The conformational equilibrium of [2.2]paracyclophanes in solution

A conformational equilibrium exists in solution between two skew forms ( s + and s − ) of [2.2]paracyclophane (1a) . The vicinal coupling constants in the 1 H‐NMR spectra of the bridge protons in the six ar ‐monosubstituted derivatives 1b–1g (RCN, NO, Br, CH 3 , CHO, and NO 2 ) indicate an increasing shift of the equilibrium towards one side in the order mentioned. The s + form (positive CCCC torsional angles of the bridges) is favored in the ( S ) enantiomer. MM2(91) and MM3(89) molecular mechanics computations suggest that this is largely caused by increasing unfavorable nonbonded interactions in the s − conformer between the substituent and the syn proton on the ortho bridge carbon atom. Thus the „effective size” of the substituent increases from 1b towards 1g .