CARBON VERSUS PHOSPHORUS SITE SELECTIVITY IN THE GAS-PHASE ANION MOLECULE REACTIONS OF DIMETHYL METHYLPHOSPHONATE

The reactions of dimethyl methylphosphonate and its conjugate base wit h 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 methylphosphonat e reacts readily with alcohols and carbonyl compounds; its reaction wi th alcohols yields products from proton transfer, proton transfer foll owed by substitution at carbon, and proton transfer followed by substi tution at phosphorus, while its reaction with carbonyl compounds gener ates 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 bas e strength from amide to hydrogen sulfide and in structure from locali zed heteroatomic bases and localized carbon bases to delocalized carba nions. Four reaction pathways account for the interaction of anions wi th dimethyl methylphosphonate: proton transfer, nucleophilic substitut ion at carbon, reductive elimination, and nucleophilic substitution at phosphorus. Proton transfer and nucleophilic substitution at carbon d ominate all reactions, while reductive elimination is observed only fo r the strongest base examined, amide. Methoxide and fluoride are the o nly 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 brackete d to be DELTAH-degrees(acid)[(CH3O)2(CH3)PO] = 373 +/- 3 kcal mol-1.