Protonation of Verkade bases: a theoretical study

Density functional calculations have been carried out to investigate the protonation pathways of phosphatranes, also known as Verkade bases, as a function of different equatorial substituents. The most favorable site of protonation is found to be at the phosphorus atom rather than the equatorial atom. The strength of transannular P⋯Nax interaction is responsible for the higher proton affinity of these bases. The proton transfer pathway from the phosphorus atom to the equatorial atom is kinetically very difficult owing to a high activation barrier. The initial deprotonation from the equatorial nitrogen atom is also favored by the presence of transannular interaction. The computed stabilization energies are found to depend largely on the phosphorus–equatorial bond strengths.