Intermolecular N–H Oxidative Addition of Ammonia, Alkylamines, and Arylamines to a Planar σ³‑Phosphorus Compound via an Entropy-Controlled Electrophilic Mechanism

Ammonia, alkyl amines, and aryl amines are found to undergo rapid intermolecular N–H oxidative addition to a planar mononuclear σ3-phosphorus compound (1). The pentacoordinate phosphorane products (1·[H]­[NHR]) are structurally robust, permitting full characterization by multinuclear NMR spectroscopy and single-crystal X-ray diffraction. Isothermal titration calorimetry was employed to quantify the enthalpy of the N–H oxidative addition of n-propylamine to 1 (nPrNH2 + 1 → 1·[H]­[NHnPr], ΔHrxn298 = −10.6 kcal/mol). The kinetics of n-propylamine N–H oxidative addition were monitored by in situ UV absorption spectroscopy and determination of the rate law showed an unusually large molecularity (ν = k[1]­[nPrNH2]3). Kinetic experiments conducted over the temperature range of 10–70 °C revealed that the reaction rate decreased with increasing temperature. Activation parameters extracted from an Eyring analysis (ΔH⧧ = −0.8 ± 0.4 kcal/mol, ΔS⧧ = −72 ± 2 cal/(mol·K)) indicate that the cleavage of strong N–H bonds by 1 is entropy controlled due to a highly ordered, high molecularity transition state. Density functional calculations indicate that a concerted oxidative addition via a classical three-center transition structure is energetically inaccessible. Rather, a stepwise heterolytic pathway is preferred, proceeding by initial amine-assisted N–H heterolysis upon complexation to the electrophilic phosphorus center followed by rate-controlling N → P proton transfer.