Substituted Heterocyclic Naphthalene Diimides with Unexpected Acidity.Synthesis, Properties, and Reactivity

摘要

Naphthalene bisimides (NDIs) with a heterocyclic 1,4-dihydro-2,3-pyrazinedione moiety have beensynthesized from both 2,6-dibromonaphthalene and 2,3,6,7-tetrabromonaphthalene bisanhydridesby means of a stepwise protocol including imidization, nucleophilic displacement of the bromineatoms by ethane-1,2-diamine, in situ reductive dehalogenation, and further oxidation. Theseheterocycles (R = n-pentyl, cyclohexyl) are yellow dyes with green emission in organic solvent,where the acid form dominates. The orange nonfluorescent conjugate base can be generatedquantitatively by CH_3COONBu_4 addition in DMSO, where it exhibits a pK,, = 7.63. The conjugatebase becomes the only detectable species (by UV-vis spectroscopy), in water solution, even underacid conditions (pH 1). In aqueous DMSO the acid/base equilibrium is a function of the DMSO/water ratio. The unexpected acidity of these heterocyclic NDIs, which justifies the reactivity with CH_2N_2, has been rationalized by DFT computational means [PBEO/6-31 + G(d,p)] in aqueoussolvent (PCM models) as a result of a strong specific solvation effect, modeled by the inclusion ofthree water molecules.