Deprotonation Equilibrium of 5‑Tropolonediazonium Salt Strongly Favors 1,2,5-Tropoquinone-5-diazide Structure in Certain Solvents

Abstract

5-Tropolonediazonium salt <b>1</b> is a well-known intermediate for the preparation of 5-substituted tropolone derivatives, but 1,2,5-tropoquinone-5-diazide <b>2</b>, which is expected to be formed by deprotonation of <b>1</b>, has not been reported. We synthesized <b>2</b>, and the structures of <b>1</b> and <b>2</b> were investigated and compared. NMR and UV spectral data indicated that <b>1</b> is easily deprotonated in water, methanol, DMSO, and DMF and exists in the form of <b>2</b> in these solvents (but not in acetone or acetonitrile) because of its strong acidity (estimated p<i>K</i>_a −2.07). Thus, the acid–base equilibrium shows strong solvent-dependence. Compound <b>2</b> may be synthetically available as a carbene precursor