Acid-catalyzed cleavage of some chromone, coumarin and pyrone derivatives of aminomethylphosphonic acid. Products and kinetics of the reaction

Treatment of chromone-2, coumarin-4 and pyrone-2 derivatives of N-benzylamino-methylphosphonic acid with strong mineral acids leads to formation of the corresponding heterocyclic amines and phosphoric acid. Kinetic studies of this cleavage reaction demonstrate that protonation has a remarkable influence on a cleavage of C-P bonds. In aq.H2SO4, cleavage of the acids 1-3 exhibits a kinetic dependence on [H+]. The measured solvent isotope effect (k(H)//k(D)) was about 1.5 for the 1 and 2 and only 1.1 for the 3. The existence of the isotope effect shows that protons are involved on the rate-determining step. The data obtained suggest that the protonated phosphonate molecule is split by a dissociative mechanism with A-S(E)2 character and this is combined with an elimination of the phosphonate group as a positive-charged phosphorus moiety.