GENERATION AND TRAPPING OF MONOMERIC METAPHOSPHORIC ACID-ESTERS IN SOLUTION - MECHANISTIC INVESTIGATION INTO THE FRAGMENTATION OF MIXED CARBONIC-PHOSPHORIC ACID ANHYDRIDES AND THE CHELOTROPIC BREAKDOWN OF CYCLIC PYROCARBONATE ESTERS

The generation of alkyl-substituted monomeric meta-phosphoric acid est ers (ROPO(2)) 4 in solution is described, utilizing two different meth ods based on the condensation of alkyl phosphorodichloridates either w ith anhydrous potassium hydrogen carbonate or with the novel disodium pyrocarbonate salt 5, which is formed in quantitative yield from dieth yl pyrocarbonate by treatment with two equivalents of sodium trimethyl silanolate in THF at 0 degrees C. The first method is believed to proc eed via a transient mixed carbonicphosphoric anhydride 3, which decomp oses at room temperature with release of 1 mol each of hydrogen chlori de and carbon dioxide to produce 4. In the other method, P-31 NMR spec troscopy indicated the involvement of a cyclic pyrocarbonate phosphate (2-alkoxy-1, 3,5-trioxaphosphorinane-4, 6-dione-2-oxide) 6, which dec omposed in situ with the release of 2 mol of carbon dioxide. In both c ares, the metaphosphate thus formed spontaneously self-condensed to pr oduce polymeric species with P-O-P bonds having characteristic P-31 NM R signals clustered in the delta -12 and -24 regions. When metaphospha te 4 is generated by either process in the presence of styrene oxide, polymerization is avoided, and trapping occurs with the exclusive form ation of a diastereomeric mixture of 2-alkoxy-1, 3,2-dioxaphospholane- 2-oxides 7 (cis, trans). In order to shed some mechanistic light on th e formation of 7, P-31 NMR studies have been carried out on the indivi dual trapping reactions between (-)-menthyl metaphosphate 4d and racem ic styrene oxide, as well as ifs optically pure forms. Based on eviden ce that these transformations proceed by inversion of configuration at the phenyl-bearing carbon of styrene oxide, a mechanism is invoked wh ereby the dipolar meta-phosphate 4d reacts both as an electrophile and as a nucleophile in a self-catalyzed process via the activated comple x 9d. (C) 1996 John Wiley & Sons, Inc.