A Beckmann Rearrangement that Forms a Lactone

Treatment of the oxime of octaethyloxochlorin under Beckmann conditions provided not the expected lactam, but the unprecendented octaethyl-1,3-oxazinochlorin, a chlorin analogue in which a pyrrole moiety of the parent oxochlorin was expanded by an oxygen atom to an 1,3-oxazinone moiety. Its mechanism of formation was demonstrated to occur along an “abnormal Beckmann” pathway, followed by intramolecular ring closure and hydrolysis. The work expands the methodologies known to convert octaethylporphyrin to pyrrole-modified porphyrin analogues with altered optical properties compared to the parent compounds.

Meehan, E.; Li, R.; Zeller, M.; Brückner, C. ‘Octaethyl-1,3-oxazinochlorin: A ß-Octaethylchlorin Analogue Made by Pyrrole Expansion’ Org. Lett. 2015, 17, 2210–2213. DOI: 10.1021/acs.orglett.5b00800.

Congratulations to Eileen’s and Ruoshi’s first paper from our group!

Collaborative Paper with Theoretician Torsten Bruhn, University of Würzburg, Published

The reduction of a free-base chlorin generally forms a bacteriochlorin (BC), while the reduction of the corresponding metallochlorin forms a metalloisobacteriochlorin (M-iBC). Nearly 50 years after its discovery, the regioselective reduction of chlorins was now explained: Bruhn, T.; Bückner, C. J. Org. Chem. 2015, 80, 4861–4868. (cover article 201580(10))

A combination of DFT-calculated average local ionization energies (ALIEs), thermodynamics of the products, and the transition-state trajectories of reduction reactions of meso-tetraaryl- and β-octaethylchlorins, as their free bases and zinc complexes, now fully delineate the theoretical basis of the reduction regioselectivity. The reactions are kinetically controlled. Steric effects originating in the conformational flexibility of the chlorin macrocycle direct the reactions toward the formation of iBCs. Only when electronic effects are strong enough to override the steric effects are BCs formed. The practical value of this work lies in the presentation of a simple predictive method toward synthetic tetrahydroporphyrins by reduction of chlorins.


Paper Rationalizing the Optical Properties of Porphyrin N‑ and Dithiaporphyrin S‑oxides Published

Following our synthetic papers on the the synthesis of porphyrin N‑ and dithiaporphyrin S‑oxides (Banerjee, S.; Zeller, M.; Brückner, C. ‘MTO/H2O2/Pyrazole-Mediated N-Oxidation of meso-Tetraarylporphyrins and –chlorins, and S-Oxidation of meso-Tetraaryldithiaporphyrin and ‑chlorin’ J. Org. Chem. 2009, 74, 4283–4288.), this theoretical paper illuminates the origin of the optical properties of these compounds. Bruhn, T.; Brückner, C. ‘Origin of the Absorption Spectra of Porphyrin N‑ and Dithiaporphyrin S‑Oxides in their Neutral and Protonated States’ Phys. Chem. Chem. Phys. 2015, 17, 3560–3569. DOI: 10.1039/c4cp04675g

Full Paper on our Quinoline-annulated Porphyrins Published

A follow-up of our communication (Akhigbe, J.; Zeller, M.; Brückner, C. ‘Quinoline-annulated Porphyrins’ Org. Lett. 2011, 13, 1322–1325.), this full paper describes the synthesis, structures, and optical properties of the quinoline-annulated porphyrins, π-extended porphyrins with particularly long wavelengths absorption spectra. Akhigbe, J.; Luciano, M.; Zeller, M.; Brückner, C. ‘Mono- and Bisquinoline-annulated Porphyrins from Porphyrin β,β’-Dione Oximes’ J. Org. Chem. 2015, 80, 499–511. DOI: 10.1021/jo502511j

Congratulations to Mike’s first paper from our group!

High pH Sensing Using a Nafion-based Optode Membrane

A further development for porpholactone-based high pH sensors: They have become water-soluble and can be incorporated into Nation membranes. Worlinsky, J. L.; Halepas, S.; Ghandehari, M.; Khalil, G.; Brückner, C. ‘Oxazolochlorins 15. High pH Sensing with Water-soluble Porpholactone Derivatives and their Incorporation into a Nafion® Optode Membrane’ Analyst 2015, 140, 190–196. DOI: 10.1039/c4an01462f

The paper was selected as HOT article and discussed on the Analyst Blog: http://blogs.rsc.org/an/