. Reported thiocyanation reactions via C-H functionalization have limited substrate scope and low RME. The ball-milling method reported here uses ammonium persulfate and ammonium thiocyanate as reagents and silica as a grinding auxiliary. It afforded aryl thiocyanates with moderate to excellent yields for a wide variety of aryl compounds (36 examples, 8-96% yield), such as anilines, phenols, anisoles, thioanisole, and indole, thus tolerating substrates with sensitive functional groups. New products such as benzo[d][1,3]oxathiol-2-ones were obtained with C-4 substituted phenols. Thus, to our knowledge, we report, for the first time, aryl thiocyanation reaction by ball-milling at room temperature and solvent-free conditions, with short reaction times and no workup. Analysis of several mass-based green metrics indicates that it is an efficient greener method.

Transition-metal-free direct C-3 cyanation of quinoxalin-2(1H)-ones with ammonium thiocyanate as the “CN” source

A practical protocol for TBHP-mediated oxidative C–H cyanation of quinoxalin-2(1H)-ones utilizing ammonium thiocyanate as the cyanide source has been developed under metal free conditions.

NCS-promoted thiocyanation and selenocyanation of pyrrolo[1,2-a]quinoxalines

An efficient NCS-promoted thiocyanation of pyrrolo[1,2-a]quinoxalines with NH₄SCN or KSCN was developed. Moreover, in the presence of KSeCN, the selenocyanation of pyrrolo[1,2-a]quinoxalines was also achieved.

Fluorium-Initiated Dealkylative Cyanation of Thioethers to Thiocyanates

Thioethers are converted to thiocyanates via fluorium-initiated dealkylative cyanation. Selectfluor is used as the oxidant, and trimethylsilyl cyanide is used as the cyanation reagent. The well-streamlined procedure is user-friendly, operationally simple, and step-economical. The current mechanistic studies show that the sulfur radical cation and cyano radical are both involved. They combine to deliver cyanosulfonium, an intermediate toward thiocyanate after dealkylation. Alternatively, a nucleophilic mechanism is also possible. Our dealkyaltive cyanation is also efficient in synthesizing thiocyanates with strongly electrophilic functionalities.

TMSCl-Catalyzed Electrophilic Thiocyano Oxyfunctionalization of Alkenes Using N-Thiocyano-dibenzenesulfonimide

Numerous electrophilic thiocyano oxyfunctionalization reactions of alkenes have been achieved using N-thiocyano-dibenzenesulfonimide, which is a new electrophilic thiocyanation reagent and could be easily prepared in two steps from dibenzenesulfonimide. This approach provides efficient, simple, and modular methods for the formation of SCN-containing heterocycles such as lactones, tetrahydrofurans, dihydrofurans, and dihydrobenzofurans in moderate to excellent yields. Meanwhile, diverse oxa-quaternary centers were rapidly constructed. Additionally, this protocol is free of transition metals and features broad substrate toleraance and mild reaction conditions.

Metal-free difunctionalization of alkynes leading to alkenyl dithiocyanates and alkenyl diselenocyanates at room temperature

A simple and practical method for the synthesis of alkenyl dithiocyanates and alkenyl diselenocyanates has been developed via stereoselective difunctionalization of alkynes with NaSCN or KSeCN at room temperature. Through this methodology, a series of alkenyl dithiocyanates and alkenyl diselenocyanates could be efficiently and conveniently obtained in moderate to good yields under mild and metal-free conditions by the simple use of oxone and PhI(OAc)2 as the oxidants.

Non-toxic cyanide sources and cyanating agents

The present review gives an overview over non-toxic cyanation agents and cyanide sources used in the synthesis of structurally diverse products containing the nitrile function. Nucleophilic as well as electrophilic agents/systems that transfer the entire CN-group were taken in consideration. Reactions in which a preexisting carbon functionality is transformed into a nitrile function by addition of nitrogen are however not covered here.

Electrophilic Cyclization of Phenylalkynediols to Naphthyl(aryl)iodonium Triflates with Chelating Hydroxyls: Preparation and X-ray Analyses

Alkynediols containing one propargylic alcohol as well as a second alcohol, which is propargylic or homopropargylic, react with PhI⁺CN^-OTf (Stang's reagent) or 3,5-(CF₃)₂C₆H₃I⁺CN^-OTf to afford naphthyl(aryl)iodonium triflates. The reaction occurs at room temperature over the course of 6-12 h and provides 36-82% yields of microcrystalline solids. Slow diffusion of Et₂O into CH₃CN solutions of the salts afforded X-ray quality crystals of five compounds with hydroxyl groups forming five- and six-membered chelation complexes with the iodine atom. Crystallizations from larger scale reactions (≥∼0.25 mmol) were generally facile from CH₂Cl₂.

Iodine(III) Reagents in Radical Chemistry

The chemistry of hypervalent iodine(III) compounds has gained great interest over the past 30 years. Hypervalent iodine(III) compounds show valuable ionic reactivity due to their high electrophilicity but also express radical reactivity as single electron oxidants for carbon and heteroatom radical generation. Looking at ionic chemistry, these iodine(III) reagents can act as electrophiles to efficiently construct C–CF₃, X–CF₃ (X = heteroatom), C–R_f (R_f = perfluoroalkyl), X–R_f, C–N₃, C–CN, S–CN, and C–X bonds. In some cases, a Lewis or a Bronsted acid is necessary to increase their electrophilicity. In these transformations, the iodine(III) compounds react as formal “CF₃⁺”, “R_f⁺”, “N₃⁺”, “Ar⁺”, “CN⁺”, and “X⁺” equivalents. On the other hand, one electron reduction of the I(III) reagents opens the door to the radical world, which is the topic of this Account that focuses on radical reactivity of hypervalent iodine(III) compounds such as the Togni reagent, Zhdankin reagent, diaryliodonium salts, aryliodonium ylides, aryl(cyano)iodonium triflates, and aryl(perfluoroalkyl)iodonium triflates. Radical generation starting with I(III) reagents can also occur via thermal or light mediated homolysis of the weak hypervalent bond in such reagents. This reactivity can be used for alkane C–H functionalization. We will address important pioneering work in the area but will mainly focus on studies that have been conducted by our group over the last 5 years.

We entered the field by investigating transition metal free single electron reduction of Togni type reagents using the readily available sodium 2,2,6,6-tetramethylpiperidine-1-oxyl salt (TEMPONa) as an organic one electron reductant for clean generation of the trifluoromethyl radical and perfluoroalkyl radicals. That valuable approach was later successfully also applied to the generation of azidyl and aryl radicals starting with the corresponding benziodoxole (Zhdankin reagent) and iodonium salts. In the presence of alkenes as radical acceptors, vicinal trifluoromethyl-, azido-, and arylaminoxylation products result via a sequence comprising radical addition to the alkene and subsequent TEMPO trapping. Electron-rich arenes also react with I(III) reagents via single electron transfer (SET) to give arene radical cations, which can then engage in arylation reactions. We also recognized that the isonitrile functionality in aryl isonitriles is a highly efficient perfluoroalkyl radical acceptor, and reaction of R_f-benziodoxoles (Togni type reagents) in the presence of a radical initiator provides various perfluoroalkylated N-heterocycles (indoles, phenanthridines, quinolines, etc.). We further found that aryliodonium ylides, previously used as carbene precursors in metal-mediated cyclopropanation reactions, react via SET reduction with TEMPONa to the corresponding aryl radicals. As a drawback of all these transformations, we realized that only one ligand of the iodine(III) reagent gets transferred to the substrate. To further increase atom-economy of such conversions, we identified cyano or perfluoroalkyl iodonium triflate salts as valuable reagents for stereoselective vicinal alkyne difunctionalization, where two ligands from the I(III) reagent are sequentially transferred to an alkyne acceptor.

Finally, we will discuss alkynyl-benziodoxoles as radical acceptors for alkynylation reactions. Similar reactivity was found for the Zhdankin reagent that has been successfully applied to azidation of C-radicals, and also cyanation is possible with a cyano I(III) reagent. To summarize, this Account focuses on the design, development, mechanistic understanding, and synthetic application of hypervalent iodine(III) reagents in radical chemistry.

Copper-catalyzed C5-selective thio/selenocyanation of 8-aminoquinolines

Copper-catalyzed direct C5-position thio/selenocyanation of quinolines using commercially available, inexpensive KSCN/SeCN as the thio/selenocyanation reagent was developed, which had good tolerance toward various aliphatic or aromatic 8-aminoquinoline derivatives.

Direct thiocyanation of ketene dithioacetals under transition-metal-free conditions

The first direct thiocyanation of ketene dithioacetals has been accomplished in the presence of N-chlorosuccinimide (NCS) and NH₄SCN under transition-metal-free conditions. Terminal alkenes and hydrazine were identified to be qualified candidates for the process as well.

Recent advances in the chemistry of organic thiocyanates

In this review the different methods for the preparation of organic thiocyantes and their synthetic applications are presented.

Oxidative trifluoromethylthiolation and thiocyanation of amines: a general approach to N–S bond formation

Herein, we described a practical and efficient one-pot oxidative trifluoromethylthiolation of amines.

Direct α-acyloxylation of organic sulfides with the hypervalent (diacyloxyiodo)benzene/tetra-n-butylammonium bromide (TBAB) reagent combination

A novel metal-free approach to directly synthesize α-acyloxy sulfides from readily available alkyl sulfides utilizing hypervalent (diacyloxyiodo)benzene and TBAB reagent combination was developed.