Santosh K. Sahoo

Intra- and Intermolecular C−S Bond Formation Using a Single Catalytic System: First Direct Access to Arylthiobenzothiazoles

We have for the first time developed two ligand-assisted Cu(I)-catalyzed sequential intra- and intermolecular S-arylations leading to the direct synthesis of arylthiobenzothiazoles in one pot without an inert atmosphere. Low catalyst loading, inexpensive metal catalyst and ligand, lower reaction temperature, and shorter reaction times make this method superior to all reported methods for the synthesis of arylthiobenzothiazole.

Acyl-isothiocyanates as Efficient Thiocyanate Transfer Reagents

An unprecedented transfer of a thiocyanate (-SCN) group from aroyl/acyl isothiocyanate to alkyl or benzylic bromide is observed in the presence of a tertiary amine. This process is most effective when the bromomethyl proton is less acidic, while the presence of a more acidic proton gives 1,3-oxathiol-2-ylidine and other related products.

Efficient Preparation of Isothiocyanates From Dithiocarbamates Using Bromineless Brominating Reagent

For the first time, the crystal structure of a ditribromide reagent 1,1′-(ethane-1,2-diyl)dipyridinium bistribromide (EDPBT) has been determined. Utilizing this thiophilic bromineless brominating agent EDPBT, highly useful synthetic intermediates (alkyl and aryl isothiocyanates) have been achieved directly from dithiocarbamates. EDPBT can be easily prepared from readily available reagents. It has been used as a thiophilic reagent, and its thiophilicity dominates over its brominating ability for substrates amenable to bromination. This is a sustainable process for the preparation of isothiocyantes because the spent reagent can be recovered, regenerated, and reused.

Cu(II) catalysed chemoselective oxidative transformation of thiourea to thioamidoguanidine/2-aminobenzothiazole

2-Haloaryl-sec-alkyl unsymmetrical thioureas (Tu) (halo = –F, –Cl) with a catalytic amount of Cu(II) salt get oxidised in situ to their disulfide intermediates followed by an imine-disulfide rearrangement to give thioamidoguanidino (Tag) moieties at room temperature. During this process Cu(II) gets reduced to Cu(I) and forms a complex with the Tag moiety from which Tag moiety can be isolated upon treatment with ammonia. However, when the same reaction was performed at an elevated temperature with a catalytic quantity of Cu(II) salt, Tu bearing o-halogens (–F, –Cl) gave 2-aminobenzothiazoles via a dehalogenative heteroarylation path and not by the Hugerschoff path involving an electrophilic substitution reaction. For thioureas containing reactive ortho halogens (such as –Br, –I) the reaction proceeds at room temperature giving 2-aminobenzothiazoles via a dehalogenative path requiring a catalytic quantity of Cu(II). No transformation of thiourea (Tu) to Tag was observed with Cu(I) salts suggesting the requirement of an oxidising Cu(II) salt for this oxidative transformation. Mild reaction conditions, environmentally benign reagents and solvent, high yields, tolerance of various functional groups are some of the essential features of this methodology.

Stable Cu(I) Complexes with Thioamidoguanidine Possessing Halide-Bridge Structure

Treatment of an aryl-sec-alkyl unsymmetrical thiourea (Tu) with Cu(II)X(2) (X = I, Br) transform the thiourea (Tu) into a thioamidoguanidino (Tag) moiety with concomitant reduction of Cu(II) to Cu(I), which forms a [Cu(2)(I)(μ(2)-X)(2)Tag(2)] (X = I (for A) and X = Br (for B)) complex. Meanwhile, the treatment of same unsymmetrical thiourea (Tu) with Cu(I)X (X = Br) forms a stable cluster with a [Cu(I)(3)(μ(2)-S)(4)Tu(4)X(3)] core (C). Single crystal X-ray diffraction revealed that compounds A and B exhibit 1D chain with a Cu(2)(μ(2)-X)(2) core, whereas compound C is a Cu(I)(3)S(4)Br(3) cluster. Compound A is centrosymmetric due to the trans orientation of two Tag units whereas compound B is ascentric due to the cis orientation of two Tag units. In compound A, the Cu(2)I(2) core is perfectly rhomboidal where the iodine atoms are trans oriented. However in compound B, the Cu(2)Br(2) core is not perfectly rhomboidal (bowl shaped) and the bromine atoms are cis oriented. It is interesting to note that although the Tag moiety in compounds A and B contain two morpholine-O atoms; only one of the morpholine-O atoms (O2) is involved in the generation of three-dimensional network. The Cu(I)(3)S(4)Br(3) cluster in compound C contains one tri- and two tetra-coordinated Cu(I) centers. The Cu(I) cluster in C contains a Cu(2)(μ(2)-S)(2) rhomboidal plane exhibiting a chair and a boat form containing the Cu(I)(3)S(3) unit. In compounds A and B, the two Cu(I) centers are μ(2)-X bridged and have a μ(1)-S linkage, whereas in compound C the linkages are opposite having four μ(2)-S bridges and three μ(1)-Br linkages.

A one-pot conversion of di-substituted thiourea to O-organyl arylthiocarbamate using FeCl3

Unsymmetrical thiourea, which on demand can generate isothiocyanate in the presence of FeCl3, can serve as a latent isothiocyanate functionality and circumvent the difficulties associated with the direct use of reactive isothiocyanate functionality. An unusual and unorthodox reactivity has been achieved during a one-pot reaction of an unsymmetrically di-substituted thiourea with an alcohol in the presence of FeCl3 leading to an expeditious synthesis of O-organyl arylthiocarbamates. In this reaction, a thiono-ester (C‒O) bond is formed at the expense of a thioamidic (C‒N) bond and works over a wide range of structurally diverse thioureas and alcohols without affecting the other functional groups.