85982-29-2Relevant academic research and scientific papers
Multistep, Reversible Redox Systems, XXXII. - Pyridyl-substituted Cyclobutane Derivatives: Syntheses and Redox Reactions of Cyanines, Cyclobutenes, and their Ring Opening Products
Horner, Michael,Huenig, Siegfried
, p. 642 - 657 (2007/10/02)
Bis(pyridinio)cyclobutanes 1 and 3 are transformed by air oxidation in basic media to give the blue cyanines 2 and 4.Methylation of 4 yields the cyanine 6 which also results from oxidative deprotonation of tetrakis(pyridinio)cyclobutane 5.In solutions of sodium ethanolate the cyanine 6 equilibrates with the "squaric acid" derivative 14.Protonation of 6 produces the cyclobutene cis-13 which changes into trans-13 on base catalysis.Electrocyclic ring opening transforms cis-13 stereoselectively into the butadiene Z,E-15 (t1/2 ca. 75 min, 60 deg C, DMSO) and trans-13 into Z,Z-15 (t1/2 ca. 6h, 30 deg C, DMSO).Finally, the most stable isomer E,E-15 is formed from Z,Z-15 by redox catalysis. - The cyanines 4, 6 and 14 are electrochemically reversibly reduced in two steps, but undergo only one-electron oxidations.The cyanine 13 is a typical two-step redox system, comparable to its open-chain analogue 16.The butadienes Z,Z-15 and E,E-15 represent other examples of very rare four-step redox systems.The effect of pyridinium groups on the redox properties of 4, 6, and 13-15 is discussed.
