Cyclophanes. XV. The synthesis of [2,2](3,3',4,4')biphenylophanes and their conformational mobility

The [2,3](3,3?,4,4?)biphenylophanes (10) and (14) have been synthesized by using standard ring-contraction procedures from the dithiacyclophane (7). Both of these cyclophanes were found to be conformationally mobile and the activation energy barriers for ring-flipping for compounds (10)and (14) have been calculated to be ΔG‡c (213 K) 43.5�2 kJ mol-1 (10.4�0.5 kcal mol-1) and ΔG‡c (261 K) 53.1�2 kJ mol-1 (12.7�0.5 kcal mol- 1) respectively. The similarity of these values indicates that, in these large macrocycles, any steric effects arising from differences in bond angles and bond lengths of the bridging functions have been effectively minimized. Both compounds (10)and (14) are considered to undergo conformational ring-flipping by either a concerted disrotation of the two constituent benzene rings in the 3,3′-biphenyl moiety, or a stepwise rotation involving two discrete steps.