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

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 1H‐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 ΔG≠ and the calculated ΔH≠ 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 1H‐NMR spectroscopy for the anti conformation.