Conformations and Rotational Barriers of (Z)-1,2-α-Naphthostilbene Crown Ethers and Their Alkali Metal Complexes

✍ Scribed by Merz, Andreas ;Gromann, Lutz ;Karl, Andreas ;Burgemeister, T. ;Kastner, F.

• Publisher: John Wiley and Sons
• Year: 1996
• Tongue: English
• Weight: 514 KB
• Volume: 1996
• Category: Article
• ISSN: 0947-3440
• DOI: 10.1002/jlac.199619961021

✦ Synopsis

Abstract

The synthesis of 2,3‐didehydro[15]crown‐5 and −[18]crown‐6 derivatives 4a, b with vicinal α‐naphthyl groups at the double bonds starting from α‐naphthoin (3) is described. The barriers to rotation of the naphthyl groups were probed by MM2 and by dynamic ^1^H‐NMR spectroscopy. An achiral syn and a chiral anti conformation were experimentally observed at −90°C for the free ligands and at −40°C for the complexes. The experimental Δ__G__^≠^ and the calculated Δ__H__^≠^ values are in fair agreement. The barriers are substantially higher in the alkali metal complexes than in the free ligands. In the presence of racemic and potassium S‐(+)‐mandelate non‐equilibrating ion pairs with 4b were observed by low‐temperature ^1^H‐NMR spectroscopy for the anti conformation.