Corannulene cyclophane

Corannulene cyclophane 105 was prepared by treating 36 with a mixture of 1,4-bis (mereaptomethyl)benzene and potassium tert-butoxide (Scheme 31) [32]. Similarly, l,5-pentadithiol/l,6-bis(bromomethyl)corannulene cyclophane (106) was prepared from 37 and 1,5-pentadithiol in 75% yield [129]. Synthesis of [3,3]seleno/l,6-dimethylcorannulene cyclophane (107), a cyclophane from two corannulenes, was not straightforward [129]. The key step is the in situ transformation of 37 to the bis(selenide anion), which reacts with the additional 37 to afford the seleno-bridged dimer 107. 

Siegel s group is the synthesis of the first coran-nulene cyclophane 80 [56]. A striking feature of its H NMR spectrum is that the endo aromatic protons (Hj.) of the benzene ring, which are positioned almost directly over the center of the corannulene system, appear at remarkably high field ( =1,89). Furthermore, with no peak broadening up to 148 °C, it was established that dynamic conformational processes in the thioether bridges and bowl-to-bowl inversion of the corannulene moiety both have lower limits of 18 1 kcal mol . ... 

The bowl depths for 105 [32] and 106 [129] in the most stable form were determined to be 0.87 and 0.96 A, respectively, based on theoretical analysis. The deep bowl depth of 106 is consistent with the experimental results showing that its solution structure indicates a very rigid conformation. Although no significant amount of strain is introduced by forming 105, the molecule is conformationally restricted. The variable temperature proton NMR study sets the minimum limit for the barrier to benzene ring flip at 18 1 kcal/mol. Thus, construction of the cyclophane effectively locks the structure into only one bowl form [129]. Two isomers - such as a head-to-tail form and a head-to-head form of 107 - are possible. The former is more stable than the latter by ca. 1.5 kcal/mol, and its calculated bowl depth is 0.86 A, slightly flattened with respect to that of corannulene. 

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