New insights into phosphorus biphilicity: Stereochemistry of monocyclic aminophosphoranes formation

Abstract

Oxidative addition of aliphatic amine on monocyclic 2-R-1,3,2-dioxaphospholane (R = methyl or phenyl) leading to phosphoranes bearing a P H axial bond was investigated using a combined theoretical and experimental approach. The reaction followed first order kinetics for both reactants. The activation parameters were consistent with a concerted mechanism exhibiting stereo, enantio, and regio specificities.Systematic density functional theory studies including reactivity descriptors, structural parameters, determination of all possible phosphorane isomers, their transition states and their formation mechanisms were carried out to delineate the reaction pathway. Depending on the nature of the atom in the final apical position (H, N of the amine, or C of the methyl), three types of phosphoranes were identified. Energy profiles, using the intrinsic reaction coordinate technique for the formation of each isomer, were established. It was shown that the phosphorus biphilicity acts in three consecutive sequences, namely nitrogen nucleophilic attack leading to a supermolecule structure, nucleophilic behavior of phosphorus in the transition state zone of influence, and electrophilic approach of the nitrogen atom to complete the reaction. Phosphorane with an equatorial P H bond was found to be a kinetic product that transforms rapidly to the thermodynamically stable isomer with axial P H via two consecutive Berry pseudo-rotations. A new tool, referred to reactive internal reaction coordinates, was introduced to represent the reaction path more clearly.