Y. Sakamoto et al., SYNTHESES, STRUCTURES, AND TRANSANNULAR PI-PI INTERACTIONS OF MULTIBRIDGED [3(N)]CYCLOPHANES, Journal of the American Chemical Society, 118(49), 1996, pp. 12267-12275
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.