Conformational studies by dynamic NMR. 64. Stereomutations of atropisomers and of conformational enantiomers in ethers of hindered naphthylcarbinols

Methyl or ethyl ethers of 1-naphthyl carbinols ArCR2OR' (Ar = 1-naphthyl, R' = Me, Et) can occur in a range of rotational conformations whose population varies with the nature of the substituents R. The passage between such conformation minima is achieved by rotation, during which one group R or OR' passes either the 2- or the 8-position of the naphthalene, and depending on the nature of R and OR', some of these interconversions are slow on the NMR time scale. Dynamic NMR experiments, supported by molecular mechanics calculations, show that different minima are preferred as the R group changes. These conformations are identified, their populations are determined, and the barriers to their interconversions are measured. In particular when R is a tert-butyl group, two atropisomers (with the OMe moiety near the 2- or 8-position) could be physically separated and their structures determined by NOE experiments in solution and X-ray diffraction in the solid state. Each of these exists as a pair of stereolabile enantiomers, with a barrier of 9-10 kcal mol(-1) for interconversion.