Structural properties of azaphosphirane and its W(CO)₅ complex. A density functional study

Properties and ring opening reactions are investigated for azaphosphirane and its P-phenyl and W(CO)₅ complex using density functional theory (B3LYP). Azaphosphirane has a relatively small N-inversion barrier of 10.8 kcal mol^-1 and a high 56.8 kcal mol^-1 'turnstile' P-inversion barrier. Its strain energy is 26.5 kcal mol^-1 at G3(MP2). The P-C bond is the weakest bond. Only 27.4 kcal mol^-1 is needed to break it, which is half that needed for both the C-N and P-N bonds. This P-C ring opening to the P,N-ylide is endothermic by 8.5 kcal mol^-1. P-phenyl substitution has little effect neither on the geometries nor on the energy of the ring opening. Complexation by W(CO)₅ leads to a tighter ring but the energy for breaking the P-C bond still requires 27.8 kcal mol^-1. The resulting P,N-ylide is only 3.9 kcal mol^-1 less stable than azaphosphirane. Cleaving either the C-N or P-N bond remain much more demanding processes. The calculations suggest that the reactivity of azaphosphirane may well have its origin in the readily accessible P,N-ylide. Its influence on the reaction of phosphinidenes with imines is discussed.