Catalytic Thiourea Promoted Electrophilic Thiocyanation of Indoles and Aromatic Amines with NCS/NH4SCN

Indole C5-Selective Bromination of Indolo[2,3-a]quinolizidine Alkaloids via In Situ-Generated Indoline Intermediate

There are many indole alkaloids that contain diverse functional groups attached to the benzene ring on the indole core. Promising biological activities of these alkaloids have been reported. Herein, we report the indole C5-selective bromination of indolo[2,3-a]quinolizidine alkaloids by adding nearly equimolar amounts of Br3 ⋅ PyH and HCl in MeOH. The resulting reaction plausibly proceeds through an indoline intermediate by the nucleophilic addition of MeOH to the C3-brominated indolenine intermediate. Data support the intermediacy of a C3-, C5-dibrominated indolenine intermediate as a brominating agent. These conditions demonstrate excellent selectivity for indole C5 bromination of natural products and their derivatives. Thus, these simple, mild, and metal-free conditions allow for selective, late-stage bromination followed by further chemical modifications. The utility of the brominated product prepared from naturally occurring yohimbine was demonstrated through various derivatizations, including a bioinspired heterodimerization reaction.

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.

Selective electrochemical para-thiocyanation of aromatic amines under metal-, oxidant- and exogenous-electrolyte-free conditions

An electrochemical oxidative para-C-H-thiocyanation of aromatic amines has been developed to construct thiocyanato aromatic compounds under metal-, oxidant-, and exogenous-electrolyte-free conditions in an undivided cell. The transformation is compatible with a range of primary, secondary, and tertiary amines and shows good functional group tolerance. This approach provides an economical and environmentally benign way for para-thiocyanation of aromatic amines.

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.

NBS-assisted palladium-catalyzed bromination/cross-coupling reaction of 2-alkynyl arylazides with KSCN: an efficient method to synthesize 3-thiocyanindoles

An efficient NBS-assisted palladium-catalyzed bromination/cross-coupling synthesis of 3-thiocyanindoles from 2-alkynyl arylazides with KSCN has been described.

A Review on Thiocyanation of Indoles

BACKGROUND

The thiocyanation of indoles is a direct way for carbon-sulfur bond formation to access 3-thiocyanato-indoles. 3-thiocyanato-indoles exhibit potent biological and pharmacological activities and also serve as building blocks to synthesize many biologically active sulfur-containing indole derivatives.

OBJECTIVE

The aim of this review is to highlight different approaches for the thiocyanation of indoles focusing on its scope and mechanism.

CONCLUSION

In this review, we have summarized various methods for the thiocyanation of indoles. Selection of new methods for the preparation of 3-thiocyanato-indoles will be done. The mechanistic aspects and significance of the methods are also briefly discussed.

Lewis Base/Brønsted Acid Cocatalysis for Thiocyanation of Amides and Thioamides

Lewis base/Brønsted acid cocatalysis for electrophilic thiocyanation of olefins is reported. Using a combination of triphenylphosphine selenide and diphenyl phosphate as a catalyst, a wide range of unsaturated amides and thioamides underwent thiocyanation to furnish thiocyanated thiazoline and oxazoline derivatives in high yields (up to 97%).

Selectfluor-initiated cyanation of disulfides to thiocyanates

A Selectfluor-initiated cyanation of disulfides to thiocyanates has been developed. In this process, Selectfluor was employed as the oxidant and trimethylsilyl cyanide was used as the cyanation reagent. It provides an eco-friendly and simple way to synthesize the thiocyanates.

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.

Synthesis and characterization of thiourea

Herein, a simple and effective method for the preparation of thiourea using a nucleophilic substitution reaction is reported. Urea and Lawesson’s reagent were used as the raw materials to prepare thiourea via a one-step method involving the sulfuration reaction, and the reaction mechanism was analyzed. The effect of the reaction time, reaction temperature, and mass ratio of the raw materials on the yield of thiourea were investigated.The most beneficial conditions used for the reaction were determined to be: Reaction time = 3.5 h, reaction temperature = 75°C, and mass ratio of urea to Lawesson’s reagent = 2:1. Under these optimal conditions, the average yield of thiourea over five replicate experiments was 62.37%. Characterization using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and thermogravimetric analysis (TGA) showed that the as-synthesized substance was thiourea. Our synthetic method has the advantages of high yield, mild reaction conditions and simplicity.

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.

An Efficient Selectfluor-Mediated Oxidative Thio- and Selenocyanation of Diversely Substituted Indoles and Carbazoles

A facile Selectfluor-mediated oxidative method for direct introduction of SCN and SeCN groups into diversely substituted indoles and carbazoles is described, by employing readily available thiocyanate and selenocyanate salts, and the scope of the method is underscored by providing 24 examples. The feasibility to sequentially introduce SCN followed by SeCN on a carbazole framework and to synthesize the corresponding S-tetrazole and Se-tetrazole derivatives is also demonstrated.

N-Thiocyanatosaccharin: A "Sweet" Electrophilic Thiocyanation Reagent and the Synthetic Applications

N-Thiocyanatosaccharin (R₁) was readily prepared from the sweet additive Saccharin in two steps with a 71% overall yield. By applying this new reagent to diverse nucleophiles such as benzothiophenes, indoles, oxindoles, aromatic amines, phenols, β-keto carbonyl compounds, and aromatic ketones, a novel electrophilic thiocyanation reaction was achieved with high yields (up to 99%). The potential recycling of Saccharin, the wide scope of substrates, and the mild reaction conditions made this protocol much more practical.

Visible light thiocyanation of N-bearing aromatic and heteroaromatic compounds using Ag/TiO2 nanotube photocatalyst

In this study, Ag/TiO₂ nanotubes (Ag/TNT) were synthesized via simple hydrothermal process, and this photocatalyst was successfully exploited in thiocyanation reactions at room temperature under visible light irradiation.