Synthesis, Structure, and Transannular π−π Interaction of Three- and Four-Layered [3.3]Paracyclophanes

The synthesis of three- and four-layered [3.3]paracyclophanes ([3.3]PCPs) 3−5 has been accomplished by utilizing the (p-ethylbenzenesulfonyl)methyl isocyanide (EbsMIC) method. The structures of the three- to four-layered [3.3]PCPs 3−5 and their diones 8, 10, and 11 have been elucidated based on the ¹H NMR spectra and finally by X-ray structural analysis. In the three-layered [3.3]PCP-dione 8, the trimethylene bridges of the [3.3]PCP unit assume a chair conformation similar to that of 2, while the [3.3]PCP-2,11-dione unit assumes a boat conformation different from that of [3.3]PCP-dione 1 with a chair conformation. On the other hand, the two [3.3]PCP units in three-layered [3.3]PCP 3 both assume a boat conformation. In the four-layered [3.3]PCP-dione 10, the two outer [3.3]PCP units assume a boat conformation while the inner dione unit has a chair conformation. The trimethylene bridges in the four-layered [3.3]PCP 4 are highly disordered even at −150 °C. All the outer benzene rings are distorted into a boat form while the inner ones are distorted into a twist form. In the electronic spectra, bathochromic shift and hyperchromic effect are observed, but the magnitude decreases with an increase in the number of layers and the spectra become structureless. In the charge-transfer (CT) bands of the three- to four-layered [3.3]PCPs 3−5 with tetracyanoethylene (TCNE), two absorption maxima (λ_max) are observed. The effect of an increase in the layers becomes significant, and the changes in the longest wavelength λ_max values from two to three and three to four are ca. 60 and 50 nm, respectively. By comparison of the stereoisomeric four-layered [3.3]PCPs 4 (meso) and 5 (racemic), the helical arrangement of the trimethylene bridges of 5 shows a more efficient transannular π-electronic interaction. In the three- to four-layered [3.3]PCP-diones, a magnitude of the CT interaction almost comparable to that of [3.3]PCP 2 was observed, and this indicates that the −CH₂COCH₂− bridges inhibit the CT interaction and that this tendency is supported by the calculated HOMO energy levels and observed oxidation potentials. Three- and four-layered [3.3]PCPs 3−5 show reversible redox processes, and 4 and 5 show an electron-donating ability almost comparable to that of [3₆]CP. Very good correlation between the λ_max of the CT bands with TCNE and the oxidation potentials is observed.