Phosphazene

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 hexachlorocyclotriphosphazene and bis(triphenylphosphine)iminium chloride.

The corresponding polymers are polyphosphazenes.

Phosphazene bases

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

Two commercially available phosphazene bases are BEMP with an acetonitrile pKa of the conjugate acid of 27.6 and the phosphorimidic triamide t-Bu-P4 (pKBH+ = 42.7) also known as Schwesinger base after one of its inventors ["Peralkylated Polyaminophosphazenes - Extremely Strong, Neutral Nitrogen Bases" Angewandte Chemie International Edition in English Volume 26, Issue 11, Date: November 1987, Pages: 1167-1169 Reinhard Schwesinger, Helmut Schlemper DOI|10.1002/anie.198711671] .

In one application t-Bu-P4 is employed in a nucleophilic addition converting the pival aldehyde to the alcohol ["Phosphazene base-promoted functionalization of aryltrimethylsilanes" Koichi Suzawa, Masahiro Ueno, Andrew E. H. Wheatley and Yoshinori Kondo Chem. Commun., 2006, 4850 - 4852, DOI| 10.1039/b611090h] :

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.

External links

* Phosphazene bases [http://www.sigmaaldrich.com/Area_of_Interest/Chemistry/Chemical_Synthesis/Product_Highlights/Phosphazenes.html Link]

References


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