New Donors with Two-Electron Oxidation. Synthesis and Electrochemical Properties of Highly Conjugated Bis(4H-pyrans), Bis(4H-thiopyrans), and Bis(flavenes)

Article abstract of DOI:10.1021/jo00164a024

Highly conjugated Δ^4,4'-bis(4H-pyrans),-(4H-thiopyrans), and -(flavenes) separated by four and six ethanediylidene groups were synthesized by condensing the bis Wittig-Horner reagent of tetraethyl 2-butene-1,4-diyldiphosphonate with the appropriate aldehydes.The bispyran separated by three ethanediylidene groups was prepared from the vinylogous pyranyl Peterson reagent generated in situ from 2,6-diphenyl-4-<(trimethylsilyl)ethenyl>-4H-pyran, which was synthesized from the corresponding acetylenic derivative by selective hydrogenation.The cyclic voltammograms of these highly conjugated donors are compared with those of the simple derivatives that are separated only by zero to two ethanediylidene groups.The two single-electron waves that are characteristic of the latter coalesce to one two-electron oxidation wave when the number of carbons in the extended conjugation separating the two pyranyl, thiopyranyl, and flavenyl moieties reaches eight.One of the extended bisthiopyrans studied, upon one-electron electrochemical oxidation, produces a 50:50 mixture of the neutral and the dicationic species in the presence of <10percent of the cation radical species in methylene chloride.