A “phosphorus derivative” of aziridines: on the importance of ring strain energy and three heteropolar bonds in azaphosphiridines†

Compared to aziridines, azaphosphiridines, which formally result from the replacement of a carbon atom by phosphorus, have been much less studied. In this work, accurate values for one of the most prominent properties, the ring strain energy (RSE), have been theoretically examined for a wide range of azaphosphiridine derivatives. Strongly related aspects of interest for developing the use of azaphosphiridines in heteroatom and polymer chemistry are ring opening reactions and polymerisations, the latter facilitated by their significantly high RSE. While methyl groups have little influence on the RSE, complexation with different metal moieties increases the RSE in all cases, and an increase was also found upon oxidation to the corresponding P-oxides and other σ⁵λ⁵-P derivatives. The highest RSE was found for the P-protonated azaphosphiridinium cation and azaphosphiridines with exocyclic double bonds. A correlation of the RSEs with the relaxed force constants of the endocyclic ring bonds and AIM-derived parameters in the ring critical points, such as the electron density, ρ(r), and the Lagrangian of the kinetic energy, G(r), was found. A relatively low barrier to P–C bond cleavage via nucleophilic attack of MeNH₂ on phosphorus points to the possibility of ring-opening polymerisation.