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Synthesis and carbanion reactions of methylphosphazenes Oakley, Richard Thomas

Abstract

The reaction of dimethyltrichlorophosphorane with methylamine hydrochloride provides a novel preparative route to the methylphosphazenes (NPMe₂)₃,₄, the separation of the two compounds being aided by the different solubilities of their salts (NPMe₂)₃. RCl and (NPMe₂)₄. 2RCl (R=H, Me) in acetonitrile. A general method for the synthesis of methylphosphazenes of large ring size (i.e. (NPMe₂)[sub n], n ≥6) by the methylation of the appropriate fluorophosphazene (NPF₂)[sub n] with methylmagnesium bromide has now been achieved. A variety of quaternary salts (NPMe₂)[sub n].RI and a number of trimethylphosphinimine derivatives have also been prepared. The differences in the properties of the cyclic and monomeric molecules have been interpreted in terms of their electronic structures. The methylphosphazenes (NPMe₂)₃₋₁₀ are crystalline solids at room temperature, and the molecular structures of (NPMe₂)[sub n] (n=4,5,7 and 8) suggest that their molecular flexibility is not suppressed by π-bonding; their conformations can be accounted for in terms of non-bonded and electrostatic interactions, which favour the staggered orientations (GT and CT) of successive ring bonds. A study has been made of the acidic properties of the P-methyl protons of methyl phosphazenes. The multiply-charged carbanions N₄P₄Me₈[sub –X](CH₂⁻)x (x=2,4) and the monocarbanion N₃P₃Ph₄Me(CH₂⁻) can be prepared by the deprotonation of the neutral methylphosphazenes with alkyllithiums. Their reactions with a variety of electrophiles (e.g. MeI, Me₃SiCl, CO₂, PhCOOEt, Br₂) establish that they are useful intermediates for the synthesis of novel organophosphazenes. The orientation of the ethyl groups in N₄P₄Me₆Et₂.2HCl shows that the second deprotonation of N₄P₄Me₈ occurs trans-antipodally to the first. The N-methyl quaternary salts (NPMe₂)₃,₄.MeI and N₃P₃Ph₄Me₂.MeI can also be deprotonated. The products are not the expected exocyclic ylids but the novel azaphosphorins Me₂[sub n-1] (NHMe)P[sub n]N[sub n-1]CH (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 (Equation 1). The driving force of the reaction is likely to [Chemical Diagrams] be the attainment of cyclic aromaticity. The ¹H n.m.r. 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 (e.g. pyridine). When KOtBu is used to deprotonate the quaternary salts, nucleophilic attack competes with proton removal, and the linear oxides (NHMe)(PMe₂N)[sub n]PMe₂O (n=2-4) have been isolated from these reactions. The azaphosphorins Me₂[sub n-1](NHMe)P[sub n]N[sub n-1]CH (n=3,4) are hydrolysed in aqueous ethanol to give the cyclic oxides Me₂[sub n-1](O)P[sub n]N[sub n-1]CH₂, and react with methyl iodide by a transylidation reaction to give the hydroiodides Me₂[sub n-1](NHMe) P[sub n]N[sub n-1]CH.HI. Their reaction with benzoyl chloride leads to the benzoylated derivatives Me₇(NHMe)P₄N₃CCOPh and Me₅ (NMeCCOPh)P₃N₂CCOPh, the initial substitution on carbon indicating that it is the primary basic centre. The molecular structures of azaphosphorins are consistent with the delocalization of charge from the endocyclic carbon into the cyclic PN skeleton. Their structural parameters can be accounted for by considering the effect of the replacement of a nitrogen atom with a =CR- group on the two equivalent components of the cyclic π-system.

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