Phosphazene Bases


Vol. 3, No. 1
Strong and Hindered Bases in Organic Synthesis
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Introduction / Phosphazene Bases / Verkade's Superbases / Traditional bases
Preservation of Reagents / Aldrich Schlenk-Type Glassware

 Phosphazene Bases Introduction / Monomeric Phosphazene Bases (P1 bases)
Dimeric Phosphazene Bases (P2) / Tetrameric Phosphazene Bases (P4)

Introduction

Phosphazene bases are extremely strong and uncharged bases, built on a unit where a nitrogen basic center is double bonded to pentavalent phosphorus.[1,2] Through oligomerization of the peralkylated triaminoiminophosphorane unit, the basicity improves dramatically (Figure 1). In the case of the monomeric phosphazene base, its basicity is about a 2-3 units beyond the basicity range of DBU (MeCNpKBH+ 24.3) and DBN, but reaches a MeCNpKBH+ of over 40 in the case of a tetrameric P4 phosphazene base (DBU = 1,8-Diazabicyclo[5.4.0]undec-7-ene, DBN = 1,5-Diazabicyclo- [4.3.0]non-5-ene).

We offer these reagents as monomeric (P1 and BEMP), dimeric (P2), and tetrameric (P4) bases, with different side chains to control their sterical hindrance. Scheme 1 shows the basicity of phosphazene bases compared to other bases (in the absolute acetonitrile scale), as well as compared to the acidity of important organic compounds or classes of compounds.

  • Properties of Phosphazene Bases:
    • High solubility in apolar to moderately polar solvents (e.g. hexane, toluene or THF)
    • Very strong solubilizing effects in appropriate weakly acidic compounds
    • Remarkably stable towards electrophilic attack, O2 and hydrolysis.
    • Depending on their base strength, slightly to extremely hygroscopic
    • Large sterical hindrance, depending on the type of side chain

     

  • Applications of Phosphazene Bases:
    • In situ generation of highly reactive "naked" anions, e.g. for alkylation reactions or for spectroscopic investigations.
    • Applicable in reactions where ionic bases cause solubility problems.
    • Applicable in reactions where ionic bases are sensitive towards oxidation or acylation.
    • Applicable in reactions where ionic bases result in Lewis-acid catalyzed side reactions (e.g. in aldol reactions, epoxide-opening, hydride shifts, elimination of alkoxide, polyanion-formation).

     

  • Benefits of using Phosphazene Bases:
    • Easier work-up through cleaner reactions.
    • Close to quantitative recovery.
    • Reaction rate enhancement.

 

References:

  1. Paquette, L. A., "Encyclopedia of Reagents for Organic Synthesis", Wiley, 1995, Vol. 6, 4110.
  2. Schwesinger, R. et al., Liebigs Ann. 1996, 1055.
  3. Schwesinger, R. et al., Chem. Ber. 1994, 127, 2435.