The Phosphorus–Carbon Prototropic Tautomerism

New types of prototropic tautomerism in which the proton migrates between a carbon atom and a phosphorus-containing functional group are examined. It is shown that in all cases the position of the tautomeric equilibrium is determined by the relative acidities of the different forms: the CH acidity of the forms in which the proton is located at a carbon atom and the PH or PXH acidity (X = O, S, etc.) of the tautomers with the proton in the phosphorus-containing group. The method for the estimation of the relative acidities of different forms with the aid of the σ^ph constants of the substituents at the phosphorus atom and the σ_C^– constants of the substituents at the central carbon atom, characterising the acidifying influence of these substituents, has attracted attention in this connection. It has been shown that in all cases the equilibrium position is determined by the general characteristics of the protolytic acid–base tautomeric equilibrium.
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