Phosphazenes refer to classes of organophosphorus compounds featuring phosphorus(V) with a double bond between P and N. One class of phosphazenes have the formula R−N=P(−NR2)3. These phosphazenes are also known as iminophosphoranes and phosphine imides. They are superbases.[1] Another class of compounds called phosphazenes are represented with the formula (−N=P(−X)2−)n, where X = halogen, alkoxy group, amide and other organyl groups. One example is hexachlorocyclotriphosphazene (−N=P(−Cl)2−)3. Bis(triphenylphosphine)iminium chloride [Ph3P=N=PPh3]+Clis also referred to as a phosphazene, where Ph = phenyl group. This article focuses on those phosphazenes with the formula R−N=P(−NR2)3.

Phosphazene bases

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Phosphazene bases are strong non-metallic non-ionic and low-nucleophilic bases. They are stronger bases than regular amine or amidine bases. Protonation takes place at a doubly bonded nitrogen atom. Related to phosphazene bases are the Verkade bases, which feature P(III) with three amido substituents and a transannular amine. The pKa's of [tert-Bu−(H)N=P(−N=P(−NR2)3)3]+, where R = Me and pyrrolidinyl, are 42.7 and 44, respectively. These are the highest pKa measured for the conjugate acid of charge-neutral molecular base.[2]

Phosphazene bases are established reagents in organic synthesis. Perhaps the best known 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.[3][clarification needed]

In one application t-Bu-P4 is employed in a nucleophilic addition converting the pivaldehyde to the alcohol:[4]

Phosphazene Base application

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

Besides organic synthesis, phosphazene bases are used as basic titrants in non-aqueous acid–base titration. Their advantages for this are: they are very strong bases in many solvents and their conjugate acids are inert and non-HBD cations.[clarification needed]

See also

References

  1. ^ Superbases for Organic Synthesis: Guanidines, Amidines, Phosphazenes and Related Organocatalysts Tsutomu Ishikawa ISBN 978-0-470-51800-7
  2. ^ Saame, Jaan; Rodima, Toomas; Tshepelevitsh, Sofja; Kütt, Agnes; Kaljurand, Ivari; Haljasorg, Tõiv; Koppel, Ilmar A.; Leito, Ivo (2016). "Experimental Basicities of Superbasic Phosphonium Ylides and Phosphazenes". The Journal of Organic Chemistry. 81 (17): 7349–7361. doi:10.1021/acs.joc.6b00872. PMID 27392255.
  3. ^ Schwesinger, Reinhard; Schlemper, Helmut (1987). "Peralkylated Polyaminophosphazenes— Extremely Strong, Neutral Nitrogen Bases". Angewandte Chemie International Edition in English. 26 (11): 1167. doi:10.1002/anie.198711671.
  4. ^ Suzawa, Koichi; Ueno, Masahiro; Wheatley, Andrew E. H.; Kondo, Yoshinori (2006). "Phosphazene base-promoted functionalization of aryltrimethylsilanes". Chemical Communications (46): 4850–4852. doi:10.1039/b611090h. PMID 17345750.