Synthesis of 5‐chloropyrazoles by chlorodediazoniation with sulfur dioxide

Abstract
A facile synthesis of 5‐chloropyrazoles 4a‐e from 5‐aminopyrazoles 2a‐e via diazotization followed by chlorodediazoniation is described. A new application of sulfur dioxide as a catalyst was demonstrated to be the best for the chlorodediazoniation of diazonium chlorides 3a‐e.

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