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-benzylami no-methylphosphonic acid with strong mineral acids leads to formation of th e corresponding heterocyclic amines and phosphoric acid. Kinetic studies of this cleavage reaction demonstrate that protonation has a remarkable influ ence on a cleavage of C-P bonds. In aq.H2SO4, cleavage of the acids 1-3 exh ibits 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 existen ce of the isotope effect shows that protons are involved on the rate-determ ining step. The data obtained suggest that the protonated phosphonate molec ule 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-charge d phosphorus moiety.