Methyl metaphosphate, electrophilicity

The reaction of 151 with methanol to give dimethyl phosphate (154) or with N-methylaniline to form the phosphoramidate 155 and (presumably) the pyrophosphate 156 complies with expectations. The formation of dimethyl phosphate does not constitute, however, reliable evidence for the formation of intermediate 151 since methanol can also react with polymeric metaphosphates to give dimethyl phosphate. On the other hand, reaction of polyphosphates with N-methylaniline to give 156 can be ruled out (control experiments). The formation of 156 might encourage speculations whether the reaction with N,N-diethylaniline might involve initial preferential reaction of monomeric methyl metaphosphate via interaction with the nitrogen lone pair to form a phosphoric ester amide which is cleaved to phosphates or pyrophosphates on subsequent work-up (water, methanol). Such a reaction route would at least explain the low extent of electrophilic aromatic substitution by methyl metaphosphate. 

These data imply that dioxane and acetonitrile suppress the electrophilic character of the reagent. The simplest interpretation of these results is that methyl metaphosphate itself is the reagent for electrophilic substitution, but since it will react readily with any unshared pair of electrons, it is deactivated by dioxane or acetonitrile to produce Compounds IV and V shown below. Since S03 reacts with dioxane to form a crystalline adduct, VI, a parallel reaction of metaphosphate appears reasonable. 

Phosphorus w) Compounds. Although the monomeric PO3 ion is severely electron-deficient, S.C.F. M.O. calculations have indicated a greater overlap population in both the a- and ir-systems for this species than in the isoelectronic nitrate ion. In view of this, it has been suggested that kinetic rather than thermodynamic reasons should be sought for the instability of PO3. The formation of monomeric methyl metaphosphate, Me0P02, reported in an earlier volume, has now been substantiated by identifying the methyl ester of p-diethyl-aminobenzenephosphonic acid as the product from the reaction with NN-diethyl-aniline at low temperatures. Substitution into the aromatic ring provided a measure of the electrophilicity of the monomer. 

Phosphorus.—Oxoanions. Evidence for the formation of the metaphosphate anion [POs] as an intermediate in acyl phosphate reactions has been reported, and ah initio calculations on this species suggest that its high electrophilicity may be due to contributions from both n and o molecular orbitals. Its methyl ester, MeOPOa, can substitute electrophilically in the aromatic ring for activated aro-... 

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