Abstract
Synthetic methods have been developed to prepare novel 1,3-benzotellurazoles carrying acylamino and arylamino moieties in position 2, in order to investigate their propensity to self-assemble to supramolecular structures. The targeted compounds were obtained in yields ranging from 44% to 67%, by reacting bis(2-aminophenyl) ditelluride with acyl- and aryl isothiocyanates, respectively, and subsequent reductive cyclization of the resulting thiourea derivatives. Seven novel 1,3-benzotellurazole derivatives were prepared: 2-benzoylamino-1,3-benzotellurazole, 2-(4-chlorobenzoylamino)-1,3-benzotellurazole, 2-(2-bromobenzoylamino)-1,3-benzotellurazole, 2-(4-bromobenzoylamino)-1,3-benzotellurazole, 2-(4-methoxybenzoylamino)-1,3-benzotellurazole, 2-phenylamino-1,3-benzotellurazole, and 2-(4-chlorophenylamino-1,3-benzotellurazole. A simplified protocol was employed to synthesize all acyl isothiocyanates needed for their preparation from benzoyl halide derivatives and potassium thiocyanate. The reductive cyclization of the intermediate thioureas was challenging, only the use of hydroxymethanesulfinate in the presence of elemental mercury provided synthetically useful product yields. A mechanism was proposed, consisting of the insertion of mercury into the Te-Te bond, followed by intramolecular nucleophilic attack of the thiocarbonyl moiety by the resulting insertion product. All 2-acylamino-1,3-benzotellurazoles are crystalline solids, which are stable to ambient light, air and moderate heat. A characterization of selected samples by X-ray crystallography indicated that they form dimers in solid state, resulting from hydrogen bonding between the exocyclic and endocyclic nitrogen atoms of two adjacent molecules. This sets them apart from 2-alkyl- and 2-aryl-1,3-benzotellurazoles, which are known to self-assemble into supramolecular wires.
Highlights
The chemistry of the five-membered Te, N-containing heterocycles has found recent interest, largely due to their tendency to self-assemble into supramolecular structures
The 1,2,5-telluradiazoles and isotellurazole Noxides have been at the focus of prior work; the sensitivity to hydrolysis of the former and low yield access to the latter make the availability of alternatives desirable [1,2,3]. 1,3-Benzotellurazoles are attractive in this respect, due to their high stability towards air, light and heat, as well as their established ability to form supramolecular wires via intermolecular Te-N coordination [4,5,6] (Figure 1)
While the preparation of thioureas derived from bis(2-aminophenyl) ditelluride proceeded smoothly, their ring closure was challenging
Summary
The chemistry of the five-membered Te, N-containing heterocycles has found recent interest, largely due to their tendency to self-assemble into supramolecular structures. Recent advances in the preparation of 1,3benzotellurazoles [7] have drastically improved access to this class of compounds Their ability to form aggregates is influenced by the presence and nature of substituents. This prompted us to seek improved synthetic access to 2-amino and 2-acylamino substituted 1,3-benzotellurazoles, to investigate the effects, which the exocyclic nitrogen has on self-assembly. Access to these compounds via the reaction of 2-iodophenylisocyanide with elemental tellurium and the lithium salts of secondary amides at low temperature has been reported, but is somewhat challenging and narrow in scope [8]. We investigated ring closures analogous to those previously reported for phenols and thiophenols carrying thiourea moieties in ortho position, which were converted to benzoxazoles and benzothiazoles by heating in the presence nickel peroxide [9], lead oxide [10] or mercuric oxide [11] in moderate to good yields (Figure 2)
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