Tuesday, February 20, 2018
4:00 pm
Noyes 147 (J. Holmes Sturdivant Lecture Hall)

Organic Chemistry Seminar

Unusual Redox Properties of π-Conjugated, Planar, Anti-Aromatic Hexapyrrolic Porphyrinoids
Chang Hee Lee, Professor, Department of Chemistry, Kangwon National University

A expanded porphyrin, hexaphyrin(1.0.1.0.1.0) derivatives, so called, rosarins are non-planar, 4nπ (24pi)- system with weakly antiaromatic character. However, annulation of the β,β′-position of the rosarin by two-carbon bridge becomes planar and strongly anti-aromatic. The noble syntheses of peripherally substituted rosarins were hurdled due to the limited accessibility of the key building blocks. We have developed a synthetic protocols of new peripherally substituted rosarin by building block approaches. The compounds displayed unusual redox chemistry and unusual spectroscopic properties.
The planar a 24π-antiaromatic rosarin displays unique redox reactivity upon protonation. For example, the compound afford a stable one-electron reduced species (4n+1)π (25pi) in the presence of acids (HCl, HBr, TFA, MSA). While, treatment of rosarin with HI yields a corresponding 26π- aromatic system by two electron reduction. The mechanistic studies indicates that the acid (HI) is quantitatively oxidized to iodine. On the other hand, simply triprotonated tricationic species was formed upon treatment with perchloric acid. Also, the time-dependent absorption spectral changes indicates that the peripheral substituents affect the redox potentials and anti-aromaticity. Rate of the the single electron reduction is faster when fluoride is substituted on the periphery. Detailed studies of the synthesized rosarin including structural property and redox chemistry will be presented. The chemistry with this study shows a model system for Proton coupled electron transfer (PCET). The PCET processes is one of the most important phenomena that serve to control a variety of chemical and biological transformations. While extensively studied in a variety of natural systems and a number of discrete metal complexes, PCET mechanisms are less well established in the case of purely organic compounds.
References
1. Protonation-coupled Redox Reaction in Planar Antiaromatic meso-Pentafluorophenyl-substituted Phenylene Bridged Annulated Rosarins. M. Ishida, S.-J. Kim, C. Preihs, K. Ohkubo, J. M. Lim, B. S. Lee, V. M. Lynch, V. V. Roznyatovskiy, P. K. Panda, C.-H. Lee, S. Fukuzumi, D. Kim, J. L. Sessler, Nature Chemistry, 2013, 5, 15-20.
2. p-Extended isomeric and expanded porphyrins, V. V. Roznyatovskiy, Chang-Hee Lee, J. L. Sessler, Chem. Soc. Rev., 2013, 42, 1921-1933.
3. A dicationic calix[4]pyrrole derivative and its use for the selective recognition and displacement-based sensing of pyrophosphate, Punidha Sokkalingam, D. S. Kim, H. Hwang, J. L. Sessler and Chang-Hee Lee, Chem. Sci., 2012, 3, 1819-1824.
4. Controlling Cesium Cation Recognition via Cation Metathesis within an Ion Pair Receptor, S. K. Kim, G. I. Vargas-Zúñiga, B. P. Hay, N. J. Young, L. H. Delmau, C. Masselin, Chang-Hee Lee, J. S. Kim, V. M. Lynch, B. A. Moyer and J. L. Sessler, J. Am. Chem.Soc. 2012, 134, 1782-1792.
5. Recent Advencements in Calix[4]pyrrole-Based Anion Receptor Chemistry, Indrajit Saha, Jeong Tae Lee, Chang-Hee Lee, Eur. J. Org. Chem. 2015, 3859-3885 (Review).
6. Endale Mulugeta, Qing He, Divya Sareen, Jonathan L.Sessler, Sung Kuk Kim, Chang-Hee Lee, Recognition, Sensing, and Trapping of Bicarbonate Anions with a Dicationic mesobis(Benzimidazolium) Calix[4]pyrrole. Chem., 2017, 3, 1-13.

Contact Victoria Brennan vicky@caltech.edu at 6151
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