Phosphazene

A phosphazene is any of a class of chemical compounds in which a phosphorus atom is covalently linked to a nitrogen atom by a double bond and to three other atoms or radicals by single bonds. Two examples are hexachlorophosphazene and bis(triphenylphosphine)iminium chloride.

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The corresponding polymers are polyphosphazenes.

Phosphazene bases

Phosphazene bases are strong non-metallic non-ionic and non-nucleophilic bases. They are stronger bases than regular amine bases such as DBU or Hünig's base. Protonation takes place at a nitrogen atom.

Two commercially available phosphazene bases are BEMP with a pKa of the conjugate acid of 27.6 and the phosphorimic triaminde t-Bu-4P (pK_BH+ = 41.9) also known as Schwesinger base after one of its inventors^[1].

In one application t-Bu-4P is employed in a nucleophilic addition converting the tert-butyl aldehyde to the alcohol^[2] :

The active nucleophile is believed to be a highly reactive phosphazenium species with full negative charge on the arene sp2 carbon.

The related proazaphosphatrane superbases have a saturated P(NR)₃ structure and protonation takes place at phosphorus.

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