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

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 reversible arrow r 2 5a were determined from quantitative variable-temperature electron paramagnetic resonance(VT-EPR) studies and were used to obtain thermochemical data for the PP bond homolysis. The calculated bond dissociation energy of 79(1) kJ?mol1 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.