Multilayered Paracyclophane

5 Multilayered Paracyclophane The three-layer paracylophane 33 was synthesized and the two stereoisomers separated. The question facing Kobayashi and colleagues was is the (-l-)-isomer of R or 5 stereochemistry Comparison of the computed and experimental ORs resolved the issue. 

They located seven conformations of (R)-33 at B3LYP/TZVP and computed the 

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In this context transannular interactions must be mentioned, although there are very few authenticated reports of such effects, and they involve solely sp2 carbon atoms. Thus, Maciel and Nakashima (256) ascribed a shielding of the carbonyl atom in 129 of approximately 10 ppm relative to 128 (X = CH2, O, S) to a transannular interaction associated with a partial charge separation (Scheme 40). Less clear-cut results were obtained from the spectra of 3- and 4-thiacyclohexanone (199,257). For the sake of completeness we note that aromatic carbon atoms experience considerable deshielding (6-9 ppm) in bi- and multilayered [2.2]paracyclophanes (258,259). This was attributed to a decrease of the excitation-energy term in the o-p expression (eq. [3], p. 222). 

Multilayered cyclophanes having three aromatic rings fixed in parallel planes above one another exhibit properties intermediate between those of the [2.2]paracyclophanes and the above-mentioned compounds 51 and 52. A cyclic compound of this type, (53), has apparently been isolated by Hubert 77>. The tetracyanoethylene complex of... 

Vogtle, F., and Hohner, G. Stereochemistry of Multibridged, Multilayered, and Multistepped Aromatic Compounds. Transanular Steric and Electronic Effects. 74, 1-29 (1978). Vogtle, F., and Neumann, P. 12.2) Paracyclophanes, Structure and Dynamics. 48, 67-129 (1974). 

Optically active triple-, quadruple-, quintuple-, and sextuple-layered [2.2]para-cyclophanes (40-43) were prepared and their CD spectra were reported [54]. The CD spectra of these multilayered [2.2]paracyclophanes were more complex than those of [2.2]paracyclophanes, exhibiting three bands of the same sign centered at ca. 270, 310, and 350 nm. The observed fine structures were ascribed to the face-to-face interactions between the distorted benzene rings. 

Nakazaki M, Yamamoto K, Tanaka S, Kametani H (1977) Syntheses of the optically active multilayered [2.2]paracyclophanes with known absolute configurations. J Org Chem 42 287-291... 

In their pioneering work in multilayered cyclophane chemistry, Longone and Chow (97) prepared a mixture of four-layered [2.2]paracyclophanes 82 and 85 by making use of the elegant procedure of Cram, which involves coupling of the p-xylylene intermediate 80 derived by a 1,6-Hofmann elimination of the corresponding quaternary ammonium base. There are available two alternatives for coupling the racemic 80 of C2 symmetry either between enantiomers of opposite chirality or between enantiomers of same chirality. The former should provide meso-82 of C2h symmetry, whereas the latter should yield 85 of D2 symmetry. 

More than 90 years after the first reported synthesis of [2.2]metacyclophane [1] and nearly 50 years after the synthesis of [2.2]paracyclophane, [2] cyclophane chemistry is still a field of ongoing research. In the beginning, work had been focused on the development of new synthetic methods yielding cyclophanes and ansa-compounds and the investigation of their physical properties. Later, the scope was extended to the incorporation of heterocycles into phanes and the more sophisticated techniques allowed the synthesis of multibridged and multilayered phanes. All this has been extensively reviewed [3]. 

All members in the series of [2 ]cyclophanes [3] up to the sixfold-clamped superphane [4] are known today. Among the [n]paracyclophanes [5] bridging with a pentamethylene chain has established an end point so far and in the case of the syntheses of multilayered [2 J cyclophanes [6] a limit of six stacked benzene rings has been achieved. 

Fig. 46 The effect of multilayer stacking of arenes. (a) Oxidation potentials of multitiered [2.2] paracyclophanes (data from [176]) (b) first and second oxidation potentials of multitiered benzo-fused bicycloundecanes (data from [177]) (c) energy of near-IR band of oxidized oligo(dibenzofulvene)s (data from [178]) and X-ray crystal structure of hexamer. Used with permission from Nakano and Yade [179]. Copyright 2003 American Chemical Society...

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