Cyclic porphyrin trimer

As an approach to biomimetic catalysis, Sanders and colleagues [67] synthesized a series of 1,1,2-linked cyclic porphyrin trimers that allow the stereo- and regiochemistry of the Diels-Alder reaction of 84 and 85 within the molecular cavity to be controlled, thereby producing prevalently or exclusively the endo 86 or the exo 87 adduct. Two examples are illustrated in Scheme 4.18. At 30 °C and in the absence of 88, the reaction furnishes a mixture of diastereoisomers, while the addition of one equivalent of trimer 88 accelerates the reaction 1000-fold and the thermodynamically more stable exo adduct 87 is the sole detectable product. 

Mackay, L.G., Wylie, R.S. and Sanders, J.K.M. (1994) Catalytic acyl transfer by a cyclic porphyrin trimer - efficient turnover without product inhibition. J. Am. Chem. Soc., 116, 3141-3142. 

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

Diels-Alder reactions have also been accelerated inside the cavities of cyclic porphyrin trimers [45]. For example, diene 52 and dienophile 53 react selectively to give the exo adduct 54 (Scheme 1-14) inside the cavity of 39 [45a,b], Cyclic porphyrin trimer 39 is an effective kinetic and thermodynamic template for this reversible Diels-Alder reaction, of which the stereochemical outcome may be reversed to give mainly the endo adduct if a smaller porphyrin trimer is used [45 c]. 

A recently reported cyclic porphyrin trimer, 9, shows an extremely high binding affinity for Ceo, with a on the order of 2.1 x 10 in cyclohexane and 1.6 x 10 in toluene. For C70, the affinity in tolnene is even greater with a of 2 X 10 M-i, and is one of the highest reported affinities for C70. The binding constant is even larger for higher fullerenes, which may be due to the increased interaction between all three of the porphyrin walls. 

Figure 10 Regioselectivity of this Diels-Alder reaction is controlled by the shape of the cyclic porphyrin trimer.

Mulholland, A. R., Woodward, C. P. and Langford, S. J. Fullerene-templated synthesis of a cyclic porphyrin trimer using olefin metathesis. Chem. Commun. 47, 1494-1496, 2011. 

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

Sanders and coworkers (45-48) developed and prepared a series of cyclic Zn(II) porphyrin trimers that not only accelerate Diels-Alder reactions between pyridine-substituted dienes and dienophiles (Scheme 3) but also influence the stereochemistry of the reaction. These oligomers lower the activation energy of the reaction by trapping the diene and dienophile close to each other inside the cavity. 

SCHEME 3 Cyclic zinc-porphyrin trimer catalysts for Diels-Alder reactions of pyridine-substituted dienes and dienophiles the structure of the catalyst dictates the stereochemistry of the reaction (45-48). 

FIGURE 13 CAChe-generated model of an exo-adduct bound inside the cavity of the 2,2,2-cyclic zinc-porphyrin trimer devised by Sanders and coworkers (45,46)... 

The Diels-Alder reaction can also be catalysed by simple artificial systems. A recent example is the reversible reaction between 7 and 8, which is accelerated by (stoicheiomet-ric amounts of) a cyclic zinc-porphyrin trimer host which binds pyridine derivatives inside the cavity. [22] The product is the exo-adduct, produced up to 1(X)0 times faster than the cor-... 

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). 

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 ... 

Anderson, H.L., A. Bashall, K. Henrick, M. McPartlin, and J.K.M. Sanders (1994). Crystal-structure of a supramolecular dimer formed by n—n interactions between 2 interlocked cyclic zinc porphyrin trimers. Angew. Chem. Int. Ed. Engl. 33, 429 31. 

The first step in this templated reaction must be the combination of two porphyrin units to yield linear dimer 5 (Fig 1). Tlien either an intramolecular cyclisation occurs or reaction witli a further monomer poiphyrin yields linear porphyrin trimer. The latter process is rendered less important by tlie Bipy template which induces the reactive ends of the linear dimer intennediate to come into close proximity and so increases the rate of intramolecular cyclisation. This reaction therefore proceeds in the same way as classical metal-cation templated macrocylisations. The yield of cyclic dimer was increased by a factor of tliree in the presence of Bipy. 

---