Synthesis of Tetraoxygenated Terephthalates via a Dichloroquinone Route: Characterization of Cross-Conjugated Liebermann Betaine Intermediates

Cross-conjugated quinoid betaines 4 (2,5-bis(alkoxycarbonyl)-3,6-dioxo-4-(1-pyridinium-1-yl)cyclohexa-1,4-dien-1-olates; Liebermann betaines) were synthesized from 2,5-dichloro-3,6-dioxocyclohexa-1,4-diene-1,4-dicarboxylates (2) and pyridines in acetone containing H₂O. Their structure was secured by NMR spectroscopy and by X-ray diffraction analysis of 4f (alkoxy = EtO, pyridine = 4-Me₂N–C₅H₄N). Betaines 4 show comparatively high reactivity towards nucleophiles as a consequence of their cross-conjugated character. Betaine 4a and hydroxy-3,4-methylenedioxybenzene (sesamol) condense to give a pyridinium quinolate salt 14 which has a bifurcate H-bond from a pyridinium N⁺–H donor to both carbonyl (C=O) and olate (C–O⁻) acceptors in the solid state. Betaine 4b hydrolyzes in aqueous solution to give diethyl 2,5-dihydroxy-3,6-dioxocyclohexa-1,4-diene-1,4-dicarboxylate (11) as a pyridinium salt, or as polymeric zinc(II) complex of the dianion of 11 in the presence of ZnCl₂. Dihydroxyquinone 11 was analytically differentiated from its independently prepared hydroquinone form, diethyl 2,3,5,6-tetrahydroxyterephthalate (12), by NMR analysis in solution and X-ray crystal structure determination of both compounds.