Templated versus non-templated synthesis of benzo-21-crown-7 and the influence of substituents on its complexing properties

Abstract

Two procedures for the synthesis of benzo-21-crown-7 have been explored. The [1+1] macrocyclization with KBF₄ as the template was found to be more efficient than the intramolecular macrocyclization without template. Pseudorotaxanes form with secondary ammonium ions bearing at least one alkyl chain narrow enough to slip into the crown ether. Substitution on benzo-21-crown-7 or on the secondary ammonium axle alters the binding affinity and binding mode. Compared to dibenzo-24-crown-8, the complexing properties of benzo-21-crown-7 turn out to be more susceptible to modifications at the crown periphery.

Keywords: benzo-21-crown-7; pseudorotaxane; self-sorting; supramolecular chemistry; template.

Scheme 1

Two synthetic procedures for the preparation of benzo-21-crown-7 (C7) and its formyl analogue 4: Top: The non-templated macrocyclization of 1 yields a mixture of crown ethers of different sizes. Bottom: With K⁺ as the template, benzo-21-crown-7 can be obtained in much better yields.

Scheme 2

Molecular structures of guests 5-H•PF₆, 6-H•PF₆, and 7-H•PF₆, and their complexes with 2-(n), C7 and 4.

Figure 1

¹H NMR spectra (500 MHz, 298 K, CDCl₃:CD₃CN = 2:1, 10.0 mM) of 5-H•PF₆ (a), mixture of 5-H•PF₆ and C7 (b), C7 (c), mixture of 5-H•PF₆ and 2-(n) (d), and 2-(n) (e). Asterisk = residual undeuterated solvent.

Figure 2

ESI-FTICR mass spectrum of a mixture of 5-H•PF₆ and 2-(n) in dichloromethane.

Figure 3

¹H NMR spectra (500 MHz, 298 K, CDCl₃, 10.0 mM) of (a) C7, (b) C7 in the presence of 1 eq. KPF₆, (c) the compound obtained after column chromatography from the KPF₆-templated reaction, and (d) 2-(n).

Figure 4

¹H NMR spectra (500 MHz, 298 K, CDCl₃:CD₃CN = 2:1, 10.0 mM) of (a) 6-HoPF₆, (b) mixture of 6-HoPF₆ and (C7+KPF₆), (c) (C7+KPF₆), (d) mixture of 5-HoPF₆ and (C7+KPF₆), (e) 5-HoPF₆. Asterisk = solvent.

Figure 5

¹H NMR spectra (500 MHz, 298 K, CDCl₃:CD₃CN = 2:1, 10.0 mM) of (a) 6-H•PF₆, equimolar mixtures of (b) 6-H•PF₆ and C7, (c) 6-H•PF₆ and 4, (d) 7-H•PF₆ and 4, and (e) 7-H•PF₆ and C7, and (f) 7-H•PF₆, Asterisk = solvent residue.