Organocatalytic Regioselective Concomitant Thiocyanation and Acylation of Oxiranes Using Aroyl Isothiocyanates

Isothiocyanates: happy-go-lucky reagents in organic synthesis

Owing to their unique structural features, isothiocyanates (ITCs) are a class of highly useful and inimitable reagents as the -NCS group serves both as electrophile and nucleophile in organic synthesis. ITCs share a rich legacy in organic, medicinal, and combinatorial chemistry. Compared to their oxygen equivalents, isocyanates, ITCs are easily available, less unpleasant, and somewhat less harmful to work with (mild conditions) which makes them happy-go-lucky reagents. Functionalized ITCs can finely tune the reactivity of the -NCS group and thus can be exploited in the late-stage functionalization processes. This review's primary aim is to outline ITC chemistry in the construction and derivatization of heterocycles through the lens of sustainability. For ease and brevity, the sections are divided based on reactive centers present in functionalized ITCs and modes of cyclisation. Scrutinizing their probable unexplored directions for future research studies is also addressed.

Base-Induced Decarboxylative 1,1-Alkoxy Thiolation via Hydrothiolation of Vinylene Carbonate

A base-mediated 1,1-difunctionalization of vinylene carbonate has been achieved using two different nucleophiles, namely, thiol and alcohol, with the assistance of air (O2). In alcoholic solvents, decarboxylation occurs at room temperature to provide a 1,1-difunctionalized product, where vinylene carbonate serves as an ethynol (C2) synthon in this three-component reaction. On the other hand, in acetonitrile, exclusive hydrothiolation occurs under the basic conditions at room temperature. This method offers a one-pot decarboxylative regioselective difunctionalization of vinylene carbonate at room temperature for the construction of α-alkoxy-β-hydroxy sulfide.

C···S Tetrel Bond Favored in the Phenyl Isothiocyanate-CO2 Complex: A Rotational Study

The rotational spectrum of the phenyl isothiocyanate-CO2 complex was investigated by pulsed-jet Fourier transform microwave spectroscopy complemented by quantum chemical calculations. Only one isomer, with CO2 almost in the plane of phenyl isothiocyanate, has been detected in the pulsed jet, of which the spectrum displays a quadrupole coupling hyperfine structure due to the presence of a 14N nucleus (I = 1). This structure is nearly equal to the lowest energy geometry obtained by B3LYP-D3(BJ)/6-311++G(d,p), which has been dominated by a C···S tetrel bond (n → π* interaction) and one secondary C-H···O hydrogen bond (n → σ* interaction). Molecular electrostatic potential and natural bond orbital analysis were used to characterize the noncovalent interactions of the complex. The results from this study would lay the groundwork for the design and advancement of materials that exhibit high efficiency in capturing CO2.

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.

Electrochemical Oxidative C–H Thiocyanation or Selenocyanation of Imidazopyridines and Arenes

Regioselective electrochemical oxidative C–H thiocyanation or selenocyanation of imidazopyridines was achieved by using an undivided electrolytic cell. Transition-metal- and oxidant-free conditions are striking features of this protocol. A library of thiocyanated imidazopyridines with a broad range of functional groups were synthesized in high yields. This method was also applicable to the thiocyanation or selenocyanation of some electron-rich arenes.

Electrochemical Oxidative C–H Thiocyanation or Selenocyanation of Imidazopyridines and Arenes

Regioselective electrochemical oxidative C–H thiocyanation or selenocyanation of imidazopyridines was achieved by using an undivided electrolytic cell. Transition-metal- and oxidant-free conditions are striking features of this protocol. A library of thiocyanated imidazopyridines with a broad range of functional groups were synthesized in high yields. This method was also applicable to the thiocyanation or selenocyanation of some electron-rich arenes.

Cu(i) catalysed annulation of isothiocyanates/isocyanates with 2-iodo-sulfonamides: synthesis of benzodithiazines, benzothiadiazinones, benzothiazinylidene-anilines and benzothiazolylidene-anilines

An efficient Cu(i)-catalysed cyclisation reaction of 2-iodobenzene sulfonamides with aryl-isothiocyanates and isocyanates that affords functionalised benzodithiazines and benzothiadiazinones, respectively, has been developed. Thus, in the reaction with aryl isothiocyanates (Ar-N[double bond, length as m-dash]C[double bond, length as m-dash]S), the C[double bond, length as m-dash]S moiety participates in the cyclisation leading to a dithiazine. By contrast, in the case of aryl isocyanates (Ar-N[double bond, length as m-dash]C[double bond, length as m-dash]O), the N[double bond, length as m-dash]C part is involved in the cyclisation and a thiadiazinone is obtained. Analogous reactions of isothiocyanates with N-tosyl-2-iodo-anilines and 2-iodo-benzyl sulfonamides afford (benzothiazin-2-ylidene)anilines and (benzothiazol-2-ylidene)anilines, respectively. Probable mechanistic pathways are briefly discussed.

A cascade synthesis of S-allyl benzoylcarbamothioates via Mumm-type rearrangement

A catalyst and solvent free synthesis of S-allyl benzoylcarbamothioates has been achieved from the in situ generated benzoylcarbonimidothioates obtained by reacting MBH alcohols with aroyl isothiocyanates. An intramolecular thia-Michael addition of the in situ generated adduct triggers a Mumm-type rearrangement leading to a stereoselective synthesis of highly functionalised S-allyl benzoylcarbamothioates.

Metal-free multicomponent cascade reactions of homopropargylic amines and acyl chlorides as well as potassium thiocyanate and diiodine: an access to thiazine imides

A novel metal-free multicomponent cascade reaction was developed for the construction of thiazine imides. This four-component cascade reaction had advantages of mild reaction conditions, wide substrate scope and good atom economy. Four new bonds were formed in one pot via a 6-exo-dig iodothiolation cyclization of homopropargylic amines. The corresponding E-configurational thiazine imide products possess an exocyclic vinyliodide functional group.

Synthesis of thiazolidine-thiones, imino-thiazolidines and oxazolidines via the base promoted cyclisation of epoxy-sulfonamides and heterocumulenes

Epoxy-sulfonamides react with heterocumulenes (CS₂/RNCS/RNCO) in the presence of a base to afford ring expansion products in good to high yields with excellent regioselectivity.

Base-Promoted Synthesis of Quinoline-4(1H)-thiones from o-Alkynylanilines and Aroyl Isothiocyanates

A base-promoted synthesis of quinoline-4(1H)-thiones has been accomplished from the in situ generated o-alkynylthiourea, obtained by reacting o-alkynylanilines with aroyl/acyl isothiocyanates. A 6-exo-dig S-cyclization of the in situ generated thiourea is followed by a rearrangement to give quinoline-4(1H)-thiones.