Conformational macrocyclic ring systems

A new type of iodonium salts constitute the conformationally rigid, tetranu-clear macrocyclic ring systems dubbed molecular boxes. The relatively simpler tetraaryltetraiodonium salts were obtained from 4,4 -bis(diacetoxyiodo)bi-phenyl and 4,4 -bis(trimethylsilyl)biphenyl [119]. The iodonium salt derived from 4-(4 -lithiophenyl)pyridine was made using the method of /J-(dichloroio-do)chloroethylene and it was used for the construction of hybrid iodonium-platinum (or palladium) cationic tetranuclear macrocyclic squares including some in which the ligand of the metal was a chiral biphosphine [120,121]. 

Ke, the intramolecular excimer equilibrium constant under conditions where the dynamics or rotational Isomeric transitions is suppressed, is examined on the basis of the theory of macrocyclization [Flory, P. J.-, Suter, U. W. Mutter, M. J. Am. Chem. Soc. 1976, 98, 5733, 5740,5745], The conformational averages of the required moments and polynomials are calculated using their exact matrix generation method. The calculations can rationalize the appearance of a maximum near m = 15 In the 1 1 M that have been reported for pyrene-(CH2lrn-pyrene. In order to achieve this agreement, It is Imperative that the theoretical analysis incorporate the probability of a proper angular correlation of the two pyrene ring systems when the separation of their centers Is 0.35 nm. 

Several detailed investigations of transannular IMDA cycloadditions have appeared in recent years. This transformation promises to become an important synthetic method owing to the potential for the rapid efficient construction of complex polycyclic ring systems. Other potenti strategic benefits include increased reactivity due to diminished entropic requirements (AS ). This is nicely illustrated by the facile cyclization of (101) with a tetrasubstituted dienophile, compared with the unsuccessful IMDA reaction of (102) (Figure 27). Although the cyclization of (101) proceeds with only moderate diastereoselection, many other examples have been reported that exhibit outstanding selectivity, e.g. cyclization of (103). It is conceivable that conformational constraints imposed by the macrocyclic system may lead to improved IMDA stereoselectivity relative to conventional acyclic trienes, but this point has not yet been demonstrated in any published examples. 

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