Tetrazoles

Tetrazoles were discovered by Bladin over 120 years ago (1885). Since then, tetrazole and its derivatives have attracted interest by their unique structure and extreme characteristics. Especially great activity in this field of chemistry has been observed since 1996. The number of publications and patents on preparation methods, properties, and applications of tetrazoles grows every year with a higher rate than with respect to other heterocyclic systems. This fact actually reflects the significant achievements in fundamental research and industry in tetrazole chemistry at the end of the twentieth and beginning of the twenty-first centuries. This chapter describes various aspects regarding tetrazoles: electronic structure, thermodynamic and theoretical problems, including aromaticity, acid–base properties, tautomerism, and also experimental structural methods. Mechanism and kinetics of unimolecular thermal and photochemical reactions of tetrazoles, molecular rearrangements, etc., are discussed. The reactivity of ring nitrogen and carbon atoms is treated: electrophilic attack at endocyclic nitrogen and carbon atoms, nucleophilic attack at endocyclic carbon, complex formation with metal ions. Considerable attention is paid to the reactivity of substituents attached to the ring nitrogen in neutral tetrazoles, tetrazolate anions, and tetrazolium cations, and also to the electrophilic and nucleophilic attack at endocyclic carbon. Various possible procedures are described for ring synthesis from acyclic compounds, and also by transformations of other heterocyclic systems. Preferred routes to tetrazole classes are indicated. In all sections of this chapter, attention is focused on new constructive ideas, efficient methods of synthesis and transformation of tetrazoles, and urgent problems of tetrazole chemistry. In the concluding sections, data are given on important compounds and applications: biologically active substances, drug applications, energetic tetrazoles, tetrazoles as promising objects in supramolecular chemistry and nanotechnology, and also as activators of chemical and biochemical reactions, etc.