On the Aromaticity and Acidity of 1-Hydroxy-4,5-benzotropylium Derivatives

The aromaticity of 1-hydroxy-4,5-benzotropylium was assessed based on experimental and theoretical investigations. An X-ray crystallographic analysis revealed a decrease of bond alternation in the seven-membered ring of 1-hydroxy-4,5-benzotropylium derivatives compared with that of the parent 4,5-benzotropones, which is indicative of an increase in aromaticity upon protonation. NICS and AICD calculation also supported the increased aromaticity of 1-hydroxy-4,5-benzotropylium. The pK_a values for a series of 1-hydroxy-4,5-benzotropylium derivatives were also determined.