Planarization of the Cyclohexane Ring by Its Incorporation Into a Cyclophane Cage: Hexahydrosuperphane

What Is the Main Feature Distinguishing the Through-Space Interactions in Cyclophanes from Their Aliphatic Analogues?

Classical cyclophanes with two benzene rings have been compared with cyclophanes with one benzene ring replaced with an aliphatic part and aliphatic compounds, which are cyclophane analogues. Analysis of geometry, atomic charges, and aromatic and steric energy and investigation of intramolecular noncovalent interactions and charge mobility show that there is no special feature that distinguishes the classical cyclophanes from aliphatic analogues, so the definition of cyclophanes can be extended to other compounds.

Structure and NMR spectra of cyclophanes with unsaturated bridges (cyclophenes)

The calculated structures of several known and hypothetical cyclophanes with ethylene bridges (cyclophenes) are reported together with experimental and calculated values of their NMR parameters. Of the exchange-correlation functionals and basis sets used in this work, only the ωB97X-D/6-311++G(2d,2p) and ωB97X-D/cc-pVQZ yielded values of the C(sp3)-C(sp3) bond length close to the experimental data, although significant differences still remain. As far as the NMR parameters are concerned, except for close-lying signals, chemical shifts and coupling constants calculated at the ωB97X-D/cc-pVQZ level reproduce in most cases the experimental trends. Contrary to the calculations of geometries, an agreement between the values of the NMR parameters obtained at ωB97X-D/cc-pVQZ level and the experimental ones is the poorest compared with that of the ωB97X-D/6-311++G(2d,2p) one. Taking into account that the results of the different calculations show the same qualitative trends in most cases, we believe that they correctly describe the structure and properties of the hypothetical molecules studied here.

Spatial structure and NMR spectra of strained [2.2.2]cyclophanes

The chemical shifts and coupling constants in the NMR spectra of the title compounds are influenced by their strain and the proximity of the aromatic rings. These parameters are studied by density functional theory (DFT) calculations for 1-4 and measured for 3 and 4. We find that, in spite of the strain, the calculations reproduce the experimental values satisfactorily. This finding is of special importance for novel hypothetical molecules like hexahydrosuperphane 5 for their future identification and in searches of hydrocarbons exhibiting unusual NMR parameters. Our results demonstrate the influence of strain on the parameters studied. Most proton and carbon chemical shifts for the molecules under study having nonplanar aromatic rings differ considerably from the corresponding values for the relatively unstrained trimethylbenzenes and ethylbenzene. In addition, the calculated values of the coupling constants through three bonds in most cases do not follow Karplus relation.