Electrosynthesis of Iminophosphoranes: Accessing P(V) Ligands from P(III) Phosphines

Iminophosphorane P(V) compounds are accessed via electrochemical oxidation of commercially available P(III) ligands, including mono-, di- and tri-dentate phosphines as well as chiral phosphines. The reaction uses inexpensive bis(trimethylsilyl)carbodiimide as an efficient and safe aminating reagent. DFT calculations, cyclic voltammetry, and NMR spectroscopic studies provide insight into the reaction mechanism. The proposed mechanism based on the data reveals a special case of sequential paired electrolysis, namely a domino electrolysis process in which intermediates generated at the cathode are subsequently oxidized at the anode, followed by an additional convergent paired electrolysis process. DFT calculations of the frontier orbitals of the iminophosphorane are compared to those of the analogous P(III) phosphines and P(V) phosphine oxides. This reveals that N-cyano-iminophosphoranes have both a higher HOMO and lower LUMO than their analogous phosphine oxide, rendering them suitable for both sigma-donating and pi-back-bonding.