Porphyrin, doubly strapped

A very different type of nitrogen-bridged expanded porphyrin is defined by two cryptand-like species recently reported by Sessler and coworkers.The first of these, the tripyrrane-strapped porphyrin 9.130, was prepared via the Schiff base-type condensation of one equivalent of diformyltripyrrane 9.128 with the a,P,a,P-atropisomer of tetrakis(2-aminophenyl)porphyrin 9.129 (Scheme 9.2.9). When two equivalents of tripyrrane 9.128 are used, the doubly strapped porphyrin 9.131 can be isolated. Both of these systems can be consistently prepared in 3CM5% yields. 

This strategy has also been independently proposed by Battersby and coworkers [87-89] to produce doubly strapped porphyrins. They used the diacid chloride of coproporphyrin I (28) as the starting compound. Coupling reactions with anthracenediol gave the single-faced-hindered porphyrin (29). 

A similar doubly-strapped porphyrin has been reported by Zhilina et al. The a,p,a,p atropisomer of mejo-tetra(o-aminophenyl)porphyrin (142) was acylated with triglycolic dichloride at room temperature in the presence of pyridine, conditions that do not cause significant isomerization of the atropisomer. The doubly-strapped porphyrin 251 was obtained in 32% yield after chromatography (Scheme 75). 

In contrast to the sequential introduction of bridging straps, Weiser and Staab used a one-step synthesis to prepare a porphyrin sandwiched between two parallel quinone units. Thus, condensation of the bis-aldehyde 252 with pyrrole in refluxing propionic acid yielded a mixture of oligomers and polymers, and three doubly-strapped porphyrins. The desired cross trans-linked porphyrin, obtained in 0.1%, was easily identified by its characteristic symmetric H-NMR spectrum. Demethylation and oxidation then yielded the desired bis-quinone porphyrin 253 (Scheme 76). 

Momenteau s route to doubly-strapped porphyrins was easily adapted to produce compounds in which an axial base was incorporated into one of the straps Condensation of tetra(o-hydroxyphenyl)porphyrin (250) (mbcture of four isomers) with one equivalent of 1,12-dibromododecane gave a mixture of two singly-linked porphyrins, depending on whether adjacent (255), or opposite (254) meso-phenyl groups were linked. This mixture was reacted with 3,5-bis(3-bromopropyl)pyridine 256 and the desired cross trans-linked isomer 257 isolated by preparative tic (5% overall yield) (Scheme 77). A similar porphyrin 259 was prepared from a,p,a,p-tetra(o-aminophenyl)porphyrin (142) in this case the straps were tied to the porphyrin skeleton by amide linkages (Scheme 78). Following iron insertion and reduction, visible absorption and H-NMR spectra of both compounds were consistent with a five-coordinate high spin (S = 2) iron(II) complex. 

With the availability of the differentially protected coproporphyrin I 261, Battersby and Hamilton adapted their syntheses to the production of doubly-strapped porphyrins... 

Modified porphyrins have been used to bind anions selectively. In 2001, Swager and coworkers reported the synthesis of the doubly strapped porphyrin 22 this compound proved to be able to coordinate two fluoride... 

Takeuchi. M. Shioya, T. Swager, T.M. Allosteric fluoride 43. anion recognition by doubly strapped porphyrin. Angew. 

Recent work has improved enantiomeric excess of epoxida-tion reaction by use of metalloporphyrins having two chiral naphthyl groups, one above and one below the porphyrin plane. This asymmetric reaction seems to arise from chiral interaction between the chiral element of the catalyst and the achiral substrate in the transition state. Doubly strapped chiral porphyrins 17 are obtainable from the coupling of a,) ,a,) -tetralds-(2-aminophenyl)porphyrin and amino-acid derivative.- " Chiral space above and below the porphyrin plane results in moderately enantio selective epoxidation. Since then, various kinds of chiral porphyrins have been synthesized as catalysts for the oxygenation of organic substrates. 

Takeuchi, M., T. Shioya, and T.M. Swager. 2001. Allosteric fluoride anion recognition by a doubly strapped porphyrin. Angew Chem Int Ed Engl 40 3372—3375. 

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