The conformational equilibrium of [2.2]paracyclophanes in solution

✍ Scribed by Ernst, Ludger

• Publisher: John Wiley and Sons
• Year: 1995
• Tongue: English
• Weight: 506 KB
• Volume: 1995
• Category: Article
• ISSN: 0947-3440
• DOI: 10.1002/jlac.199519950105

✦ Synopsis

Abstract

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 (RCN, 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 CCCC 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.