The First Synthesis of Subporphyrins

Because of the rich coordination chemistry and unique optical and electrochemical properties, porphyrin analogues have been intensively investigated. Among them, subporphyrins have been long-awaited molecules, with only their boron complexes known to date because of the crucial role of the central boron atom as a template in synthesis. The challenges related to the synthesis of a genuine subporphyrin (boron-free) have finally been met and reported in a recent article from Kim, Osuka, and Song. A key strategy of their synthesis lies in the introduction of an exocyclic double bond at the meso-position and subsequent reduction to obtain macrocyclic conjugation. Considering the fact that the development of porphyrin chemistry is ultimately linked to the availability of free base porphyrins, this seminal work will facilitate studies on coordination chemistry and applications in materials science.

Synthesis of Pyridine Containing Crowned Fused Expanded Porphyrins

New examples of nonaromatic fused expanded porphyrins containing both pyridine and crown ether moiety as a part of macrocyclic framework were synthesized by condensing pyridine based pentapyrrane with polyether based diol in CH 2 Cl 2 in the presence of one equivalent of BF 3 ·OEt 2 under inert conditions followed by oxidation with DDQ in open air. The condensation was expected to form pyridine containing crowned expanded porphyrins but resulted in the formation of fused crowned expanded porphyrins due to intramolecular fusion of two pyrrole "N"s with two adjacent inverted thiophene "C"s as revealed by X-ray crystallography obtained for one of the macrocycle. The HRMS and NMR study supported the formation of fused crowned pyridine containing expanded porphyrins, and the macrocycles showed simple well-resolved NMR spectra where all resonances were identified easily by 2D NMR spectroscopy. The macrocycles exhibited typical nonaromatic absorption features and showed one broad band with peak maxima at 535 nm and one or two shoulder bands in the higher energy region. The protonation studies resulted in clear colour change from purple to blue and absorption bands experienced bathochromic shifts with a broad band at 662 nm which was extended up to 800 nm. The electrochemical studies revealed that the macrocycles were easier to oxidize but difficult to reduce. DFT studies indicated that the macrocycle attains a very puckered and distorted 'U' shaped structure owing to the flexibility of the crown ether chain and TD-DFT studies corroborated experimental results. The preliminary studies indicated that the macrocycles could be used as colorimetric optical sensor for detection of Cu 2+ ion.

Synthesis and Structural Properties of NIR-Absorbing Pyridine-Containing Heptaphyrins

Four examples of stable nonaromatic pyridine containing heteroheptaphyrins (pyrithiaheptaphyrins) 2-5 were synthesized in 8-13% yields by [5+2] condensation of newly synthesized pyridine-based pentapyrrane 8 and bithiophene diol 9 a-d. The X-ray crystallographic analysis of macrocycle 2 proved that the macrocycle assumes a highly planar structure with two inverted thiophene rings. The heteroheptaphyrins 2-5 are asymmetric and showed a greater number of resonances in ¹ H NMR spectra compared to our previously reported symmetric heterohexaphyrin (pyrithiahexaphyrin) 1 c. Most of the macrocyclic core protons in pyrithiahepaphyrins 2-5 experienced upfield/downfield shifts compared to pyrithiahexaphyrin 1 c indicating the alteration of π-conjugation in the macrocycles. The absorption bands were significantly red-shifted and located in the NIR region in macrocycles 2-5 compared to 1 c supporting the increase of π-delocalization. The theoretical studies support the experimental findings and NICS(0) value supports the non-aromaticity of the macrocycles.

Singly and Doubly Quinoxaline-Fused BIII Subporphyrins

B-Phenyl B^III subporphyrin-α-diones prepared in a three-step reaction sequence from the parent subporphyrin were condensed with 1,2-diaminobenzenes to give the corresponding quinoxaline-fused subporphyrins in variable yields. Quinoxaline-fused B-phenyl-5,10,15-triphenyl B^III subporphyrin was transformed to the corresponding subporphyrin-α-dione in the same three-step reaction sequence, which was then condensed with 1,2-diaminobenzene to give doubly quinoxaline-fused subporphyrin. These quinoxaline-fused subporphyrins exhibit redshifted absorption and fluorescence spectra compared with the parent one. A singly quinoxaline-fused subporphyrin bearing three meso-bis(4-dimethylaminophenyl)aminophenyl substituents shows blueshifted fluorescence in less polar solvent, which has been ascribed to emission associated with charge recombination of intramolecular charge transfer (CT) state.

BIII 5-Arylsubporphyrins and BIII Subporphine

Boron(III) 5-arylsubporphyrins and B^III subporphine are promising precursors for functional B^III subporphyrins bearing asymmetric meso-substituents. Herein, we report the first synthesis of these molecules. Among many aryl acid chlorides examined, 4-nitrobenzoyl chloride gave B^III 5-(4-nitrophenyl)subporphyrin in 10 % yield in condensation with triethylamine-tri-N-tripyrromethene-borane. The nitro group of this B^III subporphyrin was reduced with NaBH₄ to prepare B^III 5-(aminophenyl)subporphyrin, which was converted into B^III 5-phenylsubporphyrin via the corresponding diazonium salt. B^III subporphine was synthesized by condensation of triethyl orthoformate with triethylamine-tri-N-tripyrromethene-borane. Progressive removal of meso-phenyl substituents leads to continuous changes in the optical properties, whereas the B^III subporphine deviates from this trend in some properties.

Inversion Triggered by Protonation-A Rubyrin with Embedded α,β'-Pyridine Moieties

Incorporation of α,β'-pyridine into a [26]hexaphyrin(1.1.0.1.1.0) frame resulted in formation of 29,33-diaza-A,D-di-p-benzi[26]hexaphyrin (A,D-di-p-pyrirubyrin), which acquires a strictly planar conformation with two nitrogen pyridine atoms directed to the center of the macrocyclic core. The mode of α,β'-pyridine incorporation results in efficient 26-electron π-conjugation, as shown by the clear spectroscopic and structural aromatic features. The formation of the trication and tetracation through protonation is accompanied by a conformational flip of one or two pyridinium moieties to result in N-confused or doubly N-confused conformers of the original macrocyclic frame.

Synthesis of Boron(III)-Coordinated Subchlorophins and Their Peripheral Modifications

A pyrrole-cleaving modification to transform boron(III) meso-triphenylsubporphyrin into boron(III) meso-triphenylsubchlorophin has been developed. Boron(III) subchlorophins thus synthesized show absorption and fluorescence spectra that are roughly similar to those of boron(III) subchlorins, but B-methoxy boron(III) subchlorophin showed considerably intensified fluorescence and a small Stokes shift. Peripheral modification reactions of B-phenyl boron(III) subchlorophin such as regioselective nitration with Cu(NO₃ )₂ ⋅3 H₂ O, ipso-substitution reactions of boron(III) α-nitrosubchlorophin with CsF and CsCl, and Pd-catalyzed cross-coupling reactions of boron(III) α-chlorosubchlorophin with arylacetylenes, have been also explored to tune the optical properties of subchlorophins.

Oxatriphyrins(2.1.1) incorporating an ortho-phenylene motif

An understanding of fundamental aspects of archetypal organic structural motifs remains a key issue faced by the experimental and theoretical chemists. Two possible bonding modes for a disubstituted benzene ring, that is a meta and para, determines the π delocalization for oligomeric structures. When the less abundant ortho-substituted variant is introduced into a triphyrin(2.1.1) skeleton an aromatic molecule is obtained and the carbocyclic ring participates in the conjugation of the macrocycle. The two-electron reduction and introduction of boron(III) changes the aromatic character and results in an anti-aromatic structure which has been confirmed by single-crystal analysis and supported by theoretical calculations.

Nucleophilic aromatic substitution reactions of meso-bromosubporphyrin: synthesis of a thiopyrane-fused subporphyrin

meso-Bromosubporphyrin undergoes nucleophilic aromatic substitution (SN Ar) reactions with arylamines, diarylamines, phenols, ethanol, thiophenols, and n-butanethiol in the presence of suitable bases to provide the corresponding substitution products. The SN Ar reactions also proceed well with pyrrole, indole, and carbazole to provide substitution products in moderate to good yields. Finally, the SN Ar reaction with 2-bromothiophenol and subsequent intramolecular peripheral arylation reaction affords a thiopyrane-fused subporphyrin.

Subporphyrins with an axial B-C bond

Axial fabrications of subporphyrins have been conveniently accomplished by the reaction of B(methoxo)triphenylsubporphyrin with Grignard reagents such as aryl-, heteroaryl-, ferrocenyl-, β-styryl-, phenylethynyl-, and ethylmagnesium bromides. The axial groups thus introduced are not conjugated with the subporphyrin core. This situation leads to effective fluorescence quenching of subporphyrins when the axial group is strongly electron donating such as 4-dimethylaminophenyl and ferrocenyl groups.

Annulation and arylation stabilize new porphyrinoids

More pi, please! A free-base, benzannelated triphyrin with a [2.1.1] bridging pattern (1) and an azulene-derived tetracationic all-carbon porphyrinoid (2) are discussed. Compound 1 can be used to form metal triphyrin complexes, and 2 shows potential in molecular electronics and as a receptor for weakly binding anions.