Synthesis and Reactions of 20 π-Electron β-Tetrakis(trifluoromethyl)-meso-tetraphenylporphyrins

Abstract

Porphyrin and their analogues usually have a stable aromatic (4n + 2) π-electron system. Porphyrin itself shows aromatic behavior with an 18 π-electron main conjugation pathway in the molecule and has been intensively investigated. 1 However, porphyrin derivatives with 4n π-electron delocation pathways have been little explored owing to their synthetic difficulty, 2,3 and the majority of them are expanded porphyrins 3 because the difference between resonance stabilization energies of 4n and (4n + 2) π-electron systems decreases for larger values of n. 4 To the best of our knowledge, the synthesis of 20 π-electron N,N′-dihydroporphryin (isophlorin) B, referred to for the first time by Woodward during the synthesis of chlorophyll about half a century ago, 5 has represented a challenge for porphyrin chemistry and remained elusive thus far because of its intensive tendency to undergo a twoelectron oxidation and deprotonation to form its corresponding 18 π-electron aromatic porphyrin A.(Scheme 1) Herein, we report the isolation and structural characterization of the 20 π-electron nonaromatic isophlorin 2 and its N-methylation reactions.During our ongoing efforts in synthesis and application of perfluoroalkylated porphyrins, 6 a facile and effective synthesis of the isophlorin 2 was achieved with the accidental discovery of a ready reduction of Cu(II) -tetrakis(trifluoromethyl)-meso-tetraphenylporphyrin (Cu1). 7 When adding activated zinc powder into a DMSO solution of Cu1 at room temperature under nitrogen, the color of the contents changed form green to brown after 1 h. TLC indicated almost complete consumption of the starting porphyrin and formation of a main orange product 2. (Scheme 2) It was relatively air stable 8 to be successfully isolated and thoroughly characterized, although being easily reoxidated to free-base -tetrakis(trifluoromethyl)-meso-tetraphenylporphyrin (H 2 1) with 2,6-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ).X-ray crystal diffraction analysis of 2 ( Figure 1) elucidated that it was a 20 π-electron isophlorin with a highly distorted skeleton that displays a nonplanar saddle conformation. 9 The average displacement for the -carbons from the least-squares plane of the meso-carbon atoms, which can be used to evaluate the magnitude of distortion in the macrocycle, was 1.36 Å for 2 in contrast with 0.89 Å for Cu1. 8 It is also a manifestation of serious distortion of the macrocycle that the pyrrole rings with trifluoromethyl groups and pyrroline rings without trifluoromethyl groups are titled on average by 69.7°and 33.5°, respectively, out of the least-squares plane. The remarkably strong twisting of the pyrrole units might be attributed to considerable steric congestion present between meso and substituents, as well as NH-hydrogen atoms. In addition, an expected feature exhibited in the structure of 2 is that clear C-C bond-length alternation is observed around the macrocycle periphery, which dramatically contrasts with its corresponding 18 π-electron aromatic porphyrin Cu1. 8 Spectroscopic data ...

Synthesis, structures, and aromaticity of phosphole-containing porphyrins and their metal complexes

“…In the pioneering studies on the structure-property relationship of core-modified isophlorins, Vogel and co-workers exemplified conformation-induced loss of paratropism in a saddle-shaped tetra-N-methyl β-octaethylporphyrin (N 4 -isophlorin) [15]. Chen's β-tetrakis(trifluoromethyl)-meso-tetraphenylporphyrin also adopts a highly distorted saddle conformation and shows almost complete conformation-induced loss of paratropicity in the 20π isophlorin system [16]. By contrast, Setsune's N 21 ,N 22 -bridged N 4 -isophlorins are not highly deviated from planarity and exhibit paratropic ring current effects in the 1 H NMR spectra [17].…”

Hydrogen bond controlled formation of trans-dihydroxo porphyrinato platinum(IV) complexes: Synthesis, characterization and catalytic activity in olefin epoxidation

“…The bond lengths for 16 p-electron [50,56,58] and 18 p-electron [46,[51][52][53] porphyrin rings in comparison with those of the Pt-complex obtained from X-ray analysis are listed in Table 4. From this table, it is found that there is much similarity in the values of bond distances of 18 p-electron rings and the Pt-complex reported here.…”

“…The macrocycle can have a versatile character; the most stable and abundant oxidation state of the macrocycle is the normal aromatic 18 p-electron ring in which the macrocycle has a À2 oxidation state. Under reducing conditions, monoanion radical 19 p-electron and anti-aromatic 20 p-electron dianion systems have been recognized [50][51][52][53]. In the oxidative environment of the enzyme, porphyrin p-cation radical with 17 p-electron system has been proposed [22,54,55].…”

Investigation of Aromaticity and Photophysical Properties in [18]/[20]π Porphycene Derivatives

“…[9][10][11][12][13][14] As a consequence, a majority of studies on [4n]p porphyrinoids have dealt with expanded porphyrins, because the difference in resonance stabilization energies between [4n]p and [4n+2]p electronic systems decreases for larger values of n. [5,15] In this regard, the isolation and stability of [4n]p porphyrinoids are the essential requirements for the reliable photophysical characterization. Chen et al [16] reported in 2007 that trifluoromethyl groups on pyrrole rings give rise to relatively air-stable [20]p porphyrin. After this work, Hayashi et al [17] also reported the successful synthesis and isolation of air-stable [20]p b-tetrakis(trifluoromethyl) porphycene ( [20]CF 3 Pc) through two-electron reduction and two protonation processes of [18]p b-tetrakis (trifluoromethyl)porphycene ([18]CF 3 Pc), which seems to be an analogue of etioporphycene ( [18]EtioPc) (Scheme 1).…”