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

Studies of the acid- catalyzed cleavage of 2-pyridyl and 4-pyridyl(amino)me thylphosphonic acids demonstrate that protonation has a profound effect on the cleavage of C-P bonds. In H2SO4, and other strong mineral acids the cle avage of aminophosphonic acids 1 and 4 exhibits a kinetic depedence on Zuck er-Hammett acidity function (h(v)), with rate law, v = k[phosphonate].h(O), and also fulfil the Bunnett approach on acidity function. The measured sol vent isotope effect k(H)/k(D)is small, and varied from 1.16 for 1b to 1.18 for 4b. The isotope effect shows that protons are involved on the rate-dete rmining step of the cleavage. All of this evidence indicates that the cleav age of 1 or 4 shows A-S(E)2 character substitution reaction. Calculated act ivation parameters and some chemical experiments indicate that the protonat ed phosphonate molecule is split by a dissociative mechanism, combined with formation of a positive-charged phosphorus moiety.