SYNTHESES, STRUCTURES, AND TRANSANNULAR PI-PI INTERACTIONS OF MULTIBRIDGED [3(N)]CYCLOPHANES

The syntheses, conformational study, and transannular pi-pi interactio n of multibridged [3(n)]cyclophanes including the ultimate member of t his series, [3(6)](1,2,3,4,5,6)cyclophane 1, are described. The stepwi se construction of trimethylene bridges starting from [3(3)](1,3,5)cyc lophane 6 led to the synthesis of 1 by way of four- and five-bridged c yclophanes 7 and 8. The variable-temperature (VT) H-1 NMR study (CD2Cl 2) and molecular mechanics calculations (MM3) of five-bridged 8 reveal ed the most stable conformer, the relative stability order of the thre e stable isomers, and energy barriers for the trimethylene bridge inve rsion. A similar VT H-1 NMR study (toluene-d(8)) of 1 suggests the pre sence of the trimethylene bridge inversion process between two C-6h co nformers. The charge transfer (CT) bands of the complexes of multibrid ged [3(n)]cyclophanes with tetracyanoethylene (TCNE) show significant bathochromic shifts with the increase in the number of the bridges, an d 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 co mplexes of [m.n]cyclophanes and multibridged benzenophanes.