title: Intermolecular N-H oxidative addition of ammonia, alkylamines, and arylamines to a planar σ(3)-phosphorus compound via an entropy-controlled electrophilic mechanism.
abstract: Ammonia, alkyl amines, and aryl amines are found to undergo rapid intermolecular N-H oxidative addition to a planar mononuclear σ(3)-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 ((n)PrNH2 + 1 → 1·[H][NH(n)Pr], ΔHrxn(298) = -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][(n)PrNH2](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.
authors.affiliation: Department of Chemistry, The Pennsylvania State University , University Park, Pennsylvania 16802, United States.
authors.forename: Sean M
authors.initials: SM
authors.lastname: McCarthy
authors.forename: Yi-Chun
authors.initials: YC
authors.lastname: Lin
authors.forename: Deepa
authors.initials: D
authors.lastname: Devarajan
authors.forename: Ji Woong
authors.initials: JW
authors.lastname: Chang
authors.forename: Hemant P
authors.initials: HP
authors.lastname: Yennawar
authors.forename: Robert M
authors.initials: RM
authors.lastname: Rioux
authors.forename: Daniel H
authors.initials: DH
authors.lastname: Ess
authors.forename: Alexander T
authors.initials: AT
authors.lastname: Radosevich