phosphazene
{{Short description|1=Organophosphorus compound with pentavalent phosphorus having P=N bonds}}
Phosphazenes refer to various classes of organophosphorus compounds featuring phosphorus(V) with a double bond between P and N. One class of phosphazenes have the formula {{chem2|R\sN\dP(\sNR2)3}}. These phosphazenes are also known as iminophosphoranes and phosphine imides. They are superbases.Superbases for Organic Synthesis: Guanidines, Amidines, Phosphazenes and Related Organocatalysts Tsutomu Ishikawa {{ISBN|978-0-470-51800-7}}
BEMP and ''t''-Bu-P<sub>4</sub>
Well known phosphazene bases are BEMP (2-tert-Butylimino-2-diEthylamino-1,3-diMethylperhydro-1,3,2-diazaPhosphorine) 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.{{Cite journal| doi =10.1002/anie.198711671| title =Peralkylated Polyaminophosphazenes— Extremely Strong, Neutral Nitrogen Bases| journal =Angewandte Chemie International Edition in English| volume =26| issue =11| pages =1167| year =1987| last1 =Schwesinger| first1 =Reinhard| last2 =Schlemper| first2 =Helmut}} BEMP and P4-t-Bu|t-Bu-P4 have attracted attention because they are low-nucleophilic, which precludes their participating in competing reactions. Being non-ionic ("charge-neutral"), they are soluble in nonpolar solvents. Protonation takes place at a doubly bonded nitrogen atom.{{cite journal |doi=10.1002/anie.199313611 |title=How Strong and How Hindered Can Uncharged Phosphazene Bases Be? |date=1993 |last1=Schwesinger |first1=Reinhard |last2=Hasenfratz |first2=Christian |last3=Schlemper |first3=Helmut |last4=Walz |first4=Leonhard |last5=Peters |first5=Eva-Maria |last6=Peters |first6=Karl |last7=von Schnering |first7=Hans Georg |journal=Angewandte Chemie International Edition in English |volume=32 |issue=9 |pages=1361–1363 }} The pKa's of {{chem2|tert\-Bu\s(H)N\dP(\sN\dP(\sNR2)3)3]+}}, where R = Me and pyrrolidinyl, are 42.7 and 44, respectively. These are the highest pKa recorded for the conjugate acid of charge-neutral molecular base.{{cite journal|doi=10.1021/acs.joc.6b00872|pmid=27392255|title=Experimental Basicities of Superbasic Phosphonium Ylides and Phosphazenes|journal=The Journal of Organic Chemistry|volume=81|issue=17|pages=7349–7361|year=2016|last1=Saame|first1 =Jaan|last2=Rodima|first2=Toomas|last3=Tshepelevitsh|first3=Sofja|last4 =Kütt|first4=Agnes|last5=Kaljurand|first5=Ivari|last6=Haljasorg|first6=Tõiv|last7=Koppel|first7=Ilmar A.|last8=Leito|first8=Ivo}}
P4-t-Bu_Strukturformel.svg|t-Bu-P4
BEMP Phosphazene.svg|BEMP
In one implemention, t-Bu-P4 catalyzes the conversion of pivaldehyde to the alcohol:{{Cite journal| doi =10.1039/b611090h| title =Phosphazene base-promoted functionalization of aryltrimethylsilanes| journal =Chemical Communications| issue =46| pages =4850–4852| year =2006| last1 =Suzawa| first1 =Koichi| last2 =Ueno| first2 =Masahiro| last3 =Wheatley| first3 =Andrew E. H.| last4 =Kondo| first4 =Yoshinori| pmid =17345750}} Phosphazene bases have been used as basic titrants in non-aqueous acid–base titrations.
Other classes of phosphazenes
Also called phosphazenes are represented with the formula {{chem2|(\sN\dP(\sX)2\s)_{n}|}}, where X = halogen, alkoxy group, amide and other organyl groups. One example is hexachlorocyclotriphosphazene {{chem2|(\sN\dP(\sCl)2\s)3}}. Bis(triphenylphosphine)iminium chloride {{chem2|[Ph3P\dN\dPPh3]+Cl-}}is also referred to as a phosphazene, where Ph = phenyl group. The present article focuses on those phosphazenes with the formula {{chem2|R\sN\dP(\sNR2)3}}.
See also
- Verkade bases feature P(III) with three amido substituents and a transannular amine
- Cyclodiphosphazane
- Hexachlorophosphazene
- Polyphosphazene