Diazaphosphinanes as Hydride, Hydrogen-atom, Proton or Electron Donors under Transition-metal-free Conditions: Thermodynamics, Kinetics, and Synthetic Applications

Exploration of new hydrogen donors is in large demand for hydrogenation chemistry. Herein, we developed a new 1,3,2-diazaphosphinane 1a, which can serve as a hydride, hydrogen-atom or proton donor without transition-metal mediation. The thermodynamics and kinetics of these three pathways of 1a, together with those of its analog 1b, were investigated in acetonitrile. Noteworthily, the reduction potentials (Ered) of the phosphenium cations 1a-[P]+ and 1b-[P]+ are extremely low, as being -1.94 and -2.39 V (vs. Fc+/0), respectively, which enable corresponding phosphinyl radicals to function as neutral super-electron-donors. Kinetic studies revealed an extraordinarily large kinetic isotope effect KIE(1a) of 31.3 for the hydrogen-atom transfer from 1a to 2,4,6-tri-(tert-butyl)-phenoxyl radical, implying a tunneling effect. Furthermore, successful applications of these diverse P-H bond energetic parameters in organic syntheses were exemplified, shedding light on more exploitations of these versatile diazaphosphinane reagents in organic chemistry