Syntheses, Structures, and Transannular π−π Interactions of Multibridged [3_n]Cyclophanes¹

The syntheses, conformational study, and transannular π−π interaction of multibridged [3_n]cyclophanes including the ultimate member of this series, [3₆](1,2,3,4,5,6)cyclophane 1, are described. The stepwise construction of trimethylene bridges starting from [3₃](1,3,5)cyclophane 6 led to the synthesis of 1 by way of four- and five-bridged cyclophanes 7 and 8. The variable-temperature (VT) ¹H NMR study (CD₂Cl₂) and molecular mechanics calculations (MM3) of five-bridged 8 revealed the most stable conformer, the relative stability order of the three stable isomers, and energy barriers for the trimethylene bridge inversion. A similar VT ¹H NMR study (toluene-d₈) of 1 suggests the presence of the trimethylene bridge inversion process between two C_6h conformers. The charge transfer (CT) bands of the complexes of multibridged [3_n]cyclophanes with tetracyanoethylene (TCNE) show significant bathochromic shifts with the increase in the number of the bridges, and this is mainly attributed to the effective hyperconjugation between the benzyl hydrogens and the benzene rings. The CT band of the TCNE−1 complex (728 nm) is the longest wavelength among those of the TCNE complexes of [m.n]cyclophanes and multibridged benzenophanes.