ENOLIZATION OF THE PHOSPHORYL GROUP

Enolization of the phosphoryl group RR'P(O)CH(Y)+PPh3X- has been studied where Y = PPh3, CN, Ts, COOEt, CONEt2; R and R' = Et, Bu, Ph, EtO, BuO, PhO; and X = Cl, Br, ClO4, BF4. It has been established that substances with Y = Ph3+PX- are phosphaenols, but in substances with Y = CONEt2 the phosphoryl group cannot be enolized under any conditions. Phosphaenolization is favored by a high acidifying ability of the Y group, the ability of the X an ion to stabilize the phosphaenolic form due to formation of a hydrogen bond OH...X with the anion, and a low electronegativity of R and R' groups. To evaluate the acidifying ability of Y, this article defines specific sigma--constants dependent on the number of substituents at the alpha-carbon atom: sigma(CH3)-, sigma(CH2)-and sigma(CH)-. Their sums characterize the enolization ability of the phosphoryl group. The enolic structure in the solid state is possible if SIGMA-sigma(CH)n- > 2. If this sum lies in the range of 2 < SIGMA-sigma(CH)n- < 2.6 the phosphoryl-phosphaenol tautomerism can be expected in appropriate solutions. Acidic properties of the investigated compounds in MeNO2 and EtOH (absolute) have been determined. Calculations of the acidity of the phosphoryl CH forms (A) and of the phosphaenol OH forms (B) have been carried out.