| Abstract: | The generation of alkyl-substituted monomeric metaphosphoric acid esters (ROPO2) 4 in solution is described, utilizing two different methods based on the condensation of alkyl phosphorodichloridates either with anhydrous potassium hydrogen carbonate or with the novel disodium pyrocarbonate salt 5, which is formed in quantitative yield from diethyl pyrocarbonate by treatment with two equivalents of sodium trimethylsilanolate in THF at 0°C. The first method is believed to proceed via a transient mixed carbonic-phosphoric anhydride 3, which decomposes at room temperature with release of 1 mol each of hydrogen chloride and carbon dioxide to produce 4. In the other method, 31P NMR spectroscopy indicated the involvement of a cyclic pyrocarbonate phosphate (2-alkoxy-1,3,5-trioxaphosphorinane-4,6-dione-2-oxide) 6, which decomposed in situ with the release of 2 mol of carbon dioxide. In both cases, the metaphosphate thus formed spontaneously self-condensed to produce polymeric species with P-O-P bonds having characteristic 31P NMR signals clustered in the δ -12 and -24 regions. When metaphosphate 4 is generated by either process in the presence of styrene oxide, polymerization is avoided, and trapping occurs with the exclusive formation of a diastereomeric mixture of 2-alkoxy-1,3,2-dioxaphospholane-2-oxides 7 (cis, trans). In order to shed some mechanistic light on the formation of 7, 31P NMR studies have been carried out on the individual trapping reactions between (—)-methyl metaphosphate 4d and racemic styrene oxide, as well as its optically pure forms. Based on evidence that these transformations proceed by inversion of configuration at the phenyl-bearing carbon of styrene oxide, a mechanism is invoked whereby the dipolar metaphosphate 4d reacts both as an electrophile and as a nucleophile in a self-catalyzed process via the activated complex 9d. © 1996 John Wiley & Sons, Inc. |
