Optical rotatory dispersion studies—131: Demonstration of a conformational isotope effect in deuterium substituted cyclopentanones

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The synthesis and chiroptical properties of three chiral, deuterium-substituted cyclopentanones are reported which shed light on subtle conformational changes. Thus, the CD spectra of (3 R)-3-deuteriocyclopentanone ( 1), (3 S, 4 S)-3,4-dideuteriocyclopentanone ( 2) and (3 S, 4 S)-2,2,3,4,5,5-hexadeuteriocyclopentanone ( 3) are found to undergo an increase (in the absolute sense) of the rotational strength on lowering the temperature. This change is associated with a shift in the conformational equilibrium towards the twist conformation with the deuterium in the quasi-axial position. Using literature estimates for the rotational strength of the involved twist conformation, enthalpy differences of approximately − 1, − 4 and − 6 cal/mol were calculated for the respective conformational equilibria of 1,2 and 3. Empirical force field calculations were found to be in qualitative agreement with the experimental results when weaker non-bonded interactions for deuterium were used. The gas-phase spectra of 1 and 2 are reported which reveal a surprisingly well resolved vibrational fine structure. Two superimposed main progressions with a spacing of 1250 cm −1 extending up to the sixth overtone are identified. It is suggested that they represent progressions in the excited state carbonyl stretch mode built upon two pseudo origins.