Atropisomers of meso Tetra(N‐Mesyl Pyrrol‐2‐yl) Porphyrins: Synthesis, Isolation and Characterization of All‐Pyrrolic Porphyrins

Leiming Zhu,Leonard Himmel,Fabian Kiessling,Jean Michél Merkes,Magnus Rueping,Srinivas Banala

doi:10.1002/chem.201905637

Leiming Zhu, Leonard Himmel + Show 4 more

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https://doi.org/10.1002/chem.201905637

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Abstract

Atropisomerism has been observed in a variety of biaryl compounds and meso‐aryl substituted porphyrins. However, in porphyrins, this phenomenon had been shown only with o‐substituted 6‐membered aromatic groups at the meso‐position. We show herein that a 5‐membered heteroaromatic (N‐mesyl‐pyrrol‐2‐yl) group at the meso‐position leads to atropisomerism. In addition, we report a ‘one‐pot’ synthetic route for the synthesis of ‘all‐pyrrolic’ porphyrin (APP) with several N‐protection groups (Boc, Cbz, Ms and Ts). Among these groups, we found that only the Ms group gave four individually separable atropisomers of meso‐tetra(N‐Ms‐pyrrol‐2‐yl) porphyrin. Furthermore, the reductive removal of Cbz‐ was achieved to obtain meso‐tetra(pyrrol‐2‐yl) porphyrin. Thus, our synthetic procedure provides an easy access to a group of APPs and stable atropisomers, which is expected to expand the application of novel APP‐based materials.

Highlights

Porphyrin chemistry has been vastly developed over the last decades, as the tetrapyrroles in hemin, chlorophyll, and vitamin B12, the ‘Pigments of Life’,[1] were valuable targets for synthetic, biomimetic, and therapeutic applications
We chose to explore NH2OH·HCl mediated condensation, as it was successfully applied to meso tetra(theinyl-2-yl) porphyrin synthesis.[21]
We anticipated that removal of electron density from the pyrrole-2-aldehyde could enhance the tendency for the condensation reaction

Summary

Introduction

Porphyrin chemistry has been vastly developed over the last decades, as the tetrapyrroles in hemin, chlorophyll, and vitamin B12, the ‘Pigments of Life’,[1] were valuable targets for synthetic, biomimetic, and therapeutic applications. Already outside of porphyrin chemistry,[13] stable atropisomers of N-aryl pyrroles were reported,[14] in which the pyrrole was substituted at the 2-position or at the N atom with an orthoaryl group.[15] 3-aryl pyrrole with an additional substituent at the 4-position yielded separable atropisomers.[16] we presumed that incorporating N-protected pyrroles in a porphyrin at meso-position (i.e. meso-(pyrrol-2-yl) porphyrin) could yield separable atropisomers, and initiated the synthesis of APPs with different N-protection groups. By choosing a variety of N-protection groups for the pyrrole-2-aldehyde, we wished to explore the formation of atropisomers, and their isolation. We report successful procedures towards those APPs, and the isolation and characterization of stable atropisomers

Results and Discussion

Conclusions

Conflict of interest