Conformations and rotational barriers of (Z)-1,2-α-naphthostilbene crown ethers and their alkali metal complexes

Article abstract of DOI:10.1002/jlac.199619961021

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 nonequilibrating ion pairs with 4b were observed by low-temperature ¹H-NMR spectroscopy for the anti conformation. VCH Verlagsgesellschaft mbH, 1996.