Axially Chiral β,β′-Bisporphyrins: Synthesis and Configurational Stability Tuned by the Central Metals

Abstract

In this paper, the synthesis of a new type of intrinsically chiral, directly beta,beta'-linked, octa-meso-aryl-substituted bisporphyrins is described, by using an optimized Suzuki-Miyaura coupling procedure. This strategy leads to a broad variety of such axially chiral 'superbiaryls', differing in their metalation states and substitution patterns. On the basis of an efficient route to as-yet-unknown beta-boronic acid esters of various meso-tetraarylporphyrins (TAPs) by a two-step bromination-borylation protocol, 18 axially chiral bisporphyrin derivatives were prepared in good to excellent yields. As compared to all other directly linked dimeric porphyrin systems, the joint presence of eight bulky meso substituents and the peripheral position of the porphyrin-porphyrin linkage is unprecedented. The axial configurations and rotational barriers of the pure atropo-enantiomers were investigated by HPLC-CD experiments on a chiral phase in combination with quantum chemical CD calculations. According to the HPLC experiments and in agreement with quantum chemical calculations by applying the dispersion-corrected BLYP method, the configurational stability of the central porphyrin-porphyrin axis strongly depends on the nature of the central metals. Cyclovoltammetric studies proved the systematic influence of the meso substituents and of the metal ions on the oxidation potentials of the bisporphyrins. The novel axially chiral bis(tetrapyrrole) compounds described here are potentially useful as substrates for asymmetric catalysis, biomimetic studies on directed electron-transfer processes, for photodynamic therapy (PDT), and for chiral recognition.

Supramolecular Chirality in Porphyrin Chemistry

“…The beta-beta linkage between two porphyrins was also successfully employed to fix the overall conformation in a series of bis-porphyrins, 43-49 ( Figure 13), and it resolved the corresponding enantiomers [42]. The obtained stereochemical assignments were in agreement with the exciton chirality method [43], whilst the rotational stability about the porphyrin-porphyrin axis was found to be dependent upon the nature of central metal ion, with the Gibbs free energy being as high as 115 kJ mol −1 (in the case of 45), as determined by the HPLC-UV measurements.…”

One-pot synthesis of 5-(8-ethoxycarbonyl-1-naphthyl)-10,15,20-triphenyl porphyrin (ENTPP) and spontaneous resolution upon crystallization

“…[1][2][3][4][5]. Chiral porphyrins are of particular interest in molecular recognition, asymmetric catalysis and nonlinear optics [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. There are several strategies for designing chiral porphyrins.…”

Synthesis of β‐Substituted meso‐Tetraaryl‐21,23‐dithiaporphyrins by Heck Reaction

“…Its formation is similar to what happens in palladium-catalyzed intramolecular cyclizations observed for regular β-brominated porphyrins, which give access to macrocycles with a fused five-membered ring. [22,23] The 1 H NMR spectrum of this compound reveals a significant loss of symmetry with four low-field doublets at δ = 9.43, 9.20, 9.06, and 8.96 ppm that can be attributed to the β-thiophene protons. The two doublets at δ = 8.20 and 8.13 ppm are assigned to the β protons of the noncyclized pyrrolic unit, and the singlet at δ = 7.28 ppm is assigned to H-8 of the cyclized unit.…”