✍ Scribed by Erwin Buncel; Ikenna Onyido
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Factors influencing C–H isotopic exchange rates in five‐membered azoles, that is imidazoles and thiazoles, under catalysis by H^+^ and M^n+^, especially transition metals, Pt(II) and Co(III) are discussed. Hydrogen ion catalysis through N(3) protonation of azoles 1–3 is generally the most efficient, with rate enhancements in the range 10^2^–10^9^ over the neutral process being attained. Metal‐ion coordination also results in effective catalysis, though less so than catalysis by protons. Catalysis of C–H exchange by M^n+^ can be studied through addition of the metal salts to a buffered solution of the heterocycle in which labile complexes exist, or on synthesized complexes such as 4–13 which are substitution‐inert thus precluding complications from unknown dissociation equilibria. A delicate balance of factors influence the ease of C–H exchange, including: (1) the magnitude of the fractional charge located at N(3) of the heterocycle through M^n+^–N(3) σ bond polarization; (2) metal‐to‐ligand π back‐bonding; (3) the electronic structure of the metal ions. These considerations have obvious consequences for deuterium‐ and tritium‐labelling of a number of biomolecules, e.g. proteins, enzymes, nucleic acids, some vitamins, as well as drugs which incorporate five‐membered azoles in their structures. Copyright © 2002 John Wiley & Sons, Ltd.
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