Acid-catalyzed cleavage of 2-pyridyl and 4-pyridyl derivatives of aminomethylphosphonic acid. Kinetic and chemical arguments for a mechanism with A-S_E2 character

Studies of the acid-catalyzed cleavage of 2-pyridyl and 4-pyridyl(amino)methylphosphonic acids demonstrate that protonation has a profound effect on the cleavage of C-P bonds. In H₂SO₄, and other strong mineral acids the cleavage of aminophosphonic acids 1 and 4 exhibits a kinetic depedence on Zucker-Hammett acidity function (h₀), with rate law, v = k[phosphonate]·h₀, and also fulfil the Bunnett approach on acidity function. The measured solvent isotope effect k_H/k_D is small, and varied from 1.46 for 1b to 1.18 for 4b. The isotope effect shows that protons are involved on the rate determining step of the cleavage. All of this evidence indicates that the cleavage of 1 or 4 shows A-S_E2 character substitution reaction. Calculated activation parameters and some chemical experiments indicate that the protonated phosphonate molecule is split by a dissociative mechanism, combined with formation of a positive-charged phosphorus moiety.