Double helices of dissymmetrical α,α'-disubstituted tripyrrins.

Dissymmetrical α,α'-disubstituted tripyrrins have been prepared using a modified synthetic protocol. Tripyrrin 2a bearing 3,5-bis(trifluoromethyl)phenyl and 4-methoxyphenyl moieties showed an anti-type dimer arrangement in the solid state. In contrast, syn-type dimers were observed for tripyrrin 2b bearing 3,5-bis(trifluoromethyl)phenyl and 3,5-di-t-butylphenyl moieties. In addition, proton-exchange NH tautomerization was observed in 2b.

Substituent Effects at the 5,10-Positions of Dianilinotripyrrins on Their Dimerization Themodynamics.

Control of the association behavior by molecular design is one of the most essential benefits in artificial supramolecular systems. 1,14-Dianilinotripyrrin has recently emerged as a novel conjugated molecule which forms a double helix in non-polar solvents with the aid of multiple interstrand hydrogen bonding interactions. In this work, we investigated the substituent effects at the 5,10-positions of tripyrrin on their association thermodynamics. This study illuminated two key findings; 1) electronic tuning by the para-substituents reduce the entropic costs thereby slightly improve the association constants, and 2) ortho-substituents force the tripyrrin core to be relatively planar, which significantly decreases the association constant due to less feasible π-stacking.

Single- and double-helices of α,α'-dibenzylaminotripyrrin: solution and solid state studies.

The dimeric association of α,α'-di(benzylamino)tripyrrin in chloroform was found to be 40 times less effective than that of previously reported α,α'-dianilinotripyrrin, which, however, led us to observe the co-crystal structure of single and double helix forms. Attachment of chiral phenylethylamines on the same tripyrrin platform was also performed to induce helical chirality.

Rational Synthesis of 5,10-Diazaporphyrins via Nucleophilic Substitution Reactions of α,α'-Dibromotripyrrin and Dihydrogenation to Give 5,10-Diazachlorins.

We report here the first rational synthesis of 5,10-diazaporphyrins via nucleophilic substitution reactions of α,α'-dibromotripyrrin. Uses of 1,3-diiminoisoindoline and 3,4-di(ethylsulfanyl)pyrrole-2,5-diimine as nucleophiles allowed for synthesis of 5,10-diazabenzoporphyrin (2) and 5,10-diaza-7,8-di(ethylsulfanyl)porphyrin (3). 3 was reduced to 5,10-diazaporphyrin (4), 5,10-diaza-2,3-dihydrogenated porphyrin (5), and 5,10-diaza-7,8-dihydrogenated porphyrin (6) with yields that were dependent upon reduction conditions. All the structures of these products were confirmed by X-ray crystallographic analysis. Their optical and electrochemical properties have been comparatively studied with those of 5,15-diazaporphyrin (7) and 5,15-diazachlorin (8). Furthermore, NH tautomers of 2 and 4 were observed as different species in solution, and the dynamic NH tautomeric behavior was studied in 4.

5,20-Diheterohexaphyrins: metal-template-free synthesis and aromaticity switching.

As the first example of expanded heteroporphyrins with heteroatoms at meso-positions, 5,20-dithiahexaphyrins and 5,20-diazahexaphyrins were synthesized via nucleophilic substitution reactions of α,α'-dibromotripyrrin as the key step. While 5,20-dithiahexaphyrins are practically nonaromatic, 5,20-[28]diazahexaphyrins show weakly antiaromatic characters and can be cleanly converted into aromatic 5,20-[26]diazahexaphyrins upon oxidation with PbO2, demonstrating distinct aromaticity switching with maintenance of dumbbell-like conformations.

Conjugated double helices via self-dimerization of α,α'-dianilinotripyrrins.

A new motif for artificial double helices was developed on the basis of α,α'-disubstituted tripyrrin. α,α'-Dibromotripyrrin 3 was prepared by gentle bromination at the pyrrolic α-positions of 5,10-diphenyltripyrrane followed by oxidation with DDQ. Nucleophilic substitution reactions of 3 with anilines proceeded efficiently to furnish a series of α,α'-dianilinotripyrrins 4-11, which displayed monomeric and dimeric forms depending upon the solvent used for crystallization and the structures of the substituted anilines. Dimeric forms show double helical structures with smooth π-conjugation as indicated by their absorption spectra. van't-Hoff plot analyses revealed that the dimerizations in CDCl3 are enthalpy-driven. Larger association constants of the dimerization are attained for 3,5-di-t-butylanilino- and 3,5-bis(trifluoromethyl)anilino-substituted tripyrrins (7 and 8) via additional multiple intermolecular interactions. In a nonpolar and aprotic solvent, tripyrrins (9 and 10) bearing bulkier 1-naphthylamino and mesitylamino groups do not dimerize but undergo unique tautomerization.

Double Ring Expansion from an Aromatic [18]Porphyrin(1.1.1.1) to an Antiaromatic [20]Porphyrin(2.1.2.1).

Double ring expansion from a 5,15-diarylporphyrin to a 5,16-diaryl-10,11,21,22-tetradehydro[20]porphyrin(2.1.2.1) occurred through a reaction sequence consisting of oxidation with PbO2 to 5,15-dioxoporphodimethene, a Corey-Fuchs reaction with tetrabromomethane in the presence of triphenylphosphine, and Fritsch-Buttenberg-Wiechell rearrangement triggered by tert-butyllithium. The obtained tetradehydro[20]porphyrin(2.1.2.1) and its mono- and dihydrogenated congeners exhibited 20 π antiaromatic character, whereas overhydrogenated congeners bearing a saturated bridge were nonaromatic owing to disrupted π conjugation.

Synthesis of Di-peri-dinaphthoporphyrins by PtCl2 -Mediated Cyclization of Quinodimethane-type Porphyrins.

Di-peri-dinaphthoporphyrins can be regarded as a key and common substructure of fused porphyrinoids. PtCl2 -mediated cycloisomerization reaction of quinodimethane-type porphyrins provided these doubly fused porphyrins, which exhibit characteristic paratropic ring currents that presumably arise from 24π antiaromatic circuit as a dominant resonance contributor. UV/Vis absorption spectra, cyclic voltammetry, and excited-state dynamics as well as theoretical calculation support this conclusion.