THE NONPLANARITY OF THE ENOL TAUTOMER OF PHOSPHORUS-SUBSTITUTED PENTANE-2,4-DIONE - INFLUENCE ON HYDROGEN-BONDING STRENGTH, ENOL CONTENT AND ENTHALPIC STABILIZATION

The X fay crystal structure of entyl)-5,5-dimethyl-2-oxo-1,3,2-dioxaph osphorinane (II) reveals significant half-chair distortion of the axia lly oriented cis-enol ring and the tilt of the methyl groups in the en ol ring away from the phosphorinane internal oxygen atoms due to their repulsive interactions. The molecule also undergoes in-plane deformat ions. R(O ... O) = 2.410 angstrom in the enol moiety indicates very st rong hydrogen bonding. The enol form, delta(OH) and stereochemical ass ignments were obtained from H-1 and P-31 NMR and IR measurements in co mparison with the planar 4,6-dimethyl isomer (I) containing an equator ially oriented enol ring. The value of DELTAH-degrees for the axial-eq uatorial conformational equilibrium in CH2Cl2 was found to be -0.440 /- 0.100 kcal mol-1 in favour of the non-planar enol. Thermodynamic pa rameters for the keto-enol equilibrium in the isomeric compounds (I) a nd (II) were obtained in CH3CN providing evidence for a smaller enthal py of enolization of the non-planar enol.