CONFORMATIONS AND ROTATIONAL BARRIERS OF (Z)-1,2-ALPHA-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 alpha-naphthyl groups at the double bonds starting from alpha-naphthoin (3) is described. The barriers to rotation of th e naphthyl groups were probed by MM2 and by dynamic H-1-NMR spectrosco py. An achiral syn and a chiral anti conformation were experimentally observed at -90 degrees C for the free ligands and at -40 degrees C fo r the complexes. The experimental Delta G and the calculated Delta H* values are in fair agreement. The barriers are substantially higher i n 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 H-1-NMR spectroscopy for the anti conformation.