Optical rotatory dispersion studies. 125. Independent evidence for preference of axial deuterium vs. axial hydrogen through variable-temperature circular dichroism spectra of (4S)-2,2-dimethyl-4-deuteriocyclohexanone and (3S)-3-deuterio-4,4-dimethylcyclohexanone

Abstract

The temperature-dependent circular dichroism (CD) spectra of (4S)-2,2-dimethyl-4-deuteriocyclohexanone(III) and (3S)-3-deuterio-4,4-dimethylcyclohexanone(IV) now present independent and additional evidence supporting earlier conclusions that the observed intensity changes of (3R)-2,2-dimethyl-3-deuteriocyclohexanone(I) and (5S)-2,2-dimethyl-5-deuteriocyclohexanone(II) reflect a shift of the equilibrium toward that conformer in which the deuterium is in the axial orientation. A significant increase in the rotational strength (R)/sub obsd/ is observed for III by lowering the temperature from 293 to 77/sup 0/K. The (R)/sub obsd/ for IV remains virtually unchanged over the same temperature range. The two chair conformations involved in the equilibrium and their octant diagrams together with the estimated rotational strengths in their axial (R)/sub ax/ and equatorial (R)/sub eq/ positions are illustrated. The decrease of the (R)/sub obsd/ with lowering the temperature leads to the unambiguous conclusion that the conformer with the deuterium in the axial position is energetically preferred. In contrast to III, the (R)/sub ax/ and (R)/sub eq/ values for IV are very small numbers since the methyl groups lie in a symmetry plane in both conformations and do not contribute to the rotational strength. The independent CD of IV has particular relevance since it excludes the alternative possibility that the observed changes of the rotational strength in I, more » II, and III are associated with the presence of twist conformations. 2 figures. « less