Tribenzotriquinacene-Based Crown Ethers: Synthesis and Selective Complexation with Ammonium Salts

Two tribenzotriquinacene-based crown ethers, TBTQ-dibenzo-24-crown-8 5 and TBTQ-benzo-21-crown-7 6, were prepared from the key TBTQ intermediate, 2,3-dihydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene (13), which in turn was synthesized in six steps using two variants of our cyclodehydration method on a multigram scale. The host-guest complexation properties of the TBTQ-based crown ethers 5 and 6 with a paraquat derivative, 16, and two secondary ammonium salts, 17 and 18, were studied by ¹H NMR spectroscopy and mass spectrometry. It was found that host 5 binds to the paraquat derivative 16 as a 1:1 complex in CDCl₃/CD₃CN solution with an association constant of K_a = (9.2 ± 1.8) × 10² M^-1, whereas no complexation was found for 17 and 18. In contrast, the TBTQ-crown ether 6 assembles selectively with dibutylammonium hexafluorophosphate 18 as a 1:1 complex in CDCl₃/CD₃COCD₃ [K_a = (5.0 ± 1.4) × 10² M^-1], for which a threaded [2]pseudorotaxane structure is assumed.