The synthesis of dibenzo[3n]crown-n by novel methods and their cation binding studied by fluorescence spectroscopy. Part IV

The dibenzo[3n]crown-n were synthesised from 1,2-bis(o-hydroxyphenoxy)ethane obtained from 1,2-bis(o-formylphenoxy)ethane via Bayer-Williger oxidations with H₂O₂/CH ₃COOH in good yields. The cyclic condensation of 1,2-bis(o-hydroxyphenoxy)ethane with dichlorides, and ditosylates of polyethylene glycols in DMF/Me₂CO₃ gave the macrocycles dibenzo[15]crown-5, dibenzo[18]crown-6, dibenzo[21]crown-7 and dibenzo[24]crown-8. The structures were identified using IR, mass, ¹H and ¹³C NMR spectroscopy. The recognition of the molecules for the cations, Li⁺, Na⁺, K⁺, Rb⁺ and Zn²⁺ were conducted quantitatively with steady state fluorescence spectroscopy. The 1:1 association constants in acetonitrile showed a good relation of the appropriate size of the macrocyclic ether towards the fitting cation radii. Namely, dibenzo[15]crown-5 was the best for Li ⁺ binding and more than 100 times better than Na⁺ and K⁺. Dibenzo[21]crown-7 was excellent for Rb⁺ binding while K⁺ is 100 times less preferred. The largest crown ether studied, dibenzo[24]crown-8, exhibited the order of binding power, Rb ⁺ > K⁺ > Na⁺. Zn²⁺displayed, however, a marked binding with only dibenzo[18]crown-6.