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

Visible-light-promoted selenylation/cyclization of o-(1-alkynyl) benzoates to access seleno-substituted isocoumarins

A simple and efficient method to access 4-selenyl-isocoumarin derivatives through visible-light-promoted selenylation/cyclization of o-(1-alkynyl) benzoates has been developed. This transformation is performed under mild conditions and has the advantages of functional group tolerance and broad substrate scope.

Introducing Alkyl Selenocyanates as Bifunctional Reagents in Photoredox Catalysis: Divergent Access to Ambident Isomers of -SeCN

Owing to their diverse biological activities and versatility as synthetic precursors, organoselonocyanes categorize themselves as vital compounds. However, a limited reagent pool restricts their utility. In the present work, alkyl selenocyanates are hereby established as new bifunctional reagents for the simultaneous transfer of an alkyl group in addition to -SeCN. These reagents, when merged with photocatalysis, provide a key to accessing organoselenocyanates from feedstock olefins in an efficient and atom-economic fashion. A route to the analogous isoselenocyanate isomers facilitated by Lewis acid catalysis is also reported, presenting a divergent strategy for accessing both ambident isomers of -SeCN in an efficient manner.

Organophotocatalytic Remote Thiocyanation Reaction via Ring-Opening Functionalization of Cycloalkanols

The remote C(sp3)-SCN bond formation via ring-opening functionalization of cycloalkanols with N-thiocyanatosaccharin as the precursor of SCN radicals and pyrylium salt as the organic photocatalyst under visible light has been developed. Thus, various terminal keto thiocyanates were prepared without transition metals and oxidants in moderate to good yields. The simplicity, wide substrate scope and mild conditions feature its synthetic application capability.

Me3SiBr-promoted cascade electrophilic thiocyanation/cyclization of ortho-alkynylanilines to synthesize indole derivatives

A Lewis acid-promoted electrophilic thiocyanation/cyclization of ortho-alkynylanilines for the synthesis of indole derivatives has been developed. The reaction utilizes Me3SiBr as the Lewis acid and N-thiocyanatosuccinimide as the thiocyanation reagent. A series of 2-aryl-3-thiocyanato indoles were prepared in moderate to high yields under mild conditions without metals and oxidants. It provides an efficient protocol for the construction of the indole skeleton and C-SCN and C-N bonds in one step as well.

Visible-Light-Promoted Aerobic α-Thiocyanation of Carbonyl Compounds with Ammonium Thiocyanate

In the present study, we successfully developed an efficient thiocyanation of carbonyl compounds by using low-toxicity and inexpensive ammonium thiocyanate as the thiocyanate source under visible light in air (O2) at room temperature. This unified strategy is very facile for thiocyanation of various carbonyl compound derivatives (β-keto esters, β-keto amides, pyrazo-5-ones, isoxazol-5-ones, etc.). More importantly, the reaction proceeded smoothly without the addition of a photocatalyst and strong oxidant, ultimately minimizing the production of chemical waste. Furthermore, this green and sustainable synthetic chemistry can be used in the late-stage functionalization (LSF) of biorelevant compounds, which offers unique opportunities to achieve smooth and clean thiocyanation of drugs under mild reaction conditions.

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.

Divergent process for the catalytic decarboxylative thiocyanation and isothiocyanation of carboxylic acids promoted by visible light

A divergent photoinduced selective synthesis of thiocyanate and isothiocyanate derivatives from readily available carboxylic acids was developed using N-thiocyanatosaccharin and a catalytic amount of base or acid. This molecular editing strategy allowed the functionalization of bioactive compounds. A mechanism for the transformation was proposed based on control experiments.

Highly Diastereoselective Preparation of Tertiary Alkyl Thiocyanates en Route to Thiols by Stereoinvertive Nucleophilic Substitution at Nonclassical Carbocations

An effective InBr3-catalyzed nucleophilic thiocyanation of cyclopropyl alcohols has been developed. The reaction takes place at the quaternary carbon stereocenter of the cyclopropyl carbinol with a complete inversion of configuration, offering a novel pathway for the creation of complex tertiary alkyl thiocyanates with high diastereopurity. These substitution reactions proceed under mild reaction conditions and tolerate several functional groups. Additionally, thiocyanates were converted to thiols using lithium aluminum hydride.

Synthetic and mechanistic study on the conjugate isothiocyanation of enones with trimethylsilyl isothiocyanate

Alkyl isothiocyanates (R-NCS) have pharmacological applications and provide a synthetic handle to various functional groups including thioureas. There are however few methods to access alkyl isothiocyanates through the creation of the C-N bond. We have developed a simple approach for the conjugate isothiocyanation of enones by trimethylsilyl isothiocyanate (TMSNCS), which proceeds through the 1,4-addition of the weak isothiocyanate nucleophile to activated enones in the absence of external promoters. This method avoids the direct use of highly toxic acids and bases, produces β-isothiocyanato carbonyl products in yields of 87-98% under mild conditions (less than 6 hours at 0 °C), and displays wide functional group tolerance. Density functional theory calculations highlighted competing cationic and anionic mechanisms, where the enone activation by the TMSNCS reagent is accelerated in protic solvents. The selective formation of the isothiocyanate vs. thiocyanate isomers is explained by the thermodynamically-controlled nature of the reaction in which only the conjugate isothiocyanation is exergonic.

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.

Visible-light-induced C(sp3)-H thiocyanation of pyrazolin-5-ones: a practical synthesis of 4-thiocyanated 5-hydroxy-1H-pyrazoles

A direct, aerobic and visible light photocatalytic approach to synthesize 4-thiocyanated 5-hydroxy-1H-pyrazoles via cross-coupling of pyrazolin-5-ones with ammonium thiocyanate is described. Under redox-neutral and metal-free conditions, a series of 4-thiocyanated 5-hydroxy-1H-pyrazoles could be easily and efficiently obtained in good to high yields by using low-toxicity and inexpensive ammonium thiocyanate as the thiocyanate source.

Paired Electrolysis Enabled Cyanation of Diaryl Diselenides with KSCN Leading to Aryl Selenocyanates

The first example of paired electrolysis-enabled cyanation of diaryl diselenides, with KSCN as the green cyanating agent, has been developed. A broad range of aryl selenocyanates can be efficiently synthesized under chemical-oxidant- and additive-free, energy-saving and mild conditions.

Photoinduced Synthesis of Thiocyanates through Hydrogen Atom Transfer and One-Pot Derivatization to Isothiocyanates

Photoinduced benzylic C-H thiocyanation is described. A series of alkyl thiocyanates were efficiently obtained by using Selectfluor as the oxidant. Moreover, we accomplished the one-pot isothiocyanation following the C-H thiocyanation. The thiocyanates and isothiocyanates were applied to the divergent transformation of pharmaceuticals.

Electrophilic Thiocyanation of Tryptamine Derivatives: Divergent Synthesis of SCN-Containing Indole Compounds

A tandem dearomative electrophilic thiocyanation/cyclization/acylation of indoles was developed for the first time, which is enabled by acyl chloride. A variety of 3-SCN pyrroloindolines were obtained with moderate to excellent yields. Interestingly, replacement of acyl chloride with methanesulfonic acid, 2-SCN tryptamines were obtained using the same starting substrates and reagents. Furthermore, catalytic enantioselective manner of thiocyanation/cyclization/acylation reaction was also studied. An enantiomer self-disproportionation effect of 3-SCN pyrroloindolines was discovered. A series of chiral 3-SCN pyrroloindolines were obtained with high enantioselectivities.

Ultrasound-Promoted Synthesis of α-Thiocyanoketones via Enaminone C═C Bond Cleavage and Tunable One-Pot Access to 4-Aryl-2-aminothiazoles

Ultrasound has been successfully employed to promote the thiocyanation of the C═C bond in enaminones for the synthesis of α-thiocyanoketones and 2-aminothiazoles. The reactions of enaminones with ammonium thiocyanate provide α-thiocyanoketones with ultrasound irradiation at room temperature. More interestingly, simply further heating the vessel after ultrasonic irradiation leads to the selective synthesis of 2-aminothiazoles with an unconventional 4-aryl substructure.

Phosphoric Acid Catalyzed Electrophilic Thiocyanation of Indoles: Access to SCN-Containing Aryl-Indole Compounds

A phosphoric acid catalyzed electrophilic thiocyanation of 3-aryl indoles, which provides an efficient and modular approach to SCN-containing 3-aryl indole compounds, was developed for the first time. A variety of 2-SCN-3-aryl indoles were obtained with moderate to excellent yields. Furthermore, catalytic asymmetric manner of this reaction was also studied. Using chiral phosphoric acid as the catalyst, axially chiral SCN-containing 3-aryl indoles were obtained in moderate to good yields with moderate enantioselectivity.

Recent advances in photochemical and electrochemically induced thiocyanation: a greener approach for SCN-containing compound formation

Techniques utilizing photo- and electrochemically induced reactions have been developed to accelerate organic processes. These techniques use light or electrical energy (electron transfer) as a direct energy source without using an initiator or reagent. Thiocyanates are found in biologically active and pharmacological compounds and can be converted into various functional groups. It is one of the most prominent organic scaffolds. Significant development in photo- and electro-chemically induced thiocyanation procedures has been made in recent years for the conception of carbon-sulfur bonds and synthesis of pharmaceutically important molecules. This review discusses different photo- and electro-chemically driven thiocyanation C(sp3)-SCN, C(sp2)-SCN, and C(sp)-SCN bond conception processes that may be useful to green organothiocyanate synthesis. We focus on the synthetic and mechanistic characteristics of organic photo- and electrochemically accelerated C-SCN bond formation thiocyanation reactions to highlight major advances in this novel green and sustainable research field.

Transition metal catalysed direct sulfanylation of unreactive C-H bonds: an overview of the last two decades

Transition metal catalysed direct sulfanylations of unreactive C-H bonds have become a unique and straightforward synthetic strategy in late-stage C-S bond formation of relevant complex molecules. Such transformations have represented...

Electrophilic Thiocyanato Reagent Assisted Oxa-Michael/Thiocyanation of α,β-Unsaturated Ketones

A route for thiocyanation-functionalization of the electron-deficient C═C double bond was developed. Regioselective thiocyanation-etherification of α,β-unsaturated ketones was achieved. The desired products were obtained in moderate to high yields under mild conditions. It was suggested that the nucleophile was activated by the electrophilic thiocyanato reagent, and difunctionalization was achieved through a 1,4-addition/thiocyanation pathway.