Porphyrins cyclophane

The distance dependencies of photoinduced electron transfer rates have been examined in anthracene-spacered porphyrin-quinone cyclophanes, and the same authors have also discussed the distance dependencies of photo-induced electron-transfer rates in benzene-, naphthalene-, and anthracene-spacered porphyrin-quinone cyclophanes and biphenylene-spacered porphyrin-quinone cyclophanes. Photoelectron transfer reactions of the porphyrin-quinone cyclophanes (3) and their zinc complexes have been examined, and in some cases at least interaction of the quinone carbonyl group with the zinc atom may be an alternative to through-space electron transfer. A study of intramolecular photoinduced electron transfer for the quinone-porphyrin cyclophane type (4) containing the especially strong acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) has appeared." The distance dependence of the TCNQ and porphyrin is of particular interest, and to this end the corresponding 2,8-naphthalenediyl-TCNQ-porphyrin has been synthesised. 

Figure 10 Structure of the iron(III) porphyrin cyclophane. Reproduced from Angew. Chem., Int. Ed. Engl., 29, 191 (1990) by pennission of VCH Verlagsgesellschaft...

Benson, D. R., Valentekovich, R., Knobler, C. B., and Diederich, F. (1991) Porphyrin-Cyclophanes Inclusion Complexation and X-ray Crystal Structure of a Zinc CX tamethyldiphenylporphyrin , Tetrahedron Lett. 47,2401-2422. 

The transition state of concerted Diels-Alder reactions has stringent regio- and stereochemical requirements and can assume settled configurations if the reaction is carried out in a molecular cavity. Cyclodextrins, porphyrin derivatives and cyclophanes are the supramolecular systems that have been most investigated. 

Detailed kinetic studies of the reaction of Fe(II) in cyclophane hemes with 02 and with CO probed polarity and steric effects the effects of deformation of the porphyrin skeleton from planarity were assessed for one compound (121). Volume profiles have been established for reactions of a lacunar Fe(II) complex with CO (122 for myoglobin with 02 and with CO and for hemerythrin with 02 (123). 

Scherer, P. L. J. and Fischer, S. F. Theoretical analysis of the photoinduced electron transfer in porphyrin-quinone cyclophanes, Chem.Phys.Lett., 190(1992), 574-580... 

Nitrogen-bearing cyclophanes like 351 [16] and 352 [17] bind larger organic anions in water due to superposition of the hydrophobic effect and electrostatic attraction. The phenanthridinium hosts like 351 have been found to form the most stable nucleotide complexes known so far. On the other hand, free tetrapyrrolic porphyrins do not bind anions since their cavity is too small to take advantage of the convergent N-H dipoles for the complex stabilization [18]. However, expanded diprotonated porphyrins like sapphyrin 353 were shown to form stable complexes with phosphate [19a] and halide [19b] anions. 

Cyclophanes are known to be efficient receptors for aromatic compounds in protic solvents. Thus, linking a cyclophane unit to a porphyrin, like in 193, provides an excellent way to study the oxidation of aromatic hydrocarbons [117]. The synthesis of 193 took advantage of an earlier protocol for the preparation of strapped porphyrins [118] using the bis-dipyrromethane 194 already linked to the cyclophane as a valuable precursor for an acid catalyzed condensation leading to the porphyrin in 9% yield (Fig. 32). 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

Computational studies followed by molecular modeling were performed on a variety of fullerene-porphyrin dyads that contained either flexible polyether or rigid steroid linkers as well as on doubly linked cyclophane-like C60-porphyrin [212]. Not surprisingly those studies confirmed that the attractive van der Waals interactions between porphyrin and C60 units cause these dyads to adopt unusual conformations in order to get together in close proximity. 

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