Steric hindrance to the syntheses and stabilities of 1,5- and 2,6-naphthalene N-permethylated diammonium salts

A series of naphthalene-1,5- and 2,6-diamines and their N-methyl derivatives were synthesized and characterized. 1H NMR spectroscopy showed that an aqueous solution of naphthalene-2,6-di(NMe3I) (2) underwent considerable demethylation over a 24 h period after its initial preparation. Exchange of the iodide nucleophilic anion by the triflate non-nucleophilic counter-ion afforded a stable quaternary aminium salt. H/D α-position exchange in D2O solutions of naphthalene-2,6-di(NH2MeCl) (3·HCl) and naphthalene-2,6-di(NHMe2Cl) (4·HCl) with acidic +NH protons was observed, but not in those of naphthalene-2,6-di(NMe3I) or triflate (2 or 5). Exhaustive per-N-methylation of naphthalene-1,5-di(NMe2) (7) afforded only the mixed naphthalene-1-NMe3I-5-NHMe2I quaternary aminium-tertiary ammonium salt due to severe 1,8-type peri-strain.