Synthesis, Conformation, and Structure of 8,ll-Bis(methoxycarbonyl)[6]paracyclophane

8,11-Bis (methoxycarbonyl) [6] paracyclophane (1c), a crystalline derivative of the smallest bridged paracyclophane system with two ester groups at the para position of the aromatic nucleus, has been successfully synthesized by utilizing thermal valence isomerization of its Dewar benzene-type isomer, [6.2.2]propelladiene (2c). The rate retardation observed in the above isomerization relative to those of the hydrocarbon 2a and the monoester 2b was attributed to development of steric repulsion between the ester groups and the benzyl hydrogens in the transition state. From the dynamic¹H NMR behavior, it has been deduced that lc occurs preferentially in conformer A rather than B (20:1 ratio) in solution and ΔG*_cfor the inversion of the bridge has been estimated to be 12.9 kcal/mol (-24 °C). The X-ray analysis of lc not only confirmed the preference of conformer A but also revealed the remarkable deformation imposed on the benzene ring and the methylene bridge. The out-of-plane bending angle of the para carbon (C(7)) is 19.4°, and that of the benzyl carbon (C(l)) is 20.2°. while the bond lengths of the bridge are normal, the bond angles of C(2) and C(3) are remarkably expanded (116.8°) from the normal angle. On the basis of the short nonbonded distance between 0(1) and H(ll) (2.43 A), it is deduced that the remarkable conformational bias in favor of conformer A over B is due to the severe nonbonded repulsion between the carbonyl oxygens and the benzyl hydrogens in the latter.