The deprotonation and rearrangement of N-methyl methylphosphazenium quaternary salts: a novel synthetic route to cyclic azaphosphorins

The N-methyl methylphosphazenium salts (NPMe2)3,4•MeI and N3P3Ph4Me2•MeI can be deprotonated by a variety of bases to yield the novel azaphosphorins Me2n−1(NHMe)PnNn−1CH (n = 3,4) and Me(NHMe)Ph4P3N2CH, formed by a rearrangement in which the methylated nitrogen atom is displaced from the PN ring by the initially produced exocyclic methylene group. The 1H nmr spectra of the azaphosphorins indicate a rapid proton exchange between the endocyclic carbon and the exocyclic nitrogen, which can be slowed by the addition of an auxiliary base. When KO-t-Bu reacts with the quaternary salts, nucleophilic attack competes with proton removal, and the linear oxides (NHMe)(PMe2N)nPMe2O (n = 2−4) have been isolated from these reactions. The azaphosphorins Me2n−1(NHMe)PnNn−1CH (n = 3, 4) are hydrolysed in aqueous ethanol to give the cyclic oxides Me2n−1(O)PnNn−1CH2, and react with methyl iodide by a proton transfer reaction to give the hydroiodides Me2n−1(NHMe)PnNn−1CH•HI. Their reaction with benzoyl chloride leads to the derivatives Me7(NHMe)P4N3CCOPh and Me7(NMeCOPh)P3N2CCOPh, the initial substitution on carbon indicating that it is the primary basic centre. Model calculations of π-electron energies suggest that both the azaphosphorin rearrangement and the proton exchange reactions depend on the relative orbital electronegativity of the ring and exocyclic atoms, the less electronegative atom being more stable in the endocyclic position.