Cyclic porphyrin dimer

Kaiser, G. Sanders, J. K. M. Synthesis under reversible conditions of cyclic porphyrin dimers nsing palladium-catalysed allyl trausesterification. Chem. 

Scheme 6.10 Sanders and coworkers template synthesis towards cyclic porphyrin dimers and trimers.

The self-assembling principle has begun to be exploited by chemists in the design of synthetic molecular receptors. Hunter and co-workers [61] describe the use of metalloporphyrin coordination chemistry to build up the self-association of a very stable cyclic porphyrin dimer 45 which recognizes derivatives of terephthalic acid (46, 47) through hydrogen-bonding interactions (Scheme 15). 

Scheme 15. Hunter s stable cyclic porphyrin dimer 45, which is able to recognize derivatives of terephthalic acid 46, 47 through hydrogen-bonding interactions.

Metal coordination also enables organic ligands to be used as templates. Sanders and co-workers have synthesized cyclic porphyrin oligomers using Glaser-Hay coupling [16,40]. This reaction oxidatively combines two terminal acetylenes to give a 1,3-buta-diyne link. The syntheses of cyclic porphyrin dimer 38 and trimer 39 were effectively templated by 4,4 -bipyridine 37 and tripyridyltriazine 22, respectively (Scheme 1-10). 

Scheme 1-10 Template directed synthesis of cyclic porphyrin dimer 38 and cyclic porphyrin trimer 39.

Sanders went on to synthesise a second generation of porphyrin hosts using guest amine-type molecules as templates. This enabled cyclic porphyrin dimers and trimers to be prepared. The trimer recognises and binds large polyoxometalate anionic clusters (e.g. PWj2O40 ) and anionic metal-carbonyl clusters containing osmium and carbon (e.g. OSioC(CO)24 -).-2 ... 

The first cyclic porphyrin dimer (cyclophane porphyrin) linked with two ester groups 15 was synthesized as a model of antenna chlorophyll dimer by condensation of a 2,12-dipropionate porphyrin and a 2,12-bis-(3-hydroxypropyl) porphyrin in high dilution in 1977." In order to clarify the mechanism of electron-transfer reactions in biological systems, a variety of porphyrin dimers have been reported as model systems of parts of the photosynthetic apparatus in the last two decades. The first synthesis of cyclic porphyrin oligomers was reported by Hamilton, Lehn and Sessler in 1986. ... 

Kamimura T, Ohkubo K, Kawashima Y et al (2013) Submillisecond-lived photoinduced charge separation in inclusion complexes composed of LrOCgo and cyclic porphyrin dimers. Chem Sci 4(4) 1451-1461... 

Fig. 8 Synthesis of cyclic porphyrin tetramer 4 from linear dimer 5 using Py4Porph as template.

Scheme 70 a Structural formula of the free-base porphyrin dimer and the porphyrin (Hj-CPDpy), and b a single crystal structure of its inclusion complex with Cgo, determined by X-ray crystallography (C6o H4-CPDpy). Reprinted from Nobukuni H, Shimazaki Y, Uno H, Naruta Y, Ohkubo K, Kojima T, Fukuzumi S, Seki S, Sakai H, Hasobe T, Tani F (2010) Supramolecular structures and photoelectronic properties of the inclusion complex of a cyclic free-base porphyrin dimer and Ceo- Chem Fur J 16(38) 11611-11623, Copyright (2010), with permission from Elsevier... 

Nobukuni H, Shimazaki Y, Uno H et al (2010) Supramolecular structures and photoelectronic properties of the inclusion complex of a cyclic free-base porphyrin dimer and Cfio. Chem Fur J 16(38) 11611-11623... 

However, we saw above that host flexibility is the key to e co-selectivity in the Diels-Alder reaction of Fig 6. Indeed the linear porphyrin dimer in Fig. 7 retains all the stereoselectivity and much of the accelerating power of its cyclic analogue even though the porphyrin units can sweep out great circles in conformational space by rotation... 

A dimer made up of two zinc porphyrins bearing a 7-azabicy-clo[2.2.1]heptadiene fused at the C2-C3 /3-positions was reported by Knapp (61). The compound was designed to dimerize with a pyrrole-over-pyrrole geometry similar to that found in the photosynthetic special pair. Dimerization at KT3 M was confirmed by VPO and JH NMR spectroscopy. Dilution to 10-5 m or addition of DMAP caused disaggregation of the complex. In the solid state, this compound assembles as a cyclic hexamer with the vicinal porphyrin planes almost perpendicular. 

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