Conformational studies by dynamic NMR. 70. Stereomutations of homochiral dicarvone in solution and in the solid state

Dynamic NMR experiments have shown that the title compound exists in two helical, P and M, forms (atropisomers) having a 99.5:0.5 ratio at -40 degrees C in methylene chloride. The experimental interconversion barrier (13.3 kcal mol(-1)) is close to that theoretically computed by a molecular mechanics (MM) method (14.7 kcal mol(-1)). A second stereomutation process has also been discovered, with a conformer having antiparallel exocyclic isopropenyl groups interconverting into a second conformer having these groups in a parallel disposition, their relative proportion being 95.5:4.5 at -135 degrees C. The corresponding MM computed barrier (7.9 kcal mol(-1)) matched satisfactorily that determined experimentally (8.4 kcal mol(-1)). Solid-state (CP-MAS) NMR at low temperatures showed that the latter motion occurs also in the crystal, the corresponding barrier being however significantly higher (14.8 kcal mol(-1)) due to the effects of the crystal lattice.