Conformational effects in compounds with 6-membered rings—XI : Study of a conformational equilibrium in the gas phase and in solvents ranging from non-polar to water: 4-methoxycyclohexanone

Abstract

The conformation free energy difference (ΔG°( 1E → 1A)) for -4methoxycyclohexanone ( 1) has been determined in 34 solvents, ranging in polarity from tetramethylsilile to trifluoracetic acid, using weighted average vicinal coupling constants and two pairs of anancomeric model compounds ( 2–5). The free energy difference in the gas phase has been estimated from the equilibrium in silicone oil (DC 550), itself derived by extrapolation from data for mixtures with hexamethyldisiloxane, together with the differential solvation energies from gas phase to silicone oil extrapolated from the solvation energies of 2–5 obtained from GLC retention data. Calculations based on Abraham's reaction field theory of solvent effects on conformational equilibria correlate quite well with the general trend in ΔG°( 1E → 1A) in “normal” solvents but fail to account even qualitatively for the difference in ΔG°( 1E → 1A) between the gas phase and non-polar solvents.