An Insight into the Reversible Dissociation and Chemical Reactivity of a Sterically Encumbered Diphosphane

Abstract

Abstract The known 1,1′,3,3′‐tetrahydro‐2,2′‐bi‐1,3,2‐diazaphosphole 4a was for the first time synthesized on a preparative scale and characterized by spectroscopy. Equilibrium constants for the dissociation 4a [rlarr2] 2 5a were determined from quantitative variable‐temperature electron paramagnetic resonance(VT‐EPR) studies and were used to obtain thermochemical data for the P–P bond homolysis. The calculated bond dissociation energy of 79(1) kJ mol –1 is substantially lower than the gas‐phase dissociation energies of small diphosphanes. Modeling of the molecular structure of 4a by DFT calculations was feasible if dispersion effects were included, but the calculations still failed to give a reasonable account of the energetics of the dissociation process. Chemical reactions of 4a with alkynes proceeded, depending on substrate structure, either as a radical‐induced diphosphanation of the triple bond or by a nonradical reaction to give an unprecedented ring metathesis.