Syntheses of Some Unusual Polypyrrole Macrocycles

Abstract

Undoubtedly, the unceasing interest in polypyrrole macrocycles is iustified by the central role which porphyrin derivatives (e.9. haemoglobin, cytochromes, chlorophylls, and, in a broader sense, vitamin B12 coenzyme) play in several essential biological processes. Thus, since Hans Flscher in 1929 proved the correctness of Kiister's cyclic tetrapyrrole structure of the porphvrin nucleus, phenomenal advances has been made in the understandinq of the physical and chemical properties of this class of compounds.') A further influence which fostered the development o f porphvrin chemistry has been the interest in the theoretical aspects of aromaticity such as the extend to which modification and expansion of cyclic systems of conjugated double bonds is possible while still retaining aromatic character. Of particular significance in this connection are pyrrole macrocycles with more than four rinqs whose spectroscopic and metal chelate forming properties should be dependent on ring size. Sapphyrins (e.g. 1) are the first example of this kind of compounds which was The present work deals with the first syntheses of two polypyrrole macrocycles analoqous to the porphyrins, i.e. havinq the same number of pyrrole rings and methine bridges. The five membered macrocycle 5 was obtained by condensation of the dipyrrvlmethane 3a with the dialdehyde 4a i n the presence of hydrobromic acid and subsequent oxidation o f the reaction mixture with chloranil. It is noteworthv that, under the same conditions, the condensationofthe diformvldipyrrylmethane 3b with the tripyrrane 4b yielded a porphyrin, instead of 5 as the product. In fact, the later approach has been found to be unsuccessful for synthesizing pentapvrrole macrocvcles by other authors also.5f6) 2 -7 9 3 -

Figure Eights, Möbius Bands, and More: Conformation and Aromaticity of Porphyrinoids

“…[12,221,235] In the absence of X-ray structural analyses, 1 H NMR spectra of these systems provided convincing evidence for the adoption of the biconcave T0 5,20 conformation. As the substitution patterns of some of the 40a-H 2 derivatives were unsymmetrical, the double inversion of meso bridges led to positional isomerism, which could be observed spectroscopically.…”

Carbaporphyrinoid Systems

“…Although meso -unsubstituted hexaphyrins such as 791 favor conformations with the pyrrolic nitrogens facing inward, hexaaryl meso -substituted hexaphyrins 792 generally prefer conformations with two inverted pyrrolic subunits (Scheme ). − This arrangement places four carbons within a porphyrinoid cavity that encapsulates two regions resembling dicarbaporphyrin binding pockets. In fact, [26]­hexaphyrins(1.1.1.1.1.1) afforded organometallic derivatives similar to those seen for carbaporphyrins such as gold­(III) complex 793 and bimetallic derivatives such as 794 incorporating gold­(III), silver­(III), copper­(III), rhodium­(III), or iridium­(III). − These types of organometallic derivatives have also been explored for larger porphyrinoid systems, and examples include palladium­(II) complexes 795 and 796 .…”

Chemistry ofmeso-Aryl-Substituted Expanded Porphyrins: Aromaticity and Molecular Twist

“…On the basis of these results, this reaction was considered to involve (i) thermal conformational change from Type-II conformer to Type-I conformer, (ii) bond formation between the two unsaturated ethynyl groups, and (iii) migration of a bulkier TIPS-ethynyl group. The overall electronic system in 86a−c can be regarded as either [26]hexaphyrin or [16]diazaannuleno [16]diazaannulene, although the latter contribution should be minor owing to the perpendicular arrangement of the central vinylene bridge. These vinylene-bridged hexaphyrins possess rigidly held structures and thus exhibited a facile redox interconversion between 26π aromatic (86a−c) and 28π antiaromatic (87a−c) states.…”