Cyclophanes, electron-transfer reactions

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. 

More useful mechanistic information is obtained from intramolecular electron-transfer reactions if the kinetics for the electron-transfer step can be isolated from the effects of diffusion. The main stimulus for making such studies is the urge to design systems that mimic some of the essential features of the photosynthetic reaction centre complex and much attention has focussed on the study of porphyrin-based photoactive dyads. Thus, a series of N-alkylporphyrins linked to a quinolinium cation has been synthesized and found to display a rich variety of photoreactions. The singlet excited state of the quinolinium cation operates in both intramolecular energy- and electron-transfer reactions while the excited singlet state of the porphyrin transfers an electron to the appended quinolinium cation. Several new porphyrin-quinone dyads have been studied,including cyclophane-derived systems where the reactants are held in a face-to-face orienta-... 

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. ... 

Among the systems proposed as models for the photosynthetic reaction center, supramolecular assemblies in which Ru(II)-polypyridine complexes and 4,4 -bipyridinium units are held together noncovalently in threaded and interlocked structures have been extensively studied [43, 82-88]. In such assemblies, connections between the molecular components rely on charge transfer interactions between the electron acceptor bipyridinium units and aromatic electron donor groups (Fig. 3). For instance, in the various pseudorotaxanes formed in acetonitrile solution at 298 K by the threading of cyclophane 4 + by the dioxybenzene-containing tethers of 192+ (Fig. 17) [84], an efficient photoinduced electron... 

However, on top of the inclusion phenomena, cyclophanes have more interesting properties to offer. In his first publication on cyclophanes, Cram [13] had already expressed his opinion about some peculiarities to be expected in cyclophanes, which he outlined as follows a) electronic interaction between aromatic rings placed face to face , b) the resulting influence on substitution reactions in the aromatic rings by transannular electronic effects, c) intramolecular charge transfer complexes and d) ring strain, steric strain and transannular strain. These effects have been studied on the parent compounds in detail [3]. 

A classic example of the formation of a macrocycle by a neutral template is that of the versatile host compound and component of molecular machines, the so-called blue box, or cyclobis paraquat-para-phenylene. Reaction of the horseshoe precursor with dibromo-para-xylene leads to the formation of a tricationic intermediate that is capable of binding the template molecule (Scheme 3), which closes the macrocycle to form the tetracationic cyclophane. The jT-ir interactions of the charge-transfer variety (the complex of the product and template is colored, whereas the components are not) assisted by the charge on ihe product are a major driving force in the process, as revealed in X-ray structures of complexes. It should be noted that the interaction is of the jr-n type assisted by the complementary positive charge on the bipyridinium residues and r-electron-rich nature of the template. This supramolecu-lar synthon can be used for other cyclophanes, catenanes, and rotaxanes (see Self-Assembly of Macromolecular Threaded Systems, Self-Assembled Links Catenanes, and Rotaxanes—Self-Assembled Links, Self-Processes). 

In metal-catalyzed reactions, steric factors usually control the selectivity, and hindered groups such as 2,4,6-trimethylphenyl (mesityl) and 2,4,6-triisopropylphenyl (TRIP) are very useful dummy groups [40, 49-51]. In the absence of steric effects, the most electron-donating aryl group is generally transferred with moderate selectivity. Common dummy groups, as well as more advanced alternatives such as cyclophane- and uracil-derived dummies [44, 52], are depicted in Scheme 2b. 

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