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Multistep Reversible Redox Systems,XLII. - Photocycloadditions of α-Styrylpyrylium Salts to 4,4'-(1,3-Cyclobutanediyl)bis(pyrylium) Salts and their Thermal and Base-induced Cycloreversions
Hesse, Konrad,Huenig, Siegfried
, p. 715 - 739 (2007/10/02)
Most of the synthesized colored α-styrylpyrylium salts 2, 4, and 6 are photodimerized in the crystalline state to give colorless cyclobutanes 7, 8, and 9 (inverse photochromism), the r-ctt configuration of which is proved.Irradiation of 2h in solution produces among other products the unexpected bispyrylium salt 10.By differential scanning calorimetry (DSC) change of crystal modification of some α-styrylpyrylium salts can be seen.This change prevents 6d from photodimerization.Thermal cycloreversion (DSC) of the cyclobutanes partly occurs at unusually low temperatures: 7c (101 deg C), 7b (148 deg C), 7d (160 - 180 deg C).The solid-state transformation 2c 7c is completely reversibel.Cyclobutanes 7, 8, and 9 suffer from base-induced (redox) cycloreversion, however, rapid two-fold deprotonation produces the bis(pyrylene)cyclobutanes 22, 23, and 24.
