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Synthesis of 5-Substituted Tetrazoles from Nitriles Catalyzed by Sulfamic Acid

Tetrazoles are five-membered heterocyclic compounds and, though not generally found in nature possess interesting applications in pharmaceuticals, agriculture, photography and as components of explosives. The tetrazole ring is employed as a metabolically-stable carboxylic acid isostere in medicinal chemistry. Tetrazole derivatives have a wide range of applications in coordination chemistry as nitrogen-containing heterocyclic ligands as well as in the preparation of imidoyl azides. They are also used as plant growth regulators, herbicides, and fungicides and possess anti-allergic, antibiotic, CCK-B antagonist, anti-hypertensive, and anti-viral activities. Recently, compounds incorporating the tetrazole motif have been widely utilized for binding of aryl thiotetrazolylacetanilides with HIV-1 reverse transcriptase. This broad range of usefulness has elicited significant effort toward the preparation of tetrazoles. The most common synthetic method for the 5-substituted tetrazoles involves the [3C2] cycloaddition of an azide with a nitrile. The reaction has been carried out using numerous catalysts such as BiCl3, 17 copper triflates, CdCl2, 19 Fe(OAc)2, 20 ZnBr2, 21 triethylamine hydrochloride, Bronsted acid catalysts, various Lewis acid catalysts such as AlCl3, 24 BF3-OEt2, 25 FeCl3, 26 n-Bu4NF, 27 InCl3, 28 I2, 29 (CH3)2SnO 30 and by using some heterogeneous catalysts such as NaHSO4 SiO2, 29 CoY zeolites, Zn/Al hydrotalcite, nanocrystalline ZnO, mesoporous ZnS nanospheres, Zn hydroxyapatite, Pd(PPh3)4, 36 Cu2O, 37 and CuFe2O4 nanoparticles. 38 The reaction of phenyl isocyanide with N-halosuccinimides in the presence of azide ion under phase-transfer catalyzed (PTC) conditions has led to a simple and facile synthesis of the corresponding 5-halo-1-phenyltetrazoles. Acid catalysts are also employed for the synthesis of tetrazoles via addition of isocyanides to hydrazoic acid. Some of these known methods suffer from drawbacks such as drastic reaction conditions, low yields, use of expensive moisture-sensitive reagents, the need to use stoichiometric amounts of catalyst, formation of side-products, toxic metal catalysts, tedious work-up, or complex isolation and recovery procedures; in addition, hydrazoic acid is toxic, volatile, and explosive. Thus, there is still a need to develop new catalytic, environmentally benign and efficient protocols for the preparation of