The non-planarity of the enol tautomer of phosphorus-substituted pentane-2,4-dione. Influence on hydrogen bonding strength, enol content and enthalpic stabilization

Abstract

The X-ray crystal structure of 2-(2′,4′-dioxo-3′-pentyl)-5,5-dimethyl-2-oxo-1,3,2-dioxaphosphorinane (II) reveals significant half-chair distortion of the axially oriented cis-enol ring and the tilt of the methyl groups in the enol ring away from the phosphorinane internal oxygen atoms due to their repulsive interactions. The molecule also undergoes in-plane deformations. R(O⋯O)=2.410 Å in the enol moiety indicates very strong hydrogen bonding. The enol form, δOH and stereochemical assignments were obtained from 1H and 31P NMR and IR measurements in comparison with the planar 4,6-dimethyl isomer (I) containing an equatorially oriented enol ring. The value of ΔH° for the axial-equatorial conformational equilibrium in CH2Cl2 was found to be -0.440 ± 0.100 kcal mol-1 in favour of the non-planar enol. Thermodynamic parameters for the keto-enol equilibrium in the isomeric compounds (I) and (II) were obtained in CH3CN providing evidence for a smaller enthalpy of enolization of the non-planar enol. © 1994.