Carbon versus phosphorus site selectivity in the gas-phase anion-molecule reactions of dimethyl methylphosphonate

Article abstract of DOI:10.1021/ja00070a031

The reactions of dimethyl methylphosphonate and its conjugate base with a variety of anions and neutral substrates, respectively, have been examined with use of the thermally equilibrated conditions (298 K) of the flowing afterglow. The conjugate base of dimethyl methylphosphonate reacts readily with alcohols and carbonyl compounds; its reaction with alcohols yields products from proton transfer, proton transfer followed by substitution at carbon, and proton transfer followed by substitution at phosphorus, while its reaction with carbonyl compounds generates products from proton transfer, Horner-Emmons-Wadsworth reaction, addition/elimination, and adduct formation. Dimethyl methylphosphonate undergoes facile reaction with a diverse set of anions ranging in base strength from amide to hydrogen sulfide and in structure from localized heteroatomic bases and localized carbon bases to delocalized carbanions. Four reaction pathways account for the interaction of anions with dimethyl methylphosphonate: proton transfer, nucleophilic substitution at carbon, reductive elimination, and nucleophilic substitution at phosphorus. Proton transfer and nucleophilic substitution at carbon dominate all reactions, while reductive elimination is observed only for the strongest base examined, amide. Methoxide and fluoride are the only anions that react at phosphorus. A reaction coordinate diagram is used to interpret the reactions of dimethyl methylphosphonate and its conjugate base. The acidity of dimethyl methylphosphonate was bracketed to be ΔH^o_acid[(CH₃O)₂(CH ₃)PO] = 373 ± 3 kcal mol^-1.