Redox-Switched Crown Ethers. 3. Cyclic-Acyclic Interconversion Coupled with Redox between Dithiol and Disulfide and Its Application to Membrane Transport

A pair of “redox-switched” monobenzo-21-crown-7 (Cr) analogues with a dithiol group at α, ω-positions (Cr_red) and a disulfide bond in the ring (Cr_ox) has been synthesized. The interconversion between these two forms was effected by treatment with redox reagents. It was found that the oxidation process (Cr_red→ Cr_ox) is remarkably subject to the metal template effect: the oxidation of Cr_red in the absence of the template gave the polymeric products (MW > 2000, 66%) in addition to Cr_ox(5.2%), cyclic dimer (15%), and cyclic trimer (3.6%), whereas the main products in the presence of Cs⁺were Cr_ox (70%) and cyclic dimer (28%) and no polymeric material was detected. The polymeric products yielded in the absence of the template depolymerized slowly to the cyclic products. Also interesting is the finding that the “kinetic” template effect is observed for the oxidation process: the pseudo-first-order rate constants for the oxidation of Cr_red by 3-methyllumiflavin are significantly enhanced with increase in the concentrations of Rb⁺ and Cs⁺. Such a rate increase could not be found for the oxidation of 1, 4-butanedithiol by 3-methyllumiflavin. The results of solvent extraction and evaluation of the association constants (K) established that (i) Cr_ox has the highest selectivity for Cs⁺, as has Cr, (ii) the Ex% (and K) of Cr_ox is smaller than that of Cr, and (iii) most importantly, Cr_redscarcely binds alkali metal cations. In ion transport across a liquid (CHC1₃) membrane, Cr_ox carried Cs⁺6.2 times faster than Cr_red. Thus, it was demonstrated that the rate of Cs⁺transport can be regulated by the interconversion between Cr_red and Cr_ox in the membrane phase. This is a new method to control the membrane transport rates by a redox-switch.