Pyrazoles, nickel -coordinated iodination

Pyrazole does not react with iodine although pyrazolylsilver is converted into 4-iodopyrazole. 3-Iodoindazole can be obtained by the reaction of iodine with the silver salt of indazole. Kinetic studies on pyrazole iodination have been carried out by Vaughan et al. (71PMH(4)55, B-76MI40402). Coordination of pyrazole by nickel(II) in aqueous solution increases the rate of iodination by factors of two at pH 6 and eight at pH 7.2 (72JA2460). 

Rate data for the iodination of pyrazole in aqueous solution showed the reaction to be first-order in both iodine and heterocycle and an inverse first-order [H+] dependence was found over the pH range 5.96-6.74 (64JA2857). A kinetic study of the aqueous iodination of pyrazole coordinated to Ni2+ showed the coordinated ligand to react more rapidly, and a [H+] dependence that differed from that of the free ligand (82JA2460). However, the results of this study should be viewed with caution, as the presence of several nickel-pyrazole complexes in solution necessarily leads to uncertainties about the exact nature of the reactive species. 

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