AN AB-INITIO STUDY OF THE MECHANISM OF THE ATHERTON-TODD REACTION BETWEEN DIMETHYL PHOSPHONATE AND CHLOROSUBSTITUTED AND FLUORO-SUBSTITUTED METHANES

A new mechanism for the amine promoted Atherton-Todd reaction is proposed, based on an ab initio theoretical study of alternative reaction pathways. Conformational analyses have been performed on a set of model phosphorus-containing species, including dimethyl phosphonate, (CH3O)2P(O)H, dimethyl chlorophosphate, (CH3O)2P(O)Cl, monomethyl phosphonate, (CH3O)(HO)P(O)H, and its anion, [(CH3O)PHOO]- phosphonic acid, (HO)2P(O)H, and its anion, [(HO)PHOO]-, dimethyl phosphite, (CH3O)2POH, and its anion, [(CH3O)2PO]-, as well as the anions of monomethyl phosphite, [(CH3O)(HO)PO]-, and phosphorous acid, [(HO)2PO]-. In addition a series of chloro- and fluoro-substituted methanes CCl4-nFn, their hydrogen derivatives CHCl3-nFn, and their corresponding anions [CCl3-nFn]-, for n = 0, 1, 2, have also been computed. Computations have also been carried out on NH3, NH4+, and CH3NH3+. Reaction pathways which correspond to the different possible steps in the mechanism of the Atherton-Todd reaction have been theoretically explored involving the location of a number of transition structures. The geometries of the studied systems have been fully optimized with the 6-31+G* basis set, whereas frozen-core Moller-Plesset (MP2) perturbation theory has been applied to correct for correlation effects. Frequency calculations have been performed at HF/6-31+G* on the optimized structures in order to ascertain the type of the located stationary points and to obtain the corresponding zero point energies. The computational results provide an explanation for the experimentally observed order of reactivity of dimethyl phosphonate toward different chloro- and fluoro-substituted methanes under the Atherton-Todd reaction conditions.