β-Trioxopyrrocorphins: pyrrocorphins of graded aromaticity.

Nivedita Chaudhri,Matthias Zeller,Ruoshi Li,Matthew J Guberman-Pfeffer,Christian Brückner

doi:10.1039/d1sc03403k

Nivedita Chaudhri, Matthias Zeller + Show 3 more

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https://doi.org/10.1039/d1sc03403k

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Abstract

Octaethyltrioxopyrrocorphins unexpectedly show macrocycle-aromatic properties, even though they contain the macrocyclic π-system of the non-aromatic pyrrocorphins (hexahydroporphyrins). Two of the four possible triketone regioisomers were first reported in 1969 by one-pot oxidation of octaethylporphyrin but remained essentially unexplored since. We detail here the targeted preparation of the remaining two triketone isomers and the optical and NMR spectroscopic properties of all isomers. All four regioisomers possess unique electronic properties, including broadly varying degrees of diatropicity that were experimentally determined using 1H NMR spectroscopy and computationally verified. Structural patterns modulating the aromaticity were recognized. These differences highlight the regioisomerically differentiated influences of the three β-oxo-functionalities. We also present the solid state structure of the two most common isomers (in their free base form or as zinc complexes), allowing further conclusions to be made about the resonance structures present in these triketones. Remarkably, also, the halochromic properties of the triketones differ sharply from those of regular (hydro)porphyrins, providing further support for the proposed 16-membered, 18 π-electron aromatic ring-current. The work conceptually expands the understanding of tris-modified hydroporphyrinoid analogues and the factors that enable and control porphyrinoid aromaticity.

Highlights

Porphyrins are characterized by the presence of a Huckelaromatic 18 p electron system, cross-conjugated with two additional double bonds, resulting in what is typically described as the porphyrinic 18 + 4 p-system (Fig. 1)
While their free bases are preferentially in the porphyrinogen tautomeric form (5,10,15,20,22,24-hexahydroporphyrin), their nickel(II) complexes are in the pyrrocorphin form (2,3,7,8,12,13-hexahydroporphyrin) (Scheme 1).4a aDepartment of Chemistry, University of Connecticut, Storrs, CT 06269-3060, USA
Better studied are the porpholactone-based meso-arylporphyrin derivatives prepared by Zhang and co-workers that contain two pyrrolines and one oxazolone or one pyrroline and two oxazolones; they exhibited intermediate aromaticity and strong regioisomeric in uences with respect to the two possible relative orientations of the lactone moieties.[7,9]

Summary

Introduction

Porphyrins are characterized by the presence of a Huckelaromatic 18 p electron system, cross-conjugated with two additional double bonds, resulting in what is typically described as the porphyrinic 18 + 4 p-system (Fig. 1).Reactions that convert one or both cross-conjugated b,b0double bonds to single bonds generate chlorins, bacteriochlorins or isobacteriochlorins, respectively.[1]. The work provides new concepts of how three b-oxo functionalities modulate the chemical and electronic properties of porphyrinoids to the extent that the usually nonaromatic psystems of pyrrocorphins assume the spectroscopic and structural characteristics of an aromatic macrocycle, whereby stark regiochemical differences are noted.

Results

Conclusion