Regioselective routes to disubstituted dibenzo crown ethers and their complexations

Abstract

Two isomers of bis(carbomethoxybenzo)-24-crown-8 (cis-BCMB24C8, 1, and trans-BCMB24C8, 2) were synthesized regiospecifically with acceptable to excellent yields. Cyclization in the presence of a template reagent, KPF₆, led to an essentially quantitative yield of the potassium complex of the crown ether 1; the isolated cyclization yield of pure 1 was a remarkable 89%! The methods not only avoid the very difficult separation of the isomers, but also greatly shorten the synthesis time by eliminating syringe pump usage during cyclization. The complexations of the isomeric BCMB24C8 with dibenzylammonium hexafluorophosphate (10) were studied by NMR; association constants (K_a) for 1 and 2 with the dibenzylammonium cation are 190 and 312 M⁻¹, respectively. The X-ray crystal structures of crown ether 2 and the complexes 1·KPF₆, 2·KPF₆ and pseudorotaxane 2·10 were determined.