The sulfonation and protonation of a series of azulenes. Observation of stable σ‐complexes in the sulfonation of 4,6,8‐trimethylazulene, guaiazulene and bis(3‐guaiazulenyl)methane

The protonation and sulfonation of a series of azulenes (1–9) has been studied. The protonations occur at the 1‐ and 3‐positions, and with6at the formyl oxygen. Reaction of9in CF3CO22H leads to a preferred deuteration of the 3‐ as compared with the 1‐postion (k1/k30.64) which is ascribed to relief of steric strain between the 3‐ and 4‐substituents on deuteration at C(3). Reaction of the azulenes with SO3in (2H8)dioxane or (2H3)nitromethane leads to sulfodeprotonation at the 1‐ and/or 3‐position and, if these positions are blocked bytert‐butyl groups, to sulfodeprotonation at the less reactive 5‐ and 7‐positions in accordance with predictions based on localization energies. Reaction of2and3in 105.1% H2SO4leads to sulfonation of the respective corresponding 1‐ and 3‐azulenium ions with formation of the 2‐sulfonic acids, whereas reaction of7in 98.4% H2SO4leads to sulfonation of its 1‐azulenium ion at thepara‐position of the 2‐ or 3‐phenyl group. Reaction of2, 3and9with SO3in (2H3)nitromethane leads to the formation of the stable sulfonation σ‐complexes10 + 11, 14 + 16 + 17and20, respectively. The relatively high stability of the σ‐complexes is due to the high rate of addition of SO3to the azulenes at the 1‐ and 3‐positions, combined with the very low rate of proton transfer of theC(sp3)‐bonded hydrogen of the reaction centre caused by the largeperi‐strain that results on bringing the sulfo group into the plane of the azulene skeleton. Addition of 1,3‐dimethoxybenzene to the mixtures formed from both2and3with 2 or 3 equivalents of SO3in nitromethane leads to sulfonation of the added substrate. The trans‐sulfonating species derived from2are11and13(of which the latter is the indirect one), and that derived from3is17.