Protonation of Polymethylnaphthalenes and Hexahydropyrene. Formation of Stable Naphthalenium Ions and Observation of Isomerizations Based on 1H and 13C Nuclear Magnetic Resonance Studies

Some tri- and tetramethylnaphthalenes and hexahydropyrene have been protonated with FSO3H-SbF5 (1:1) and/or FSO3H in SO2C1F, and the results have been classified in terms of distinct equilibria, isomerization processes, and the formation of selected stable naphthalenium ions, as determined by and 13C NMR spectroscopy. The formation of stable “free” α, ipso α-, and “free” β-naphthalenium ions and a β,β-naphthalenium dication are reported and the electronic features of the ions discussed on the basis of the 13C NMR data. Both 1,4,5-tri- and 1,4,5,8-tetramethylnaphthalene are subject to kinetic vs. thermodynamically controlled protonation and 1,2-methyl migration occurs in the ions upon temperature changes. For the temperature-dependent protonation of 1,4,6,7-tetramethylnaphthalene a dynamic proton exchange and isomerization cycle is proposed.