The Mechanism of Nucleophilic Displacements at Phosphorus in Chloro-Substituted Methylphosphonate Esters: P-O vs P-C Bond Cleavage: A DFT Study

Potential energy surfaces for the nucleophilic displacements at phosphorus in dimethyl methyl-, chloromethyl-, dichloromethyl-, and trichloromethylphosphonatcs have been computed at the B3LYP/6-31+G* level of theory, using IEF-PCM to account for the solvent effect. The results reveal that sequential addition of chlorine substituents on the methyl phosphonates increases the stability of transition states and intermediates which facilitate P-C bond cleavage. Thus, while nonsubstituted dimethyl methylphosphonate and dimethyl chloromethylphosphonate may undergo exclusive P-O bond cleavage, the trichlorinated analogue exclusively undergoes P-C bond dissociation. Dichloromethylphosphonic acid derivatives were found to be borderline cases: while P-O fission is the preferred process, P-C scission might also be feasible. The increase in stability of the corresponding transition states and intermediates can account for the enhancement in the apicophilicity of the methyl ligand upon substitution with chlorine atoms.