Phosphazene Bases

Verkade's Superbases | Traditional Bases | Preservation of Reagents | Schlenk Type Glassware

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 (^MeCNpK_BH+ 24.3) and DBN, but reaches a ^MeCNpK_BH+ 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.