Synthesis, structure, and transannular pi-pi interaction of multilayered [3.3]metacyclophanes

The synthesis of a series of three- to six-layered [3.3]metacyclophanes ([3.3]MCPs) 3-6 has been successfully accomplished by the (p-tolylsulfonyl)methyl isocyanide (TosMIC) method as a critical coupling reaction. Their important synthetic intermediates are the two- and three-layered bis(bromomethyl) compounds 11, 17, 21, and tetrakis(bromomethyl) compounds 25 and 28. The structures of the three- to six-layered [3.3]MCPs (3-6) as well as three- to six-layered [3.3]MCP-di- (22-24) and tetraones (26, 27, and 29) as the synthetic intermediates have been elucidated based on the 1H NMR data and X-ray structural analysis. These multilayered cyclophanes are constructed with two different geometries, syn-[3.3]MCP and anti-[3.3]MCP-2,11-dione. In principle, their geometries are maintained in the multilayered [3.3]MCPs, but deformation of the dihedral angle of the two benzene rings of the syn-[3.3]MCP moiety is generally observed. In the four-layered MCP 4, the central [3.3]MCP moiety takes an anti geometry. These data indicate the structural flexibility of the [3.3]MCP moiety. In the electronic spectra, rather simple and structureless absorption curves are observed, and the most significant spectral change is observed for the two to three layers and becomes less effective even if it is more layered. In the charge-transfer (CT) bands of the multilayered [3.3]MCPs with tetracyanoethylene (TCNE), the lambdamax gradually shifts to the longer wavelength region, but the extent of the shift is much smaller as the number of layers increases. In the multilayered [3.3]MCP-di- and tetraones, the anti-[3.3]MCP-dione moiety works as an insulator. Therefore, the CT interaction of the four- and five-layered [3.3]MCPs with one anti-[3.3]MCP-dione moiety (23 and 24) shows the almost comparable magnitude of the interaction with the two- and three-layered [3.3]MCPs (2 and 3), respectively. The tetraones of the three and four-layered MCPs (29 and 26) do not show CT interactions except for the six-layered MCP 27.