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 he lical, 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 kc al mol(-1)) is close to that theoretically computed by a molecular mechanic s (MM) method (14.7 kcal mol(-1)). A second stereomutation process has also been discovered, with a conformer having antiparallel exocyclic isopropeny l groups interconverting into a second conformer having these groups in a p arallel disposition, their relative proportion being 95.5:4.5 at -135 degre es C. The corresponding MM computed barrier (7.9 kcal mol(-1)) matched sati sfactorily that determined experimentally (8.4 kcal mol(-1)). Solid-state ( CP-MAS) NMR at low temperatures showed that the latter motion occurs also i n the crystal, the corresponding barrier being however significantly higher (14.8 kcal mol(-1)) due to the effects of the crystal lattice.